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Publisher: Elsevier   (Total: 3177 journals)

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Showing 1 - 200 of 3177 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: 448, SJR: 0.758, CiteScore: 2)
Acta Automatica Sinica     Full-text available via subscription   (Followers: 2)
Acta Biomaterialia     Hybrid Journal   (Followers: 30, SJR: 1.967, CiteScore: 7)
Acta Colombiana de Cuidado Intensivo     Full-text available via subscription   (Followers: 3)
Acta de Investigación Psicológica     Open Access   (Followers: 2)
Acta Ecologica Sinica     Open Access   (Followers: 11, SJR: 0.18, CiteScore: 1)
Acta Histochemica     Hybrid Journal   (Followers: 5, SJR: 0.661, CiteScore: 2)
Acta Materialia     Hybrid Journal   (Followers: 325, SJR: 3.263, CiteScore: 6)
Acta Mathematica Scientia     Full-text available via subscription   (Followers: 5, SJR: 0.504, CiteScore: 1)
Acta Mechanica Solida Sinica     Full-text available via subscription   (Followers: 9, SJR: 0.542, CiteScore: 1)
Acta Oecologica     Hybrid Journal   (Followers: 12, SJR: 0.834, CiteScore: 2)
Acta Otorrinolaringologica (English Edition)     Full-text available via subscription  
Acta Otorrinolaringológica Española     Full-text available via subscription   (Followers: 2, SJR: 0.307, CiteScore: 0)
Acta Pharmaceutica Sinica B     Open Access   (Followers: 2, SJR: 1.793, CiteScore: 6)
Acta 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: 7, 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: 12, SJR: 2.611, CiteScore: 8)
Additives for Polymers     Full-text available via subscription   (Followers: 23)
Advanced Drug Delivery Reviews     Hybrid Journal   (Followers: 193, 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: 1, SJR: 0.686, CiteScore: 2)
Advances in Cancer Research     Full-text available via subscription   (Followers: 35, SJR: 3.043, CiteScore: 6)
Advances in Carbohydrate Chemistry and Biochemistry     Full-text available via subscription   (Followers: 9, SJR: 1.453, CiteScore: 2)
Advances in Catalysis     Full-text available via subscription   (Followers: 5, SJR: 1.992, CiteScore: 5)
Advances in Cell Aging and Gerontology     Full-text available via subscription   (Followers: 5)
Advances in Cellular and Molecular Biology of Membranes and Organelles     Full-text available via subscription   (Followers: 14)
Advances in Chemical Engineering     Full-text available via subscription   (Followers: 29, SJR: 0.156, CiteScore: 1)
Advances in Child Development and Behavior     Full-text available via subscription   (Followers: 11, SJR: 0.713, CiteScore: 1)
Advances in Chronic Kidney Disease     Full-text available via subscription   (Followers: 11, SJR: 1.316, CiteScore: 2)
Advances in Clinical Chemistry     Full-text available via subscription   (Followers: 26, SJR: 1.562, CiteScore: 3)
Advances in Colloid and Interface Science     Full-text available via subscription   (Followers: 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: 9, SJR: 0.694, CiteScore: 2)
Advances in Medicinal Chemistry     Full-text available via subscription   (Followers: 6)
Advances in Microbial Physiology     Full-text available via subscription   (Followers: 5, SJR: 1.158, CiteScore: 3)
Advances in Molecular and Cell Biology     Full-text available via subscription   (Followers: 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: 11)
Advances in Plant Pathology     Full-text available via subscription   (Followers: 6)
Advances in Porous Media     Full-text available via subscription   (Followers: 5)
Advances in Protein Chemistry     Full-text available via subscription   (Followers: 19)
Advances in Protein Chemistry and Structural Biology     Full-text available via subscription   (Followers: 20, SJR: 0.791, CiteScore: 2)
Advances in Psychology     Full-text available via subscription   (Followers: 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: 433, 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: 37, SJR: 2.208, CiteScore: 4)
Advances in Veterinary Medicine     Full-text available via subscription   (Followers: 20)
Advances in Veterinary Science and Comparative Medicine     Full-text available via subscription   (Followers: 15)
Advances in Virus Research     Full-text available via subscription   (Followers: 6, SJR: 2.262, CiteScore: 5)
Advances in Water Resources     Hybrid Journal   (Followers: 54, SJR: 1.551, CiteScore: 3)
Aeolian Research     Hybrid Journal   (Followers: 6, SJR: 1.117, CiteScore: 3)
Aerospace Science and Technology     Hybrid Journal   (Followers: 388, 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: 488, SJR: 1.238, CiteScore: 3)
Agri Gene     Hybrid Journal   (Followers: 1, SJR: 0.13, CiteScore: 0)
Agricultural and Forest Meteorology     Hybrid Journal   (Followers: 18, SJR: 1.818, CiteScore: 5)
Agricultural Systems     Hybrid Journal   (Followers: 32, SJR: 1.156, CiteScore: 4)
Agricultural Water Management     Hybrid Journal   (Followers: 45, 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: 54, SJR: 4.66, CiteScore: 10)
Alzheimer's & Dementia: Diagnosis, Assessment & Disease Monitoring     Open Access   (Followers: 6, SJR: 1.796, CiteScore: 4)
Alzheimer's & Dementia: Translational Research & Clinical Interventions     Open Access   (Followers: 6, SJR: 1.108, CiteScore: 3)
Ambulatory Pediatrics     Hybrid Journal   (Followers: 5)
American Heart J.     Hybrid Journal   (Followers: 58, SJR: 3.267, CiteScore: 4)
American J. of Cardiology     Hybrid Journal   (Followers: 67, SJR: 1.93, CiteScore: 3)
American J. of Emergency Medicine     Hybrid Journal   (Followers: 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: 37, SJR: 2.973, CiteScore: 4)
American J. of Medicine     Hybrid Journal   (Followers: 50)
American J. of Medicine Supplements     Full-text available via subscription   (Followers: 3, SJR: 1.967, CiteScore: 2)
American J. of Obstetrics and Gynecology     Hybrid Journal   (Followers: 272, 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: 217, 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: 230, 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: 325  
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 1359-6454
Published by Elsevier Homepage  [3177 journals]
  • Dislocation-induced Breakthrough of Strength and Ductility Trade-off in a
           Non-equiatomic High-Entropy Alloy
    • Abstract: Publication date: Available online 30 November 2019Source: Acta MaterialiaAuthor(s): Wenqi Guo, Jing Su, Wenjun Lu, Christian H. Liebscher, Christoph Kirchlechner, Yuji Ikeda, Fritz Körmann, Xuan Liu, Yunfei Xue, Gerhard DehmIn conventional metallic materials, strength and ductility are mutually exclusive, referred to as strength-ductility trade-off. Here, we demonstrate an approach to improve the strength and ductility simultaneously by introducing micro-banding and the accumulation of a high density of dislocations in single-phase high-entropy alloys (HEAs). We prepare two compositions (Cr10Mn50Fe20Co10Ni10 and Cr10Mn10Fe60Co10Ni10) with distinctive different stacking fault energies (SFEs) as experimental materials. The strength and ductility of the Cr10Mn50Fe20Co10Ni10 HEA are improved concurrently by grain refinement from 347.5±216.1 µm to 18.3±9.3 µm. The ultimate tensile strength increases from 543±4 MPa to 621±8 MPa and the elongation to failure enhances from 43±2% to 55±1%. To reveal the underlying deformation mechanisms responsible for such a strength-ductility synergy, the microstructural evolution upon loading is investigated by electron microscopy techniques. The dominant deformation mechanism observed for the Cr10Mn50Fe20Co10Ni10 HEA is the activation of micro-bands, which act both as dislocation sources and dislocation barriers, eventually, leading to the formation of dislocation cell structures. By decreasing grain size, much finer dislocation cell structures develop, which are responsible for the improvement in work hardening rate at higher strains (>7 %) and thus for the increase in both strength and ductility. In order to drive guidelines for designing advanced HEAs by tailoring their SFE and grain size, we compute the SFEs of Cr10MnxFe70–xCo10Ni10 (10 ≤ x ≤ 60) based on first principles calculations. Based on these results the overall changes on deformation mechanism can be explained by the influence of Mn on the SFE.Graphical Image, graphical abstract
  • Crossing twin of Ni-Mn-Ga 7M martensite induced by thermo-mechanical
    • Abstract: Publication date: Available online 29 November 2019Source: Acta MaterialiaAuthor(s): Zongbin Li, Dong Li, Jiaxing Chen, Bo Yang, Naifu Zou, Yudong Zhang, Claude Esling, Xiang Zhao, Liang ZuoThermo-mechanical treatment is an effective way to tune the microstructure and enhance the output strain of NiMn-based magnetic shape memory alloys through field driven variant reorientation or structural transformation. Comprehensive knowledge on the microstructural feature and related transformation crystallography is of great importance for the microstructure control and property optimization. In this work, we demonstrate the crossing twin of sever-layered modulated (7M) martensite assembled in herringbone shape induced by thermo-mechanical treatment in a directionally solidified Ni50Mn30Ga20 alloy. Based on the electron backscatter diffraction (EBSD) measurements, it is found that the crossing twin is composed of four 7M martensite variants with two mutually crossing systems of twin, i.e., type I twin and compound twin. The type I twin in the crossing twin, being with {1 –2 10}7M as the twinning plane, is quite different from that of the self-accommodated martensite (i.e., {1 –2 –10}7M as the twinning plane). A novel transformation orientation relationship between austenite and 7M martensite in crossing twin is resolved to be {1 0 1}A//{1 –2 10}7M and A//7M, in contrast to that of self-accommodated case, i.e., {1 0 1}A//{1 –2 –10}7M and A//7M. By using the experimentally determined orientation relationship between two phases to construct the deformation gradient matrix, the underlying mechanism for the selection of twinning system is further discussed.Graphical abstractImage, graphical abstract
  • Interactions between Zirconia-Yttria-Tantala Thermal Barrier Oxides and
           Silicate Melts
    • Abstract: Publication date: Available online 29 November 2019Source: Acta MaterialiaAuthor(s): Najeb M. Abdul-Jabbar, Abel N. Fernandez, R. Wesley Jackson, Daesung Park, Carlos G. LeviThe effects of TaO2.5 additions to Y4Zr3O12 (YZO) on its interactions with calcium-magnesium alumino-silicate (CMAS) melts are investigated. YZO is a candidate material for the protection of novel ZrO2-YO1.5-TaO2.5 based thermal barrier coatings (TBCs) against CMAS attack, but thermochemical considerations dictate that YZO contain TaO2.5 for interfacial compatibility. Oxides based on Y4Zr3-xTaxO12+0.5x (x=0, 0.7, 1.4) were mixed with various ternary and quinary silicate compositions ranging in Ca:Si ratio from 0.35 to 1.13 and equilibrated at 1300°C. The results show that all combinations form the desired apatite phase except for the mixture of the higher Ta5+ content with the lowest Ca:Si melt. It is found that the addition of Ta5+ increases the capture of Y3+ by the reprecipitated fluorite, decreasing the amount available to form apatite and eventually suppressing it at low Ca:Si. The effect is ascribed to the attractive interactions between Y3+ and Ta5+ in solid solution, similar to that found in tetragonal zirconia, but with concurrent incorporation of anion vacancies associated with excess Y3+ that stabilize the cubic form. As the Ca2+ content of the melt increases, so does the amount of Ca2+ incorporated in fluorite. The latter also interacts attractively with Ta5+ and leads to a reduction of captured Y3+ in fluorite as the Ca content in the reacting deposit increases. This serves to increase the amount of Y3+ available in the melt to form apatite in TBC-CMAS interactions.Graphical abstractImage, graphical abstract
  • The significance of strain weakening and self-annealing in a superplastic
           Bi-Sn eutectic alloy processed by high-pressure torsion
    • Abstract: Publication date: Available online 29 November 2019Source: Acta MaterialiaAuthor(s): Chuan Ting Wang, Yong He, Terence G. LangdonA cast Bi-Sn eutectic alloy was processed by high-pressure torsion (HPT) at room temperature and stored at room temperature for durations of up to 91 days in order to investigate the effect of self-annealing. The HPT processing produces grain refinement but hardness measurements show there is strain softening with lower microhardness values than in the initial as-cast material and with hardness values that decrease with increasing amounts of imposed torsional strain. This softening is attributed to a loss of precipitates within the Sn and Bi phases during the processing operation. In self-annealing at room temperature, the microhardness increases significantly due to reprecipitation and there is also a minor increase in grain size with increasing time of storage. It is demonstrated by tensile testing that the HPT-processed Bi-Sn alloy exhibits superplastic behavior with elongations of up to>1000% after storage for 35 days and with an associated strain rate sensitivity close to ∼0.5. Grain boundary sliding plays an important role in superplastic flow and it is shown that maximum sliding occurs on the Bi-Bi interfaces where this is consistent with estimates of the coefficients for grain boundary diffusion in the Bi-Sn alloy.Graphical abstractImage, graphical abstract
  • Microstructural control in metal laser powder bed fusion additive
           manufacturing using laser beam shaping strategy
    • Abstract: Publication date: Available online 29 November 2019Source: Acta MaterialiaAuthor(s): Rongpei Shi, Saad A. Khairallah, Tien T. Roehling, Tae Wook Heo, Joseph T. McKeown, Manyalibo J. MatthewsAdditive manufacturing (AM) promises to revolutionize manufacturing by producing complex parts with tailored mechanical properties through local microstructure control. The main challenge is to control or prevent columnar (elongated) growth morphology which is prevalent in AM parts. Here, we elucidate mechanisms of microstructure control that promote favorable equiaxed grains (aspect ratio close to 1) using a laser beam shaping strategy. This requires an accurate thermal profile that is only captured using advanced predictive simulation that couples full laser ray tracing, ultra-fast hydrodynamic melt flow and the cellular automata method for grain growth. We investigate columnar to equiaxed microstructure transition during single-track laser powder bed fusion processing of 316L stainless steel using Gaussian (circular) and elliptical (transverse and longitudinal) laser beam shapes. We demonstrate that the propensity to produce equiaxed grains through nucleation events correlates with large beam width as delivered by an elliptical transverse laser beam. In addition, we reveal different microstructure evolution mechanisms during transient states such as at start and end of a scan track when the laser is respectively turned on and off. Columnar growth is hard to prevent at the start of a track and the growth morphology in the absence of heat input is dictated by the melt pool width and depth achieved and the degree of thermal undercooling. We expect this fundamental understanding of the physics of local beam shaping for microstructural control would have implications on future complex beam shape designs as well as beam modulation.Graphical abstractImage, graphical abstract
  • A Decohesion Pathway for Hydrogen Embrittlement in Nickel: Mechanism and
           Quantitative Prediction
    • Abstract: Publication date: Available online 29 November 2019Source: Acta MaterialiaAuthor(s): A. Tehranchi, X. Zhou, W.A. CurtinHydrogen embrittlement (HE) is a ubiquitous and catastrophic mode of fracture in metals. Here, embrittlement is considered as an intrinsic ductile-brittle transition at the crack tip, where H at the crack tip can reduce the stress intensity KIc for cleavage below the value KIe required for ductile dislocation emission and blunting. Specifically, cleavage fracture along (111) planes in Ni occurs due to the formation of just 3 planar layers of H interstitial occupation at a sharp crack tip. During the cleavage process, the sub-surface H in the upper and lower layers can rapidly diffuse to the fracture surface, lowering the net fracture free energy to KIc 
  • Interplay between internal stresses and matrix stiffness influences
           hydrothermal ageing behaviour of zirconia-toughened-alumina
    • Abstract: Publication date: Available online 29 November 2019Source: Acta MaterialiaAuthor(s): C. Wei, G. Montagnac, B. Reynard, N. Le Roux, L. GremillardZirconia based bio-ceramics such as ZTA (Zirconia-toughened alumina) composite are often used in orthopaedic and dental implants. The hydrothermal ageing behaviour of zirconia in humid environments is an important factor affecting the lifetime of these biomaterials. It is a tetragonal-to-monoclinic phase transformation that has been shown to be both temperature- and stress-dependant. Here, ZTA composites with zirconia inclusion particles of different sizes were prepared. The influence of zirconia inclusions on internal stresses and thus on ageing behaviour was studied by Raman micro-spectroscopy and X-ray diffraction. The results show that the ageing kinetics result from a balance between stiffer matrix and increased tensile stress in the zirconia inclusion as the local alumina fraction increase. As a result, the maximum monoclinic fraction formed after1200 h ageing at 134°C decreases with increasing alumina content, showing the predominance of the stabilization of tetragonal particles by a stiff matrix over their de-stabilization by tensile stress.Graphical abstractImage, graphical abstract
  • Solute segregation induced sandwich structure in Al-Cu(-Au) alloys
    • Abstract: Publication date: 1 February 2020Source: Acta Materialia, Volume 184Author(s): Yunhe Zheng, Yixian Liu, Nick Wilson, Shiqi Liu, Xiaojun Zhao, Houwen Chen, Jinfeng Li, Ziqiao Zheng, Laure Bourgeois, Jian-Feng NieIn this work atomic-resolution techniques of high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDXS)-STEM and first-principles calculations have been combined to study a hitherto unreported sandwich structure formed in aged samples of Al-Cu and Al-Cu-Au alloys. This sandwich structure comprises a stack of regularly spaced plates of metastable precipitate phases of GP zones, θ″ and θ′. Within the sandwich structure, the separation between the broad surface of θ′ and its adjacent GP zone, as well as that between two neighbouring GP zones, is always three {002}α planes. This sandwich structure is observed for θ′ precipitates of various thicknesses. Based on experimental results and calculations, it is inferred that the formation of the sandwich structure is induced by the θ′/Al interfacial segregation of Cu atoms, rather than the misfit associated with θ′ formation. In the sandwich structure formed in the ternary alloy, Au atoms distribute mainly in the central part of θ′ plate, but not at the θ′/Al interface or in the GP zone. Calculations confirm the experimental observations and further indicate that, energetically, Au atoms prefer to substitute for Cu, rather than Al, atoms within θ′.Graphical abstractImage, graphical abstract
  • The significance of spatial length scales and solute segregation in
           strengthening rapid solidification microstructures of 316L stainless steel
    • Abstract: Publication date: 1 February 2020Source: Acta Materialia, Volume 184Author(s): Tatu Pinomaa, Matti Lindroos, Martin Walbrühl, Nikolas Provatas, Anssi LaukkanenSelective laser melting (SLM) can produce outstanding mechanical properties in 316L stainless steel. Nonetheless, the technique can lead to considerable variation in quality. This reflects an incomplete understanding and control of the process-structure-properties linkage. This paper demonstrates how length-scale informed micromechanical behavior can be linked to solidification microstructures and how these structures depend on SLM process conditions. This linkage is produced by sequential phase field and crystal plasticity simulations. Rapid solidification is described with a recent quantitative phase field model with solute trapping kinetics, where a range of process conditions are considered in terms of thermal gradients and pulling speeds. The predicted morphological transitions (dendritic-cellular-planar) are consistent with experiments, including segregation-free microstructures, which emerge in planar growth conditions. The predicted cell spacing vs. cooling rate data are also consistent with experiments. The simulated cellular structures produced through phase field modeling are then analyzed with a Cosserat crystal plasticity model with calibrated length-scale and hardening effects and with a solid solution strengthening description that depends on the local microsegregation. It is found that the length scale characteristics and solute segregation greatly influence the overall hardening behavior and affect plastic localization and the evolution of geometrically necessary dislocation (GND) type hardening. Our results suggest that the material strength of SLM 316L steel is more sensitive to cell spacing (microstructural length scale) than to the magnitude of solute segregation. Pulling speed (solidification velocity) is identified as the main process condition determining the material micromechanical behavior. Further analysis of idealized polycrystalline structures demonstrated that plastic incompatibilities and subgrain cell interactions with grain boundaries lead to notable strengthening. The presented sequential phase field-crystal plasticity modeling scheme is a proof-of-concept for systematically investigating and discovering new compositions, process conditions and microstructures for SLM.Graphical abstractGraphical abstract for this article
  • 1 x Sr x Al2Si2O8:1%Eu 2 + , 1%Pr 3 + +anorthite&rft.title=Acta+Materialia&rft.issn=1359-6454&">Relating structural phase transitions to mechanoluminescence: The case of
           the Ca 1 − x Sr x Al2Si2O8:1%Eu 2 + , 1%Pr 3 +
    • Abstract: Publication date: 15 January 2020Source: Acta Materialia, Volume 183Author(s): Ang Feng, Simon Michels, Alfredo Lamberti, Wim Van Paepegem, Philippe F. SmetThe 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
  • Characterization of polyhedral nano-oxides and helium bubbles in an
           annealed nanostructured ferritic alloy
    • Abstract: Publication date: 15 January 2020Source: Acta Materialia, Volume 183Author(s): Tiberiu Stan, Yuan Wu, Jim Ciston, Takuya Yamamoto, G. Robert OdetteNanostructured ferritic alloys (NFAs) contain an ultra-high density of 2–4 nm fcc pyrochlore Y2Ti2O7 nano-oxides (NOs) embedded in a bcc Fe-14Cr ferritic matrix. Characterization of helium interactions with NOs and associated Fe-Y2Ti2O7 interfaces is important to the development of structural materials for nuclear fusion and fission applications. A benchmark 14YWT NFA was first annealed to coarsen the NOs, then insoluble helium was implanted at 700 °C to produce a high number density of bubbles. High-resolution scanning transmission electron microscopy characterization shows two dominant Fe-Y2Ti2O7 crystallographic orientation relationships (cube-on-edge and cube-on-cube). The smallest NOs (≈ 2 nm) are associated with the smaller bubbles (≈ 1.5 nm), while some of the largest NOs (> 6 nm) have larger, and sometimes multiple, bubbles. NO corner {111} facets are the preferred sites for He bubble nucleation. A refined sequence of events for He trapping and bubble formation is presented. These observations offer new insight on He management in NFAs, and provide a foundation for detailed modeling studies.Graphical abstractImage, graphical abstract
  • Hafnia-doped silicon bond coats manufactured by PVD for SiC/SiC CMCs
    • Abstract: Publication date: 15 January 2020Source: Acta Materialia, Volume 183Author(s): Ronja Anton, Vito Leisner, Philipp Watermeyer, Michael Engstler, Uwe SchulzSiC/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 crystallised 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 abstractImage, graphical abstract
  • { 10 1 ¯ 2 } +twin+banding+in+magnesium&rft.title=Acta+Materialia&rft.issn=1359-6454&">Characterization and modeling of { 10 1 ¯ 2 } twin banding in magnesium
    • Abstract: Publication date: 15 January 2020Source: Acta Materialia, Volume 183Author(s): YubRaj Paudel, Joseph Indeck, Kavan Hazeli, Matthew W. Priddy, Kaan Inal, Hongjoo Rhee, Christopher D. Barrett, Wilburn R. Whittington, Krista R. Limmer, Haitham El KadiriWe experimentally and numerically examined the localization behavior of {101¯2} extension twins in strongly and weakly textured AZ31 and ZEK100 magnesium alloys as they depart from the surface toward the center of the sheet under the mechanical action of three-point bending. Strain measurement analysis using digital image correlation revealed that twin banding occurs only in the sharply textured AZ31, but with a characteristic spacing between the bands which decreases with the twin height and applied strain. ZEK100 did not exhibit any ordered localization of bands and behaved as a “true” polycrystal. This finding is in agreement with micromechanical calculations by the present authors . We show that full-field plasticity finite element crystal plasticity model was unable to capture many important 2D features of the twin banding. This suggest that twin shear transfer, non-local effects, and the third dimension effects should be included into this model.Graphical abstractGraphical abstract for this article
  • The effect of alloying elements on cementite coarsening during martensite
    • Abstract: Publication date: 15 January 2020Source: Acta Materialia, Volume 183Author(s): Y.X. Wu, W.W. Sun, X. Gao, M.J. Styles, A. Arlazarov, C.R. HutchinsonThe effects of anti-segregating elements (Si and Al) and segregating elements (Cr and Mo) on cementite precipitation during tempering of Fe-C-Mn martensites have been quantitatively investigated in terms of both cementite chemistry and particle size evolution. Si and Al additions both accelerate the Mn partitioning kinetics and decelerate the particle growth kinetics. They are trapped in cementite during the initial stage of precipitation but are rejected from cementite after prolonged tempering. By comparison, small Cr and Mo additions do not strongly affect the Mn partitioning kinetics or particle growth kinetics, even though both Cr and Mo partition to cementite. The experimental measurements of cementite coarsening with time-dependent chemistry evolution are compared with a cementite coarsening model recently proposed by Wu et al. that has been generalised to multicomponent steels. The model can describe well the partitioning kinetics of segregating elements Mn/Cr/Mo and cementite particle growth kinetics simultaneously in a range of higher order systems. At 600 °C, the model works very well under local equilibrium (LE) and volume diffusion assumptions. The retardation by Si/Al on cementite coarsening and resulting accelerated Mn partitioning can be understood by the kinetic barrier from the migration of Si/Al spikes in front of growing cementite. At lower temperatures of 500 °C and 400 °C, the model can also reasonably well describe the Mn/Cr/Mo partitioning kinetics (and particle growth kinetics). However, to better describe the particle growth kinetics, the interfacial condition has to be treated with a deviation from LE due to Si/Al solute trapping. The Cr and Mo mobilities in cementite are enhanced by the same factor as for Mn mobility in cementite previously tuned by Wu et al., and the simulated Mn/Cr/Mo partitioning kinetics indicate these substitutional diffusion data are reasonably well estimated, which is further demonstrated in the simulation of Al rejection kinetics.Graphical abstractImage, graphical abstract
  • Growth-twins in CrN/AlN multilayers induced by hetero-phase interfaces
    • Abstract: Publication date: Available online 29 November 2019Source: Acta MaterialiaAuthor(s): Zhuo Chen, Qinqin Shao, Matthias Bartosik, Paul H. Mayrhofer, Hong Chen, Zaoli ZhangWe carry out a detailed transmission electron microscopy study of growth-twins in a high stacking-fault energy transition-metal nitride (TMN) multilayer comprising of 4.2 nm-thin CrN and 1 to ∼8 nm-thin AlN layers. A high density of rock-salt TMN twins with Σ3{112} incoherent twin boundaries (ITB) were found in the {111} {0002} textured film area near the substrate. The extensive high-resolution transmission electron microscopy (HRTEM) observations reveal that rock-salt TMN twins with ITBs are frequently formed in wurtzite {0002} interface with a single-atomic-layer terrace (1 × d{0002}w-AlN). However, twins with ITBs were hardly observed in the wurtzite {0002} interface with a double atomic-layer terrace (2 × d{0002}w-AlN). The formation of twins with ITBs can be interpreted by the rs-CrN/w-AlN interface structure (with a mirror-symmetry)-induced energetically stable nucleation. Furthermore, we see that the growth-twins with Σ3 {111} coherent twin boundaries (CTB) appear in the non-textured film area further away from the substrate. Based on the HRTEM observations and atomic model analyses, supplemented with theoretical calculations, several nucleation modes of twins with Σ3 {112} ITB and Σ3 {111} CTB are proposed. These findings offer a new perspective on the formation mechanism of growth-twins in transition-metal nitride materials.Graphical abstractImage, graphical abstract
  • Geometrical Constraints on the Bending Deformation of Penta-Twinned Silver
    • Abstract: Publication date: Available online 28 November 2019Source: Acta MaterialiaAuthor(s): Hu Zhao, Alexander S. Eggeman, Christopher P. Race, Brian DerbyPenta-twinned and single crystal silver nanowires were bent in-situ for TEM structural analysis using high spatial resolution scanning precession electron diffraction. Machine learning methods were used to determine the structure of the localised bends in the wires. Single crystal nanowires showed both dispersed large radius bend deformation and abrupt, angular bends with narrow deformation zones accommodated by a high angle boundary plane. The penta-twinned wires were found to display simple in-plane bends with narrow deformation zones in all 5 sub-crystals of the wire, with a common boundary plane. A coincident site lattice model has been developed to explain the bending angles seen in individual nanowires and the observed distribution of angles in a population of penta-twinned nanowires. The geometrical constraints of the model explain the difference in this distribution when compared to that from a population of single crystal nanowires.Graphical Image, graphical abstract
  • Investigating active slip planes in tantalum under compressive load:
           crystal plasticity and slip trace analyses of single crystals
    • Abstract: Publication date: Available online 28 November 2019Source: Acta MaterialiaAuthor(s): Hojun Lim, Jay D. Carroll, Joseph R. Michael, Corbett C. Battaile, Shuh Rong Chen, J. Matthew D. LaneActive slip systems in body centered cubic (BCC) metals remain ambiguous and controversial. In this work, active slip planes in tantalum are investigated using single crystal experiments and simulations. Four tantalum single crystals with [001], [011], [1¯11] and [1¯49] orientations along the loading direction are analyzed after quasi-static compression tests and dynamic Taylor impact tests. Mechanical behavior, deformed shape, texture evolution and slip traces of single crystals are analyzed and compared with crystal plasticity finite element simulations to investigate active slip systems in tantalum. Both experimental observations and modeling results support dominant dislocation slip along {112} planes in tantalum under compressive load.Graphical abstractGraphical abstract for this article
  • Atomic scale investigation of the crystal structure and interfaces of the
           B′ precipitate in Al-Mg-Si alloys
    • Abstract: Publication date: Available online 28 November 2019Source: Acta MaterialiaAuthor(s): Haonan Chen, Jiangbo Lu, Yi Kong, Kai Li, Tong Yang, Arno Meingast, Mingjun Yang, Qiang Lu, Yong DuABSTRACTB′ is a common type of metastable precipitate in over-aged Al-Mg-Si alloys, which is long regarded as a variation of the Q precipitate in Al-Mg-Si-Cu alloys due to the similarity between their lattice parameters. Atomic-resolution high angle annular dark field scanning transmission electron microscopy and energy dispersive X-ray elemental mapping at low beam damage conditions, as well as first-principles calculations were used to explore the atomistic structure of B′. It has a hexagonal unit cell with a space group P and lattice parameters a = 10.3(1) Å, c = 4.05 Å. The M sites in B′, which are analogous to Cu sites in the Q structure, were inferred with 50% Si atoms and 50% vacancies. The chemical nature of other sites agrees well with the model predicted by Ravi et al.. The determined model Al3Mg9Si8 has the lowest formation enthalpy and the smallest lattice misfit with the Al matrix along the [0001] growth direction. Step-like boundaries with alternately arranged Mg-Si-Mg-Si atoms were observed at the coherent interfaces (100)B′ // (10)Al. A layer of defect structures sandwiched in the B′ precipitate was found geometrically necessary to allow both (10)Al interfaces to arrange coherently in 3 dimensions and thus to relieve the strain of the surrounding matrix. The transformation from U1 → U2 → B′ was evidenced and the reverse transformation B′ → U2 nucleating at the incoherent B′/Al interfaces (150)Al is also likely. These results will provide new insight into the aging precipitation and compositional design of Al-Mg-Si(-Cu) alloys.Graphical abstractImage, graphical abstract
  • Deformation mechanisms and ductile fracture characteristics of a friction
           stir processed transformative high entropy alloy
    • Abstract: Publication date: Available online 26 November 2019Source: Acta MaterialiaAuthor(s): S. Sinha, S.S. Nene, M. Frank, K. Liu, R.A. Lebensohn, R.S. MishraDeformation mechanisms of a friction stir processed Fe40Mn20Co20Cr15Si5 transformative high entropy alloy using three different process parameters were studied to explain the microstructural dependence of tensile response in these specimens. The relative strain hardening contribution due to transformation and twinning effects was different for the three different microstructural conditions and thus resulted in different magnitude of work hardening. Crystal plasticity simulations confirmed that synergistic activity of face centered cubic and hexagonal close packed slip and twin mechanisms resulted in sustained work hardening and enhanced uniform elongation. The non-uniform ductility and extent of void nucleation and growth in these specimens were limited. Nevertheless, fractography and X-ray microscopy of fractured tensile specimens verified that microstructural flexibility induced different propensities for void growth and ductile fracture mode. Thus, alloy design induced phase stability, adaptive phase evolution due to friction stir processing, extent and kinetics of martensitic transformation and related work hardening capability of the individual phases combined together to determine the overall tensile response. The crystallographic orientation dependence of deformation induced phase transformation was also thoroughly studied. This study included quantitative determination of the crystallographic orientation where resolved shear stress on leading and trailing partial dislocations favored separation of the partials to cause transformation at a critical value of applied stress.Graphical abstractImage, graphical abstract
  • Hierarchy of Domain Reconstruction Processes due to Charged Defect
           Migration in Acceptor Doped Ferroelectrics
    • Abstract: Publication date: Available online 26 November 2019Source: Acta MaterialiaAuthor(s): Ivan S. Vorotiahin, Anna N. Morozovska, Yuri A. GenenkoEvolution of a stripe array of polarization domains triggered by the oxygen vacancy migration in an acceptor doped ferroelectric is investigated in a self-consistent manner. A comprehensive model based on the Landau-Ginzburg-Devonshire approach includes semiconductor features due to the presence of electrons and holes, and effects of electrostriction and flexoelectricity especially significant near the free surface and domain walls. A domain array spontaneously formed in the absence of an external field is shown to undergo a reconstruction in the course of the gradual oxygen vacancy migration driven by the depolarization fields. The charge defect accumulation near the free ferroelectric surface causes a series of phenomena: (i) symmetry breaking between the positive and negative c-domains, (ii) appearance of an effective dipole layer at the free surface followed by the formation of a surface electrostatic potential, (iii) tilting and recharging of the domain walls, especially pronounced at higher acceptor concentrations. An internal bias field determined by the gain in the free energy of the structure exhibits dependences of its amplitude on time and dopant concentration well comparable with available experimental results on aging in BaTiO3.Graphical Image, graphical abstract
  • Subgrain dynamics during recovery of partly recrystallized aluminium
    • Abstract: Publication date: Available online 30 October 2019Source: Acta MaterialiaAuthor(s): S.R. Ahl, H. Simons, C. Detlefs, D. Juul Jensen, H.F. PoulsenThe recovery dynamics of 498 individual subgrains within the non-recrystallized matrix of a 50% recrystallized aluminium sample is studied using high resolution X-ray diffraction at a synchrotron source. The subgrains’ change in size, orientation, axial strain and internal disorder are monitored as functions of temperature. The subgrains are surprisingly active, and the individual subgrains exhibit markedly different growth kinetics. Contradictory to common knowledge we find that the individual subgrains’ sizes change significantly even though the average size remain constant. Furthermore we observe no correlation between subgrain size and growth rate. An analysis of the volume changes of the individual subgrains indicates a broad spectrum of critical temperatures above which the subgrains grow or shrink. Possible mechanisms are discussed and related to relaxation of local stresses.Graphical Image, graphical abstract
  • Evolution of β-phase precipitates in an aluminum-magnesium alloy at
           the nanoscale
    • Abstract: Publication date: Available online 25 October 2019Source: Acta MaterialiaAuthor(s): Daniel L. Foley, Asher Leff, Andrew C. Lang, Mitra L. TaheriAluminum alloys in the 5xxx series are susceptible to sensitization due to the formation of β (Al3Mg2) at grain boundaries at moderate to low temperatures. Little is known about the mechanism of β phase formation, which is thought to be preceded by the metastable phases β’’ and β’. Using high-resolution transmission electron microscopy (HRTEM), energy dispersive x-ray spectroscopy (EDS), and precession electron diffraction (PED), we determine a parameter space for β phase precipitates at various sensitization temperatures and investigate their growth habits and local matrix strain states along grain boundaries. Our findings reveal that metastable β-related phases are present at low aging temperatures, while the equilibrium β phase is present at temperatures well above the previously described solvus of similar alloys. Furthermore, the phases were found to prefer particular grain boundary planes and contribute to the local grain boundary strain state differently. Overall, these findings present a unified view of β phase evolution and its contribution to lattice strain environments in aluminum magnesium alloys, which serves as a foundation for use in a range of temperatures and environments.Graphical Image, graphical abstract
  • Solid solution formation in Mg2(Si,Sn) and shape of the
           miscibility gap
    • Abstract: Publication date: Available online 25 November 2019Source: Acta MaterialiaAuthor(s): M. Yasseri, A. Sankhla, H. Kamila, R. Orenstein, D.Y.N. Truong, N. Farahi, J. de Boor, E. MuellerInvestigation of the thermochemical stability of Mg2(Si,Sn) thermoelectric materials is crucial for further development of thermoelectric modules. There is a miscibility gap reported for the quasibinary Mg2Si–Mg2Sn series, though the exact compositions of its limits are disputed. Gaining a better understanding of intersolubility limits in Mg2(Si,Sn) is important for further optimization of material performance by exploiting the gap-induced phase segregation. For a better understanding on the boundaries of the miscibility gap below 700°C, two approaches were taken to provide evidence of thermodynamic stable phases and, hereby, monitor the borders of the miscibility gap. The approaches cover the homogenization of Mg2SixSn1-x at 700°C and diffusion couple experiments at 600°C, 525°C, and 450°C. For 600°C we find two ranges where Mg2Si and Mg2Sn are not miscible, namely x = 0.35 ± 0.05 and x = 0.75 ± 0.05 for miscibility gap I and 0.85 ± 0.05 < x < 0.95 ± 0.05 for the second gap. The deduced complex temperature dependence of the mixing / demixing behavior helps to understand the previously observed, apparently contradicting experimental results. We also show that there is no demixing for the compositions with the best thermoelectric properties at temperatures above 700°C.Graphical Image, graphical abstract
  • Synthesis of α”-Fe16N2 foils with an ultralow temperature coefficient
           of coercivity for rare-earth-free magnets
    • Abstract: Publication date: Available online 25 November 2019Source: Acta MaterialiaAuthor(s): Jinming Liu, Guannan Guo, Xiaowei Zhang, Fan Zhang, Bin Ma, Jian-Ping WangMagnets with a low-temperature coefficient of coercivity (TCC) have various applications in a varying temperature environment. Rare-earth magnets like NdFeB are widely used, but they usually have a large negative TCC. Here, it is first experimentally demonstrated that α”-Fe16N2 foils, as a candidate for rare-earth-free magnets, have an ultralow positive TCC (0.4 Oe/K) from 300 K to 425 K. It is two orders of magnitude smaller than that of the commercial NdFeB magnets in this temperature range. The α”-Fe16N2 foils are made from as-rolled iron foils (25 μm) by a low-temperature nitridation process (< 473 K). The microstructure of these iron foils is tuned before the nitridation by a combined oxidation and reduction process to induce pores and defects that could significantly enhance the diffusivity of nitrogen atoms in the foils. The fabricated α”-Fe16N2 foils exhibit the specific saturation magnetization up to 222 emu/g at 300 K (reduced iron foils ∼ 205 emu/g) and the coercivity 1.1 kOe. The synthesized α”-Fe16N2 foils have an ultralow TCC and a high saturation magnetization besides its usages of low-cost and environment-friendly raw materials. These combined unique features make α”-Fe16N2 a promising rare-earth-free alternative for many applications required less temperature dependence of magnetic properties.Graphical abstractIron foils (25 μm thick) are used the first time to prepared α”-Fe16N2, a promising candidate of rare-earth-free magnets at low temperature (
  • Large influence of vacancies on the elastic constants of cubic epitaxial
           tantalum nitride layers grown by reactive magnetron sputtering
    • Abstract: Publication date: Available online 25 November 2019Source: Acta MaterialiaAuthor(s): Grégory Abadias, Chen-Hui Li, Laurent Belliard, Qing Miao Hu, Nicolas Greneche, Philippe DjemiaWe report a comprehensive study on the structure and elastic constants of epitaxial cubic δ-TaxN (x≈1) layers deposited by reactive magnetron sputtering at 570°C on MgO(001), MgO(110) and MgO(111) substrates. X-ray diffraction shows that all layers are single-phase and have a single-crystal rocksalt-structure, growing with an epitaxial cube-on-cube relationship with respect to their MgO substrates. For (001)-orientation, Brillouin light scattering provides the measurement of c44 (137 ± 3 GPa) and c12 (130 ± 5 GPa) single-crystal elastic constants, while picosecond laser ultrasonics (PLU) provides the third independent one, c11 (530 ± 10 GPa). For (110)- and (111)-orientations, PLU provides selectively the following combinations, (c11+c12+2c44)/2 and (c11+2c12+4c44)/3, respectively. Electrical resistivity measurements show evidence of point defects in the films despite a composition close to 1:1 stoichiometry. Point defects (metal and nitrogen vacancies) are considered in our first-principles calculations to find closer agreement between experimental and theoretical lattice parameter, mass density, sound velocities and elastic constants. Sound velocities and elastic constants are found to be particularly sensitive to the presence of vacancies, resulting into a more compliant and less anisotropic material. While only ∼ − 1 % mismatch is found between predicted defect-free and experimental lattice parameters, huge differences of −27 % and +180 % are observed for c11 and c44, respectively. These discrepancies can be rationalized by considering the presence of point defects (metal vacancy and/or Schottky defect) in concentration ∼11% in the as-deposited TaN films, a compositional value that corresponds to the lowest formation energy of δ-TaNx structure.Graphical abstractImage, graphical abstract
  • The effect of anisotropic microstructure on the crack growth and fatigue
           overload behaviour of ultrafine-grained nickel
    • Abstract: Publication date: Available online 25 November 2019Source: Acta MaterialiaAuthor(s): W. Zhang, C.A. Simpson, T. Leitner, X. Zhang, R. Pippan, P.J. WithersChanges in crack growth rate associated with overload events during fatigue are poorly understood, especially for materials with anisotropic microstructures. Here overload fatigue tests are reported for compact tension samples cut in two different orientations from high pressure torsion disc samples. During growth the crack planes reoriented either slightly, or significantly, to align with the elongated grain structure leading to low, and high, levels of mixed mode fatigue loading respectively. In both cases the ultrafine grained microstructure led to macroscopically flat crack faces. The fatigue crack growth rate was around 3.4 times slower for the case with the high mode II component than for the low. A 100% overload was then introduced and synchrotron X-ray diffraction and digital image correlation (DIC) were applied in-situ to map the bulk crack-tip elastic strain field (plane strain) and surface displacement field (plane stress) respectively prior to, during and after overload. The high mode II case displayed a larger degree of retardation after overload. Residual stress and plasticity-induced crack closure were found to be the primary causes for the retardation as the crack grows into the overload plastic zone. Significant crack face contact was observed for the high mode II case along with significant levels of compressive stress transferred across the crack faces at minimum load. Compared with conventional (coarse) grain Ni, the ultrafine grained Ni is less retarded by overload, because of its relatively flatter crack path and higher yield stress and thus less plasticity and residual stress induced closure.Graphical abstractImage, graphical abstract
  • Half-Heusler alloys: Enhancement of ZT after severe plastic deformation
           (ultra-low thermal conductivity)
    • Abstract: Publication date: 15 January 2020Source: Acta Materialia, Volume 183Author(s): Gerda Rogl, Sanyukta Ghosh, Lei Wang, Jiri Bursik, Andriy Grytsiv, Michael Kerber, Ernst Bauer, Ramesh Chandra Mallik, Xing-Qiu Chen, Michael Zehetbauer, Peter RoglSeveral 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
  • Phase transformation induces plasticity with negligible damage in
           ceria-stabilized zirconia-based ceramics
    • Abstract: Publication date: 15 January 2020Source: Acta Materialia, Volume 183Author(s): Aléthéa Liens, Helen Reveron, Thierry Douillard, Nicholas Blanchard, Vanni Lughi, Valter Sergo, René Laquai, Bernd R. Müller, Giovanni Bruno, Sven Schomer, Tobias Fürderer, Erik Adolfsson, Nicolas Courtois, Michael Swain, Jérôme ChevalierCeramics and their composites are in general brittle materials because they are predominantly made up of ionic and covalent bonds that avoid dislocation motion at room temperature. However, a remarkable ductile behavior has been observed on newly developed 11 mol.% ceria-stabilized zirconia (11Ce-TZP) composite containing fine alumina (8 vol.% Al2O3) and elongated strontium hexa-aluminate (8 vol.% SrAl12O19) grains. The as-synthesized composite also has shown full resistance to Low Temperature Degradation (LTD), relatively high strength and exceptionally high Weibull modulus, allowing its use in a broader range of biomedical applications. In this study, to deepen the understanding of plastic deformation in Ce-TZP based composites that could soon be used for manufacturing dental implants, different mechanical tests were applied on the material, followed by complete microstructural characterization. Distinct from pure Ce-TZP material or other zirconia-based ceramics developed in the past, the material here studied can be permanently strained without affecting the Young modulus, indicating that the ductile response of tested samples cannot be associated to damage occurrence. This ductility is related to the stress-induced tetragonal to monoclinic (t-m) zirconia phase transformation, analogue to Transformation-Induced Plasticity (TRIP) steels, where retained austenite is transformed to martensite. The aim of this study is to corroborate if the observed plasticity can be associated exclusively to the zirconia t-m phase transformation, or also to microcraking induced by the transformation. The t-m transformed-zones produced after bending and biaxial tests were examined by X-ray refraction and SEM/TEM coupled with Raman. The results revealed that the observed elastic-plastic behavior occurs without extensive microcracking, confirming a purely elastic-plastic behavior driven by the phase transformation (absence of damage).Graphical abstractImage, graphical abstract
  • Solidification-driven orientation gradients in additively manufactured
           stainless steel
    • Abstract: Publication date: 15 January 2020Source: Acta Materialia, Volume 183Author(s): Andrew T. Polonsky, William C. Lenthe, McLean P. Echlin, Veronica Livescu, George T. Gray, Tresa M. PollockA 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
  • c + a +screw+dislocations+and+implications+for+ductility+in+Mg+alloys&rft.title=Acta+Materialia&rft.issn=1359-6454&">Analysis of double cross-slip of pyramidal I 〈 c + a 〉 screw
           dislocations and implications for ductility in Mg alloys
    • Abstract: Publication date: 15 January 2020Source: Acta Materialia, Volume 183Author(s): Rasool Ahmad, Zhaoxuan Wu, W.A. CurtinSolute accelerated cross-slip of pyramidal 〈c+a〉 screw dislocations has recently been recognized as a crucial mechanism in enhancing the ductility of solid-solution Mg alloys. In pure Mg, cross-slip is ineffective owing to the energy difference between the high energy pyramidal I and low energy pyramidal II 〈c+a〉 screw dislocations. A small addition of solutes, especially rare earth (RE) elements, can reduce this energy difference and accelerate cross-slip, thus enabling enhanced ductility. With increasing solute concentrations, the pyramidal I dislocation can become energetically favorable, which switches the primary 〈c+a〉 slip plane and alters the cross-slip process. Here, the transition path and energetics for double cross-slip of pyramidal I 〈c+a〉 dislocations are analysed in the regime where the pyramidal I dislocation is energetically more favorable than the pyramidal II. This is achieved using nudged elastic band simulations on a proxy MEAM potential for Mg designed to favor the pyramidal I over pyramidal II. The minimum energy transition path for pyramidal I double cross-slip is found to initiate with cross-slip onto a pyramidal II plane followed by cross-slip onto a pyramidal I plane parallel to the original pyramidal I plane. A previous mechanistic model for ductility is then extended to higher solute concentrations where pyramidal I is favorable. The model predicts an upper limit of solute concentrations beyond which ductility again becomes poor in Mg alloys. The model predictions are consistent with limited experiments of Mg-RE alloys at high concentrations and motivate further experimental studies in the high concentration regime.Graphical abstractGraphical abstract for this article
  • Segmentation crack formation dynamics during air plasma spraying of
    • Abstract: Publication date: 15 January 2020Source: Acta Materialia, Volume 183Author(s): Shalaka V. Shinde, Edward J. Gildersleeve V, Curtis A. Johnson, Sanjay SampathAir Plasma Sprayed (APS) Yttria Stabilized Zirconia (YSZ) Thermal Barrier Coatings (TBCs) is a well-established technology in the gas turbine industry. A conventional APS TBC is a layered structure with heterogenous distribution of defects (microcracking, pores, etc.) which allow it to simultaneously possess low thermal conductivity and elastic modulus compared to its bulk counterpart. However, interfacial defects can be a source of delamination failure during thermal cycling. In addition, conventional porous coatings can experience sintering during sustained exposure, which augments failure through stiffening-induced delamination. Electron Beam Physical Vapor Deposition (EB-PVD) TBC coatings, due to their dense columnar structures, are less susceptible to both sintering and delamination. This has led to the consideration of more economically-applied APS TBCs that are dense with periodic vertical segmentation cracks. Such dense vertically cracked coatings (DVCs) have been successfully developed and implemented in gas turbine engines. These microstructures are produced in-situ through control of the process conditions with high deposition temperatures. However, the mechanism of such segmentation cracks is unclear. In this study, formation dynamics of segmentation cracks were observed through in-situ beam curvature monitoring during deposition in combination with microstructural evaluations. It was observed the initial layers of the coating are dense without segmentation cracking. As subsequent layers are deposited, periodic macrocracking initiates and typically propagates through the remaining coating thickness. The in-situ in-plane coating stress is significantly reduced after segmentation cracking begins. These results are reconciled through interpretation of thin-film fracture literature, and an initial framework to interpret the experimental observations is provided.Graphical abstractImage, graphical abstract
  • Real time observation of martensite transformation for a 0.4C low alloyed
           steel by neutron diffraction
    • Abstract: Publication date: Available online 21 November 2019Source: Acta MaterialiaAuthor(s): Yanxu Wang, Yo Tomota, Takahito Ohmura, Satoshi Morooka, Wu Gong, Stefanus HarjoA high-intensity and high-resolution neutron diffractometer with a thermomechanically controlled processing simulator was employed in-situ to investigate martensite transformation behavior with and without ausforming for a medium-carbon low-alloy steel. Serial neutron diffraction profiles have revealed that the transformation behavior could be successfully monitored during quenching with and without the ausforming process. The fresh martensite exhibits a body-centered tetragonal structure when it forms immediately below the martensite start (Ms) temperature, and its c/a ratio decreases rapidly as time elapses. The lattice parameter and the full width at half maximum of austenite peaks significantly decreases and increases upon martensite transformation, respectively. After ausforming, the data reveal that lattice parameters are larger in austenite whereas smaller in martensite compared with those in the non-ausformed case, which is ascribed to the introduced dislocations. Thus, the lattice defects affect the lattice parameter during martensite transformation. Ausforming also slightly raises the Ms temperature and increases the amount of retained austenite at room temperature as a result of different dislocation densities. The cutting-edge operant quantitative measurements with neutron diffraction for steel production is demonstrated.Graphical abstractImage, graphical abstract
  • Degradation of thermal transport properties in fine-grained isotropic
           graphite exposed to swift heavy ion beams
    • Abstract: Publication date: Available online 21 November 2019Source: Acta MaterialiaAuthor(s): Alexey Prosvetov, Georges Hamaoui, Nicolas Horny, Mihai Chirtoc, Florent Yang, Christina Trautmann, Marilena TomutIsotropic polycrystalline graphite samples were irradiated with ∼1 GeV 197Au and 238U ions of fluences up to 5 × 1013 ions/cm2. Beam-induced changes of thermophysical properties were characterized using frequency domain photothermal radiometry (PTR) and the underlying structural transformations were monitored by Raman spectroscopy. The ion range (∼60 µm) was less than the sample thickness, therefore thermal diffusivity contributions of the irradiated as well as non-irradiated layer were considered when analyzing the PTR data. At the highest applied fluences, the thermal effusivity of the damaged layer degrades down to 20% of the pristine value and the corresponding calculated values of thermal conductivity decrease from 95 Wm−1K−1 for pristine material to 4 Wm−1K−1, a value characteristic for the glassy carbon allotrope. This technique provides quantitative data on thermal properties of ion-irradiated polycrystalline graphite and is very valuable for the prediction of lifetime expectancy in long-term applications in extreme radiation environments.Graphical abstractImage, graphical abstract
  • Antiphase boundaries, magnetic domains, and magnetic vortices in Ni-Mn-Ga
           single crystals
    • Abstract: Publication date: Available online 20 November 2019Source: Acta MaterialiaAuthor(s): Marek Vronka, Ladislav Straka, Marc De Graef, Oleg HeczkoThe interplay between antiphase boundaries (APBs), magnetic domain structure and functional properties was investigated in the martensitic state of Ni-Mn-Ga single crystals which showed magnetically induced martensite reorientation (MIR) with 6% strain. The APB density was controlled by different heat treatments. The APBs and magnetic domains were observed by Lorentz transmission electron microscopy (LTEM). Slow cooling at ∼1 K/min resulted in a low density (
  • Effect of Vacancy Creation and Annihilation on Grain Boundary Motion
    • Abstract: Publication date: Available online 20 November 2019Source: Acta MaterialiaAuthor(s): G.B. McFadden, W.J. Boettinger, Y. MishinInteraction of vacancies with grain boundaries (GBs) is involved in many processes occurring in materials, including radiation damage healing, diffusional creep, and solid-state sintering. We analyze a model describing a set of processes occurring at a GB in the presence of a non-equilibrium, non-homogeneous vacancy concentration. Such processes include vacancy diffusion toward, away from, and across the GB, vacancy generation and absorption at the GB, and GB migration. Numerical calculations within this model reveal that the coupling among the different processes gives rise to interesting phenomena, such as vacancy-driven GB motion and accelerated vacancy generation/absorption due to GB motion. The key combinations of the model parameters that control the kinetic regimes of the vacancy-GB interactions are identified via a linear stability analysis. Possible applications and extensions of the model are discussed.Graphical abstractGraphical abstract for this article
  • Ultra-high strength and plasticity mediated by partial dislocations and
           defect networks: Part I: Texture effect
    • Abstract: Publication date: Available online 20 November 2019Source: Acta MaterialiaAuthor(s): Ruizhe Su, Dajla Neffati, Qiang Li, Sichuang Xue, Jaehun Cho, Jin Li, Jie Ding, Yifan Zhang, Cuncai Fan, Haiyan Wang, Yashashree Kulkarni, Xinghang ZhangDeformation mechanisms governing the strength of nanostructured metallic multilayers have been studied extensively for various applications. In general, size effect is the most effective way to tailor the mechanical strength of multilayers. Here we report that three Cu/Co multilayer systems with identical layer thickness but different types of layer interfaces exhibit drastically different mechanical behavior. In situ micropillar compression tests inside a scanning electron microscope show that coherent FCC (100) and (110) Cu/Co multilayer systems have low yield strength of about 600 MPa, and prominent shear instability. In contrast, the incoherent Cu/ HCP Co multilayers show much greater yield strength, exceeding 2.4 GPa, and significant plasticity manifested by a cap on the deformed pillar. Molecular dynamics simulations reveal an unexpected interplay between pre-existing twin boundaries in Cu, stacking faults in HCP Co, and incoherent layer interfaces, which leads to partial dislocation dominated high strength, and outstanding plasticity. This study provides fresh insights for the design of strong, deformable nanocomposites by using a defect network consisting of twin boundaries, stacking faults and layer interfaces.Graphic abstractImage, graphical abstract
  • Characterisation of the oxidation and creep behaviour of novel Mo-Si-Ti
    • Abstract: Publication date: Available online 20 November 2019Source: Acta MaterialiaAuthor(s): Susanne Obert, Alexander Kauffmann, Martin HeilmaierThe oxidation and creep behaviour of novel eutectic-eutectoid Mo-Si-Ti alloys were studied and compared to previously investigated entirely eutectic Mo-20Si-52.8Ti (at%) and eutectoid Mo-21Si-34Ti reference alloys [Schliephake et al. in Intermetallics 104 (2019) 133-142]. While the latter reference alloys showed either outstanding oxidation behaviour in the temperature range of 800 to 1200°C (eutectic alloy) or reasonable creep resistance (eutectoid alloy), a combination of both was successfully achieved in a Ti-rich alloy variant (Mo-21Si-43.4Ti). The ubiquitous catastrophic oxidation (“pesting”) of Mo-based alloys at 800°C is suppressed in this alloy and reasonable oxidation resistance at higher temperatures is observed. For the first time, the unexpected oxidation resistance of the alloys exhibiting eutectic volume fractions of more than 50 vol% is rationalised by a systematic deconvolution of mass gain by scale formation and mass loss by evaporation of volatile species. Furthermore, creep is revealed to be based on similar creep mechanisms throughout the alloy series. Therefore, the observed improvement in creep resistance of the pesting-resistant Ti-rich alloy variant over the eutectic alloy is attributed to the decreasing homologous temperature when testing both at 1200°C.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. SteinLaves 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
  • Electrical conduction mechanism of rare-earth calcium oxyborate high
           temperature piezoelectric crystals
    • Abstract: Publication date: 15 January 2020Source: Acta Materialia, Volume 183Author(s): Shiwei Tian, Lili Li, Xinyu Lu, Fapeng Yu, Yanlu Li, Chao Jiang, Xiulan Duan, Zhengping Wang, Shujun Zhang, Xian ZhaoThe electrical conduction behavior of piezoelectric crystals is critical in the design of piezoelectric sensors for use at elevated temperatures. The electrical resistivity and dielectric properties of rare-earth calcium oxyborate crystals ReCa4O(BO3)3 (ReCOB, Re = Y, Gd and Pr) are investigated in this report. The relationships between the electronic structures and electrical properties are then determined using X-ray photoelectron spectra and first principle calculations. Among the ReCOB type crystals, YCOB is found to possess the highest electrical resistivity and lowest dielectric loss. Nonlinearity of the electrical resistivity as a function of temperature for ReCOB crystals is then confirmed, corresponding to different conduction mechanisms. It is also revealed that the electrical conductivity of ReCOB type crystals is heavily influenced by oxygen vacancy defects at relatively lower temperatures (below ∼600 °C), while both vacancy defects and band gap contribute to conductivity at elevated temperatures (above ∼600 °C).Graphical abstractThe electrical conduction mechanism of ReCa4O(BO3)3 (Re: rare-earth elements and Y) type high-temperature piezoelectric crystals.Image, graphical abstract
  • Impurity and Texture Driven HCP-to-FCC Transformations in Ti-X Thin Films
           During in situ TEM Annealing and FIB Milling
    • Abstract: Publication date: Available online 20 November 2019Source: Acta MaterialiaAuthor(s): Rachel Traylor, Ruopeng Zhang, Josh Kacher, James O. Douglas, Paul A.J. Bagot, Andrew M. MinorA hexagonal close-packed (HCP) to face-centered cubic (FCC) phase transition has been observed in freestanding alpha-titanium (α-Ti) thin foils under two separate conditions: (1) after focused ion beam (FIB) irradiation, and (2) during in situ heating in a transmission electron microscope (TEM). The FCC phase is not found on the Ti single-component equilibrium phase diagram, however, both FCC structures were found to be stable after formation under these conditions. Here, we combine analytical TEM and Atom Probe Tomography (APT) investigations into the chemical nature of these anomalous FCC Ti-X phases. Both occurrences of the FCC phase were observed in thin films containing (initial) prismatic HCP surface plane texturing and appear to be facilitated by hydrogen and oxygen impurities. Our results suggest that the FIB-induced FCC Ti-X is a form of titanium hydride (δ-TiH2 and/or γ-TiH), while the thermally-induced FCC Ti-X appears to be tied to the incorporation of oxygen.Graphical abstractImage, graphical abstract
  • The Effect of Thermal Cycling on the Fracture Toughness of Metallic
    • Abstract: Publication date: Available online 20 November 2019Source: Acta MaterialiaAuthor(s): Jittisa Ketkaew, Rui Yamada, Hui Wang, Derek Kuldinow, Benjamin Sol Schroers, Wojciech Dmowski, Takeshi Egami, Jan SchroersA wide range of behaviors, including non-monotonic rejuvenation and relaxation, and the ability to qualitatively change the effect by varying the structural state of the glass was observed during thermal cycling of bulk metallic glasses. For this, we considered various bulk metallic glasses, Zr44Ti11Cu10Ni10Be25, Pd43Cu27Ni10P20, Pt57.5Cu14.7Ni5.3P22.5, and La55Al25Ni20, at various fictive temperatures to study the effect of thermal cycling on structure, thermal signature, and fracture toughness. For some BMGs and conditions considered here, thermal cycling results in a looser structure and an increase in fracture toughness. We found that for certain other BMGs and conditions, thermal cycling results in relaxation, reflected in a denser structure, and a decrease in fracture toughness. All these responses are non-monotonic and reveal a pronounced extremum with fracture toughness values of ± 50% of the original value, before approaching a value similar to the original value prior to thermal cycling. Such richness in response to thermal cycling suggests incompleteness of the previous picture where monotonically decreasing local stresses resulting in a homogenization of the structure with increasing cycle number. Our finding suggests that relative comparisons of various contributions including activation barriers for α-relaxation have to be considered which are also constantly changing, to decide if further cycling results in an increase or a decrease in fracture toughness. The fracture toughness’ response to thermal cycling can be correlated with the average atomic structures’ response to thermal cycling, while the thermal response does not exhibit an obvious correlation.Graphical abstractImage, graphical abstract
  • Nanoindentation-induced plasticity in cubic zirconia up to 500°C
    • Abstract: Publication date: Available online 19 November 2019Source: Acta MaterialiaAuthor(s): Hiroshi Masuda, Koji Morita, Takahito OhmuraOrientation-dependent crystal plasticity in cubic zirconia ceramics (8 mol% yttria-stabilized zirconia) was investigated from 25°C to 500°C, which is below the macroscopic brittle–ductile transition temperature. Nanoindentation experiments and subsequent electron microscope analysis revealed that dislocation activities on {001} planes predominantly governed small-scale plasticity at the grain interior for each crystal orientation in this temperature range. The maximum shear stress at yielding, τmax, necessary for plastic yielding, was comparable to the theoretical shear strength, τth, estimated from the measured elastic modulus at low temperatures, while their ratio, τmax/τth, almost monotonically decreased at elevated temperatures. This suggests that plastic yielding was predominantly mediated by atomistic shear processes at low temperatures and increasingly affected by thermally assisted processes at elevated temperatures. The orientation-dependent behavior switched in the vicinity of ∼ 25° from impression axes. In the near-[001] axis, plastic yielding occurred more smoothly at lower stresses, which can be attributed for the surface nucleation and forest-cutting interaction of dislocations on multiple {001} slip planes along and across the impression axes. In near-[101] and [111] axes, on the other hand, yielding was more burst-like and typically accompanied by pop-in events at higher stresses, which could result from the homogeneous dislocation nucleation and free-gliding motion of dislocations on {001} planes with considerable Schmid factors for the impression axes.Graphical abstractImage, graphical abstract
  • Influence of the stress state on the cross-slip free energy barrier in Al:
           an atomistic investigation
    • Abstract: Publication date: Available online 19 November 2019Source: Acta MaterialiaAuthor(s): G. Esteban-Manzanares, R. Santos-Güemes, I. Papadimitriou, E. Martínez, J. LLorcaAbstractThe influence of the stress state on the cross-slip rate in Al was analyzed by means of molecular dynamics simulations and transition state theory. The activation energy barrier in the absence of thermal energy was determined through the nudged elastic band method while the cross-slip rates were determined using molecular dynamics simulations for different magnitudes of the Schmid stress on the cross-slip plane, and of the Escaig stresses on the cross-slip and glide planes. The enthalpy barrier and the effective attempt frequency were determined from the average rates of cross-slip obtained from the molecular dynamics simulations. It was found that the different stress states influence the cross-slip rate assuming harmonic transition state theory. Moreover, the theoretical contributions to the enthalpy barrier (configurational and due to the interaction of the applied stress with the local stress field created by the defect) were identified from the atomistic simulations while the entropic contribution to the activation energy could be estimated by the Meyer-Neldel rule. Based on these results, an analytical expression of the activation enthalpy for cross-slip in Al as a function of the different combinations of Schmid and Escaig stress states was developed and validated. This expression can be easily used in dislocation dynamics simulations to evaluate the probability of cross-slip of screw dislocation segments.
  • Size dependent strength, slip transfer and slip compatibility in
           nanotwinned silver
    • Abstract: Publication date: Available online 19 November 2019Source: Acta MaterialiaAuthor(s): Maya K. Kini, Gerhard Dehm, Christoph KirchlechnerPerfect slip transfer through single coherent Σ3 twin boundaries is known to be a cross-slip-like mechanism occurring at low stresses, which is expected to strongly depend on material properties like stacking fault energy. In the present study, we extend the argument of perfect slip transfer to (i) multiple closely spaced coherent twin boundaries in a nanotwinned thin film and (ii) to materials with very low stacking fault energy. The slip transfer is indicated by the continuity of slip steps and observed across up to 100 coherent Σ3 boundaries. The study addresses size scaling due to multiple weak obstacles for dislocation motion and discusses the underlying deformation mechanisms. The importance of strain compatibility is further extended to incoherent twin boundaries.Graphical abstractImage, graphical abstract
  • Electric-field-induced structure and domain texture evolution in
           PbZrO3-based antiferroelectric by in-situ high-energy synchrotron X-ray
    • Abstract: Publication date: Available online 19 November 2019Source: Acta MaterialiaAuthor(s): Hui Liu, Longlong Fan, Shengdong Sun, Kun Lin, Yang Ren, Xiaoli Tan, Xianran Xing, Jun ChenAntiferroelectrics (AFEs) have a great potential for modern electronic devices by virtue of the large strain during the antiferroelectric-to-ferroelectric (AFE-FE) phase transition under external electric fields. Although the fascinating macroscopic properties of AFE materials have been extensively studied, it is still unclear how the underlying structure evolution engenders their defining properties. Here we employ an electric biasing in-situ high-energy synchrotron X-ray diffraction technique to reveal the phase, domain texture, and lattice evolution in a high performance PbZrO3-based AFE material. During the reversible AFE-FE transition triggered by electric fields, the evolution of the superstructure for AFE pseudo-tetragonal and FE rhombohedral phase is found to display strong dependence on the angle with respect to the field direction. In contrast to previous prediction, it is found that there is no obvious domain reorientation in the AFE phase, when the system is far away from the AFE-FE transitions. The electric-field-induced FE rhombohedral phase exhibits an unusual microscopic behavior, distinguished from the normal one, presenting small changes in domain texture and lattice strain with electric field, and leading to a small piezoelectric response. The longitudinal, transverse, and volume strains estimated from the XRD peak profiles are well consistent with the macroscopic strain measurements. It is demonstrated that the large strain arises from the structural change associated with anisotropic lattice strain and highly preferential domain reorientation during the AFE-FE transitions. The AFE-FE switching sequence is constructed based on the present study, which provides a further understating of AFE materials.Graphical abstractImage, graphical abstract
  • On the Assessment of Creep Damage Evolution in Nickel-Based Superalloys
           Through Correlative HR-EBSD and cECCI Studies
    • Abstract: Publication date: Available online 19 November 2019Source: Acta MaterialiaAuthor(s): Sabin Sulzer, Zhuangming Li, Stefan Zaefferer, Seyed Masood Hafez Haghighat, Angus Wilkinson, Dierk Raabe, Roger ReedThe evolution of dislocation density with creep strain in single-crystal superalloys is studied quantitatively using high-resolution electron backscatter diffraction (HR-EBSD) and electron channelling contrast imaging under controlled diffraction conditions (cECCI). Data regarding dislocation density/structure is measured for deformation at 900 °C and 450 MPa up to ≈ 1% plastic strain. Effects of chemical composition are elucidated via three purpose-designed superalloys of differing rhenium and ruthenium contents. The evidence indicates that dislocation avalanching is already prevalent at plastic strains of ≈ 0.1%; thereafter, an exponential decay in the dislocation multiplication rate is indicative of self-hardening due to dislocation constriction within the matrix channels, as confirmed by the imaging. The results are rationalised using discrete dislocation dynamics modelling: a universal dislocation evolution law emerges, which will be useful for alloy design efforts.Graphical abstractImage 1
  • Uncertainty Propagation in a Multiscale CALPHAD-Reinforced Elastochemical
           Phase-field Model
    • Abstract: Publication date: Available online 18 November 2019Source: Acta MaterialiaAuthor(s): Vahid Attari, Pejman Honarmandi, Thien Duong, Daniel J. Sauceda, Douglas Allaire, Raymundo ArroyaveICME approaches provide decision support for materials design by establishing quantitative process-structure-property relations. Confidence in the decision support, however, must be achieved by establishing uncertainty bounds in ICME model chains. The quantification and propagation of uncertainty in computational materials science, however, remains a rather unexplored aspect of computational materials science approaches. Moreover, traditional uncertainty propagation frameworks tend to be limited in cases with computationally expensive simulations. A rather common and important model chain is that of CALPHAD-based thermodynamic models of phase stability coupled to phase-field models for microstructure evolution. Propagation of uncertainty in these cases is challenging not only due to the sheer computational cost of the simulations but also because of the high dimensionality of the input space. In this work, we present a framework for the quantification and propagation of uncertainty in a CALPHAD-based elastochemical phase-field model. We motivate our work by investigating the microstructure evolution in Mg2SixSn1−x thermoelectric materials. We first carry out a Markov Chain Monte Carlo-based inference of the CALPHAD model parameters for this pseudobinary system and then use advanced sampling schemes to propagate uncertainties across a high-dimensional simulation input space. Through high-throughput phase-field simulations we generate 200,000 time series of synthetic microstructures and use machine learning approaches to understand the effects of propagated uncertainties on the microstructure landscape of the system under study. The microstructure dataset has been curated in the Open Phase-field Microstructure Database (OPMD), available at abstractGraphical abstract for this article
  • Equilibrium viscosity and structural change in the Cu47.5Zr45.1Al7.4 bulk
           glass-forming liquid
    • Abstract: Publication date: Available online 18 November 2019Source: Acta MaterialiaAuthor(s): Hao-Ran Jiang, Benedikt Bochtler, Maximilian Frey, Qi Liu, Xian-Shun Wei, Yang Min, Sascha S. Riegler, Dan-Dan Liang, Ralf Busch, Jun ShenThe low temperature viscosities of Cu47.5Zr45.1Al7.4 glass-forming liquid are measured by thermomechanical analysis in three-point beam bending mode. The experimental equilibrium values are fitted with the Vogel-Fulcher-Tammann (VFT) function, and the fragility parameter of the supercooled liquid is determined to be D* = 20.6. Viscosity measurements during isothermal annealing at 710 K show an initial relaxation from the glassy state into the supercooled liquid, followed by an anomalous viscosity increase of about two orders of magnitude. In-situ transmission electron microscope (TEM) and spherical aberration-corrected TEM investigations reveal that the effects of primary crystallization, nanoscale decomposition and structural ordering might be responsible for the anomalous viscosity behavior. The change in dynamic properties of the supercooled liquid, which is reflected by the larger VFT fitting parameter D* = 41.9 of the new annealed state reached before the final crystallization, is suggested to be not only related to the nanoscale structure change, but may also be caused by the change in chemical composition of the new annealed glass due to the primary crystallization. Furthermore, possible mechanisms for the nanoscale phase separation and the formation of atomic clusters are discussed based on the results of high-resolution TEM experiments.Graphical abstractImage, graphical abstract
  • Deformation in nanocrystalline ceramics: A microstructural study of
    • Abstract: Publication date: 15 January 2020Source: Acta Materialia, Volume 183Author(s): Barak Ratzker, Avital Wagner, Maxim Sokol, Louisa Meshi, Sergey Kalabukhov, Nachum FrageContrary 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 TsujiMicrostructural 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
    • Abstract: Publication date: 15 January 2020Source: Acta Materialia, Volume 183Author(s): M. Eynbeygui, J. Arghavani, A.H. Akbarzadeh, R. NaghdabadiThe 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. WenmanDensity 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. BoyceDuctile 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 ArroyaveMAX 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. SteinExtended 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. MakineniWe 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 HuangSamples 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. LiThe 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
  • 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. PéczThe 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 RaabeThe 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 KeBy 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 ZhangWhereas 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 SongTi 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. ManiattyMicrostructure 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
    • Abstract: Publication date: Available online 12 November 2019Source: Acta MaterialiaAuthor(s): Jia-Mian Hu, Tiannan Yang, Long-Qing ChenUnderstanding 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 EisenlohrCrystallographic 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
    • 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 SnyderNanoporous 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 KirchlechnerWe 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. RitchieBulk-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.GraphicalImage, 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. DemkowiczSevere 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. BakerTantalum 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
  • 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 ZhangDuplex 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-CastilloA 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 SehitogluThis 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 JiangDeformation 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
  • 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 ChenThe 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
  • 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. FrankelThis 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
  • 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. EvansThe 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 CollianderNi-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-RocaTin 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
    • 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 WangStrain 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
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