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Showing 1 - 200 of 3183 Journals sorted alphabetically
Academic Pediatrics     Hybrid Journal   (Followers: 38, SJR: 1.655, CiteScore: 2)
Academic Radiology     Hybrid Journal   (Followers: 26, SJR: 1.015, CiteScore: 2)
Accident Analysis & Prevention     Partially Free   (Followers: 102, SJR: 1.462, CiteScore: 3)
Accounting Forum     Hybrid Journal   (Followers: 28, SJR: 0.932, CiteScore: 2)
Accounting, Organizations and Society     Hybrid Journal   (Followers: 40, 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: 436, SJR: 0.758, CiteScore: 2)
Acta Automatica Sinica     Full-text available via subscription   (Followers: 2)
Acta Biomaterialia     Hybrid Journal   (Followers: 28, SJR: 1.967, CiteScore: 7)
Acta Colombiana de Cuidado Intensivo     Full-text available via subscription   (Followers: 3)
Acta de Investigación Psicológica     Open Access   (Followers: 3)
Acta Ecologica Sinica     Open Access   (Followers: 11, SJR: 0.18, CiteScore: 1)
Acta Histochemica     Hybrid Journal   (Followers: 5, SJR: 0.661, CiteScore: 2)
Acta Materialia     Hybrid Journal   (Followers: 311, 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: 1, SJR: 1.793, CiteScore: 6)
Acta Poética     Open Access   (Followers: 4, SJR: 0.101, CiteScore: 0)
Acta Psychologica     Hybrid Journal   (Followers: 26, SJR: 1.331, CiteScore: 2)
Acta Sociológica     Open Access   (Followers: 1)
Acta Tropica     Hybrid Journal   (Followers: 6, SJR: 1.052, CiteScore: 2)
Acta Urológica Portuguesa     Open Access  
Actas Dermo-Sifiliograficas     Full-text available via subscription   (Followers: 3, SJR: 0.374, CiteScore: 1)
Actas Dermo-Sifiliográficas (English Edition)     Full-text available via subscription   (Followers: 2)
Actas Urológicas Españolas     Full-text available via subscription   (Followers: 3, SJR: 0.344, CiteScore: 1)
Actas Urológicas Españolas (English Edition)     Full-text available via subscription   (Followers: 1)
Actualites Pharmaceutiques     Full-text available via subscription   (Followers: 7, SJR: 0.19, CiteScore: 0)
Actualites Pharmaceutiques Hospitalieres     Full-text available via subscription   (Followers: 3)
Acupuncture and Related Therapies     Hybrid Journal   (Followers: 8)
Acute Pain     Full-text available via subscription   (Followers: 15, SJR: 2.671, CiteScore: 5)
Ad Hoc Networks     Hybrid Journal   (Followers: 11, SJR: 0.53, CiteScore: 4)
Addictive Behaviors     Hybrid Journal   (Followers: 18, SJR: 1.29, CiteScore: 3)
Addictive Behaviors Reports     Open Access   (Followers: 9, SJR: 0.755, CiteScore: 2)
Additive Manufacturing     Hybrid Journal   (Followers: 11, SJR: 2.611, CiteScore: 8)
Additives for Polymers     Full-text available via subscription   (Followers: 23)
Advanced Drug Delivery Reviews     Hybrid Journal   (Followers: 183, 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: 29, SJR: 0.126, CiteScore: 0)
Advances in Antiviral Drug Design     Full-text available via subscription   (Followers: 2)
Advances in Applied Mathematics     Full-text available via subscription   (Followers: 12, SJR: 0.992, CiteScore: 1)
Advances in Applied Mechanics     Full-text available via subscription   (Followers: 12, SJR: 1.551, CiteScore: 4)
Advances in Applied Microbiology     Full-text available via subscription   (Followers: 24, SJR: 2.089, CiteScore: 5)
Advances In Atomic, Molecular, and Optical Physics     Full-text available via subscription   (Followers: 15, SJR: 0.572, CiteScore: 2)
Advances in Biological Regulation     Hybrid Journal   (Followers: 4, SJR: 2.61, CiteScore: 7)
Advances in Botanical Research     Full-text available via subscription   (Followers: 2, SJR: 0.686, CiteScore: 2)
Advances in Cancer Research     Full-text available via subscription   (Followers: 34, SJR: 3.043, CiteScore: 6)
Advances in Carbohydrate Chemistry and Biochemistry     Full-text available via subscription   (Followers: 9, SJR: 1.453, CiteScore: 2)
Advances in Catalysis     Full-text available via subscription   (Followers: 5, SJR: 1.992, CiteScore: 5)
Advances in Cell Aging and Gerontology     Full-text available via subscription   (Followers: 5)
Advances in Cellular and Molecular Biology of Membranes and Organelles     Full-text available via subscription   (Followers: 14)
Advances in Chemical Engineering     Full-text available via subscription   (Followers: 29, SJR: 0.156, CiteScore: 1)
Advances in Child Development and Behavior     Full-text available via subscription   (Followers: 11, SJR: 0.713, CiteScore: 1)
Advances in Chronic Kidney Disease     Full-text available via subscription   (Followers: 10, SJR: 1.316, CiteScore: 2)
Advances in Clinical Chemistry     Full-text available via subscription   (Followers: 26, SJR: 1.562, CiteScore: 3)
Advances in Colloid and Interface Science     Full-text available via subscription   (Followers: 20, SJR: 1.977, CiteScore: 8)
Advances in Computers     Full-text available via subscription   (Followers: 14, SJR: 0.205, CiteScore: 1)
Advances in Dermatology     Full-text available via subscription   (Followers: 15)
Advances in Developmental Biology     Full-text available via subscription   (Followers: 13)
Advances in Digestive Medicine     Open Access   (Followers: 12)
Advances in DNA Sequence-Specific Agents     Full-text available via subscription   (Followers: 7)
Advances in Drug Research     Full-text available via subscription   (Followers: 26)
Advances in Ecological Research     Full-text available via subscription   (Followers: 43, 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: 51, 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: 65, 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: 10, 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: 25)
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: 14, SJR: 3.027, CiteScore: 2)
Advances in Medical Sciences     Hybrid Journal   (Followers: 8, SJR: 0.694, CiteScore: 2)
Advances in Medicinal Chemistry     Full-text available via subscription   (Followers: 6)
Advances in Microbial Physiology     Full-text available via subscription   (Followers: 5, SJR: 1.158, CiteScore: 3)
Advances in Molecular and Cell Biology     Full-text available via subscription   (Followers: 24)
Advances in Molecular and Cellular Endocrinology     Full-text available via subscription   (Followers: 8)
Advances in Molecular Toxicology     Full-text available via subscription   (Followers: 7, SJR: 0.182, CiteScore: 0)
Advances in Nanoporous Materials     Full-text available via subscription   (Followers: 5)
Advances in Oncobiology     Full-text available via subscription   (Followers: 2)
Advances in Organ Biology     Full-text available via subscription   (Followers: 2)
Advances in Organometallic Chemistry     Full-text available via subscription   (Followers: 18, SJR: 1.875, CiteScore: 4)
Advances in Parallel Computing     Full-text available via subscription   (Followers: 7, SJR: 0.174, CiteScore: 0)
Advances in Parasitology     Full-text available via subscription   (Followers: 5, SJR: 1.579, CiteScore: 4)
Advances in Pediatrics     Full-text available via subscription   (Followers: 27, SJR: 0.461, CiteScore: 1)
Advances in Pharmaceutical Sciences     Full-text available via subscription   (Followers: 19)
Advances in Pharmacology     Full-text available via subscription   (Followers: 17, SJR: 1.536, CiteScore: 3)
Advances in Physical Organic Chemistry     Full-text available via subscription   (Followers: 9, SJR: 0.574, CiteScore: 1)
Advances in Phytomedicine     Full-text available via subscription  
Advances in Planar Lipid Bilayers and Liposomes     Full-text available via subscription   (Followers: 3, SJR: 0.109, CiteScore: 1)
Advances in Plant Biochemistry and Molecular Biology     Full-text available via subscription   (Followers: 10)
Advances in Plant Pathology     Full-text available via subscription   (Followers: 6)
Advances in Porous Media     Full-text available via subscription   (Followers: 5)
Advances in Protein Chemistry     Full-text available via subscription   (Followers: 19)
Advances in Protein Chemistry and Structural Biology     Full-text available via subscription   (Followers: 20, SJR: 0.791, CiteScore: 2)
Advances in Psychology     Full-text available via subscription   (Followers: 67)
Advances in Quantum Chemistry     Full-text available via subscription   (Followers: 6, SJR: 0.371, CiteScore: 1)
Advances in Radiation Oncology     Open Access   (Followers: 1, 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: 421, 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: 53, SJR: 1.551, CiteScore: 3)
Aeolian Research     Hybrid Journal   (Followers: 6, SJR: 1.117, CiteScore: 3)
Aerospace Science and Technology     Hybrid Journal   (Followers: 385, 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: 476, SJR: 1.238, CiteScore: 3)
Agri Gene     Hybrid Journal   (Followers: 1, SJR: 0.13, CiteScore: 0)
Agricultural and Forest Meteorology     Hybrid Journal   (Followers: 18, SJR: 1.818, CiteScore: 5)
Agricultural Systems     Hybrid Journal   (Followers: 31, SJR: 1.156, CiteScore: 4)
Agricultural Water Management     Hybrid Journal   (Followers: 44, SJR: 1.272, CiteScore: 3)
Agriculture and Agricultural Science Procedia     Open Access   (Followers: 4)
Agriculture and Natural Resources     Open Access   (Followers: 3)
Agriculture, Ecosystems & Environment     Hybrid Journal   (Followers: 58, SJR: 1.747, CiteScore: 4)
Ain Shams Engineering J.     Open Access   (Followers: 5, SJR: 0.589, CiteScore: 3)
Air Medical J.     Hybrid Journal   (Followers: 8, SJR: 0.26, CiteScore: 0)
AKCE Intl. J. of Graphs and Combinatorics     Open Access   (SJR: 0.19, CiteScore: 0)
Alcohol     Hybrid Journal   (Followers: 12, SJR: 1.153, CiteScore: 3)
Alcoholism and Drug Addiction     Open Access   (Followers: 12)
Alergologia Polska : Polish J. of Allergology     Full-text available via subscription   (Followers: 1)
Alexandria Engineering J.     Open Access   (Followers: 2, SJR: 0.604, CiteScore: 3)
Alexandria J. of Medicine     Open Access   (Followers: 1, SJR: 0.191, CiteScore: 1)
Algal Research     Partially Free   (Followers: 11, SJR: 1.142, CiteScore: 4)
Alkaloids: Chemical and Biological Perspectives     Full-text available via subscription   (Followers: 2)
Allergologia et Immunopathologia     Full-text available via subscription   (Followers: 1, SJR: 0.504, CiteScore: 1)
Allergology Intl.     Open Access   (Followers: 5, SJR: 1.148, CiteScore: 2)
Alpha Omegan     Full-text available via subscription   (SJR: 3.521, CiteScore: 6)
ALTER - European J. of Disability Research / Revue Européenne de Recherche sur le Handicap     Full-text available via subscription   (Followers: 10, SJR: 0.201, CiteScore: 1)
Alzheimer's & Dementia     Hybrid Journal   (Followers: 53, SJR: 4.66, CiteScore: 10)
Alzheimer's & Dementia: Diagnosis, Assessment & Disease Monitoring     Open Access   (Followers: 6, SJR: 1.796, CiteScore: 4)
Alzheimer's & Dementia: Translational Research & Clinical Interventions     Open Access   (Followers: 6, SJR: 1.108, CiteScore: 3)
Ambulatory Pediatrics     Hybrid Journal   (Followers: 5)
American Heart J.     Hybrid Journal   (Followers: 58, SJR: 3.267, CiteScore: 4)
American J. of Cardiology     Hybrid Journal   (Followers: 63, SJR: 1.93, CiteScore: 3)
American J. of Emergency Medicine     Hybrid Journal   (Followers: 46, SJR: 0.604, CiteScore: 1)
American J. of Geriatric Pharmacotherapy     Full-text available via subscription   (Followers: 12)
American J. of Geriatric Psychiatry     Hybrid Journal   (Followers: 14, SJR: 1.524, CiteScore: 3)
American J. of Human Genetics     Hybrid Journal   (Followers: 37, SJR: 7.45, CiteScore: 8)
American J. of Infection Control     Hybrid Journal   (Followers: 29, SJR: 1.062, CiteScore: 2)
American J. of Kidney Diseases     Hybrid Journal   (Followers: 36, SJR: 2.973, CiteScore: 4)
American J. of Medicine     Hybrid Journal   (Followers: 50)
American J. of Medicine Supplements     Full-text available via subscription   (Followers: 3, SJR: 1.967, CiteScore: 2)
American J. of Obstetrics and Gynecology     Hybrid Journal   (Followers: 254, 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: 66, SJR: 0.138, CiteScore: 0)
Anaesthesia Critical Care & Pain Medicine     Full-text available via subscription   (Followers: 24, SJR: 0.411, CiteScore: 1)
Anales de Cirugia Vascular     Full-text available via subscription   (Followers: 1)
Anales de Pediatría     Full-text available via subscription   (Followers: 3, SJR: 0.277, CiteScore: 0)
Anales de Pediatría (English Edition)     Full-text available via subscription  
Anales de Pediatría Continuada     Full-text available via subscription  
Analytic Methods in Accident Research     Hybrid Journal   (Followers: 5, SJR: 4.849, CiteScore: 10)
Analytica Chimica Acta     Hybrid Journal   (Followers: 44, SJR: 1.512, CiteScore: 5)
Analytica Chimica Acta : X     Open Access  
Analytical Biochemistry     Hybrid Journal   (Followers: 210, SJR: 0.633, CiteScore: 2)
Analytical Chemistry Research     Open Access   (Followers: 13, SJR: 0.411, CiteScore: 2)
Analytical Spectroscopy Library     Full-text available via subscription   (Followers: 14)
Anesthésie & Réanimation     Full-text available via subscription   (Followers: 2)
Anesthesiology Clinics     Full-text available via subscription   (Followers: 25, SJR: 0.683, CiteScore: 2)
Angiología     Full-text available via subscription   (SJR: 0.121, CiteScore: 0)
Angiologia e Cirurgia Vascular     Open Access   (Followers: 1, SJR: 0.111, CiteScore: 0)
Animal Behaviour     Hybrid Journal   (Followers: 222, SJR: 1.58, CiteScore: 3)
Animal Feed Science and Technology     Hybrid Journal   (Followers: 6, 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: 311  
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 1359-6454
Published by Elsevier Homepage  [3183 journals]
  • Effects of 3d Electron Configurations on Helium Bubble Formation and Void
           Swelling in Concentrated Solid-Solution Alloys
    • Abstract: Publication date: Available online 11 October 2019Source: Acta MaterialiaAuthor(s): Yanwen Zhang, Xing Wang, Yuri N. Osetsky, Yang Tong, Robert Harrison, Stephen E. Donnelly, Di Chen, Yongqiang Wang, Hongbin Bei, Brian C. Sales, Karren L. More, Pengyuan Xiu, Lumin Wang, William J. Weber Elemental specific chemical complexity is known to play a critical role in microstructure development in single-phase concentrated solid-solution alloys (SP-CSAs), including both He bubble formation and irradiation-induced void swelling. While cavity formation and evolution under ion irradiation at elevated temperature are complex nonequilibrium processes, chemical effects are revealed at the level of electrons and atoms herein in a simplified picture, using Ni and a special set of Ni-based SP-CSAs composed of 3d transition metals as model alloys. Based on Ni and the model alloys with minimized variables (e.g., atomic mass, size, and lattice structure), we discuss the effects of chemically-biased energy dissipation, defect energetics, sluggish diffusion, and atomic transport on cavity formation and evolution under both self-ion Ni irradiation and He implantation. The observed difference in microstructure evolution is attributed to the effects of d electron interactions in their integrated ability to dissipate radiation energy. The demonstrated impact of alloying 3d transition metals with larger differences in the outermost electron counts suggests a simple design strategy for tuning defect properties to improve radiation tolerance in structural alloys.Graphical abstractImage, graphical abstract
  • Phase transformation assisted twinning in face-centered-cubic
           FeCrNiCoAl 0.36
    • Abstract: Publication date: Available online 11 October 2019Source: Acta MaterialiaAuthor(s): Peijun Yu, Rui Feng, Junping Du, Shuhei Shinzato, Jyh-Pin Chou, Bilin Chen, Yu-Chieh Lo, Peter K. Liaw, Shigenobu Ogata, Alice Hu The FeNiCoCr-based high entropy alloys (HEAs) exhibit excellent mechanical properties, such as twin-induced plasticity (TWIP) and phase transformation plasticity (TRIP) that can reach a remarkable combination of strength and ductility. In this work, the face-centered-cubic (FCC) single-crystal FeNiCoCrAl0.36 HEAs were studied, using the density functional theory (DFT) combined with the phonon calculation to estimate the stacking fault energies, temperature-dependent phase stabilities of different structures. And the kinetic Monte Carlo (kMC) is used to predict the substructures evolution based on the transition state energies obtained from DFT calculations. We proposed two different formation paths of nano-twins in this Al-composited HEA and found that short-range hexagonal-close-packed (HCP)-stacking could occur in this HEA. The DFT calculations suggest that this HEA has negative stacking fault energy, HCP formation energy, and twin-formation energy at 0 K. Phonon calculations represent that at the finite temperature, the competing FCC/HCP phase stability and propensity for twinning make it possible to form HCP-like twin boundaries. The kMC simulations suggest that under deformation, TWINs could form within the HCP laths which differs from the study of others. With the great agreement of results from kMC simulations and experiments, this twin-hcp laminated substructure formation path offers a new concept of designing TWIP HEAs containing tunable twin structures with HCP and TWIN lamellae structures, which could result in better mechanical properties of HEAs.Graphical abstractImage, graphical abstract
  • Twinning and sequential kinking in lamellar Ti-6Al-4V alloy
    • Abstract: Publication date: Available online 11 October 2019Source: Acta MaterialiaAuthor(s): Xiaodong Zheng, Shijian Zheng, Jian Wang, Yingjie Ma, Hao Wang, Yangtao Zhou, Xiaohong Shao, Bo Zhang, Jiafeng Lei, Rui Yang, Xiuliang Ma Fully lamellar Ti-6Al-4V alloys comprise body-centered cubic (BCC) β lamellae in large-sized, hexagonal close-packed (HCP) α colonies and exhibit outstanding toughness. Although α/β interfaces are considered to play a key role in plastic deformation connected to the toughness, the interface effects have not been revealed so far. In this work, we studied underlying deformation mechanisms of interface-related deformation modes at an atomic scale. After the cyclic loading, {11¯02} deformation twins were observed in the vicinity of fatigue crack surfaces. Moreover, the α/β interface structures before and after cyclic loading deformation were characterized via transmission electron microscopy (TEM). The initial α/β interfaces can be described by the terrace ledge kink model, consisting of (011¯0)α (1¯21)β terrace plane and (1¯100)α (1¯01)β ledge plane. TEM investigations reveal that deformation twins nucleate at the α/β interface and the corresponding nucleation is ascribed to the dissociation of basal type dislocations. More importantly, these twins can continuously propagate through multiple β phase lamella. The continuous propagation of twinning is accomplished through double kinking mechanism. In this manner, twinning in α phases and sequential kinking in β phases can effectively release the stress intensification at the crack tip and dissipate plastic work/energy, correspondingly enhancing fracture toughness of fully lamellar Ti-6Al-4V.Graphical Image, graphical abstract
  • Ternary diagrams of the phase, optical bandgap energy and
           photoluminescence of mixed-halide perovskites
    • Abstract: Publication date: Available online 11 October 2019Source: Acta MaterialiaAuthor(s): Se-Yun Kim, Ho-Chang Lee, Yujin Nam, Yeonghun Yun, Si-Hong Lee, Dong Hoe Kim, Jun Hong Noh, Joon-Hyung Lee, Dae-Hwan Kim, Sangwook Lee, Young-Woo Heo Halide perovskites attract enormous attention as promising light absorption and emission materials for photovoltaics and optoelectronic applications. Here we report ternary diagrams of the phase, optical bandgap energy (Eg) and photoluminescence intensity of methylammonium lead halide (MAPbX3, where X = I, Br and Cl) perovskites, with three vertices of MAPbI3, MAPbBr3 and MAPbCl3. All the compositions were synthesized via a facile mechanochemical reaction at room temperature, to ensure the desired stoichiometries of the final products. Through structural study on MAPbX3, the phase diagram comprising a single phase region and two multi-phase regions was obtained. In the single phase region, the a-axis lattice constant increases almost linearly with increasing the average size of the X site ions. Interestingly, Eg decreases almost linearly with increasing the average size of the X site ions, giving negligible deviation from Vegard's law. As the result, a certain bandgap value, in the range of 1.55 - 2.9 eV, can be easily designed with infinite numbers of compositions. For the last, the ternary diagram of the photoluminescence intensity reveals the effective compositions for red, green and blue light emission. The comprehensive structural and optical information reported in this study is useful for designing halide perovskites for various applications. In addition, our approach for compositional mapping various characteristics using a solid-state reaction is an efficient and robust way to studying halide perovskites.Graphical abstractImage, graphical abstract
  • Flash Sintering Activated by Bulk Phase and Grain Boundary Complexion
    • Abstract: Publication date: Available online 11 October 2019Source: Acta MaterialiaAuthor(s): Yuanyao Zhang, Jiuyuan Nie, Jian Luo A naturally-occurring coupled thermal and electric runaway, resulted from an Arrhenius temperature-dependent specimen conductivity, can trigger flash sintering in many ceramics. This study reveals another possibility to activate flash sintering: a bulk phase transformation or a grain boundary (phase-like) complexion transition can cause an abrupt rise in the specimen conductivity to jump start flash sintering (prior to the occurrence of a natural thermal runaway). In undoped and Al2O3-doped ZnO, the flash sintering is activated by natural thermal runways that can be quantitively predicted from an Arrhenius extrapolation of low-temperature specimen conductivity. In contrast, a bulk eutectic reaction and the associated formation of premelting-like intergranular films (IGFs) in Bi2O3-doped ZnO can lead to a nonlinear rise in the specimen conductivity (above the Arrhenius extrapolation) to trigger flash sintering prior to the occurrence of the predicted natural thermal runaway. Yet, a natural thermal runaway can still take place in Bi2O3-doped ZnO before the occurrence of the interfacial and bulk transformation if the initial electric field is increased to a sufficiently high level. All five cases can be fully explained in a consistent framework so that this set of experiments systematically validate our theory of flash initiation. This work uncovers the roles of the bulk phase and interfacial (phase-like) complexion transformations in initiating flash sintering, thereby suggesting a new direction to understand and tailor the flash sintering process. An observation of ultra-fast field-induced migration of aliovalent cations during the flash sintering of Al2O3-doped ZnO is also reported.Graphical Image, graphical abstract
  • 2020 Acta Award Recipients
    • Abstract: Publication date: Available online 10 October 2019Source: Acta MaterialiaAuthor(s):
  • Insights into the fivefold symmetry of the amorphous Sb-based change
           materials in the rapid phase change from first principles
    • Abstract: Publication date: Available online 10 October 2019Source: Acta MaterialiaAuthor(s): Kewu Bai In materials science, it is widely believed that the fivefold symmetry in the amorphous phase will suppress crystallization because of its structural incompatibility with the crystal phase. The Sb-based phase change materials dominated by the fivefold rings, however, display the ultrafast growth-dominated phase change from amorphous to crystalline phase and exhibit significant property contrast consequently when they are heated by a laser or electrical pulse. To resolve the paradox, the long-time ab initio molecular dynamics calculation is carried out to simulate the crystallization cycle of the amorphous Ge15Sb85 under non-isothermal condition. The calculation results are in a reasonable agreement with experimental data. In particular, it is found that a transient phase state exists just before the crystallization onset temperature of the amorphous Ge15Sb85, in which the predominant fivefold rings in half-chair conformation undergo rapid conversion into the puckered sixfold rings. It is further demonstrated that such rings conversion via a route of the bond exchange model involves small atomic displacements and results in the structural motif similarity between the transient and the crystalline states. This thus reduces the crystal–amorphous interfacial energy promoting crystal nucleation consequently. The subsequent electronic calculations indicated that such conversion of the dominant rings in the amorphous Ge15Sb85 may be triggered by the increased pp orbital hybridizations and modulated by the diminished sp electron mixing. It is anticipated that the findings presented will provide a stepping-stone for the rational design of the Sb-based phase change materials.Graphical abstractImage, graphical abstract
  • Quantitative identification of constituent phases in a Nd-Fe-B-Cu sintered
           magnet and temperature dependent change of electron density of Nd2Fe14B
           studied by synchrotron X-ray diffraction
    • Abstract: Publication date: Available online 8 October 2019Source: Acta MaterialiaAuthor(s): Hiroyuki Okazaki, David Billington, Naruki Tsuji, Wakana Ueno, Yoshinori Kotani, Shogo Kawaguchi, Kunihisa Sugimoto, Kentaro Toyoki, Tomoki Fukagawa, Takeshi Nishiuchi, Kazuhiro Hono, Satoshi Hirosawa, Tetsuya Nakamura We have measured the temperature dependent XRD profiles of an isotropic Nd-Fe-B-Cu sintered magnet during the annealing process. Through Rietveld refinement, we demonstrate the changes in the volume fractions of a Nd2Fe14B main-phase and the other secondary phases as a function of increasing temperature up to 1047°C. The secondary phases mainly include dhcp-Nd, fcc-NdOx, and hcp-Nd2O3 at room temperature and fcc-Nd at elevated temperatures. The main phase starts melting above 900°C but remains relatively stable up to 600°C, while the dhcp-Nd phase completely disappears at around 600°C. Taking an advantage of the excellent quality of the XRD profiles in the powdered single crystal sample, we have also investigated the electron density distribution of main-phase by MEM/Rietveld analysis in order to elucidate the origin of the large magnetic anisotropy. The change in asymmetric part of electron density is derived by the subtraction of the electron density distributions between those recorded at -123 and -173°C, where the spin reorientation transition at -138°C changes the magnetic anisotropy. This change is attributed to be a difference of electron density distribution between Nd f and g sites of the Nd2Fe14B, relating to the magnetic anisotropy.Graphical abstractImage, graphical abstract
  • Optimizing composition in MnBi permanent magnet alloys
    • Abstract: Publication date: Available online 7 October 2019Source: Acta MaterialiaAuthor(s): B.A. Jensen, W. Tang, X.B. Liu, G. Ouyang, K.W. Dennis, J. Cui MnBi is an attractive rare-earth-free permanent magnetic material due to its low materials cost, high magnetocrystalline anisotropy (1.6  ×  106 J m−3), and good magnetization (81 emu g−1) at room temperature. Although the theoretical maximum energy product (BH)max of 20 MGOe is lower than that of NdFeB-based magnets, the low temperature phase (LTP) of MnBi has a positive temperature coefficient of coercivity, up to 200 °C, which makes it a potential candidate for high temperature applications such as permanent magnet motors. However, the oxygen sensitivity of the MnBi compound and the peritectic reaction between Mn and Bi make it difficult to synthesize into a material with high purity. This challenge is partly offset by adding excess Mn to the alloy, with composition close to Mn55Bi45 resulting in the highest saturation magnetization after common processing techniques such as arc melting, casting, melt spinning, and ball milling. Here we report a systematic process which reduces the amount of excessive Mn, while simultaneously providing a large saturation magnetization (MS) of 79 emu g−1 at 300 K in the annealed Mn52Bi48 ribbons. We also report excellent magnetic properties in the ball powders, resulting in 0.5-5 µm particles with MS of 75.5 emu g−1, coercivity Hci of 10.8 kOe, and (BH)max of 13 MGOe using 9 T applied field at 300 K. A secondary annealing treatment on various ball milled powders increased Hci by up to 21%, and also resulted in an increase in MS up to 78.8 emu g−1.Graphical Image, graphical abstract
  • Effect of hardening on toughness captured by stress-based damage
           nucleation in 6061 aluminum alloy
    • Abstract: Publication date: November 2019Source: Acta Materialia, Volume 180Author(s): Tom Petit, Jacques Besson, Claire Ritter, Kimberly Colas, Lukas Helfen, Thilo F. Morgeneyer A deterioration of fracture toughness, especially of the tearing modulus, with aging time and associated strength increase is observed for aluminum 6061 and reproduced here numerically thanks to a stress-based damage nucleation criterion.A correlative multiscale analysis by scanning electron microscopy, atom probe tomography as well as 3D X-ray laminography shows that coarse particles and the characteristic damage mechanisms do not depend on aging time: the fracture mechanism is typically ductile and transgranular as shown by electron backscatter diffraction analysis of sections of compact tension specimens containing interrupted cracks. Large Mg2Si inclusions fracture at very low plastic strain, and defects nucleate at large (Fe,Si)-rich inclusions with increasing plastic deformation. Only the hardening nanoprecipitation increases with aging time: aging favors the precipitation of nm-size Mg2Si precipitates which causes hardening of the matrix so that damage nucleation at coarse inclusions becomes easier - thus leading to a decrease in toughness. Indeed, larger clusters and a substantially higher area fraction of iron based intermetallic particles are found on the fracture surfaces of the longest aging time CT samples compared to the shortest aging time samples.Based on these observations, a Gurson-Tvergaard Needleman type model is proposed to simulate the tearing tests using Finite Elements. It uses damage nucleation kinetics which depend on the maximum principal stress, since a classical strain-based nucleation is not sufficient to reproduce the deterioration of the tearing modulus.Graphical abstractImage 1
  • Designed materials with the giant magnetocaloric effect near room
    • Abstract: Publication date: November 2019Source: Acta Materialia, Volume 180Author(s): Anis Biswas, Arjun K. Pathak, Nikolai A. Zarkevich, Xubo Liu, Yaroslav Mudryk, Viktor Balema, Duane D. Johnson, Vitalij K. Pecharsky The coupling between structural and magnetic degrees of freedom is crucial for realization of interesting physical phenomena associated with magneto-structural transformations resembling austenite-to-martensite transitions. Despite substantial efforts in design and discovery of materials with strong magnetocaloric effects, a majority of viable candidates are composed of non-earth-abundant and toxic elements, while others involve challenging syntheses and post processing. Guided by advanced density functional theory calculations, we report a new family of compounds, i.e., Mn0.5Fe0.5NiSi1-xAlx [x = 0.045–0.07] exhibiting a giant magnetocaloric effect (MCE) that is tunable near room temperature. Their MCE functionality arises from a distinct magneto-structural transformation between a paramagnetic hexagonal Ni2In-type phase and ferromagnetic orthorhombic TiNiSi-type phase that can be actuated by magnetic field and/or pressure. As the transition is sensitive to external hydrostatic pressure, the same materials should also exhibit a strong barocaloric response in addition to the giant MCE.Graphical abstractImage, graphical abstract
  • Collaborative ductile rupture mechanisms of high-purity copper identified
           by in situ X-ray computed tomography
    • Abstract: Publication date: Available online 5 October 2019Source: Acta MaterialiaAuthor(s): Brendan P. Croom, Helena Jin, Philip J. Noell, Brad L. Boyce, Xiaodong Li The competition between ductile rupture mechanisms in high-purity Cu and other metals is sensitive to the material composition and loading conditions, and subtle changes in the metal purity can lead to failure either by void coalescence or Orowan Alternating Slip (OAS). In situ X-ray computed tomography tensile tests on 99.999% purity Cu wires have revealed that the rupture process involves a sequence of damage events including shear localization; growth of micron-sized voids; and coalescence of microvoids into a central cavity prior to the catastrophic enlargement of the coalesced void via OAS. This analysis has shown that failure occurs in a collaborative rather than strictly competitive manner. In particular, strain localization along the shear band enhanced void nucleation and drove the primary coalescence event, and the size of the resulting cavity and consumption of voids ensured a transition to the OAS mechanism rather than continued void coalescence. Additionally, the tomograms identified examples of void coalescence and OAS growth of individual voids at all stages of the failure process, suggesting that the transition between the different mechanisms was sensitive to local damage features, and could be swayed by collaboration with other damage mechanisms. The competition between the different damage mechanisms is discussed in context of the material composition, the local damage history, and collaboration between the mechanisms.Graphical Image, graphical abstract
  • Misorientation Dependence Grain Boundary Complexions in <111>
           Symmetric Tilt Al Grain Boundaries
    • Abstract: Publication date: Available online 4 October 2019Source: Acta MaterialiaAuthor(s): Prakash Parajuli, David Romeu, Viwanou Hounkpati, Rubén Mendoza-Cruz, Jun Chen, Miguel José Yacamán, Jacob Flowers, Arturo Ponce Since polycrystalline alloys consist of a complex network of various types of grain boundaries (GBs), detailed atomic-scale analysis of how some impurities are distributed at every type of GBs is necessary to fully understand the implications of GB segregation on material’s performance. In this study, we present the atomic-scale structural combined with a chemical analysis of segregation induced GB complexions across the various types of Al alloy 7075 GBs using aberration-corrected microscopy and crystal orientation mapping assisted with precession electron diffraction. The result shows multilayer Cu GB segregation containing non-uniformly segregated mixed atomic columns across the interfaces. Two distinct types of Cu GB segregation behavior were observed, point and parallel array, analyzed by means of a displacement field obtained from the dichromatic pattern. Atomistic simulations were performed to test the energetic feasibility of the observed segregation behavior. As per the knowledge of the authors, this is the first report on experimental analysis of segregation induced periodic ordered structured GB complexions on Al alloy system. Furthermore, every GBs of the films were segregated uniquely forming ordered structures along the interface. The distance between two consecutive high segregated units was periodic for the point segregated GBs and followed a trend of a theoretical model of dislocation spacing. Based on the distance between two high segregated units, it is inferred that highly misorientated GBs are more segregated than low misoriented GBs. This study demonstrates that the misorientation between the neighboring grains significantly influences the segregation behavior across the interface and consequently, the structure of segregation-induced GB complexions.Graphical abstractImage 1
  • Mesoscale Characterization of Continuous Fiber Reinforced Composites
           Through Machine Learning: Fiber Chirality
    • Abstract: Publication date: Available online 4 October 2019Source: Acta MaterialiaAuthor(s): Samuel Sherman, Jeff Simmons, Craig Przybyla A method of quantifying fiber chirality, the twist of continuous fibers through a volume, is defined and applied to both phantom data and real data. Specifically, a field quantity termed the fiber chirality based on the anti-symmetric part of the gradient of the fiber orientation is introduced. For this method of estimation, the input is the set of fiber positions gathered from the stack of images which represent the sample volumes. The phantom sample is generated and several different real continuous fiber reinforced matrix composites are experimentally characterized. Each phantom dataset contains a bundle of fibers that rotate about a center axis with a user-defined angle at each step in the z-direction. The chirality of the fibers is calculated based on the pre-characterized positions using a machine learning algorithm. To validate the method of quantification, our chirality estimation method results in a colormap with an angle of rotation that becomes increasingly more similar to the user-defined angle with decreasing inter-slice distance. Fiber positions from real data are then input into the estimation method and the results are compared.Graphical abstractGraphical abstract for this article
  • Synergetic effects of solute and strain in biocompatible Zn-based and
           Mg-based alloys
    • Abstract: Publication date: Available online 4 October 2019Source: Acta MaterialiaAuthor(s): Y.Q. Guo, S.H. Zhang, I.J. Beyerlein, D. Legut, S.L. Shang, Z.K. Liu, R.F. Zhang Zn-based and Mg-based alloys have been considered highly promising biodegradable materials for cardiovascular stent applications due to their excellent biocompatibility and moderate in vitro degradation rates. However, their strength is too poor for use in cardiovascular stents. The strength of these metals can be related to the sizes of the dislocation cores and the threshold stresses needed to activate slip, i.e., the Peierls stress. Using density functional theory (DFT) and an ab initio-informed semi-discrete Peierls-Nabarro model, we investigate the coupled effect of the solute element and mechanical straining on the stacking fault energy, basal dislocation core structures and Peierls stresses in both Zn-based and Mg-based alloys. We consider several biocompatible solute elements, Li, Al, Mn, Fe, Cu, Mg and Zn, in the same atomic concentrations. The combined analysis here suggests some elements, like Fe, can potentially enhance strength in both Zn-based and Mg-based alloys, while other elements, like Li, can lead to opposing effects in Zn and Mg. We show that the effect of solute strengthening and longitudinal straining on SFEs is much stronger for the Zn-based alloys than for the Mg-based alloys. DFT investigations on electronic structure and bond lengths reveal a coupled chemical-mechanical effect of solute and strain on electronic polarization, charge transfer, and bonding strength, which can explain the weak mechanical effect on Zn-based alloys and the variable strengthening effect among these solutes. These findings can provide critical information needed in solute selection in Zn-based and Mg-based alloy design for biomedical applications.Graphical abstractImage, graphical abstract
  • Non-synchronized rotation of layered spin configurations in
           La0.825Sr0.175MnO3 /SrTiO3 film
    • Abstract: Publication date: Available online 3 October 2019Source: Acta MaterialiaAuthor(s): Xin Li, Jingzhi Han, Xiongzuo Zhang, Rui Wu, Yinfeng Zhang, Haidong Tian, Mingzhu Xue, Xin Wen, Zhichao Li, Shunquan Liu, Wenyun Yang, Changsheng Wang, Honglin Du, Xiaodong Zhang, Yingchang Yang, Jinbo Yang Magnetic properties of single perovskite-structure epitaxial film are reported to have layered distribution recently. However, the different responses of spin configurations in individual layers to the variation of temperature and strain state, especially the subtle response of layered film due to structural phase transformation of SrTiO3 (STO) substrate around 105 K, have not been revealed completely. Drastic drop and concomitant abnormal increase of remnant magnetic moments (REM) and coercivity of low-doped La1-xSrxMnO3 (x=0.175) /SrTiO3 film were observed around 105 K only in the in-plane direction, and layered magnetic structures were further inferred based on investigation of microstructure, strain distribution and chemical inhomogeneity. Abnormal change of magnetic properties around 105 K was discussed by a three-layer model, in which softer ferromagnetic layer was supposed to form in the middle layer of strain-induced layered structure, and the spin configuration of middle layer underwent non-synchronous transformation relative to other parts of the film, which may be attributed to the change of in-plane strain inside the film around 105 K. Our work reveals the variation of individual magnetic layers with the increase of temperature through utilizing the remnant magnetic field in measuring system, and the abrupt reversal of spin configurations at the intermediate layer can also serve the design of novel spintronics devices in the future.Graphical abstractImage, graphical abstract
  • Thermodynamic and structural evolution of mechanically milled and swift
           heavy ion irradiated Er2Ti2O7 pyrochlore
    • Abstract: Publication date: Available online 20 September 2019Source: Acta MaterialiaAuthor(s): Cheng-Kai Chung, Eric C. O'Quinn, Joerg C. Neuefeind, Antonio F. Fuentes, Hongwu Xu, Maik Lang, Alexandra Navrotsky Design and synthesis of thermodynamically metastable yet kinetically achievable materials possessing various desired functional and physical properties have recently drawn tremendous scientific-attention. In addition to conventional heat treatments and wet chemistry approaches, energy deposition into materials can induce unique nonequilibrium phases with distinct structures, chemistry, energetics, and properties. Mechanochemical synthesis and ion beam irradiation are two processing techniques that provide access to phases and states far from equilibrium. By a combination of high temperature oxide melt solution calorimetry, differential scanning calorimetry (DSC), neutron pair distribution function (PDF) analysis, and supplementary powder X-ray diffraction (XRD), the energetics and multiscale structural evolution on annealing of ball milled and swift heavy ion irradiated Er2Ti2O7 pyrochlore were investigated. Despite very similar structural modifications of local atomic arrangements and only minor differences in the long range structure, both types of damage yield significant difference in the energetics of the produced material. The energy of destabilization in the milled sample (70.2 ± 8.2 kJ/mol) is much less endothermic than that in the irradiated sample (457.3 ± 8.0 kJ/mol). The DSC profiles, supported by neutron scattering, X-ray diffraction, and solution calorimetry, reveal decoupled annealing events in different temperature ranges, separating crystallization of long range pyrochlore structure from annealing of short range weberite-like domains.Graphical Image, graphical abstract
  • First-principles and Machine Learning Predictions of Elasticity in
           Severely Lattice-distorted High-Entropy Alloys with Experimental
    • Abstract: Publication date: Available online 20 September 2019Source: Acta MaterialiaAuthor(s): George Kim, Haoyan Diao, Chanho Lee, A.T. Samaei, Tu Phan, Maarten de Jong, Ke An, Dong Ma, Peter K. Liaw, Wei Chen Stiffness usually increases with the lattice-distortion-induced strain, as observed in many nanostructures. Partly due to the size differences in the component elements, severe lattice distortion naturally exists in high entropy alloys (HEAs). The single-phase face-centered-cubic (FCC) Al0.3CoCrFeNi HEA, which has large size differences among its constituent elements, is an ideal system to study the relationship between the elastic properties and lattice distortion using a combined experimental and computational approach based on in-situ neutron-diffraction (ND) characterizations, and first-principles calculations. Analysis of the interatomic distance distributions from calculations of optimized special quasi random structure (SQS) found that the HEA has a high degree of lattice distortion. When the lattice distortion is explicitly considered, elastic properties calculated using SQS are in excellent agreement with experimental measurements for the HEA. The calculated elastic constant values are within 5% of the ND measurements. A comparison of calculations from the optimized SQS and the SQS with ideal lattice sites indicate that the lattice distortion results in the reduced stiffness. The optimized SQS has a bulk modulus of 177 GPa compared to the ideal lattice SQS with a bulk modulus of 194 GPa. Machine learning (ML) modeling is also implemented to explore the use of fast, and computationally efficient models for predicting the elastic moduli of HEAs. ML models trained on a large dataset of inorganic structures are shown to make accurate predictions of elastic properties for the HEA. The ML models also demonstrate the dependence of bulk and shear moduli on several material features which can act as guides for tuning elastic properties in HEAs.Graphical abstractImage, graphical abstract
  • The effect of large plastic deformation on elevated temperature mechanical
           behavior of dynamic strain aging Al-Mg alloys
    • Abstract: Publication date: Available online 20 September 2019Source: Acta MaterialiaAuthor(s): C. Meng, W. Hu, S. Sandlöbes, S. Korte-Kerzel, G. Gottstein The tensile behavior at elevated temperature of heavily plastically deformed Al-Mg alloys with an Mg content of 1 to 5% was investigated. Large plastic deformation was imposed by confined channel die pressing at room temperature up to 18 passes. During heating to test temperature the specimens were observed to undergo recovery and partly recrystallization. The strain rate sensitivity was found to increase with increasing test temperature, and the previously reported asymmetry of the stress differential upon instantaneous up and down strain rate changes disappeared at elevated temperatures. The transition temperature from the low temperature to the high temperature behavior depended on the amount of pre-deformation. The observed phenomena were attributed to a change in deformation mechanism which occurred the earlier, i.e. at lower temperature, the higher the dislocation density introduced by pre-deformation.Graphical abstractImage, graphical abstract
  • The Effects of Ultra-Fine-Grained Structure and Cryogenic Temperature on
           Adiabatic Shear Localization in Titanium
    • Abstract: Publication date: Available online 11 September 2019Source: Acta MaterialiaAuthor(s): Zezhou Li, Shiteng Zhao, Bingfeng Wang, Shuang Cui, Renkun Chen, Ruslan Z. Valiev, Marc A. Meyers The deformation at low temperatures (173 K and 77 K) in ultrafine-grained (100 and 500 nm) titanium is investigated and its effect on adiabatic shear localization is established. In comparison with coarse-grained titanium, the strength of ultrafine-grained titanium is higher due to the classic Hall-Petch effect while the strain hardening approaches zero. Our results show that shear localization in dynamic deformation is also altered. The width of the shear band of coarse-grained titanium decreases from 30 to 18 μm (by 40%) with decreasing the initial deformation temperature to 77 K. In contrast, for 100 nm titanium, the width of shear band decreases more significantly, from 4 μm at room temperature to 1 μm (a 75% decrease) at 77 K. This difference is attributed to the combined effects of the decrease in the thermal conductivity and specific heat capacity, and the increase in thermal softening rate. These changes in the width are consistent with the predictions of the Grady and Bai-Dodd theories. Ultrafine- and nano-recrystallized grains are observed inside the bands which are dependent on initial grain size and initial deformation temperature. The dislocation evolution is evaluated for the different conditions using a Kocks-Mecking formulation; the rotational dynamic recrystallization mechanism responsible for forming the ultrafine/nanosized grains (40 to 200 nm) is successfully modeled incorporating the differences in initial temperature and grain size. Our results and analysis are important in enhancing the understanding of the structural evolution processes under high strain-rates and cryogenic temperatures.Graphical abstractImage 1
  • Helium-plasma–Induced Straight Nanofiber Growth on HCP Metals
    • Abstract: Publication date: Available online 3 October 2019Source: Acta MaterialiaAuthor(s): Shin Kajita, Tomohiro Nojima, Tatsuki Okuyama, Yuta Yamamoto, Naoaki Yoshida, Noriyasu Ohno Low-energy helium (He) plasma irradiations were conducted on ruthenium (Ru) and rhenium (Re), which have hexagonal close packed (HCP) crystal structures. Growth of linear shaped fiberform nanostructures were identified on the surfaces of the both metals after the He plasma irradiation. We also conducted He plasma irradiation while Re particles were deposited on tungsten substrate; 3-mm-thick large scale fiberform nanostructures were grown on the surface. The crystal orientation was analyzed using diffraction patterns of Re and Ru nanofibers together with detailed transmission electron microscope observations. It was found that the growth of linear nanofibers has a preferential crystal orientation in the growth direction and it is always in the c-direction of the HCP crystals. Potential growth processes and mechanisms are proposed based on the experimental observations.Graphical abstractGraphical abstract for this article
  • Ultra-broad Temperature Insensitive Ceramics with Large Piezoelectricity
           by Morphotropic Phase Boundary Design
    • Abstract: Publication date: Available online 2 October 2019Source: Acta MaterialiaAuthor(s): Haiyan Zhao, Yudong Hou, Xiaole Yu, Mupeng Zheng, Mankang ZhuABSTRACTImproving the operating characteristics of piezoelectric devices in high temperature environments urgently requires the development of piezoceramics with both high piezoelectric coefficient and excellent temperature stability. However, it is difficult for existing piezoceramics to take care of both at the same time. Generally, high piezoelectricity can be obtained at morphotropic phase boundary (MPB) in the binary systems, such as PbZrO3-PbTiO3 and Pb(Mg1/3Nb2/3)O3-PbTiO3, but the temperature stability is unsatisfactory, which seriously restricts the practical application. Here, an optimum composition having excellent comprehensive properties is constructed by designing multiple MPBs in the novel xPb(In1/2Nb1/2)O3-yBiScO3-zPbTiO3 ternary system. When x = 0.04, y = 0.345 and z = 0.615, the specimen has a high piezoelectric coefficient d33 of 478 pC/N at 200°C, meanwhile, the fluctuation of d33 is less than ± 10% over an ultra-broad temperature range of 50-350°C. Combined with a variety of in situ analysis techniques, it can be determined that the temperature-insensitive high piezoelectric coefficient is related to the multiple MPBs design, which is beneficial to the optimization of the hierarchical domain configuration. The developed phase boundary design strategy paves a new way to building next generation high performance high temperature piezoceramics.Graphical abstractImage, graphical abstract
  • Low temperature deformation of MoSi2 and the effect of Ta, Nb
           and Al as alloying elements
    • Abstract: Publication date: Available online 2 October 2019Source: Acta MaterialiaAuthor(s): Carolin Zenk, James S.K.-L. Gibson, Verena Maier-Kiener, Steffen Neumeier, Mathias Göken, Sandra Korte-Kerzel Molybdenum disilicide (MoSi2) is a very promising material for high temperature structural applications due to its high melting point (2030∘C), low density, high thermal conductivity and good oxidation resistance. However, MoSi2 has limited ductility below 900∘C due to its anisotropic plastic deformation behaviour and high critical resolved shear stresses on particular slip systems.Nanoindentation of MoSi2 microalloyed with aluminium, niobium or tantalum showed that all alloying elements cause a decrease in hardness. Analysis of surface slip lines indicated the activation of the additional {1 1 0} slip system in microalloyed MoSi2, which is not active below 300∘C in pure MoSi2. This was confirmed by TEM dislocation analysis of the indentation plastic zone. Further micropillar compression experiments comparing pure MoSi2 and the Ta-alloyed sample enabled the determination of the critical resolved shear stresses of individual slip systems even in the most brittle [0 0 1] crystal direction.Graphical abstractImage 1
  • On the high creep strength of the W containing IRIS-TiAl alloy at
    • Abstract: Publication date: Available online 1 October 2019Source: Acta MaterialiaAuthor(s): Alain Couret, Jean-Philippe Monchoux, Daniel Caillard This paper presents a study of the creep at 850°C under 150 MPa of the IRIS alloy (Ti-Al48-W2-B0.1) densified by spark plasma sintering. The dislocation microstructure in a sample strained up to 1.5% was studied by post-mortem transmission electron microscopy. The deformation is mainly due to ordinary dislocations. Several populations of dislocations are evidenced. Their Burgers vectors, the plane in which they are moving and the corresponding deformation mechanisms are determined. In the discussion section, the deformation mechanisms, the factors controlling their activation and the role of tungsten as hardening element are examined.Graphical abstractImage, graphical abstract
  • Understanding solid solution strengthening at elevated temperatures in a
           creep-resistant Mg-Gd-Ca alloy
    • Abstract: Publication date: Available online 1 October 2019Source: Acta MaterialiaAuthor(s): Ning Mo, Ingrid McCarroll, Qiyang Tan, Anna Ceguerra, Ying Liu, Julie Cairney, Hajo Dieringa, Yuanding Huang, Bin Jiang, Fusheng Pan, Michael Bermingham, Ming-Xing Zhang The present work studies the strengthening mechanisms of a creep-resistant Mg-0.5Gd-1.2Ca (at.%) alloy at both room and elevated temperatures. Although peak-ageing (T6) at 180 ∘C for 32 h led to a significant increase in room temperature strength due to the precipitation strengthening by three types of precipitates (Mg2Ca, Mg5Gd on prismatic planes and a new type of Mg-Gd-Ca intermetallic compound on the basal plane), the as-solid solution treated (T4) alloy exhibited better resistance to temperature softening during compression and to stress relaxation at 180 °C and better creep resistance at 210 °C/100 MPa. The Gd-Ca co-clusters with short-range order in the Mg solid solution, which was verified, at the first time, by atom probe tomography (APT) analysis and atomic-resolution high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), were responsible for the solid solution hardening, offering a more effective strengthening effect through local order-strengthening. Such solid solution strengthening increased the thermal stability of the alloy structure at elevated temperatures, at least at early stage of the creep. Subsequently, dynamic precipitation started contributing to the creep resistance due to the formation of higher density of precipitates. However, in the T6 alloy, creep testing at elevated temperatures, particularly at 210 °C that was higher than the ageing temperature, led to coarsening of the precipitates, which acted as over ageing. As a result of such over ageing, the resistance of the T6 alloy to heat-induced softening was weakened, leading to lower creep resistance than the T4 alloy.Graphical abstractImage, graphical abstract
  • Effects of Zn and Cr Additions on Precipitation and Creep Behavior of a
           Dilute Al-Zr-Er-Si Alloy
    • Abstract: Publication date: Available online 1 October 2019Source: Acta MaterialiaAuthor(s): Richard A. Michi, Jacques Perrin Toinin, Amir R. Farkoosh, David N. Seidman, David C. Dunand The effects of adding 0.08 at.% Cr or 1 at.% Zn to a dilute Al-0.11Zr-0.005Er-0.02Si at.% alloy are studied in terms of the precipitation behavior of Al3Zr (L12-structure) nanoprecipitates and the resulting alloy's creep resistance. Although Cr and Zn additions do not affect measurably the precipitation kinetics or coarsening resistance, the modified alloys exhibit changes in dislocation creep resistance at 300°C: the creep threshold stress is decreased by 2 MPa in the Zn-modified alloy and increased by 3 MPa in the Cr-modified alloy. This is attributed to a modification of the lattice parameter of Al3Zr(L12), which affects the ease with which matrix dislocations climb over the nanoprecipitates. The Zn-modified alloy exhibits Al3Zr(L12) nanoprecipitates containing 6–7 at.% Zn, as determined by atom-probe tomography, which reduces the lattice parameter by 0.17% as a result of Zn substituting for Al on its sublattice, as calculated utilizing density functional theory. The Al3Zr(L12) nanoprecipitates in the Cr-modified alloy contain 0.10–0.20 at.% Cr (1.2 –2.5 times more than the matrix) and 0.28–0.51 at.% Er (a 60- to 100-fold enrichment); the Er increases the lattice parameter misfit of Al3Zr(L12), with the Al matrix. Erbium is confirmed to be particularly potent in increasing the creep resistance of aluminum alloys containing L12 nanoprecipitates. An alloy design methodology for creep resistance is also validated, whereby precipitate compositions measured by APT are input to DFT calculations to determine their effects on lattice parameter misfit.Graphical Image, graphical abstract
  • On distances between grain interfaces in macroscopic parameter space
    • Abstract: Publication date: Available online 30 September 2019Source: Acta MaterialiaAuthor(s): A. Morawiec There is a considerable activity in acquisition of large data sets of macroscopic grain boundary parameters. Analysis of these sets requires grouping boundaries with similar parameters, and thus one needs a measure of similarity or a distance between interfaces. A number of such functions have been previously defined. Interface distances are constructed in two steps: first, an underlying metric is devised, and then it is modified to account for equivalences between points in the space of macroscopic parameters. The paper characterizes and compares the known underlying distance functions. Features important for a function to be applicable as the interface distance are considered. The main aspects are the invariance with respect to symmetries in the parameter space and the property of metric intrinsicality which is essential for analysis of interface distributions. Also ways of devising alternative metrics are described, and suggestions for choosing a standard metric which may serve in conventional computations are given. Results of the metric characterization are of importance for laying out foundations of the statistical analysis of macroscopic grain boundary data.Graphical abstractGraphical abstract for this article
  • A eutectic dual-phase design towards superior mechanical properties of
           heusler-type ferromagnetic shape memory alloys
    • Abstract: Publication date: Available online 30 September 2019Source: Acta MaterialiaAuthor(s): Zhigang Wu, Zhiwen Liang, Yajiu Zhang, Zhuhong Liu, Junsong Zhang, Fakhrodin Motazedian, Sam Bakhtiari, Bashir Samsam Shariat, Yinong Liu, Yang Ren, Hong Yang Heusler-type ferromagnetic shape memory alloys possess attractive multifunctional properties, including magnetic field induced shape memory effect, magnetoresistance and magnetocaloric effect, owing to the unique concurrent magnetic and martensitic transformations. However, these intermetallics generally exhibit intrinsic high brittleness and low strength, which severely impede their workability for processing and applicability in real use. In this study, we demonstrate a new grain refining strategy by means of eutectic solidification to improve the mechanical properties in Ni-Mn-Sn-Fe alloys. In a fully eutectic microstructure, the average γ lamellae thickness was refined to ∼170 nm and the composite showed a compressive strength of 1950 MPa, ductility of 19.5%, Young's Modulus of 38 GPa, pseudoelasticity of 3.2% and high mechanical cyclic stability. The high mechanical performance is attributed to the effect of departmentalization of the brittle Heusler alloy by the densely distributed γ phase fine lamellae in resisting crack propagation. The eutectic Heusler composite exhibited a metamagnetic phase transformation, with a magnetic entropy change of 10.2 J/kg•K and a refrigeration capacity of 168 J/kg in a field change of 5 T.Graphical abstractImage, graphical abstract
  • Experimental observations of amorphization in stoichiometric and
           boron-rich boron carbide
    • Abstract: Publication date: Available online 30 September 2019Source: Acta MaterialiaAuthor(s): Ankur Chauhan, Mark C. Schaefer, Richard A. Haber, Kevin J. Hemker Boron carbide is extremely hard but has been shown to undergo stress-induced amorphization when subjected to large nonhydrostatic stresses. This localized amorphization has been associated with the sudden loss of shear strength and poor ballistic performance. Recent quantum mechanics predictions suggest that boron-enrichment may be used to mitigate amorphization in boron carbide. As a means to test this hypothesis, stoichiometric boron carbide (nominally B4C) and a novel composition of B-rich boron carbide (nominally B6.3C) were investigated. Nanoindentation followed by Raman spectroscopy revealed an obvious reduction in the Raman peaks associated with amorphization in the B-rich material. Transmission electron microscopy observations of the region below the nanoindents facilitated direct observation of amorphization, confirmed the Raman finding that amorphization is reduced in the B-rich specimens, and provided additional insight into deformation mechanisms. It is surmised that boron-rich alloys offer a path to reducing local amorphization in boron carbide.Graphical abstractImage, graphical abstract
  • Development of strong and ductile metastable face-centered cubic
           single-phase high-entropy alloys
    • Abstract: Publication date: Available online 29 September 2019Source: Acta MaterialiaAuthor(s): Daixiu Wei, Xiaoqing Li, Stephan Schönecker, Jing Jiang, Won-Mi Choi, Byeong-Joo Lee, Hyoung Seop Kim, Akihiko Chiba, Hidemi Kato Face-centered cubic (fcc)-phase high-entropy alloys (HEAs) have attracted much academic interest, with the stacking fault energy (SFE) playing an important role in regulating their mechanical behaviors. Here, we revealed the principles for regulating both the elastic and plastic behaviors by composition modification and Mo addition in an fcc-phase quaternary CoCrFeNi system with the assistance of ab initio and thermodynamics calculations. An increase in Co content and a decrease in Fe and Ni contents reduced the fcc phase stability and SFE, but enhanced the elastic modulus, anisotropy, and lattice friction stress. A minor substitution of Co by Mo increased the lattice constant, but decreased the SFE and elastic modulus. Based on these findings, we developed a series of strong and ductile metastable fcc-phase CoxCr25(FeNi)70-xMo5 (x= 30, 40, 50) HEAs with mechanical properties superior to those of the CoCrFeNi HEAs. The careful investigation revealed that the enhanced mechanical properties are due to the Mo-addition-induced strengthening accompanied with a low-SFE-induced restriction of planar behavior of dislocations, mechanical twinning, and strain-induced martensitic transformation. The findings shed light on the development of high-performance HEAs.Graphical abstractImage, graphical abstract
  • Work Hardening Behavior of Fe40Mn40Cr10Co10 High Entropy Alloy Single
           Crystals Deformed by Twinning and Slip
    • Abstract: Publication date: Available online 29 September 2019Source: Acta MaterialiaAuthor(s): S. Picak, J. Liu, C. Hayrettin, W. Nasim, D. Canadinc, K. Xie, Y.I. Chumlyakov, I.V. Kireeva, Ibrahim Karaman The orientation dependence of tensile deformation in Fe40Mn40Co10Cr10 high entropy alloy (HEA) was investigated in [111], [001] and [123] oriented single crystals. Transmission electron microscopy investigations revealed three major mechanisms controlling the deformation stages, depending on the orientation: (i) deformation twinning, (ii) planar slip and (iii) dislocation wall/network formation. While twinning and planar slip were strongly orientation dependent, dislocation walls were observed in all orientations. Twinning was the dominant deformation mode in [111] crystals, while only multi-slip was observed in [001]. Both twins and planar slip were activated in [123] crystals. [111] crystals exhibited the highest strain hardening coefficients and ultimate tensile strength due to the strong twin-twin and twin-slip interactions where twin boundaries reduce the mean free path of dislocations, leading to dynamic Hall-Petch hardening. The decent ductility levels (∼45%) were attained in [111] due to nanoscale internal twins and tertiary twin system forming at the later stages of deformation and suppressing necking. In contrast, no twins or stacking faults were observed in [001] crystals, which is consistent with the Copley-Kear effect. [123] crystals had outstanding tensile ductility (∼65%), due to the activation of planar slip and twinning. Overall, in this off-stoichiometric HEA, we have determined the stacking faculty energy and critical resolved shear stresses for both twinning and slip, and demonstrated the formation of high dislocation density walls and wavy slip in [001], while the hardening stages of [123] and [111] are primarily governed by planar slip and twinning, which can be rationalized by the Copley-Kear effect.Graphical Image, graphical abstract
  • Oxygen effects on ω and α phase transformations in a metastable
           β Ti-Nb alloy
    • Abstract: Publication date: Available online 29 September 2019Source: Acta MaterialiaAuthor(s): Kathleen Chou, Emmanuelle A. Marquis Oxygen is known to have substantial influence on metastable β titanium alloys through martensite suppression and phase stability changes that significantly affect mechanical behavior. Here, we have investigated the influence of oxygen in solid solution on ω and α precipitation during ageing in a metastable β-type Ti-20Nb atomic (at.) % alloy with up to about 5 at. % O obtained through an oxidation exposure. Ageing results show that elevated oxygen induced a shape change for ω precipitates from an ellipsoid shape to an elongated rod shape and resulted in a higher ω number density. Additionally, the growth rate of ω precipitates was slowed with oxygen. Oxygen partitioned to the ω phase during ageing and was shown to expand the region of ω phase stability to higher temperatures, suggesting that oxygen increases ω phase stability. Prolonged ageing revealed that α eventually nucleated at all oxygen levels. However, the rate of α precipitation depended on oxygen content, and the slowest rate was observed with intermediate levels of oxygen (∼2-3 at. %) compared to elevated and minimal levels. A mechanism for this non-linear effect on α precipitation is discussed based on oxygen acting as both an ω-stabilizer and α-stabilizer in β titanium alloys.Graphical Image, graphical abstract
  • A phase-field model for hydride formation in polycrystalline metals:
           Application to δ-hydride in zirconium alloys
    • Abstract: Publication date: Available online 28 September 2019Source: Acta MaterialiaAuthor(s): Tae Wook Heo, Kimberly B. Colas, Arthur T. Motta, Long-Qing Chen We report a phase-field model for simulating metal hydride formation involving large volume expansion in single- and polycrystals. As an example, we consider δ-hydride formation in α-zirconium (Zr), which involves both displacive crystallographic structural change and hydrogen diffusion process. Thermodynamic Gibbs energy functions are extracted from the available thermodynamic database based on the sublattice model for the interstitial solid solutions. Solute-grain boundary interactions and inhomogeneous elasticity of polycrystals are taken into consideration within the context of diffuse-interface description. The stress-free transformation strains of multiple variants for hcp-Zr (α) to fcc-hydride (δ) transformation are derived based on the well-established orientation relationship between the α and δ phases as well as the corresponding temperature-dependent lattice parameters. In particular, to account for the large volume expansion, we introduced the mixed interfacial coherency concept between those phases—basal planes are coherent and prismatic planes are semi-coherent in computing the strain energy contribution to the thermodynamics. We analyzed the morphological characteristics of hydrides involving multiple structural variants and their interactions with grain boundaries. Moreover, our simulation study allows for the exploration of the possible hydride re-orientation mechanisms when precipitating under applied tensile load, taking into account the variation in the interfacial coherency between hydrides and matrix, their elastic interactions with the applied stress, as well as their morphology-dependent interactions with grain boundaries. The phase-field model presented here is generally applicable to hydride formation in any binary metal-hydrogen systems.Graphical abstractImage, graphical abstract
  • The mechanical response of a α 2(Ti3Al) + γ(TiAl)-submicron grained
           Al2O3 cermet under dynamic compression: modeling and experiment
    • Abstract: Publication date: Available online 28 September 2019Source: Acta MaterialiaAuthor(s): B. Amirian, HY. Li, J.D. Hogan Novel experimental data, obtained using an advanced digital image correlation technique coupled to ultra-high-speed photography, have been used to develop and validate a microstructure-dependent constitutive model for a α2(Ti3Al) + γ(TiAl)-submicron grained Al2O3 cermet. Utilizing experimental characterization for important simulation inputs (e.g., microstructural features size, constituent stiffness), the numerical model makes use of a variational form of the Gurson model, based on the nonlinear homogenization approach, to account for the experimentally observed deformation features in this composite (e.g., void deformation and growth, particle fracture). By considering the variability in microstructural features (e.g., particle shape, size, and aspect ratio), as well as densely packed ceramic particles, the proposed model is evaluated by comparing the numerical responses to experimental results for quasi-static and dynamic stress-strain behavior of the material. The results show that the proposed approach is able to accurately predict the mechanical response and deformation of the microstructure. Once validated, the model is expanded for studying the predominant damage mechanisms in this material, as well as determining important mechanical response features such as transitional strain rates, flow stress hardening, extensive flow softening, and energy absorbing efficiency of the material as a function of void and particle volume fraction under high strain rate loading. The totality of this work opens promising avenues for qualitative (damage micromechanisms) and quantitative (stress-strain curve) understanding of ceramic-metal composites under various loading conditions, and offer insights for designing and optimizing cermet microstructures.Graphical abstractGraphical abstract for this article
  • Grain Boundary Serration in Nickel Alloy Inconel 600: Quantification and
    • Abstract: Publication date: Available online 26 September 2019Source: Acta MaterialiaAuthor(s): Yuanbo T. Tang, Phani Karamched, Junliang Liu, Jack C. Haley, Roger C. Reed, Angus J. Wilkinson The serration of grain boundaries in Inconel 600 caused by heat treatment is studied systematically. A new method based on Fourier transforms is used to analyse the multiple wave-like character of the serrated grain boundaries. A new metric – the serration index – is devised and utilised to quantify the degree of serration and more generally to distinguish objectively between serrated and non-serrated boundaries. By considering the variation of the serration index with processing parameters, a causal relationship between degree of serration and solution treatment/cooling rate is elucidated. Processing maps for the degree of serration are presented. Two distinct formation mechanisms arise which rely upon grain boundary interaction with carbides: (i) Zener-type dragging which hinders grain boundary migration and (ii) a faceted carbide growth-induced serration.Graphical abstractGraphical abstract for this article
  • Interaction of precipitation with austenite-to-ferrite phase
           transformation in vanadium micro-alloyed steels
    • Abstract: Publication date: Available online 26 September 2019Source: Acta MaterialiaAuthor(s): Chrysoula Ioannidou, Zaloa Arechabaleta, Alfonso Navarro-López, Arjan Rijkenberg, Robert M. Dalgliesh, Sebastian Kölling, Vitaliy Bliznuk, Catherine Pappas, Jilt Sietsma, Ad A. van Well, S. Erik Offerman The precipitation kinetics of vanadium carbides and its interaction with the austenite-to-ferrite phase transformation is studied in two micro-alloyed steels that differ in vanadium and carbon concentrations by a factor of two, but have the same vanadium-to-carbon atomic ratio of 1:1. Dilatometry is used for heat-treating the specimens and studying the phase transformation kinetics during annealing at isothermal holding temperatures of 900, 750 and 650°C for up to 10 h. Small-Angle Neutron Scattering (SANS) and Atom Probe Tomography (APT) measurements are performed to study the vanadium carbide precipitation kinetics. Vanadium carbide precipitation is not observed after annealing for 10 h at 900 and 750°C, which is contrary to predictions from thermodynamic equilibrium calculations. Vanadium carbide precipitation is only observed during or after the austenite-to-ferrite phase transformation at 650°C. The precipitate volume fraction and mean radius continuously increase as holding time increases, while the precipitate number density starts to decrease after 20 min, which corresponds to the time at which the austenite-to-ferrite phase transformation is finished. This indicates that nucleation and growth are dominant during the first 20 min, while later precipitate growth with soft impingement (overlapping diffusion fields) and coarsening take place. APT shows gradual changes in the precipitate chemical composition during annealing at 650°C, which finally reaches a 1:1 atomic ratio of vanadium-to-carbon in the core of the precipitates after 10 h.Graphical Image, graphical abstract
  • Controlling surface morphology by nanocrystalline/amorphous competitive
           self-phase separation in thin films: thickness-modulated reflectance and
           interference phenomena
    • Abstract: Publication date: Available online 26 September 2019Source: Acta MaterialiaAuthor(s): A. Borroto, S. Bruyère, S. Migot, J.F. Pierson, T. Gries, F. Mücklich, D. Horwat Controlling surface morphology is a key issue for obtaining functional materials with surface-based properties. In this paper, we explore the possibility of using the self-separation of phases as a way of controlling the surface morphology features. We demonstrate using X-ray diffraction and transmission electron microscopy that a competitive self-separation of a nanocrystalline and an amorphous phases occurs in co-sputtered Zr-Mo thin films with a Mo content of 60 at%, corresponding to a composition intermediate to those necessary to form single-phased amorphous and nanocrystalline films. The dependence of the residual stress with the thickness at the biphased composition is discussed in terms of the morphology evolution and a possible mechanism for the self-separation of phases is presented. We show that the self-separation of phases as presented here is not limited to Zr-Mo alloys and can be extended to other systems. By changing the film thickness, it is possible to change the surface morphology of the films at the biphasic composition, due to the competitive growth of the nanocrystalline phase in the amorphous phase. In this way, it was possible to control the surface roughness and, because of this, tuning the film reflectance at a determined wavelength. The occurrence of an interference pattern in the reflectance spectra was discussed and associated to the presence of two different height levels at the film surface.Graphical abstractGraphical abstract for this article
  • Thermodynamics of solute capture during the oxidation of multicomponent
    • Abstract: Publication date: Available online 25 September 2019Source: Acta MaterialiaAuthor(s): Q.C. Sherman, P.W. Voorhees, L.D. Marks In the classical theories of oxidation of metals it is assumed that the interface between the oxide and metal is in thermodynamic equilibrium. However, in many cases this is not true, the oxide grows too fast or the fluxes through the interface are too large for local interfacial equilibrium to exist, leading to nonequilibrium solute capture. We present a thermodynamic analysis using both an available database as well as density functional theory calculations of the thermodynamic conditions for this during the oxidation of Ni-Cr alloys. The analysis indicates that nickel atoms can be captured in the rocksalt or corundum crystallographies for a very wide range of compositions, consistent with recent experimental observations. The density functional theory analysis also provides information about the electronic structure of these oxides which is important to understand their properties, and also indicates that interpretation of spectroscopic data is not simple as mixed valence states as well as Cr4+ can occur under oxidizing conditions. We point out that across at least the first transition row of elements the thermodynamic conditions for nonequilibrium solute capture can easily be met.Graphical abstractillustration of a moving oxidation front and how the velocity of the interface connects to equilibrium or nonequilibrium formation of the oxide Image, graphical abstract
  • Transition from ductilizing to hardening in Tungsten: the dependence on
           Rhenium distribution
    • Abstract: Publication date: Available online 25 September 2019Source: Acta MaterialiaAuthor(s): Yu-Hao Li, Hong-Bo Zhou, Linyun Liang, Ning Gao, Huiqiu Deng, Fei Gao, Gang Lu, Guang-Hong Lu Mechanical responses of tungsten (W) and its alloys are strongly controlled by the properties of 1/2 screw dislocations. Rhenium (Re), as a typical alloying and transmutation element in W, can substantially modify the properties of the dislocations, thus the plasticity of the materials. In this study, we investigate the interaction of Re and Re clusters with the screw dislocations in W by first-principles calculations in combination with theoretical models. Specifically, we propose two competing and Re-distribution dependent mechanisms, i.e. “ductilizing effect” and “hardening effect”; both are crucial to the mechanical properties of W. For the ductilizing effect, dispersed Re atoms weaken the surrounding interatomic interaction and reduce the shear resistance, thus facilitating the motion of the dislocation. In contrast, for the hardening effect, Re clusters formed by aggregated Re atoms due to irradiation can increase the Peierls stress and energy, thus hindering the motion of the dislocations. The proposed mechanisms shed light on the experimental observations that there is a Re-induced transition from ductilizing to hardening due to irradiation. The current work provides a theoretical guidance to the development of W-based future fusion materials in search of ductilizing alloying elements.Graphical abstractImage, graphical abstract
  • High Temperature Stability and Mechanical Quality Factor of Donor-acceptor
           Co-doped BaTiO3 Piezoelectrics
    • Abstract: Publication date: Available online 25 September 2019Source: Acta MaterialiaAuthor(s): Ruixuan Song, Yu Zhao, Weili Li, Yang Yu, Jie Sheng, Ze Li, Yulei Zhang, Hetian Xia, Wei-Dong Fei Low temperature stability has limited the applications at elevated temperature for ABO3-type lead-free ceramics. And multi-grade resonances of piezoelectric ceramics are hardly obtained although the resonances are very important in some applications such as filter and resonator. High piezoelectric properties and large mechanical quality factors with multi-grade resonances can be obtained in (Li+-La3+) co-doped BaTiO3 ceramics, and excellent temperature stabilities are achieved in the ceramics. It has been shown that the thermal treatments both with and without electric field at 200 °C improve the piezoelectric properties and thermal stability further. Large piezoelectric constant (272 pC N−1) and huge mechanical quality factor (2010) was obtained after thermal-electrical treatment, which is caused by Li+-La3+ ionic pair aligning along the external electric filed during thermal-electrical treatment. Moreover, the present study provides an effective method to design combination properties with high temperature stability for ABO3 perovskite ferroelectric ceramics.Graphical Image, graphical abstract
  • Electron beam induced rejuvenation in a metallic glass film during in-situ
           TEM tensile straining
    • Abstract: Publication date: Available online 25 September 2019Source: Acta MaterialiaAuthor(s): Christian Ebner, Jagannathan Rajagopalan, Christina Lekka, Christian Rentenberger Rejuvenation of an amorphous TiAl thin film under external tensile stress by high energy electron irradiation is observed via in-situ transmission electron microscopy (TEM). Electron beam (e-beam) irradiation results in a characteristic change of the elastic properties over time, as measured by the atomic-level elastic strain contained in the TEM diffraction pattern. Specifically, a time dependent increase/decrease of elastic strain is observed along the tensile direction, the saturation value of which correlates linearly with the preceding stress increment/decrement but shows little dependence on the e-beam condition. The low sensitivity of the saturation value to the e-beam condition indicates that the elastic strain change is induced by the structural transitions of a population (dependent on stress increment/decrement) of unstable atomic configurations to local, elastically soft areas. Classical molecular dynamics (MD) simulations including high energy electron scattering events are performed under tensile load to obtain insights into the structural modification that leads to time dependent changes in elastic strain under irradiation. The simulations reveal a change in quantities that are characteristic of structural rejuvenation, with a reduction of the local shear modulus manifesting as time dependent increase in the atomic-level elastic strain at fixed external stress. This link to the experimental data is confirmed by tracking elliptic distortions of simulated diffraction patterns calculated from MD configurations. The presented findings are highly relevant for experimental characterization of amorphous materials using TEM and give a new perspective on local structural modifications by electron irradiation.Graphical abstractGraphical abstract for this article
  • Tracking pores during solidification of a Ni-based superalloy using 4D
           synchrotron microtomography.
    • Abstract: Publication date: Available online 24 September 2019Source: Acta MaterialiaAuthor(s): Emeric. Plancher, Pauline Gravier, Edouard Chauvet, Jean-Jacques Blandin, Elodie Boller, Guilhem Martin, Luc Salvo, Pierre Lhuissier Time-resolved in situ microtomography is employed to track the nucleation and growth of individual pores during solidification of a commercial nickel-based superalloy. Three cooling rates (0.1, 0.5 and 1°C/s) are investigated to evaluate the effect of this key processing parameter on the formation of porosity. Phase contrast obtained with a coherent X-ray beam is used to visualize the evolution of dendritic structures in absence of a sufficient absorption contrast. Two mechanisms leading to shrinkage pores have been identified. The first mechanism (mechanism A) is associated with the coalescence of secondary dendrite arms at temperature during the early stages of solidification. The second mechanism (mechanism B) is related to insufficient liquid feeding in the interdendritic region during the last stages of solidification, at lower temperatures. A variation of cooling rate by a factor 2 does not affect the nucleation rate of pores generated through mechanism B. However, it seems to affect the nucleation rate of small pores obtained through the mechanism A. The kinetics of growth for the majority of individual pores can be described using an exponential-like function. This kinetics is faster for mechanism B compared to mechanism A.Graphical abstractImage, graphical abstract
  • Compression-compression fatigue behaviour of Gyroid-type Triply Periodic
           Minimal Surface porous structures fabricated by Selective Laser Melting
    • Abstract: Publication date: Available online 24 September 2019Source: Acta MaterialiaAuthor(s): Lei Yang, Chunze Yan, Wenchao Cao, Zhufeng Liu, Bo Song, Shifeng Wen, Cong Zhang, Yusheng Shi, Shoufeng Yang Triply Periodic Minimal Surface (TPMS) porous structures are recognized as the most promising bionic artificial structures for tissue engineering. The fatigue properties of additive manufactured porous structures are essential for long-term use in a dynamical bio-skeletal environment. The aim of this study is to study the compression-compression fatigue behaviour and the underlying fatigue mechanism of Gyroid cellular structures (GCS), a typical TPMS porous structure. The high-cycle fatigue results show that both cyclic ratcheting and fatigue damage phenomena contribute to the failure of GCS during fatigue testing. For most fatigue loading stress, the failure samples have nearly 45° fracture bands along the diagonal surface. The fatigue ratio of GCS reaches 0.35 for as-built samples and can be raised to 0.45 after sandblasting treatment. The fatigue ratio values are higher than most of the other bending-dominated lattice structures, suggesting superior fatigue resistance properties of GCSs due to the smooth surface connection between struts. Besides, a systematic investigation of the crack initiation and propagation was conducted by both deformation analysis and finite element method to support experimental phenomena. The results also indicate that the fatigue resistance properties of GCSs are significantly enhanced by sandblasting post-treatment, through removing the adhered powder particles, inducing compressive residual stress on the surface and generating a nanocrystalline zone.Graphical abstractImage, graphical abstract
  • Interfacial origins of visible-light photocatalytic activity in ZnS-GaP
    • Abstract: Publication date: Available online 24 September 2019Source: Acta MaterialiaAuthor(s): Paria Sadat Musavi Gharavi, Lin Xie, Richard Francis Webster, Collin Keon Young Park, Yun Hau Ng, Jiaqing He, Judy Nancy Hart, Nagarajan Valanoor The origins of recently reported visible-light photoelectrochemical activity in ZnS-GaP (ZG) multilayer films are investigated using aberration-corrected scanning transmission electron microscopy (STEM). It is revealed that the multilayers carry a large volume fraction of defects, specifically stacking faults and twins, at the interfaces. The defects act as excellent channels for diffusion. For each ZG interface, a ∼5 nm-interdiffused region with an effective chemical composition of a ZnS-GaP solid solution is observed. Previous theoretical calculations have found that ZnS-GaP solid solutions possess a lower band gap than either GaP or ZnS and thus are expected to have better visible-light photo-activity. These findings are thus able to explain the observed commensurate increase in the visible-light photoelectrochemical response with increasing number of ZG layers. This work suggests that interfaces with intentionally designed lattice imperfections and/or intentionally driven interdiffusion leading to local solid solution formation provide a new materials design strategy for achieving efficient visible-light photo-activity.Graphical abstractImage, graphical abstract
  • Dissecting the influence of nanoscale concentration modulation on
           martensitic transformation in multifunctional alloys
    • Abstract: Publication date: Available online 24 September 2019Source: Acta MaterialiaAuthor(s): Jiaming Zhu, Hong-Hui Wu, Xu-Sheng Yang, He Huang, Tong-Yi Zhang, Yunzhi Wang, San-Qiang Shi Nanoscale concentration modulation (CM) is a novel and effective approach of manipulating martensitic transformations (MTs) for developing next-generation high-performance shape memory alloys (SMAs). Spinodal decomposition is one of the most economic methods to obtain bulk compositionally modulated materials for practical applications. The wavelength, amplitude, and statistical distribution of CM generated by spinodal decomposition are tunable via adjusting the ageing temperature, or the ageing time. However, how these features influence the effect of CM on MTs still remains largely unexplored. In this study, theoretical analyses and computer simulations are combined to dissect the influence of these features on the kinetic process of MTs and mechanical properties of SMAs. The findings of this study provide insights and guidance on the design of SMAs for desired mechanical properties via CM engineering. Moreover, the findings are applicable to not only SMAs but also other materials that have MTs, e.g. steels and high-entropy alloys.Graphic abstractImage, graphical abstract
  • In Situ Characterization of Work Hardening and Springback in Grade 2
           α-Titanium Under Tensile Load
    • Abstract: Publication date: Available online 24 September 2019Source: Acta MaterialiaAuthor(s): K. Sofinowski, M. Šmíd, S. Van Petegem, S. Rahimi, T. Connolley, H. Van Swygenhoven Plastic effects during sheet metal forming can lead to undesirable distortions in formed components. Here, the three-stage work hardening and plastic strain recovery ("springback") in a cold-rolled, α-phase commercially pure titanium is examined. Interrupted standard tensile tests with in situ x-ray diffraction and quasi-in situ electron backscatter diffraction show that twinning plays a minor role in both of these phenomena. The experiments give evidence that the observed work hardening plateau is the result of an abrupt activation and multiplication of 〈c+a〉 slip and a subsequent redistribution of load between grain families. The springback can be attributed to inelastic backwards motion and annihilation of dislocations, driven by backstresses from dislocation-based hardening during loading. The peak broadening behavior, observed by x-ray diffraction, suggests that the internal stress state is highest in the rolling direction, resulting in consistently higher springback magnitude along this direction.Graphical_abstractImage, graphical abstract
  • Helical Dislocations: Observation of vacancy defect bias of screw
           dislocations in neutron irradiated Fe-9Cr
    • Abstract: Publication date: Available online 23 September 2019Source: Acta MaterialiaAuthor(s): J.C. Haley, F. Liu, E. Tarleton, A.C.F. Cocks, G.R. Odette, S. Lozano-Perez, S.G. Roberts We have analysed the microstructure of a model alloy of Fe9Cr irradiated with neutrons to a dose of 1.6 dpa at 325°C. Helical dislocations comprise a major part of the damage; these formed from the interaction of pre-existing screw dislocations with irradiation-induced defects. We have investigated the process behind how these helices form, and how they cause local clustering of dislocation loops. Specifically, we have shown experimentally that the interaction of vacancy defects with pre-existing screw dislocations causes the formation of mixed screw-edge helical dislocations. Interstitials and vacancies were generated in equal numbers, which shows that the screw dislocations must have acted as vacancy-biased sinks.Helical dislocations in general were analysed from a theoretical perspective, and three Dimensional Discrete Dislocation Dynamics (3D-DDD) was used to develop a model for the formation and growth of a vacancy-fed helical dislocation.Since the helical dislocations cause the removal of vacancies from the local microstructure, this leaves a higher supersaturation of interstitials close to the dislocations. We argue that this supersaturation is responsible for enhanced interstitial loop coarsening, leading to a higher proportion of visible interstitial clusters in the vicinity of helical dislocations. These findings offer a new perspective on how dislocations affect the spatial homogeneity of radiation damage.Graphical abstractImage, graphical abstract
  • Influence of phase decomposition on mechanical behavior of an equiatomic
           CoCuFeMnNi high entropy alloy
    • Abstract: Publication date: Available online 22 September 2019Source: Acta MaterialiaAuthor(s): Benjamin E. MacDonald, Zhiqiang Fu, Xin Wang, Zhiming Li, Weiping Chen, Yizhang Zhou, Dierk Raabe, Julie Schoenung, Horst Hahn, Enrique J. Lavernia Phase decomposition is commonly observed experimentally in single-phase high entropy alloys (HEAs). Hence, it is essential for the consideration of HEAs for structural applications to study and understand the nature of phase decomposition in HEAs, particularly the influence it has on mechanical behavior. This paper describes the phase decomposition in the equiatomic CoCuFeMnNi HEA and how the reported secondary phases influence mechanical behavior. Thermomechanical processing, followed by systematic post deformation annealing treatments, revealed the formation of two distinct secondary phases within the equiatomic face-centered cubic (FCC) matrix phase. Low temperature annealing treatments at 600°C and below led to the nucleation of Fe-Co rich ordered B2 precipitates that contributed precipitation hardening while sufficiently small in size, on the order of 140 nm in diameter. At temperatures < 800°C Cu segregation, due to its immiscibility with the other constituents, eventually forms a Cu-rich disordered FCC phase that is determined to increase the yield strength of the alloy while reducing the ductility, likely attributable to the presence of additional interfaces. The thermal stability and chemistry of these phases are compared to those predicted on the basis of calculated phase diagram (CALPHAD) analyses.Graphical Image, graphical abstract
  • Microstructural evolution of helium-implanted 6H-SiC subjected to
           different irradiation conditions and annealing temperatures: a multiple
           characterization study
    • Abstract: Publication date: Available online 22 September 2019Source: Acta MaterialiaAuthor(s): N. Daghbouj, B.S. Li, M. Callisti, H.S. Sen, M. Karlik, T. Polcar The microstructural phenomena occurring in 6H-SiC subjected to different irradiation conditions and annealing temperatures were investigated to assess the suitability of 6H-SiC as a structural material for nuclear applications. To this aim, a single crystal of 6H-SiC was subjected to He+ irradiation at 300 keV with different fluences and at temperatures ranging from 25 to 750°C. Rutherford backscattering/channeling (RBS/C), X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses were combined to shed light on the microstructural changes induced by irradiation and subsequent annealing (750 to 1500°C). At room temperature, amorphization starts to occur at a fluence of 2.5 × 1016 cm−2 (0.66 dpa). On the contrary, amorphization was prevented at high irradiation temperatures and fluences. Furthermore, a thin and highly strained region located around the maximum He concentration (Rp) formed. This region results from the accumulation of interstitial atoms which are driven toward the highly damaged region under the actions of a strain gradient and high temperature. Regardless of the fluence and irradiation temperature, the material stores elastic energy, which leads to the trapping of He in dissimilar defect geometries. For irradiation temperatures below 750°C, helium was accumulated in bubbles which coarsened after annealing. On the other hand, for an implantation temperature of 750°C, helium was trapped in platelets (even for medium fluence), which evolved into a homogenous dense array of cavities during annealing. DFT calculations show that the bubbles are under high pressure and contribute to developing the overall tensile strain in the single crystal 6H-SiC.Graphical abstractImage, graphical abstract
  • A 3D analysis of the onset of slip activity in relation to the degree of
           micro-texture in Ti-6Al-4V
    • Abstract: Publication date: Available online 22 September 2019Source: Acta MaterialiaAuthor(s): S. Hémery, A. Naït-Ali, M. Guéguen, J. Wendorf, A.T. Polonsky, M.P. Echlin, J.C. Stinville, T.M. Pollock, P. Villechaise The mechanical properties of titanium alloys result from their complex multi-scale microstructural features, including micron scale precipitates and millimeter scale microtextured regions (MTRs). While previous investigations have revealed that the presence of mm-scale MTRs can degrade mechanical properties, particularly fatigue, the accompanying strain localization processes that operate at the microscale within the α grains in MTRs are not well understood. The present work is a mechanistic investigation of MTRs using crystal plasticity simulations of mm3-scale experimentally captured and synthetically generated 3D microstructure datasets. The explicit modeling of both the α grains and MTRs in Ti-6Al-4V enables assessment of the effect microtexture and local structure variations within the MTR on overall deformation behavior and the onset of plastic slip in MTRs. The presence of MTRs with a dominant [0001] orientation results in both stress and plastic strain hotspots during the early stages of straining. Crystal plasticity predictions are compared to previous digital image correlation studies on early strain localization. The influence of MTRs on the local stress and strain fields is discussed with regard to the monotonic tension, fatigue and dwell-fatigue behavior of titanium alloys.Graphical abstractImage, graphical abstract
  • Spectrum of Grain Boundary Segregation Energies in a Polycrystal
    • Abstract: Publication date: Available online 21 September 2019Source: Acta MaterialiaAuthor(s): Malik Wagih, Christopher A. Schuh Solute segregation at grain boundaries (GBs) is emerging as an alloy design tool, uses of which include the stabilization of nanocrystalline alloys. To predict the equilibrium segregation state in a given alloy, most thermodynamic models treat the full network of GBs as a single “entity”, and thus use an “effective” segregation energy to describe it. This simplification ignores the spectral nature of available GB segregation energies in a polycrystal, which we elucidate here computationally for a Mg solute in an Al polycrystal; the distribution is found to be captured accurately with a skew-normal function. A thermodynamic segregation isotherm that incorporates this spectrum is outlined and employed to study the effect of such a spectrum on predictions of the equilibrium GB segregation state. The ramifications for experimentally-extracted GB segregation energies are shown to be potentially significant, and nanocrystalline stability criteria are extended to account for this spectral nature of GB segregation.Graphical abstractImage, graphical abstract
  • CuMnNiSi precipitate evolution in irradiated reactor pressure vessel
           steels: Integrated Cluster Dynamics and experiments
    • Abstract: Publication date: November 2019Source: Acta Materialia, Volume 180Author(s): Mahmood Mamivand, Peter Wells, Huibin Ke, Shipeng Shu, G. Robert Odette, Dane Morgan An improved Cluster Dynamics (CD) model of Cu rich and Mn–Ni–Si phase co-precipitation was developed to provide insights on the combined effects of the flux, fluence, temperature and alloy composition on irradiation enhanced precipitation leading to the embrittlement of reactor pressure vessel steels. The CD model was calibrated using a large microstructural database, and key parameters (e.g., interfacial energies) were fitted to minimize the predicted versus measured errors. The CD model was further validated against data not used in the fitting. The CD model predicts that: a) even 0.05% Cu reduces the Mn, Ni, Si precipitation threshold fluence; b) precipitate number densities increase, while their sizes and volume fractions, decrease with increasing flux; c) precipitate number densities and volume fractions increase with decreasing temperature; and, d) most of the matrix Cu precipitates in the early years of vessel service, while MnNiSi precipitates continue to grow up to very high extended life fluence.Graphical abstractImage 1
  • Effect of non-stoichiometry on the crystal nucleation and growth in oxide
    • Abstract: Publication date: Available online 19 September 2019Source: Acta MaterialiaAuthor(s): Vladimir M. Fokin, Alexander S. Abyzov, Alisson M. Rodrigues, Rogerio Z. Pompermayer, Guilherme de Silva Macena, Edgar D. Zanotto, Eduardo B. Ferreira Non-stoichiometric glasses (NSG) are much more common than stoichiometric compositions. However, due to inherent difficulties, fundamental studies of crystallization kinetics of NSG are much less frequent. To shed light on the crystal nucleation and growth kinetics of NSG, we adopted a nucleation kinetics model, leaving the interfacial energy and diffusion coefficient as free parameters, to explain experimental nucleation data of glasses of three compositions in the pseudo-binary Li2O·2SiO2–BaO·2SiO2 model system. We show that, as the glass composition approaches the eutectic, the nucleation rates drop drastically, mainly due to an increase in the interfacial energy. This result corroborates the common empirical observation that eutectic compositions tend to show good glass-forming ability. We also found that the structural relaxation times are significantly shorter than the characteristic nucleation times, validating the widely used and scarcely tested assumption that relaxation does not play a noticeable role in crystal nucleation. For a stoichiometric glass the fitted diffusion coefficient determining nucleation, D, is lower than that determining the growth of macro crystals, DU, and this difference significantly increases with decreasing temperature. On the other hand, the diffusion coefficient calculated from viscosity, Dη, is close to DU at high temperatures and approaches D in the glass transition range. Finally, after crystallization of the primary phase (lithium disilicate), barium disilicate and more lithium disilicate crystals precipitate in the diffusion zone existing in the residual glass.Graphical abstractImage 1
  • in situ High Energy X-ray Diffraction Measurement of Strain and
           Dislocation Density ahead of Crack Tips Grown in Hydrogen
    • Abstract: Publication date: Available online 18 September 2019Source: Acta MaterialiaAuthor(s): Matthew Connolly, May Martin, Peter Bradley, Damian Lauria, Andrew Slifka, Robert Amaro, Christopher Looney, Jun-Sang Park The deformation fields near fatigue crack tips grown in hydrogen and in air were measured using high-energy x-ray diffraction. A larger magnitude of elastic strain was observed in the hydrogen case compared to the air case. The magnitude of elastic strain was quantified through an effective crack tip stress intensity factor. The dislocation profile ahead of the crack was probed via x-ray line broadening and electron back-scatter diffraction was used to assess the crack path (intergranular vs. transgranular). Ahead of the crack tip grown in hydrogen, an order of magnitude lower dislocation density, compared to a baseline density far from the crack, was observed. This decrease in dislocation density was not observed in the air case. These differences are discussed in terms of two leading hydrogen embrittlement mechanisms, Hydrogen Enhanced Localized Plasticity (HELP) and Hydrogen Enhanced Decohesion (HEDE). We have observed a decrease in transgranular cohesion (transgranular HEDE), as well as an increase in intergranular fracture. The measurements of dislocation activity support a model of a decrease in intergranular cohesion (intergranular HEDE) which is likely facilitated by the HELP mechanism. This suggests that the increase in fatigue crack growth rate is due to a sum of the two effects of hydrogen, in which the crack grows faster in the transgranular fracture mode and faster due to an increase in a new mode of intergranular fracture.Graphical abstractImage 1
  • Revealing the deformation mechanisms of nanograins in gradient
           nanostructured Cu and CuAl alloys under tension
    • Abstract: Publication date: Available online 16 September 2019Source: Acta MaterialiaAuthor(s): J.J. Wang, N.R. Tao, K. Lu A gradient nanostructured (GNS) surface layer was induced on coarse-grained (CG) Cu and CuAl alloys by means of surface mechanical grinding treatment. The GNS/CG Cu-4.5Al sample subjected to tensile tests yields at a higher strength and fails at a higher uniform elongation (∼42%) in comparison with the GNS/CG Cu and Cu-2.2Al samples. The microstructures of the GNS/CG samples before and after tension at different strains were systematically investigated by transmission electron microscope. It is revealed that grain coarsening dominates the plastic deformation of nanograins in the GNS/CG Cu sample while the propensity of deformation twinning in nanograins increases in the GNS/CG CuAl samples. The experimental results suggested a transition of deformation mechanism of nanograins from grain coarsening to the partial dislocation associated deformation twinning in the GNS/CG Cu and CuAl alloys with increasing Al solute concentration. The obvious activation of deformation twinning accommodates the large tensile plasticity of the surface nanograins in the GNS/CG Cu-4.5Al sample. This work demonstrated that the partial dislocation associated deformation twinning is an effective deformation mechanism to retard the strain localization and to improve the tensile ductility of nanograins.Graphical abstractImage 1
  • Role of surface oxide layers in the hydrogen embrittlement of austenitic
           stainless steels: a TOF-SIMS study
    • Abstract: Publication date: Available online 14 September 2019Source: Acta MaterialiaAuthor(s): Chika Izawa, Stefan Wagner, Martin Deutges, Mauro Martín, Sebastian Weber, Richard Pargeter, Thorsten Michler, Haru-Hisa Uchida, Ryota Gemma, Astrid Pundt Hydrogen environment embrittlement (HEE) of low-nickel austenitic stainless steels (AISI 300 series) with different chemical compositions was studied focusing on the impact of the steels surface oxides, grain sizes and dislocation arrangements. The susceptibility of the steels to HEE is judged with respect to the relative reduction of area (RRA), where the HEE susceptibility is lower for larger RRA values.For many AISI 300 steels a linear trend is observed correlating RRA and the probability of strain induced martensite formation in tensile tests. Some steels, however, depart from the general trend, revealing greater HEE resistances.A careful examination of possible factors influencing HEE of the investigated steels reveals that high RRA values are linked to a specific type of oxide layer, namely the “high constant level oxide”, as categorized by TOF-SIMS evaluation. Thus, this type of oxide layer may be able to lower the steels HEE susceptibility. Other types of surface oxides, grain sizes and dislocation arrangements in the matrix of the particular AISI 300 steels appear to be of secondary importance.Graphical abstractImage 1
  • Effect of solutes on strength and ductility of Mg alloys
    • Abstract: Publication date: Available online 13 September 2019Source: Acta MaterialiaAuthor(s): D.F. Shi, M.T. Pérez-Prado, C.M. Cepeda-Jiménez This work investigates the origin of the simultaneous increase in strength and ductility that takes place in Mg polycrystals alloyed with Al and Zn solutes. With that purpose, twelve polycrystalline binary Mg-Zn and Mg-Al alloys, with up to 2 wt. % of alloying additions and average grain sizes comprised between 3 and 42 μm, were prepared by casting, hot rolling and annealing and were tested at room temperature and quasi-static strain rates. Electron backscattered diffraction-assisted slip trace analysis was then utilized to characterize slip activity, and the latter was related to the grain size, to the texture, and to the topology of the grain boundary network. Basal slip was found to be the dominant deformation mechanism in all the binary alloys, irrespective of composition and grain size. Alloying additions were observed to have little influence on texture development but acted as strong modifiers of the topology of the grain boundary network developed during processing. In particular, they reduced the connectivity of grains that are well oriented for basal slip, preventing intergranular slip localization and, in turn, leading to considerable strengthening of basal slip. Solutes act also as enhancers of diffuse slip within individual grains. It is proposed that the simultaneous increase in strength and ductility of Mg alloys by the addition of solutes must be understood as a multiscale phenomenon resulting from the coupling of solute-dislocation interactions at the atomic scale with alterations of the topology of the grain boundary network at the mesoscale.Graphical abstractImage 1
  • Efficient Use of Multiple Information Sources in Material Design
    • Abstract: Publication date: Available online 13 September 2019Source: Acta MaterialiaAuthor(s): Seyede Fatemeh Ghoreishi, Abhilash Molkeri, Raymundo Arróyave, Douglas Allaire, Ankit Srivastava We present a general framework for the design/optimization of materials that is capable of accounting for multiple information sources available to the materials designer. We demonstrate the framework through the microstructure-based design of multi-phase microstructures. Specifically, we seek to maximize the strength normalized strain-hardening rate of a dual-phase ferritic/martensitic steel through a multi-information source Bayesian optimal design strategy. We assume that we have multiple sources of information with varying degrees of fidelity as well as cost. The available information from all sources is fused through a reification approach and then a sequential experimental design is carried out. The experimental design seeks not only to identify the most promising region in the materials design space relative to the objective at hand, but also to identify the source of information that should be used to query this point in the decision space. The selection criterion for the source used accounts for the discrepancy between the source and the ‘ground truth’ as well as its cost. It is shown that when there is a hard constraint on the budget available to carry out the optimization, accounting for the cost of querying individual sources is essential.Graphical abstractImage 1
  • Atomistic Investigation into Interfacial Effects on the Plastic Response
           and Deformation Mechanisms of the Pearlitic Microstructure
    • Abstract: Publication date: Available online 13 September 2019Source: Acta MaterialiaAuthor(s): Matthew Guziewski, Shawn P. Coleman, Christopher R. Weinberger Atomistic modeling is used to investigate the mechanical response and deformation mechanisms at 5 K temperature within the commonly reported orientation relationships between ferrite and cementite within pearlite: Bagaryatskii, Pitsch-Petch, Isaichev, and their associated near orientations. For each orientation, compressive and tensile simulations were performed in the transverse and longitudinal directions for a range of ferrite to cementite volume ratios. Important mechanical properties such as peak stress, flow stress, and the activated slip systems in both lamella are reported. Significant variation in mechanical response is found between the various orientation relationships. In the transverse direction, the responses are well described by composite theory; longitudinal loading requires further consideration of the strain compatibility of the interface. Plasticity within the ferrite is found to initiate from the interface and is well described by Schmid factors; slip and failure in the cementite are affected by slip transfer mechanisms across the the interface between the lamella. Simulation results are used to create a simple model for predicting deformation behavior in pearlite, allowing for greater understanding of the plasticity and failure mechanisms within the various reported orientations, and raising the possibility of the targeted creation of specific microstructures based on the intended mechanical loading.Graphical abstractImage 1
  • Effect of Ag addition on the precipitation evolution and interfacial
           segregation for Al-Mg-Si alloy
    • Abstract: Publication date: Available online 12 September 2019Source: Acta MaterialiaAuthor(s): Yaoyao Weng, Lipeng Ding, Zezhong Zhang, Zhihong Jia, Boyang Wen, Yingying Liu, Shinji Muraishi, Yanjun Li, Qing Liu The effect of Ag addition on the precipitation evolution and interfacial segregation for Al-Mg-Si alloys was systematically investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), atom probe tomography (APT) and density functional theory (DFT) calculation. At the early aging stage, Ag atoms could enter clusters and refine the distribution of these clusters. Then, Ag atoms preferentially segregate at the GP zone/α-Al and β"/α-Al interfaces at the peak aging stage by the replacement of Al atoms in FCC matrix. With prolonging aging time, Ag atoms generally incorporate into the interior of β" precipitate, facilitating the formation of QP lattice (a hexagonal network of Si atomic columns) and the local symmetry substructures, Ag sub-unit (1) and Ag sub-unit (2). At the over-aged stage, the Ag sub-unit (1) and Ag sub-unit (2) could transform to the β′Ag (i.e. β′Ag1 and β′Ag2.) and Q′Ag unit cells, respectively. All the precipitates at the over-aging stage have a composite and disordered structure due to the coexistence of different unit cells (β′Ag1, β′Ag2, Q′Ag and β′) and the non-periodic arrangement of Ag atoms within the precipitate. In the equilibrium stage, the incorporated Ag atoms in the precipitates release into the α-Al matrix as solute atoms or form Ag particles. In general, Ag atoms undergo a process of “segregate at the precipitate/matrix interface → incorporate into the interior of precipitate → release into the α-Al matrix” during the precipitation for Al-Mg-Si-Ag alloys. Besides, Ag segregation is found at the interfaces of almost all metastable phases (including GP zone, β″, β′/β′Ag phase) in Al-Mg-Si-Ag alloys. The Ag segregation at the β′/α-Al interface could increase the length/diameter ratio of β′ phase and thus promote the additional strengthening potential of these alloys. These findings provide a new route for precipitation hardening by promoting the nucleation and morphology evolution of precipitates.Graphical abstractImage 1
  • Swamps of Hydrogen in Equiatomic FeCuCrMnMo Alloys: First-principles
    • Abstract: Publication date: Available online 12 September 2019Source: Acta MaterialiaAuthor(s): X.L. Ren, P.H. Shi, W.W. Zhang, X.Y. Wu, Q. Xu, Y.X. Wang High-entropy alloys (HEAs) merit promising applications in nuclear reactors. A FeCuCrMnMo HEA with low activation under neutron irradiation was studied, which was first focused on the search of an equilibrium state by a hybrid method combining the Metropolis Monte Carlo method and density functional theory (DFT). As a transmutation byproduct in fission reactions or fuel for fusion reactions, the evolution of hydrogen, in the HEA was then investigated through a systematic analysis of H solution and diffusion using DFT and molecular dynamics simulations. Rooted in the unique distortion of HEA lattices, i.e., the destroyed translational symmetry of the energy landscape, tetrahedral and octahedral interstitial positions have no significant difference in the priority of H residing. Diffusion of H, as a guest atom, also presents a sluggish effect. The dramatic increases and decreases in potential energy generate a great number of insurmountable barriers pervading the matrix and largely suppressing the mobility of H. However, this effect originates not only from the difference between the potential energies of interstitial positions as observed with host atoms, which increases the fluctuation of migration barriers and decreases the effective atomic jump frequency, but also from the destabilization of interstitial positions for H residing. This blocks the diffusion channels of H and further decreases the atomic jump probability in Einstein’s equation. The present investigation provides fundamental insight into H behavior in HEAs and clues for the application of HEAs as materials of tritium permeation barriers or resistance to hydrogen irradiation.Graphical abstractImage 1
  • Recoverability of large strains and deformation twinning in martensite
           during tensile deformation of NiTi shape memory alloy polycrystals
    • Abstract: Publication date: Available online 11 September 2019Source: Acta MaterialiaAuthor(s): Yuchen Chen, Orsolya Molnárová, Ondřej Tyc, Lukáš Kadeřávek, Luděk Heller, Petr Šittner Superelastic NiTi wires were deformed in tension up to gradually increasing total strains at various temperatures, recoverable strains were evaluated and lattice defects left in the microstructure of deformed wires were analyzed by TEM. The recoverable strains evaluated in tensile tests: i) are surprisingly large - exceed 10 % strain at low temperatures T ≤ 20 °C, ii) are not significantly restricted by large plastic deformation up to 55% at low temperatures, iii) display local maxima in dependence on total strain in tensile tests at low temperatures, iv) decrease with increasing test temperatures and iv) increase with increasing total strain in tensile tests at high temperatures T ≥ 50 °C. Besides slip dislocations, key lattice defects created by the tensile deformation beyond the martensite yield point are deformation bands containing {114} austenite twins or R-phase. No other austenite twins were found statistically relevant.It is concluded that the plastic deformation of NiTi wires is initiated by deformation twinning in oriented martensite, accompanied by dislocation slip in austenite and/or martensite in extent depending on the test temperature and total strain. The deformation twinning is found to play an ambiguous role in NiTi deformation. When it proceeds at low temperatures (T ≤ 20 °C) beyond the yield point, it raises recoverable strain up to 13 %, while at high temperatures (T ≥ 50 °C) when it proceeds already within the plateau range as intermediate step of the stress induced B2=>B19’=>B2T martensitic transformation, it restricts the recoverability of transformation strains. The repeated deformation twinning in oriented martensite enables low temperature deformation of NiTi wires with specific microstructure up to ∼50 % strain and causes severe microstructure refinement during the mechanical and thermomechanical processing of NiTi.Graphical abstractImage 1
  • Resistance to amorphisation in Ca1-xLa2x/3TiO3 perovskites – a bulk
           ion-irradiation study
    • Abstract: Publication date: Available online 10 September 2019Source: Acta MaterialiaAuthor(s): Sebastian M. Lawson, Neil C. Hyatt, Karl R. Whittle, Amy S. Gandy The changes induced from 1 MeV Kr+ and 5 MeV Au+ ion irradiation at room temperature have been utilised to determine the impact of cation vacancies on the radiation damage response on bulk Ca1-xLa2x/3TiO3 perovskite structured ceramics. Perovskite systems have long been considered as candidate waste forms for the disposition of actinide wastes and doping with multi-valent elements such as Pu may lead to cation deficiency. Based on GAXRD and TEM analysis, two regions of resistance/susceptibility to amorphisation have been confirmed with reference to CaTiO3. Increased resistance to amorphisation has been observed for 0.1 ≤ x ≤ 0.4, with an increased susceptibility to amorphisation for x ≥ 0.5. It is proposed that these processes are induced by enhanced recovery from radiation damage for 0.1 ≤ x ≤ 0.4, and reduced tolerance for disorder/the increasingly covalent nature of the A-O bond for x ≥ 0.5. Lattice parameter analysis of the x = 0 and 0.5 samples showed a saturation in radiation damage induced volume swelling at 4.7 ± 0.1 % and 1.8 ± 0.1 %, respectively, while the saturation limit for the b parameter was lower than the respective a and c orthorhombic parameters. In the x = 0.2 and 0.4 samples, amorphisation was not observed, however the b parameter was found to swell to a lesser extent than the a and c parameters. Swelling was not observed for the ion irradiated x ≥ 0.6 samples.Graphical abstractImage 1
  • Rearrangement of interstitial defects in alpha-Fe under extreme condition
    • Abstract: Publication date: Available online 9 September 2019Source: Acta MaterialiaAuthor(s): A. Chartier, M.-C. Marinica In this study, by theoretical means, we reveal the main mechanisms that underpin the microstructure evolution driven by the formation of self-interstitial atoms (SIAs) clusters in body centered cubic iron under extreme conditions. Using Frenkel pairs accumulation simulations we point the complex interplay between the two families of interstitial defects, the dislocation loops with Burgers vectors and ½ and the tridimensional C15 clusters. We reconcile the previous sparse understanding of microstructure evolution that put in opposition various mechanisms of defects formation by showing that both ½ loops self-interactions and C15 clusters transformations produce loops. Moreover, we exhibit the fact that these tri-dimensional clusters can form under irradiations with only the Frenkel pair accumulation that mimics electron irradiation and not only in high-energy cascades as it was previously stated. Finally, we show that the tridimensional C15 clusters even precede production of loops under irradiation.Graphical abstractImage 1
  • Corrigendum to Multiscale investigations of nanoprecipitate nucleation,
           growth, and coarsening in annealed low-Cr oxide dispersion strengthened
           FeCrAl powder [Acta Mater. 166 (2019) 1–17]
    • Abstract: Publication date: November 2019Source: Acta Materialia, Volume 180Author(s): Caleb P. Massey, Sebastien N. Dryepondt, Philip D. Edmondson, Matthew G. Frith, Kenneth C. Littrell, Anoop Kini, Baptiste Gault, Kurt A. Terrani, Steven J. Zinkle
  • Fracture behavior and deformation mechanisms in nanolaminated
           crystalline/amorphous micro-cantilevers
    • Abstract: Publication date: Available online 7 September 2019Source: Acta MaterialiaAuthor(s): Y.Q. Wang, R. Fritz, D. Kiener, J.Y. Zhang, G. Liu, O. Kolednik, R. Pippan, J. Sun In order to quantify the fracture toughness and reveal the failure mechanism of crystalline/amorphous nanolaminates (C/ANLs), in-situ micro-cantilever bending tests were performed on Ag/Cu-Zr and Mo/Cu-Zr C/ANLs in a scanning electron microscope over a wide range of cantilever widths from several microns to the submicron scale. The results demonstrate that the fracture behavior was strongly influenced by sample size and constituent phases, respectively. The Ag/Cu-Zr micro-cantilevers failed in a ductile manner, with fracture toughnesses higher than the Mo/Cu-Zr samples that exhibited brittle failure. Both materials also displayed different cantilever width-dependences of fracture toughness. The Ag/Cu-Zr beams showed a fracture toughness that increases with the cantilever width, mainly due to a size-dependent constraining effect on the deformation of the crystalline phase. For the Mo/Cu-Zr beams, the fracture toughness decreased gradually to a low plateau as the cantilever width exceeded ∼1500 nm, which can be rationalized by a transition in stress condition. The underlying fracture mechanism of the Ag/Cu-Zr micro-cantilevers was identified as the interconnection of microcracks initiated in the amorphous Cu-Zr layers, compared to a catastrophically penetrating crack propagation in the Mo/Cu-Zr samples. The discrepancy in size-dependent fracture behavior between the two material systems is discussed in terms of plastic energy dissipation of ductile phases, crack tip blunting, crack bridging and the effect of strain gradient in the plastic zone on crack propagation.Graphical abstractImage 1
  • Hydrogen pickup during oxidation in aqueous environments: the role of
           nano-pores and nano-pipes in zirconium oxide films
    • Abstract: Publication date: Available online 7 September 2019Source: Acta MaterialiaAuthor(s): Jing Hu, Junliang Liu, Sergio Lozano-Perez, Chris Grovenor, Mikael Christensen, Walter Wolf, Erich Wimmer, Erik V. MaderABSTRACTOxidation of metals by water generates hydrogen which can enter the solid causing serious degradation of its mechanical properties and may also influence the corrosion rate. The present work focuses on hydrogen pickup during the corrosion of zirconium alloys in an aqueous environment. Transmission electron microscopy using Fresnel imaging on three different samples of oxidized Zr has been used to study the type, distribution, concentration and connectivity of nano-porosity as a function of depth through the oxide layer. Extensive interconnected nano-pipes are found in the non-protective outer part of the oxide, while in the protective barrier layer closer to the metal-oxide interface, continuous nano-pipes turn into individual nano-pores. Ab initio calculations show that molecular hydrogen is formed spontaneously by the reaction of water with oxygen vacancies in zirconium oxide. Molecular dynamics simulations reveal that these H2 molecules can diffuse rapidly through nano-pores and nano-pipes as small as 0.5 nm in the oxide layer. Calculations demonstrate that molecular hydrogen dissociates spontaneously on surfaces of suboxides found experimentally at the metal-oxide interface. Oxygen vacancies in ZrO enable the ingress and diffusion of H atoms with an energy barrier of approximately 65 kJ/mol. Further diffusion of hydrogen through oxygen-saturated α-Zr metal is fast, leading to the formation of thermodynamically stable zirconium hydrides. Thus, formation and diffusion of molecular hydrogen through nano-pores in the bulk oxide and ingress of H atoms via suboxides is a possible mechanism of hydrogen pickup in any metal or alloy covered by an oxide scale that contains nano-porosity.Graphical abstractImage 1
  • Critical role of atomic-scale defect disorders for high-performance
           nanostructured half-Heusler thermoelectric alloys and their thermal
    • Abstract: Publication date: Available online 7 September 2019Source: Acta MaterialiaAuthor(s): Ho Jae Lee, Kyu Hyoung Lee, Liangwei Fu, GyeongTak Han, Hyun-Sik Kim, Sang-Il Kim, Young-Min Kim, Sung Wng Kim Atomic-scale defects are essential for improving thermoelectric (TE) performance of most state-of-the-art materials by simultaneously tuning the electronic and thermal properties. However, because the plural atomic-scale defects are generally inherent and disordered in nanostructured TE materials, their complexity and ambiguity on determining TE performance remain a challenge to be solved. Furthermore, the thermal stability of atomic-scale defects in nanostructured TE materials has not been studied much so far. Herein, we report that the atomic-scale defect disorders are indispensable for high TE performance of nanostructured Ti1–xHfxNiSn1–ySby half-Heusler alloys, but gradually degraded at over 773 K, deteriorating the TE performance. It is found from the thermal annealing of nanostructured Ti0.5Hf0.5NiSn0.98Sb0.02 alloys that the annihilation of Ti,Hf/Sn antisite defects primarily reduces atomic-scale defect disorders and largely contributes to the increase of lattice thermal conductivity. Moreover, it is verified that the Ni interstitial defects mainly dominate the electronic transport properties, leading to the enhancement of power factor. Direct atomic structure observations clearly demonstrate the inherent Ni interstitial defects and the thermal vulnerability of Ti,Hf/Sn antisite defects. These results provide an important guide for the application of half-Heusler alloys with highly disordered atomic-scale defects.Graphical abstractImage 1
  • Local Structural Investigation of Hafnia-Zirconia Polymorphs in Powders
           and Thin Films by X-ray Absorption Spectroscopy
    • Abstract: Publication date: Available online 6 September 2019Source: Acta MaterialiaAuthor(s): Tony Schenk, Andris Anspoks, Inga Jonane, Reinis Ignatans, Brienne S. Johnson, Jacob L. Jones, Massimo Tallarida, Carlo Marini, Laura Simonelli, Philipp Hönicke, Claudia Richter, Thomas Mikolajick, Uwe Schroeder Despite increasing attention for the recently found ferro- and antiferroelectric properties, the polymorphism in hafnia- and zirconia-based thin films is still not sufficiently understood. In the present work, we show that it is important to have a good quality X-ray absorption spectrum to go beyond an analysis of the only the first coordination shell. Equally important is to analyze both EXAFS and XANES spectra in combination with theoretical modelling to distinguish the relevant phases even in bulk materials and to separate structural from chemical effects. As a first step toward the analysis of thin films, we start with the analysis of bulk references. After that, we successfully demonstrate an approach that allows us to extract high-quality spectra also for 20 nm thin films. Our analysis extends to the second coordination shell and includes effects created by chemical substitution of Hf with Zr to unambiguously discriminate the different polymorphs. The trends derived from X-ray absorption spectroscopy agree well with X-ray diffraction measurements. In this work we clearly identify a gradual transformation from monoclinic to tetragonal phase as the Zr content of the films increases. We separated structural effects from effects created by chemical disorder when ration of Hf:Zr is varied and found differences for the incorporation of the substitute atoms between powders and thin films, which we attribute to the different fabrication routes. This work opens the door for further in-depth structural studies to shine light into the chemistry and physics of these novel ferroelectric thin films that show high application relevance.Graphical abstractImage 1
  • Size Effects in the Martensitic Transformation Hysteresis in Ni-Mn-Sn
           Heusler Alloy Films
    • Abstract: Publication date: Available online 5 September 2019Source: Acta MaterialiaAuthor(s): Yijia Zhang, Julia Billman, Patrick J. Shamberger Understanding the effect of small characteristic length scales on phase transformations requires microscopic observations to identify mechanisms which may influence the progression of the transformation. Here, we report thickness-dependent hysteresis in electrochemically deposited Ni-Mn-Sn Heusler alloy films, as observed by optical microscopy. This approach allows for analyzing size dependent phase transformation behavior within individual grains from films with decreasing thickness. Hysteresis is not correlated with grain size, but increases with decreasing film thickness following a power law relationship. This behavior is attributable to internal friction-induced energy dissipation at the film/substrate interface in microscale alloy films.Graphical abstractImage 1
  • In situ study on fracture behaviour of white etching layers
           formed on rails
    • Abstract: Publication date: Available online 4 September 2019Source: Acta MaterialiaAuthor(s): A. Kumar, A.K. Saxena, C. Kirchlechner, M. Herbig, S. Brinkmann, R.H. Petrov, J. Sietsma Failure in engineering materials like steels is strongly affected by in-service deleterious alterations in their microstructure. White Etching Layers (WELs) are an example of such in-service alterations in the pearlitic microstructure at the rail surface. Cracks initiate in the rails due to delamination and fracture of these layers and propagate into the base material posing severe safety concerns. In this study, we investigate the microscale fracture behaviour of these WELs. We use in situ elastic-plastic fracture mechanics using J-integral to quantify the fracture toughness. Although usually assumed brittle, the fracture toughness of 21 – 25 MPa√m reveals a semi-brittle nature of WELs. Based on a comparison of the fracture toughness and critical defect size of WELs with the undeformed pearlitic steels, WELs are detrimental for rails. In the micro fracture tests, WELs show crack tip blunting, branching, and significant plasticity during crack growth due to their complex microstructure. The fracture behaviour of the WELs is governed by their microstructural constituents such as phases (martensite/austenite), grain size, dislocation density and carbon segregation to dislocations and grain boundaries. We observed dislocation annihilation in some martensitic grains in the WELs which also contributes to their fracture behaviour. Additionally, the strain-induced transformation from austenite to martensite affects the crack growth and fracture.Graphical abstractImage 1
  • Grain Refinement Mechanism of Nickel-Based Superalloy by Severe Plastic
           Deformation - Mechanical Machining Case
    • Abstract: Publication date: Available online 3 September 2019Source: Acta MaterialiaAuthor(s): Zhirong Liao, Mikhail Polyakov, Oriol Gavalda Diaz, Dragos Axinte, Gaurav Mohanty, Xavier Maeder, Johann Michler, Mark Hardy This paper studied the formation mechanism of white layer of a next generation nickel-based superalloy formed under severe plastic deformation induced by a mechanical material removal process. A graded microstructure of the white layer in the nickel-based superalloy has been revealed for the first time, which is composed of (i) a “dynamic recrystallisation” layer formed by nanocrystalline (∼200 nm) grains at the vicinity of the surface and (ii) a “dynamic recovery” layer with subgrain microstructures extending further into the subsurface. The mechanism of surface grain refinement was identified based on the results obtained via crystallographic and chemical analysis, as well as in-situ micro-mechanics experiments in the scanning electron microscope. It is found that in the top surface layer not only grain refinement but also the γ’ phase dissolution occurs, changing drastically from the bulk material. Furthermore, it is shown how the high plastic strain and cutting temperature along the subsurface causes grain refinement in the white layer and grain elongation in the subsurface. The γ’ precipitates in the recrystallisation layer are dissolved during the machining process, while the ultra-high cooling rate suppresses the further precipitation of this phase, resulting in the supersaturation of γ grains or minimized γ’ precipitates in the top surface layer. Hence, the grain refinement does not result in an increase of mechanical stiffness but a deterioration of mechanical properties due to the dissolution of the strengthening phase γ’, which leads to a lower strength and increased ductility. Machining is generally treated as a cold-working process. However, according to our findings hot-working with dynamic recrystallisation and recovery, as well as phase evolution, occurs in the white layer of nickel-based superalloys.Graphical abstractImage 1
  • Rapid solidification of Nd1+XFe11Ti compounds: phase formation and
           magnetic properties
    • Abstract: Publication date: Available online 31 August 2019Source: Acta MaterialiaAuthor(s): F. Maccari, L. Schäfer, I. Radulov, L.V.B. Diop, S. Ener, E. Bruder, K. Skokov, O. Gutfleisch The effects of compositional variations and different annealing regimes in Nd(Fe,Ti)12 alloys were studied in terms of phase formation and magnetic properties analysis. NdxFe11Ti (x=1.05, 1.10, 1.15, 1.20) alloys were produced by rapid solidification through suction casting technique. The effect of Nd content and post annealing were investigated in the temperature range of 700-1200°C. Single 1:12 phase samples were obtained at temperatures between 1150-1200°C for compositions with Nd concentration of 1.15 and 1.20. Intrinsic magnetic properties and magnetization reversal were studied for 1:12 single phase samples, revealing uniaxial anisotropy with anisotropy field (HA) of 1.08T and saturation magnetization of 137Am2kg-1 at room temperature. In addition, the demagnetization mechanism in bulk polycrystalline samples was analyzed by means of Kerr microscopy under applied magnetic fields. Magnetization reversal process starts at the twin boundary, which acts as a nucleation center for the reversal domain, and coupling between adjacent grains is also observed. These may be part of the reasons for the observed low coercivity in the NdFe11Ti systems. The findings of the present study leads to a better understanding of the relation between magnetic properties and microstructure, and can open new strategies to obtain coercivity in this 1:12 phase system and, possibly, in the corresponding nitride.Graphical abstractImage 1
  • Experimental and computational analysis of binary Fe-Sn ferromagnetic
    • Abstract: Publication date: Available online 30 August 2019Source: Acta MaterialiaAuthor(s): Bahar Fayyazi, Konstantin P. Skokov, Tom Faske, Ingo Opahle, Michael Duerrschnabel, Tim Helbig, Ivan Soldatov, Urban Rohrmann, Leopoldo Molina-Luna, Konrad Güth, Hongbin Zhang, Wolfgang Donner, Rudolf Schäfer, Oliver Gutfleisch Ferromagnetic Fe3Sn, Fe5Sn3 and Fe3Sn2 single crystals were synthesized using the reactive flux technique. Derived from single crystal x-ray diffraction and Transmission Electron Microscopy (TEM), a new structural model is proposed for the Fe5Sn3 crystals - the threefold twinning of an orthorhombic unit cell with (3+1) dimensional space group Pbcm(α00)0s0. The spontaneous magnetization (Ms) and the anisotropy constants K1 and K2 of Fe3Sn, Fe5Sn3 and Fe3Sn2 single crystals were determined in a wide temperature range using M(H) dependencies and a modified Sucksmith-Thompson technique. Ms and K1 were also evaluated in the framework of Density Functional Theory (DFT) and an overall good agreement was observed between the calculated and experimental results. Furthermore, a critical evaluation of different analytical models for the assessment of magnetocrystalline anisotropy was performed, which are restricted to the analysis of uniaxial magnetic domain patterns, and it is shown that such high-throughput techniques can lead to unrealistic results. Finally, a DFT high-throughput screening of the Fe-Sn phase diagram was used to identify Fe-Sn based phases with potential to be stabilized upon alloying, and their magnetization and magnetocrystalline anisotropy were evaluated. The results show that a similar strong anisotropy as observed in Fe3Sn may also be found in other Fe-Sn based phases, having higher potential to be used as hard magnetic material.Graphical abstractImage 1
  • Structural Perspective on Revealing Energy Storage Behaviors of Silver
           Vanadates Cathode in Aqueous Zinc-Ion Batteries
    • Abstract: Publication date: Available online 30 August 2019Source: Acta MaterialiaAuthor(s): Shan Guo, Guozhao Fang, Shuquan Liang, Minghui Chen, Xianwen Wu, Jiang ZhouABSTRACTExploitation and improvement of electrode materials mainly rely on the understanding of electrochemical reaction mechanisms. Here we provide a comprehensive perspective of zinc ions storage behaviors in silver vanadates (e.g. Ag0.33V2O5, Ag1.2V3O8, Ag2V4O11, β-AgVO3, Ag4V2O7), which exhibit electrochemical redox multi-mechanisms. Ag0.33V2O5 with stable tunnel structure and low mole ratio of Ag/V demonstrates a combination of reversible displacement/intercalation reaction with good cyclic stability. Ag1.2V3O8 and Ag2V4O11 with layer structure and higher mole ratio of Ag/V show a reversible insertion/extraction reaction accomplished by an irreversible displacement reaction to form a highly conductive Ag0 matrix, leading to the high rate performance. The chain-like β-AgVO3 and isolated island-like Ag4V2O7 with unstable structure and the highest mole ratio of Ag/V reveal irreversible phase transition mechanism to form the amorphous matrix. The crystal structure is the decisive factor in the basic electrochemical properties, providing a new insight into battery storage mechanism.Graphical abstractWe provide a comprehensive perspective of zinc ions storage behaviors in bimetallic cathode materials (e.g. Ag0.33V2O5, Ag1.2V3O8, β-AgVO3, Ag2V4O11, Ag4V2O7), which exhibit electrochemical redox multi-mechanisms. This work provides a new structural insight into energy storage mechanism in aqueous zinc-ions battery system.Image 1
  • a +basal+screw+dislocations+in+hexagonal+titanium+alloys&rft.title=Acta+Materialia&rft.issn=1359-6454&">Influence of simple metals on the stability of 〈 a 〉 basal screw
           dislocations in hexagonal titanium alloys
    • Abstract: Publication date: Available online 29 August 2019Source: Acta MaterialiaAuthor(s): Piotr Kwasniak, Emmanuel Clouet Basal slip acts as a secondary deformation mode in hexagonal close-packed titanium and becomes one of the primary mechanisms in titanium alloyed with simple metals. As these solute elements also lead to a pronounced reduction of the energy of the basal stacking fault, one can hypothesize that they promote basal dissociation of dislocations which can then easily glide in the basal planes. Here, we verify the validity of this hypothesis using ab initio calculations to model the interaction of a screw dislocation with indium (In) and tin (Sn). These calculations confirm that these simple metals are attracted by the stacking fault existing in the dislocation core when it is dissociated in a basal plane, but this interaction is not strong enough to stabilize a planar configuration, even for a high solute concentration in the core. Energy barrier calculations reveal that basal slip, in the presence of In and Sn, proceeds without any planar dissociation, with the dislocation being spread in pyramidal and prismatic planes during basal slip like in pure Ti. The corresponding energy barrier is higher in presence of solute atoms, showing that In and Sn do not ease basal slip but increase the corresponding lattice friction. This strengthening of basal slip by solute atoms is discussed in view of available experimental data.Graphical abstractImage 1
  • Processing-induced secondary phase formation in Mo-substituted lanthanum
           tungstate membranes
    • Abstract: Publication date: Available online 29 August 2019Source: Acta MaterialiaAuthor(s): Ke Ran, Wendelin Deibert, Hongchu Du, Daesung Park, Mariya E. Ivanova, Wilhelm A. Meulenberg, Joachim Mayer The compositional homogeneity of a technically relevant hydrogen separation membrane, La5.4W0.8Mo0.2O12-δ (LWO-Mo20), was studied using comprehensive transmission electron microscopy (TEM) techniques. The membrane is predominantly composed of dense LWO-Mo20 grains with a defect fluorite structure. In addition to the primary phase, the observed secondary phase (SP) grains were identified as La2/3(Mg1/2W1/2)O3, with the W sites partially occupied by Mo, Fe and Al. Part of the SP grains were incorporated into single LWO-Mo20 grains through smart orientations, in which massive structural defects at the interface of the LWO-Mo20 and SP grains are efficiently avoided. Slight elemental disorder is limited within a few atomic layers. In contrast, the LWO-Mo20 grains share barely common features with neighboring SP grains, and are unstable under electron beam irradiation. The formation of the SP was tracked back to the traces of impurities in the precursors. Excluding such impurities is technically challenging and unacceptable in terms of cost. Hence, our results here show an opportunity to remedy these impurities through engineering the SP into individual primary grains, in which even a significant cost reduction could thus be realized.Graphical abstractImage 1
  • Growth and coarsening kinetics of gamma prime precipitates in CMSX-4 under
           simulated additive manufacturing conditions
    • Abstract: Publication date: Available online 28 August 2019Source: Acta MaterialiaAuthor(s): Benjamin Wahlmann, Florian Galgon, Andreas Stark, Sören Gayer, Norbert Schell, Peter Staron, Carolin Körner Additive manufacturing of superalloys offers new opportunities for alloy design but also poses significant processing difficulties. While the γ′ phase is responsible for the excellent high-temperature resistance of these alloys, it also induces cracking by precipitation hardening during manufacturing. Using small-angle X-ray scattering, we characterized the dynamic precipitation, dissolution, coarsening, and morphological evolution of the γ′ phase in situ during simulated additive manufacturing conditions. For this purpose, a CMSX-4 cylinder was subjected to cyclic heat treatment with heating and quenching rates up to 300 K/s. A specialized setup employing aluminum lenses to focus the X-ray beam was utilized to extend the q-range to small scattering vectors up to 0.035 nm-1. It was shown that the γ′ phase precipitates extremely fast without any measurable undercooling but remains below the equilibrium fraction throughout the process. Coarsening is readily measurable over timespans of only several seconds. A fraction of the γ′ phase that was dissolved during heating reprecipitated by forming new particles instead of growing on already existing precipitates. The findings provide new insight into the dynamic behavior of the γ′ phase during additive manufacturing and may prove valuable in designing new superalloys and processing strategies for additive manufacturing.Graphical abstractImage 1
  • Model for Ratchet Growth in PBX 9502
    • Abstract: Publication date: Available online 28 August 2019Source: Acta MaterialiaAuthor(s): R.B. Schwarz Polycrystalline solids composed of crystals with anisotropic thermal expansion coefficients exhibit ratchet growth (RG), a phenomenon characterized by a cumulative and irreversible volume expansion that develops upon exposing the material to cyclic excursions in temperature. We developed a statistical model for RG. The model attributes RG to the formation of intergranular cracks caused by tessellated internal stresses that develop during the thermal excursions. It is postulated that different sets of internal cracks form upon heating and cooling the polycrystalline solid. The model reproduces RG measurements in pressed TATB and PBX 9502 energetic materials and suggests an explanation for why the amplitude of RG generated by a heating excursion is larger than that generated by a subsequent cooling excursion of the same amplitude.
  • Achieving high strength and ductility in traditionally brittle soft
           magnetic intermetallics via additive manufacturing
    • Abstract: Publication date: Available online 28 August 2019Source: Acta MaterialiaAuthor(s): Tomas F. Babuska, Mark A. Wilson, Kyle L. Johnson, Shaun R. Whetten, John F. Curry, Jeffrey M. Rodelas, Cooper Atkinson, Ping Lu, Michael Chandross, Brandon A. Krick, Joseph R. Michael, Nicolas Argibay, Donald F. Susan, Andrew B. Kustas Intermetallic alloys possess exceptional soft magnetic properties, including high permeability, low coercivity, and high saturation induction, but exhibit poor mechanical properties that make them impractical to bulk process and use at ideal compositions. We used laser-based Additive Manufacturing to process traditionally brittle Fe-Co and Fe-Si alloys in bulk form without macroscopic defects and at near-ideal compositions for electromagnetic applications. The binary Fe-50Co, as a model material, demonstrated simultaneous high strength (600-700 MPa) and ductility (35%) in tension, corresponding to a ∼300% increase in strength and an order-of-magnitude improvement in ductility relative to conventionally processed material. Atomic-scale toughening and strengthening mechanisms, based on engineered multiscale microstructures, are proposed to explain the unusual combination of mechanical properties. This work presents an instance in which metal Additive Manufacturing processes are enabling, rather than limiting, the development of higher-performance alloys.Graphical abstractImage 1
  • Relations between Material Properties and Barriers for Twin Boundary
           Motion in Ferroic Materials
    • Abstract: Publication date: Available online 14 August 2019Source: Acta MaterialiaAuthor(s): Bar Danino, Gil Gur-Arieh, Doron Shilo, Dan Mordehai Ferroic materials typically exhibit a microstructure that contains twins or domains separated by twin boundaries (walls). The deformation of these materials is governed by twin boundary motion under mechanical/electrical/magnetic driving force. The Landau-Ginzburg model is a widely accepted phenomenological model used to describe twin boundary properties. However, it is incapable of describing energy barriers for motion due to the lack of atomistic description. In this work, we present a model interatomic potential for studying the relations between the lattice barrier for twin boundary motion and measurable material properties. The interatomic potential emulates the continuum Landau-Ginzburg model and reproduces known results of twin boundary thickness and energy as a function of the model parameters. An atomic model system is constructed, with a single twin boundary separating crystals of different orientations and we employ the Nudged Elastic Band method to calculate the energy barriers for the motion of twin boundaries with different thicknesses under different externally-applied shear stresses. The results are summarized in a closed-form expression relating the energy barriers with material properties and the external loading. The energy barrier function extends the Landau-Ginzburg model and allows treating the motion of twin boundary as a thermally activated process.Graphical abstractImage 1
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