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Publisher: Elsevier   (Total: 3181 journals)

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Showing 1 - 200 of 3181 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: 105, SJR: 1.462, CiteScore: 3)
Accounting Forum     Hybrid Journal   (Followers: 28, SJR: 0.932, CiteScore: 2)
Accounting, Organizations and Society     Hybrid Journal   (Followers: 42, 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: 443, SJR: 0.758, CiteScore: 2)
Acta Automatica Sinica     Full-text available via subscription   (Followers: 2)
Acta Biomaterialia     Hybrid Journal   (Followers: 29, SJR: 1.967, CiteScore: 7)
Acta Colombiana de Cuidado Intensivo     Full-text available via subscription   (Followers: 3)
Acta de Investigación Psicológica     Open Access   (Followers: 3)
Acta Ecologica Sinica     Open Access   (Followers: 11, SJR: 0.18, CiteScore: 1)
Acta Histochemica     Hybrid Journal   (Followers: 5, SJR: 0.661, CiteScore: 2)
Acta Materialia     Hybrid Journal   (Followers: 319, 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 Poética     Open Access   (Followers: 4, SJR: 0.101, CiteScore: 0)
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  
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: 11, SJR: 2.611, CiteScore: 8)
Additives for Polymers     Full-text available via subscription   (Followers: 23)
Advanced Drug Delivery Reviews     Hybrid Journal   (Followers: 187, SJR: 4.09, CiteScore: 13)
Advanced Engineering Informatics     Hybrid Journal   (Followers: 12, 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: 2, SJR: 0.686, CiteScore: 2)
Advances in Cancer Research     Full-text available via subscription   (Followers: 34, 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: 10, 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: 20, 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: 15)
Advances in Developmental Biology     Full-text available via subscription   (Followers: 13)
Advances in Digestive Medicine     Open Access   (Followers: 12)
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: 44, SJR: 2.524, CiteScore: 4)
Advances in Engineering Software     Hybrid Journal   (Followers: 29, SJR: 1.159, CiteScore: 4)
Advances in Experimental Biology     Full-text available via subscription   (Followers: 8)
Advances in Experimental Social Psychology     Full-text available via subscription   (Followers: 52, SJR: 5.39, CiteScore: 8)
Advances in Exploration Geophysics     Full-text available via subscription   (Followers: 1)
Advances in Fluorine Science     Full-text available via subscription   (Followers: 9)
Advances in Food and Nutrition Research     Full-text available via subscription   (Followers: 67, 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: 11, SJR: 12.74, CiteScore: 13)
Advances in Geophysics     Full-text available via subscription   (Followers: 7, 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: 3, 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: 15, SJR: 3.027, CiteScore: 2)
Advances in Medical Sciences     Hybrid Journal   (Followers: 8, 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: 25)
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: 5, 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: 9, 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: 10)
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: 68)
Advances in Quantum Chemistry     Full-text available via subscription   (Followers: 6, SJR: 0.371, CiteScore: 1)
Advances in Radiation Oncology     Open Access   (Followers: 2, 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: 6)
Advances in Space Research     Full-text available via subscription   (Followers: 423, SJR: 0.569, CiteScore: 2)
Advances in Structural Biology     Full-text available via subscription   (Followers: 5)
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: 38, 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: 54, SJR: 1.551, CiteScore: 3)
Aeolian Research     Hybrid Journal   (Followers: 6, SJR: 1.117, CiteScore: 3)
Aerospace Science and Technology     Hybrid Journal   (Followers: 383, 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: 482, 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: 31, SJR: 1.156, CiteScore: 4)
Agricultural Water Management     Hybrid Journal   (Followers: 44, 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: 53, 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: 66, SJR: 1.93, CiteScore: 3)
American J. of Emergency Medicine     Hybrid Journal   (Followers: 47, 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: 14, SJR: 1.524, CiteScore: 3)
American J. of Human Genetics     Hybrid Journal   (Followers: 37, 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: 36, 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: 265, SJR: 2.7, CiteScore: 4)
American J. of Ophthalmology     Hybrid Journal   (Followers: 66, 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: 28, 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: 5, 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: 210, 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: 226, SJR: 1.58, CiteScore: 3)
Animal Feed Science and Technology     Hybrid Journal   (Followers: 7, SJR: 0.937, CiteScore: 2)
Animal Reproduction Science     Hybrid Journal   (Followers: 7, SJR: 0.704, CiteScore: 2)

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Similar Journals
Journal Cover
Acta Materialia
Journal Prestige (SJR): 3.263
Citation Impact (citeScore): 6
Number of Followers: 319  
 
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 1359-6454
Published by Elsevier Homepage  [3181 journals]
  • Deformation in nanocrystalline ceramics: A microstructural study of
           MgAl2O4
    • Abstract: Publication date: 15 January 2020Source: Acta Materialia, Volume 183Author(s): Barak Ratzker, Avital Wagner, Maxim Sokol, Louisa Meshi, Sergey Kalabukhov, Nachum Frage Contrary to the characteristic strengthening of polycrystalline ceramics with a decrease in grain size, extremely fine nanocrystalline ceramics exhibit softening, increased plasticity and an inverse Hall-Petch relation. Despite experimental evidence, questions remain regarding the underlying deformation mechanisms governing this abnormal mechanical behavior. In the present study, an in-depth microstructural examination was performed on nanostructured transparent magnesium aluminate spinel (MgAl2O4) subjected to microhardness tests. Microstructural observations revealed regions strained to various degrees below the point of indentation, containing varying amounts of dislocations and nano-cavities. Furthermore, the residual strain in different areas was estimated by local electron diffraction. These observations and analysis provided evidence for grain boundary (GB) mediated mechanisms (e.g., GB sliding and rotation). Moreover, shear bands formed and were found to be associated with micro-cracking. By combining the microstructural analysis with suitable models, it was concluded that these mechanisms govern plastic deformation. By elucidating how strain is accommodated within nanocrystalline ceramics, a deeper understanding of their unique mechanical behavior is gained.Graphical abstractImage, graphical abstract
       
  • Unique high-temperature deformation dominated by grain boundary sliding in
           heterogeneous necklace structure formed by dynamic recrystallization in
           HfNbTaTiZr BCC refractory high entropy alloy
    • Abstract: Publication date: 15 January 2020Source: Acta Materialia, Volume 183Author(s): Rajeshwar R. Eleti, Atul H. Chokshi, Akinobu Shibata, Nobuhiro Tsuji Microstructural evolution of dynamically recrystallized (DRX) grains and grain boundary sliding (GBS) in the heterogeneous necklace structure of HfNbTaTiZr refractory high entropy alloy (RHEA) was studied systematically during high temperature deformation. Uniaxial compression testing was carried out to different strains at 1000 °C and a strain rate 10−3 s−1. Significant bulging of grain boundaries led initially to the formation of DRX grains. The fraction of DRX grains increased with strain, and typical necklace structures of fine (d ≤ 1.5 µm) DRX grains formed at strain ε ≥ 0.3. The DRX grains showed very limited grain growth, and heterogeneous microstructures composed of coarse unrecrystallized regions surrounded by the characteristic DRX necklace structure were formed at larger strains. Interrupted testing with marker grids revealed that DRX grains deformed by a GBS mechanism. The DRX necklace regions connected mesoscopically and also displayed diamond network morphologies, with unique “Y-shaped”, “T-shaped” and “X-shaped” junctions. The formation of different types of junctions were rationalized on the basis of GBS accommodated by local dislocation slip in unrecrystallized regions. The unrecrystallized regions showed preferred / micro-texture, consistent with conventional dislocation slip in the BCC crystals. On the other hand, the newly formed DRX grains initially had similar orientations to those of the parent grains, but they displayed a random texture with increasing strain, as expected from GBS. The randomized texture of DRX grains and the stability of DRX grains size represented GBS in the DRX necklace regions.Graphical abstractImage, graphical abstract
       
  • Anisotropic elastic-plastic behavior of architected pyramidal lattice
           materials
    • Abstract: Publication date: 15 January 2020Source: Acta Materialia, Volume 183Author(s): M. Eynbeygui, J. Arghavani, A.H. Akbarzadeh, R. Naghdabadi The initial and subsequent yield surfaces for architected pyramidal lattice materials are investigated analytically. Considering lattice struts as elastic-perfectly plastic thin beams subjected to both axial force and bending moment, a set of nonlinear elastic-plastic constitutive relations for a strut is proposed. Moreover, we phenomenologically present anisotropic pressure-dependent yield functions for pyramidal lattices. Comparison of planar yield surfaces of pyramidal lattices predicted by analytical approach to the ones obtained from phenomenological models shows a good agreement for the type of external loads and range of strains investigated in this study. Investigating the normality between the plastic strain vectors and yield surfaces, the obtained results demonstrate the associative nature of the flow rule. We have also emphasized on utilizing hollow-tapered struts as the constituent of pyramidal lattices. To this end, we analytically found that hollow-tapered struts can remarkably improve the effective stiffness and yield strength of bending-dominant pyramidal lattices.Graphical abstractImage, graphical abstract
       
  • G-phase strengthened iron alloys by design
    • Abstract: Publication date: Available online 16 November 2019Source: Acta MaterialiaAuthor(s): D.J.M. King, Mujin Yang, T.M. Whiting, Xingjun Liu, M.R. Wenman Density functional theory (DFT) calculations were used to model G-phase precipitates of formula X6M16Si7 where X is Cr, Hf, Mn, Mo, Nb, Ta, Ti, V, W and Zr and M is either Fe or Ni. It was found that the occupancy of the d-orbital is correlated to the formation enthalpies of each structure. Past thermal expansion coefficient data was used to predict the lattice misfit between each G-phase and body centred cubic (BCC) Fe. All except Hf and Zr containing G-phases were predicted to have zero misfit between 581−843 K. Of the Ni containing G-phases, Mn6Ni16Si7 was predicted to have the most similar elastic properties to BCC Fe. DFT calculations of the substitution energies of Al, Cr Cu, Fe, Ge, Hf, Mo, Nb, P, Ta, Ti, V, Zr, and vacancies onto the Mn6Ni16Si7 G-phase from BCC Fe were performed. It was predicted that Cu, P and vacancies favour G-phase substitution. Suppression of the G-phase is predicted when Si content is reduced by half, at which point the BCC phase is favoured. It is hypothesised that including Zr to form a (Mn,Zr)6Ni16Si7 precipitate will allow for higher ageing temperature and expediate nucleation in an Fe alloy. Thermocalc was used to predict that a mixture of FebalCr9Ni4Si2(Mn0.6Zr0.4)1.2 (at.%) will produce a G-phase strengthened Fe alloy with potential for a good balance of strength, ductility and oxidation/corrosion resistance at room temperature. This alloy composition was experimentally determined to precipitate the G-phase in ≤24 h with cube-on-cube orientation to the BCC Fe matrix.Graphical abstractImage, graphical abstract
       
  • Nanoscale conditions for ductile void nucleation in copper: vacancy
           condensation and the growth-limited microstructural state
    • Abstract: Publication date: Available online 15 November 2019Source: Acta MaterialiaAuthor(s): Philip J. Noell, Julian E.C. Sabisch, Douglas L. Medlin, Brad L. Boyce Ductile rupture or tearing usually involves structural degradation from the nucleation and growth of voids and their coalescence into cracks. Although some materials contain preexisting pores, the first step in failure is often the formation of voids. Because this step can govern both the failure strain and the fracture mechanism, it is critical to understand the mechanisms of void nucleation and the enabling microstructural configurations which give rise to nucleation. To understand the role of dislocations during void nucleation, the present study presents ex-situ cross-sectional observations of interrupted deformation experiments revealing incipient, subsurface voids in a copper material containing copper oxide inclusions. The local microstructural state was evaluated using electron backscatter diffraction (EBSD), electron channeling contrast (ECC), transmission electron microscopy (TEM), and transmission kikuchi diffraction (TKD). Surprisingly, before substantial growth and coalescence had occurred, the deformation process had resulted in the nucleation of a high density of nanoscale (≈50 μm) voids in the deeply deformed neck region where strains were on the order of 1.5. Such a proliferation of nucleation sites immediately suggests that the rupture process is limited by void growth, not nucleation. With regard to void growth, analysis of more than 20 microscale voids suggests that dislocation boundaries facilitate the growth process. The present observations call into question prior assumptions on the role of dislocation pile-ups and provide new context for the formulation of revised ductile rupture models. While the focus of this study is on damage accumulation in a highly ductile metal containing small, well-dispersed particles, these results are also applicable to understanding void nucleation in engineering alloys.Graphical Image, graphical abstract
       
  • The Effect of Chemical Disorder on Defect Formation and Migration in
           Disordered MAX Phases
    • Abstract: Publication date: Available online 15 November 2019Source: Acta MaterialiaAuthor(s): Prashant Singh, Daniel Sauceda, Raymundo Arroyave MAX phases have attracted increased attention due to their unique combination of ceramic and metallic properties. Point-defects are known to play a vital role in the structural, electronic and transport properties of alloys in general and this system in particular. As some MAX phases have been shown to be stable in non-stoichiometric compositions, it is likely that such alloying effects will affect the behavior of lattice point defects. This problem, however, remains relatively unexplored. In this work, we investigate the alloying effect on the structural-stability, energy-stability, electronic-structure, and diffusion barrier of point defects in MAX phase alloys within a first-principles density functional theory framework. The vacancy (VM, VA, VX) and antisite (M-A; M-X) defects are considered with M and A site disorder in (Zr-M)2(AA’)C, where M=Cr,Nb,Ti and AA’=Al, Al-Sn, Pb-Bi. Our calculations suggest that the chemical disorder helps lower the VA formation energies compared to VM and VX. The VA diffusion barrier is also significantly reduced for M-site disorder compared to their ordered counterpart. This is very important finding because reduced barrier height will ease the Al diffusion at high-operating temperatures, which will help the formation of passivating oxide layer (i.e., Al2O3 in aluminum-based MAX phases) and will slow down or stop the material degradation. We believe that our study will provide a fundamental understanding and an approach to tailor the key properties that can lead to the discovery of new MAX phases.Graphical abstractImage, graphical abstract
       
  • Crystal structure and composition dependence of mechanical properties of
           single-crystalline NbCo2 Laves phase
    • Abstract: Publication date: Available online 15 November 2019Source: Acta MaterialiaAuthor(s): W. Luo, C. Kirchlechner, J. Zavašnik, W. Lu, G. Dehm, F. Stein Extended diffusion layers of the cubic C15 and hexagonal C14 and C36 NbCo2 Laves phases with concentration gradients covering their entire homogeneity ranges were produced by the diffusion couple technique. Single-phase and single-crystalline micropillars of the cubic and hexagonal NbCo2 Laves phases were prepared in the diffusion layers by focused ion beam (FIB) milling. The influence of chemical composition, structure type, orientation and pillar size on the deformation behavior and the critical resolved shear stress (CRSS) was studied by micropillar compression tests. The pillar orientation influences the activated slip systems, but the deformation behavior and the CRSS are independent of orientation. The deformation of the smallest NbCo2 micropillars (0.8 μm in top diameter) appears to be dislocation nucleation controlled and the CRSS approaches the theoretical shear stress for dislocation nucleation. The CRSS of the 0.8 μm-sized NbCo2 micropillars is nearly constant from 26 to 34 at.% Nb where the C15 structure is stable. It decreases as the composition approaches the Co-rich and Nb-rich boundaries of the homogeneity range where the C15 structure transforms to the C36 and the C14 structure, respectively. The decrease in the CRSS at these compositions is related to the reduction of shear modulus and stacking fault energy. As the pillar size increases, stochastic deformation behavior and large scatter in the CRSS values occur and obscure the composition effect on the CRSS.Graphical abstrractImage, graphical abstract
       
  • On the atomic solute diffusional mechanisms during compressive creep
           deformation of a Co-Al-W-Ta single crystal superalloy
    • Abstract: Publication date: Available online 15 November 2019Source: Acta MaterialiaAuthor(s): J. He, C.H. Zenk, X. Zhou, S. Neumeier, D. Raabe, B. Gault, S.K. Makineni We investigated the solute diffusional behavior active during compressive creep deformation at 150 MPa / 975°C of a Co-Al-W-Ta single crystal superalloy in the [001] orientation. We report the formation of shear-bands that involves re-orientation of γ/γʹ rafts to {111} from {001} planes, referring to as γ/γ′ raft-rotation. In the shear-band regions, we observed abundant micro-twins, stacking faults (SFs), disordered zones within the γʹ termed as ‘γ pockets’ and also few geometrically-close-packed (GCP) phases. We used a correlative approach blending electron microscopy and atom probe tomography to characterize the structure and composition of these features. The SFs were identified as intrinsic and exhibit a W enrichment up to 14.5 at.% and an Al deficiency down to 5.1 at.%, with respect to the surrounding γʹ phase. The micro-twin boundaries show a solute enrichment similar to the SFs with a distinct W compositional profile gradients perpendicular from the boundaries into the twin interior, indicating solute diffusion within the micro-twins. The γ-pockets have a composition close to that of γ but richer in W/Ta. Based on these observations, we propose (i) a solute diffusion mechanism taking place during micro-twinning, (ii) a mechanism for the γ/γʹ raft-rotation process and evaluate their influence on the overall creep deformation of the present Co-based superalloy.Graphical Image, graphical abstract
       
  • Transitions in mechanical behavior and in deformation mechanisms enhance
           the strength and ductility of Mg-3Gd
    • Abstract: Publication date: Available online 15 November 2019Source: Acta MaterialiaAuthor(s): Xuan Luo, Zongqiang Feng, Tianbo Yu, Junqian Luo, Tianlin Huang, Guilin Wu, Niels Hansen, Xiaoxu Huang Samples of Mg-3Gd (wt. %) were prepared by accumulative roll-bonding followed by annealing at different temperatures to produce samples with average grain sizes ranging from 3.3 μm to 114 μm. The samples were tensile-tested at room temperature to characterize their strength and ductility, both of which were found to be significantly affected by transitions in mechanical behavior and deformation mechanisms. These transitions occurred with decreasing grain size and are described by: (i) a transition in the mechanical behavior from continuous flow to discontinuous flow associated with a yield point phenomenon, and (ii) a transition in the deformation mechanisms from slip and twinning to and slip. The dislocation structures and deformation twins in the tensile samples have been characterized by transmission electron microscopy and electron backscatter diffraction, respectively. Dislocations of and type were identified based on two-beam diffraction contrast experiments. The results reveal that dislocations and tension twins dominate in the samples with grain sizes larger than 10 μm, while and dislocations dominate in the samples with grain sizes smaller than 5 μm. In parallel, a consistent trend for both the strength and ductility to increase with decreasing grain size is observed. The appearance of a yield point phenomenon at small grain sizes has a significant effect on both strength and ductility, illustrated by an increase in boundary (Hall-Petch) strengthening and an increase in the total elongation to 36.6%. These results demonstrated a positive effect of a superposition of the transitions on both the strength and ductility of Mg-3Gd.Graphical abstractImage, graphical abstract
       
  • Relationship between the thermal stability of coercivity and the aspect
           
    • Abstract: Publication date: Available online 15 November 2019Source: Acta MaterialiaAuthor(s): Xin Tang, J. Li, Y. Miyazaki, H. Sepehri-Amin, T. Ohkubo, T. Schrefl, K. HonoThe influence of the aspect ratio of grains on the thermal stability of coercivity of Nd-Fe-B hot-deformed magnets was systematically investigated by experimental and numerical approaches. With increasing amount of Nb doping from 0.2 at.% to 0.6 at.% in the hot-deformed magnets, the aspect ratio of grains, defined as the ratio of the width to the height of a Nd2Fe14B grain (Dc/Dab), decreased while the average grain size was preserved. No change of temperature coefficient of coercivity (β) was observed in the as hot-deformed samples with different grain aspect ratios. However, the reduction of aspect ratio (increase of the height and reduction of the width of platelet shaped Nd2Fe14B grains) improves β for the sample infiltrated with a Nd-Cu eutectic alloy; i.e., from β = −0.42%/°C to −0.40%/°C for the aspect ratio reduction from 4.7 to 3.1. Micromagnetic simulations indicated that the grain aspect ratio is not a dominant factor to the thermal stability of coercivity for exchange-coupled anisotropic polycrystals. While smaller aspect ratio reduces demagnetizing field caused by magnetically isolated grains, resulting in the improvement of the thermal stability of the coercivity for Nd-Cu infiltrated hot-deformed magnets.Graphical abstractImage, graphical abstract
       
  • Growth evolution and formation mechanism of η′-Cu6Sn5 whiskers on
           η-Cu6Sn5 intermetallics during room-temperature ageing
    • Abstract: Publication date: Available online 14 November 2019Source: Acta MaterialiaAuthor(s): Z.H. Zhang, C.W. Wei, J.J. Han, H.J. Cao, H.T. Chen, M.Y. Li The phase-transformation-induced damage of Cu6Sn5 is an emerging reliability issue in the manufacturing of 3D ICs. Although the retarded phase transformation from η-Cu6Sn5 to η′-Cu6Sn5 at room temperature can produce a large expansion in volume, how the transformation stress threatens the joint reliability during usage is poorly understood. In this paper, the evolution characteristics of quenched η-Cu6Sn5 bumps were observed during ageing at 25°C for 1–40 d. Due to the retarded phase transformation, η′-Cu6Sn5 whiskers spontaneously nucleated and grew on the surfaces of η-Cu6Sn5 bumps. The orientation relationship between the two phases favourable for whisker growth was confirmed, and two necessary conditions for whisker formation were discussed. In addition, the potential harmfulness of whisker growth was analysed. The study will help expose the phase-transformation-induced damage of Cu6Sn5 during room-temperature ageing and may reduce the failure risk of entire Cu6Sn5 intermetallic joints in future large-scale applications of 3D ICs.Graphical abstractImage, graphical abstract
       
  • NiAl-34Cr&rft.title=Acta+Materialia&rft.issn=1359-6454&rft.date=&rft.volume=">Phase-field study of eutectic colony formation in        class="sans-serif">NiAl-34Cr
    • Abstract: Publication date: 1 January 2020Source: Acta Materialia, Volume 182Author(s): Michael Kellner, Johannes Hötzer, Ephraim Schoof, Britta Nestler The formation of two-phase eutectic colonies is often observed in microstructures of directionally solidified ternary alloys. Their formation is driven by microscopic instabilities in a macroscopic planar solidification front, due to impurities of the minor component, diffusing from the two solidifying phases into the liquid. The growth conditions for eutectic colonies, their interactions and their responses to the microstructure during growth are the focus of the current work. Therefore, phase-field simulations based on a grand potential formalism are performed for the high-performance material NiAl-34Cr. To enable the evolution of eutectic colonies in two-dimensional simulations, a concentration-driven nucleation mechanism is introduced into a multiphase-field framework and is subsequently validated. With this mechanism, two-dimensional large-scale phase-field simulations are conducted to study the influence of the applied temperature gradient on the evolving colonies. The patterns are quantitatively analyzed by measuring their number, size and height. Furthermore, the adjustment processes between the eutectic colonies during the directional solidification are investigated. The results demonstrate the ability of the presented phase-field approach with integrated nucleation mechanism for the formation of eutectic colonies in two-dimensional simulations.Graphical abstractGraphical abstract for this article
       
  • Mechanistic origin of high strength in refractory BCC high entropy alloys
           up to 1900K
    • Abstract: Publication date: 1 January 2020Source: Acta Materialia, Volume 182Author(s): Francesco Maresca, William A. Curtin The body centered cubic (BCC) high entropy alloys (HEAs) MoNbTaW and MoNbTaVW show exceptional strength retention up to 1900K. The mechanistic origin of the retained strength is unknown yet is crucial for finding the best alloys across the immense space of BCC HEA compositions. Experiments on Nb-Mo, Fe-Si and Ti-Zr-Nb alloys report decreased mobility of edge dislocations, motivating a theory of strengthening of edge dislocations in BCC alloys. The strength of BCC HEAs can be controlled by edge dislocations, especially at high temperatures, because the random field of solutes in these high-concentration alloys creates large energy barriers for thermally-activated edge glide. A parameter-free theory for this mechanism of edge motion in BCC alloys qualitatively and quantitatively captures the strength versus temperature for both MoNbTaW and MoNbTaVW alloys and other BCC HEAs. A reduced analytic version of the theory, embodying the edge-strengthening mechanism, then enables screening over  > 600,000 compositions in the Mo-Nb-Ta-V-W family to identify promising new compositions with high retained strength and/or reduced mass density. Overall, the theory reveals an unexpected mechanism - edge dislocation motion in the random alloy - as responsible for high temperature strength in BCC alloys and paves the way for theory-guided design of new high-performance HEAs.Graphical abstractGraphical abstract for this article
       
  • Grain-boundary segregation of boron in high-strength steel studied by
           nano-SIMS and atom probe tomography
    • Abstract: Publication date: 1 January 2020Source: Acta Materialia, Volume 182Author(s): G. Da Rosa, P. Maugis, A. Portavoce, J. Drillet, N. Valle, E. Lentzen, K. Hoummada High resolution imaging by secondary ion mass spectrometry and atom probe tomography have been employed to investigate boron segregation at austenite grain boundaries (γGBs) after soaking in a high-strength low-carbon steel. The combined use of these two analytical techniques is shown to be powerful for quantifying solute and segregated boron levels. Quenching was performed after soaking aiming to clarify the temperature effect on boron distribution under thermal equilibrium. Boron depletion in the γGBs vicinity was observed in the as-quenched states from high temperatures, suggesting that the cooling rate was not fast enough to limit boron diffusion during cooling. We found that boron segregation at γGBs increases with temperature. This is due to the increase of solute boron concentration in the grains, resulting from boride precipitate dissolution. It appears that the segregation magnitude still follows the equilibrium laws as a function of temperature. From our investigations, it was possible to determine the boron equilibrium segregation enthalpy. These results have important practical consequences for controlling the levels of segregated boron in steels.Graphical abstractImage, graphical abstract
       
  • Theory of screw dislocation strengthening in random BCC alloys from dilute
           to “High-Entropy” alloys
    • Abstract: Publication date: 1 January 2020Source: Acta Materialia, Volume 182Author(s): Francesco Maresca, William A. Curtin Random body-centered-cubic (BCC) “High Entropy” alloys are a new class of alloys, some having high strength and good ductility at room temperature and some having exceptional high-temperature strength. There are no theories of strengthening of screw dislocations in BCC metals that span naturally from the dilute limit to the multi-component, non-dilute concentrations typical of the high-entropy domain. Here, such a theory is developed and validated. Unlike low-temperature elemental BCC metals and very dilute BCC alloys, strength is not controlled by the kink-pair nucleation mechanism. Rather, screw dislocations naturally adopt a kinked structure as the minimum total energy configuration in the field of random alloying atoms. The characteristic length and energy scales for the low-energy kinked screw dislocation are derived for random alloys, leading to a characteristic spacing of both kinks and cross-kinks that depends on the kink formation energy and a characteristic collective solute/screw dislocation interaction energy parameter. Glide motion of this initially-kinked screw dislocation occurs via Peierls-type motion, lateral kink glide, and failure of cross-kinks. All these features are observed in molecular dynamics simulations. The resulting strength versus temperature, strain rate, and composition is analytic. The theory is validated by comparison to experiments on non-dilute Fe-Si, Nb-Mo, Nb-W, and Ti-Nb-Zr-based high entropy alloys versus composition and temperature. The theory provides a framework for tailored design of new high-performance BCC alloys.Graphical abstractGraphical abstract for this article
       
  • Uncovering the eutectics design by machine learning in the
           Al–Co–Cr–Fe–Ni high entropy system
    • Abstract: Publication date: 1 January 2020Source: Acta Materialia, Volume 182Author(s): Qingfeng Wu, Zhijun Wang, Xiaobing Hu, Tao Zheng, Zhongsheng Yang, Feng He, Junjie Li, Jincheng Wang Eutectics in high entropy alloys (HEAs) have shown excellent properties and promising applications. With empirical rules, various of eutectic high entropy alloys (EHEAs) have been proposed. The current design strategies shed light on the formation of eutectics in HEAs, but they are incapable of confirming multiple variables quantitatively in the selection of a specific system. In the present study, the eutectic formation in the multi-principal element systems is uncovered via data mining with machine learning (ML), where the critical elements and strongly associated elements were discovered. Taking the Al–Co–Cr–Fe–Ni system as an example, Al is confirmed to be the critical element for the eutectic formation and Cr is the strongly associated element with Al, Ni, Co, Fe and minor additions with comparably large solid solubility can be considered overall. With these understandings, a three-step approach can be summarized for designing EHEAs in a given system. Within the designed EHEAs, properties can be tested for optimization of application orientated design. The findings can not only accelerate the exploitation of EHEAs with better performance but also provide new ideas for designing compositionally complex alloys.Graphical abstractImage, graphical abstract
       
  • Evaluating the accuracy of common γ-Al2O3 structure models by selected
           area electron diffraction from high-quality crystalline γ-Al2O3
    • Abstract: Publication date: 1 January 2020Source: Acta Materialia, Volume 182Author(s): Henry O. Ayoola, Stephen D. House, Cecile S. Bonifacio, Kim Kisslinger, Wissam A. Saidi, Judith C. Yang Single crystal and textured polycrystalline γ-Al2O3 thin films were synthesized by oxidation of NiAl(110) in air at 850 °C for 1 and 2 h, respectively, and used to evaluate the accuracy of two spinel-based and two nonspinel models by comparison of selected-area electron diffraction (SAED). The lattice interplanar distances derived from the polycrystalline SAED pattern most closely matched the cubic spinel γ-Al2O3 model. The single-crystal SAED spot pattern showed symmetry consistent with both the cubic spinel and tetragonal nonspinel models, however, the Al cation distribution better matched the cubic spinel model based on the relative intensities of diffraction spots. Our work indicates that the traditional cubic spinel model is a more accurate model of γ-Al2O3 than the other models considered. The spinel-based monoclinic model is also more accurate than the monoclinic nonspinel model. The understanding of the relative accuracy of the different models is key for simulating γ-Al2O3 containing systems and is of general interest for the metal oxide and ceramic communities.Graphical abstractImage, graphical abstract
       
  • In situ TEM study of κ→β and κ→γ phase transformations in Ga2O3
    • Abstract: Publication date: Available online 14 November 2019Source: Acta MaterialiaAuthor(s): I. Cora, Z.S. Fogarassy, R. Fornari, M. Bosi, A. Rečnik, B. Pécz The temperature-driven phase transformation of metastable κ-Ga2O3 layers deposited on sapphire was studied by high resolution TEM. Annealing experiments up to 1000°C were performed either in situ in vacuum within the TEM or ex situ in ambient air. This allowed for the detection of the atomistic mechanisms at the basis of κ to β phase transition. In the case of in situ TEM observations we could even record in real time the atomic rearrangement. We provide in this paper the relevant crystallographic relations between original κ and new β lattice.Surprisingly, the ex situ experiments demonstrated the additional formation of a γ-Ga2O3 intermediate phase at 820°C. The remarkably different behaviour between in situ and ex situ annealing experiments is explained in terms of ambient (ambient air or high vacuum) and heating rate.An extensive investigation of γ-Ga2O3, also a metastable phase, showed that it has a cubic defect spinel structure (Fd3-m) with disordered vacancies. Repeated observations of the metastable γ-Ga2O3 after two months show that the vacancies tend to order, and that the vacancies are fully ordered after one year.Graphical Image, graphical abstract
       
  • Dependence of hydrogen embrittlement mechanisms on microstructure-driven
           hydrogen distribution in medium Mn steels
    • Abstract: Publication date: Available online 14 November 2019Source: Acta MaterialiaAuthor(s): Binhan Sun, Waldemar Krieger, Michael Rohwerder, Dirk Ponge, Dierk Raabe The risk of hydrogen embrittlement (HE) is currently one important factor impeding the use of medium Mn steels. However, knowledge about HE in these materials is sparse. Their multiphase microstructure with highly variable phase conditions (e.g. fraction, percolation and dislocation density) and the feature of deformation-driven phase transformation render systematic studies of HE mechanisms challenging. Here we investigate two austenite-ferrite medium Mn steel samples with very different phase characteristics. The first one has a ferritic matrix (∼74 vol.% ferrite) with embedded austenite and a high dislocation density (∼1014 m−2) in ferrite. The second one has a well recrystallized microstructure consisting of an austenitic matrix (∼59 vol.% austenite) and embedded ferrite. We observe that the two types of microstructures show very different response to HE, due to fundamental differences between the HE micromechanisms acting in them. The influence of H in the first type of microstructure is explained by the H-enhanced local plastic flow in ferrite and the resulting increased strain incompatibility between ferrite and the adjacent phase mixture of austenite and strain-induced α'-martensite. In the second type of microstructure, the dominant role of H lies in its decohesion effect on phase and grain boundaries, due to the initially trapped H at the interfaces and subsequent H migration driven by deformation-induced austenite-to-martensite transformation. The fundamental change in the prevalent HE mechanisms between these two microstructures is related to the spatial distribution of H within them. This observation provides significant insights for future microstructural design towards higher HE resistance of high-strength steels.Graphical abstractImage, graphical abstract
       
  • First-principles modeling of the hydrogen evolution reaction and its
           application in electrochemical corrosion of Mg
    • Abstract: Publication date: Available online 13 November 2019Source: Acta MaterialiaAuthor(s): Hui Ma, Liping Wu, Chen Liu, Mingfeng Liu, Changgang Wang, Dianzhong Li, Xing-Qiu Chen, Junhua Dong, Wei Ke By means of first-principles calculations, we have proposed an ab initio modeling to establish a formula between the hydrogen evolution rate and its overpotential of hydrogen evolution reaction (HER), relating with three different rate determining mechanisms, when Volmer reaction, Tafel reaction and Heyrowsky reaction are the rate determining steps of the entire reaction, respectively. Within this modeling, the free energy (ΔGH*) of the adsorbed hydrogen atom and the concentration of hydrogen ions in the solution have been correlated to the exchange current density. The hydrogen evolution modeling has been validated by available experimental results. Furthermore, by combining the previously proposed first-principles modeling of the anodic dissolution and this modeling of the HER in the electrochemical corrosion, the polarization curves of the 18 crystallographic surfaces of pure Mg have been theoretically derived. It has been found that in a neutral solution (pH=7) the corrosion current densities (icorr) of the 18 crystallographic surfaces range from 10−3.477 to 10−0.455 A/cm2 and their corresponding corrosion potentials (Ecorr) range from -1.36 to -0.892 VSHE, respectively. The base (0001) surface exhibits a lower corrosion rate of 10−3.345 A/cm2, whereas the crystal (213¯0) surface has a fast corrosion rate of 10−0.455A/cm2. The calculations even reveal that except for Ag, all the other alloying elements considered here accelerate the rates of the cathodic HER. In agreement with theoretical results, the experimentally measured polarization curves of the Mg-1Zn and Mg-2Sn alloys verify that both Zn and Sn additions accelerate the rate of the HER of Mg.Graphical abstractGraphical abstract for this article
       
  • Severe local lattice distortion in Zr/Hf-containing refractory
           multi-principal element alloys
    • Abstract: Publication date: Available online 13 November 2019Source: Acta MaterialiaAuthor(s): Yang Tong, Shijun Zhao, Hongbin Bei, Takeshi Egami, Yanwen Zhang, Fuxiang Zhang Whereas exceptional mechanical and radiation performances have been found in the emergent body-centered cubic (BCC) refractory multi-principal element alloys (RMPEAs), the importance of their complex atomic environment, reflecting diversity in atomic size and chemistry, has been largely unexplored at the atomic level. Here, we adopt a local structure approach based on the atomic pair distribution function measurements in combination with density functional theory (DFT) calculations to investigate a series of BCC RMPEAs. Our results demonstrate that all the analyzed RMPEAs exhibit local lattice distortions (LLD) to some extent, but a severe LLD, a breakdown of the 15% atomic size difference in Hume-Rothery rules, occurs only in the Zr- and/or Hf-containing RMPEAs. In addition, through the DFT calculations we show that charge transfer among the elements profoundly reduces the size mismatch effect in average to stabilize this energy-unfavorable severe LLD. The observed competitive coexistence between LLD and charge transfer demonstrates the importance of the electronic effects on the local environments in RMPEAs.Graphical abstractImage, graphical abstract
       
  • Crystallographic orientation dependence of hydride precipitation in
           commercial pure titanium
    • Abstract: Publication date: Available online 13 November 2019Source: Acta MaterialiaAuthor(s): Qian Wang, Shun Xu, Jean-Sébastien Lecomte, Christophe Schuman, Laurent Peltier, Xiao Shen, Wenwen Song Ti and its alloys have a variety of applications in aerospace industry and medical implants. The formation of hydride has been used in biomedical areas and can significantly influence the mechanical performance of materials. In this work, we investigate the orientation dependence of hydride precipitation in commercially pure titanium via interrupted in-situ electron backscatter diffraction (EBSD) measurements. The results reveal that hydrides during hydrogen charging at room temperature exhibit two types of orientation relationships with α-titanium, i.e., {0001}α//{11¯1}δ α//δ with interface plane {101¯3}α//{11¯0}δ (B-type), and {0001}α//{001}δ α//δ with interface plane {101¯0}α//{11¯0}δ (P-type). Significant orientation dependence of hydride precipitation is observed, especially when {101¯3}, {101¯0}, {0001} or {112¯0} planes of the parent grains are parallel to the diffusion surface. The displacement gradient tensor based accommodation shows that the orientation dependence is attributed to the strain relaxation of hydride transition. Three types of hydride platelets are characterized: parallel hydride platelets (Type I), crossed hydride platelets (Type II) and clustered hydride platelets (Type III). The multiple morphologies of hydride platelets resulting from the hydride variant selection and interaction are dependent on the crystal orientation of the matrix.Graphical Image, graphical abstract
       
  • Active Grain Growth Control with Distributed Heating
    • Abstract: Publication date: Available online 12 November 2019Source: Acta MaterialiaAuthor(s): Chengjian Zheng, Yixuan Tan, John T. Wen, Antoinette M. Maniatty Microstructure affects the physical properties and behavior of materials. While metallurgists have long studied microstructure characterization and evolution, thermo-mechanical material processing to achieve a desired microstructure remains largely experience-based. This paper presents a distributed thermal control methodology for the microstructure evolution. We consider the problem of achieving a uniform microstructure, starting from a non-uniform initial distribution. This is a common goal in material processing, as uniform microstructure implies consistent macroscopic properties. To illustrate the approach, we consider an example process with a multi-zone micro-heater array controlling the grain growth of a copper thin film. Cascaded temperature and grain-growth models characterize the process dynamics – finite element method (FEM) models the temperature field in response to the heater input, which in turn drives the microstructure evolution through a biased Monte-Carlo (MC) model. The high order combined FEM/MC model is used as the validation “truth” model. For the control design and analysis, a simplified model is developed to only capture the essential trend in the full model. Using the simplified model and dividing the copper thin film into multiple spatial zones with measurable grain statistics in each zone, we obtain a nonlinear multi-input/multi-output control design model. Using the simplified model, this paper presents the development and comparison of three control methods: 1. Direct output feedback from the measured mean local grain sizes to the heater current. 2; Model predictive control (MPC) using a finite horizon optimization to compute the required heat input at each control step; 3. Inner-outer loop control with temperature as the surrogate input for the outer loop and using the heater current to achieve the required temperature in the inner loop. All three methods achieve uniform microstructure in grain growth in the higher order FEM/MC simulation. Direct output feedback is the simplest to implement, but has the slowest convergence. MPC shows fast convergence but requires model-dependent on-line optimization. Inner-outer loop demonstrates good compromise between model-dependence and rate of convergence.Graphical abstractGraphical abstract for this article
       
  • Redistribution of carbon caused by butterfly defects in bearing steels
    • Abstract: Publication date: Available online 12 November 2019Source: Acta MaterialiaAuthor(s): M.E. Curd, T.L. Burnett, J. Fellowes, P. Yan, P.J. WithersABSTRACTButterfly defects initiate from inclusions in the subsurface of steel bearing components subject to rolling contact. The white etching matter (WEM) microstructure is a characteristic of butterflies and is related to the dissolution of carbides and thus generally believed to be enriched with carbon, in supersaturated solid solution, relative to the parent microstructure. Here, several butterflies are investigated using wavelength dispersive spectroscopy (WDS), soft x-ray emission spectroscopy (SXES) and electron microscopy (EM). Contrary to established thinking, in all cases investigated the butterfly-neighbouring WEM was found to be depleted in carbon, relative to parent material, by around 27% (measured in counts). Furthermore, the carbon level was shown to be lower than the matrix itself, suggesting that solute carbon is also expelled from the WEM during its formation due to the low level of solubility of carbon in ferrite. This was observed in both AISI 52100 and 18NiCrMo14-6 bearing steels and it is suggested to be due to the low solubility of carbon in ferrite. In spite of this, nano-indentation found that WEM in both alloys was ∼17% harder than the parent material. This may explain the strings of micro-voids observed near the WEM-parent interface, which appear to play a role in the growth of the butterfly cracks. It is suggested that the increased hardness of the WEM is mainly due to microstructural changes, rather changes in solute carbon concentration.Graphical abstractImage, graphical abstract
       
  • Stability and dynamics of skyrmions in ultrathin magnetic nanodisks under
           strain
    • Abstract: Publication date: Available online 12 November 2019Source: Acta MaterialiaAuthor(s): Jia-Mian Hu, Tiannan Yang, Long-Qing Chen Understanding the switching mechanism of magnetic skyrmions is critical for realizing their potential applications in future spintronic devices. Here we study the thermodynamic stability and dynamics of a Néel skyrmion in an ultrathin magnetic nanodisk under biaxial in-plane strains using a combination of phase-field simulations and analytical theory. We demonstrated the switching of a circular skyrmion to a variety of magnetic configurations, including an out-of-plane monodomain or an in-plane vortex under isotropic strains and to an elliptical skyrmion or a stripe domain under anisotropic strains. We successfully formulated a Lagrangian-mechanics-based model to analytically describe the switching dynamics of a skyrmion. Both our simulations and analytical model revealed that the strain-mediated breathing dynamics of skyrmions lead to a counter-intuitive phenomenon in which a lager strain may lead to slower skyrmion-to-monodomain switching.Graphical Image, graphical abstract
       
  • Assessment of surface and bulk-dominated methodologies to measure critical
           resolved shear stresses in hexagonal materials
    • Abstract: Publication date: Available online 12 November 2019Source: Acta MaterialiaAuthor(s): Aritra Chakraborty, Chen Zhang, Shanoob Balachandran, Thomas R. Bieler, Philip Eisenlohr Crystallographic slip in hexagonal metals involves a number of geometrically distinct slip families characterized by their slip direction and slip plane (basal, prismatic, and pyramidal). Owing to the low symmetry of hexagonal lattices, each of these slip families only have a few symmetrically equivalent slip systems (family members). Furthermore, different slip families become active at different resolved shear stress, i.e. , they have different critical resolved shear stress values (CRSS). The plastic anisotropy of hexagonal materials renders the numerical prediction of their plastic behavior challenging and depends critically on the knowledge of CRSS values. The present contribution assesses the reliability of three proposed methods (with additional variations) to quantify CRSS values of the different hexagonal slip families. Those methods (a to c) rely on: (a) the statistics of observed surface slip traces in a (slightly) deformed polycrystal; (b) an iterative adjustment of CRSS values until a simulated single crystal indentation matches the corresponding experiment in terms of load–displacement response and residual surface topography of the indent; (c) in-situ high-energy X-ray diffraction to measure the evolution of resolved stress (from lattice strains) in grains for which single-family slip can be deduced from specific lattice reorientation conditions. Virtual experiments are performed on synthetic microstructures such that the (predicted) CRSS values resulting from simulating the different methodologies can be rigorously compared against the (target) CRSS values that are installed in the phenomenological constitutive material description used in those simulations. The resulting CRSS values of methods (a) exhibit a strong dependence on, and deterioration with, decreasing level of slip trace observability, which is an uncertain quantity in experimental measurements. For the inverse indentation method (b), the predicted CRSS values are within 8% of their reference CRSS values for the two investigated cases. The high-energy X-ray diffraction method (c) most reliably determines CRSS values for basal and prism slip, but lacks a strict grain selection criterion to assess pyramidal slip.Graphical abstractGraphical abstract for this article
       
  • Nanoporous Metals from Thermal Decomposition of Transition Metal
           Dichalcogenides
    • Abstract: Publication date: Available online 12 November 2019Source: Acta MaterialiaAuthor(s): Swarnendu Chatterjee, Anton Anikin, Debjit Ghoshal, James L. Hart, Yawei Li, Saad Intikhab, D.A. Chareev, O.S. Volkova, A.S. Vasiliev, Mitra L. Taheri, Nikhil Koratkar, Goran Karapetrov, Joshua Snyder Nanoporous metals (np-M) have emerged as promising materials owing to their high surface area-to-volume ratio and electrical/thermal conductivity. There exists a group of processing methodologies by which np-M are formed through a top-down nanostructure evolution driven by the selective removal of a sacrificial component, all of which are a variation of dealloying. Nanoporosity evolution through current dealloying methodologies, however, is governed by strict requirements including sufficient separation in “reactivity” of the participating components and a homogeneous solid solution precursor alloy. This limits the viable alloy systems that may be used and the range of np-M's that may be formed. Here, we report thermal decomposition of crystalline transition metal dichalcogenides (TMDs) as a new processing methodology for np-M formation, adding to the spectrum of dealloying protocols. We demonstrate application of this process to the formation of a broader class of np-M including W, Re, Mo, and Ta with feature sizes below 100 nm. The presented facile thermal treatment of TMDs offers a new methodology for the evolution of nanoporosity in a broad range of metals.Graphical abstractImage, graphical abstract
       
  • On the mechanical heterogeneity in dual phase steel grades: Activation of
           slip systems and deformation of martensite in DP800
    • Abstract: Publication date: Available online 12 November 2019Source: Acta MaterialiaAuthor(s): Chunhua Tian, Dirk Ponge, Leon Christiansen, Christoph Kirchlechner We used micropillar compression experiments to study the plasticity of ferrite and martensite of two commercial dual phase steel grades (DP800). The activation of all three slip plane families, namely {110}, {112}, {123}, was observed in single crystalline ferrite pillars. They exhibit a comparable mean critical resolved shear stress (CRSS) of 147 ± 6, 143 ± 9, 146 ± 4 MPa for 3 µm pillars and are predominantly following Schmid´s law. A distinct size effect occurs when comparing the CRSS of 2 µm and 3 µm pillars. Martensite islands show uniform deformation and exhibit high compressive yield strength up to nearly 3 GPa. In most cases martensite pillars deform in an isotropic fashion without distinct slip traces. Despite the identical ultimate tensile stress of two steel grades their ferrite CRSS and martensite strength are largely different. It is found that the softer ferrite results in a lower macroscopic yield strength and a higher elongation to failure during macroscopic tensile testing. The results suggest that an increased local strain hardening capability suppresses global damage. The data provided here can serve as input parameter for crystal plasticity modelling.Graphical abstractImage, graphical abstract
       
  • Intrinsic Toughness of the Bulk-Metallic Glass Vitreloy 105 Measured Using
           Micro-Cantilever Beams
    • Abstract: Publication date: Available online 11 November 2019Source: Acta MaterialiaAuthor(s): Daniel Sorensen, Eric Hintsala, Joseph Stevick, Jesse Pischlar, Bernard Li, Daniel Kiener, Jason C. Myers, Hui Jin, Jia Liu, Douglas Stauffer, Antonio. J. Ramirez, Robert O. Ritchie Bulk-metallic glasses (BMGs) are a class of structural materials with many attractive processing featuers such as the ability to be processed into parts with fine features, dimensional precision, and repeatability; however, their fracture behavior is complex and size-dependent. Previous work has shown that BMGs can display strong size effects on toughness, where multiple mechanisms on different length-scales, e.g., crack bridging and bifurication, shear band spacing and length, can significantly affect the properies. This length-scale dependence on the fracture toughness has importance not only for advancing the understanding of fracture processes in these materials, but also for the potential future applications of BMGs, such as for microdevices. Here, using in situ scanning electron microscopy (SEM), we report on notched micro-cantilever bending experiments to address the lack of data regarding fracture properties of BMGs at the microscale. Sudden catastrophic propagation of shear bands resulted in failure for these specimens at stress intensities much lower than the bulk material, which may be due to a lack of extrinsic toughening mechanisms at these dimensions. This is explored further with post mortem SEM and transmission electron microscopy (TEM) analysis of the fractured beams while the fracture toughness results are verified using finite element modeling. The excellent agreement between model and micro cantilever beam bending experiments suggests that the intrinsic fracture toughness of Vitreloy 105, 9.03±0.59 MPa.m½, is being reported for the first time.Graphical Image, graphical abstract
       
  • Distinct driven steady states emerge from diverse initial textures in
           rolled nanocomposites
    • Abstract: Publication date: Available online 11 November 2019Source: Acta MaterialiaAuthor(s): Ian Chesser, Elizabeth A. Holm, Michael J. Demkowicz Severe plastic deformation is a widespread method of making high-performance metallic materials. Single-phase polycrystalline metals undergoing severe plastic deformation develop steady-state textures that are characteristic of the mode of deformation. By contrast, we show that two-phase, Cu-Nb nano-laminate composites reach a variety of different steady-state textures under a single mode of deformation. Using molecular statics simulations and a novel algorithm for crystal rotation analysis, we observe that the final, steady state texture and interface character in these materials depends on the initial texture of the composite. This finding suggests that the range of bulk Cu-Nb nano-composite textures that may be made by severe plastic deformation is larger than previously demonstrated, with multiple plastically-driven steady states accessible, depending on initial texture. We propose a modification of accumulative roll bonding with highly textured seed layers as a means of accessing different driven steady states in layered composites.Graphical abstractImage, graphical abstract
       
  • Effect of sputter pressure on microstructure and properties of
           β-Ta thin films
    • Abstract: Publication date: Available online 9 November 2019Source: Acta MaterialiaAuthor(s): Elizabeth A.I. Ellis, Markus Chmielus, Shangchen Han, Shefford P. Baker Tantalum thin films may be deposited in two phases. The stable bulk alpha phase is well known, but the metastable tetragonal beta phase is relatively poorly understood. We reported previously on a series of 100% β-Ta films deposited under varying sputter pressures in a low-oxygen environment, and discussed texture, stresses, and phase selection. Here, we discuss microstructure, morphology, and properties of these same β-Ta films. Grain size increases with sputter pressure, which can be explained by the energies of incident species at the growing film. Mechanical properties were measured by nanoindentation. Hardness decreases with grain size in accordance with the Hall-Petch relation while comparison of indentation modulus with biaxial modulus measurements indicates that the β phase is elastically anisotropic, and much stiffer in the [001] direction than in others. Finally, a canonical resistivity value for virtually oxygen-free, 100% β-Ta films of 169 ± 5 μΩcm is reported for the first time.Graphical abstractImage, graphical abstract
       
  • Atomic scale configuration of planar defects in the Nb-rich C14 laves
           phase NbFe2
    • Abstract: Publication date: Available online 8 November 2019Source: Acta MaterialiaAuthor(s): M. Šlapáková, A. Zendegani, C.H. Liebscher, T. Hickel, J. Neugebauer, T. Hammerschmidt, A. Ormeci, J. Grin, G. Dehm, K.S. Kumar, F. Stein Laves phases belong to the group of tetrahedrally close-packed intermetallic phases, and their crystal structure can be described by discrete layer arrangements. They often possess extended homogeneity ranges and the general notion is that deviations from stoichiometry are accommodated by anti-site atoms or vacancies. The present work shows that excess Nb atoms in a Nb-rich NbFe2 C14 Laves phase can also be incorporated in various types of planar defects. Aberration-corrected scanning transmission electron microscopy and density functional theory calculations are employed to characterize the atomic configuration of these defects and to establish stability criteria for them. The planar defects can be categorized as extended or confined ones. The extended defects lie parallel to the basal plane of the surrounding C14 Laves phase and are fully coherent. They contain the characteristic Zr4Al3-type (O) units found in the neighboring Nb6Fe7 µ phase. An analysis of the chemical bonding reveals that the local reduction of the charge transfer is a possible reason for the preference of this atomic arrangement. However, the overall layer stacking deviates from that of the perfect µ phase. The ab initio calculations establish why these exceptionally layered defects can be more stable configurations than coherent nano-precipitates of the perfect µ phase. The confined defects are observed with pyramidal and basal habit planes. The pyramidal defect is only ∼1 nm thick and resembles the perfect µ phase. In contrast, the confined basal defect can be regarded as only one single O unit and it appears as if the stacking sequence is disrupted. This configuration is confirmed by ab initio calculations to be metastable.Graphical Image, graphical abstract
       
  • Reconstructing the decomposed ferrite phase to achieve toughness
           regeneration in a duplex stainless steel
    • Abstract: Publication date: Available online 8 November 2019Source: Acta MaterialiaAuthor(s): Xuebing Liu, Wenjun Lu, Xinfang Zhang Duplex stainless steels suffer from thermal aging embrittlement that results from severe phase decomposition in ferrite phase after a long-term service at temperatures of 550–700 K, leading to the severe performance deterioration of duplex stainless steels. To ensure reliability and extend the service life of fabricated components made of duplex stainless steels, the development of techniques to efficiently and completely regenerate the deteriorated performance induced by spinodal decomposition and precipitation are extremely important. In this study, a novel pathway–an external electric field, is developed to eliminate the emerging Cr-rich (α′) phase and Fe-rich (α) phase resulting from spinodal decomposition as well as to dissolve the precipitates of G-phase in ferrite by introducing extra electrical free energy. The investigation is evidenced by microstructural and mechanical analyses using atom probe tomography, transmission electron microscopy, and nanoindentation. This high-efficiency (performance recovery above 90 %), low-energy consumption, online repair at the service temperature (700 K) is considerably superior to the traditional heat treatment process, which requires off-site repair at high temperatures (> 823 K). This new concept of manipulating precipitates using electric current to reconstruct the decomposed microstructure and achieve performance regeneration is expected to further stimulate the interest of researchers to extend the service life of materials by this means.Graphical abstractImage, graphical abstract
       
  • Microstructural evolution and strain-hardening in TWIP Ti alloys
    • Abstract: Publication date: Available online 8 November 2019Source: Acta MaterialiaAuthor(s): Guo-Hua Zhao, Xin Xu, David Dye, Pedro E.J. Rivera-Díaz-del-Castillo A multiscale dislocation-based model was built to describe, for the first time, the microstructural evolution and strain-hardening of {332}⟨113⟩ TWIP (twinning-induced plasticity) Ti alloys. This model not only incorporates the reduced dislocation mean free path by emerging twin obstacles, but also quantifies the internal stress fields present at β-matrix/twin interfaces. The model was validated with the novel Ti-11Mo-5Sn-5Nb alloy (wt.%), as well as an extensive series of alloys undergoing {332}⟨113⟩ twinning at various deformation conditions. The quantitative model revealed that solid solution hardening is main contributor to the yield stress, where multicomponent alloys or alloys containing eutectoid β-stabilisers exhibited higher yield strength. The evolution of twinning volume fraction, intertwin spacing, dislocation density and flow stress were successfully described. Particular attention was devoted to investigate the effect of strain rate on the twinning kinetics and dislocation annihilation. The modelling results clarified the role of each strengthening mechanism and established the influence of phase stability on twinning enhanced strain-hardening. The origin of strain-hardening is owing to the formation of twin obstacles in early stages, whereas the internal stress fields provide a long-lasting strengthening effect throughout the plastic deformation. A tool for alloy design by controlling TWIP is presented.Graphical abstractGraphical abstract for this article
       
  • Modeling the interface structure of Type II twin boundary in B19′ NiTi
           from an atomistic and topological standpoint
    • Abstract: Publication date: Available online 8 November 2019Source: Acta MaterialiaAuthor(s): Ahmed Sameer Khan Mohammed, Huseyin Sehitoglu This study addresses fundamental quandaries in the understanding of Type II twin interface in B19′ NiTi. A combined atomistic-topological approach is proposed to resolve a longstanding debate on the interface structure, affirming the hypothesis of a semi-coherent ledged geometry comprising of disconnected terraces. Atomic registry across the terrace is shown to require interface coherence strains. The twinning plane is shown to be a non-crystallographic virtual boundary separating the strained twin variants. Consequently, the issue of lattice offset arises and is addressed by an atomistic evaluation of interface energetics upon parametric variation of an offset parameter. Required atomic movements for migration of the terrace are established from a crystallographic analysis of the strained interface structure, and validated by a Molecular Statics (MS) simulation of the twin migration segment in the Generalized Planar Fault Energy (GPFE) curve. The GPFE calculation estimates a twinning partial magnitude consistent with an earlier ab initio prediction. This twinning partial serves as a “perfect” interface dislocation which, along with the coherence strain, feed into a topological model causally explaining the known irrational indices of the effective Twin Boundary (TB). A complete mechanistic picture of diffusionless TB migration is presented, the importance of which is discussed.Graphical abstractImage, graphical abstract
       
  • Experimental and crystal plasticity study on deformation bands in single
           crystal and multi-crystal pure aluminium
    • Abstract: Publication date: Available online 8 November 2019Source: Acta MaterialiaAuthor(s): Qinmeng Luan, Hui Xing, Jiao Zhang, Jun Jiang Deformation bands (DBs) formed in metals even in single crystals are known to give rise to the microstructural heterogeneities, thus contributing to some long-standing microstructure formation problems, such as the occurrence of recrystallization on the basis of deformed microstructure. Previous experimental transmission electron microscope (TEM) work has identified two types of DBs in the microscopic scale, i.e. kink bands and bands of secondary slips, showing the importance of understanding the slip activation for DBs. To extend the theory in mesoscale, single crystal and multi-crystal pure aluminium, as well as their corresponding crystal plasticity finite element (CPFE) models, are used in this paper to explore the effect of grain orientation, strain level and neighbouring grains on the formation of DBs. It is demonstrated that slip band intersection of primary and secondary slips is predicted to constrain the lattice sliding but facilitate the lattice rotation for the formation of DBs regarding the wall of DBs and its orientation. It is found that the impact of the above factors on the formation of DBs is caused by the slip field of primary slips. A sufficient amount of primary slips activated inside grains would be the key to the formation of distinct DBs with high area fraction and aspect ratio.Graphical abstractImage, graphical abstract
       
  • Complex phase transitions and associated electrocaloric effects in
           different oriented PMN-30PT single crystals under multi-fields of electric
           field and temperature
    • Abstract: Publication date: Available online 8 November 2019Source: Acta MaterialiaAuthor(s): Jianting Li, Ruowei Yin, Xiaopo Su, Hong-Hui Wu, Junjie Li, Shiqiang Qin, Shengdong Sun, Jun Chen, Yanjing Su, Lijie Qiao, Dong Guo, Yang Bai The phase composition and their evolution in ferroelectrics are very complex near the morphotropic phase boundary (MPB), especially under multiple fields of electric field and temperature, which results in versatile behaviors of electrocaloric effect (ECE). This paper systematically studied the phase transitions and associated ECEs in the , and oriented 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-30PT) single crystals with the chemical composition in the MPB area. Based on the dielectric and polarization responses under multi-fields, the refined electric field-temperature phase diagrams are established, wherein the ECE properties are closely related to the electric-field-induced phase transitions and exhibit a complex evolution across positive and negative values. The negative ECE originates from the transition from a monoclinic phase to a tetragonal or orthogonal phase under the noncollinear electric field, so it only appears in the or oriented crystals. Conversely, the transition from the tetragonal phase to the rhombohedral or orthogonal phase in the or oriented crystals induces a positive ECE peak with ∆Tmax=0.40K (@10kV/cm). Just above Tm, the positive ECE reaches ∆Tmax=0.61K, 0.63K and 0.68K in the , and oriented crystals, respectively. In addition, there is an abnormal net endothermic phenomenon around the monoclinic-tetragonal or monoclinic-orthogonal phase boundaries due to the electric-field-induced irreversible transition.Graphical Image, graphical abstract
       
  • Half-Heusler alloys: Enhancement of ZT after Severe Plastic Deformation
           (ultra-low Thermal Conductivity)
    • Abstract: Publication date: Available online 8 November 2019Source: Acta MaterialiaAuthor(s): Gerda Rogl, Sanyukta Ghosh, Lei Wang, Jiri Bursik, Andriy Grytsiv, Michael Kerber, Ernst Bauer, Ramesh C. Mallik, Xingqiu Chen, Michael Zehetbauer, Peter Rogl Several n- and p-type Half-Heusler (HH) thermoelectric materials (Ti0.5Zr0.5NiSn-based and NbFeSb-based) have been processed by high-pressure torsion (HPT) to improve their thermoelectric performance via a drastic reduction towards ultra-low thermal conductivity. This reduction occurs due to grain refinement and a high concentration of deformation-induced defects, i.e. vacancies and dislocations as inferred by this severe plastic deformation and documented via SEM and TEM investigations. In most cases the figure of merit, ZT, and the thermo-electric conversion efficiency were enhanced up to η ∼ 10% for the thermally stable HPT-processed sample. Raman spectroscopy, backed by DFT calculations, proves that HPT induces a stiffening of the lattice and as a consequence, a blue-shift of the lattice vibrations occurs.Furthermore for all investigated specimens Vickers hardness values after HPT were significantly higher, whereas the change in the elastic moduli was less than 5% in comparison to the HP reference sample.Graphical abstractImage, graphical abstract
       
  • Presence of a purely tetragonal phase in ultrathin BiFeO3 films:
           thermodynamics and phase-field simulations
    • Abstract: Publication date: Available online 8 November 2019Source: Acta MaterialiaAuthor(s): Yang Zhang, Fei Xue, Zuhuang Chen, Jun-Ming Liu, Long-Qing Chen The stability of a purely tetragonal phase relative to the nominal rhombohedral phase in ultrathin BiFeO3 films is investigated using thermodynamics and phase-field simulations. The thermodynamic analysis demonstrates the possible presence of a purely tetragonal state primarily due to the interfacial effect from the constraint of the adjacent layer although the built-in potential and compressive in-plane strain also play a role. Phase-field simulations of the corresponding ultrathin films reveal the coexistence of tetragonal and rhombohedral phases at certain film thickness arising from strain phase separation. It is shown that the piezoelectric coefficient d33 of the two-phase mixture is up to 200% higher than that of the rhombohedral single phase.Graphical abstractA purely tetragonal phase can be stabilized in ultrathin BiFeO3 films primarily due to the interfacial constraint of bottom layers according to the thermodynamic analysis. The phase-field simulations further reveal the coexistence of tetragonal and rhombohedral phases at certain film thickness arising from strain phase separation, leading to the enhancement of piezoelectric performances.Image, graphical abstract
       
  • Solidification-driven Orientation Gradients in Additively Manufactured
           Stainless Steel
    • Abstract: Publication date: Available online 8 November 2019Source: Acta MaterialiaAuthor(s): Andrew T. Polonsky, William C. Lenthe, McLean P. Echlin, Veronica Livescu, George T. Gray, Tresa M. Pollock A sample of 304L stainless steel manufactured by Laser Engineered Net Shaping (LENS) was characterized in 3D using TriBeam tomography. The crystallographic, structural, and chemical properties of the as-deposited microstructure have been studied in detail. 3D characterization reveals complex grain morphologies and large orientation gradients, in excess of 10∘, that are not easily interpreted from 2D cross-sections alone. Misorientations were calculated via a methodology that locates the initial location and orientation of grains that grow during the build process. For larger grains, misorientation increased along the direction of solidification. For grains with complex morphologies, K-means clustering in orientation space is demonstrated as a useful approach for determining the initial growth orientation. The gradients in misorientation directly tracked with gradients in chemistry predicted by a Scheil analysis. The accumulation of misorientation is linked to the solutal and thermal solidification path, offering potential design pathways for novel alloys more suited for additive manufacturing.Graphical abstractGraphical abstract for this article
       
  • The stability of irradiation-induced defects in Zr3AlC2, Nb4AlC3 and
           (Zr0.5,Ti0.5)3AlC2 MAX phase-based ceramics
    • Abstract: Publication date: Available online 7 November 2019Source: Acta MaterialiaAuthor(s): D. Bowden, J. Ward, S. Middleburgh, S. de Moraes Shubeita, E. Zapata-Solvas, T. Lapauw, J. Vleugels, K. Lambrinou, W.E. Lee, M. Preuss, P. Frankel This work is a first assessment of the radiation tolerance of the nanolayered ternary carbides (MAX phases), Zr3AlC2, Nb4AlC3 and (Zr0.5,Ti0.5)3AlC2, using proton irradiation followed by post-irradiation examination based primarily on x-ray diffraction analysis. These specific MAX phase compounds are being evaluated as candidate coating materials for fuel cladding applications in advanced nuclear reactor systems. The aim of using a MAX phase coating is to protect the substrate fuel cladding material from corrosion damage during its exposure to the primary coolant. Proton irradiation was used in this study as a surrogate for neutron irradiation in order to introduce radiation damage into these ceramics at reactor-relevant temperatures. The post-irradiation examination of these materials revealed that the Zr-based 312-MAX phases, Zr3AlC2 and (Zr0.5,Ti0.5)3AlC2 have a superior ability for defect-recovery above 400°C, whilst the Nb4AlC3 does not demonstrate any appreciable defect recovery below 600°C. Density functional theory calculations have demonstrated that the structural differences between the 312 and 413-MAX phase structures govern the variation of the irradiation tolerance of these materials.Graphical abstractImage, graphical abstract
       
  • 1 x Sr x Al2Si2O8:1%Eu 2 + , 1%Pr 3 + +Anorthite&rft.title=Acta+Materialia&rft.issn=1359-6454&rft.date=&rft.volume=">Relating Structural Phase Transitions to Mechanoluminescence: The Case of
           the Ca 1 − x Sr x Al2Si2O8:1%Eu 2 + , 1%Pr 3 +
           Anorthite
    • Abstract: Publication date: Available online 7 November 2019Source: Acta MaterialiaAuthor(s): Ang Feng, Simon Michels, Alfredo Lamberti, Wim Van Paepegem, Philippe F. Smet The phenomenon of mechanoluminescence (ML), where phosphors emit light when pressure is applied, is considered to be closely related to the crystallographic structure of those phosphors. In this work we unravel this connection for the anorthite solid solution Ca1−xSrxAl2Si2O8, which displays two important phase transitions as a function of strontium content x (denoted as xSr), i.e., the nearly second-order P1¯-I1¯ transition and the ferroelastic I1¯-I2c transition at ambient temperature and pressure. The spontaneous strains reveal that the ferroelastic transition takes place when xSr ∈ (0.70, 0.75), while other optical methods suggest that the second-order P1¯−I1¯ transition takes place when xSr is around 0.4. The ML intensity reaches its maximum when the second order transition takes place and drops to zero when the phosphors undergo the ferroelastic transition. The first transition already brings significant changes to electron occupations at traps in this solid solution. The structural phase transitions in the anorthite solid solutions are reflected in specific ML properties, such as the ML intensity and the load threshold. Further analysis suggests this is due to the structural change of the hosts and the trap properties (trap density and electron population function). Analysis of the ML dynamics may therefore serve as a useful tool to investigate phase transitions in ML phosphors.Graphical abstractGraphical abstract for this article
       
  • Hafnia-doped Silicon Bond Coats manufactured by PVD for SiC/SiC CMCs
    • Abstract: Publication date: Available online 7 November 2019Source: Acta MaterialiaAuthor(s): Ronja Anton, Vito Leisner, Philipp Watermeyer, Michael Engstler, Uwe Schulz SiC/SiC ceramic matrix composites (CMCs) demand an environmental barrier coating (EBC) system when implemented in the hot section of a turbine engine. The connection between EBC and CMC is provided by a bond coat (BC). Numerous reasons make silicon the state-of-the-art BC material but it has some disadvantages regarding long time mechanical behaviour and oxidation resistance. To overcome this, a Si-BC doped with the refractory metal oxide HfO2 is introduced. Two different compositions have been deposited on monolithic SiC by magnetron sputtering. After deposition the coatings are X-ray amorphous, homogenous, columnar structured and virtually free of cracks and pores. Furnace cycle tests up to 1000 cycles were performed at 1523 K. The evolution of microstructure and phases of the coatings were examined employing Scanning Electron Microscopy (SEM), Focused Ion Beam (FIB) serial sectioning, Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD). During high temperature exposure, the coatings crystallized and the silicon phase started to form a mixed thermally grown oxide (mTGO) layer. The BCs showed evenly distributed hafnia precipitates within the silicon. During testing Ostwald ripening of the precipitates took place. Hafnia slowly reacted with silicon oxide to hafnon (HfSiO4). Compared to a pure silicon reference BC, the doped coatings show a better resistance towards crack initiation and spallation up to 1000 h testing time. The results demonstrate that sputtered hafnia-doped Si-BCs are more advantageous for SiC/SiC CMCs with respect to longevity, TGO adherence, and protection of the underlying SiC in comparison to pure Si bond coats.Graphical Image, graphical abstract
       
  • Controlling the domain structure of ferroelectric nanoparticles using
           tunable shells
    • Abstract: Publication date: Available online 7 November 2019Source: Acta MaterialiaAuthor(s): Anna N. Morozovska, Eugene A. Eliseev, Yevhen M. Fomichov, Yulian M. Vysochanskii, Victor Yu. Reshetnyak, Dean R. Evans The possibility of controlling the domain structure in spherical nanoparticles of uniaxial and multiaxial ferroelectrics using a shell with tunable dielectric properties is studied in the framework of Landau-Ginzburg-Devonshire theory. Finite element modeling and analytical calculations are performed for Sn2P2S6 and BaTiO3 nanoparticles covered with polymer, temperature dependent isotropic paraelectric strontium titanate, or anisotropic liquid crystal shells with a strongly temperature dependent dielectric permittivity tensor. It appeared that the “tunable” paraelectric shell with a temperature dependent high dielectric permittivity (∼300 – 3000) provides much more efficient screening of the nanoparticle polarization than the polymer shell with a much smaller (∼10) temperature-independent permittivity. The tunable dielectric anisotropy of the liquid crystal shell (∼ 1 – 100) adds a new level of functionality for the control of ferroelectric domains morphology (including a single-domain state, domain stripes and cylinders, meandering and labyrinthine domains, and polarization flux-closure domains and vortexes) in comparison with isotropic paraelectric and polymer shells. The obtained results indicate the opportunities to control the domain structure morphology of ferroelectric nanoparticles covered with tunable shells, which can lead to the generation of new ferroelectric memory and advanced cryptographic materials.Graphical abstractImage, graphical abstract
       
  • Microstructure-dependent deformation behaviour of a low γ′ volume
           fraction Ni-base superalloy studied by in-situ neutron diffraction
    • Abstract: Publication date: Available online 7 November 2019Source: Acta MaterialiaAuthor(s): Nitesh Raj Jaladurgam, Hongjia Li, Joe Kelleher, Christer Persson, Axel Steuwer, Magnus Hörnqvist Colliander Ni-base superalloys are critical materials for numerous demanding applications in the energy and aerospace sectors. Their complex chemistry and microstructure require detailed understanding of the operating deformation mechanisms and interaction between the matrix and the hardening phase during plastic deformation. Here we use in-situ neutron diffraction to show that the dependence of the deformation mechanisms and load redistribution on γ′ particle size in a Ni-base superalloy with a γ′ volume fraction of around 20% can exhibit distinct differences compared to their high volume fraction counterparts. In particular, the load redistribution in the coarse microstructure occurs immediately upon yielding in the present case, whereas high γ′ volume fractions have been observed to initially lead to shear mediated co-deformation before work hardening allows looping to dominate and cause load partitioning at higher stresses. The fine microstructure, on the other hand, behaved similar to high volume fraction alloys, exhibiting co-deformation of the phases due to particle shearing. A recently developed elasto-plastic self-consistent (EPSC) crystal plasticity model, specifically developed for the case of coherent multi-phase materials, could reproduce experimental data with good accuracy. Furthermore, the finite strain formulation of the EPSC model allowed deformation induced texture predictions. The correct trends were predicted by the simulations, but the rate of lattice rotation was slower than experimentally observed. The insights point towards necessary model developments and improvements in order to accurately predict e.g. texture evolution during processing and effect of texture and microstructure on component properties.Graphical abstractGraphical abstract for this article
       
  • Structural and vibrational properties of α- and π-SnS polymorphs for
           photovoltaic applications
    • Abstract: Publication date: Available online 6 November 2019Source: Acta MaterialiaAuthor(s): Maxim Guc, Jacob Andrade-Arvizu, Ibbi Y. Ahmet, Florian Oliva, Marcel Placidi, Xavier Alcobé, Edgardo Saucedo, Alejandro Pérez-Rodríguez, Andrew L. Johnson, Victor Izquierdo-Roca Tin sulphide (SnS) has attracted the attention of the photovoltaic (PV) community due to the combination of desirable optical properties, and its binary and earth abundant elemental composition, which should lead to relatively simple synthesis. However, currently the best SnS based PV device efficiency remains at 4.36 %. Limited performance of this material is attributed to band gap alignment issues, deviations in doping concentration and poor film morphology. In this context Raman spectroscopy (RS) analysis can be useful as it facilitates the accurate evaluation of material properties. In this study we present a RS study, supported by X-ray diffraction and wavelength dispersive X-ray measurements, of α- and π-SnS thin films. In particular a complete description of SnS vibrational properties is made using six excitation wavelengths, including excitation energies coupled with certain optical band to band transitions, which leads to close to resonance measurement conditions. This study describes an in-depth analysis of the Raman spectra of both SnS structural polymorphs, including the differences in the number of observed peaks, with their relative intensities and Raman shift. Additionally, we evaluate the impact of low temperature heat treatment on SnS. These results explicitly present how the variation of the [S]/[Sn] ratio in samples deposited by different methods can lead to significant and correlated shifts in the relative positions of Raman peaks, which is only observed in the α-SnS phase. Furthermore, we discuss the suitability of using Raman spectroscopy based methodologies to extract fine stoichiometric variations in different α-SnS samples.Graphical Image, graphical abstract
       
  • Magnetic-field-induced strain-glass-to-martensite transition in a Fe-Mn-Ga
           alloy
    • Abstract: Publication date: Available online 6 November 2019Source: Acta MaterialiaAuthor(s): Xiaoming Sun, Daoyong Cong, Yang Ren, Klaus-Dieter Liss, Dennis E. Brown, Zhiyuan Ma, Shijie Hao, Weixing Xia, Zhen Chen, Lin Ma, Xinguo Zhao, Zhanbing He, Jian Liu, Runguang Li, Yandong Wang Strain glass is a frozen disordered strain state with local strain order manifested by nano-sized strain domains, which is formed as a result of doping sufficient point defects into the normal martensitic system. Exploration of the transition between strain glass and long-range strain-ordered martensite is of both great fundamental importance and practical interest. However, it remains a mystery whether magnetic field can induce a transition from strain glass to martensite. Here, we report for the first time the magnetic-field-induced strain-glass-to-martensite transition, in a model system Fe-Mn-Ga. It was found that the martensitic transformation temperature of the Fe43-xMn28Ga29+x alloys decreases rapidly with increasing x and the martensitic transformation disappears when x reaches the critical value xc = 2.0. Strain glass transition occurs in the alloy with x = 2.0 (Fe41Mn28Ga31), which is confirmed by the invariance of the average structure during cooling, the frequency dispersion of the ac storage modulus and internal friction following the Vogel-Fulcher relation, and the formation of nanodomains. The magnetic-field-induced transition from strain glass to non-modulated tetragonal martensite in Fe41Mn28Ga31 was indicated by the abrupt magnetization jump on the M(H) curve and directly evidenced by the crystal structure evolution with magnetic field change revealed by in-situ neutron diffraction experiments. The microscopic mechanism for this magnetic-field-induced strain-glass-to-martensite transition is discussed. The present study may not only help establish the unified theory for strain-glass-to-martensite transition under external fields but also open a new avenue for designing advanced materials with novel functional properties.Graphical abstractImage, graphical abstract
       
  • A multiscale study on the morphology and evolution of slip bands in a
           nickel-based superalloy during low cycle fatigue
    • Abstract: Publication date: 1 January 2020Source: Acta Materialia, Volume 182Author(s): F.D. León-Cázares, R. Schlütter, T. Jackson, E.I. Galindo-Nava, C.M.F. Rae Plastic deformation during low cycle fatigue in fcc materials with low stacking fault energy is accumulated in slip bands, which become preferential sites for crack initiation. Whilst these dislocation structures have been studied before, little has been done to assess the effect and evolution of the individual slip lines within them. In this study, samples of a γ′ precipitate strengthened nickel-based superalloy are fatigued at room temperature and 700 ∘C for 1, 40 and 500 cycles. The resulting dislocation structures are characterised via Electron Channeling Contrast Imaging and Transmission Electron Microscopy. We introduce a new methodology to measure slip band parameters such as the slip line spacing and shear step length by analysing the holes left by sheared precipitates in γ′-etched secondary electron micrographs. Statistics of these parameters are obtained and compared for different conditions. Advantages of this technique include resolution at the scale of individual planes, acquisition of true three-dimensional data and applicability in the bulk of the material. The combination of these techniques provides a unique mechanistic and quantitative insight into the slip band and precipitate morphology evolution.Graphical abstractGraphical abstract for this article
       
  • Irradiation induced creep in nanocrystalline high entropy alloys
    • Abstract: Publication date: 1 January 2020Source: Acta Materialia, Volume 182Author(s): Gowtham Sriram Jawaharram, Christopher M. Barr, Anthony M. Monterrosa, Khalid Hattar, Robert S. Averback, Shen J. Dillon Irradiation induced creep (IIC) compliance in NiCoFeCrMn high entropy alloys is measured as a function of grain size (30 < x < 80 nm) and temperature (23–500 °C). For 2.6 MeV Ag3+ irradiation at a dose rate of 1.5×10–3 dpa−1s−1 the transition from the recombination to sink limited regimes occurs at ∼ 100 °C. In the sink-limited regime, the IIC compliance scales inversely with grain size, consistent with a recently proposed model for grain boundary IIC. The thermal creep rate is also measured; it does not become comparable to the IIC rate, however, until ∼ 650 °C. The results are discussed in context of defect kinetics in irradiated HEA systems.Graphical abstractImage, graphical abstract
       
  • The effect of grain boundary structure on the intergranular degradation
           behavior of solution annealed Alloy 690 in high temperature, hydrogenated
           water
    • Abstract: Publication date: Available online 30 October 2019Source: Acta MaterialiaAuthor(s): Wenjun Kuang, Gary S. Was The environmental degradation of four different types of grain boundaries were investigated on alloy 690 following slow strain rate tensile tests in 360 °C hydrogenated water. Random high angle boundaries (RHABs) support fast Cr diffusion that promotes the formation of a continuous surface oxide film and grain boundary migration. Surprisingly, coherent twin boundaries (CTBs) are susceptible to intergranular oxidation and do not exhibit Cr diffusion or grain boundary migration. When CTBs are changed to transformed twin boundaries (TTBs) by cold work, they behave like RHABs. Finally, incoherent twin boundaries (ITBs) undergo intergranular oxidation with limited Cr depletion but no boundary migration beyond the oxide. The Cr diffusivity along grain boundary in this alloy is directly related to the density of coincident site in the grain boundary plane and determines the morphology of oxide formed near the grain boundary. CTBs are still highly resistant to stress corrosion cracking (SCC) due to the semi-coherent interface between the intergranular chromia and grain matrix. In contrast, the intergranular oxides formed along RHABs inherit highly-disordered boundary structure from the original grain boundaries and show much higher SCC susceptibility. The grain boundary structure dependence of SCC resistance should be understood from its effects on solute diffusivity, structure of intergranular oxide and the local stress-strain state.Graphical abstractImage, graphical abstract
       
  • Tailoring ultra-strong nanocrystalline tungsten nanofoams by reverse phase
           dissolution
    • Abstract: Publication date: Available online 30 October 2019Source: Acta MaterialiaAuthor(s): Mingyue Zhao, Inas Issa, Manuel J. Pfeifenberger, Michael Wurmshuber, Daniel Kiener Bulk nanoporous tungsten as an extremely strong and low density nanocrystalline material was for the first time created to satisfy the need for advanced high performance materials that can endure harsh environments. Synthesis of nanoporous tungsten was achieved by a unique procedure involving severe plastic deformation of a coarse-grained tungsten-copper composite followed by selective dissolution of the nobler copper phase. The used ammonium persulfate etching solution, in which the less noble tungsten is chemically stable, is proved to be effective in removing the nobler copper phase. A nanoporous tungsten microstructure characterized by a network of interconnected nanocrystalline tungsten ligaments and interconnected nanopores was obtained. Based on a high-resolution interface analysis, the underlined mechanisms for the formation of nanoporous tungsten structure were elucidated. Moreover, using nanoindentation we demonstrate that, due to the nanoscale microstructure, the created nanoporous tungsten possesses outstanding strength, making it an attractive material for applications in radiation shielding fields.Graphical abstractImage, graphical abstract
       
  • Cyclic deformation behavior and related micro-mechanisms of a special CVD
           Ni processed with bimodal grain structures: ultrafine (UF) grains and
           large grains with UF/nano twins
    • Abstract: Publication date: Available online 18 October 2019Source: Acta MaterialiaAuthor(s): Shaohua Fu, Tzu-Yin Jean Hsu, Zhirui Wang Stress-controlled cyclic tests were conducted on bulk sheet Ni-carbonyl Chemical Vapor Deposited material (CVD Ni) with bimodal grain structures: ultrafine (UF) grains and large grains with UF/nano twins. The tests were run with the cyclic stress ratio R=0.05 and peak stress level of 0.9 to 1.5 times of the material's yield strength. Results show that within the applied peak stress ratio of 0.9-1.1, the material demonstrated cyclic hardening behavior first, followed by stress-strain saturation till fracture; upon increasing the ratio to 1.4-1.5, an additional softening stage was activated and continued till fracture. By transmission electron microscope (TEM) examination, it was found that such cyclic deformation responses were associated with the stability of the ultrafine- and nano-twin structures. Initial hardening was found mainly due to the increase in dislocation density and the activities of dislocations especially with their strong interactions with the dense twin boundaries (TBs). The saturation was contributed by the simultaneous operations of the softening effect due to massive detwinning and the hardening behavior due to dislocation interactions with existing TBs. Newly formed dislocation walls and cell structures were further found in samples with stress ratio of 1.4 at fracture, corresponding to the softening stage. Furthermore, such microstructural evolution, which were observed also through annealing and monotonic deformation of the same material, is identified as a consistent energy reduction path for the material. Thus, an energy criterion is further established to predict the massive detwinning events that cause the major softening phenomena under cyclic deformation.Graphical abstractImage, graphical abstract
       
  • Kinetic Pathways of Ordering and Phase Separation Using Classical Solid
           State Models within the Steepest-Entropy-Ascent Quantum Thermodynamic
           Framework
    • Abstract: Publication date: Available online 14 October 2019Source: Acta MaterialiaAuthor(s): Ryo Yamada, Michael R. von Spakovsky, William T. Reynolds The kinetics of ordering and concurrent ordering and phase separation are analyzed with an equation of motion initially developed to account for dissipative processes in quantum systems. A simplified energy eigenstructure, or pseudo-eigenstructure, is constructed from a static concentration wave method to describe the configuration-dependent energy in a binary alloy. This pseudo-eigenstructure is used in conjunction with an equation of motion that follows steepest entropy ascent to calculate the kinetic path that leads to ordering and phase separation in a series of hypothetical alloys. By adjusting the thermodynamic solution parameters, it is demonstrated that the model can predict: (a) the stable equilibrium state, (b) the unique thermodynamic path and kinetics of continuous or discontinuous ordering, and (c) the kinetics of concurrent processes involving simultaneous ordering and phase separation.Graphical abstractGraphical abstract for this article
       
  • Deformation induced grain boundary segregation in nanolaminated Al-Cu
           alloy
    • Abstract: Publication date: Available online 29 October 2019Source: Acta MaterialiaAuthor(s): W. Xu, X.C. Liu, X.Y. Li, K. Lu A gradient nanostructured surface layer was formed in an Al-4 wt.% Cu alloy processed by means of surface mechanical grinding treatment at liquid nitrogen temperature. Within the deformed surface layer, laminated structures with a wide range of thickness were formed. With a decreasing depth from the treated surface, lamellae thickness decreases accompanied by an increased fraction of high angle grain boundaries (HAGBs) from 10% to 70%. In the topmost surface layer, nanolaminated (NL) structures were found with an average thickness as small as 28 nm and a HAGB fraction of 70%. Composition analysis indicated that Cu atoms segregate at NL boundaries in the as-prepared sample, Cu concentration is about 3-4 times higher than that in the lattice. The obvious grain boundary (GB) segregation of Cu induced by cryogenic plastic deformation is attributed dynamic interaction between solute atoms with gliding dislocations. GB segregation of Cu is responsible for the stabilization of the NL structures with a much finer structural size than that in pure Al, resulting in higher hardness. The deformation-induced GB segregation provides an alternative strategy to achieving stable high strength nanostructures in Al alloys.Graphical abstractImage, graphical abstract
       
  • Contribution of intragranular misorientations to the cold rolling textures
           of ferritic stainless steels
    • Abstract: Publication date: Available online 29 October 2019Source: Acta MaterialiaAuthor(s): A. Després, M. Zecevic, R.A. Lebensohn, J.D. Mithieux, F. Chassagne, C.W. Sinclair A combined experimental and simulation study of intragranular misorientation and texture development in ferritic stainless steels is presented. Cold rolling was performed on materials having different grain shapes to reveal variations of misorientations and texture with variations in microstructure. The experimental results were compared with predictions of the Visco-Plastic Self-Consistent (VPSC) model and the recently developed Grain-Fragmentation Visco-Plastic Self-Consistent (GF-VPSC) model. It is shown that the GF-VPSC model, incorporating the development of intragranular misorientations, provides a much better prediction of the texture strength compared to the standard VPSC model. The predictions of intragranular misorientation are also in good agreement with experimental measurements. Both experiments and simulations point to the importance of anisotropy of intragranular misorientation distributions in determining texture development and, importantly, texture strength.Graphical abstractGraphical abstract for this article
       
  • Dynamic Martensitic Phase Transformation in Single-crystal Silver
           Microcubes
    • Abstract: Publication date: Available online 26 October 2019Source: Acta MaterialiaAuthor(s): Ramathasan Thevamaran, Claire Griesbach, Sadegh Yazdi, Mauricio Ponga, Hossein Alimadadi, Olawale Lawal, Seog-Jin Jeon, Edwin L. Thomas The ability to transform the crystal structure of metals in the solid state enables tailoring their physical, mechanical, electrical, thermal, and optical properties in unprecedented ways. We demonstrate a martensitic phase transformation from a face-centered-cubic (fcc) structure to a hexagonal-close-packed (hcp) structure that occurs in nanosecond timescale in initially near-defect-free single-crystal silver (Ag) microcubes impacted at supersonic velocities. Impact-induced high pressure and high strain rates in Ag microcubes cause impact orientation dependent extreme micro- and nano-structural transformations. When a microcube is impacted along the [100] crystal symmetry direction, the initial fcc structure transforms into an hcp crystal structure, while impact along the [110] direction does not produce phase transformations, suggesting the predominant role played by the stacking faults generated in the [100] impact. Molecular dynamics simulations at comparable high strain rates reveal the emergence of such stacking faults that coalesce, forming large hcp domains. The formation of hcp phase through the martensitic transformation of fcc Ag shows new potential to dramatically improve material properties of low-stacking-fault energy materials.Graphical abstractImage, graphical abstract
       
  • On solute depletion zones along grain boundaries during segregation
    • Abstract: Publication date: Available online 25 October 2019Source: Acta MaterialiaAuthor(s): D. Scheiber, T. Jechtl, J. Svoboda, F.D. Fischer, L. Romaner We propose a model for predicting the depletion zone arising next to grain boundaries during non-equilibrium segregation. The model directly links to distribution of segregation energies as provided by atomistic simulations. We expose the theoretical framework based on the thermodynamic extremal principle and propose an efficient algorithm to solve the underlying equations. The example of the W-25at%Re is discussed to illustrate the main features of the model. Multi-component segregation kinetics is discussed for segregation of B, C, and N in Mo to illustrate site-competition scenarios. Comparison with earlier results obtained without depletion illustrates the importance of this effects. Finally the depletion zones along GBs are investigated for many different compositions to asses for which material composition and heat treatments they may be observed experimentally. We find that extended depletion zones arise for very small solute concentrations.Graphical abstractImage, graphical abstract
       
  • Mesoscale Modeling of Jet Initiation Behavior and Microstructural
           Evolution during Cold Spray Single Particle Impact
    • Abstract: Publication date: Available online 25 October 2019Source: Acta MaterialiaAuthor(s): Sumit Suresh, Seok-Woo Lee, Mark Aindow, Harold D. Brody, Victor K. Champagne, Avinash M. Dongare Quasi-coarse-grained dynamics (QCGD) simulations are carried out to investigate the mesoscale deformation behavior during the impact of a 20 µm pure aluminum particle onto a substrate of pure aluminum at time and length scales relevant to cold spray deposition. A rigorous analysis of the evolution of pressure, temperature, strain, flow stress and microstructure is carried out to investigate the jetting mechanisms over a range of process parameters (impact velocity and particle temperature). The QCGD simulations identify a critical role of the pressure wave propagation in the initiation of a jet, i.e. outward flow of material at the particle/substrate interface periphery (edge). Jetting is observed to initiate when the shock wave interacts with the edge and results in localized softening of the metal in this region. This localized softening enables outward flow of the material and is accompanied by a release of the pressures in the particle and the substrate at the interface. Observations of final splat microstructures of systems that showed jetting revealed several new “small” grains in the range of 2-4 µm. These grains are mainly found at the interface, suggesting that recrystallization is favored in cold sprayed impacts of aluminum.Graphical abstractThe temporal evolution of pressure in a thin vertical section through the center of the particle is plotted in Figure 2 for an impact velocity of 1300 m/s, where “jetting” is observed. The contour levels are chosen to provide a clear visual inspection of the propagation of a compressive shock wave generated as shown in (a), interaction with the particle edge as shown in (b), if any, and its role in initiating a jet. In this case the shock wave arrives the particle edge at t = 3.6 ns as shown in Figure 2 (c), that results in outward flow of the materials and a particle jet is initiated.Image, graphical abstract
       
  • Multi-layered domain morphology in relaxor single crystals with
           nano-patterned composite electrode
    • Abstract: Publication date: Available online 24 October 2019Source: Acta MaterialiaAuthor(s): Chengtao Luo, Wei-Yi Chang, Min Gao, Chih-Hao Chang, Jiefang Li, Dwight Viehland, Jian Tian, Xiaoning Jiang (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) single crystals, especially with compositions near its morphotropic phase boundary (MPB), have been employed for a broad range of applications such as ultrasound transducers, sensors, and actuators. To further enhance the properties of PMN-PT, electrode patterning, as a method of domain engineering, was proved to be an effective approach. In our previous report, a 200 nm grating pattern electrode (Ti/Au-MnOx) (nano-electrode) was prepared on one surface of PMN-PT crystal, exhibiting 30% d33 enhancement. In this work, the multi-layered domain morphology and the domain engineering from nano-electrode were characterized using piezoresponse force microscopy (PFM). A hypothetical domain engineering model for nano-electrodes is established to explain the experimental results as well as the property enhancement from the nano-electrode. The electrode patterning proves that the nano-scale modification can tune the macro-scale piezoelectric properties of the bulk material.Graphical abstractNano-patterned composite electrode (Nano-electrode), as a method of domain engineering, exhibits 30% d33 enhancement on (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) single crystal. In this work, the mechanism of the property enhancement is studied through the multi-layered domain morphology by piezoresponse force microscopy (PFM). A hypothetical domain engineering model for nano-electrodes is established in the end. Image, graphical abstract
       
  • Microstructural Evolution in Amorphous-Nanocrystalline ZrCu Alloy under
           Neutron Irradiation
    • Abstract: Publication date: Available online 23 October 2019Source: Acta MaterialiaAuthor(s): Fan Xiong, Ming-Fei Li, Babafemi Malomo, Liang Yang An extensive investigation on the microstructural evolution of an amorphous-nanocrystalline alloy (ANA) under neutron irradiation has been conducted by molecular dynamics simulation. The phenomenon of rapid and full annihilation of irradiation-induced vacancies was found in the nanocrystal zone and after structural relaxation, free volumes in the amorphous matrix were systematically self-recovered. An effective self-healing behavior of the nanocrystal zone subsequently sufficed, regardless of the thermal degradation effect caused by the intensity of collision cascades during quenching. As knocked-on atoms were arrested at the phase boundary, it is self-evident that, the mechanism of atomic diffusion was non-existent at the interface between the nanocrystal grain and the neighbouring amorphous zone. Consequently, from the foregoing, ANA materials have been found to demonstrate excellent resistance to neutron irradiation and prospectively, the results of this study will potentially facilitate the development of advanced materials with high irradiation resistance.Graphical Image, graphical abstract
       
  • High electrostrictive strain in lead-free relaxors near the morphotropic
           phase boundary
    • Abstract: Publication date: Available online 23 October 2019Source: Acta MaterialiaAuthor(s): Tangyuan Li, Chang Liu, Xiaoqin Ke, Xiao Liu, Liqiang He, Peng Shi, Xiaobing Ren, Yunzhi Wang, Xiaojie Lou Thanks to its small hysteresis, large electrostrictive strain in relaxor ferroelectrics is superior than piezoelectric strain for applications in precision microactuators. Although relaxor ferroelectrics exhibit the largest electrostrictive strain in ceramics, the magnitude of the strain is limited to ∼0.20% at room temperature due to the large amount of non-ferroelectric defects existing in relaxors. In this work, we develop a relaxor with a morphotropic phase boundary (MPB) by doping a rhombohedral (R3m) ferroelectric BaZr0.2Ti0.8O3 into a tetragonal (P4mm) ferroelectric 0.89Bi0.5Na0.5TiO3-0.11BaTiO3. A high electrostrictive strain of 0.27% is achieved at room temperature in the relaxor sample. Experimental results illustrate that the composition is near the MPB and exhibits the existence of nanodomains, favoring the achievement of high electrostrictive strain. Moreover, phase field simulations show that the high electrostrictive strain obtained at this composition originates from the low defect fields needed to induce relaxor as a result of small polarization anisotropy at the phase boundary as compared to conventional relaxors away from phase boundaries. Our work provides a new design strategy for the next generation of high-performance ferroelectric relaxors.Graphical Image, graphical abstract
       
  • Modeling sintering anisotropy in ceramic stereolithography of silica
    • Abstract: Publication date: Available online 23 October 2019Source: Acta MaterialiaAuthor(s): Charles Manière, Gabriel Kerbart, Christelle Harnois, Sylvain Marinel In the domain of ceramic additive manufacturing, sintering is a key step for controlling the final shape and mechanical strength of a 3D object. The thermal treatment of the printed green objects has a high influence on the specimen density, debinding, and sintered microstructure. This work focuses on the shrinkage anisotropy phenomenon that occurs during sintering. We demonstrate by dilatometry and interrupted sintering microstructure analysis that this phenomenon originates from non-ideal particle packing between the printed layers, which generates an anisotropic porosity distribution at the mesoscale. Based on this, a sintering model is developed and specially adapted for the numerical prediction of the sintering anisotropy. This model is formulated in analytic equations that can easily identify all the model parameters and reproduce the experimental dimensional changes. This numerical tool can be of great assistance in the prediction of additive manufacturing object dimensional changes during sintering.Graphical Image, graphical abstract
       
  • Nanocalorimetry and Ab Initio Study of Ternary Elements in
           CuZr-based Shape Memory Alloy
    • Abstract: Publication date: Available online 23 October 2019Source: Acta MaterialiaAuthor(s): Yucong Miao, Ruben Villarreal, Anjana Talapatra, Raymundo Arróyave, Joost J. Vlassak We present a computational-experimental study on the ternary alloying effect of CuZr-based shape memory alloy. The transformation behavior, including crystallization, martensite-austenite transformation temperature and hysteresis of Cu-Zr-X (X = Ni, Co, Hf) thin-film samples were investigated by nanocalorimetry. We used ab initio simulations to determine the B2-Cm transformation pathway, evaluate the lattice parameters, the relative phase stability, and the twin boundary energy as a function of composition. Experimental results show that alloying with Ni or Co reduces the hysteresis of the martensitic transformation, while Hf increases it. These observations are in agreement with the trend of the middle eigenvalue of the martensitic transformation matrix. The energy difference between the pure phases obtained from simulations suggests that both Co and Ni stabilize martensite against austenite. However, experiments show that Co decreases transformation temperature, while Ni increases it. We attribute this observation to the larger twin boundary energy and strain energy in the Co-containing alloy. Our results indicate that ab initio simulations are a helpful tool in the development of new shape memory alloys, provided the energy terms associated with the fine twin structure of the martensite are taken into account.Graphical Image, graphical abstract
       
  • Amorphization and dislocation evolution mechanisms of single crystalline
           6H-SiC
    • Abstract: Publication date: Available online 23 October 2019Source: Acta MaterialiaAuthor(s): Zhonghuai Wu, Weidong Liu, Liangchi Zhang, Sean Lim The amorphization and dislocation evolution mechanisms of a single crystal 6H-SiC were systematically investigated by using nano-indentation, high-resolution transmitted electron microscope (HRTEM), molecular dynamics (MD) simulations and the generalized stacking fault (GSF) energy surface analysis. Two major plastic deformation mechanisms of 6H-SiC under nano-indentation were revealed by HRTEM, i.e., (1) an amorphization region near the residual indentation mark, and (2) dislocations below the amorphization region in both the basal and prismatic planes. MD results showed that the amorphization process corresponds to the first “pop-in” event of the indentation load-displacement curve, while the dislocation nucleation and propagation are related to the consequent “pop-in” events. The amorphization is confirmed to achieve via an initial transformation from wurtzite structure to an intermediate structure, and then a further amorphization process.Graphical Image, graphical abstract
       
  • Dislocation density distribution at slip band-grain boundary intersections
    • Abstract: Publication date: Available online 23 October 2019Source: Acta MaterialiaAuthor(s): Yi Guo, David M. Collins, Edmund Tarleton, Felix Hofmann, Angus J. Wilkinson, T. Ben Britton We study the mechanisms of slip transfer at a grain boundary, in titanium, using Differential Aperture X-ray Laue Micro-diffraction (DAXM). This 3D characterisation tool enables measurement of the full (9-component) Nye lattice curvature tensor and calculation of the density of geometrically necessary dislocations (GNDs). We observe dislocation pile-ups at a grain boundary, as the neighbour grain prohibits easy passage for dislocation transmission. This incompatibility results in local micro-plasticity within the slipping grain, near to where the slip planes intersect the grain boundary, and we observe bands of GNDs lying near the grain boundary. We observe that the distribution of GNDs can be significantly influenced by the formation of grain boundary ledges that serve as secondary dislocation sources. This observation highlights the non-continuum nature of polycrystal deformation and helps us understand the higher order complexity of grain boundary characteristics.Graphical abstractImage, graphical abstract
       
  • Effect of AC field on uniaxial viscosity and sintering stress of ceria
    • Abstract: Publication date: Available online 23 October 2019Source: Acta MaterialiaAuthor(s): Chen Cao, Robert Mücke, Olivier Guillon The production of traditional and advanced ceramics is an energy-intensive activity, which requires high temperatures and long dwelling times to activate diffusional processes necessary for densification. Electric field assisted processing has received considerable attention recently, due to its potential to significantly reduce the costs of required heat treatments. However, the effect of electric fields on the densification and coarsening of oxide ceramics still not completely understood, and the mechanisms behind, in particular for fields, are still under debate. The potential influence of electric field on the sintering parameters (uniaxial viscosity and uniaxial sintering stress) and microstructure of polycrystalline yttria doped ceria were studied. Sintering parameters were measured without and with AC electric fields (14 V/cm and 28 V/cm, 50 Hz) which were below the “flash regime”. During all sintering measurements, the sample temperature was adjusted by lowering the furnace temperature according to the temperature measurements using densified samples. Major findings are: (i) The densification behavior is clearly modified by these moderate electric fields, although temperature increase due to macroscopic Joule heating is excluded. (ii) The densification rate remains proportional to the applied stress under electrical fields. (iii) Sintering parameters are significantly affected by the applied electric fields.Graphical abstractImage, graphical abstract
       
  • Electric field compensation effect driven strain temperature stability
           enhancement in potassium sodium niobate ceramics
    • Abstract: Publication date: Available online 22 October 2019Source: Acta MaterialiaAuthor(s): Ting Zheng, Jiagang Wu In view of unsolved key scientific problems of (K,Na)NbO3 (KNN)-based ceramics, this work focuses on intrinsic structure elucidation and physical mechanism of temperature stability in both piezoelectricity d33 and strain property d33* through domain strategy and crystal structure engineering. The introduction of BiFeO3 into KNN-BNH can stabilize multiphase coexistence structure and then results in high performance (d33=400∼450 pC/N, TC=280∼320 °C, and Suni=0.16∼0.18%) with composition in-sensitivity (x=0∼0.6%). Different from the viewpoint of electric field induced diffuse phase transition, we first verified that electric field compensation effect can greatly offset the negative effects induced by polycrystalline phase transition and thermal depolarization, leading to much better temperature stability of d33* than that of d33. We believe that this work has clarified the origin of the difference in temperature stability between d33 and d33*, which can provide some useful clues for further improving the stability of KNN-based ceramics, especially d33 temperature stability for sensor and transducer applications.Graphical abstractImage, graphical abstract
       
 
 
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