Publisher: Elsevier   (Total: 3147 journals)

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Showing 1 - 200 of 3147 Journals sorted alphabetically
Academic Pediatrics     Hybrid Journal   (Followers: 39, SJR: 1.655, CiteScore: 2)
Academic Radiology     Hybrid Journal   (Followers: 26, SJR: 1.015, CiteScore: 2)
Accident Analysis & Prevention     Partially Free   (Followers: 106, SJR: 1.462, CiteScore: 3)
Accounting Forum     Hybrid Journal   (Followers: 28, SJR: 0.932, CiteScore: 2)
Accounting, Organizations and Society     Hybrid Journal   (Followers: 44, SJR: 1.771, CiteScore: 3)
Achievements in the Life Sciences     Open Access   (Followers: 7)
Acta Anaesthesiologica Taiwanica     Open Access   (Followers: 6)
Acta Astronautica     Hybrid Journal   (Followers: 447, SJR: 0.758, CiteScore: 2)
Acta Automatica Sinica     Full-text available via subscription   (Followers: 2)
Acta Biomaterialia     Hybrid Journal   (Followers: 30, SJR: 1.967, CiteScore: 7)
Acta Colombiana de Cuidado Intensivo     Full-text available via subscription   (Followers: 3)
Acta de Investigación Psicológica     Open Access   (Followers: 2)
Acta Ecologica Sinica     Open Access   (Followers: 12, SJR: 0.18, CiteScore: 1)
Acta Histochemica     Hybrid Journal   (Followers: 5, SJR: 0.661, CiteScore: 2)
Acta Materialia     Hybrid Journal   (Followers: 325, SJR: 3.263, CiteScore: 6)
Acta Mathematica Scientia     Full-text available via subscription   (Followers: 5, SJR: 0.504, CiteScore: 1)
Acta Mechanica Solida Sinica     Full-text available via subscription   (Followers: 9, SJR: 0.542, CiteScore: 1)
Acta Oecologica     Hybrid Journal   (Followers: 12, SJR: 0.834, CiteScore: 2)
Acta Otorrinolaringologica (English Edition)     Full-text available via subscription  
Acta Otorrinolaringológica Española     Full-text available via subscription   (Followers: 2, SJR: 0.307, CiteScore: 0)
Acta Pharmaceutica Sinica B     Open Access   (Followers: 2, SJR: 1.793, CiteScore: 6)
Acta Psychologica     Hybrid Journal   (Followers: 26, SJR: 1.331, CiteScore: 2)
Acta Sociológica     Open Access   (Followers: 1)
Acta Tropica     Hybrid Journal   (Followers: 6, SJR: 1.052, CiteScore: 2)
Acta Urológica Portuguesa     Open Access   (Followers: 1)
Actas Dermo-Sifiliograficas     Full-text available via subscription   (Followers: 3, SJR: 0.374, CiteScore: 1)
Actas Dermo-Sifiliográficas (English Edition)     Full-text available via subscription   (Followers: 2)
Actas Urológicas Españolas     Full-text available via subscription   (Followers: 3, SJR: 0.344, CiteScore: 1)
Actas Urológicas Españolas (English Edition)     Full-text available via subscription   (Followers: 1)
Actualites Pharmaceutiques     Full-text available via subscription   (Followers: 7, SJR: 0.19, CiteScore: 0)
Actualites Pharmaceutiques Hospitalieres     Full-text available via subscription   (Followers: 3)
Acupuncture and Related Therapies     Hybrid Journal   (Followers: 8)
Acute Pain     Full-text available via subscription   (Followers: 15, SJR: 2.671, CiteScore: 5)
Ad Hoc Networks     Hybrid Journal   (Followers: 11, SJR: 0.53, CiteScore: 4)
Addictive Behaviors     Hybrid Journal   (Followers: 18, SJR: 1.29, CiteScore: 3)
Addictive Behaviors Reports     Open Access   (Followers: 9, SJR: 0.755, CiteScore: 2)
Additive Manufacturing     Hybrid Journal   (Followers: 13, SJR: 2.611, CiteScore: 8)
Additives for Polymers     Full-text available via subscription   (Followers: 22)
Advanced Drug Delivery Reviews     Hybrid Journal   (Followers: 190, SJR: 4.09, CiteScore: 13)
Advanced Engineering Informatics     Hybrid Journal   (Followers: 13, SJR: 1.167, CiteScore: 4)
Advanced Powder Technology     Hybrid Journal   (Followers: 17, SJR: 0.694, CiteScore: 3)
Advances in Accounting     Hybrid Journal   (Followers: 9, SJR: 0.277, CiteScore: 1)
Advances in Agronomy     Full-text available via subscription   (Followers: 17, SJR: 2.384, CiteScore: 5)
Advances in Anesthesia     Full-text available via subscription   (Followers: 30, SJR: 0.126, CiteScore: 0)
Advances in Antiviral Drug Design     Full-text available via subscription   (Followers: 2)
Advances in Applied Mathematics     Full-text available via subscription   (Followers: 12, SJR: 0.992, CiteScore: 1)
Advances in Applied Mechanics     Full-text available via subscription   (Followers: 12, SJR: 1.551, CiteScore: 4)
Advances in Applied Microbiology     Full-text available via subscription   (Followers: 24, SJR: 2.089, CiteScore: 5)
Advances In Atomic, Molecular, and Optical Physics     Full-text available via subscription   (Followers: 15, SJR: 0.572, CiteScore: 2)
Advances in Biological Regulation     Hybrid Journal   (Followers: 4, SJR: 2.61, CiteScore: 7)
Advances in Botanical Research     Full-text available via subscription   (Followers: 1, SJR: 0.686, CiteScore: 2)
Advances in Cancer Research     Full-text available via subscription   (Followers: 35, SJR: 3.043, CiteScore: 6)
Advances in Carbohydrate Chemistry and Biochemistry     Full-text available via subscription   (Followers: 9, SJR: 1.453, CiteScore: 2)
Advances in Catalysis     Full-text available via subscription   (Followers: 5, SJR: 1.992, CiteScore: 5)
Advances in Cell Aging and Gerontology     Full-text available via subscription   (Followers: 5)
Advances in Cellular and Molecular Biology of Membranes and Organelles     Full-text available via subscription   (Followers: 14)
Advances in Chemical Engineering     Full-text available via subscription   (Followers: 29, SJR: 0.156, CiteScore: 1)
Advances in Child Development and Behavior     Full-text available via subscription   (Followers: 11, SJR: 0.713, CiteScore: 1)
Advances in Chronic Kidney Disease     Full-text available via subscription   (Followers: 11, SJR: 1.316, CiteScore: 2)
Advances in Clinical Chemistry     Full-text available via subscription   (Followers: 26, SJR: 1.562, CiteScore: 3)
Advances in Colloid and Interface Science     Full-text available via subscription   (Followers: 21, SJR: 1.977, CiteScore: 8)
Advances in Computers     Full-text available via subscription   (Followers: 14, SJR: 0.205, CiteScore: 1)
Advances in Dermatology     Full-text available via subscription   (Followers: 16)
Advances in Developmental Biology     Full-text available via subscription   (Followers: 14)
Advances in Digestive Medicine     Open Access   (Followers: 13)
Advances in DNA Sequence-Specific Agents     Full-text available via subscription   (Followers: 7)
Advances in Drug Research     Full-text available via subscription   (Followers: 26)
Advances in Ecological Research     Full-text available via subscription   (Followers: 45, SJR: 2.524, CiteScore: 4)
Advances in Engineering Software     Hybrid Journal   (Followers: 30, SJR: 1.159, CiteScore: 4)
Advances in Experimental Biology     Full-text available via subscription   (Followers: 9)
Advances in Experimental Social Psychology     Full-text available via subscription   (Followers: 51, SJR: 5.39, CiteScore: 8)
Advances in Exploration Geophysics     Full-text available via subscription   (Followers: 2)
Advances in Fluorine Science     Full-text available via subscription   (Followers: 9)
Advances in Food and Nutrition Research     Full-text available via subscription   (Followers: 68, SJR: 0.591, CiteScore: 2)
Advances in Fuel Cells     Full-text available via subscription   (Followers: 17)
Advances in Genetics     Full-text available via subscription   (Followers: 21, SJR: 1.354, CiteScore: 4)
Advances in Genome Biology     Full-text available via subscription   (Followers: 12, SJR: 12.74, CiteScore: 13)
Advances in Geophysics     Full-text available via subscription   (Followers: 8, SJR: 1.193, CiteScore: 3)
Advances in Heat Transfer     Full-text available via subscription   (Followers: 26, SJR: 0.368, CiteScore: 1)
Advances in Heterocyclic Chemistry     Full-text available via subscription   (Followers: 11, SJR: 0.749, CiteScore: 3)
Advances in Human Factors/Ergonomics     Full-text available via subscription   (Followers: 26)
Advances in Imaging and Electron Physics     Full-text available via subscription   (Followers: 4, SJR: 0.193, CiteScore: 0)
Advances in Immunology     Full-text available via subscription   (Followers: 37, SJR: 4.433, CiteScore: 6)
Advances in Inorganic Chemistry     Full-text available via subscription   (Followers: 10, SJR: 1.163, CiteScore: 2)
Advances in Insect Physiology     Full-text available via subscription   (Followers: 2, SJR: 1.938, CiteScore: 3)
Advances in Integrative Medicine     Hybrid Journal   (Followers: 6, SJR: 0.176, CiteScore: 0)
Advances in Intl. Accounting     Full-text available via subscription   (Followers: 3)
Advances in Life Course Research     Hybrid Journal   (Followers: 9, SJR: 0.682, CiteScore: 2)
Advances in Lipobiology     Full-text available via subscription   (Followers: 1)
Advances in Magnetic and Optical Resonance     Full-text available via subscription   (Followers: 8)
Advances in Marine Biology     Full-text available via subscription   (Followers: 21, SJR: 0.88, CiteScore: 2)
Advances in Mathematics     Full-text available via subscription   (Followers: 17, SJR: 3.027, CiteScore: 2)
Advances in Medical Sciences     Hybrid Journal   (Followers: 9, SJR: 0.694, CiteScore: 2)
Advances in Medicinal Chemistry     Full-text available via subscription   (Followers: 6)
Advances in Microbial Physiology     Full-text available via subscription   (Followers: 5, SJR: 1.158, CiteScore: 3)
Advances in Molecular and Cell Biology     Full-text available via subscription   (Followers: 26)
Advances in Molecular and Cellular Endocrinology     Full-text available via subscription   (Followers: 8)
Advances in Molecular Toxicology     Full-text available via subscription   (Followers: 7, SJR: 0.182, CiteScore: 0)
Advances in Nanoporous Materials     Full-text available via subscription   (Followers: 5)
Advances in Oncobiology     Full-text available via subscription   (Followers: 2)
Advances in Organ Biology     Full-text available via subscription   (Followers: 2)
Advances in Organometallic Chemistry     Full-text available via subscription   (Followers: 18, SJR: 1.875, CiteScore: 4)
Advances in Parallel Computing     Full-text available via subscription   (Followers: 7, SJR: 0.174, CiteScore: 0)
Advances in Parasitology     Full-text available via subscription   (Followers: 6, SJR: 1.579, CiteScore: 4)
Advances in Pediatrics     Full-text available via subscription   (Followers: 27, SJR: 0.461, CiteScore: 1)
Advances in Pharmaceutical Sciences     Full-text available via subscription   (Followers: 19)
Advances in Pharmacology     Full-text available via subscription   (Followers: 17, SJR: 1.536, CiteScore: 3)
Advances in Physical Organic Chemistry     Full-text available via subscription   (Followers: 10, SJR: 0.574, CiteScore: 1)
Advances in Phytomedicine     Full-text available via subscription  
Advances in Planar Lipid Bilayers and Liposomes     Full-text available via subscription   (Followers: 3, SJR: 0.109, CiteScore: 1)
Advances in Plant Biochemistry and Molecular Biology     Full-text available via subscription   (Followers: 11)
Advances in Plant Pathology     Full-text available via subscription   (Followers: 6)
Advances in Porous Media     Full-text available via subscription   (Followers: 5)
Advances in Protein Chemistry     Full-text available via subscription   (Followers: 19)
Advances in Protein Chemistry and Structural Biology     Full-text available via subscription   (Followers: 20, SJR: 0.791, CiteScore: 2)
Advances in Psychology     Full-text available via subscription   (Followers: 69)
Advances in Quantum Chemistry     Full-text available via subscription   (Followers: 7, SJR: 0.371, CiteScore: 1)
Advances in Radiation Oncology     Open Access   (Followers: 3, SJR: 0.263, CiteScore: 1)
Advances in Small Animal Medicine and Surgery     Hybrid Journal   (Followers: 3, SJR: 0.101, CiteScore: 0)
Advances in Space Biology and Medicine     Full-text available via subscription   (Followers: 7)
Advances in Space Research     Full-text available via subscription   (Followers: 432, SJR: 0.569, CiteScore: 2)
Advances in Structural Biology     Full-text available via subscription   (Followers: 6)
Advances in Surgery     Full-text available via subscription   (Followers: 13, SJR: 0.555, CiteScore: 2)
Advances in the Study of Behavior     Full-text available via subscription   (Followers: 37, SJR: 2.208, CiteScore: 4)
Advances in Veterinary Medicine     Full-text available via subscription   (Followers: 20)
Advances in Veterinary Science and Comparative Medicine     Full-text available via subscription   (Followers: 15)
Advances in Virus Research     Full-text available via subscription   (Followers: 6, SJR: 2.262, CiteScore: 5)
Advances in Water Resources     Hybrid Journal   (Followers: 57, SJR: 1.551, CiteScore: 3)
Aeolian Research     Hybrid Journal   (Followers: 6, SJR: 1.117, CiteScore: 3)
Aerospace Science and Technology     Hybrid Journal   (Followers: 396, SJR: 0.796, CiteScore: 3)
AEU - Intl. J. of Electronics and Communications     Hybrid Journal   (Followers: 8, SJR: 0.42, CiteScore: 2)
African J. of Emergency Medicine     Open Access   (Followers: 6, SJR: 0.296, CiteScore: 0)
Ageing Research Reviews     Hybrid Journal   (Followers: 12, SJR: 3.671, CiteScore: 9)
Aggression and Violent Behavior     Hybrid Journal   (Followers: 490, SJR: 1.238, CiteScore: 3)
Agri Gene     Hybrid Journal   (Followers: 1, SJR: 0.13, CiteScore: 0)
Agricultural and Forest Meteorology     Hybrid Journal   (Followers: 18, SJR: 1.818, CiteScore: 5)
Agricultural Systems     Hybrid Journal   (Followers: 32, SJR: 1.156, CiteScore: 4)
Agricultural Water Management     Hybrid Journal   (Followers: 47, SJR: 1.272, CiteScore: 3)
Agriculture and Agricultural Science Procedia     Open Access   (Followers: 4)
Agriculture and Natural Resources     Open Access   (Followers: 3)
Agriculture, Ecosystems & Environment     Hybrid Journal   (Followers: 58, SJR: 1.747, CiteScore: 4)
Ain Shams Engineering J.     Open Access   (Followers: 5, SJR: 0.589, CiteScore: 3)
Air Medical J.     Hybrid Journal   (Followers: 8, SJR: 0.26, CiteScore: 0)
AKCE Intl. J. of Graphs and Combinatorics     Open Access   (SJR: 0.19, CiteScore: 0)
Alcohol     Hybrid Journal   (Followers: 12, SJR: 1.153, CiteScore: 3)
Alcoholism and Drug Addiction     Open Access   (Followers: 12)
Alergologia Polska : Polish J. of Allergology     Full-text available via subscription   (Followers: 1)
Alexandria Engineering J.     Open Access   (Followers: 2, SJR: 0.604, CiteScore: 3)
Alexandria J. of Medicine     Open Access   (Followers: 1, SJR: 0.191, CiteScore: 1)
Algal Research     Partially Free   (Followers: 11, SJR: 1.142, CiteScore: 4)
Alkaloids: Chemical and Biological Perspectives     Full-text available via subscription   (Followers: 2)
Allergologia et Immunopathologia     Full-text available via subscription   (Followers: 1, SJR: 0.504, CiteScore: 1)
Allergology Intl.     Open Access   (Followers: 5, SJR: 1.148, CiteScore: 2)
Alpha Omegan     Full-text available via subscription   (SJR: 3.521, CiteScore: 6)
ALTER - European J. of Disability Research / Revue Européenne de Recherche sur le Handicap     Full-text available via subscription   (Followers: 11, SJR: 0.201, CiteScore: 1)
Alzheimer's & Dementia     Hybrid Journal   (Followers: 55, SJR: 4.66, CiteScore: 10)
Alzheimer's & Dementia: Diagnosis, Assessment & Disease Monitoring     Open Access   (Followers: 6, SJR: 1.796, CiteScore: 4)
Alzheimer's & Dementia: Translational Research & Clinical Interventions     Open Access   (Followers: 6, SJR: 1.108, CiteScore: 3)
Ambulatory Pediatrics     Hybrid Journal   (Followers: 5)
American Heart J.     Hybrid Journal   (Followers: 58, SJR: 3.267, CiteScore: 4)
American J. of Cardiology     Hybrid Journal   (Followers: 67, SJR: 1.93, CiteScore: 3)
American J. of Emergency Medicine     Hybrid Journal   (Followers: 48, SJR: 0.604, CiteScore: 1)
American J. of Geriatric Pharmacotherapy     Full-text available via subscription   (Followers: 13)
American J. of Geriatric Psychiatry     Hybrid Journal   (Followers: 15, SJR: 1.524, CiteScore: 3)
American J. of Human Genetics     Hybrid Journal   (Followers: 40, SJR: 7.45, CiteScore: 8)
American J. of Infection Control     Hybrid Journal   (Followers: 29, SJR: 1.062, CiteScore: 2)
American J. of Kidney Diseases     Hybrid Journal   (Followers: 37, SJR: 2.973, CiteScore: 4)
American J. of Medicine     Hybrid Journal   (Followers: 50)
American J. of Medicine Supplements     Full-text available via subscription   (Followers: 3, SJR: 1.967, CiteScore: 2)
American J. of Obstetrics and Gynecology     Hybrid Journal   (Followers: 266, SJR: 2.7, CiteScore: 4)
American J. of Ophthalmology     Hybrid Journal   (Followers: 67, SJR: 3.184, CiteScore: 4)
American J. of Ophthalmology Case Reports     Open Access   (Followers: 5, SJR: 0.265, CiteScore: 0)
American J. of Orthodontics and Dentofacial Orthopedics     Full-text available via subscription   (Followers: 6, SJR: 1.289, CiteScore: 1)
American J. of Otolaryngology     Hybrid Journal   (Followers: 25, SJR: 0.59, CiteScore: 1)
American J. of Pathology     Hybrid Journal   (Followers: 32, SJR: 2.139, CiteScore: 4)
American J. of Preventive Medicine     Hybrid Journal   (Followers: 30, SJR: 2.164, CiteScore: 4)
American J. of Surgery     Hybrid Journal   (Followers: 39, SJR: 1.141, CiteScore: 2)
American J. of the Medical Sciences     Hybrid Journal   (Followers: 12, SJR: 0.767, CiteScore: 1)
Ampersand : An Intl. J. of General and Applied Linguistics     Open Access   (Followers: 7)
Anaerobe     Hybrid Journal   (Followers: 4, SJR: 1.144, CiteScore: 3)
Anaesthesia & Intensive Care Medicine     Full-text available via subscription   (Followers: 67, SJR: 0.138, CiteScore: 0)
Anaesthesia Critical Care & Pain Medicine     Full-text available via subscription   (Followers: 25, SJR: 0.411, CiteScore: 1)
Anales de Cirugia Vascular     Full-text available via subscription   (Followers: 1)
Anales de Pediatría     Full-text available via subscription   (Followers: 3, SJR: 0.277, CiteScore: 0)
Anales de Pediatría (English Edition)     Full-text available via subscription  
Anales de Pediatría Continuada     Full-text available via subscription  
Analytic Methods in Accident Research     Hybrid Journal   (Followers: 6, SJR: 4.849, CiteScore: 10)
Analytica Chimica Acta     Hybrid Journal   (Followers: 44, SJR: 1.512, CiteScore: 5)
Analytica Chimica Acta : X     Open Access  
Analytical Biochemistry     Hybrid Journal   (Followers: 216, SJR: 0.633, CiteScore: 2)
Analytical Chemistry Research     Open Access   (Followers: 13, SJR: 0.411, CiteScore: 2)
Analytical Spectroscopy Library     Full-text available via subscription   (Followers: 14)
Anesthésie & Réanimation     Full-text available via subscription   (Followers: 2)
Anesthesiology Clinics     Full-text available via subscription   (Followers: 25, SJR: 0.683, CiteScore: 2)
Angiología     Full-text available via subscription   (SJR: 0.121, CiteScore: 0)
Angiologia e Cirurgia Vascular     Open Access   (Followers: 1, SJR: 0.111, CiteScore: 0)
Animal Behaviour     Hybrid Journal   (Followers: 239, SJR: 1.58, CiteScore: 3)
Animal Feed Science and Technology     Hybrid Journal   (Followers: 8, SJR: 0.937, CiteScore: 2)
Animal Reproduction Science     Hybrid Journal   (Followers: 7, SJR: 0.704, CiteScore: 2)
Annales d'Endocrinologie     Full-text available via subscription   (Followers: 3, SJR: 0.451, CiteScore: 1)

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Similar Journals
Journal Cover
Acta Materialia
Journal Prestige (SJR): 3.263
Citation Impact (citeScore): 6
Number of Followers: 325  
 
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 1359-6454
Published by Elsevier Homepage  [3147 journals]
  • Shuffle-nanodomain regulated strain glass transition in Ti-24Nb-4Zr-8Sn
           alloy
    • Abstract: Publication date: March 2020Source: Acta Materialia, Volume 186Author(s): Qianglong Liang, Dong Wang, Yufeng Zheng, Shuangshuang Zhao, Yipeng Gao, Yulin Hao, Rui Yang, Dipankar Banerjee, Hamish L. Fraser, Yunzhi WangThe unprecedented properties of multi-functional metastable β-Ti alloys, including superelasticity over a wide temperature range, ultra-low modulus, and Invar and Elinvar anomalies, have attracted a great deal of attention. Persistent research efforts have been made towards the understanding of the origins of these unique properties. In this article we report a novel shuffle-nanodomain regulated strain glass transition in a metastable β-Ti alloy, Ti-24Nb-4Zr-8Sn (wt.%, Ti2448), which could be the dominant transformation pathway that offers these unique properties. Using the ex-situ aberration-corrected scanning transmission electron microscopy and in-situ cooling transmission electron microscopy, we find that randomly distributed {011}〈01¯1〉β O′ phase (orthorhombic, shuffle only) nanodomains embedded in the β phase (BCC) matrix at room temperature transform to α″ phase (orthorhombic) with a continuous increase in the amount of {21¯1}〈1¯1¯1〉β shear upon cooling or loading. Crystallographic analysis shows that the shuffle of the O′ phase will restrain the twelve possible shears that transform a BCC lattice to α″ martensite to only two. Thus, the randomly distributed O′ nanodomains prevent the formation of long-range-ordered, self-accommodating transformation-strain domain patterns seen in normal martensitic transformations and suppress completely the sharp first-order, auto-catalytic and avalanche-like martensitic transformation into a high-order-like (continuous) strain glass transition. Such a continuous β  →  O′  →  α″ strain glass transition has been confirmed by dynamic mechanical analysis, resistivity and differential scanning calorimetric measurement. This unique transition pathway allows us to offer new insights into the unique properties found in this alloy.Graphical abstractImage, graphical abstract
       
  • Suppression of Shear Localization in Nanocrystalline Al-Ni-Ce Via
           Segregation Engineering
    • Abstract: Publication date: Available online 24 January 2020Source: Acta MaterialiaAuthor(s): Glenn H. Balbus, Fulin Wang, Daniel S. GianolaShear localization in nanocrystalline metals is a severely limiting factor precluding their use as practical engineering materials. While several strategies exist to enhance the thermal and mechanical behavior of these materials, there are still many outstanding questions regarding the effects of chemical segregation on shear localization of FCC nanocrystalline materials. In this paper we investigate the mechanical response of a ternary aluminum alloy with a sub-10 nm nanocrystalline microstructure subject to various thermal treatments. Contrary to previous observations, our results suggest that annealing up to 0.7 Tm reduces the propensity for shear localization and increases strength, as demonstrated by a transition in deformation morphology from pronounced strain localization to more homogeneous deformation during indentation. This behavior coincides with the formation of an amorphous intergranular film during annealing, causing intragranular dislocation plasticity to be favored over other grain boundary dominated deformation mechanisms, in turn resulting in a lower propensity for long range plastic localization.Graphical abstractGraphical abstract for this article
       
  • Angular dependence and thermal stability of coercivity of Nd-rich Ga-doped
           Nd-Fe-B sintered magnet
    • Abstract: Publication date: Available online 23 January 2020Source: Acta MaterialiaAuthor(s): J. Li, Xin Tang, H. Sepehri-Amin, T.T. Sasaki, T. Ohkubo, K. HonoWe studied the angular dependence and thermal stability of coercivity of Nd-rich Ga-doped Nd-Fe-B sintered magnets. Experimentally measured angular dependence of coercivity for the as-sintered magnet agrees well with the micromagnetic simulation results for exchange-coupled polycrystalline model that assumes the magnetization reversal is dominated by the domain wall depinning at grain boundaries. On the other hand, the measured angular dependence of coercivity of the post-sinter annealed sample with an enhanced coercivity deviates from fully exchange-decoupled model in spite of previous experimental results on inter-grain exchange decoupling. Based on micromagnetic simulations and Kronmüller equation, it is suggested that the entire elimination of magnetic exchange coupling between Nd2Fe14B grains can further improve the coercivity and its thermal stability of the 5µm-grain-sized Dy-free magnet.Graphical Image, graphical abstract
       
  • i n s i t u +experiments+and+atomistic+simulations&rft.title=Acta+Materialia&rft.issn=1359-6454&rft.date=&rft.volume=">Origins of strengthening and failure in twinned Au nanowires: Insights
           from i n − s i t u experiments and atomistic simulations
    • Abstract: Publication date: Available online 23 January 2020Source: Acta MaterialiaAuthor(s): Zhuocheng Xie, Jungho Shin, Jakob Renner, Aruna Prakash, Daniel S. Gianola, Erik BitzekThe deformation behavior of ⟨110⟩-oriented twinned Au nanowires (NWs) with multiple longitudinal coherent twin boundaries (CTBs) under tension is studied using in−situ experiments and molecular dynamics (MD) simulations. The twinned NWs show higher yield strength than the single-crystalline NWs with similar diameter. Postmortem observations using electron microscopy and MD simulations show that the presence of CTBs transitions the governing mechanism from twinning-mediated deformation in single-crystalline NWs to strongly localized deformation. MD simulations reveal that the intersection of deposited partial dislocations at the CTB with the free surfaces plays an important role in the transmission of the dislocation, leading to the formation of full dislocations instead of partial dislocations and twinning in the case of single-crystalline NWs. The repeated activation of full dislocation slip leading to localized deformation is furthermore dependent on the relative orientation of surface facets to the activated Burgers vectors. The results of this work enhance the understanding of deformation mechanisms of twinned nano-objects and suggest design strategies for mechanical systems at the nanoscale.Graphical abstractGraphical abstract for this article
       
  • Insight into Si poisoning on grain refinement of Al-Si/Al-5Ti-B system
    • Abstract: Publication date: Available online 23 January 2020Source: Acta MaterialiaAuthor(s): Yang Li, Bin Hu, Bin Liu, Anmin Nie, Qinfen Gu, Jianfeng Wang, Qian LiSi poisoning on Al-5Ti-B master alloys has been restraining the effectiveness of grain refinement of hypoeutectic Al-Si casting alloys for over 60 years, and yet the underlying mechanism of this phenomenon remains unclear. In this work, Si poisoning in Al-Si/Al-5Ti-B system was systematically investigated by combining state-of-the-art electron microscopy, first-principles calculations and thermodynamic calculations. Different from the common belief that silicides coat and therefore poison TiB2, this study demonstrates that the segregation of Si atoms at the TiB2/α-Al interface is likely the cause of Si poisoning. Silicide was found to be thermodynamically unfavorable to form even in an alloy with 10 wt.%Si. On the other hand, an appreciable amount of Si (5∼20 at. %) was found to segregate in the TiAl3 two-dimensional compound (2DC) which is critical for triggering the nucleation of α-Al on TiB2. The formation of Ti-Si covalent bond within TiAl3 2DC disturbs its lattice and reduces its chemical interaction with α-Al, which both obstruct the epitaxial nucleation of α-Al and hence leads to Si poisoning. This study suggests that composition engineering of TiAl3 2DC and TiB2 with elements less attractive to Si could be a viable way to mitigate Si poisoning.Graphical Image, graphical abstract
       
  • Giant current performance in lead-free piezoelectrics stem from local
           structural heterogeneity
    • Abstract: Publication date: Available online 23 January 2020Source: Acta MaterialiaAuthor(s): Xiaodong Yan, Mupeng Zheng, Xin Gao, Mankang Zhu, Yudong HouPiezoelectric materials, which convert mechanical vibration energy into electric energy, are essential for vibration energy harvester. Nevertheless, the piezoelectric energy harvesting also encounters the bottleneck that piezoelectric materials generally produce only micro scale current due to its high impedance. In this work, the developed lead-free (Ba0.85Ca0.15)0.9985Sm0.001(Ti0.9Zr0.1)O3 (BCSm0.001TZ) piezoelectric ceramic possesses both high piezoelectric charge constant and low impedance, boosting its energy harvesting performance: an extremely high short-circuit current (60 µA) and large power density (2.1 µW/mm3) in a cantilever-type energy harvester, which is superior to those of other reported ones. Due to the giant current performance, the related charging speed of BCSm0.001TZ energy harvester possessed ∼100% enhancement relative to the non-doped counterpart. Electrical property measurement and nanostructure observation revealed that the outstanding electromechanical properties as well as energy harvesting performances in BCSm0.001TZ ceramic benefits from a complex domain architecture and low concentration of defect, which is closely associated with the local structural heterogeneity induced by the rare-earth Sm doping. This work provides a new important paradigm for developing high-performance viable green energy materials and devices, especially for the fast recharging microelectronic storage devices in wireless sensor network.Graphical abstractA giant current of 60 µA was achieved in (Ba0.85Ca0.15)0.9985Sm0.001(Ti0.9Zr0.1)O3 lead-free piezoelectrics, thus significantly shorten the charging time.Image, graphical abstract
       
  • Charged Domain Wall Modulation of Resistive Switching with Large ON/OFF
           Ratios in High Density BiFeO3 Nano-Islands
    • Abstract: Publication date: Available online 23 January 2020Source: Acta MaterialiaAuthor(s): M.J. Han, Y.L. Tang, Y.J. Wang, Y.L. Zhu, J.Y. Ma, W.R. Geng, Y.P. Feng, M.J. Zou, N.B. Zhang, X.L. MaFerroelectrics exhibit polarization tunable resistance switching behaviors, which are promising for next-generation non-volatile memory devices. For technological applications, thinner nanoscale arrays are expected, which feature with higher density and larger ON/OFF (RON/OFF) ratios in the metal/ferroelectrics/semiconductor heterojunction. Here, we acquire high density BiFeO3 (BFO) nano-islands around 10 nm in thickness displaying a high RON/OFF ratio of 103, comparable to the tunnel junctions. Moreover, both the macroscopic and microscopic resistive switching behaviors of the present BiFeO3 films reveal an unexpected the filamentary-type resistive switching which is modified by the charged domain walls in nano-islands dominated by the center-type domains. Particularly, the charged domain walls spontaneously formed within the BFO nano-islands are proposed as the conductive paths based on the redistribution of carriers under the applied voltages. Potential applications for memories with large RON/OFF ratios of such kind of configurable charged domain walls are demonstrated.Graphical abstractImage, graphical abstract
       
  • The combined effects of hydrogen and aging condition on the deformation
           and fracture behavior of a precipitation-hardened nickel-base superalloy
    • Abstract: Publication date: Available online 22 January 2020Source: Acta MaterialiaAuthor(s): Zachary D. Harris, Jishnu J. Bhattacharyya, Joseph A. Ronevich, Sean R. Agnew, James T. BurnsThe effect of hydrogen (H) on the deformation behavior of Monel K-500 in various isothermal heat treatment conditions (non-aged, under-aged, peak-aged, and over-aged) was assessed via uniaxial mechanical testing. H-charged and non-charged specimens were strained to failure to facilitate a comparison of ductility, fracture surface morphology, strength, and work hardening behavior. For all examined heat treatment conditions, H charging leads to a significant reduction in ductility, which is accompanied by a consistent change in fracture surface morphology from ductile microvoid coalescence to brittle intergranular fracture. While H charging led to a systematic enhancement in the yield strength of all heat treatments, the three age-hardened conditions exhibited a more than 2-fold increase relative to the non-aged heat treatment. This suggests that H modifies the dislocation-precipitate interactions, which also manifest themselves through changes in work hardening metrics related to the dislocation storage and recovery rates. In particular, the H-charged peak-aged specimen exhibited a significant increase in initial hardening (dislocation storage) rate relative to the H-charged under-aged specimen. Transmission electron microscopy of these samples revealed the onset of widespread dislocation looping in the H-charged peak-aged sample, in addition to the planar slip bands characteristic of the non-charged condition. This result suggests that hydrogen induces the particle shearing-to-looping transition at smaller particle sizes. Possible mechanistic explanations for this observed behavior are presented.Graphical Image, graphical abstract
       
  • Instabilities in the periodic arrangement of elastically interacting
           precipitates in nickel-base superalloys
    • Abstract: Publication date: Available online 22 January 2020Source: Acta MaterialiaAuthor(s): M. Degeiter, Y. Le Bouar, B. Appolaire, M. Perrut, A. FinelNickel-base superalloys display cuboidal precipitates aligned along the cubic directions, which are the elastic soft directions. At high precipitate volume fraction, the microstructure is often described as a regular array of precipitates organized on a simple cubic macro-lattice. In the present work, we use a stability analysis and 3D phase field simulations to show that such a regular array is in fact unstable whatever the volume fraction of precipitates. The two main instability modes are the longitudinal [100] mode and the transverse [110] mode along the [11¯0] eigenvector. We argue that these instabilities lead to formation of configurational defects closely related to experimentally observed branches and herringbone patterns. The rôles of elastic anisotropy and elastic homogeneity are also discussed.Graphical abstractGraphical abstract for this article
       
  • Quantum-Mechanical Oxidation States of Metal Ions in the Solid-State
           Binary Sulfides
    • Abstract: Publication date: Available online 22 January 2020Source: Acta MaterialiaAuthor(s): Bingyun AoABSTRACTThe solid-state metal sulfides have versatile technological and industrial applications; however, a comprehensive understanding of their chemical states remains in lack mainly due to the complicated bonding behavior of sulfur. Herein I conduct the systematic first-principles DFT + U calculations on the solid-state metal (all transition metals and the first five actinide metals Th, Pa, U, Np, Pu) binary sulfides, focusing on quantitative determination of the quantum-mechanical oxidation states (OSqm) of metal ions by counting d or f orbital occupation numbers. The results show the variation of OSqm of a specific metal ion with sulfur stoichiometry and the trend of OSqm of all considered metal ions. The most remarkable aspect is that OSqm in many sulfides are not consistent with formal OS (OSf) assigned by ionic approximation, especially for the sulfides with the highest sulfur composition. After detailed analysis of OSqm in actinide sulfides, I conclude that the relatively weak bonding between metal and sulfur, the relatively strong S-S bonding interaction, and the localization/itinerancy dual nature of d and f electrons can rationally elucidate the deviation of OSqm from OSf. Such quantitative determination of OSqm in the binary sulfides is expected to offer an alternative for the further exploration of more complicated sulfides and the theoretical design of novel sulfides.Graphical abstractImage, graphical abstract
       
  • A Model of Grain Boundary Complexion Transitions and Grain Growth in
           Yttria-Doped Alumina
    • Abstract: Publication date: Available online 22 January 2020Source: Acta MaterialiaAuthor(s): Philp E. Goins, William E. FrazierIn this work, we present a physically-parameterized microstructure evolution model for the Yttria-doped alumina system. Yttria-doped alumina is a well-known ceramic system which undergoes first-order phase-like transitions at grain boundaries, which can radically alter interface properties. The change in interfacial properties in turn can radically change microstructure outcomes during processing, including the induction of abnormal grain growth modes. In this work, we develop a simulation that evolves alumina microstructure as a function of yttria concentration and temperature. In the window studied, we achieve strong agreement with reviewed experimental results in identifying the windows for large grains, small grains, abnormal grain growth, and complexion transition kinetics (as measured by JMAK analysis). We then apply the model to study and demonstrate how the possible inclusion of second-phase particles or uneven solute distribution profiles will impact microstructure evolution. It is found that particles do not significantly affect abnormal grain growth in the window studied (but do lead to reduced grain size through pinning effects). It is found that even modest amounts of solute inhomogeneity will result in substantial changes in microstructure outcomes, frequently leading to clusters of abnormal grains. This model largely corroborates the expectations and hypotheses made from recent experimental studies in oxide-doped alumina systems. Further, it is found that there exists a peak transition fraction for the system at which abnormal grain size tends to be maximized.Graphical abstractImage, graphical abstract
       
  • Effect of Heterostructure and Hetero-deformation Induced Hardening on the
           Strength and Ductility of Brass
    • Abstract: Publication date: Available online 22 January 2020Source: Acta MaterialiaAuthor(s): X.T. Fang, G.Z. He, C. Zheng, X.L. Ma, D. Kaoumi, Y.S. Li, Y.T. ZhuHeterostructured materials have been reported to possess superior combinations of strength and ductility, which is attributed to hetero-deformation induced (HDI) strengthening and work hardening. However, the influence of heterostructural parameters on the evolution of HDI stress and mechanical behavior during tensile deformation is not well understood. In this paper, heterostructured brass (Cu-30% Zn) was fabricated by cold rolling and partial annealing, to produce heterostructures with different heterostructural parameters, including domain volume fraction, domain thickness/spacing and domain misorientation. It was found that HDI hardening was dominant when the tensile strain was less than ∼ 4.5%, while conventional dislocation hardening became more effective at higher strain levels. Quick accumulation of geometrically necessary dislocations was found in the domain boundary regions, leading to high HDI stress. Higher domain misorientation was found more effective in developing HDI. These findings elucidate the effect of heterostructure on strength and ductility, which can help with the design of heterostructured materials for superior mechanical properties.Graphical abstractHeterogeneous lamella structured (HLS) brass with superior combination of stress and ductility has been fabricated and the hetero-deformation induced (HDI) hardening has been demonstrated to significantly influence the mechanical properties of the (HLS) brass.Image, graphical abstract
       
  • Development of a segregation model beyond McLean based on atomistic
           simulations
    • Abstract: Publication date: Available online 22 January 2020Source: Acta MaterialiaAuthor(s): T. Krauß, S.M. EichIn the present atomistic study, layer-specific segregation to the surface is investigated for an exemplary (100) surface in iron–chromium alloys using an embedded-atom potential. Through a continuous variation of the chemical potential difference in the semi-grandcanonical ensemble, the full composition range is explored at temperatures between 600 K to 1400 K. The obtained layer-specific segregation curves demonstrate the well-known limitations of the widely used McLean model for interface segregation, i.e. a monolayer model imposing ideal behavior. However, keeping the original idea of McLean, the segregation model is extended in two ways in order to provide a complete analytical description of segregation: Firstly, the entire surface is hypothetically replaced by an equivalent single layer with identical segregation properties, which accounts for all subsurface layer effects, but also enables the application of an effective monolayer model with only one single energy of segregation. Secondly, directly following from the general treatment assuming non-ideal solution behavior, the energy of segregation becomes composition-dependent. The validity and accuracy of the proposed analytical model is confirmed by fitting the composition-dependent energy of segregation to the thermodynamically unambiguous solute excess. The change in surface formation energy according to the interfacial adsorption equation can be described excellently over the entire composition range for all investigated temperatures. The model can be applied to experimental data and directly transferred to grain boundaries.Graphical abstractGraphical abstract for this article
       
  • Weak influence of ferrite growth rate and strong influence of driving
           force on dispersion of VC interphase precipitation in low carbon steels
    • Abstract: Publication date: Available online 21 January 2020Source: Acta MaterialiaAuthor(s): Y.-J. Zhang, G. Miyamoto, K. Shinbo, T. FuruharaDispersion of nano-sized alloy carbides formed by interphase precipitation at migrating ferrite/austenite interphase boundaries is expected to be affected by local conditions at the boundaries, including ferrite growth rate and driving force for precipitation. The effects of these two factors were systematically investigated by changing alloy composition and transformation temperature. Quantitative analyses by using three-dimensional atom probe reveal that VC interphase precipitation becomes higher in number density and smaller in size at lower transformation temperature or with higher V content. The dispersion of VC is also slightly refined by lower Mn or higher Si content, but remains almost unchanged by increasing the C content. Such variations show good correlations with the driving force for its precipitation that finer VC precipitates can be obtained by enlarging the driving force. In contrast, the influence of the ferrite growth rate on the dispersion in most cases is quite small in the range of this study, which can be explained by considering nucleation kinetics of VC at migrating ferrite /austenite interface.Graphical Image, graphical abstract
       
  • Phase prediction in high entropy alloys with a rational selection of
           materials descriptors and machine learning models
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): Yan Zhang, Cheng Wen, Changxin Wang, Stoichko Antonov, Dezhen Xue, Yang Bai, Yanjing SuMaterials informatics employs machine learning (ML) models to map the relationship between a targeted property and various materials descriptors, providing new avenues to accelerate the discovery of new materials. However, the possible ML models and materials descriptors are numerous, and a rational recipe to rapidly choose the best combination of the two is needed. In the present study, we propose a systematic framework that utilizes a genetic algorithm (GA) to efficiently select the ML model and materials descriptors from a huge number of alternatives and demonstrated its efficiency on two phase formation problems in high entropy alloys (HEAs). The optimized classification model allows an accuracy for identifying solid-solution and non-solid-solution HEAs to be up to 88.7% and further for recognizing body-centered-cubic (BCC), face-centered-cubic (FCC), and dual-phase HEAs to reach 91.3%. Furthermore, by employing an active learning approach, several HEAs with largest classification uncertainties were selected, experimentally synthesized and phase-identified, and augmented to the initial dataset to iteratively improve the ML model. The method serves as a general algorithm to select materials descriptors and ML models for various material problems including classification and optimization of targeted properties.Graphical abstractGraphical abstract for this article
       
  • Microstructural evolution and thermal stability of AlCr(Si)N hard coatings
           revealed by in-situ high-temperature high-energy grazing incidence
           transmission X-ray diffraction
    • Abstract: Publication date: Available online 18 January 2020Source: Acta MaterialiaAuthor(s): N. Jöger, M. Meindlhumer, S. Spor, H. Hruby, J. Julin, A. Stark, F. Nahif, J. Keckes, C. Mitterer, R. DanielAn extensive understanding about the microstructural evolution and thermal stability of the metastable AlCr(Si)N coating system is of considerable importance for applications facing high temperatures, but it is also a challenging task since several superimposed processes simultaneously occur at elevated temperatures. In this work, three AlCr(Si)N coatings with 0 at.%., 2.5 at.% and 5at.% Si were investigated by in-situ high-temperature high-energy grazing incidence transmission X-ray diffraction (HT-HE-GIT-XRD) and complementary differential scanning calorimetry and thermogravimetric analysis measurements combined with conventional ex-situ X-ray diffraction. The results revealed (i) a change in the microstructure from columnar to a fine-grained nano-composite, (ii) a reduced decomposition rate of CrN to Cr2N, also shifted to higher onset temperatures from  ∼ 1000∘C to above  ∼ 1100∘C and (iii) an increase of lattice defects and micro strains resulting in a significant increase of compressive residual strain with increasing Si content. While the Si-containing coatings in the as-deposited state show a lower hardness of 28GPa compared to AlCrN with 32GPa, vacuum annealing at  ∼ 1100∘C led to an increase in hardness to 29GPa for the coatings containing Si and a decrease in hardness to 26GPa for AlCrN. Furthermore, the in-situ HT-HE-GIT-XRD method allowed for simultaneously accessing temperature-dependent variations of the coating microstructure (defect density, grain size), residual strain state and phase stability up to  ∼ 1100∘C. Finally, the results established a deeper understanding about the relationships between the elemental composition of the materials, the resulting microstructure including crystallographic phases and residual strain state, and the coating properties from room temperature up to  ∼ 1100∘C.Graphical abstractGraphical abstract for this article
       
  • New insights into high-temperature deformation and phase transformation
           mechanisms of lamellar structures in high Nb-containing TiAl alloys
    • Abstract: Publication date: Available online 16 January 2020Source: Acta MaterialiaAuthor(s): Lin Song, Fritz Appel, Li Wang, Michael Oehring, Xingguo Hu, Andreas Stark, Junyang He, Uwe Lorenz, Tiebang Zhang, Junpin Lin, Florian PyczakThe paper describes the microstructure evolution by high-temperature compression of a high Nb-containing TiAl alloy. The paper extends a previous publication [L. Song, et al. Intermetallics 109 (2019) 91-96], in which a unique twin-like morphology in the α2 (Ti3Al) phase was reported. However, the origin of these structures could not be clarified without doubt. The present study is focused on phase transformations that in this multiphase alloy can be associated with deformation. Particular attention is paid to local transformations of the α2 phase into O phase or ω-related phases, which, because of structural and chemical similarity of these phases with α2, can easily occur and could mistakenly be considered as a twin structure. The details of the atomic processes involved are elucidated by electron microscopy. Given the large shufflings and the atomic site interchanges required for the operation of this twinning system, it is concluded that twinning of the α2 phase is a diffusive-displacive process. Within the α2 phase, ωo is heterogeneously nucleated. The nucleation sites are defect-rich areas, which are subjected to high local stresses. The study strongly emphasizes the close relationship between high-temperature deformation and phase transformations in multiphase titanium aluminide alloys.Graphical Image, graphical abstract
       
  • Cold Sintering of ZnO-PTFE: utilizing polymer phase to promote ceramic
           anisotropic grain growth
    • Abstract: Publication date: Available online 16 January 2020Source: Acta MaterialiaAuthor(s): Thomas Herisson de Beauvoir, Kosuke Tsuji, Xuetong Zhao, Jing Guo, Clive RandallAbstractDensification of ZnO-PTFE (polytetrafluoroethylene) composites is permitted by the Cold Sintering Process, having no effect on the stability of both materials. Highly dense samples can be obtained by this technique at extremely low temperatures in just a few minutes. Interestingly, the obtained samples show an anisotropy impacting: crystalline, microstructural and electrical properties. While the Wurztite ZnO crystals show a preferential growth along (00l) direction, microstructure observations show a grain growth along the in-plane (perpendicular to pressure application direction) up to 240%. Electrical conductivity is also influenced and is related to microstructure. In this situation, the addition of PTFE insulating phase allows to increase the conductivity in plane compared to the pure cold sintered ZnO sample. A mechanism is proposed to explain this phenomenon which involves PTFE transient distribution competing with the transient liquid driving densification and grain growth associated with cold sintering. This is further confirmed by the observation of a curvature of microstructure direction while approaching die edges. These observations offer a large variety of designs for further orientation driven properties.
       
  • Interface-mediated plasticity of nanoscale Al -Al2Cu eutectics
    • Abstract: Publication date: Available online 16 January 2020Source: Acta MaterialiaAuthor(s): Guisen Liu, Shujuan Wang, Amit Misra, Jian WangLaser surface re-melted Al-Al2Cu eutectic alloy with α-Al and θ-Al2Cu nanoscale lamellae exhibits high strength and good plasticity at room temperature, implying that the nanoscale θ-Al2Cu lamellae plastically co-deform with α-Al. Microscopy characterization reveal that plastic deformation of θ-Al2Cu lamellae is accommodated by localized shear on unusual slip planes of {121}Al2Cu. Herein, we elucidate interface-mediated deformation mechanisms of nanoscale Al -Al2Cu eutectics and investigate the structure and properties of the {001}Al‖{001}Al2Cu interface using atomistic simulations. Simulation results reveal that the interface is composed of two sets of misfit dislocations with the displacement shift complete vectors as Burgers vectors. We then conduct simple shear simulations to explore shear response and corresponding mechanisms of the Al-Al2Cu interface, and reveal that interfacial shear is accomplished through the gliding of misfit dislocations on the interface. Plasticity of nanoscale θ-Al2Cu lamellae is examined to be associated with localized shears on {121}Al2Cu planes, which are ascribed to the continuity of slip systems across Al-Al2Cu interfaces and accumulated lattice dislocations at interfaces.Graphical abstractImage, graphical abstract
       
  • Tunable pyroelectricity, depolarization temperature and energy harvesting
           density in Pb(Lu0.5Nb0.5)O3-xPbTiO3 ceramics
    • Abstract: Publication date: Available online 16 January 2020Source: Acta MaterialiaAuthor(s): Xiaoming Yang, Fangping Zhuo, Chenxi Wang, Ying Liu, Zujian Wang, Chao He, Xifa LongThe ferroelectric to antiferroelectric (FE-AFE) phase boundary design based on orthorhombic AFE phase and tetragonal FE phase is an effective method to develop high-performance pyroelectric materials due to the complete release of large electrical polarization in FE-AFE phase transition. Herein, we report the phase structure evolution in (1-x)Pb(Lu0.5Nb0.5)O3-xPbTiO3 (abbreviated as PLNT100x) ceramic system based on the relationship of tolerance factors versus electronegativity differences. The composition/temperature effects on FE-AFE phase transition behavior, pyroelectricity, depolarization temperature (Td) and energy harvesting performance were investigated systematically. Obviously, PLNT system displays superior pyroelectric characteristics as well as high Td. The maximum pyroelectric peak was 4.50 μC•cm−2•K−1 over a wide temperature range from 28°C to 167°C. In addition, the obtained maximum pyroelectric energy harvesting density was 1.66 J/cm3 which was much higher than the currently reported values, indicating a potential candidate for pyroelectric energy conversion applications. Based on the modified Ginzburg−Landau−Devonshire (GLD) phenomenology, the composition/temperature driven phase transitions were discussed, and the temperature−electric field (T−E) phase diagram was accordance with actual phase diagram based on the experimental data.Graphical abstractImage, graphical abstract
       
  • Direct measurements of slip irreversibility in a nickel-based superalloy
           using high resolution digital image correlation
    • Abstract: Publication date: March 2020Source: Acta Materialia, Volume 186Author(s): J.C. Stinville, P.G. Callahan, M.A. Charpagne, M.P. Echlin, V. Valle, T.M. PollockFatigue crack nucleation in crystalline materials typically develops due to highly localized cyclic slip. During a fatigue cycle, reverse slip differs locally from slip in the forward direction particularly in precipitate-containing materials such as superalloys. In this paper we report the first direct measurements of irreversibility at the scale of individual slip bands by high-resolution digital image correlation (DIC) in a polycrystalline nickel-based superalloy. Quantitative measurements of the slip irreversibility are challenging for regions of material that have a size that captures the microstructure and its variability. High spatial resolution at the nanometer scale during experimental measurements is needed to observe slip localization during deformation. Moreover, large fields are also needed to obtain the material response over statistically representative populations of microstructural configurations. Recently, high resolution scanning electron microscope (SEM) digital image correlation (DIC) has been extended for quantitative analysis of discontinuities induced by slip events using the Heaviside-DIC method. This novel method provides quantitative measurements of slip localization at the specimen surface. In this paper, the Heaviside-DIC method is used to measure slip irreversibility and plastic strain accumulation in a nickel-based superalloy. The method detects bands with high levels of irreversibility early in cycling that ultimately form fatigue cracks upon further cycling. The local microstructural configurations that induce large amounts of plasticity and slip irreversibility are correlated to crack nucleation locations.Graphical abstractGraphical abstract for this article
       
  • Solidification orientation relationships between Al3Ti and
           TiB2
    • Abstract: Publication date: March 2020Source: Acta Materialia, Volume 186Author(s): Y. Cui, D.J.M. King, A.P. Horsfield, C.M. GourlayAbstractOrientation relationships (ORs) can form during solidification by a variety of mechanisms that are often difficult to distinguish after solidification. Here we study three ORs formed by the nucleation of Al3Ti on TiB2, and by the pushing and engulfment of TiB2 by growing Al3Ti facets in hyperperitectic Al-rich melts. The nucleation OR is identified by growing a relatively large TiB2 crystal, solidifying multiple small Al3Ti crystals on one (0001) facet of TiB2, and measuring the resulting OR by electron backscatter diffraction (EBSD). Pushing and engulfment ORs are investigated by statistical analysis of EBSD measurements, density functional theory (DFT) calculations of interface energies, and imaging of cross-sections of TiB2 particles being pushed and engulfed by Al3Ti facets. It is shown that the lowest energy OR is formed by nucleation as well as by pushing/engulfment. The higher energy ORs, formed by pushing and engulfment, correspond to local interfacial energy minima and can be explained by rotation of TiB2 particles on Al3Ti facets during pushing.
       
  • The evolution of cube ({001}<100>) texture in non-oriented electrical
           steel
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): Mehdi Mehdi, Youliang He, Erik J. Hilinski, Leo A.I. Kestens, Afsaneh EdrisyDue to the alignment of two easy 〈100〉 axes in the sheet plane, the cube orientation ({001}) is an ideal texture for non-oriented electrical steel sheets used as core lamination for electric motors. However, this magnetically favorable texture was rarely able to be produced using conventional rolling and annealing routes in non-oriented electrical steels. In this research, inclined cold rolling − a simple rolling scheme to alter the initial texture before cold rolling − was applied to a 2.8 wt% Si non-oriented electrical steel, in order to intentionally “create” a rotated Goss ({110}) texture before cold rolling, which was not commonly observed in hot-rolled electrical steels. Plane-strain compression (rolling) of the rotated Goss was able to produce cube crystallites within the matrix, at the grain boundaries and within the shear bands of the deformed rotated Goss grains. The cube crystallites within the shear bands had lower stored energy than their surroundings, and served as the initial seeds for nucleation. Upon annealing, the cube crystallites preferentially nucleated from the shear bands and competitively grew out of the surrounding substructure, forming a strong cube texture in the final sheet. The formation of the initial cube crystallites within the shear bands of the deformed microstructure was believed to be necessary for the development of a final cube texture in the annealed electrical steel sheet. Although inclined rolling may be difficult to be implemented in industrial production, its unique capability to produce uncommon initial texture before cold rolling provides an interesting technique for the study of texture involution during thermomechanical processing.Graphical abstractImage, graphical abstract
       
  • Interfacial energy as the driving force for diffusion bonding of ceramics
    • Abstract: Publication date: Available online 14 January 2020Source: Acta MaterialiaAuthor(s): S. Kovacevic, R. Pan, D.P. Sekulic, S.Dj. MesarovicDiffusion bonding of ceramics with a metallic interlayer can give a variety of very complex joint microstructures, which are highly influenced by ceramic compositions, the material and thickness of the interlayer, bonding temperature as well as time at the peak bonding temperature. Experiments with a diffusion bonding of ZrC using a Ti interlayer clearly show that under a certain bonding condition, a seamless joint with the total dissolution of the interlayer can be obtained. They also indicate the existence of the critical interlayer thickness, below which the seamless homogeneous joint domain is obtained, and above which the joint does not homogenize. The key process leading to these outcomes is the diffusion of carbon from ZrC into Ti, which, when the critical carbon concentration is reached, initiates the phase transformation of bcc Ti to TiC, while the binary Zr/Ti diffusion is then driven by entropy and results in a seamless Zr(Ti)C joint.We first show that the dependence of ZrC/Ti interfacial energy on the carbon concentration jump across the interface is the main thermodynamic driving force of the diffusion of carbon from ZrC to the Ti interlayer. Then, we show that the characteristic length (critical thickness of the interlayer) arises as the ratio of this driving force (energy/area) and the bulk energy densities which oppose the carbon diffusion. Finally, we develop a diffuse interface (phase-field) model to simulate the process. The novelty in the phase-field model is the introduction of a dependence of the interfacial energy on the carbon concentrations on the two sides of the interface. The critical thickness of the interlayer is estimated employing both models and good agreement with experimental findings is obtained.Graphical Image, graphical abstract
       
  • A combinatorial guide to phase formation and surface passivation of
           tungsten titanium oxide prepared by thermal oxidation
    • Abstract: Publication date: March 2020Source: Acta Materialia, Volume 186Author(s): Sebastian Siol, Noémie Ott, Casey Beall, Michael Stiefel, Yeliz Unutulmazsoy, Max Döbeli, S. David Tilley, Patrik Schmutz, Lars P.H. Jeurgens, Claudia CancellieriTiO2 and WO3 are two of the most important earth-abundant electronic materials with applications in countless industries. Recently alloys of WO3 and TiO2 have been investigated leading to improvements of key performance indicators for a variety of applications ranging from photo-electrochemical water splitting to electrochromic smart windows. These positive reports and the complexity of the ternary W-Ti-O phase diagram motivate a comprehensive experimental screening of this phase space. Using combinatorial thermal oxidation of solid solution W1-xTix precursors combined with bulk and surface analysis mapping we investigate the oxide phase formation and surface passivation of tungsten titanium oxide in the entire compositional range from pure WO3 to TiO2. The system shows a remarkable structural transition from monoclinic over cubic to tetragonal symmetry with increasing Ti concentration. In addition, a strong Ti surface enrichment is observed for precursor Ti-concentrations in excess of 55 at.%, resulting in the formation of a protective rutile-structured TiO2 surface layer. Despite the structural transitions, the optical properties of the oxide alloys remain largely unaltered demonstrating an independent control of multiple functional properties in W1-xTixOn. The results from this study provide valuable guidelines for future development of W1-xTixOn for electronic and energy applications, but also novel engineering approaches for surface functionalization and additive manufacturing of Ti-based alloys.Graphical abstractUsing combinatorial thermal oxidation of solid solution W1-xTix precursors combined with bulk and surface analysis mapping we investigate the oxide phase formation and surface passivation of tungsten titanium oxide in the entire compositional range from pure WO3 to TiO2.Image, graphical abstract
       
  • Characterizing microscale deformation mechanisms and macroscopic tensile
           properties of a high strength magnesium rare-earth alloy: A combined
           experimental and crystal plasticity approach
    • Abstract: Publication date: March 2020Source: Acta Materialia, Volume 186Author(s): A. Githens, S. Ganesan, Z. Chen, J. Allison, V. Sundararaghavan, S. DalyThe effect of aging on the accumulation of microscale plasticity, and the resulting macroscopic mechanical behavior, were examined in the magnesium alloy WE43 under uniaxial tension. Full-field strains on the length scale of the microstructure, and their relation to the underlying crystallography, were captured using a combination of electron backscatter diffraction, custom nanoparticle patterning processes for corrosion-susceptible alloys, scanning electron microscopy (SEM), in-SEM uniaxial tensile and compressive loading, and distortion-corrected digital image correlation. The as-received material exhibited an average grain size of 12  µm. The strain incurred on individual slip traces in magnesium was resolved for the first time. Insights into slip activation across the microstructure revealed that using Schmid's Law with the nominal Schmid Factor appeared to be predictive for basal and non-basal slip. The DIC results were compared with simulation using an advanced open-source crystal plasticity finite element (CPFE) code, PRISMS-Plasticity. The PRISMS-Plasticity model is a more precise determination of the local Schmid Factor and was used to simulate variations in slip and twin activity within each grain. Such simulations provide an avenue for physically interpreting the various slip traces observed in the dense DIC data and an improved understanding of the critical resolved shear stress of the various slip systems.Graphical abstractImage, graphical abstract
       
  • Amorphous intergranular films mitigate radiation damage in nanocrystalline
           Cu-Zr
    • Abstract: Publication date: Available online 11 January 2020Source: Acta MaterialiaAuthor(s): Jennifer D. Schuler, Charlette M. Grigorian, Christopher M. Barr, Brad L. Boyce, Khalid Hattar, Timothy J. RupertNanocrystalline metals are promising radiation tolerant materials due to their large interfacial volume fraction, but irradiation-induced grain growth can eventually degrade any improvement in radiation tolerance. Therefore, methods to limit grain growth and simultaneously improve the radiation tolerance of nanocrystalline metals are needed. Amorphous intergranular films are unique grain boundary structures that are predicted to have improved sink efficiencies due to their increased thickness and amorphous structure, while also improving grain size stability. In this study, ball milled nanocrystalline Cu-Zr alloys are heat treated to either have only ordered grain boundaries or to contain amorphous intergranular films distributed within the grain boundary network, and are then subjected to in situ transmission electron microscopy irradiation and ex situ irradiation. Differences in defect density and grain growth due to grain boundary complexion type are then investigated. When amorphous intergranular films are incorporated within the material, fewer and smaller defect clusters are observed while grain growth is also limited, leading to nanocrystalline alloys with improved radiation tolerance.Graphical Image, graphical abstract
       
  • Mechanistic investigation of a low-alloy Mg–Ca-based extrusion alloy
           with high strength–ductility synergy
    • Abstract: Publication date: Available online 10 January 2020Source: Acta MaterialiaAuthor(s): Hucheng Pan, Rui Kang, Jingren Li, Zhuoran Zeng, Hongbo Xie, Qiuyan Huang, Changlin Yang, Yuping Ren, Gaowu QinHigh strength–ductility synergy is difficult to achieve in Mg alloys. Although high strength has been achieved through considerable alloying addition and low-temperature extrusion, these techniques result in low ductility (2–5%). In this work, a novel low-alloy Mg–Ca-based alloy that overcomes this strength–ductility trade-off is designed. The alloy has an excellent tensile yield strength (∼425 MPa) and exhibits a reasonably high elongation capacity (∼11%). A microstructure examination reveals that a high density of submicron grains and nano-precipitates provides the alloy high strength, and the leaner alloy additions and higher extrusion temperatures initially improve ductility. As a result, the density of residual dislocations is reduced, and the formation of low-angle grain boundaries (LAGBs) is enhanced. With fewer residue dislocations, it becomes less probable for the newly activated mobile dislocations to be impeded and transformed into an immobile type during the subsequent tensile test. The LAGBs function as potential sites to emit new dislocations, thus enhancing the dislocation–multiplication capability. More importantly, they can induce evident sub-grain refinement hardening and guarantee that the alloy achieves high strength. The findings lead to a controllable Mg alloy design strategy that can simultaneously afford high strength and ductility.Graphical abstractImage, graphical abstract
       
  • Joint investigation of strain partitioning and chemical partitioning in
           ferrite-containing TRIP-assisted steels
    • Abstract: Publication date: Available online 9 January 2020Source: Acta MaterialiaAuthor(s): Xiaodong Tan, Dirk Ponge, Wenjun Lu, Yunbo Xu, Huansheng He, Jun Yan, Di Wu, Dierk RaabeWe applied two types of hot-rolling direct quenching and partitioning (HDQ&P) schemes to a low-C low-Si Al-added steel and obtained two ferrite-containing TRIP-assisted steels with different hard matrix structures, viz, martensite or bainite. Using quasi in-situ tensile tests combined with high-resolution electron back-scattered diffraction (EBSD) and microscopic digital image correlation (µ-DIC) analysis, we quantitatively investigated the TRIP effect and strain partitioning in the two steels and explored the influence of the strain partitioning between the soft and hard matrix structures on the TRIP effect. We also performed an atomic-scale analysis of the carbon partitioning among the different phases using atom probe tomography (APT). The results show that the strain mainly localizes in the ferrite in both types of materials. For the steel with a martensitic hard-matrix, a strong strain contrast exists between ferrite and martensite, with the local strain difference reaching up to about 75% at a global strain of 12.5%. Strain localization bands initiated in the ferrite rarely cross the ferrite/martensite interfaces. The low local strain (2%-10%) in the martensite regions leads to a slight TRIP effect with a transformation ratio of the retained austenite of about 7.5%. However, for the steel with bainitic matrix, the ferrite and bainite undergo more homogeneous strain partitioning, with an average local strain in ferrite and bainite of 15% and 8%, respectively, at a global strain of 12.5%. The strain localization bands originating in the ferrite can cross the ferrite/bainite (F/B) interfaces and increase the local strain in the bainite regions, resulting in an efficient TRIP effect. In that case the transformation ratio of the retained austenite is about 41%. The lower hardness difference between the ferrite and bainite of about 178 HV, compared with that between the ferrite and martensite of about 256 HV, leads to a lower strain contrast at the ferrite/bainite interfaces, thus retarding interfacial fracture. Further microstructure design for TRIP effect optimization should particularly focus on adjusting the strength contrast among the matrix structures and tuning strain partitioning to enhance the local strain partitioning into the retained austenite.Graphical abstractImage, graphical abstract
       
  • Deformation mechanisms and strain rate sensitivity of bimodal
           and ultrafine-grained copper
    • Abstract: Publication date: Available online 9 January 2020Source: Acta MaterialiaAuthor(s): J. Bach, M. Stoiber, L. Schindler, H.W. Höppel, M. GökenMaterials with ultrafine grain size in the range from 100 nm to 1 µm exhibit very high strength paired with a satisfactory ductility when compared to their coarse grained (CG) counterparts. Although this typical behavior is already well known, the dominating deformation mechanisms are still controversially discussed in literature. One idea to explain the deformation behavior of ultrafine-grained metals is that deformation is mainly triggered by grain boundary sliding. Another explanation is that deformation in ultrafine-grained materials is controlled by the thermally activated dislocation annihilation of dislocations at grain boundaries. To gain deeper insights to the relevant deformation mechanisms in UFG metals a systematic study was conducted where the deformation behavior of UFG and bimodal copper (consisting of UFG and CG grains) is compared to the behavior of their CG counterparts. The UFG microstructure was obtained by equal channel angular pressing (ECAP). To achieve a bimodal or coarsened microstructure, specimens were annealed at 125°C or, respectively, at 140°C subsequent to the ECAP-process. Mechanical characterization and investigation on the strain-rate sensitivity were done by compression strain-rate jump tests at room-temperatures and elevated temperatures. It turned out clearly that the degree of bimodality determines the dominant deformation mechanism and the strain-rate sensitivity. In the UFG-state thermally activated annihilation of dislocations at the grain boundaries govern the mechanical behavior. For the bimodal microstructure the annihilation of dislocation at the interface of coarsened grains to the surrounding ultrafine-grained matrix dominate the mechanical behavior. For the fully coarsened state plastic deformation is mainly governed by dislocation interaction in the grain interior. In this regime, annihilation at grain boundaries plays only a minor role.Graphical abstractMaterials with ultrafine grain size in the range from 100 nm to 1 µm exhibit very high strength paired with a satisfactory ductility when compared to their coarse grained (CG) counterparts. Although this typical behavior is already well known, the dominating deformation mechanisms are still controversially discussed in literature. One idea to explain the deformation behavior of ultrafine-grained metals is that deformation is mainly triggered by grain boundary sliding. Another explanation is that deformation in ultrafine-grained materials is controlled by the thermally activated dislocation annihilation of dislocations at grain boundaries. To gain deeper insights to the relevant deformation mechanisms in UFG metals a systematic study was conducted where the deformation behavior of UFG and bimodal copper (consisting of UFG and CG grains) is compared to the behavior of their CG counterparts. The UFG microstructure was obtained by equal channel angular pressing (ECAP). To achieve a bimodal or coarsened microstructure, specimens were annealed at 125°C or, respectively, 140°C subsequent to the ECAP-process. Mechanical characterization and investigation on the strain-rate sensitivity were done by compression strain-rate jump tests at room-temperatures and elevated temperatures. It turned out clearly that the degree of bimodality determines the dominant deformation mechanism and the strain-rate sensitivity. In the UFG-state thermally activated annihilation of dislocations at the grain boundaries govern the mechanical behavior. For the bimodal microstructure the annihilation of dislocation at the interface of coarsened grains to the surrounding ultrafine-grained matrix define the mechanical behavior. For the fully coarsened state plastic deformation is mainly governed by dislocation interaction in the grain interior. Annihilation at grain boundaries plays only a minor role.Image, graphical abstract
       
  • Tracing intermediate phases during crystallization in a Ni-Zr
           metallic glass
    • Abstract: Publication date: Available online 9 January 2020Source: Acta MaterialiaAuthor(s): S.Y. Liu, Q.P. Cao, X. Mu, T.D. Xu, D. Wang, K. Ståhl, X.D. Wang, D.X. Zhang, C. Kübel, J.Z. JiangCrystallization of metallic glasses (MGs) is a complex dynamic process, driven by thermodynamics and limited by kinetics, which often involves in the phase transformation from the metastable amorphous state, via intermediates, to final stable crystalline states. The intermediate structural state remains mysterious at present but very crucial to a deeper understanding of the physics and mechanisms of the crystallization process. Detailed structural characterization of the complicated intermediate crystalline phases using transmission electron microscopy (TEM) provides a unique platform to study such issues. Here, we monitor the evolution of the crystallization process for Ni65Zr35 (at.%) MG ribbon with structural heterogeneities. Direct visualization combined with compositional analysis reveal that the intermediate phase with Zr concentration higher than that of MG consists of the inter-stacked nanometer-sized layers of Ni-rich units (Ni at.%> 67 %) and Ni10Zr7-like units, where the thin Ni-rich single layer gradually disappears with increasing the annealing temperature. Our findings provide insight into the key role of Ni in the structural transition process, improving the understanding of the atomic diffusion-dominated crystallization in MGs.Graphical Image, graphical abstract
       
  • { 10 1 ¯ 1 } +twin+in+magnesium&rft.title=Acta+Materialia&rft.issn=1359-6454&rft.date=&rft.volume=">Dislocation ↔ twin transmutations during interaction between prismatic
           slip and { 10 1 ¯ 1 } twin in magnesium
    • Abstract: Publication date: Available online 8 January 2020Source: Acta MaterialiaAuthor(s): Peng Chen, Jamie Ombogo, Bin LiVery unusual and interesting interaction between matrix prismatic dislocations and {101¯1} twin boundaries (TBs) in Magnesium (Mg) was observed in atomistic simulations. When the first prismatic dislocation impinged on the TB, the incoming dislocation was transmuted into a thin layer of {112¯1} twin inside the {101¯1} twin. When successive prismatic dislocations on the same slip plane impinged on the same location at the {101¯1} TB, the {112¯1} twin kept growing toward the opposite {101¯1} TB. Eventually, the {112¯1} twin reached the opposite TB and was then transmuted back to prismatic dislocations that exited the {101¯1} twin and glided into the matrix. Hence, the matrix prismatic dislocations temporarily lose their dislocation identity during twin-slip interaction and then resume their dislocation identity after the interaction is complete. The net effect of these interactions is that the matrix prismatic dislocations transmit across the {101¯1} twin. Lattice correspondence analysis for {101¯1} twinning was performed to understand the mechanism of the interactions. The results show that, the prismatic slip plane is exactly the corresponding plane of {112¯1} twinning. Such a correspondence is consistent with the crystallographic calculations based on classical twinning theory.Graphical abstractMatrix prismatic dislocations are transmuted into a {112¯1} twin when interacting with a {101¯1} twin boundary. At the opposite {101¯1} twin boundary, the {112¯1} twin is transmuted back to matrix prismatic dislocations Image, graphical abstract
       
  • Dynamic observation of Joule heating-induced structural and domain
           transformation in smart shape-memory alloy
    • Abstract: Publication date: Available online 8 January 2020Source: Acta MaterialiaAuthor(s): Abdul Karim, Chaoshuai Guan, Bin Chen, Yong Li, Junwei Zhang, Liu Zhu, Xia Deng, Yang Hu, Kaiqi Bi, Hongli Li, Yong Peng, Lingwei LiFerromagnetic shape-memory alloys can realize smart functional sensors and actuators due to their distinctive capabilities of field-induced strain, which have attracted extensive interests. However, their response and transformation kinetics under the stimulus of Joule heating are still lack, which limits their extensive application into electrical-related sensors. In this work, we firstly report the dynamic structural transformation and magnetic domain evolution of smart shape memory alloy induced by Joule heating in-situ TEM with thermoelectric holder by using Ni2MnGa as experimental model. The stimulus of low-power Joule heating induces the reversible structural and magnetic domain transformations of the smart Ni2MnGa shape memory alloy. Whilst, high-power Joule heating induces an irreversible process of structural transformation and magnetic domain evolution. Our work should be significant to expand the smart shape memory alloy into electrical-related applications.Graphical abstractImage, graphical abstract
       
  • On the Observation of Annealing Twins during Simulating β-Grain
           Refinement in Ti-6Al-4V High Deposition Rate AM with In-Process
           Deformation
    • Abstract: Publication date: Available online 8 January 2020Source: Acta MaterialiaAuthor(s): J. Donoghue, A.E. Davis, C.S. Daniel, A. Garner, F. Martina, J. Quinta da Fonseca, P.B. PrangnellABSTRACTAdditive Manufacture (AM) of Ti-6Al-4V frequently leads to undesirable, coarse, columnar β-grain structures with a strong fibre texture. In Wire-Arc AM (WAAM), it has been found that the application of a low plastic strain, by methods such as inter-pass rolling, can disrupt β columnar growth and produce a refined, equiaxed grain structure that is more randomly orientated. The origin of this desirable effect has been investigated by thermo-mechanical simulation, direct in-situ EBSD observation, as well as by real-time synchrotron X-ray diffraction (SXRD) during rapid heating. These complementary approaches have shown that, when starting with a WAAM microstructure, the grain refinement process produces a unique micro-texture represented by a four-pole motif symmetrically centred on the parent grain {100} orientations. These new β-grain orientations can be reproduced by a double {112} twinning operation, which produces 12 new, unique, β-orientation variants. High-resolution orientation-mapping techniques and in-situ SXRD heating simulations suggest that the prior β does not twin during deformation, but rather the grain refinement and related texture may be caused by annealing twinning during β re-growth on rapid re-heating of the deformed AM microstructure. Although this is the first time such a unique texture has been observed in a deformed and β annealed Ti-6Al-4V material, it was only found to dominate under the unusual conditions that occur in AM of rapid heating – a fine, lightly deformed α transformation microstructure, with a very coarse starting β-grain structure.Graphical abstractImage, graphical abstract
       
  • Sensitivity of twin boundary movement to sample orientation and magnetic
           field direction in Ni-Mn-Ga
    • Abstract: Publication date: Available online 8 January 2020Source: Acta MaterialiaAuthor(s): Medha Veligatla, Christian Titsch, Welf-Guntram Drossel, Carlos J. Garcia-Cervera, Peter MüllnerWhen applying a magnetic field parallel or perpendicular to the long edge of a parallelepiped Ni-Mn-Ga stick, twin boundaries move instantaneously or gradually through the sample. We evaluate the sample shape dependence on twin boundary motion with a micromagnetics computational study of magnetic domain structures and their energies. Due to the sample shape, the demagnetization factor varies with the direction of external magnetic field. When the external magnetic field is applied perpendicular to the long edge of the sample, i.e. in the direction in which the demagnetizing field is highest, the magnetic energy intermittently increases when the strength of the applied magnetic field is low. This energy gain hinders the twin boundary motion and results in a gradual switching, i.e. a gradual magnetization reversal as the applied magnetic field is increased. The formation of 180⁰ magnetic domains offsets this effect partially. In contrast, when the applied magnetic field is parallel to the long edge of the sample, i.e. in the direction in which the demagnetizing field is lowest, the energy decreases with each subsequent magnetization domain reversal and the twin boundary moves instantaneously with ongoing switching. The actuation mode with the field parallel to the long sample edge lends itself for on-off actuators whereas the actuation mode with the field perpendicular to the long sample edge lends itself to gradual positioning devices.Graphical abstractImage, graphical abstract
       
  • On the Remarkable Fracture Toughness of 90 to 97W-NiFe Alloys Revealing
           Powerful New Ductile Phase Toughening Mechanisms
    • Abstract: Publication date: Available online 8 January 2020Source: Acta MaterialiaAuthor(s): M.E. Alam, G.R. OdetteTungsten is generally too brittle to serve a robust structural function. Here, we explore the fracture toughness of 90 to 97 wt.%W Fe-Ni liquid phase sintered tungsten heavy alloys (WHAs). The room temperature (RT) maximum load fracture toughness (KJm ≈ 69 to 107 MPa√m) of the WHA, containing only 3 to 10 wt.% of a Ni-Fe ductile phase (DP), is ≈ 9 to 13 times higher than KIc typical of monolithic W (≈ 8 MPa√m). All the WHAs show extensive stable crack growth, and increasing blunting line toughness averaging ≈ 170 MPa√m, prior to significant crack extension. In contrast to classical ductile phase toughening, that is primarily due to macrocrack bridging, the WHA toughness increase mainly involves new mechanisms associated with arrest, blunting and bridging of numerous dilatational shielding process zone microcracks in the macrocrack process zone. Tests down to -196°C, to partially emulate irradiation hardening, show decreasing toughness and a transition to elastic fracture at a temperature of -150°C for 90W to -25°C for 97W. However, even at -196°C, the leanest DP 97W WHA KIc is ≈ 3 times that of monolithic W at RT. Possible effects of the small specimen size used in this study are briefly summarized.Graphical abstractA schematic illustration of the process zone microcracking toughening mechanisms (left) that results in the very high room temperature maximum load KJm in the 90-97W-NiFe alloys. Image, graphical abstract
       
  • Strongly correlated and strongly coupled s-wave superconductivity of the
           high entropy alloy Ta1/6Nb2/6Hf1/6Zr1/6Ti1/6 compound
    • Abstract: Publication date: Available online 8 January 2020Source: Acta MaterialiaAuthor(s): Gareoung Kim, Min-Ho Lee, Jae Hyun Yun, Soon-Gil Jung, Woongjin Choi, Tae-Soo You, Jong-Soo RhyeeHigh entropy alloy (HEA) is a random mixture of multiple elements stabilized by high mixing entropy. We synthesized a Ta1/6Nb2/6Hf1/6Zr1/6Ti1/6 bulk HEA compound as a body-centered cubic structure with lattice parameter a = 3.38 Å based on arc melting. From the electronic and magnetic property measurements, we obtained the superconducting properties such as electron-phonon coupling constant λel-ph, electron-phonon potential Vel-ph, density of states at the Fermi level D(EF), superconducting energy gap 2Δ(0)/kBTc, upper-critical field Hc2(0), coherence length ξ, and critical current density Jc. The compound showed a superconducting transition at Tc = 7.85 K. The compound has relatively sizeable specific heat jump (ΔC/γTc), high effective mass of carrier (29 me), and high Kadowaki-Woods ratio (A/γ2, which plays an important role in the heavy Fermi compounds), indicating that it resides within the strongly coupled s-wave superconductor within a dirty limit. Its vortex pinning force is described by the Dew-Huges double exponential pinning model, implying that there are two types of pinning mechanisms. The possible coexistence of strongly correlated behavior in s-wave superconductivity in HEA compounds is noteworthy because many of the strongly correlated superconductors, such as heavy-fermion and high Tc cuprate superconductors, have nodal gap symmetry. The HEA compound suggests exploiting different types of superconductivity with the current strongly correlated superconductors as well as metallic superconductors.Graphical abstractKawadoki-Woods plot for transition metals (aTM = 0.4 μΩ cm mol2 K2 J−2, dark blue dashed line) and for heavy fermion system (aHF = 10 μΩ cm mol2 K2 J−2, orange dash-dotted line). The inset shows the temperature dependent electrical resistivity above the Tc fitted by the formula ρ(T) = ρ0+AT2.Image, graphical abstract
       
  • Accelerated design of novel W-free high-strength Co-base superalloys with
           extremely wide γ/γʹ region by machine learning and CALPHAD methods
    • Abstract: Publication date: Available online 7 January 2020Source: Acta MaterialiaAuthor(s): Jingjing Ruan, Weiwei Xu, Tao Yang, Jinxin Yu, Shuiyuan Yang, Junhua Luan, Toshihiro Omori, Cuiping Wang, Ryosuke Kainuma, Kiyohito Ishida, Chain Tsuan Liu, Xingjun LiuSince half a century ago, researchers have continuously focused on developing γʹ-strengthened Co-base superalloys to achieve an increased power and efficiency; these alloys can supposedly operate at higher temperatures than Ni-base superalloys. However, the yielded results have failed to meet the expectations. Herein, we successfully design novel W-free Co-V-Ta-base alloys by employing machine learning algorithm and CALPHAD methods, which exhibit low mass density (8.67-8.86 g/cm3), an extremely wide γ/γʹ region, a high γʹ solvus temperature (up to 1044 °C), and a high strength. The atom probe tomography results show that titanium is an extremely strong γʹ-former; therefore, it is expected to improve the thermodynamic stability of the γʹ phase. Furthermore, besides the very high tensile strength (18.7 GPa) of γʹ phase, indicated by first-principles calculations, the strength of Ti-incorporated alloy is higher than that of γʹ-strengthened Co-base superalloys; especially, the reported strength value is higher than that of the well-known Co-9Al-9W alloy by approximately 322 MPa at 750 °C, which is comparable to that of a few commercial Ni-base superalloys. Therefore, the possibility of the Co-V-Ta-base system being a candidate for developing novel Co-base superalloys is strongly suggested in this study.Graphical abstractImage, graphical abstract
       
  • Metal hetero-diffusion along the metal-ceramic interfaces: a case study of
           Au diffusion along the Ni-sapphire interface
    • Abstract: Publication date: Available online 6 January 2020Source: Acta MaterialiaAuthor(s): Hagit Barda, Eugen RabkinWe propose a new method for determining the hetero-diffusion coefficient of metals along the metal-ceramic interfaces. The samples for diffusion studies are produced by partial dewetting of thin metal film deposited on ceramic substrate, followed by the deposition of an ultrathin layer of a diffuser. The latter covers both the partially dewetted thin film and the dewetting holes exposing the substrate. During diffusion anneal, the diffuser penetrates from the triple lines along the film-substrate interface. We apply this method for studying the diffusion of Au along the Ni-sapphire interface. Our experiments yield the pre-exponential factor and activation energy of 2.217−2.208+535×10−6m2s and 179±33kJmol−1, respectively. Our results demonstrate that in the examined temperature range (450 – 550°C), interface diffusion is slower than grain boundary hetero-diffusion, but faster than bulk diffusion of Au in Ni.Graphical Image, graphical abstract
       
  • Pressureless Two-Step Sintering of Ultrafine-Grained Tungsten
    • Abstract: Publication date: Available online 6 January 2020Source: Acta MaterialiaAuthor(s): Xingyu Li, Lin Zhang, Yanhao Dong, Rui Gao, Mingli Qin, Xuanhui Qu, Ju LiThe challenge of producing dense ultrafine-grained (UFG) tungsten is hereby addressed by a simple pressureless two-step sintering method. It provides a uniform microstructure with ∼99% theoretical density and ∼700 nm grain size, which is among the best sintering practice of pure tungsten reported in the literature. Benefitting from the finer and more uniform microstructures, two-step sintered samples show better mechanical properties in bending strength and hardness. While parabolic grain growth kinetics is verified, a transition in the nominal grain boundary mobility was observed at 1400 °C, above which the effective activation enthalpy is ∼6.1 eV, below which grain boundary motion is rapidly frozen with unusually large activation enthalpy of ∼12.9 eV. Such highly nonlinear behavior in activation parameters with respect to temperature suggests that activation entropy and maybe collective behavior play a role. We believe the as-reported two-step sintering method should also be applicable to other refractory metals and alloys, and may be generalized to multi-step or continuous-cooling sintering design using machine learning.Graphical abstractA simple pressureless two-step sintering method has been successfully applied to pure tungsten, producing dense ultrafine-grained samples with uniform microstructure and improved mechanical properties.Image, graphical abstract
       
  • Critical assessment of the evaluation of thermal desorption spectroscopy
           data for duplex stainless steels: a combined experimental and numerical
           approach
    • Abstract: Publication date: Available online 6 January 2020Source: Acta MaterialiaAuthor(s): L. Claeys, V. Cnockaert, T. Depover, I. De Graeve, K. VerbekenThe present study evaluates thermal desorption spectroscopy (TDS) data measured for UNS S32205 duplex stainless steel. Variations in the TDS spectra are obtained by electrochemical hydrogen charging for different times and by applying different heating rates for desorption to evaluate the desorption activation energy. Good agreement is found when comparing the experimental TDS curves with the desorption flux based on a numerical diffusion model using a homogeneous average hydrogen diffusion coefficient for the two-phase (ferrite-austenite) duplex microstructure. Trapping cannot be distinguished from the experimental TDS data since hydrogen diffusion in austenite is the rate-determining process during desorption. An average diffusion activation energy of 43.4 kJ/mol is determined from the experiments. Moreover, similar findings are obtained with a finite-element model that includes the heterogeneous hydrogen-related properties of the two phases of this duplex stainless steel.Graphical abstractImage, graphical abstract
       
  • “Self-sharpening” tungsten high-entropy alloy
    • Abstract: Publication date: Available online 6 January 2020Source: Acta MaterialiaAuthor(s): X.F. Liu, Z.L. Tian, X.F. Zhang, H.H. Chen, T.W. Liu, Y. Chen, Y.J. Wang, L.H. Dai“Self-sharpening”, a material maintaining its acute head shape during penetration, is highly desirable in a wide range of engineering applications. However, it remains a great challenge to make it occur in conventional single-principal-element alloys. Here, we develop a new chemical-disordered multi-phase tungsten high-entropy alloy that exhibits outstanding self-sharpening capability, in sharp contrast to conventional tungsten alloys only showing mushrooming. This alloy consists of a BCC dendrite phase and a rhombohedral μ phase embedded in the continuous FCC matrix, and such a unique microstructure leads to a 10-20% better penetration performance than conventional tungsten heavy alloys. We show that emergence of the self-sharpening is triggered by the ultrastrong μ phase stimulated dynamic recrystallization softening mediated shear banding. This study sheds light on the origin of self-sharpening and might open new opportunities for developing high-performance penetrator materials.Graphical abstractImage, graphical abstract
       
  • Dislocation nucleation and evolution at the ferrite-cementite interface
           under cyclic loadings
    • Abstract: Publication date: Available online 6 January 2020Source: Acta MaterialiaAuthor(s): Lun-Wei Liang, Yun-Jiang Wang, Yan Chen, Hai-Ying Wang, Lan-Hong DaiFatigue is of significant importance to the engineering applications of the structural materials. High-strength pearlite steel consisting of a ductile ferrite phase and a brittle cementite phase is a widely used structural metal for extreme load-bearing applications. However, the fatigue mechanisms of such important materials remain elusive, in particular, the atomic-scale dislocation behaviors at interface are poorly understood. We used molecular dynamics simulations to probe the mechanical response and deformation mechanism of the Bagaryatskii-oriented ferrite-cementite interface in pearlite. The interface was subjected to a hundred symmetric tension-compression deformation cycles. Three different loading schemes with strain magnitudes of 4.0%, 6.0%, and 9.0% are sophisticatedly designed to explore the cyclic plastic mechanisms under different conditions corresponding to pure elasticity, elasticity in tension but plasticity in compression, and plasticity in both tension and compression, respectively. During cyclic deformation, rapid dislocation accumulation occurs in the first 30 cycles, after which dislocation density decreases to a stable value in ferrite. It is found that the onset of plasticity is governed by dislocation nucleation from the ferrite-cementite interface. After slip into the ferrite phase, some dislocations annihilate at the interface. After a few tens of cycles, the dislocation nucleation and annihilation rates become equal, leading to a steady-state flow in cyclic deformation. Up to high cycles with large strain magnitude, the magnitude of plastic strain in pearlite is higher than critical values and slip crosses the interface from the ferrite phase to the brittle cementite phase. Dislocation slip in cementite will destroy the interface structure, which may be the plastic mechanism of final fatigue failure. Our simulations agree with experimental observations of dislocation evolution in the ratchetting of pearlite steels and provide further atomic-scale mechanisms to explain the fatigue failure of these materials.Graphical abstractImage, graphical abstract
       
  • Precipitation-induced mitigation of recrystallization in ultra-thin,
           cold-rolled AlScZrMn(Mg) sheets at brazing temperatures: the critical
           effect of alloy composition and thermal processing route
    • Abstract: Publication date: Available online 6 January 2020Source: Acta MaterialiaAuthor(s): Qingshan Dong, Andrew Howells, Mary Gallerneault, Vahid FallahUsing advanced electron microscopy techniques, statistical analysis and analytical investigation of precipitates/dispersoids evolution, we demonstrate the critical effect of alloy composition (Sc, Mn, and Mg content) and thermal processing route (heating rate and pre-aging) on the recrystallization behaviour of AlScZrMn(Mg) alloys. Two major types of second phases, namely Al3(Sc,Zr) precipitates and α-Al(Mn,Fe)Si dispersoids, were identified in the thermally-treated cold-rolled sheets (of 0.3 mm thickness). Both phases were observed to maintain coherency with the Al matrix at abnormally large sizes (>100 nm and>500 nm, respectively), as well as exhibiting unprecedented levels of thermal stability (i.e., high coarsening resistance). The recrystallization behaviour and strength evolution were shown to be a strong function of the size and aerial number density evolution of the precipitates/dispersoids which, in turn, are controlled by the alloy composition and thermal history. Particularly, the recrystallization was effectively mitigated at a slow ramp to 590°C (a typical brazing temperature for AlMn alloys) whereas a full recrystallization occurred during a faster ramp. Such behaviour was explained by the competitive kinetics of Al3(Sc,Zr) precipitation and recrystallization phenomenon at intermediate and high-temperature ranges upon heating to 590°C. The introduction of a pre-aging treatment within the intermediate temperature range (i.e., 250-400°C), prior to the fast ramp, was shown to prevent recrystallization due to the stabilization effect of a large aerial number density of finely-dispersed Al3(Sc,Zr) precipitates. A higher Sc content in the alloy enhances such a stabilization effect. Mn additions not only enhance the mitigation of recrystallization (through a refinement of Al3(Sc,Zr) precipitates) but also refines the evolution of α-Al(Mn,Fe)Si dispersoids resulting in a higher yield strength. The Mg addition, on the other hand, has no impact on the evolution of Al3(Sc,Zr) precipitates nor on the recrystallization status, though it causes a refinement of α-Al(Mn,Fe)Si dispersoids and thus leads to a higher final yield strength. The extraordinary high-temperature stability of cold-rolled thin sheets, obtained by the alloy and process design in this study, can be effectively utilized for many light-weight applications of AA3xxx Al alloys.Graphical abstractImage, graphical abstract
       
  • From Coherent to Semicoherent–Evolution of Precipitation
           Crystallography in an fcc/bcc System
    • Abstract: Publication date: Available online 6 January 2020Source: Acta MaterialiaAuthor(s): Fu-Zhi Dai, Zhi-Peng Sun, Wen-Zheng ZhangHow are crystallographic features developed in phase transformation systems with anisotropic misfits (e.g., bcc/fcc systems)? Comprehensive knowledge about this question is still very limited, which impedes the thorough understanding and the quantitative modeling of microstructure evolution. In this work, we simulated the early stage growth of a Cr precipitate (bcc) in a Cu matrix (fcc) by combining Monte Carlo and molecular dynamics. The simulation results reveal fine details about the evolution of crystallographic features, including the orientation relationship (OR) between the two phases, the precipitate morphology, and the interfacial dislocation structures. The governing event during the evolution process is the generation of a dominant set of dislocations. The selection of the dominant dislocations is rationalized based on minimization of the interfacial energy in the major facet, which contains a single set of dislocations. The initial OR between the coherent precipitate and the matrix is close to the Nishiyama-Wassermann OR. In response to the generation of the dominant dislocations, the OR jumps towards the ideal OR corresponding to the O-line condition, which is close to the Kurdjumov-Sachs OR. This tendency reflects the experimental observations in a Cu-Cr system and provides helpful insight into the actual evolution of crystallographic features.Graphical abstractImage, graphical abstract
       
  • Comparative Study of Radiation Defects in Ion Irradiated Bulk and
           Thin-Foil Tungsten
    • Abstract: Publication date: Available online 6 January 2020Source: Acta MaterialiaAuthor(s): Ruo-Yao Zheng, Wei-Zhong HanIn this study, we employ transmission electron microscope (TEM) to analyze radiation defects in helium (He) and krypton (Kr) ions implanted bulk and thin-foil tungsten. For bulk tungsten, subgrains are formed near the surface region under both He+ and Kr+ irradiation. Dislocation loops are observed beyond ion implanted range. These observations are related to self-interstitial atoms (SIAs) diffusion and clustering. Ordered bubbles are formed in He+ implantation, while no cavities are detected in Kr+ irradiation. In thin-foil tungsten, line up of dislocation loops is found mainly aligns along {101} and {112} slip planes. Both 1/2 and dislocation loops are identified. Compared to He+ irradiation, more loops are detected in Kr+ irradiation due to higher energy collision cascade. Nanocavities are detected in irradiated thin-foil tungsten besides the formation of high density of interstitial loops. The number density of dislocation loops and the volume fractions of cavities are higher in He+ irradiation than in Kr+ irradiation. The differences in nature of radiation defects is attributed to the higher recombination rate of vacancies and interstitials in bulk sample, the significant surface sink effect in thin-foil irradiation and the chemical and physical effect of implanted ions.Graphical abstractImage, graphical abstract
       
  • Structural evolution of topologically closed packed phase in a Ni-based
           single crystal superalloy
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): Haibo Long, Shengcheng Mao, Yinong Liu, Hong Yang, Hua Wei, Qingsong Deng, Yanhui Chen, Ang Li, Ze Zhang, Xiaodong HanThis work investigates the evolution of the crystal structure of the topologically closed packed orthorhombic P phase precipitate in a Re-containing Ni-based single crystal superalloy during thermal exposure. The P phase is formed with a thin needle morphology. The precipitate is formed from the matrix with an initial complex atomic arrangement which continues to evolve during the process of thermal exposure. Based on the experimental evidence of this study and the theoretically predicted structure reported in the literature, a mechanism of the structural transformation is proposed. The initial structure is composed of a parallelogram (P) atomic arrangement configuration, which gradually evolve into a rectangle (R) atomic arrangement configuration in the [100]P projection. In the [010] projection, the initial structure is composed of alternating rows of a larger parallelogram (P') and a larger rectangle (R') configurations, which gradually evolve into an intricate structure of P'-R' along the length [001]P direction and P'P'-R'R' along the transverse [100]P direction. The initial structure is formed for its structural similarities to the γ phase to minimize lattice mismatch. The final structure is evolved over time to conform to its thermodynamically more stable state. The structural evolution is achieved by collective atomic shuffling. These intricate atomic arrangements gives rise to the very faint and highly dense parallel striation lines along the transverse [010]P direction in [100]P view and along the length [010]P direction in [010]P view under transmission electron microscopy observation.Graphical abstractImage, graphical abstract
       
  • The key role played by dislocation core radius and energy in hydrogen
           interaction with dislocations
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): Ping Yu, Yanguang Cui, Guo-zhen Zhu, Yao Shen, Mao WenIt is generally believed that the H-induced reduction in dislocation energy plays a key role in modifying dislocation behaviors in the process of hydrogen embrittlement. Here, we examine the factors that lead to H reducing the line energies of the edge and screw dislocations in bcc Fe by atomistic simulations. Grand canonical Monte Carlo simulations are conducted to obtain the distribution of H around the dislocations. We find that H mainly aggregates at the dislocation cores and the H concentration in the elastic field of dislocations is extremely low. The direct consequences of such a distribution pattern of H are as follows. (i) In contrast with previous studies, H induces no change in the shear modulus of the systems containing dislocations. (ii) H increases the core radii and decreases the core energies of the dislocations, which are the only factors leading to the reduction of dislocation line energy by H. (iii) H brings little effect on the stress field of either the edge or screw dislocation, implying that H induces almost no stress-shielding effect on dislocations. A linear relation between the critical shear stress for homogeneous dislocation nucleation and logarithmic bulk H concentration is thus revealed, based on the atomistic result of the H-induced increase in the core radius and decrease in the core energy of the dislocations. The present results indicate that the dislocation-dislocation interaction in the presence of H, which is the key ingredient for the H-enhanced localized plasticity mechanism for hydrogen embrittlement, can be easily evaluated by the linear elastic theory of dislocations if the core radius and energy of dislocations are properly described.Graphical abstractImage, graphical abstract
       
  • Reversible dislocation movement, martensitic transformation and
           nano-twinning during elastic cyclic loading of a metastable high entropy
           alloy
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): S.M. Vakili, A. Zarei-Hanzaki, A.S. Anoushe, H.R. Abedi, M.H. Mohammad-Ebrahimi, M. Jaskari, Seok Su Sohn, D. Ponge, L.P. KarjalainenThe present study contends with the room temperature microstructural response of a non-equiatomic metastable high entropy alloy to the elastic cyclic deformation. The stress and strain-induced martensite formation and reversion are recognized as the main microstructural evolutions which are directly correlated with the reversibility of dislocation movement. Two different patterns of reversion for deformation driven epsilon martensite are identified. Full reversion of stress-induced epsilon martensite results in development of nano-twined matrix, the various aspects of which have been described through a dislocation-based model. The strain-induced martensite also goes through partial reversion leading to lath fragmentation which in-turn significantly influences the martensite stability. Interestingly, the presence of a well-developed dislocation substructure is characterized within the martensite bands, which seems to be phenomenal owing to the low imposed strain, low temperature and low stacking fault energy of the experimented material. The development of vein and wall/channel structures is justified through proposing a conceptual based model regarding the interaction of the stacking faults and subsequent generation of the perfect dislocations.Graphical abstractImage, graphical abstract
       
  • The effects of temperature and alignment state of nanofillers on the
           thermal conductivity of both metal and nonmetal based graphene
           nanocomposites
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): Jie Wang, Jackie J. Li, George J. Weng, Yu SuTemperature and the alignment state of nanofillers are two important factors that can greatly affect the thermal conductivity of graphene nanocomposites, but the simultaneous influence of these two issues have never been considered in any theoretical treatment. In this work we incorporate the contributions of electron-phonon coupling and phonon-phonon interaction into the diffuse mismatch model to establish the temperature dependence of filler-matrix interfacial thermal resistance and filler-filler contact resistance, both crucial for thermal conduction in graphene nanocomposites. The electron and phonon transport mechanisms, suitable for metal and nonmetal matrices respectively, are both taken into account. Then through an effective-medium approximation based on Maxwell's far-field matching, the temperature-dependent thermal conductivity of both metal and nonmetal-based graphene nanocomposites is derived. By further introducing a confinement angle with respect to the main alignment direction wherein the graphene nanofillers are supposed to be dispersed, the influence of the alignment state of graphene nanofillers is also investigated. We highlight this newly developed theory with direct comparisons to several sets of experimental data, and demonstrate the significant effects of temperature level and alignment state in thermal conductivity of graphene metal and nonmetal nanocomposites.Graphical abstractImage, graphical abstract
       
  • Effect of twin boundary formation on the growth rate of the GaSb{111}
           plane
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): Keiji Shiga, Kensaku Maeda, Haruhiko Morito, Kozo FujiwaraThe top surface of a crystal–melt interface was observed directly during directional solidification of polycrystalline GaSb at a constant cooling rate, and the effect of {111}Σ3 twin boundary formation on the rate of grain growth parallel to the 〈111〉 direction was investigated. A {111}Σ3 twin boundary was generated by the decomposition of another {111}Σ3 twin boundary with formation of a Σ9 grain boundary. The growth of the crystal–melt interface in the direction parallel to the B direction was stalled during the nucleation and propagation of the new {111}Σ3 twin boundary. The growth rate of the crystal–melt interface after twin formation was approximately half of that before. This is considered to be due to {111} polarity reversal by twinning. The dangling bond density on the {111}A plane of GaSb is almost ten times of that on the {111}B plane, and the dangling bond density is related to the attachment energy of atoms of the plane; therefore, the growth rate in the A direction would be significantly lower than that in the B direction. These results indicate different growth rates in opposite 〈111〉 directions and a significant effect of the polarity on the competition of grain growth during the directional solidification of polycrystalline GaSb.Graphical abstractImage, graphical abstract
       
  • Size-dependent elastic modulus of nanoporous Au nanopillars
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): Santhosh Mathesan, Dan MordehaiThe deformation response of nanoporous gold nanopillars under compression, with varying nanopillar and ligament diameters, is analyzed from a series of molecular dynamics simulations. We have measured the ligament size-dependent elastic modulus of nanoporous nanopillars from the elastic region of stress–strain curves, showing that the elastic modulus at a given solid fraction depends on both the geometrical characteristics of the nanopillars and topology of the ligament structure. We modified the Gibson–Ashby scaling law for nanopillars, to predict the size-dependency found in the simulations. To compare the modified model and the simulation results, we have developed a method to estimate the average number of load-bearing ligaments by employing a unique post-processing technique. The comparison between the scaling law and the simulation results lead us to the conclusion that the elastic response of the nanopillars is predominantly due to the compression of ligaments. The size effect due to the new dimensionality vanishes at larger nanopillar diameter and eventually, the scaling laws resemble the classical laws, which is a function of solid fraction only. Besides the size effect, the need to generate nanoporous nanopillars with different topologies and to ensemble average over the various topologies is discussed.Graphical abstractImage, graphical abstract
       
  • Mechanisms behind the spontaneous growth of Tin whiskers on the
           Ti2SnC ceramics
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): Yushuang Liu, Chengjie Lu, Peigen Zhang, Jin Yu, Yamei Zhang, Zheng Ming SunThe spontaneous growth of Tin whiskers has been a reliability issue in electronic assemblies for around 70 years, but the underlying physics is still ambiguous. Herein, Sn whiskers formed on the Ti2SnC, a layered ternary carbide ceramic MAX phase, were studied to trace the atomic motion in whisker growth process. Free Sn source was found to be necessary for the formation of Sn whiskers, however, the interface microstructure suggests that Sn atoms feeding whiskers are diffusing through the Ti2SnC lattice. According to the simulation results on the formation and migration energies of vacancies in Ti2SnC, a low diffusion barrier is expected for Sn atoms to diffuse along the basal planes. When Sn atoms diffuse out of the substrate, Sn whiskers bounded by low-energy planes form to minimize the total energy. In contrast, the whiskers maintain the striated morphology inherited from the whisker root in the environment containing oxygen, which is mainly attributed to the confinement of the oxide film on whisker surface. The findings will also shed new light on understanding the general whiskering problems found in other materials.Graphical abstractImage, graphical abstract
       
  • Mobility of dislocations in aluminum: The role of non-Schmid stress state
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): Khanh Dang, Darshan Bamney, Laurent Capolungo, Douglas E. SpearotThe extent that non-Schmid stresses influence the mobility of dislocations in Al is studied using molecular dynamics (MD) simulations. Specifically, mobility laws for straight screw, 30°, 60° and edge dislocations are atomistically derived for different combinations of Escaig stress, τe, and stress normal to the (111) slip plane, σn. MD simulations show that the mobility of each dislocation is distinctly influenced by non-Schmid stresses. Furthermore, MD simulations of dislocation shear loop expansion show that mobility laws derived from straight dislocations are applicable to describe the more complex behavior of stress-state dependent expansion of a dislocation loop. Data describing the dislocation mobility laws are incorporated into discrete dislocation dynamics (DDD) simulations using a hierarchical multiscale approach. DDD simulations of isolated dislocation loop expansion show strong agreement with MD simulation results, validating the nonlinear multiscale implementation. DDD simulations of dislocation network evolution show that the use of stress state dependent dislocation mobility laws provides quantifiable changes to the plastic deformation path, which leads to different final microstructures. The results presented here demonstrate the key role that local stresses, experienced by the dislocation core, play on the evolution of a dislocation network during plastic deformation.Graphical abstractImage, graphical abstract
       
  • Design and analysis of negative permittivity behaviors in barium
           titanate/nickel metacomposites
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): Zhongyang Wang, Kai Sun, Peitao Xie, Qing Hou, Yao Liu, Qilin Gu, Runhua FanAs an important supplement to the metamaterials, negative permittivity in 'natural' materials has attracted increasing attention, while some fundamental regulation mechanisms and common theoretical principles have not yet been systematically explored. In this paper, the permittivity transition is investigated along with insulator-conductor conversion in BaTiO3/Ni composite. When the Ni content in BaTiO3/Ni composites is lower than and higher than the percolation threshold, Lorentz-like and Drude-type negative permittivity behaviors is obtained, respectively. In addition, the fundamental principles of negative permittivity are discussed, especially the validity of the Kramers–Kronig relations and the relationship between the negative permittivity and the reactance characters. Further, a universal regulatory mechanism of the Drude-type negative permittivity profile is qualitatively analyzed. Negative permittivity and negative permeability are achieved simultaneously in BaTiO3/Ni composites with 35.56 vol% of Ni loading, which show potential application in electromagnetic wave absorption and shielding.Graphical abstractImage, graphical abstract
       
  • Origins of strength and plasticity in the precious metal based
           high-entropy alloy AuCuNiPdPt
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): F. Thiel, D. Geissler, K. Nielsch, A. Kauffmann, S. Seils, M. Heilmaier, D. Utt, K. Albe, M. Motylenko, D. Rafaja, J. FreudenbergerThe precious metal based High-Entropy Alloy (HEA) AuCuNiPdPt crystallises in a face-centred cubic structure and is single phase without chemical ordering after homogenisation. However, a decomposition is observed after annealing at intermediate temperatures. This HEA shows extended malleability during cold work up to a logarithmic deformation degree of φ=2.42. The yield strength ranges from 820 MPa in the recrystallised state to 1170 MPa when strain hardened by cold working with a logarithmic deformation degree of φ > 0.6. This work hardening behaviour is traced back to a steep increase in dislocation density as well as in deformation twinning occurring at low strain. The microstructure and the mechanical properties of AuCuNiPdPt are assessed in detail by various methods. EBSD and TEM analyses reveal mechanical twinning as an important deformation mechanism. The high strength in the recrystallised state is evaluated and found to originate predominantly upon solid solution strengthening.Graphical abstractGraphical abstract for this article
       
  • Multifunctional elastic metasurface design with topology optimization
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): Junjie Rong, Wenjing YeMetasurfaces have emerged as promising materials for wave manipulation due to their compact form. While various innovative designs have been proposed and impressive wave-shaping capabilities have been demonstrated, the practical applications of these materials require additional features and functionalities such as high-energy transmission and robust manipulation, which are difficult to realize based on physics-driven design approaches. In this work, we propose a systematic design method based on topology optimization for the design of elastic metasurfaces with multiple objectives/requirements. Theoretical analyses were conducted to identify the conditions for achieving high transmission and robust manipulation. Based on the analyses, a topology optimization formulation is proposed for achieving specific phase delays and high transmission within a given frequency band. Genetic algorithm is adopted to solve the optimization problem, with special treatments on the structural connectivity. Both locally resonant and non-resonant structures can be obtained from this method, and there is no need to pre-determine the type of structures to be designed. Depending on the design emphasis and the working frequency range, the proposed design method can automatically determine the best type of structures that meet the design requirements. With this method, metasurfaces for anomalous refraction of longitudinal waves were designed. Nearly full transmission with robust performance has been confirmed by numerical simulations. To demonstrate the generality of the proposed method, a multi-functional metasurface for simultaneously control of longitudinal and shear waves has also been designed, which is the first realization of such functionality.Graphical abstractImage, graphical abstract
       
  • Mechanistic approach of Goss abnormal grain growth in electrical steel:
           Theory and argument
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): Soran Birosca, Ali Nadoum, Diween Hawezy, Fiona Robinson, Winfried KockelmannThe first Si-Fe electrical steel was produced in 1905, and the grain-oriented steel was discovered in 1930 after Goss demonstrated how optimal combinations of heat treatment and cold rolling could produce a texture giving Si-Fe strip good magnetic properties when magnetised along its rolling direction. This technology has reduced the power loss in transformers greatly and remains the basis of the manufacturing process today. Since then many postulations reported on the mechanism on abnormal grain growth (AGG) which is the key for Si-Fe superior magnetic properties, however, none have provided a concrete understanding of this phenomenon. Here, we established and demonstrated a new theory that underlines the fundamental mechanistic approach of abnormal grain growth in 3% Si-Fe steel. It is demonstrated, that the external heat flux direction applied during annealing and Si atom positions in the solid solution disordered α-Fe cube unit cell that cause lattice distortions and BCC symmetry reduction are the most influential factors in the early stage of Goss AGG than what was previously thought to be dislocation related stored energy, grain boundary characteristics and grain size/orientation advantages.Graphical abstractImage, graphical abstract
       
  • Anisotropic polycrystal plasticity due to microstructural heterogeneity: A
           multi-scale experimental and numerical study on additively manufactured
           metallic materials
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): S. Amir H. Motaman, Franz Roters, Christian HaaseAdditively manufactured (AM) metallic parts exhibit substantially different microstructures compared to those that are conventionally produced. Characterization studies have revealed that the microstructure of as-built AM metallic materials is highly heterogeneous in many respects. The strongly anisotropic mechanical response under plastic deformation observed in AM metals, compared to their conventionally manufactured counterparts, lies in the aforementioned inherent microstructural disparities. In this study, we have focused on a high-manganese steel (HMnS) processed by laser powder bed fusion (LPBF), which exhibits twinning-induced plasticity (TWIP). The as-built microstructure is carefully characterized by electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques. To unfold the potential of metal additive manufacturing, it is essential to understand the microstructure of AM products and its connection with the mechanical properties by means of numerical modeling and simulation. The mechanical response of AM components under plastic deformation is highly complex and its simulation requires advanced modeling and numerical methods. In the present study, in order to simulate the anisotropic plasticity of the LPBF-HMnS, we used the full field method for computational polycrystal homogenization combined with physics-based crystal plasticity constitutive and statistical microstructure modeling. The impact of different process-induced microstructural heterogeneity characteristics on macroscopic strain hardening behavior of the material has been comprehensively and systematically investigated. Finally, it has been argued why the chosen AM material with the selected processing parameters and chemical composition represented an ideal candidate for a generic assessment of the anisotropic polycrystal plasticity due to microstructural heterogeneity.Graphical abstractImage, graphical abstract
       
  • Finite interface dissipation phase field modeling of Ni–Nb under
           additive manufacturing conditions
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): Kubra Karayagiz, Luke Johnson, Raiyan Seede, Vahid Attari, Bing Zhang, Xueqin Huang, Supriyo Ghosh, Thien Duong, Ibrahim Karaman, Alaa Elwany, Raymundo ArróyaveDuring laser powder bed fusion (L-PBF) parts undergo multiple rapid heating-cooling cycles, leading to complex microstructures with nonuniform properties. In the present work, a computational framework which weakly couples a finite element thermal model to a non-equilibrium PF model was developed to investigate the rapid solidification microstructure of a Ni–Nb alloy during L-PBF. The framework is utilized to predict the spatial variation of the morphology and size of cellular segregation structures as well as the differences in melt pool microstructures obtained under different process conditions. A solidification map demonstrating the variation of microstructural features as a function of the temperature gradient and growth rate is presented. A planar to cellular transition is predicted in the majority of keyhole mode melt pools, while a planar interface is predominant in conduction mode melt pools. The predicted morphology and size of the cellular segregation structure agree well with experimental measurements.Graphical abstractGraphical abstract for this article
       
  • Probing defect relaxation in ultra-fine grained Ta using micromechanical
           spectroscopy
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): Markus Alfreider, Inas Issa, Oliver Renk, Daniel KienerThe study of grain boundaries (GBs) in polycrystalline materials is a field of major interest, as many fundamental properties are influenced by their actual structure. One of the main challenges in investigating GBs is that electron microscopy techniques capable to resolve their atomistic arrangement require very small sample volumes of a few tens of nanometers and might therefore change the natural state of the GB via removal of occurring material constraint. To counteract this influence one could apply indirect measurements such as internal friction to probe for changes of the GB structure. However, with the drive towards smaller volumes most of these techniques are at their limit. The current work proposes a new approach based on mechanical spectroscopy of micron sized specimens applied in-situ in a scanning electron microscope. Applying this novel concept to ultra-fine grained Ta we investigate changes in the defect structure between the as-deformed condition and after a heat treatment. In fact, we detect a decrease in internal friction of about 50% upon annealing. This pronounced change occurs in conjunction with an increase in flow stress and hardness and is related to thermally induced GB relaxation.Graphical abstractImage, graphical abstract
       
  • Solid solution strengthening and deformation behavior of single-phase
           Cu-base alloys under tribological load
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): Stephan Laube, Alexander Kauffmann, Friederike Ruebeling, Jens Freudenberger, Martin Heilmaier, Christian GreinerThe influence of solid solution strengthening on the evolution of the microstructure under cyclic dry sliding is still not fully rationalized. One reason is that alloying needed for solid solution strengthening alters the stacking fault energy at the same time and, hence, the mode of dislocation slip in face-centered cubic metals and alloys. Both aspects determine the details of plastic deformation and, therefore, lead to different results under tribological load. A series of Cu-Mn alloys was investigated in the present study, which exhibit wavy slip mode and an almost constant stacking fault energy over a wide solute concentration range. Solid solution strengthening is the main contribution to the hardness in these alloys. The sole impact of changing strength and hardness on the tribological response along with microstructure evolution during tribological load is assessed. After the reciprocating, tribological loading a linear correlation between the wear track width and hardness could be ascertained. Electron microscopy reveals a horizontal discontinuity of the dislocation structure beneath the surface in all alloys at a similar depth. An evaluation of the Hamiltonian elastic stress field model indicates that the depth of the dislocation feature after one sliding pass correlates with the stress distribution as well as the critical stress for dislocation motion. The subsurface microstructure features a transition from the dislocation feature to subgrain formation after about five to ten cycles. Beyond ten cycles, oxide clusters are formed on the sliding surface and the grains elongate in the sliding direction.Graphical abstractImage, graphical abstract
       
  • Atomic-scale investigation of the interface precipitation in a TiB2
           nanoparticles reinforced Al–Zn–Mg–Cu matrix composite
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): Yu Ma, Ahmed Addad, Gang Ji, Ming-Xing Zhang, Williams Lefebvre, Zhe Chen, Vincent JiThe effects of nanosized reinforcement particles on precipitation reactions in age-hardenable Al alloy matrix composites have been largely unknown. In this work, an Al–Zn–Mg–Cu matrix composite reinforced with uniformly distributed TiB2 nanoparticles was successfully produced. The solid-soluted, peak-aged and overaged materials were then characterized, at the atomic scale using (high-resolution) scanning transmission electron microscopy, to provide a fundamental insight into the interface precipitation. Our results demonstrated that the facetted TiB2 nanoparticles have a significant impact on the precipitation in matrix areas adjacent to the TiB2/Al interfaces. The interfaces after solid-solution treatment are tightly-bonded and oxide-free. The TiB2 particles and Al matrix display two orientation relationships (ORs): the well-reported [21¯1¯0]TiB2//[101]Al, (0001)TiB2//(1¯11)Al (OR1) and the new [21¯1¯0]TiB2//[101]Al, (011¯0)TiB2//(111¯)Al (OR2). The interface precipitates (i.e., interphase) having the size of several tens of nanometers were formed after ageing and were determined to be (Zn1.5Cu0.5)Mg phase. Their formations were only related to the initial OR1 and OR2 where the mutual ORs between the TiB2, interphase and Al matrix were further developed. Periodically spaced misfit dislocations were revealed at the semi-coherent TiB2/Al interfaces, which are generally considered beneficial to the heterogeneous precipitation. They not only reduced nucleation energy barrier, but also acted as short-circuit diffusion paths for transporting solute atoms and vacancies, accelerating growth rate. However, the growth of interphase at the interface parallel to close-packed {111} Al planes was suppressed by the ultra-low accommodation factor. In addition, such an interface precipitation reduced the mismatch of the TiB2/Al interface, increasing the overall coherency and being potential for effective interface strengthening.Graphical abstractImage, graphical abstract
       
  • Nanoparticles in Bi0.5Sb1.5Te3: A prerequisite defect structure to scatter
           the mid-wavelength phonons between Rayleigh and geometry scatterings
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): Kyu Hyoung Lee, Hyun-Sik Kim, Weon Ho Shin, Se Yun Kim, Jae-Hong Lim, Sung Wng Kim, Sang-il KimNanoparticles in thermoelectric alloys has been considered as one of the most important ingredients to enhance their thermoelectric figure of merit zT mainly by reducing the lattice thermal conductivity due to intensified phonon scattering. However, the scattering mechanism of phonon with respect to wavelengths, which provides the comprehensive design rules for nanocomposites with enhanced zT, has not been fully understood. Here, we report a critical role of nanoparticles for the lattice thermal conductivity reduction from the theoretical and experimental analysis of the temperature-dependent thermal and electronic transport properties of p-type Ag/Cu nanoparticles-embedded Bi0.5Sb1.5Te3 with respect to their electronic, bipolar, and lattice thermal conductivities. It was found that the introduction of the Ag/Cu nanoparticles reduced the lattice thermal conductivity through the additional phonon scattering based on the changeover between the Rayleigh and geometrical scatterings, indicating the indispensability of nanoparticles to scatter phonons that cannot be scattered effectively by either point defects or grain boundaries.Graphical abstractImage, graphical abstract
       
  • Fracture in nanoporous gold: An integrated computational and experimental
           study
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): Nathan Beets, Joshua Stuckner, Mitsuhiro Murayama, Diana FarkasWe present simulation/experimental integrated study examining crack propagation in nanoporous gold. We utilized a simulation technique that combines continuum fracture mechanics with molecular dynamics. We applied this to a large atomistic digital sample of nanoporous gold to implement mode-1 crack propagation. Crack propagation tests were also conducted on an experimental sample, prepared via chemical dealloying and observed via in-situ TEM microscopy. We observed cracks in both samples propagating by the same mechanisms of sequential individual ligament failure. A series of nanowire computational deformation tests were also conducted to understand individual ligament behavior, and how this influences the overall sample fracture. This iterative direct experiment/simulation comparison provides new insight into the failure response of nanoporous gold.Graphical abstractImage, graphical abstract
       
  • The nucleation and growth of η phase in nickel-based superalloy during
           long-term thermal exposure
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): Gang Liu, Xueshan Xiao, Muriel Véron, Soran BiroscaThe microstructure degradation and subsequent phase transformations in Waspaloy nickel-based superalloy during thermal exposure at 780 °C for 10,000 h were investigated. Two paths of η phase formation in the centre of extra-large γ’ (EL-γ’) following the formation of EL-γ’ were observed: (i) η phase directly precipitated within EL-γ’ when the coalescence of γ’ reached a critical stage; (ii) η phase precipitated at the interface of small size MC carbide and EL-γ’, with both MC and η embedded inside EL-γ’. The phase transformation process including the formation of EL-γ’ were experimentally observed and the formation sequences were schematically suggested. Two criteria of η formation and growth within EL-γ’ were established: (i) stacking faults formation in the nucleation site and (ii) sufficient atom diffusion during nuclei growth. The study of kinetics of η formation through two different paths revealed the critical role of small size carbides in promoting η nucleation and growth. It is concluded that η formation may be suppressed by controlling the size and density of MC carbides during materials processing.Graphical abstractImage, graphical abstract
       
  • Interfacial nanophases stabilize nanotwins in high-entropy alloys
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): Wenjun Lu, Christian H. Liebscher, Fengkai Yan, Xufei Fang, Linlin Li, Jianjun Li, Wenqi Guo, Gerhard Dehm, Dierk Raabe, Zhiming LiNanostructuring metals through nanograins and nanotwins is an efficient strategy for strength increase as the mean free path of dislocations is reduced. Yet, nanostructures are thermally often not stable, so that the material properties deteriorate upon processing or during service. Here, we introduce a new strategy to stabilize nanotwins by an interfacial nanophase design and realize it in an interstitial high-entropy alloy (iHEA). We show that nanotwins in a carbon-containing FeMnCoCrNi iHEA can remain stable up to 900 °C. This is enabled by co-segregation of Cr and C to nanoscale 9R structures adjacent to incoherent nanotwin boundaries, transforming the 9R structures into elongated nano-carbides in equilibrium with the nanotwin boundaries. This nanoscale 9R structures assisted nano-carbide formation leads to an unprecedented thermal stability of nanotwins, enabling excellent combination of yield strength (~1.1 GPa) and ductility (~21%) after exposure to high temperature. Stimulating the formation of nanosized 9R phases by deformation together with interstitial doping establishes a novel interfacial-nanophase design strategy. The resulting formation of nano-carbides at twin boundaries enables the development of strong, ductile and thermally stable bulk nanotwinned materials.Graphical abstractImage, graphical abstract
       
  • Role of twinning on the omega-phase transformation and stability in
           zirconium
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): M. Arul Kumar, N. Hilairet, R.J. McCabe, T. Yu, Y. Wang, I.J. Beyerlein, C.N. ToméGroup-IV transition metal zirconium is used in nuclear and chemical industries as a choice material for operating in extreme environments. At ambient-conditions, zirconium has a stable hexagonal-close-packed structure (α-phase), but under high-pressures it transforms into a simple-hexagonal structure (ω-phase). Experimental studies involving high-pressures have reported retention of ω-phase upon recovery to ambient-pressures, which is undesirable since the ω-phase is brittle compared to the α-phase. Understanding the α-to-ω transformation is relevant for enhancing the applicability of transition metals. In this work using in-situ synchrotron X-ray diffraction, we show that deformation twins in the α-phase lower the transformation pressure and increase the amount of retained ω-phase. Our analysis concludes that the characteristics of the stress fields associated with the twins promote the α-to-ω transformation while making the reverse transformation energetically unfavorable. This work reveals a plausible way to design Zr microstructure for high-pressure applications via controlling twinning and retained ω-phase.Graphical abstractImage, graphical abstract
       
  • ZnO bilayer thin film transistors using H2O and O3 as oxidants by atomic
           layer deposition
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): Xue Chen, Jiaxian Wan, Hao Wu, Chang LiuThis work demonstrates a novel design of thin film transistors composed of a bilayer structure of ZnO thin films (O3 as oxidant/H2O as oxidant) prepared by atomic layer deposition. Based on this structure, a large ION/IOFF ratio of 108 at VDS = 0.1 V, a high field-effect mobility of 31.1 ± 0.26 cm2 V−1 s−1, a low threshold voltage of 0.14 ± 0.07 V and a proper subthreshold swing of 0.21 ± 0.02 V/decade as well as an excellent positive bias stress stability have been obtained. The improved performance of the bilayer structured devices is attributed to the first channel layer of ZnO (H2O as oxidant) decreasing the interfacial trap density and providing high mobility, and the second channel layer of ZnO (O3 as oxidant) suppressing the off-state current as well as a newly formed sub-channel at the interface between the two channel layers.Graphical abstractZnO bilayer thin film transistors using H2O and O3 as oxidants by atomic layer deposition, Xue Chen, Jiaxian Wan, Hao Wu, and Chang Liu.Image, graphical abstract
       
  • Enhanced photocatalytic hydrogen production on GaN–ZnO oxynitride by
           introduction of strain-induced nitrogen vacancy complexes
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): Kaveh Edalati, Ryoko Uehiro, Shuhei Takechi, Qing Wang, Makoto Arita, Motonori Watanabe, Tatsumi Ishihara, Zenji HoritaOxynitrides are considered as new potential candidates for photocatalysis due to their lower bandgap compared with traditional oxide photocatalysts. However, the formation of native nitrogen monovacancies during the synthesis of oxynitrides reduces their photocatalytic activity due to the vacancy-induced electron/hole recombination. This study shows that a transition from the native nitrogen monovacancies to the nitrogen-based vacancy complexes in a GaN–ZnO oxynitride not only diminishes the recombination but also enhances the photocatalytic hydrogen production. Here, the vacancy complexes are introduced by mechanical straining via the high-pressure torsion (HPT) method and it is shown that the vacancy complexes reduce the bandgap and increase the over-potential for hydrogen production on the conduction band. The current results introduce a simple but effective approach to turn the nitrogen vacancies to favorable defects for photocatalysis.Graphical abstractImage, graphical abstract
       
  • Prediction of a wide variety of linear complexions in face centered cubic
           alloys
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): Vladyslav Turlo, Timothy J. RupertLinear complexions are defect states that have been recently discovered along dislocations in body centered cubic Fe-based alloys. In this work, we use atomistic simulations to extend this concept and explore segregation-driven structural transitions at dislocations in face centered cubic alloys. We identify a variety of stable, nanoscale-size structural and chemical states, which are confined near dislocations and can be classified as linear complexions. Depending on the alloy system and thermodynamic conditions, such new states can preserve, partially modify, or relax the original dislocation cores they were born at. By considering different temperatures and compositions, we construct linear complexion diagrams that are similar to bulk phase diagrams, defining the important conditions for complexion formation while also specifying an expected complexion size and type. Several notable new complexion types were predicted here: (1) nanoparticle arrays comprised of L12 phases in Ni-Fe, Ni-Al, and Al-Zr, (2) replacement of stacking faults with layered complexions comprised of (111) planes from the Cu5Zr intermetallic phase in Cu-Zr, (3) platelet arrays comprised of two-dimensional Guinier-Preston zones in Al-Cu, and finally (4) coexistence of multiple linear complexions containing both Guinier-Preston zones and L12 phases in ternary Al-Cu-Zr. All of these new complexion states are expected to alter material properties and affect the stability of the dislocations themselves, offering a unique opportunity for future materials design.Graphical abstractImage, graphical abstract
       
  • Inheritance from glass to liquid: β relaxation depresses the
           nucleation of crystals
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): L.J. Song, M. Gao, W. Xu, J.T. Huo, J.Q. Wang, R.W. Li, W.H. Wang, J.H. PerepezkoBoth crystallization and relaxation are intrinsic characteristics that derive from the metastable nature of metallic glasses. However, the interactions between the two reactions are not understood fully. In this paper, enthalpy evolution and phase existence diagrams composed of liquid, glass and crystal are established to illustrate the phase transitions upon heating and cooling a metallic glass-forming material. We find that metallic glasses located in different enthalpy levels in the diagrams exhibit quite different resistance to crystallization. The crystal nucleation rate for the fast-cooled glass is smaller compared to the slow-cooled glass, which is verified to be correlated with the larger amount of β relaxation in the fast-cooled sample. These findings provide new evidence for the inheritance between glass and liquid, and suggest that modulating the glassy state is a new route to control the crystallization kinetics of supercooled liquids.Graphical abstractA faster-cooled metallic glass is composed of more β relaxation and is more difficult to crystallize.Image, graphical abstract
       
  • Cyclic phase transformation behavior of nanocrystalline NiTi at microscale
    • Abstract: Publication date: 15 February 2020Source: Acta Materialia, Volume 185Author(s): Peng Hua, Kangjie Chu, Fuzeng Ren, Qingping SunCuboidal micropillars of nanocrystalline superelastic NiTi shape memory alloys with an average grain size of 65 nm were fabricated by focused ion beam and then subjected to cyclic compression. It is found that the micropillars have maintained superelasticity for over 106 full-transformation cycles under a maximum compressive stress of 1.2 GPa. Functional degradation of the micropillars mainly occurs in the first 104 cycles where hysteresis loop area and forward transformation stress rapidly decrease from initial 11 MPa (MJ/m3) and 586 MPa to 6 MPa and 271 MPa. In the 104 ~ 106 cycles, stress-strain responses of the micropillars show asymptotic stabilization. Residual strain is accumulated to 3.3% and multiple ~50 nm wide extrusions are found at the surface of the micropillars after 106 cycles. SEM and TEM studies indicate that cyclic phase transformation results in formation and glide of transformation-induced dislocations that create surface steps and the extrusions. The dislocations inhibit reverse transformation and result in residual martensite and residual stresses. The dislocations and the residual martensite lead to the functional degradation. The role of the residual martensite in the functional degradation is further verified by 21% recovery of the residual strain and an increase of 278 MPa in the forward transformation stress after heating up the cyclically deformed micropillars to 100 °C. The recorded over 106 phase transformation cycles under a maximum stress of 1.2 GPa of the NiTi shape memory alloys at microscale open up new avenues for applications of the material in microscale devices and engineering.Graphical abstractImage, graphical abstract
       
  • Designing rare-earth free permanent magnets in Heusler alloys via
           interstitial doping
    • Abstract: Publication date: Available online 3 January 2020Source: Acta MaterialiaAuthor(s): Qiang Gao, Ingo Opahle, Oliver Gutfleisch, Hongbin ZhangBased on high-throughput density functional theory calculations, we investigated the effects of light interstitial H, B, C, and N atoms on the magnetic properties of cubic Heusler alloys, with the aim to design new rare-earth free permanent magnets. It is observed that the interstitial atoms induce significant tetragonal distortions, leading to 32 candidates with large ( >  0.4 MJ/m3) uniaxial magneto-crystalline anisotropy energies (MAEs) and 10 cases with large in-plane MAEs. Detailed analysis following the the perturbation theory and chemical bonding reveals the strong MAE originates from the local crystalline distortions and thus the changes of the chemical bonding around the interstitials. This provides a valuable way to tailor the MAEs to obtain competitive permanent magnets, filling the gap between high performance Sm-Co/Nd-Fe-B and widely used ferrite/AlNiCo materials.Graphical abstractGraphical abstract for this article
       
  • Unveiling the pinning behavior of charged domain walls in BiFeO3 thin
           films via vacancy defects
    • Abstract: Publication date: Available online 2 January 2020Source: Acta MaterialiaAuthor(s): W.R. Geng, X.H. Tian, Y.X. Jiang, Y.L. Zhu, Y.L. Tang, Y.J. Wang, M.J. Zou, Y.P. Feng, B. Wu, W.T. Hu, X.L. MaManipulation of electronic states in functional ferroelectrics is promising for next generation electronics devices. The charged domain walls in ferroelectric materials especially facilitate the electronic state modulation and are promising for developing interface-based devices. However, the major challenges impeding the application are their intentional manipulation and the elusive pinning behavior. Here, results that charged domain walls in BiFeO3 films can be pinned and regulated by oxygen vacancy planar distributions controlled by oxygen pressure during film growth are reported. Using aberration-corrected scanning transmission electron microscopy complemented by theoretical simulations, rich pinning behavior of tail-to-tail charged domain walls by oxygen vacancy plates is revealed. At high annealing oxygen pressure, 71° charged domain walls are stabilized by narrow vacancy plates. Decreasing the oxygen pressure, the transformation from 71° to 109° charged domain walls happens by expanding the vacancy plates, as collaborated by phase field simulations. Besides, the 71°-109° charged domain wall pairs are stabilized due to further interaction between two neighboring vacancy plates. These results provide the active modulation of the electronic states and illuminate the rich pinning behavior of domain walls by vacancy defects in ferroelectrics, which in turn could provide implications for designing potential electronics devices.Graphical abstractImage, graphical abstract
       
  • Martensitic Twin Boundary Migration as a Source of Irreversible Slip in
           Shape Memory Alloys
    • Abstract: Publication date: Available online 31 December 2019Source: Acta MaterialiaAuthor(s): Ahmed Sameer Khan Mohammed, Huseyin SehitogluThe mechanistic origin of fatigue in Shape Memory Alloys (SMAs) is addressed using atomistic simulations. A causal explanation is proposed for the known agreement between the fatigue-activated slip system and the martensitic twinning system. As a model system, the Type II twin boundary (TB) in NiTi B19′ martensite phase is analyzed. TEM-based models have established the presence of disconnections on the TB. Topological models establish the TB migration to depend on the motion of twinning partials on these disconnections. A disconnection is setup within a Molecular Statics (MS) framework. A twinning partial is positioned on it by enforcing continuum displacement fields external to a prescribed core of atoms which is subsequently relaxed under governance of the interatomic potential. The displacement fields are calculated from the anisotropic Eshelby-Stroh formalism and enforced in a non-Cauchy-Born adherent manner to obtain the right core structure. TB migration is simulated as a motion of this disconnection under applied load. In the presence of a barrier to this motion, a dislocation reaction occurs where a stacking fault emits at the TB while returning a residual negated partial. The emissary fault partial is proposed as a precursor to the resulting slip observed in reverse-transformed austenite.Graphical Image, graphical abstract
       
  • A universal scaling relationship between the strength and Young’s
           modulus of dealloyed porous Fe0.80Cr0.20
    • Abstract: Publication date: Available online 30 December 2019Source: Acta MaterialiaAuthor(s): Yi-Hou Xiang, Ling-Zhi Liu, Jun-Chao Shao, Hai-Jun JinThe fact that ligament strength is strongly affected by size has become an obstacle to understanding the relationship between structural topology and mechanical response of dealloyed nanoporous metals. Herein we studied the mechanical properties of porous Fe0.80Cr0.20 prepared by liquid metal dealloying. The ligament diameters of these samples are stabilized at  ∼ 4 μm, so that the ligament strength is constant in all samples. The variation of strength (or flow stress) and Young’s modulus with relative density, on a log–log scale, is nonlinear. Both properties decrease more steeply with decreasing relative density at lower relative density. These results are similar to the observations in nanoporous gold prepared by (electro)chemical dealloying but deviate from Gibson–Ashby (G-A) scaling laws. However, the strength of the porous Fe0.80Cr0.20 plotted against the Young’s modulus on a log–log scale exhibits a linear relation in the full range, with a slope of  ∼ 3/4 that matches perfectly with the standard G-A prediction. This confirms the significant role played by “dangling ligaments” in the deformation of dealloyed porous materials: Coarsening-induced pinch-off of some ligaments is responsible for the anomalously low strength and stiffness of dealloyed porous materials; the load-bearing network remains self-similar, and its mechanical response follows the standard G-A scaling laws, despite the fact that the porous material itself may not do so. Our study confirms that, for dealloyed porous materials, the G-A scaling relations are valid if the apparent relative density or, alternatively, genus-density-related prefactors are introduced.Graphical abstractGraphical abstract for this article
       
  • The role of plasticity and hydrogen flux in the fracture of a tempered
           martensitic steel: a new design of mechanical test until fracture to
           separate the influence of mobile from deeply trapped hydrogen
    • Abstract: Publication date: Available online 27 December 2019Source: Acta MaterialiaAuthor(s): D. Guedes, L. Cupertino Malheiros, A. Oudriss, S. Cohendoz, J. Bouhattate, J. Creus, F. Thébault, M. Piette, X. FeaugasABSTRACTThe design of an electrochemical permeation device on a tensile machine has allowed to control the hydrogen flux and to isolate the effects of trapped and mobile hydrogen on the hydrogen embrittlement of a martensitic steel. Based on a local approach of fracture, tensile tests on several notched specimens were completed in order to investigate the impact of hydrostatic stress, equivalent plastic strain, hydrogen concentration and flux on the damage processes. Analysis of the fracture surfaces revealed that trapped hydrogen favors ductile fracture, enhancing nucleation and growth of voids by reducing the interface energy between precipitates/inclusions and matrix. Mobile hydrogen leads to quasi-cleavage along the substructure (lath and/or blocks) boundaries at mainly the {101} planes. For both mechanisms, the mutual interaction between hydrogen and dislocations (drag process increasing hydrogen diffusion and hydrogen favoring dislocations mobility) has a large contribution to the hydrogen embrittlement of the martensitic steel.Graphical abstractImage, graphical abstract
       
  • Hollow/porous-walled SnO2 via nanoscale Kirkendall diffusion
           with irregular particles
    • Abstract: Publication date: Available online 24 December 2019Source: Acta MaterialiaAuthor(s): Bo-In Park, Jin-Sung Park, Seunggun Yu, So-Hye Cho, Ji Young Byun, Jihun Oh, Seung Yong LeeHollow/porous structured SnO2 nanoparticles were synthesized by simple oxidation of dense metal chalcogenide precursors via nanoscale Kirkendall diffusion effect. First, tin chalcogenide (SnS, SnSe) nanoparticles were synthesized by mechanochemical method, which is considered a facile, scalable, and eco-friendly process. Hollow/porous-walled SnO2 nanoparticles were synthesized by simple oxidation of the prepared Sn chalcogenide precursors, for which the transformation mechanism was verified in detail. Nanoscale Kirkendall diffusion process was thoroughly investigated by morphological, crystallographic, and elemental analyses performed at various oxidation temperatures and times. To examine the morphological effect of hollow/porous-walled SnO2 nanoparticles on the electrochemical performance, the synthesized nanoparticles were applied as anode material in a lithium-ion battery. Anode material showed highly improved electrochemical properties compared to its dense counterpart, with 83% capacity retention from the second cycle at the 400th cycle and capacity of 302 mA h g−1 at a high current density of 30 A g−1.Graphical abstract (Table of contents)Sn-chalcogenides as precursors are synthesized by facile mechanochemical method. Sn-chalcogenides are oxidized under air atmosphere in a controlled manner. Finally, peculiar hollow/porous-walled SnO2 particles are formed via nanoscale Kirkendall diffusion effect. When the SnO2 particles are used as anode, the structural benefits result in improvement of electrochemical properties of lithium-ion battery.Image, graphical abstract
       
  • Mechanical Properties of Metallic Lithium: from Nano to Bulk Scales
    • Abstract: Publication date: Available online 23 December 2019Source: Acta MaterialiaAuthor(s): Cole D. Fincher, Daniela Ojeda, Yuwei Zhang, George M. Pharr, Matt PharrDespite renewed interest in lithium metal anodes, unstable electrodeposition of Li during operation has obstructed progress in practical battery applications. While deformation mechanics likely play a key role in Li's mechanical stability as an anode material, reports of Li's mechanical properties vary widely, perhaps due to variations in testing procedures. Through bulk tensile testing and nanoindentation, we provide a comprehensive assessment of the strain-rate and length-scale dependent mechanical properties of Li in its most commonly used form: high purity commercial foil. We find that bulk Li exhibits a yield strength between 0.57 and 1.26 MPa for strain rates from 5E-4 s−1 to 5E-1 s−1. For indentation tests with target  P˙/P = 0.05 s−1, the hardness decreases precipitously from nearly 43 MPa to 7.5 MPa as the indentation depth increases from 250 nm to 10 μm. The plastic properties measured from bulk and nanoindentation testing exhibit strong strain-rate dependencies, with stress exponents of n = 6.55 and 6.9, respectively. We implement finite element analysis to relate the indentation depth to length scales of relevance in battery applications. Overall, the results presented herein may provide important guidance in designing Li anode architectures and charging conditions to mitigate unstable growth of Li during electrochemical cycling.Graphical abstractImage, graphical abstract
       
  • An Ultra-High Strength Martensitic Steel Fabricated using Selective Laser
           Melting Additive Manufacturing: Densification, Microstructure, and
           Mechanical Properties
    • Abstract: Publication date: Available online 23 December 2019Source: Acta MaterialiaAuthor(s): Raiyan Seede, David Shoukr, Bing Zhang, Austin Whitt, Sean Gibbons, Philip Flater, Alaa Elwany, Raymundo Arroyave, Ibrahim KaramanMartensitic steels have gained renewed interest recently for their use in automotive, aerospace, and defense applications due to their ultrahigh yield strengths and reasonable ductility. A recently discovered low alloy martensitic steel, AF9628, has been shown to exhibit strengths greater than 1.5GPa with more than 10% tensile ductility, due to the formation of ε-carbide phase. In an effort to produce high strength parts with a high degree of control over geometry, the work herein presents the effects of selective laser melting (SLM) parameters on the microstructure and mechanical properties of this new steel. An optimization framework to determine the process parameters for building porosity-free parts is introduced. This framework utilizes the computationally inexpensive Eager-Tsai model, calibrated with single track experiments, to predict the melt pool geometry. A geometric criterion for determining maximum allowable hatch spacing is also developed in order to avoid lack of fusion induced porosity in the as-printed parts. Using this framework, fully dense samples were successfully fabricated over a wide range of process parameters, allowing the construction of an SLM processing map for AF9628. The as-printed samples displayed tensile strengths of up to 1.4GPa, the highest reported to date for any 3D printed alloy, with up to 11% elongation. The demonstrated flexibility in process parameter selection, while maintaining full density, opens up the possibility of local microstructural refinement and parameter optimization for improved mechanical properties in as-printed parts. The process optimization framework introduced here is expected to allow successful printing of new materials in an accelerated fashion.Graphical abstractImage, graphical abstract
       
  • Fracture, Fatigue, and Sliding-Wear Behavior of Nanocomposites of Alumina
           and Reduced Graphene-Oxide
    • Abstract: Publication date: Available online 23 December 2019Source: Acta MaterialiaAuthor(s): Qizhong Wang, Cristina Ramírez, Connor S. Watts, Oscar Borrero-López, Angel L. Ortiz, Brian W. Sheldon, Nitin P. PadtureThere is growing interest in using 2D graphene-related reinforcements to toughen brittle ceramics in nanocomposites. However, there is a lack of fundamental understanding of the toughening mechanisms and microstructural effects in such nanocomposites. To address this paucity, fully-dense nanocomposites of aluminum oxide (Al2O3) matrix and reduced graphene-oxide (rGO) reinforcements (∼5 vol%) of different average-thicknesses and orientations are fabricated and characterized. The interactions between stably propagating cracks and rGO in the Al2O3/rGO nanocomposites are observed in situ inside a scanning electron microscope (SEM). Toughening by pullout of thick rGO in the crack-tip wake in the cross-section orientation is found to be the most effective, which is consistent with the highest fracture toughness (KIC∼6.7 MPa.m0.5) measured in those Al2O3/rGO nanocomposites. Interestingly, upon unloading and reloading, the intact rGO crack-bridges appear to crinkle and uncrinkle without a remnant crease, respectively, which is a unique deformation property of multi-layer graphene-like materials. This points to a possible new cyclic-fatigue resistance mechanism in those nanocomposites. Sliding-wear properties of the Al2O3/rGO nanocomposites are also studied, where the hardness and microstructural heterogeneities are found to play dominant roles. The results from this study have implications for the creation of high-toughness, fatigue-resistant, and wear-resistant graphene-reinforced ceramic nanocomposites of the future.Graphical abstractImage, graphical abstract
       
  • Grain Boundary Properties of Elemental Metals
    • Abstract: Publication date: Available online 19 December 2019Source: Acta MaterialiaAuthor(s): Hui Zheng, Xiang-Guo Li, Richard Tran, Chi Chen, Matthew Horton, Donny Winston, Kristin Aslaug Persson, Shyue Ping OngThe structure and energy of grain boundaries (GBs) are essential for predicting the properties of polycrystalline materials. In this work, we use high-throughput density functional theory calculations workflow to construct the Grain Boundary Database (GBDB), the largest database of DFT-computed grain boundary properties to date. The database currently encompasses 327 GBs of 58 elemental metals, including 10 common twist or symmetric tilt GBs for body-centered cubic (bcc) and face-centered cubic (fcc) systems and the Σ7 [0001] twist GB for hexagonal close-packed (hcp) systems. In particular, we demonstrate a novel scaled-structural template approach for HT GB calculations, which reduces the computational cost of converging GB structures by a factor of ∼3−6. The grain boundary energies and work of separation are rigorously validated against previous experimental and computational data. Using this large GB dataset, we develop an improved predictive model for the GB energy of different elements based on the cohesive energy and shear modulus. The open GBDB represents a significant step forward in the availability of first principles GB properties, which we believe would help guide the future design of polycrystalline materials.Graphical abstractGraphical abstract for this article
       
  • Grain boundary structure and mobility in high-entropy alloys: A
           comparative molecular dynamics study on a Σ11 symmetrical tilt grain
           boundary in face-centered cubic CuNiCoFe
    • Abstract: Publication date: Available online 19 December 2019Source: Acta MaterialiaAuthor(s): Daniel Utt, Alexander Stukowski, Karsten AlbeWe employ atomistic computer simulations to study the structure and migration behavior of a Σ11 symmetrical tilt grain boundary in a 4-component model FCC high-entropy alloy (HEA) (CuNiCoFe). The results are compared to grain boundaries in elemental metals and a so-called ‘average-atom’ sample. We find that the repeating structural units characterizing the static grain boundary structure show the same repeating structural units for all samples, while the high temperature equilibrium grain boundary structure is most strongly influenced by presence of stacking faults. Under an applied synthetic driving force, this GB migrates by a mechanism assisted by partial dislocations in all materials. For this reason the grain boundary mobilities and stacking fault energies are directly related. Moreover, the HEA sample and the average-atom sample show almost identical mobilities suggesting that local chemical fluctuations play a minor role. Solute segregation to the GB in the HEA suppresses GB migration up to very high temperatures and might be the main cause for reduced grain growth in FCC HEAs.Graphical abstractGraphical abstract for this article
       
  • Neutron irradiation-induced microstructure damage in ultra-high
           temperature ceramic TiC.
    • Abstract: Publication date: Available online 13 December 2019Source: Acta MaterialiaAuthor(s): S. Agarwal, T. Koyanagi, A. Bhattacharya, L. Wang, Y. Katoh, X. Hu, M. Pagan, S.J. ZinkleTitanium carbide (TiC) is an ultra-high temperature ceramic with potential as a structural material candidate for advanced reactor concepts. However, the irradiation tolerance of TiC is not well understood. Here, we reveal the key irradiation damage microstructure degradation processes in TiC using mixed spectrum neutron irradiations at dose of ∼2 displacements per atom (dpa) at temperatures of ∼220, 620, and 1115°C, combined with state-of-art microstructure characterization using transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). The helium injection (∼65 atomic parts per million) produced by neutron transmutation also occurred in the sample. TiC was observed to form irradiation-induced interstitial-type dislocation loops and He-stabilized cavities. At 220 and 620°C, the analysis of the electron diffraction patterns, rel-rod imaging and HRTEM revealed that the dislocation loops were faulted Frank loops with Burgers vector bFrank = 1/3 lying on {111} planes. A detailed Burgers vector identification performed by the g.b technique revealed that the dislocation loops forming at 1115°C were unfaulted, edge-type, with Burgers vectors b=a and a/2 with corresponding {100} and {110} habit planes. Using continuum mechanics, we estimated the critical radius at which a faulted dislocation loop transitions to a perfect loop to be 9 nm. Further, no amorphization occurred in TiC under-investigated irradiation conditions while macroscopic swelling under point defect swelling regime was observed. He-stabilized cavities were detected at 1115°C, but not at lower temperature irradiation. These observations indicate the onset of long-range vacancy migration in TiC occurs between 620 and 1115°C.Graphical abstractImage, graphical abstract
       
  • Dislocation-induced Breakthrough of Strength and Ductility Trade-off in a
           Non-equiatomic High-Entropy Alloy
    • Abstract: Publication date: Available online 30 November 2019Source: Acta MaterialiaAuthor(s): Wenqi Guo, Jing Su, Wenjun Lu, Christian H. Liebscher, Christoph Kirchlechner, Yuji Ikeda, Fritz Körmann, Xuan Liu, Yunfei Xue, Gerhard DehmIn conventional metallic materials, strength and ductility are mutually exclusive, referred to as strength-ductility trade-off. Here, we demonstrate an approach to improve the strength and ductility simultaneously by introducing micro-banding and the accumulation of a high density of dislocations in single-phase high-entropy alloys (HEAs). We prepare two compositions (Cr10Mn50Fe20Co10Ni10 and Cr10Mn10Fe60Co10Ni10) with distinctive different stacking fault energies (SFEs) as experimental materials. The strength and ductility of the Cr10Mn50Fe20Co10Ni10 HEA are improved concurrently by grain refinement from 347.5±216.1 µm to 18.3±9.3 µm. The ultimate tensile strength increases from 543±4 MPa to 621±8 MPa and the elongation to failure enhances from 43±2% to 55±1%. To reveal the underlying deformation mechanisms responsible for such a strength-ductility synergy, the microstructural evolution upon loading is investigated by electron microscopy techniques. The dominant deformation mechanism observed for the Cr10Mn50Fe20Co10Ni10 HEA is the activation of micro-bands, which act both as dislocation sources and dislocation barriers, eventually, leading to the formation of dislocation cell structures. By decreasing grain size, much finer dislocation cell structures develop, which are responsible for the improvement in work hardening rate at higher strains (>7 %) and thus for the increase in both strength and ductility. In order to drive guidelines for designing advanced HEAs by tailoring their SFE and grain size, we compute the SFEs of Cr10MnxFe70–xCo10Ni10 (10 ≤ x ≤ 60) based on first principles calculations. Based on these results the overall changes on deformation mechanism can be explained by the influence of Mn on the SFE.Graphical Image, graphical abstract
       
  • Crossing twin of Ni-Mn-Ga 7M martensite induced by thermo-mechanical
           treatment
    • Abstract: Publication date: Available online 29 November 2019Source: Acta MaterialiaAuthor(s): Zongbin Li, Dong Li, Jiaxing Chen, Bo Yang, Naifu Zou, Yudong Zhang, Claude Esling, Xiang Zhao, Liang ZuoThermo-mechanical treatment is an effective way to tune the microstructure and enhance the output strain of NiMn-based magnetic shape memory alloys through field driven variant reorientation or structural transformation. Comprehensive knowledge on the microstructural feature and related transformation crystallography is of great importance for the microstructure control and property optimization. In this work, we demonstrate the crossing twin of sever-layered modulated (7M) martensite assembled in herringbone shape induced by thermo-mechanical treatment in a directionally solidified Ni50Mn30Ga20 alloy. Based on the electron backscatter diffraction (EBSD) measurements, it is found that the crossing twin is composed of four 7M martensite variants with two mutually crossing systems of twin, i.e., type I twin and compound twin. The type I twin in the crossing twin, being with {1 –2 10}7M as the twinning plane, is quite different from that of the self-accommodated martensite (i.e., {1 –2 –10}7M as the twinning plane). A novel transformation orientation relationship between austenite and 7M martensite in crossing twin is resolved to be {1 0 1}A//{1 –2 10}7M and A//7M, in contrast to that of self-accommodated case, i.e., {1 0 1}A//{1 –2 –10}7M and A//7M. By using the experimentally determined orientation relationship between two phases to construct the deformation gradient matrix, the underlying mechanism for the selection of twinning system is further discussed.Graphical abstractImage, graphical abstract
       
  • Interactions between Zirconia-Yttria-Tantala Thermal Barrier Oxides and
           Silicate Melts
    • Abstract: Publication date: Available online 29 November 2019Source: Acta MaterialiaAuthor(s): Najeb M. Abdul-Jabbar, Abel N. Fernandez, R. Wesley Jackson, Daesung Park, Carlos G. LeviThe effects of TaO2.5 additions to Y4Zr3O12 (YZO) on its interactions with calcium-magnesium alumino-silicate (CMAS) melts are investigated. YZO is a candidate material for the protection of novel ZrO2-YO1.5-TaO2.5 based thermal barrier coatings (TBCs) against CMAS attack, but thermochemical considerations dictate that YZO contain TaO2.5 for interfacial compatibility. Oxides based on Y4Zr3-xTaxO12+0.5x (x=0, 0.7, 1.4) were mixed with various ternary and quinary silicate compositions ranging in Ca:Si ratio from 0.35 to 1.13 and equilibrated at 1300°C. The results show that all combinations form the desired apatite phase except for the mixture of the higher Ta5+ content with the lowest Ca:Si melt. It is found that the addition of Ta5+ increases the capture of Y3+ by the reprecipitated fluorite, decreasing the amount available to form apatite and eventually suppressing it at low Ca:Si. The effect is ascribed to the attractive interactions between Y3+ and Ta5+ in solid solution, similar to that found in tetragonal zirconia, but with concurrent incorporation of anion vacancies associated with excess Y3+ that stabilize the cubic form. As the Ca2+ content of the melt increases, so does the amount of Ca2+ incorporated in fluorite. The latter also interacts attractively with Ta5+ and leads to a reduction of captured Y3+ in fluorite as the Ca content in the reacting deposit increases. This serves to increase the amount of Y3+ available in the melt to form apatite in TBC-CMAS interactions.Graphical abstractImage, graphical abstract
       
  • The significance of strain weakening and self-annealing in a superplastic
           Bi-Sn eutectic alloy processed by high-pressure torsion
    • Abstract: Publication date: Available online 29 November 2019Source: Acta MaterialiaAuthor(s): Chuan Ting Wang, Yong He, Terence G. LangdonA cast Bi-Sn eutectic alloy was processed by high-pressure torsion (HPT) at room temperature and stored at room temperature for durations of up to 91 days in order to investigate the effect of self-annealing. The HPT processing produces grain refinement but hardness measurements show there is strain softening with lower microhardness values than in the initial as-cast material and with hardness values that decrease with increasing amounts of imposed torsional strain. This softening is attributed to a loss of precipitates within the Sn and Bi phases during the processing operation. In self-annealing at room temperature, the microhardness increases significantly due to reprecipitation and there is also a minor increase in grain size with increasing time of storage. It is demonstrated by tensile testing that the HPT-processed Bi-Sn alloy exhibits superplastic behavior with elongations of up to>1000% after storage for 35 days and with an associated strain rate sensitivity close to ∼0.5. Grain boundary sliding plays an important role in superplastic flow and it is shown that maximum sliding occurs on the Bi-Bi interfaces where this is consistent with estimates of the coefficients for grain boundary diffusion in the Bi-Sn alloy.Graphical abstractImage, graphical abstract
       
  • A Decohesion Pathway for Hydrogen Embrittlement in Nickel: Mechanism and
           Quantitative Prediction
    • Abstract: Publication date: Available online 29 November 2019Source: Acta MaterialiaAuthor(s): A. Tehranchi, X. Zhou, W.A. CurtinHydrogen embrittlement (HE) is a ubiquitous and catastrophic mode of fracture in metals. Here, embrittlement is considered as an intrinsic ductile-brittle transition at the crack tip, where H at the crack tip can reduce the stress intensity KIc for cleavage below the value KIe required for ductile dislocation emission and blunting. Specifically, cleavage fracture along (111) planes in Ni occurs due to the formation of just 3 planar layers of H interstitial occupation at a sharp crack tip. During the cleavage process, the sub-surface H in the upper and lower layers can rapidly diffuse to the fracture surface, lowering the net fracture free energy to KIc 
       
  • Interplay between internal stresses and matrix stiffness influences
           hydrothermal ageing behaviour of zirconia-toughened-alumina
    • Abstract: Publication date: Available online 29 November 2019Source: Acta MaterialiaAuthor(s): C. Wei, G. Montagnac, B. Reynard, N. Le Roux, L. GremillardZirconia based bio-ceramics such as ZTA (Zirconia-toughened alumina) composite are often used in orthopaedic and dental implants. The hydrothermal ageing behaviour of zirconia in humid environments is an important factor affecting the lifetime of these biomaterials. It is a tetragonal-to-monoclinic phase transformation that has been shown to be both temperature- and stress-dependant. Here, ZTA composites with zirconia inclusion particles of different sizes were prepared. The influence of zirconia inclusions on internal stresses and thus on ageing behaviour was studied by Raman micro-spectroscopy and X-ray diffraction. The results show that the ageing kinetics result from a balance between stiffer matrix and increased tensile stress in the zirconia inclusion as the local alumina fraction increase. As a result, the maximum monoclinic fraction formed after1200 h ageing at 134°C decreases with increasing alumina content, showing the predominance of the stabilization of tetragonal particles by a stiff matrix over their de-stabilization by tensile stress.Graphical abstractImage, graphical abstract
       
  • Growth-twins in CrN/AlN multilayers induced by hetero-phase interfaces
    • Abstract: Publication date: Available online 29 November 2019Source: Acta MaterialiaAuthor(s): Zhuo Chen, Qinqin Shao, Matthias Bartosik, Paul H. Mayrhofer, Hong Chen, Zaoli ZhangWe carry out a detailed transmission electron microscopy study of growth-twins in a high stacking-fault energy transition-metal nitride (TMN) multilayer comprising of 4.2 nm-thin CrN and 1 to ∼8 nm-thin AlN layers. A high density of rock-salt TMN twins with Σ3{112} incoherent twin boundaries (ITB) were found in the {111} {0002} textured film area near the substrate. The extensive high-resolution transmission electron microscopy (HRTEM) observations reveal that rock-salt TMN twins with ITBs are frequently formed in wurtzite {0002} interface with a single-atomic-layer terrace (1 × d{0002}w-AlN). However, twins with ITBs were hardly observed in the wurtzite {0002} interface with a double atomic-layer terrace (2 × d{0002}w-AlN). The formation of twins with ITBs can be interpreted by the rs-CrN/w-AlN interface structure (with a mirror-symmetry)-induced energetically stable nucleation. Furthermore, we see that the growth-twins with Σ3 {111} coherent twin boundaries (CTB) appear in the non-textured film area further away from the substrate. Based on the HRTEM observations and atomic model analyses, supplemented with theoretical calculations, several nucleation modes of twins with Σ3 {112} ITB and Σ3 {111} CTB are proposed. These findings offer a new perspective on the formation mechanism of growth-twins in transition-metal nitride materials.Graphical abstractImage, graphical abstract
       
  • Investigating active slip planes in tantalum under compressive load:
           crystal plasticity and slip trace analyses of single crystals
    • Abstract: Publication date: Available online 28 November 2019Source: Acta MaterialiaAuthor(s): Hojun Lim, Jay D. Carroll, Joseph R. Michael, Corbett C. Battaile, Shuh Rong Chen, J. Matthew D. LaneActive slip systems in body centered cubic (BCC) metals remain ambiguous and controversial. In this work, active slip planes in tantalum are investigated using single crystal experiments and simulations. Four tantalum single crystals with [001], [011], [1¯11] and [1¯49] orientations along the loading direction are analyzed after quasi-static compression tests and dynamic Taylor impact tests. Mechanical behavior, deformed shape, texture evolution and slip traces of single crystals are analyzed and compared with crystal plasticity finite element simulations to investigate active slip systems in tantalum. Both experimental observations and modeling results support dominant dislocation slip along {112} planes in tantalum under compressive load.Graphical abstractGraphical abstract for this article
       
  • Atomic scale investigation of the crystal structure and interfaces of the
           B′ precipitate in Al-Mg-Si alloys
    • Abstract: Publication date: Available online 28 November 2019Source: Acta MaterialiaAuthor(s): Haonan Chen, Jiangbo Lu, Yi Kong, Kai Li, Tong Yang, Arno Meingast, Mingjun Yang, Qiang Lu, Yong DuABSTRACTB′ is a common type of metastable precipitate in over-aged Al-Mg-Si alloys, which is long regarded as a variation of the Q precipitate in Al-Mg-Si-Cu alloys due to the similarity between their lattice parameters. Atomic-resolution high angle annular dark field scanning transmission electron microscopy and energy dispersive X-ray elemental mapping at low beam damage conditions, as well as first-principles calculations were used to explore the atomistic structure of B′. It has a hexagonal unit cell with a space group P and lattice parameters a = 10.3(1) Å, c = 4.05 Å. The M sites in B′, which are analogous to Cu sites in the Q structure, were inferred with 50% Si atoms and 50% vacancies. The chemical nature of other sites agrees well with the model predicted by Ravi et al.. The determined model Al3Mg9Si8 has the lowest formation enthalpy and the smallest lattice misfit with the Al matrix along the [0001] growth direction. Step-like boundaries with alternately arranged Mg-Si-Mg-Si atoms were observed at the coherent interfaces (100)B′ // (10)Al. A layer of defect structures sandwiched in the B′ precipitate was found geometrically necessary to allow both (10)Al interfaces to arrange coherently in 3 dimensions and thus to relieve the strain of the surrounding matrix. The transformation from U1 → U2 → B′ was evidenced and the reverse transformation B′ → U2 nucleating at the incoherent B′/Al interfaces (150)Al is also likely. These results will provide new insight into the aging precipitation and compositional design of Al-Mg-Si(-Cu) alloys.Graphical abstractImage, graphical abstract
       
  • Subgrain dynamics during recovery of partly recrystallized aluminium
    • Abstract: Publication date: Available online 30 October 2019Source: Acta MaterialiaAuthor(s): S.R. Ahl, H. Simons, C. Detlefs, D. Juul Jensen, H.F. PoulsenThe recovery dynamics of 498 individual subgrains within the non-recrystallized matrix of a 50% recrystallized aluminium sample is studied using high resolution X-ray diffraction at a synchrotron source. The subgrains’ change in size, orientation, axial strain and internal disorder are monitored as functions of temperature. The subgrains are surprisingly active, and the individual subgrains exhibit markedly different growth kinetics. Contradictory to common knowledge we find that the individual subgrains’ sizes change significantly even though the average size remain constant. Furthermore we observe no correlation between subgrain size and growth rate. An analysis of the volume changes of the individual subgrains indicates a broad spectrum of critical temperatures above which the subgrains grow or shrink. Possible mechanisms are discussed and related to relaxation of local stresses.Graphical Image, graphical abstract
       
  • Evolution of β-phase precipitates in an aluminum-magnesium alloy at
           the nanoscale
    • Abstract: Publication date: Available online 25 October 2019Source: Acta MaterialiaAuthor(s): Daniel L. Foley, Asher Leff, Andrew C. Lang, Mitra L. TaheriAluminum alloys in the 5xxx series are susceptible to sensitization due to the formation of β (Al3Mg2) at grain boundaries at moderate to low temperatures. Little is known about the mechanism of β phase formation, which is thought to be preceded by the metastable phases β’’ and β’. Using high-resolution transmission electron microscopy (HRTEM), energy dispersive x-ray spectroscopy (EDS), and precession electron diffraction (PED), we determine a parameter space for β phase precipitates at various sensitization temperatures and investigate their growth habits and local matrix strain states along grain boundaries. Our findings reveal that metastable β-related phases are present at low aging temperatures, while the equilibrium β phase is present at temperatures well above the previously described solvus of similar alloys. Furthermore, the phases were found to prefer particular grain boundary planes and contribute to the local grain boundary strain state differently. Overall, these findings present a unified view of β phase evolution and its contribution to lattice strain environments in aluminum magnesium alloys, which serves as a foundation for use in a range of temperatures and environments.Graphical Image, graphical abstract
       
  • Solid solution formation in Mg2(Si,Sn) and shape of the
           miscibility gap
    • Abstract: Publication date: Available online 25 November 2019Source: Acta MaterialiaAuthor(s): M. Yasseri, A. Sankhla, H. Kamila, R. Orenstein, D.Y.N. Truong, N. Farahi, J. de Boor, E. MuellerInvestigation of the thermochemical stability of Mg2(Si,Sn) thermoelectric materials is crucial for further development of thermoelectric modules. There is a miscibility gap reported for the quasibinary Mg2Si–Mg2Sn series, though the exact compositions of its limits are disputed. Gaining a better understanding of intersolubility limits in Mg2(Si,Sn) is important for further optimization of material performance by exploiting the gap-induced phase segregation. For a better understanding on the boundaries of the miscibility gap below 700°C, two approaches were taken to provide evidence of thermodynamic stable phases and, hereby, monitor the borders of the miscibility gap. The approaches cover the homogenization of Mg2SixSn1-x at 700°C and diffusion couple experiments at 600°C, 525°C, and 450°C. For 600°C we find two ranges where Mg2Si and Mg2Sn are not miscible, namely x = 0.35 ± 0.05 and x = 0.75 ± 0.05 for miscibility gap I and 0.85 ± 0.05 < x < 0.95 ± 0.05 for the second gap. The deduced complex temperature dependence of the mixing / demixing behavior helps to understand the previously observed, apparently contradicting experimental results. We also show that there is no demixing for the compositions with the best thermoelectric properties at temperatures above 700°C.Graphical Image, graphical abstract
       
  • Effect of Vacancy Creation and Annihilation on Grain Boundary Motion
    • Abstract: Publication date: Available online 20 November 2019Source: Acta MaterialiaAuthor(s): G.B. McFadden, W.J. Boettinger, Y. MishinInteraction of vacancies with grain boundaries (GBs) is involved in many processes occurring in materials, including radiation damage healing, diffusional creep, and solid-state sintering. We analyze a model describing a set of processes occurring at a GB in the presence of a non-equilibrium, non-homogeneous vacancy concentration. Such processes include vacancy diffusion toward, away from, and across the GB, vacancy generation and absorption at the GB, and GB migration. Numerical calculations within this model reveal that the coupling among the different processes gives rise to interesting phenomena, such as vacancy-driven GB motion and accelerated vacancy generation/absorption due to GB motion. The key combinations of the model parameters that control the kinetic regimes of the vacancy-GB interactions are identified via a linear stability analysis. Possible applications and extensions of the model are discussed.Graphical abstractGraphical abstract for this article
       
  • Ultra-high strength and plasticity mediated by partial dislocations and
           defect networks: Part I: Texture effect
    • Abstract: Publication date: Available online 20 November 2019Source: Acta MaterialiaAuthor(s): Ruizhe Su, Dajla Neffati, Qiang Li, Sichuang Xue, Jaehun Cho, Jin Li, Jie Ding, Yifan Zhang, Cuncai Fan, Haiyan Wang, Yashashree Kulkarni, Xinghang ZhangDeformation mechanisms governing the strength of nanostructured metallic multilayers have been studied extensively for various applications. In general, size effect is the most effective way to tailor the mechanical strength of multilayers. Here we report that three Cu/Co multilayer systems with identical layer thickness but different types of layer interfaces exhibit drastically different mechanical behavior. In situ micropillar compression tests inside a scanning electron microscope show that coherent FCC (100) and (110) Cu/Co multilayer systems have low yield strength of about 600 MPa, and prominent shear instability. In contrast, the incoherent Cu/ HCP Co multilayers show much greater yield strength, exceeding 2.4 GPa, and significant plasticity manifested by a cap on the deformed pillar. Molecular dynamics simulations reveal an unexpected interplay between pre-existing twin boundaries in Cu, stacking faults in HCP Co, and incoherent layer interfaces, which leads to partial dislocation dominated high strength, and outstanding plasticity. This study provides fresh insights for the design of strong, deformable nanocomposites by using a defect network consisting of twin boundaries, stacking faults and layer interfaces.Graphic abstractImage, graphical abstract
       
  • On the Assessment of Creep Damage Evolution in Nickel-Based Superalloys
           Through Correlative HR-EBSD and cECCI Studies
    • Abstract: Publication date: Available online 19 November 2019Source: Acta MaterialiaAuthor(s): Sabin Sulzer, Zhuangming Li, Stefan Zaefferer, Seyed Masood Hafez Haghighat, Angus Wilkinson, Dierk Raabe, Roger ReedThe evolution of dislocation density with creep strain in single-crystal superalloys is studied quantitatively using high-resolution electron backscatter diffraction (HR-EBSD) and electron channelling contrast imaging under controlled diffraction conditions (cECCI). Data regarding dislocation density/structure is measured for deformation at 900 °C and 450 MPa up to ≈ 1% plastic strain. Effects of chemical composition are elucidated via three purpose-designed superalloys of differing rhenium and ruthenium contents. The evidence indicates that dislocation avalanching is already prevalent at plastic strains of ≈ 0.1%; thereafter, an exponential decay in the dislocation multiplication rate is indicative of self-hardening due to dislocation constriction within the matrix channels, as confirmed by the imaging. The results are rationalised using discrete dislocation dynamics modelling: a universal dislocation evolution law emerges, which will be useful for alloy design efforts.Graphical abstractImage 1
       
 
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