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Publisher: Elsevier   (Total: 3162 journals)

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Showing 1 - 200 of 3162 Journals sorted alphabetically
A Practical Logic of Cognitive Systems     Full-text available via subscription   (Followers: 9)
AASRI Procedia     Open Access   (Followers: 15)
Academic Pediatrics     Hybrid Journal   (Followers: 33, SJR: 1.655, CiteScore: 2)
Academic Radiology     Hybrid Journal   (Followers: 23, SJR: 1.015, CiteScore: 2)
Accident Analysis & Prevention     Partially Free   (Followers: 95, SJR: 1.462, CiteScore: 3)
Accounting Forum     Hybrid Journal   (Followers: 25, SJR: 0.932, CiteScore: 2)
Accounting, Organizations and Society     Hybrid Journal   (Followers: 36, SJR: 1.771, CiteScore: 3)
Achievements in the Life Sciences     Open Access   (Followers: 5)
Acta Anaesthesiologica Taiwanica     Open Access   (Followers: 7)
Acta Astronautica     Hybrid Journal   (Followers: 411, SJR: 0.758, CiteScore: 2)
Acta Automatica Sinica     Full-text available via subscription   (Followers: 2)
Acta Biomaterialia     Hybrid Journal   (Followers: 27, SJR: 1.967, CiteScore: 7)
Acta Colombiana de Cuidado Intensivo     Full-text available via subscription   (Followers: 2)
Acta de Investigación Psicológica     Open Access   (Followers: 3)
Acta Ecologica Sinica     Open Access   (Followers: 10, SJR: 0.18, CiteScore: 1)
Acta Haematologica Polonica     Free   (Followers: 1, SJR: 0.128, CiteScore: 0)
Acta Histochemica     Hybrid Journal   (Followers: 3, SJR: 0.661, CiteScore: 2)
Acta Materialia     Hybrid Journal   (Followers: 249, 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: 27, 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: 6, SJR: 0.19, CiteScore: 0)
Actualites Pharmaceutiques Hospitalieres     Full-text available via subscription   (Followers: 3)
Acupuncture and Related Therapies     Hybrid Journal   (Followers: 6)
Acute Pain     Full-text available via subscription   (Followers: 14, SJR: 2.671, CiteScore: 5)
Ad Hoc Networks     Hybrid Journal   (Followers: 11, SJR: 0.53, CiteScore: 4)
Addictive Behaviors     Hybrid Journal   (Followers: 16, SJR: 1.29, CiteScore: 3)
Addictive Behaviors Reports     Open Access   (Followers: 8, SJR: 0.755, CiteScore: 2)
Additive Manufacturing     Hybrid Journal   (Followers: 9, SJR: 2.611, CiteScore: 8)
Additives for Polymers     Full-text available via subscription   (Followers: 22)
Advanced Drug Delivery Reviews     Hybrid Journal   (Followers: 148, SJR: 4.09, CiteScore: 13)
Advanced Engineering Informatics     Hybrid Journal   (Followers: 11, SJR: 1.167, CiteScore: 4)
Advanced Powder Technology     Hybrid Journal   (Followers: 17, SJR: 0.694, CiteScore: 3)
Advances in Accounting     Hybrid Journal   (Followers: 8, SJR: 0.277, CiteScore: 1)
Advances in Agronomy     Full-text available via subscription   (Followers: 12, SJR: 2.384, CiteScore: 5)
Advances in Anesthesia     Full-text available via subscription   (Followers: 28, 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: 10, SJR: 0.992, CiteScore: 1)
Advances in Applied Mechanics     Full-text available via subscription   (Followers: 11, SJR: 1.551, CiteScore: 4)
Advances in Applied Microbiology     Full-text available via subscription   (Followers: 23, SJR: 2.089, CiteScore: 5)
Advances In Atomic, Molecular, and Optical Physics     Full-text available via subscription   (Followers: 14, 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: 32, SJR: 3.043, CiteScore: 6)
Advances in Carbohydrate Chemistry and Biochemistry     Full-text available via subscription   (Followers: 8, 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: 3)
Advances in Cellular and Molecular Biology of Membranes and Organelles     Full-text available via subscription   (Followers: 12)
Advances in Chemical Engineering     Full-text available via subscription   (Followers: 27, SJR: 0.156, CiteScore: 1)
Advances in Child Development and Behavior     Full-text available via subscription   (Followers: 10, 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: 29, SJR: 1.562, CiteScore: 3)
Advances in Colloid and Interface Science     Full-text available via subscription   (Followers: 19, 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: 12)
Advances in Digestive Medicine     Open Access   (Followers: 9)
Advances in DNA Sequence-Specific Agents     Full-text available via subscription   (Followers: 5)
Advances in Drug Research     Full-text available via subscription   (Followers: 25)
Advances in Ecological Research     Full-text available via subscription   (Followers: 44, SJR: 2.524, CiteScore: 4)
Advances in Engineering Software     Hybrid Journal   (Followers: 28, SJR: 1.159, CiteScore: 4)
Advances in Experimental Biology     Full-text available via subscription   (Followers: 7)
Advances in Experimental Social Psychology     Full-text available via subscription   (Followers: 44, 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: 58, SJR: 0.591, CiteScore: 2)
Advances in Fuel Cells     Full-text available via subscription   (Followers: 16)
Advances in Genetics     Full-text available via subscription   (Followers: 16, SJR: 1.354, CiteScore: 4)
Advances in Genome Biology     Full-text available via subscription   (Followers: 8, SJR: 12.74, CiteScore: 13)
Advances in Geophysics     Full-text available via subscription   (Followers: 6, SJR: 1.193, CiteScore: 3)
Advances in Heat Transfer     Full-text available via subscription   (Followers: 21, SJR: 0.368, CiteScore: 1)
Advances in Heterocyclic Chemistry     Full-text available via subscription   (Followers: 12, SJR: 0.749, CiteScore: 3)
Advances in Human Factors/Ergonomics     Full-text available via subscription   (Followers: 22)
Advances in Imaging and Electron Physics     Full-text available via subscription   (Followers: 2, SJR: 0.193, CiteScore: 0)
Advances in Immunology     Full-text available via subscription   (Followers: 36, SJR: 4.433, CiteScore: 6)
Advances in Inorganic Chemistry     Full-text available via subscription   (Followers: 8, 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: 8, 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: 9)
Advances in Marine Biology     Full-text available via subscription   (Followers: 17, SJR: 0.88, CiteScore: 2)
Advances in Mathematics     Full-text available via subscription   (Followers: 11, SJR: 3.027, CiteScore: 2)
Advances in Medical Sciences     Hybrid Journal   (Followers: 6, SJR: 0.694, CiteScore: 2)
Advances in Medicinal Chemistry     Full-text available via subscription   (Followers: 5)
Advances in Microbial Physiology     Full-text available via subscription   (Followers: 4, SJR: 1.158, CiteScore: 3)
Advances in Molecular and Cell Biology     Full-text available via subscription   (Followers: 22)
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: 3)
Advances in Oncobiology     Full-text available via subscription   (Followers: 1)
Advances in Organ Biology     Full-text available via subscription   (Followers: 1)
Advances in Organometallic Chemistry     Full-text available via subscription   (Followers: 17, 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: 24, SJR: 0.461, CiteScore: 1)
Advances in Pharmaceutical Sciences     Full-text available via subscription   (Followers: 11)
Advances in Pharmacology     Full-text available via subscription   (Followers: 16, SJR: 1.536, CiteScore: 3)
Advances in Physical Organic Chemistry     Full-text available via subscription   (Followers: 8, 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: 9)
Advances in Plant Pathology     Full-text available via subscription   (Followers: 5)
Advances in Porous Media     Full-text available via subscription   (Followers: 5)
Advances in Protein Chemistry     Full-text available via subscription   (Followers: 18)
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: 62)
Advances in Quantum Chemistry     Full-text available via subscription   (Followers: 6, SJR: 0.371, CiteScore: 1)
Advances in Radiation Oncology     Open Access   (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: 395, 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: 10, SJR: 0.555, CiteScore: 2)
Advances in the Study of Behavior     Full-text available via subscription   (Followers: 33, SJR: 2.208, CiteScore: 4)
Advances in Veterinary Medicine     Full-text available via subscription   (Followers: 17)
Advances in Veterinary Science and Comparative Medicine     Full-text available via subscription   (Followers: 13)
Advances in Virus Research     Full-text available via subscription   (Followers: 5, SJR: 2.262, CiteScore: 5)
Advances in Water Resources     Hybrid Journal   (Followers: 46, SJR: 1.551, CiteScore: 3)
Aeolian Research     Hybrid Journal   (Followers: 6, SJR: 1.117, CiteScore: 3)
Aerospace Science and Technology     Hybrid Journal   (Followers: 340, 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: 11, SJR: 3.671, CiteScore: 9)
Aggression and Violent Behavior     Hybrid Journal   (Followers: 448, SJR: 1.238, CiteScore: 3)
Agri Gene     Hybrid Journal   (Followers: 1, SJR: 0.13, CiteScore: 0)
Agricultural and Forest Meteorology     Hybrid Journal   (Followers: 17, SJR: 1.818, CiteScore: 5)
Agricultural Systems     Hybrid Journal   (Followers: 31, SJR: 1.156, CiteScore: 4)
Agricultural Water Management     Hybrid Journal   (Followers: 42, SJR: 1.272, CiteScore: 3)
Agriculture and Agricultural Science Procedia     Open Access   (Followers: 3)
Agriculture and Natural Resources     Open Access   (Followers: 3)
Agriculture, Ecosystems & Environment     Hybrid Journal   (Followers: 57, SJR: 1.747, CiteScore: 4)
Ain Shams Engineering J.     Open Access   (Followers: 5, SJR: 0.589, CiteScore: 3)
Air Medical J.     Hybrid Journal   (Followers: 6, SJR: 0.26, CiteScore: 0)
AKCE Intl. J. of Graphs and Combinatorics     Open Access   (SJR: 0.19, CiteScore: 0)
Alcohol     Hybrid Journal   (Followers: 11, SJR: 1.153, CiteScore: 3)
Alcoholism and Drug Addiction     Open Access   (Followers: 9)
Alergologia Polska : Polish J. of Allergology     Full-text available via subscription   (Followers: 1)
Alexandria Engineering J.     Open Access   (Followers: 1, 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: 9, SJR: 0.201, CiteScore: 1)
Alzheimer's & Dementia     Hybrid Journal   (Followers: 52, SJR: 4.66, CiteScore: 10)
Alzheimer's & Dementia: Diagnosis, Assessment & Disease Monitoring     Open Access   (Followers: 4, SJR: 1.796, CiteScore: 4)
Alzheimer's & Dementia: Translational Research & Clinical Interventions     Open Access   (Followers: 4, SJR: 1.108, CiteScore: 3)
Ambulatory Pediatrics     Hybrid Journal   (Followers: 6)
American Heart J.     Hybrid Journal   (Followers: 50, SJR: 3.267, CiteScore: 4)
American J. of Cardiology     Hybrid Journal   (Followers: 54, SJR: 1.93, CiteScore: 3)
American J. of Emergency Medicine     Hybrid Journal   (Followers: 45, SJR: 0.604, CiteScore: 1)
American J. of Geriatric Pharmacotherapy     Full-text available via subscription   (Followers: 10)
American J. of Geriatric Psychiatry     Hybrid Journal   (Followers: 14, SJR: 1.524, CiteScore: 3)
American J. of Human Genetics     Hybrid Journal   (Followers: 34, SJR: 7.45, CiteScore: 8)
American J. of Infection Control     Hybrid Journal   (Followers: 28, SJR: 1.062, CiteScore: 2)
American J. of Kidney Diseases     Hybrid Journal   (Followers: 34, SJR: 2.973, CiteScore: 4)
American J. of Medicine     Hybrid Journal   (Followers: 46)
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: 208, SJR: 2.7, CiteScore: 4)
American J. of Ophthalmology     Hybrid Journal   (Followers: 63, 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: 28, 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: 38, 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: 62, SJR: 0.138, CiteScore: 0)
Anaesthesia Critical Care & Pain Medicine     Full-text available via subscription   (Followers: 17, SJR: 0.411, CiteScore: 1)
Anales de Cirugia Vascular     Full-text available via subscription  
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: 43, SJR: 1.512, CiteScore: 5)
Analytical Biochemistry     Hybrid Journal   (Followers: 175, SJR: 0.633, CiteScore: 2)
Analytical Chemistry Research     Open Access   (Followers: 11, SJR: 0.411, CiteScore: 2)
Analytical Spectroscopy Library     Full-text available via subscription   (Followers: 11)
Anesthésie & Réanimation     Full-text available via subscription   (Followers: 2)
Anesthesiology Clinics     Full-text available via subscription   (Followers: 23, 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: 192, SJR: 1.58, CiteScore: 3)

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Journal Cover
Acta Materialia
Journal Prestige (SJR): 3.263
Citation Impact (citeScore): 6
Number of Followers: 249  
 
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 1359-6454
Published by Elsevier Homepage  [3162 journals]
  • Micromechanical modeling of non-linear stress-strain behavior of
           polycrystalline microcracked materials under tension
    • Abstract: Publication date: Available online 13 October 2018Source: Acta MaterialiaAuthor(s): Giovanni Bruno, Mark Kachanov, Igor Sevostianov, Amit Shyam The stress-strain behavior of microcracked polycrystalline materials (such as ceramics or rocks) under conditions of tensile, displacement-controlled, loading is discussed. Micromechanical explanation and modeling of the basic features, such as non-linearity and hysteresis in stress-strain curves, is developed, with stable microcrack propagation and “roughness” of intergranular cracks playing critical roles. Experiments involving complex loading histories were done on large- and medium grain size β-eucryptite ceramic. The model is shown to reproduce the basic features of the observed stress-strain curves.Graphical abstractThe stress-strain behavior of microcracked polycrystalline materials under conditions of tensile, displacement-controlled, loading is discussed. Micromechanical explanation and modeling of the basic features, such as non-linearity and hysteresis in stress-strain curves, is developed, with “roughness” of intergranular cracks playing critical roles: in forward loading, roughness profiles of crack faces get mismatched when nonlinearity starts due to crack propagation (point 2); at unloading (point 4) the faces get “stuck” (their displacement at peak load C is locked).Image 1
       
  • Point defect structure of La-doped SrTiO3 ceramics with
           colossal permittivity
    • Abstract: Publication date: Available online 13 October 2018Source: Acta MaterialiaAuthor(s): Mengjie Qin, Feng Gao, Jakub Cizek, Shengjie Yang, Xiaoli Fan, Lili Zhao, Jie Xu, Gaogao Dong, Mike Reece, Haixue Yan Sr1-xLaxTiO3 (SLTO) ceramics with colossal permittivity were fabricated by conventional solid-state reaction method. The point defects of pure STO and SLTO ceramics were analyzed by Positron Annihilation Lifetime Spectroscopy (PALS) and Coincidence Doppler Broadening (CDB). The charge compensation mechanisms and dielectric properties of ceramics were investigated. The results indicated that the intrinsic defects in pure STO ceramics were mainly VTi″″. The charge compensation mechanism of SLTO ceramics was predominantly formation of VSr″. With increasing La content, εr of SLTO ceramics increased up to ∼70000 at room temperature. The results of first-principle calculations indicated that the colossal permittivity came from a sharp polarization increase caused by dipole structure of defects. tanδ of SLTO ceramics showed obvious Debye relaxation at high temperatures and the relaxation showed a multiple relaxation times derived from different kinds of polarization mechanism. The main polarization mechanism of SLTO ceramics gradually changed from ion displacement polarization to defect dipole polarization influenced by the concentration of La dopants.Graphical abstractImage 1
       
  • Corrigendum to ‘Probing deformation mechanisms of a FeCoCrNi
           high-entropy alloy at 293 and 77 K using in situ neutron diffraction’
           [Acta Mater. 154C (2018) 79–89]
    • Abstract: Publication date: Available online 12 October 2018Source: Acta MaterialiaAuthor(s): Yiqiang Wang, Bin Liu, Kun Yan, Minshi Wang, Saurabh Kabra, Yu-Lung Chiu, David Dye, Peter D. Lee, Yong Liu, Biao Cai
       
  • Effects of the stacking fault energy fluctuations on the strengthening of
           alloys
    • Abstract: Publication date: Available online 12 October 2018Source: Acta MaterialiaAuthor(s): Yifei Zeng, Xiaorong Cai, Marisol Koslowski In alloys and high entropy alloys the stacking fault energy varies with the local composition. The effects of these energy fluctuations on the strengthening are studied using dislocation dynamics simulations that track the evolution of partial dislocations in FCC metals at zero temperature. Different values of the intrinsic stacking fault energy are assigned to regions with size in the range of 0.5 nm–12 nm in the slip plane, while the other mechanical properties are left constant. A theoretical model is derived and compared to the simulation results. In the model and the simulations the predicted value of the yield stress grows with larger fluctuations of the stacking fault energy. Furthermore, a strong size dependency is observed, with a maximum in the strength attained when the mean region size approaches the average equilibrium stacking fault width. In summary, the strength of high entropy alloys can be improved by introducing disorder in the chemical misfit with a characteristic length scale of the order of the average stacking fault width.Graphical abstractImage 1
       
  • Dislocation evolution at a crack-tip in a hexagonal close packed metal
           under plane-stress conditions
    • Abstract: Publication date: Available online 12 October 2018Source: Acta MaterialiaAuthor(s): Zhouyao Wang, Christopher Cochrane, Travis Skippon, Qingshan Dong, Mark R. Daymond Understanding the stress state and microstructural features at a growing crack-tip is critical to understanding the failure mechanisms of engineering structures. To investigate the strain and dislocation evolution at a crack-tip, electron backscatter diffraction and geometrically necessary dislocation analysis were performed on fully annealed zirconium foils at room temperature. Different levels of macroscopic plastic strain were applied: 0.0%, 0.22%, 0.84%, 1.2%. Based on their different Burgers vectors and line vectors, prismatic , basal , screw , screw and pyramidal geometrically necessary dislocation densities were estimated during crack blunting and subsequent propagation. Most of the plastic deformation was accommodated by screw and pyramidal dislocations. Screw dislocations were found to be dominant over the as might be expected. Instead of twinning, pyramidal slip accommodated the strain along the c-axis caused by contraction at the crack-tip. Dislocation densities at the crack-tip were plotted according to the angle relative to the applied tension direction and the distance from the tip, and were compared with plastic strains simulated from a 3D static finite element model. Crack-tip singularity was observed and total geometrically necessary dislocation densities were in qualitatively good agreement with the equivalent plastic strain distribution predicted by the finite element method (FEM).Graphical abstractImage 1
       
  • Vapor phase dealloying: a versatile approach for fabricating 3D porous
           materials
    • Abstract: Publication date: Available online 11 October 2018Source: Acta MaterialiaAuthor(s): Jiuhui Han, Cheng Li, Zhen Lu, Hao Wang, Zhili Wang, Kentaro Watanabe, Mingwei Chen Three-dimensional porous materials with bicontinuous open porosity represent a new class of functional materials for various applications. Top-down dealloying has been demonstrated to be one of the most effective ways to fabricate 3D porous materials. Vapor phase dealloying, which makes use of the saturated vapor pressure difference between the constituent components in an alloy for selectively removing a less stable element or phase, is a promising versatile method for fabricating porous materials from active metals to inorganic elements. Here, using nickel-zinc and germanium-zinc alloys as the prototypes of single-phase and two-phase precursors, respectively, we report the fabrication of 3D bicontinuous porous Ni and Ge by vapor phase dealloying on the basis of selective element or selective phase evaporations. We also show the incorporation of vapor phase dealloying with chemical vapor deposition for the one-pot growth of 3D nanoporous graphene and the functional applications of vapor phase dealloyed porous Ge as Li ion battery electrodes. This study shines lights on the versatility of vapor phase dealloying for the fabrication of bicontinuous porous materials for a wide range of functional applications.Graphical abstractImage 1
       
  • Application of Onsager's variational principle to the dynamics of a solid
           toroidal island on a substrate
    • Abstract: Publication date: Available online 11 October 2018Source: Acta MaterialiaAuthor(s): Wei Jiang, Quan Zhao, Tiezheng Qian, David J. Srolovitz, Weizhu Bao In this paper, we consider the capillarity-driven evolution of a solid toroidal island on a flat rigid substrate, where mass transport is controlled by surface diffusion. This problem is representative of the geometrical complexity associated with the solid-state dewetting of thin films on substrates. We apply Onsager's variational principle to develop a general approach for describing surface diffusion-controlled problems. Based on this approach, we derive a simple, reduced-order model and obtain an analytical expression for the rate of island shrinking and validate this prediction by numerical simulations based on a full, sharp-interface model. We find that the rate of island shrinking is proportional to the material constants B and the surface energy density γ0, and is inversely proportional to the island volume V0. This approach represents a general tool for modeling interface diffusion-controlled morphology evolution.Graphical abstractImage 1
       
  • Hierarchical microstructure design to tune the mechanical behavior of an
           interstitial TRIP-TWIP high-entropy alloy
    • Abstract: Publication date: Available online 11 October 2018Source: Acta MaterialiaAuthor(s): Jing Su, Dierk Raabe, Zhiming Li We demonstrate a novel approach of utilizing a hierarchical microstructure design to improve the mechanical properties of an interstitial carbon doped high-entropy alloy (HEA) by cold rolling and subsequent tempering and annealing. Bimodal microstructures were produced in the tempered specimens consisting of nano-grains (∼50 nm) in the vicinity of shear bands and recovered parent grains (10-35 μm) with pre-existing nano-twins. Upon annealing, partial recrystallization led to trimodal microstructures characterized by small recrystallized grains (
       
  • The influence of stacking fault energy on plasticity mechanisms in
           triode-plasma nitrided austenitic stainless steels: implications for the
           structure and stability of nitrogen-expanded austenite
    • Abstract: Publication date: Available online 11 October 2018Source: Acta MaterialiaAuthor(s): Xiao Tao, Xingguang Liu, Allan Matthews, Adrian Leyland Austenitic stainless steels (ASSs), especially AISI type 304 and 316 ASSs, have been extensively studied after thermochemical diffusion treatments (e.g. nitriding, carburising) to resolve the anomalous lattice expansion after supersaturation of interstitial elements under paraequilibium conditions. The known issues are i) plastic deformation of surfaces under nitrogen-introduced strain at low treatment temperatures and ii) degradation in surface corrosion performance in association with chromium nitride formation at elevated treatment temperatures (and/or longer treatment times). In this study, a nitrogen-containing high-manganese ASS and a high-nickel ASS (i.e. Fe-17Cr-20Mn-0.5N and Fe-19Cr-35Ni, in wt.%) were triode-plasma nitrided under a high nitrogen gas volume fraction and low (and close to monoenergetic) ion energy of ∼200 eV at 400°C, 425°C and 450°C for 4hrs and 20hrs, respectively. Auxiliary radiant heating was used to facilitate different treatment temperatures at a deliberately controlled and constant substrate current density of ∼0.13 mA/cm2, under which material surface crystallographic structure was mainly influenced by the different treatment temperatures and times applied during nitriding. With respect to stacking fault energy (SFE), we illustrate and discuss i) the analogy of composition-induced plastic deformation phenomena to mechanical deformation processes, ii) two possible types of dislocation-mediated plasticity mechanism in γN, iii) two possible types of diffusional decomposition mechanism for γN, and iv) the lattice structures formed at low to moderate nitriding temperatures.Graphical abstractImage 1
       
  • Characterization of (Ti,Mo,Cr)C nanoprecipitates in an austenitic
           stainless steel on the atomic scale
    • Abstract: Publication date: Available online 11 October 2018Source: Acta MaterialiaAuthor(s): N. Cautaerts, R. Delville, E. Stergar, D. Schryvers, M. Verwerft Nanometer sized (Ti,Mo,Cr)C (MX-type) precipitates that grew in a 24% cold worked Ti-stabilized austenitic stainless steel (grade DIN 1.4970, member of the 15-15Ti alloys) after heat treatment were fully characterized with transmission electron microscopy (TEM), probe corrected high angle annular dark field scanning transmission electron microscopy (HR-HAADF STEM), and atom probe tomography (APT). The precipitates shared the cube-on-cube orientation with the matrix and were facetted on {111} planes, yielding octahedral and elongated octahedral shapes. The misfit dislocations were believed to have burgers vectors a/6 which was verified by geometrical phase analysis (GPA) strain mapping of a matrix-precipitate interface. The dislocations were spaced five to seven atomic planes apart, on average slightly wider than expected for the lattice parameters of steel and TiC. Quantitative atom probe tomography analysis of the precipitates showed that precipitates were significantly enriched in Mo, Cr and V, and that they were hypostoichiometric with respect to C. These findings were consistent with a reduced lattice parameter. The precipitates were found primarily on Shockley partial dislocations originating from the original perfect dislocation network. These novel findings could contribute to the understanding of how TiC nanoprecipitates interact with point defects and matrix dislocations. This is essential for the application of these Ti-stabilized steels in high temperature environments or nuclear fast reactors.Graphical abstractImage 1
       
  • Investigation of Interactions between Defect Clusters in Stainless Steels
           by In Situ Irradiation at Elevated Temperatures
    • Abstract: Publication date: Available online 10 October 2018Source: Acta MaterialiaAuthor(s): Dongyue Chen, Kenta Murakami, Hiroaki Abe, Zhengcao Li, Naoto Sekimura In recent years, modeling studies on the interactions between defect clusters have been extensively conducted to predict the behavior of stainless steels in reactors. However, comparable experimental results are desired to validate existing results and provide guidelines for future modeling. The size of defect clusters is expected to be a key factor influencing cluster interactions. Thus, in this work, the effects of growing clusters on their surrounding microstructure were quantitatively examined by in situ transmission electron microscopy during ion irradiation. To this end, a high-purity 316L stainless steel model alloy was irradiated by 2 MeV Fe2+ to 0.2 dpa at 400°C and 300°C separately. At 300°C, the number density of defect clusters monotonically increased with increasing fluence, whereas at 400°C, the number density decreased almost immediately after the rapid nucleation regime. This decrease could be explained by the distinct growth of some clusters, which suppressed the nucleation of clusters around them as well as the lifetime of neighboring tiny clusters. Large interstitial-type clusters approximately 6–11 nm in size could be absorbed by neighboring interstitial-type clusters of similar sizes via interstitial crowdion diffusion. The absorption would not occur until both clusters grew large enough to permit a diffusion path between them.Graphical abstractImage 1
       
  • Morphological instability of iron-rich precipitates in Cu-Fe-Co alloys
    • Abstract: Publication date: Available online 10 October 2018Source: Acta MaterialiaAuthor(s): K.X. Chen, P.A. Korzhavyi, G. Demange, H. Zapolsky, R. Patte, J. Boisse, Z.D. Wang The mechanical properties of metallic materials are determined by their microstructure, and in particular, the different morphologies of precipitates lead to distinct strengthening effects. Usually, the shape of precipitates changes during growth and coarsening regimes, leading to modification of the macroscopic properties of the materials. Thus, understanding of this phenomenon is key to tailoring the precipitate strengthening of industrial alloys. In this article, a general approach to explain the shape instability of iron-rich nanoparticles in Cu-Fe-Co alloys during casting and ageing processes is proposed. The evolution of particle shape from sphere to cuboid to petal and finally splitting into eight sub-nanoparticles is observed using transmission electron microscopy. Phase-field modelling and thermodynamic calculations are combined into a general model that describes and elucidates the morphological evolution of precipitates in alloys in terms of particle size, interfacial and elastic strain energy, and chemical driving force. These findings have the potential to promote new microstructural design approaches for a wide range of materials.Graphical abstractImage 1
       
  • Differentiation of γ′- and γ″- precipitates in Inconel 718 by a
           complementary study with small-angle neutron scattering and analytical
           microscopy
    • Abstract: Publication date: Available online 10 October 2018Source: Acta MaterialiaAuthor(s): R. Lawitzki, S. Hassan, L. Karge, J. Wagner, D. Wang, J. von Kobylinski, C. Krempaszky, M. Hofmann, R. Gilles, G. Schmitz We present an experimental method to distinguish and quantify the two strengthening phases γ′ and γ″ in the nickel-based superalloy Inconel 718. So far, this was only achieved by techniques that evaluated sample volumes in the nano-to micrometer range. In this study, reliable volume fractions of the precipitates were obtained and calculated from ex-situ small-angle neutron scattering (SANS) on differently heat treated specimens. For interpretation of the SANS curves, a structural model was set up, using complementary information from measurements by transmission electron microscopy (TEM) and atom probe tomography (APT). Phase identification, as well as size distributions, morphologies and crystallographic information of the precipitates were obtained by TEM. APT provided compositional information, which is necessary to calculate the scattering contrast of each phase. As a benefit of using bulk neutron diffraction for quantification, volumes of a few tens of cubic millimeters are analyzed and thus, significantly better statistics are obtained. The measured γ″ volume fractions are remarkably lower than stated in previous works, but now well fulfill the chemical mass balance.Graphical abstractImage 1
       
  • Strengthening mechanisms acting in extruded Mg-based long-period stacking
           ordered (LPSO)-phase alloys
    • Abstract: Publication date: Available online 10 October 2018Source: Acta MaterialiaAuthor(s): Koji Hagihara, Zixuan Li, Michiaki Yamasaki, Yoshihito Kawamura, Takayoshi Nakano The unusual increase in the strength by extrusion is a unique feature of recently developed Mg alloys containing the LPSO phase. In this study, we first elucidated the detailed mechanisms that induce this drastic strengthening. The dependencies of the deformation behavior of a Mg88Zn4Y7 extruded alloy, which contains ∼86-vol% LPSO phase, on the temperature, loading orientation, and extrusion ratio were examined. It was found that the yield stress of the alloy is drastically increased by extrusion, but the magnitude of the increase in the yield stress is significantly different depending on the loading orientation. That is, the strengthening of the LPSO phase by extrusion shows a strong anisotropy. By the detailed analyses, this was clarified to be derived from the variation in the deformation mechanisms depending on the loading orientation and extrusion ratio. Basal slip was found to govern the deformation behavior when the loading axis was inclined at a 45° to the extrusion direction, while the predominant deformation mechanism varies from basal slip to the formation of deformation kink bands as the extrusion ratio increased when the loading axis was parallel to the extrusion direction. Moreover, it was clarified in this study that the introduction of a deformation-kink-band boundary during extrusion effectively acts as a strong obstacle to basal slip. That is, "the kink band strengthening" was first quantitatively elucidated, which contributes to the drastic increase in the yield stress of the extruded LPSO-phase alloys in the wide temperature range below 400 ºC.Graphical abstractImage 1
       
  • Determination of the structure and properties of an edge dislocation in
           rutile TiO2
    • Abstract: Publication date: Available online 9 October 2018Source: Acta MaterialiaAuthor(s): Emile Maras, Mitsuhiro Saito, Kazutoshi Inoue, Hannes Jónsson, Yuichi Ikuhara, Keith P. McKenna A global optimization procedure is used to predict the structure and electronic properties of the b=c[001] edge dislocation in rutile TiO2. Over 1,000 different atomic configurations have been generated using both semi-empirical and density functional theory estimates of the energy of the system to identify the most stable structure. Both stoichiometric and oxygen deficient dislocation core structures are predicted to be stable depending on the oxygen chemical potential. The latter is associated with Ti3+ species in the dislocation core. The dislocation is predicted to act as a trap for electrons but not for holes suggesting they are not strong recombination centers. The predicted structures and properties are found to be consistent with experimental results obtained using scanning transmission electron microscopy and electron energy loss spectroscopy on samples produced using the bicrystal approach.Graphical abstractImage 1
       
  • Quantification of precipitate hardening of twin nucleation and growth in
           Mg and Mg-5Zn using micro-pillar compression
    • Abstract: Publication date: Available online 9 October 2018Source: Acta MaterialiaAuthor(s): Jiangting Wang, Mahendra Ramajayam, Eric Charrault, Nicole Stanford In polycrystalline materials, the stress corresponding to twin nucleation is difficult to separate from twin growth because these events occur concurrently during deformation. In this work, we separate the nucleation stress and growth stress of {101¯2} twinning by compression of micro-pillars containing pre-existing twins through their centre. Micro-pillar compression results showed a strong size effect on both twin nucleation and twin growth stresses for pure Mg and Mg-5Zn alloys. Taking this into account, the critical stress for growth of twins in pure magnesium is found to be ∼7 MPa which is consistent with previously published measurements on macroscopic single crystals. These experiments have been used to deduce the precipitate hardening of twin growth, and for the present precipitate dispersion this has been measured to be ∼30 MPa. Back-stress calculations based on elastic bending of the precipitates showed close agreement to the measured precipitate hardening, and this model therefore accounts well for the observed strengthening. Site-specific atom probe tomography of the twin boundaries showed that room temperature ageing is sufficient to produce segregation of zinc to the twin boundary. This was found to immobilize the twin, and is believed to be the first report of solute locking of twins from room temperature exposure.Graphical abstractImage 1
       
  • Semi-solid deformation of Al-Cu alloys: a quantitative comparison between
           real-time imaging and coupled LBM-DEM simulations
    • Abstract: Publication date: Available online 9 October 2018Source: Acta MaterialiaAuthor(s): T.C. Su, C. O’Sullivan, T. Nagira, H. Yasuda, C.M. Gourlay Semi-solid alloys are deformed in a wide range of casting processes; an improved understanding and modelling capability is required to minimise defect formation and optimise productivity. Here we combine thin-sample in-situ X-ray radiography of semisolid Al-Cu alloy deformation at 40 – 70% solid with 2D coupled lattice Boltzmann method - discrete element method (LBM-DEM) simulations. The simulations quantitatively capture the key features of the in-situ experiments, including (i) the local contraction and dilation of the grain assembly during shear deformation; (ii) the heterogeneous strain fields and localisation features; (iii) increases in local liquid pressure in regions where liquid was expelled from the free surface in the experiment; and (iv) decreases in liquid pressure in regions where surface menisci are sucked-in in experiments. The verified DEM simulations provide new insights into the role of initial solid fraction on the stress-deformation response and support the hypothesis that the behaviour of semi-solid alloys can be described using critical state soil mechanics.Graphical abstractImage 1
       
  • Morphological Control and Kinetics in Three Dimensions for Hierarchical
           Nanostructures Growth by Screw Dislocations
    • Abstract: Publication date: Available online 8 October 2018Source: Acta MaterialiaAuthor(s): Yanhui Chu, Siyi Jing, Da Liu, Jinchao Liu, Yunlong Zhao The precise control and in-depth understanding of the anisotropic crystal screw dislocation growth could yield further optimization of nanomaterial design and broader applications, yet the studies of rational control and kinetics for more complex nanostructures are still insufficient. In this work, by programming synthesis conditions, we achieve a controllable three-dimensional (3D) screw dislocation growth of hierarchical nanostructures, including nanowires, nanoplates, and previously unreported hierarchical hollow nanobelts, via a facile chemical vapor deposition approach. Notably, the screw dislocation growth in nanobelts is confirmed by the clear observations of the stepwise spiral terraces with initial hexagonal to octagonal shapes and the hollow cores in the growth spiral centers, as well as the fundamental Burton-Cabrera-Frank crystal growth theoretical calculations. The formation of the nanowires and nanoplates can be well interpreted by the previously reported screw dislocation growth model, while a new 3D screw dislocation growth model with considering of transition in growth velocities and directions is proposed to interpret the formation of the nanobelts and other potential complex nanostructures. This study not only enriches our understanding of the screw dislocation growth kinetics but also guides us to achieve the precise morphological design and control in nanosynthesis.Graphical abstractBy programming synthesis conditions, we achieve a controllable 3D screw dislocation growth of previously unreported hierarchical hollow nanobelts via a facile CVD approach. More importantly, we propose a new 3D screw dislocation growth model with considering of transition in growth velocities and directions to interpret the formation of the nanobelts.Image 1
       
  • Influence of metal/semiconductor interface on attainable piezoelectric and
           energy harvesting properties of ZnO
    • Abstract: Publication date: Available online 6 October 2018Source: Acta MaterialiaAuthor(s): Nikola Novak, Peter Keil, Till Frömling, Florian H. Schader, Alexander Martin, Kyle G. Webber, Jürgen Rödel The piezoelectric coefficient is a measure to quantify the potential use of a material in energy harvesting and sensor applications. High concentration of free charge carriers in piezoelectric materials can significantly impede the use of generated piezoelectric charge. However, in piezoelectric semiconductors such as ZnO, high conductivity drastically reduces the attainable piezoelectric coefficient and consequently the harvesting performance. Typically, acceptor doping and a decrease in operation temperature are employed to reduce the conductivity and retain the piezoelectric properties of the material. In piezotronics, however, the creation of a resistive space charge layer at the metal-semiconductor interface (Schottky contact) retains piezoelectric properties in spite of a highly conductive bulk material. Nevertheless, the piezoelectric coefficient has never been determined using a Schottky contact. Thus, the energy harvesting properties of a single contact have so far not been quantified. To this end In this study, undoped semiconducting ZnO single crystals with both Ohmic and Schottky contacts were prepared to quantify the effective piezoelectric response at temperatures from 20020 °C to -140 °C and frequencies of mechanical loading from 0.5 Hz to 160 Hz. It was demonstrated that the formation of an electrostatic potential barrier at the metal-semiconductor interface increases the overall resistance, which provides access to unbiased piezoelectric coefficients of ZnO single crystals even at room temperature. These findings were verified using semiconducting ZnO for energy harvesting at room temperature and relatively low loading frequency.Graphical abstractImage 1
       
  • On the heterogeneous nature of deformation in a strain-transformable beta
           metastable Ti-V-Cr-Al alloy
    • Abstract: Publication date: Available online 6 October 2018Source: Acta MaterialiaAuthor(s): L. Lilensten, Y. Danard, C. Brozek, S. Mantri, P. Castany, T. Gloriant, P. Vermaut, F. Sun, R. Banerjee, F. Prima Ti-10V-4Cr-1Al wt% (TVCA) is a new grade of titanium alloy, developed to combine twinning induced plasticity (TWIP) and transformation induced plasticity (TRIP) effects. The TVCA alloy exhibits a very high strain-hardening rate and an excellent balance between strength and ductility for great potential in aerospace applications. Deformation mechanisms are investigated using in-situ techniques as synchrotron X-ray diffraction (SXRD) and in-situ electron backscatter diffraction (EBSD) analysis during tensile strain, as well as transmission electron microscopy (TEM). The results reveal that permanent {332} mechanical twinning and an unstable orthorhombic α” martensite are the major deformation products. This study aims at unveiling the interaction and co-deformation of the various deformation features, that lead to the outstanding mechanical properties of the alloy. The very high strain hardening rate could be explained by the simultaneous activation of two different deformation modes, the primary TRIP mode on one side, and the hybrid TWIP and secondary TRIP mode on the other one, in different grains, resulting in in-grain dynamic hardening (Hall-Petch)/softening (α” martensite) effects and meso-scale dynamic mechanical contrast. Selection of the deformation mechanism – TRIP or TWIP –, which seems to be inhomogeneous, at both the inter- and the intra-granular level, is investigated.Graphical abstractImage 1
       
  • The formation mechanism of a novel interfacial phase with high thermal
           stability in a Mg-Gd-Y-Ag-Zr alloy
    • Abstract: Publication date: Available online 6 October 2018Source: Acta MaterialiaAuthor(s): L.R. Xiao, Y. Cao, S. Li, H. Zhou, X.L. Ma, L. Mao, X.C. Sha, Q.D. Wang, Y.T. Zhu, X.D. Han Due to their unique precipitation behavior, magnesium-rare earth (Mg-RE) alloys exhibit excellent mechanical properties and decent thermal stability. In this work, a Mg-Gd-Y-Ag-Zr alloy was employed to investigate the segregation and interfacial phase formation at grain boundaries after plastic deformation and heat treatment. The interfacial phase was unequivocally investigated by aberration-corrected high-angle annular dark-filed scanning transmission electron microscopy (HAADF-STEM) from three different crystal directions and modeling, which reveals a hitherto unknown crystal structure (monoclinic: β = 139.1°, a = 1.20 nm, b = 1.04 nm and c = 1.59 nm). Its orientation relationship with the Mg matrix is: [101]//[110]α, [302]//[100]α and (010)//(0001)α. Different from the precipitates in matrix, the size of the interfacial phase was not sensitive to annealing temperature between 250 °C and 400 °C. Transformation of twin boundaries to coaxial grain boundaries via multiple twinning led to the generation of many high strain sites along the boundaries, which promoted the formation of the interfacial phase. The interfacial phase was stable up to 400 °C, which was about 100 °C higher than the dissolution temperature of β′ and γ" precipitates.Graphical abstractImage 1
       
  • Low-Temperature-Solderable Intermetallic Nanoparticles for 3D Printable
           Flexible Electronics
    • Abstract: Publication date: Available online 4 October 2018Source: Acta MaterialiaAuthor(s): Ying Zhong, Rong An, Huiwen Ma, Chunqing Wang Functional materials for flexible and wearable smart devices have attracted much attention in recent years. This paper describes structure and properties of uniquely prepared, functional interconnectable nanoparticles (NPs) of Cu6Sn5 intermetallic compound that can allow 3D flexible packaging and nano-circuits. In situ TEM analysis confirms that size-controllable Cu6Sn5 NPs as small as ∼6.40 nm can be made sinterable at the start temperature as low as ∼130 °C, which is much lower than its bulk melting point (MP) of 415 oC. After sintering, its high MP provides mechanical and thermal stability. Based on the in situ TEM observation and calculation, particle size and distribution affects the sintering process. More interestingly, the relative orientations of adjacent particles also play an important role. A new orientation related sintering mechanism noted as orientation unification (OU) is revealed as two adjacent particles exhibit orientation change to slowly match their orientation with each other during the heating process. The interesting interaction between nano-Cu6Sn5 and micro-Cu substrate during in situ TEM heating gives first hand atomic level proof of the formation of Cu3Sn. The nano-Cu6Sn5 joints possess high enough bonding strength and great high temperature working capability. This intermetallic nano-soldering approach can pioneer a novel strategy of circuit connection, by providing high working temperature interconnection materials for 3D flexible packaging and ultra-high-density micro/nano interconnections.Graphical abstractImage 1
       
  • An Investigation of the Microstructure and Ductility of Annealed
           Cold-Rolled Tungsten
    • Abstract: Publication date: Available online 4 October 2018Source: Acta MaterialiaAuthor(s): Chai Ren, Z. Zak Fang, Lei Xu, Jonathan P. Ligda, James D. Paramore, Brady G. Butler Tungsten is notoriously brittle metal at room temperature. Furthermore, contrary to most metals, plastic deformation increases ductility and recrystallization decreases ductility of tungsten. The fundamentals that govern this behavior have challenged academia and industry for decades. This paper focuses on understanding the controlling factors of ductility through a systematic investigation of the changes in microstructure and mechanical properties of cold-rolled tungsten that occur during annealing. Cold-rolled tungsten samples were annealed at temperatures up to 1400 °C, and mechanical testing and microstructural analysis was performed before and after annealing. Furthermore, a dislocation mobility model based on the Orowan equation was applied. The mechanisms of deformation are discussed within the context of deformed and annealed microstructures. The high fraction of low angle grain boundaries and high density of edge dislocations were found to be the most important factors for ductility. Although there were gradual changes in microstructure and mechanical properties, the ductility of cold-rolled tungsten was maintained up to 1300 °C. The material recrystallized when annealed above this temperature, had no ductility, and suffered brittle fracture. Microstructural characterizations of the as-rolled material revealed a typical BCC texture, with grains elongated in rolling direction and a large amount of edge dislocations and low angle grain boundaries. The level of texturing and the fraction of low angle grain boundaries diminished after recrystallization. It was found that, compared to the recrystallized material, as-rolled tungsten can accommodate over 7 orders of magnitude higher deformation velocity due to the high density of edge dislocations.Graphical abstractImage 1
       
  • Elucidation of Cold-Spray Deposition Mechanism by Auger Electron
           Spectroscopic Evaluation of Bonding Interface Oxide Film
    • Abstract: Publication date: Available online 4 October 2018Source: Acta MaterialiaAuthor(s): Yuji Ichikawa, Ryotaro Tokoro, Masatoshi Tanno, Kazuhiro Ogawa The relationship between the cold spray deposition mechanism, microstructure, and strength of the resulting film must be understood for this innovative process to be practical. Previous studies have suggested that the coating mechanism is reliant on breaking the natural oxide film such that metallic bonding occurs through direct contact between the metal surfaces. In this study, the proposed model was experimentally verified by a small tensile adhesion test and auger electron spectroscopy analysis of the bonding interface. Since shear deformation does not occur at the tip (south pole) of the incoming particle, the oxide film is not broken, such that the bonding strength is weak. In contrast, at the outer edge of the particle, metallic bonding occurs, attaining a level of strength that exceeds that of the base material due to the huge plastic deformation. This phenomenon is known as the “south-pole problem,” and can lead to a decrease in the overall adhesion strength despite the local adhesion being strong. However, detailed observations revealed, in parts of the deposits, particles that had adhered across their entire surface. This suggests that, provided the collision state can be controlled, it is possible to overcome the south-pole problem and improve the adhesion strength.Graphical abstractImage 1
       
  • Architected Cellular Piezoelectric Metamaterials:
           Thermo-Electro-Mechanical Properties
    • Abstract: Publication date: Available online 3 October 2018Source: Acta MaterialiaAuthor(s): J. Shi, A.H. Akbarzadeh Advances in additive manufacturing have recently made possible the manufacturing of smart materials with arbitrary microarchitectures, which leads to developing lightweight smart metamaterials with unprecedented multifunctional properties. In this paper, asymptotic homogenization (AH) method is developed for predicting the effective thermo-electro-mechanical properties of architected cellular piezoelectric metamaterials. The effect of pore microarchitecture (relative density and cell topology) and polarization direction on elastic, dielectric, piezoelectric, pyroelectric and thermal properties of periodic cellular piezoelectric metamaterials is explored. The pore topology is determined by Fourier series expansion. Alternative pore microarchitectures are considered by tailoring shape parameters, scaling factor, and rotation angle of the constitutive pore. Smart cellular metamaterials made of both single-phase (BaTiO3) and bi-phase (BaTiO3-expoy) piezoelectric materials are considered. Apart from effective thermo-electro-mechanical properties, a series of figures of merit for the cellular piezoelectric metamaterials, i.e. piezoelectric coupling constant, acoustic impedance, piezoelectric charge coefficient, hydrostatic figure of merit, current responsivity, voltage responsivity and pyroelectric harvesting figures of merit, are presented and the reason for difference between the figures of merit of different types of piezoelectric metamaterials is discussed. The figures of merit shed lights on the effect of microarchitecture on optimizing the multifunctional performance of smart cellular metamaterials for applications as structurally efficient multifunctional energy harvesters. It is found that the piezoelectric and pyroelectric figures of merit of cellular piezoelectric metamaterials can be significantly improved compared to the commonly used honeycomb cellular materials and composite materials with solid circular inclusion if an appropriate microarchitecture is selected for the pore. For example, piezoelectric charge coefficient (dh) for a transversely polarized single-phase cellular piezoelectric metamaterial with a solid volume fraction of 0.4 can be 350% higher than the corresponding figures of merit of honeycomb piezoelectric material; voltage responsivity of transversely polarized bi-phase cellular piezoelectric metamaterials with an inclusion volume fraction of 0.3 can be also 249% higher than the corresponding value of composite materials with a solid circular inclusion.Graphical abstractImage 1
       
  • hcp → ω phase transition mechanisms in shocked zirconium: A machine
           learning based atomic simulation study
    • Abstract: Publication date: Available online 3 October 2018Source: Acta MaterialiaAuthor(s): Hongxiang Zong, Yufei Luo, Xiangdong Ding, Turab Lookman, Graeme J. Ackland There has been much controversy over the behavior of zirconium under shock strong enough to cause the pressure-induced hcp → ω phase transformation. Due to the short time- and length scales involved, direct measurements of the microstructure are extremely challenging. We have performed molecular dynamics simulations to investigate this issue, with Zr described by a machine-learned interatomic potential. Two different orientation relationships (ORs) between the hcp and ω phases are observed under shock driven conditions. Unlike the case with Ti that is in the same group, the ORs between the hcp and ω phases show less anisotropic phase transition sensitivity and in most cases follow the Silcock relationship with(0001)α (12¯10)ω. Furthermore, we find that the α→ ω transformation in shocked Zr occurs via an intermediate metastable bcc structure during the loading process, whereas no such intermediate is found during the reverse ω→α transition when the shock releases.Graphical abstractImage 1
       
  • Plastic Flow Resistance of NiTiCu Shape Memory Alloy-Theory and
           Experiments
    • Abstract: Publication date: Available online 3 October 2018Source: Acta MaterialiaAuthor(s): S. Alkan, H. Sehitoglu The NiTiCu alloys belong to a class of materials with excellent shape memory properties. The limitations in shape memory properties arise due to onset of slip at interfaces and also in austenite domains. As slip mediated plasticity is a source of irreversibility, it is imperative to understand the glide resistance of austenite which can be rather complex. In this paper, we develop a model to precisely derive the CRSS for slip substantiated with the uniaxial loading experiments on single crystals in a wide range of orientations employing Digital Image Correlation. We illustrate the core spreading of {011} dislocations and evaluate the role of non-Schmid stress components which introduces profound anisotropy in CRSS levels. The model matches the experimental findings in single crystals with excellent agreement. The theory and experiments show significant crystal orientation dependence of plasticity which must be taken into account when designing with these alloys.Graphical abstractImage 1
       
  • Graphene-Size-Tuned Mechanical Serration Behaviors in Nanocarbons
    • Abstract: Publication date: Available online 3 October 2018Source: Acta MaterialiaAuthor(s): Bo Li, Yanli Nan, Xiang Zhao, Peng Zhang, Xiaolong Song Two vastly different types of load-displacement responses observed in graphitic nanostructures under nano-compression are compared in terms of serration behaviors. Different from commonly encountered linear/nonlinear elastic deformation, a periodic serration behavior related to plastic flow is observed in amorphous carbon nanospheres. The true stress-strain relation exhibits a sole feature of type C serration, and comprehensive statistical, dynamical and fractal analyses further demonstrate a chaotic characteristic of dynamics for those serration events. When entering a quasi-steady flow stage, the elastic stress in each serration event could maintain a relatively stable level near ∼135 MPa, very close to the interlayer shear stress (ISS) of single crystalline graphite (∼140 MPa). This finding indicates the dependence of shear deformation on weak van der Waals interaction (elastic constant C44), instead of other structural factors associated with high elastic constants of graphite cells. Based on the experimental results, a microscale ISS-driven shearing mechanism is proposed. The local flexibility induced by small graphene lamellas may facilitate interfacial slip between neighboring domains with commensurate contact. Such slip mode may be responsible for the mechanical serration phenomenon in graphitic materials.Graphical abstractImage 1
       
  • Atomistic insight into the dislocation nucleation at
           crystalline/crystalline and crystalline/amorphous interfaces without full
           symmetry
    • Abstract: Publication date: Available online 3 October 2018Source: Acta MaterialiaAuthor(s): Y.Y. Xiao, X.F. Kong, B.N. Yao, D. Legut, T.C. Germann, R.F. Zhang Misfit dislocations at bimetal interfaces play a decisive role in determining various deformation behaviors by carrying the shear sliding, serving as a barrier for dislocation transmission and a source of dislocation nucleation. However, when the interface does not possess the distinct feature of misfit dislocations, the nucleation mechanism of lattice dislocations at the interfaces cannot be simply quantified by previously developed atomistic mechanisms based on characteristic misfit dislocations. Using crystalline/crystalline interfaces with a large lattice mismatch and crystalline/amorphous interfaces without local symmetry as prototypes, we show for the first time that the dislocation nucleation at such interfaces is attributable to the localized strain heterogeneities by modifying the volumetric and shear strain components at the atomic level to mechanically respond to different loadings. Using atomic strain tensor analysis, we found that in-plane localized shearing plays a critical role in the emission of lattice dislocations from interfaces, while the corresponding normal components of the volumetric strain tensor will dominate the character of the nucleated lattice dislocation by modifying the atomic excess volume at the interface to overcome the barrier to dislocation nucleation. Further exploration of various crystalline/amorphous interfaces by varying the chemical composition of the amorphous side indicates that chemical heterogeneity may substantially change the strain heterogeneity by forming a different clustered structure at the interface, resulting in the preferred choice of nucleation sites at the boundary regions that can be defined as nano shear traces (NSTs). These results provide a foundation to investigate the effects of strain and chemical heterogeneities in order to provide a realistic explanation of interface mediated deformation mechanisms and an efficient solution to tune interface dominated plasticity.Graphical abstractImage 1
       
  • Helium induced microstructure damage, nano-scale grain formation and
           helium retention behaviour of ZrC
    • Abstract: Publication date: Available online 3 October 2018Source: Acta MaterialiaAuthor(s): Shradha Agarwal, Arunodaya Bhattacharya, Patrick Trocellier, Steven J. Zinkle Transition-metal ultra-high temperature ceramics are promising materials for nuclear structural applications. However, an understanding of their response to high-temperature irradiation and helium is vastly limited. This paper presents a study of helium effects in zirconium carbide (ZrC) by performing room temperature 3 MeV 3He+ ion irradiations up to 5x1020 ions.m-2 and high-temperature annealing (1273–1873 K), coupled with state-of-art characterization using transmission electron microscopy (TEM), scanning electron microscopy (SEM) and nuclear reaction analysis (NRA). We reveal that ZrC is susceptible to irradiation damage in terms of helium bubble formation. After annealing at 1373 K, tiny bubbles (1-2 nm) formed aligned clusters which were highly over-pressurized, producing strain contrast in TEM. At 1773 K, significant bubble growth occurred. Additionally, at 1773 K, a combined TEM/SEM analysis revealed dramatic matrix damage due to helium-induced surface blistering. Underneath blister caps, the microstructure evolved into ultra-fine nano-scale grains, similar to high burn-up structures observed in nuclear fuels, but consisting of numerous nano-cracks. We hypothesize that such structures are formed due to high gas pressure build-up and its subsequent release. This phenomenon initiated at the grain boundaries. Blister top surface consisted of inter-granular and trans-granular cracks. NRA depth profiling revealed that helium was present as double peaks with major portion lying at the end-of-the-range (EOR) and the rest as TEM invisible clusters in a shoulder extending to the surface. ZrC started releasing helium after 1373 K. Helium release increased significantly at higher temperatures, with majority helium loss occurring from EOR, rather than from near-surface regions.Graphical abstractImage 1
       
  • Globularization using Heat Treatment in Additively Manufactured Ti-6Al-4V
           for High Strength and Toughness
    • Abstract: Publication date: Available online 3 October 2018Source: Acta MaterialiaAuthor(s): Rushikesh Sabban, Sumit Bahl, Kaushik Chatterjee, Satyam Suwas A bimodal globularized microstructure in contrast to martensitic laths is known to impart high strength and toughness in Ti-6Al-4V. Heat treatment for the phase transformation of the laths to the globularized microstructure must be preceded by plastic deformation. This work reports an innovative strategy to obtain the bimodal microstructure consisting of globular α in additively manufactured Ti-6Al-4V alloy by heat treatment alone. The heat treatment schedule involves repeated thermal cycling close to but below the β transus temperature to form globular α eliminating the need for plastic deformation prior to heat treatment. A new mechanism of globularization other than known in literature is proposed to explain the formation of globular α. The inherent dislocation sub-structure of the martensitic laths initiates globularization by thermal grooving and boundary splitting but is unable to completely globularize the microstructure. Mechanisms such as cylinderization and edge spheroidization also do not lead to globularization. The purposefully designed thermal cycling causes oscillations in the volume fractions of α and β phases that in synergism with the slow cooling segments of the cycle globularize the α phase by epitaxial growth. The bimodal microstructure thus produced led to a significant improvement in the ductility by 80% and the toughness by 66 %, which are desirable for structural applications. Furthermore, beneficial compressive stresses were generated in the alloy because of cyclic heat treatment. It is envisaged that the exceptional combination of mechanical properties observed here will lead to the fabrication of SLM printed Ti-6Al-4V parts that could leverage the advantages of additive manufacturing with material properties that are comparable to those obtained by conventional fabrication routes.Graphical abstractImage 1
       
  • Deformation of lamellar γ-TiAl below the general yield stress
    • Abstract: Publication date: Available online 2 October 2018Source: Acta MaterialiaAuthor(s): Thomas Edward James Edwards, Fabio Di Gioacchino, Amy Jane Goodfellow, Gaurav Mohanty, Juri Wehrs, Johann Michler, William John Clegg The occurrence of plasticity below the macroscopic yield stress during tensile monotonic loading of nearly lamellar Ti-45Al-2Nb-2Mn(at%)-0.8vol% TiB2 at both 25 °C and 700 °C, and in two conditions of lamellar thickness, was measured by digital image correlation strain mapping of a remodelled Au surface speckle pattern. Such initial plasticity, not necessarily related to the presence of common stress concentrators such as hard particles or cracks, could occur at applied stresses as low as 64 % of the general yield stress. For a same applied strain it was more prominent at room temperature, and located as slip and twinning parallel to, and near to or at (respect.) lamellar interfaces of all types in soft mode-oriented colonies. These stretched the full colony width and the shear strain was most intense in the centre of the colonies. Further, the most highly operative microbands of plasticity at specimen fracture were not those most active prior to yielding. The strain mapping results from polycrystalline tensile loading were further compared to those from microcompression testing of soft-mode stacks of lamellae milled from single colonies performed at the same temperatures. Combined with post-mortem transmission electron microscopy of the pillars, the initial plasticity by longitudinal dislocation glide was found to locate within 30 – 50 nm of the lamellar interfaces, and not at the interfaces themselves. The highly localised plasticity that precedes high cycle fatigue failure is therefore inherently related to the lamellar structure, which predetermines the locations of plastic strain accumulation, even in a single loading cycle.Graphical abstractImage 1
       
  • An atomistic investigation of the interaction of dislocations with
           Guinier-Preston zones in Al-Cu alloys
    • Abstract: Publication date: Available online 1 October 2018Source: Acta MaterialiaAuthor(s): G. Esteban-Manzanares, E. Martínez, J. Segurado, L. Capolungo, J. LLorca The interaction between edge dislocations and Guinier-Preston zones in an Al-Cu alloy was analyzed by means of atomistic simulations. The different thermodynamic functions that determine the features of these obstacles for the dislocation glide were computed using molecular statics, molecular dynamics and the nudged elastic band method. It was found that Guinier-Preston zones are sheared by dislocations and the rate at which dislocations overcome the precipitate is controlled by the activation energy, ΔU, in agreement with the postulates of the harmonic transition state theory. Moreover, the entropic contribution to the Helmholtz activation free energy was in the range 1.3–1.8 kb, which can be associated with the typical vibrational entropy of solids. Finally, an estimation of the initial shear flow stress as a function of temperature was carried out from the thermodynamic data provided by the atomistic simulations. Comparison with experimental results showed that the effect of the random precipitate distribution and of the dislocation character and dislocation/precipitation orientation has to be taken into account in the simulations to better reproduce experiments.Graphical abstractImage 1
       
  • Dynamic precipitation, segregation and strengthening of an Al-Zn-Mg-Cu
           alloy (AA7075) processed by high-pressure torsion
    • Abstract: Publication date: Available online 28 September 2018Source: Acta MaterialiaAuthor(s): Yidong Zhang, Shenbao Jin, Patrick W. Trimby, Xiaozhou Liao, Maxim Y. Murashkin, Ruslan Z. Valiev, Jizi Liu, Julie M. Cairney, Simon P. Ringer, Gang Sha Combining transmission Kikuchi diffraction, high resolution transmission electron microscopy and atom probe tomography, we investigated an Al-Zn-Mg-Cu alloy (AA7075) processed by high-pressure torsion (HPT) at room temperature and 200 °C, with an objective to reveal the deformation-induced precipitation and segregation of elements at grain boundaries, and to study their appearance at different processing regimes. Although HPT processing at the two temperatures both induced the formation of ŋ phase, ŋ precipitates formed at the two temperatures have different chemical compositions. The increase of the HPT processing temperature increased significantly segregation of Mg and Cu at grain boundaries. The HPT–induced segregation and decomposition of the alloy have a significant effect on its mechanical strength. Our results open a way for achieving advanced mechanical properties in nanostructured metals and alloys by designing their precipitation and segregation through the control of SPD processing regimes.Graphical abstractImage 1
       
  • Erratum to ‘Micromechanistic study of textured multiphase polycrystals
           for resisting cold dwell fatigue’ [Acta Mater. (2018) 254–265]
    • Abstract: Publication date: Available online 28 September 2018Source: Acta MaterialiaAuthor(s): Zhen Zhang
       
  • Two Way Shape Memory Effect in NiTiHf High Temperature Shape Memory Alloy
           Tubes
    • Abstract: Publication date: Available online 27 September 2018Source: Acta MaterialiaAuthor(s): C. Hayrettin, O. Karakoc, I. Karaman, J.H. Mabe, R. Santamarta, J. Pons Two-way shape memory effect (TWSME) in nano-precipitation hardened, Ni50.3Ti29.7Hf20 high temperature shape memory alloy (HTSMA) thin walled tubes and its thermal stability were investigated. Torsional TWSME was induced in the thin wall tubes by repeated thermal cycling across their martensitic transformation under applied shear stress. The effects of training parameters and geometric factors, such as the number of training cycles, shear stress levels, and thickness of the tube walls, on the resulting TWSME were evaluated. Thermal stability of TWSME was characterized as a function of annealing treatments at elevated temperatures. It was found that under 200MPa, 600 thermal cycles were sufficient to reach a two-way shape memory strain (TWSMS) as high as 2.95%, which was shown to be stable upon annealing up to 400°C for 30 minutes. This TWSMS was 85% of the maximum measured actuation strain under 200MPa. The microstructure after thermo-mechanical training was investigated using transmission electron microscopy (TEM), which did not indicate a significant change in precipitate structure and size after the training. However, small amount of remnant austenite was revealed at 100°C below the martensite finish temperature, with notable amount of dislocations. Overall, it was found that nano-precipitation hardened Ni50.3Ti29.7Hf20 shows relatively high TWSMS and stable actuation response after much less number of training cycles as compared to binary NiTi and nickel lean NiTiHf compositions. Tube wall thickness and training stress levels have been found to have negligible effect on shape memory strains and number of cycles to reach the desired training level, for the ranges studied.Graphical abstractImage 1
       
  • The Formation of Highly Ordered Graphitic Interphase Around Embedded CNTs
           Controls the Mechanics of Ultra-Strong Carbonized Nanofibers
    • Abstract: Publication date: Available online 27 September 2018Source: Acta MaterialiaAuthor(s): Jizhe Cai, Mohammad Naraghi Templating graphitization process, i.e., the transformation of certain polymers to highly-ordered graphitic (HOG) domains upon pyrolysis in the vicinity of graphitic nanomaterials, such as carbon nanotubes (CNTs), is known to be an effective approach to modify the microstructure of carbon nanofibers (CNFs). In this work, the microstructure of CNFs subjected to the templating effect of functionalized single-walled CNTs (f-SWNTs) and the effect of templating on mechanical properties of CNF/f-SWNTs hybrid nanofiber are studied. The CNF/f-SWNTs were fabricated via pyrolysis of electrospun polyacrylonitrile precursors with CNT inclusions. Prior to pyrolysis, the precursors were subjected to thermomechanical treatments, known as hot-drawing, to enhance chain and CNT alignment and packing. The study of the microstructure of the precursor and CNFs indicates the crucial role of precursor hot-drawing in enhancing the microstructure of the precursor and CNFs, leading to drastically enhanced templating effect, as evidenced from the thickness of the HOG that forms around CNTs. Mechanical tests on single nanofibers using custom-designed microdevices led to the realization that the templating effect of CNTs on CNFs, when properly implemented via precursor hot-drawing, can considerably increase the strength of CNFs. The average tensile strength and modulus of CNF/f-SWNTs in which HOG domains had clearly formed were measured to be 7.6 and 268 GPa, respectively, which are the highest value reported to date among similar types of materials. The existence and evolution of the HOG around CNTs inside CNFs and mechanical reinforcing of HOG were thoroughly discussed in conjunction with finite element models of building blocks of CNFs, alluding to the stress fields around HOG and CNTs in the CNF. The high-performance 1-D hybrid graphitic nanostructure developed here, CNF/f-SWNTs, can serve as an outstanding reinforcement material for weight sensitive applications.Graphical abstractImage 1
       
  • First-order reversal curve analysis of a Nd-Fe-B sintered magnet with soft
           X-ray magnetic circular dichroism microscopy
    • Abstract: Publication date: Available online 26 September 2018Source: Acta MaterialiaAuthor(s): Kazunori Miyazawa, Satoshi Okamoto, Takahiro Yomogita, Nobuaki Kikuchi, Osamu Kitakami, Kentaro Toyoki, David Billington, Yoshinori Kotani, Tetsuya Nakamura, Taisuke Sasaki, Tadakatsu Ohkubo, Kazuhiro Hono First-order reversal curve (FORC) diagram, which visualizes the variation of magnetic susceptibility on a field plane, has been applied to a Nd-Fe-B sintered magnet. The FORC diagram exhibits the characteristic behavior of two remarkable spots in low-field and high-field regions. The high-field FORC spot corresponds to the irreversible magnetization reversal at a coercive field, whereas the low-field FORC spot indicates the appearance of a large magnetic susceptibility state during the demagnetization process. Moreover, this low-field FORC spot becomes dominant at high temperature, accompanied by a significant reduction in coercivity. These results suggest that the low-field FORC spot has a strong correlation with the degradation of magnetic properties of a Nd-Fe-B sintered magnet. To clarify the actual magnetization reversal processes corresponding to these two FORC spots, soft X-ray magnetic circular dichroism (XMCD) microscopy observation was employed with similar field sequences of the FORC measurements. Consequently, the low-field FORC spot is mainly attributed to the domain wall motion in multi-domain grains, whereas the high-field FORC spot corresponds to the magnetization reversal of single-domain grains. These indicate that a FORC diagram is a powerful evaluation method for the magnetization reversal processes of permanent magnets.Graphical abstractImage 1
       
  • Structural evolution of directionally freeze-cast iron foams during
           oxidation/reduction cycles immediately prior to returning your
           corrections. -->
    • Abstract: Publication date: Available online 26 September 2018Source: Acta MaterialiaAuthor(s): Stephen K. Wilke, David C. Dunand Cyclical oxidation/reduction behavior of iron-based powders and porous pellets is of great interest for iron-air batteries, steam-iron, and chemical looping processes, but extended cycling is limited by degradation via sintering or pulverization. To address these problems, we use directional freeze casting to fabricate porous iron foams, consisting of colonies of parallel iron lamellae and open channels of sufficient width (20–40 and 20–50 μm, respectively) to accommodate iron/iron oxide volume changes during redox cycling. Iron foams of three different initial channel porosities (48, 61 and 65 vol.%) are fabricated via water-based freeze casting of Fe2O3 powders followed by reduction with H2 and sintering. The evolution of these iron foams is examined after 5 and 10 redox cycles between Fe3O4 and Fe at 800 °C (via steam and H2) using optical microscopy, scanning electron microscopy, and synchrotron X-ray tomography. Redox cycling causes a macroscopic foam shrinkage as the iron lamellae grow closer together, decreasing (and even sometimes eliminating) the channel width between lamellae. Smaller micropores within individual iron lamellae are partially preserved, consistent with new porosity formation via vacancy diffusion and clustering in the oxide phase. Additionally, a dense Fe shell forms on the exterior surface of most samples, caused by lamellae contacting and sintering during oxidation, followed by formation of an impermeable Fe layer during reduction. Strategies are proposed to reduce both channel constriction and shell formation, which are undesirable as they restrict gas phase transport.Graphical abstractImage 1
       
  • Fracture Toughness of NiTi–Towards Establishing Standard Test Methods
           for Phase Transforming Materials
    • Abstract: Publication date: Available online 26 September 2018Source: Acta MaterialiaAuthor(s): Behrouz Haghgouyan, Ceylan Hayrettin, Theocharis Baxevanis, Ibrahim Karaman, Dimitris C. Lagoudas A new test methodology for measuring the fracture toughness of shape memory alloys using the critical value of J-integral as the fracture criterion is proposed. The method relies on the ASTM standard method for measuring the fracture toughness of conventional ductile materials extended to account for the martensitic transformation/martensite reorientation-induced changes in the apparent elastic properties. A comprehensive set of nominally-isothermal fracture experiments is carried out on near-equiatomic NiTi compact tension specimens at three distinct temperatures: (i) below the martensite-finish temperature, Mf; (ii) between the martensite-start temperature, Ms, and the martensite desist temperature, Md, above which the stress-induced martensitic transformation is suppressed; and (iii) above Md. At these temperatures, the material either remains in the martensite state throughout the loading (martensitic material, case (i)) or transforms from austenite to martensite close to the crack tip (transforming material, case (ii)) or remains always in the austenite state (austenitic material, case (iii)), respectively. The critical J-values for crack growth, i.e., the fracture toughness, reported in all three cases, result in extrapolated stress intensity factors that are much higher than the corresponding values reported in literature on the basis of linear elastic fracture mechanics. Moreover, contrary to literature, the fracture toughness of martensitic and transforming materials is found to be approximately the same while the fracture toughness of stable austenite is considerably higher. This mechanics-aided test method can be potentially utilized for measuring the fracture toughness of martensitically transforming materials beyond shape memory alloys.Graphical abstractImage 1
       
  • Site occupancy of alloying elements in the L12 structure determined by
           channeling enhanced microanalysis in γ/γ’ Co-9Al-9W-2X alloys
    • Abstract: Publication date: Available online 26 September 2018Source: Acta MaterialiaAuthor(s): Li Wang, Michael Oehring, Yong Liu, Uwe Lorenz, Florian Pyczak Knowledge about the sublattice site preference of alloying elements in the L12-γ’ phase of novel Co-base superalloys is a necessary pre-requisite to understand their influence on the properties of the alloys in general and the γ’ phase in particular. In the present study, the atomic site occupancy of the alloying elements in the L12-γ’ structure in Co-9Al-9W-2X quaternary alloys after long-term annealing at 900 °C for 5000 hours was determined using the atom location by channeling enhanced microanalysis (ALCHEMI) technique in combination with energy-dispersive X-ray spectroscopy (EDX) composition analysis in a transmission electron microscope (TEM). The experimental ALCHEMI data were evaluated by comparing them with those calculated by the program ‘Inelastic Cross Section Calculator’ (ICSC). The results show that Co mainly occupies one sublattice site and Al/W are located at the other sublattice site in the L12 unit cell in the ternary alloy. The additional elements Ti, V, Mo and Ta which partition strongly to the γ’ phase tend to occupy the Al/W sublattice site, and Cr which partitions more to the γ phase also favors the Al/W sublattice site, while Ni weakly partitions into the γ’ phase and favors the Co sublattice site. The results of this study can provide evidence to the predictions on the site preference in literature based on the phase composition or on theoretical studies.Graphical abstractImage 1
       
  • Interplay between chemical strain, defects and ordering in Sr1-xLaxFeO3
           materials
    • Abstract: Publication date: Available online 26 September 2018Source: Acta MaterialiaAuthor(s): V.V. Sereda, D.S. Tsvetkov, I.L. Ivanov, A.Yu. Zuev Different point defect interactions were found to govern the defect chemistry of SrFeO3-δ and La0.6Sr0.4FeO3-δ. Conventional defect structure model based on two reactions – oxygen release by oxide lattice and the charge disproportionation in Fe-sublattice – can be applied successfully to La0.6Sr0.4FeO3-δ. Successful verification of this model using the available data on the oxygen nonstoichiometry gives virtually zero standard entropy and comparatively high standard enthalpy of iron disproportionation (116.54±1.14 kJ/mol). In turn, the chemical strain of La0.6Sr0.4FeO3-δ was predicted successfully using the simple dimensional model, based on the ionic radii formalism and verified defect structure model. For SrFeO3-δ, a reference set of nonstoichiometry data was chosen from among the multitudinous literature data by comparing the calculated and calorimetrically determined oxide’s reduction enthalpies. Some aspects of perovskite – brownmillerite phase transition in SrFeO3-δ were discussed and the defect structure model for this oxide was proposed and then verified using the chosen data set. Introduction of the vacancy cluster formation in the defect structure model was shown to be necessary since SrFeO3-δ is highly nonstoichiometric with respect to oxygen and tends to form various ordered structures. As a consequence, chemical expansivity of SrFeO3-δ with respect to the oxygen nonstoichiometry was found to be much more complex than that of La0.6Sr0.4FeO3-δ. According to our findings, unusually high values and the anomalous character of chemical strain of SrFeO3-δ are likely to be attributed to the vacancy cluster formation (i.e. short-range ordering) and some degree of long-range vacancy ordering, respectively.Graphical abstractImage 1
       
  • Common mechanism for controlling polymorph selection during
           crystallization in supercooled metallic liquids
    • Abstract: Publication date: Available online 26 September 2018Source: Acta MaterialiaAuthor(s): Simin An, Rui Su, Yuan-Chao Hu, Jianbo Liu, Yong Yang, Baixin Liu, Pengfei Guan Despite the fundamental and technological importance, the physical scenario of polymorph selections during crystallization in metals remains poorly understood. Here, through the extensive molecular dynamics simulation, we study the crystallization pathway of face-centered cubic (fcc) metals aluminum and copper. We reveal two different crystallization pathways at ambient pressure: involving the metastable body-centered cubic (bcc) polymorph (Cu) or not involving the bcc polymorph (Al). Interestingly, by varying pressure on Al, we successfully control polymorph selection and observe crystallites containing the metastable bcc polymorph. These findings are in line with the phonon dispersion analyses, which indicate that the bcc structure of Cu at ambient pressure and of Al at high pressure remain stable whereas that of Al at low pressure is thermally unstable. More importantly, we demonstrate that it is the profile of the free energy surface related to the phase stability, rather than the strong cohesive interactions of metals, encodes the crystallization pathway in metals. Our findings not only shed important light on a common thermodynamic mechanism of polymorph selections, but also pave a new way for better controlling the crystallization pathway in industrial and metallurgical applications by regulating the stability of the intermediate metastable phases.Graphical abstractImage 1
       
  • Stable antiferroelectricity with incompletely reversible phase transition
           and low volume-strain contribution in BaZrO3 and CaZrO3 substituted NaNbO3
           ceramics
    • Abstract: Publication date: Available online 26 September 2018Source: Acta MaterialiaAuthor(s): Ruzhong Zuo, Jian Fu, He Qi In this work a reproducible double polarization versus electric field (P-E) hysteresis loop clearly reveals a room-temperature stable antiferroelectricity (AFE) in virgin NaNbO3 (NN) samples doped with 6 mol% BaZrO3 and 3 mol% CaZrO3. Inconsistent but sprout-like strain versus field (S-E) curves without any negative strains between the first and non-first cycles yet display an incompletely reversible field induced AFE-ferroelectric (FE) phase transition. In/ex-situ synchrotron x-ray diffraction results suggest irrecoverable AFE states with orthorhombic and monoclinic structures before and after electric cycling, respectively, which generate an irreversible strain contribution including both irreversible volume (22%) and lattice (78%) strains. Frequency-dependent measurements of P/S-E curves demonstrate the rest of remanent strains from the time effect of back-switching from field induced FE to AFE equivalently existing in strain loops of any cycles at a fixed frequency. Compared with virgin samples, post-cycled samples exhibit a completely reversible monoclinic AFE-monoclinic FE phase transition but a relatively small poling strain. It is of particular note that the contribution of the volume strain to the poling strain is only 20% in AFE NN-based ceramics, in which a positive longitudinal strain and a negative transverse strain are concurrently observed, challenging a common knowledge that both strains in two directions should be positive for traditional Pb-based AFE ceramics. The experimental results provide new insights into the AFE phase stability of NN and an exciting possibility to take advantage of NN-based antiferroelectric ceramics for large-displacement actuators in future.Graphical abstractImage 1
       
  • Exceptional phase-transformation strengthening of ferrous medium-entropy
           alloys at cryogenic temperatures
    • Abstract: Publication date: Available online 26 September 2018Source: Acta MaterialiaAuthor(s): Jae Wung Bae, Jae Bok Seol, Jongun Moon, Seok Su Sohn, Min Ji Jang, Ho Yong Um, Byeong-Joo Lee, Hyoung Seop Kim High-entropy alloys (HEAs) are a newly emerging class of materials that show attractive mechanical properties for structural applications. Particularly, face-centered cubic (fcc) structured HEAs and medium-entropy alloys (MEAs) such as FeMnCoNiCr and CoNiCr alloys, respectively, which exhibit superior fracture toughness and tensile properties at liquid nitrogen temperature, are the potential HEA materials available for cryogenic applications. Here, we report a ferrous Fe60Co15Ni15Cr10 (at%) MEA exhibiting combination of cryogenic tensile strength of ∼1.5 GPa and ductility of ∼87% due to the multiple-stage strain hardening. Astonishingly, detailed microstructural observations at each stage reveal the sequential operation of deformation-induced phase transformation from parent fcc to newly formed bcc (body-centered cubic) phases. No compositional heterogeneity is observed at phase boundaries, indicating diffusionless phase transformation, as confirmed by atom probe tomography. The transformation to bcc phase occurs predominantly along grain boundaries (GBs) at the early stage of plastic deformation. Simultaneously, numerous deformation-induced shear bands (SBs) having stacking faults associated to the Shockley partial dislocations and thin hcp plates, form within fcc grains. Further deformation leads to the intense nucleation and growth of the bcc phase at the intersections of SBs within fcc grains. These micro-processes consecutively enhance the strain hardening rate, which play a key role in the multiple strain hardening behavior. The in-situ neutron diffraction studies make it clear that the martensite formation and the concurrent load partitioning between the fcc and bcc phases play an important role in the increase in strength. Furthermore, replacing high-cost alloying elements cobalt and nickel with iron, as well as introduction of metastability-engineering at liquid nitrogen temperature, distinguishes the new ferrous MEAs from previously reported equiatomic HEAs. This result underlines insights to provide expanded opportunities for the future development of HEAs for cryogenic applications.Graphical abstractImage 1
       
  • Evolution of structure and residual stress of a fcc/hex-AlCrN
           multi-layered system upon thermal loading revealed by cross-sectional
           X-ray nano-diffraction
    • Abstract: Publication date: Available online 25 September 2018Source: Acta MaterialiaAuthor(s): N. Jäger, S. Klima, H. Hruby, J. Julin, M. Burghammer, J.F. Keckes, C. Mitterer, R. Daniel Understanding the influence of process conditions and coating architecture on the microstructure and residual stress state of multi-layered coatings is essential for the development of novel thermally and mechanically stable coatings and requires advanced depth resolving characterization techniques. In this work, an arc-evaporated multi-layered coating, consisting of alternating Al70Cr30N and Al90Cr10N sublayers with an individual layer thickness between 120nm and 380nm, was investigated. The as-deposited state of the multi-layered coating and the state after vacuum annealing at 1000°C for 30min was studied along its cross-section by synchrotron X-ray nano-diffraction using a beam with a diameter of 50nm. The results revealed sublayers with alternating cubic and hexagonal phase, causing repeated interruption of the grain growth at the interfaces. The in-plane residual stress depth distribution across the coating thickness could be tuned in a wide range between pronounced compressive and slight tensile stress by combining the effects of the coating architecture and the modulated incident particle energy controlled by the substrate bias voltage ranging from -30V to -250V. This resulted in an oscillatory stress profile fluctuating between -2 GPa and -4.5 GPa or pronounced stress gradients with values between -4 GPa and 0.5 GPa. Finally, the decomposition routes of the metastable cubic Al70Cr30N phase could be controlled by the Al90Cr10N sublayers which acted as nucleation sites and governed the texture of the decomposition products as Cr2N. The results demonstrate that the cross sectional combinatorial approach, relying on a sophisticated multi-layer architecture combining various materials synthesized under tailored conditions, allowed for resolving structural variations and stress profiles in the individual layers within the complex architecture and pioneers the path for knowledge-based development of multi-layered coatings with predefined microstructure and a dedicated stress design.Graphical abstractImage 1
       
  • Micromechanical behavior and thermal stability of a dual-phase α+α’
           titanium alloy produced by additive manufacturing
    • Abstract: Publication date: Available online 25 September 2018Source: Acta MaterialiaAuthor(s): Charlotte de Formanoir, Guilhem Martin, Frédéric Prima, Sébastien Allain, Thibaut Dessolier, Fan Sun, Solange Vivès, Benjamin Hary, Yves Bréchet, Stéphane GodetABSTRACTIn order to improve the tensile properties of additively manufactured Ti-6Al-4V parts, specific heat treatments have been developed. Previous work demonstrated that a sub-transus thermal treatment at 920 °C followed by water quenching generates a dual-phase α+α' microstructure with a high work-hardening capacity inducing a desirable increase in both strength and ductility. The present study investigates the micromechanical behavior of this α+α' material as well as the thermal stability of the metastable α’ martensite. To that end, annealing of the α+α' microstructure is performed and the resulting microstructural evolution is analyzed, along with its impact on the tensile properties. A deeper understanding of the micromechanics of the multiphase microstructure both before and after annealing is achieved by performing in-situ tensile testing within a SEM, together with digital image correlation for full-field local strain measurements. This approach allows the strain partitioning to be quantified at a microscale and highlights a significant mechanical contrast between the two phases. In the α+α' microstructure, the α' phase is softer than the α phase, which is confirmed by nanoindentation measurements. Partial decomposition of the martensite during annealing induces a substantial hardening of the α' phase, which is attributed to fine-scale precipitation and solution strengthening. A scale transition model based on the iso-work assumption and describing the macroscopic tensile behavior of the material depending on the individual mechanical behavior of each phase is also proposed. This model enables to provide insights into the underlying deformation and work-hardening mechanisms.Graphical abstractImage 1
       
  • Elemental site occupancy in the L12 A3B ordered intermetallic phase in
           Co-based superalloys and its influence on the microstructure
    • Abstract: Publication date: Available online 25 September 2018Source: Acta MaterialiaAuthor(s): P. Pandey, S.K. Makineni, A. Samanta, A. Sharma, S.M. Das, B. Nithin, C. Srivastava, A.K. Singh, D. Raabe, B. Gault, K. Chattopadhyay We explore the effects of the elemental site occupancy in γ'-A3B (L12) intermetallic phases and their partitioning across the γ/γ' interface in a class of multicomponent W-free Co-based superalloys. Atom probe tomography and first principles density functional theory calculations (DFT) were used to evaluate the Cr site occupancy behavior in the γ' phase and its effect on the γ/γ' partitioning behavior of other solutes in a series of Co-30Ni-10Al-5Mo-2Ta-2Ti-xCr alloys, where x is 0, 2, 5, and 8 at.% Cr, respectively. The increase in Cr content from 0 to 2 to 5 at.% leads to an inversion of the partitioning behavior of the solute Mo from the γ' phase (KMo>1) into the γ matrix (KMo
       
  • Mechanisms of pearlitic spheroidization: Insights from 3D phase-field
           simulations
    • Abstract: Publication date: Available online 25 September 2018Source: Acta MaterialiaAuthor(s): P.G.Kubendran Amos, Avisor Bhattacharya, Britta Nestler, Kumar Ankit Morphological evolution of eutectoid phases determine the spheroids’ size and distribution post sub-critical annealing of steel. In this work, the spheroidization of the 3-dimensional cementite plates is investigated via phase-field modeling to enhance our understanding of the underlying capillary-mediated mechanisms. Since the interfacial energy plays a key role in the spheroidization process, a phase-field model which efficiently avoids any contribution of the bulk free energy in the interface is employed to recover the sharp interface solutions. It is identified that depending on the cementite aspect ratio, the spheroidization mechanism adopted by the plate, varies. In plates of smaller aspect ratios (
       
  • Multi-scale modeling of the complex microstructural evolution in
           structural phase transformations
    • Abstract: Publication date: Available online 25 September 2018Source: Acta MaterialiaAuthor(s): Kang Wang, Lin Zhang, Feng Lius Modeling the microstructural evolution in structural phase transformations remains challenging, mostly due to the competitions among the potential product phases and the multi-scale nature. To develop a practical tool for such a scientifically and technologically important issue, a multi-scale framework is proposed, where a coarse graining scheme based on the probability density distribution of the representative volume elements (RVEs) of product phases is coupled with the maximal entropy production principle (MEPP) to model the competitions among the multiple product phases as the selection of dissipative paths, and a Fokker-Planck type equation is obtained for the evolution of multiple microstructural parameters (MPs) for the product phases. Applied to precipitation in Al-Cu alloys, the present model, free of adjustable parameters, predicts a correct sequence of precipitation, i.e. GP zone → θꞌꞌ → θꞌ, and yields the accurate precipitation kinetics for θꞌꞌ and θꞌ as compared with the previous experimental data, thus demonstrating the inherent correlation between the MPs and thermodynamics and kinetics of the transformation. For the complex transformations in engineering alloys, the current framework, starting from the general statistical principles and the MEPP, can incorporate the specific MPs for a given transformation following the same scheme.Graphical abstractImage 1
       
  • Dislocation slip transmission through a coherent Σ3{111} copper twin
           boundary: Strain rate sensitivity, activation volume and strength
           distribution function
    • Abstract: Publication date: Available online 25 September 2018Source: Acta MaterialiaAuthor(s): N.V. Malyar, B. Grabowski, G. Dehm, C. Kirchlechner We present the first measurement of the strain rate sensitivity of the ideal dislocation slip transmission through a coherent Σ3{111} copper twin boundary. For this purpose we have deformed 129 geometrically identical samples at different strain rates. The micron-sized samples are either single crystalline (87 pillars) or contain one vertical Σ3{111} twin boundary (42 pillars). The strain rate sensitivity of the ideal slip transmission event is 0.015 ± 0.009. This value is considerably lower than the strain rate sensitivity observed for nano-twinned bulk materials, which is addressed to multiple simultaneously activated deformation processes present in the latter case. The activation volume of the ideal slip transmission points towards a cross-slip like transmission process of dislocations through the twin boundary. Furthermore, the high number of geometrically identical samples is used to discuss the ability to identify the strength distribution function of micropillars.Graphical abstractImage 1
       
  • Recipients of the 2017 Acta Student Awards
    • Abstract: Publication date: Available online 25 September 2018Source: Acta MaterialiaAuthor(s):
       
  • Impact of boron diffusion at MgO grain boundaries on magneto-transport
           properties of MgO/CoFeB/W magnetic tunnel junctions
    • Abstract: Publication date: Available online 24 September 2018Source: Acta MaterialiaAuthor(s): X.D. Xu, K. Mukaiyama, S. Kasai, T. Ohkubo, K. Hono Controlling the B diffusion in CoFeB-MgO based magnetic tunnel junctions (MTJs) is important to realize the large tunneling magnetoresistance (TMR) ratio, which is required to realize high density magnetic random access memories (MRAMs). Here we investigate the detailed microstructure of the MgO-CoFeB MTJs having W underlayer and overlayer in comparison with well-studied MTJs with Ta underlayer and overlayer. Detailed microstructure analysis based on transmission electron microscopy and electron energy loss spectroscopy revealed that B diffused into Ta under-/overlayers and crystallization of CoFeB to CoFe efficiently occurred upon annealing at 300 °C and 400 °C in MTJs with Ta underlayers. In contrast, B diffused into the [001] tilt grain boundaries of MgO barrier in the case of MTJs with W under-/overlayers. These structural differences well explain the difference in post annealing temperature dependence of TMR ratio and resistance area product (RA) of MTJs with Ta and W under-/overlayers.Graphical abstractImage 1
       
  • Strain relaxation in low-mismatched
           GaAs/GaAs1-xSbx/GaAs heterostructures
    • Abstract: Publication date: Available online 22 September 2018Source: Acta MaterialiaAuthor(s): Abhinandan Gangopadhyay, Aymeric Maros, Nikolai Faleev, David J. Smith The creation of structural defects in low-mismatched GaAs/GaAs0.92Sb0.08/GaAs(001) heterostructures and their evolution during strain relaxation have been studied using transmission electron microscopy as well as high-resolution x-ray diffraction and atomic force microscopy. These GaAsSb films had thicknesses in the range of 50 to 4000 nm with 50-nm-thick capping layers and were grown using molecular beam epitaxy. The strain relaxation had three distinct phases as the film thickness was increased, whereas the thin GaAs capping layers exhibited only the initial sluggish stage of relaxation in heterostructures with thick GaAsSb films. The character of the misfit dislocations at the two interfaces was determined using g.b analysis, and atomic-scale structural information was obtained using aberration-corrected electron microscopy. Stage-I relaxation took place primarily by glide of dissociated 60° dislocations. Although the films were mostly free of threading dislocations, many curved dislocations extended into the substrate side for heterostructures that had undergone Stage-II and Stage-III relaxation. Investigation of dislocation density evolution at the cap/film interface and morphological evolution of the growth surface revealed a strong correlation. The smoother growth surface in the heterostructure with 4000-nm-thick film resulted in a reduced areal density of surface troughs that acted as nucleation sites for dislocations, which explained the decreased dislocation density at the cap/film interface. Overall, these results prove that heterogeneously nucleated surface half-loops are the primary source of threading dislocations in low-mismatched heterostructures.Graphical abstractImage 1
       
  • Influence of electronic vs nuclear energy loss in radiation damage of
           Ti3SiC2
    • Abstract: Publication date: Available online 21 September 2018Source: Acta MaterialiaAuthor(s): William A. Hanson, Maulik K. Patel, Miguel L. Crespillo, Yanwen Zhang, William J. Weber The thermal conductivity and stability of MAX phases has led to irradiation studies of these materials for their possible application in the hostile environments of high temperature and radiation. Numerous neutron and ion irradiation studies have been conducted that demonstrate that radiation induced modifications consists of lattice strain and increased formation of TiC. Ion beams have been used to simulate damage created by neutrons; however, what is not clear is the effect of varying electronic to nuclear energy loss on the damage evolution within this material. In the present work, changes in c/a within Ti3SiC2 are monitored as a function of energy deposition at constant damage dose following high fluence 9 MeV Ti ion irradiations at room temperature. The results reveal that there is an apparent threshold in the electronic energy loss, above which the c/a ratio and TiC concentration starts to increase with increasing electronic energy loss. Interestingly, this change is independent of the damage dose in displacements per atom. This suggests that inelastic energy dissipation is of paramount importance when selecting ions for simulating damage by energetic neutrons.Graphical abstractImage 1
       
  • Phase stability and kinetics of σ-phase precipitation in CrMnFeCoNi
           high-entropy alloys
    • Abstract: Publication date: Available online 20 September 2018Source: Acta MaterialiaAuthor(s): G. Laplanche, S. Berglund, C. Reinhart, A. Kostka, F. Fox, E.P. George Although the phase stability of high-entropy alloys in the Cr-Mn-Fe-Co-Ni system has received considerable attention recently, knowledge of their thermodynamic equilibrium states and precipitation kinetics during high-temperature exposure is limited. In the present study, an off-equiatomic Cr26Mn20Fe20Co20Ni14 high-entropy alloy was solutionized and isothermally aged at temperatures between 600°C and 1000°C for times to 1000 h. In the original single-phase fcc matrix, an intermetallic σ phase was found to form at all investigated temperatures. Its morphology and composition were determined and the precipitation kinetics analyzed using the Johnson-Mehl-Avrami-Kolmogorov equation and an Arrhenius type law. From these analyses, a time-temperature-transformation diagram (TTT diagram) is constructed for this off-equiatomic alloy. We combine our findings with theories of precipitation kinetics developed for traditional polycrystalline fcc alloys to calculate a TTT diagram for the equiatomic CrMnFeCoNi HEA. The results of our investigation may serve as a guide to predict precipitation kinetics in other complex alloys in the Cr-Mn-Fe-Co-Ni system.Graphical abstractImage 1
       
  • Microstructural effects on strain rate and dwell sensitivity in dual-phase
           titanium alloys
    • Abstract: Publication date: Available online 20 September 2018Source: Acta MaterialiaAuthor(s): Sana Waheed, Zebang Zheng, Daniel S. Balint, Fionn P.E. Dunne In this study, stress relaxation tests are performed to determine and compare the strain rate sensitivity of different α−β titanium alloy microstructures using discrete dislocation plasticity (DDP) and crystal plasticity finite element (CPFE) simulations. The anisotropic α and β phase properties of alloy Ti-6242 are explicitly included in both the thermally-activated DDP and CPFE models together with direct dislocation penetration across material-interfaces in the DDP model. Equiaxed pure α, colony, Widmanstatten and basketweave microstructures are simulated together with an analysis of the effect of α grain size and dislocation penetration on rate sensitivity. It is demonstrated that alloy morphology and texture significantly influence microstructural material rate sensitivity in agreement with experimental evidence in the literature, whereas dislocation penetration is found not to be as significant as previously considered for small deformations. The mechanistic cause of these effects is argued to be changes in dislocation mean free-path and the total propensity for plastic slip in the specimen. Comparing DDP results with corresponding CPFE simulations, it is shown that discrete aspects of slip and hardening mechanisms have to be accounted for to capture experimentally observed rate sensitivity. Finally, the dwell sensitivity in a polycrystalline dual-phase titanium alloy specimen is shown to be strongly dependent on its microstructure.Graphical abstractImage 1
       
  • Spinodal decomposition during isothermal gas-solid equilibration – its
           effects and implications
    • Abstract: Publication date: Available online 20 September 2018Source: Acta MaterialiaAuthor(s): K.N. Sasidhar, S.R. Meka The possibility for a system to sample non-classical chemical pathways while attempting to attain thermodynamic equilibrium under conditions of restricted kinetics has been elucidated in this work through the analogy of gas phase nitriding of iron based alloys. Detailed thermodynamic assessment has for the first time shown evidence for the occurrence of in-situ isothermal spinodal decomposition during controlled nitridation of ferritic Fe-Cr alloys. Recognition of this phenomenon has led to the explanation of certain reported anomalous experimental observations: dependence of nitridation kinetics of ferritic Fe-Cr alloys on N diffusion, and the paradox of Cr-nitrides precipitation sequence not following the predictions of the phase diagram in nitrided Fe-Cr alloys. Finally, the nitridation kinetics of Fe-Al alloys being controlled by Al diffusion has been attributed to the absence of a possibility for spinodal decomposition during nitriding of Fe-Al alloys.Graphical abstractImage 1
       
  • Conventional vs Harmonic-structured β-Ti-25Nb-25Zr alloys: a comparative
           study of deformation mechanisms
    • Abstract: Publication date: Available online 20 September 2018Source: Acta MaterialiaAuthor(s): F. Mompiou, D. Tingaud, Y. Chang, B. Gault, G. Dirras Harmonic alloys processed by powder metallurgy are constituted by a core of coarse grains embedded in an interconnected small grains shell. They have attracted attention due to their excellent strength combined with large ductility, the two properties being rather antagonist from the classical metallurgy point of view. In contrast, conventional β-Ti alloys are currently vastly studied owing their excellent properties especially for biomedical applications. In the present study, we explore at the micron scale the deformation mechanisms operating both in standard and harmonic-structured β-Ti-25Nb-25Zr alloys using transmission electron microscopy (TEM). Although we show some similarities, deformation mechanisms appear significantly different due to the activation of martensitic transformation in conventional samples. The combined use of automated crystal orientation in TEM and in-situ TEM straining reveals that deformation bands nucleate and grow according to a mechanism involving both martensitic transformation and twinning. The comparison between in-situ and post-mortem experiments shows globally a good agreement and highlights a strain relaxation mechanism between martensite and twin. More importantly, a cross-glide mechanism similar to what is observed in dilute solid solutions is proposed to explain the dynamics of dislocation motion. Stress estimations derived from the observations of dislocation curvature between pinning points, show a reasonable good agreement with macroscopic values. The observation of deformation mechanisms operating both in core and shell structures of the harmonic-structured alloy allows us to propose a scenario of plastic deformation in the early stages.Graphical abstractImage 1
       
  • Evidence for Deformation-Induced Phase Transformation in a High Sn Content
           Zirconium Alloy
    • Abstract: Publication date: Available online 20 September 2018Source: Acta MaterialiaAuthor(s): C. Cochrane, M.A. Gharghouri, M.R. Daymond The zirconium alloy Excel has been shown to retain a metastable β phase upon quenching from temperature within the α+β phase field. Using a combination of neutron diffraction and EBSD, we show that it is possible to initiate a phase transformation from β→α during tensile deformation of Excel in the quenched state. We present a model to explain the observed behaviour. EBSD results show that twin-like α laths are formed in the β grains during deformation. This β→α transformation shows a strong variant selection, as expected for a phase transformation that accommodates plastic strain. Secondary transformation laths were also observed which did not follow the expected variant rule. We present a simple model that explains quantitatively the behaviour of the β→α phase transformation during straining and accurately predicts the variant selection.Graphical abstractImage 1
       
  • Grain Boundary Segregation and Intermetallic Precipitation in Coarsening
           Resistant Nanocrystalline Aluminum Alloys
    • Abstract: Publication date: Available online 20 September 2018Source: Acta MaterialiaAuthor(s): A. Devaraj, W. Wang, R. Vemuri, L. Kovarik, X. Jiang, M. Bowden, J.R. Trelewicz, S. Mathaudhu, A. Rohatgi In-spite of all of the unique properties of nanocrystalline materials, they are notorious when it comes to their susceptibility to thermally induced grain coarsening, thus imposing an upper limit to their application temperature. In this study, we demonstrate a coupled Monte Carlo-molecular dynamics simulation-guided experimental approach of improving the resistance to thermally induced grain coarsening in light-weight nanocrystalline Al-Mg alloys. The structure, grain boundary segregation of Mg, and extent of grain coarsening of the Al-Mg alloys were characterized using plan view and cross-sectional transmission electron microscopy and atom probe tomography. Coarsening resistance is attributed to a combination of thermodynamic stabilization of grain boundaries by controlled Mg segregation, and kinetic stabilization through pinning of the boundaries with nanoscale intermetallic precipitates. Thus, we highlight the opportunities in extending the upper limit of application temperature for nanocrystalline alloys by using a complementary thermodynamic and kinetic stabilization approach.Graphical abstractImage 1
       
  • Thermodynamics of Solid Sn and Pb-Sn Liquid Mixtures using Molecular
           Dynamics Simulations
    • Abstract: Publication date: Available online 19 September 2018Source: Acta MaterialiaAuthor(s): Seyed Alireza Etesami, Michael I. Baskes, Mohamed Laradji, Ebrahim Asadi We present a new set of modified embedded-atom method parameters for the Pb-Sn system that describes many 0K and high temperature properties including melting point, elastic constants, and enthalpy of mixing for solid and liquid Pb-Sn alloys in agreement with experiments. Then, we calculate the phase diagram of the Sn-rich side of Pb-Sn alloys utilizing a hybrid Molecular Dynamics/Monte Carlo simulation that agrees with experimental solidus and liquidus curves as well as stability of α-Sn and β-Sn. In addition, we present structure factors of Pb-Sn liquid alloys as well as temperature-dependent thermal expansion coefficients and heat capacity. Our simulations show that the ratios of the heights of the second and third peaks over the first peak for Pb-Sn liquid mixtures are maximum at Pb-0.6Sn concentration.Graphical abstractImage 1
       
  • Improved very high cycle bending fatigue behavior of Ni microbeams with Au
           coatings
    • Abstract: Publication date: Available online 19 September 2018Source: Acta MaterialiaAuthor(s): Saurabh Gupta, Nick England, Alejandro Barrios, Olivier N. Pierron This work investigated the effect of an 850-nm-thick electroplated Au coating on the very high cycle bending fatigue behavior of electroplated Ni microbeams tested under resonance in air at high frequencies (∼9 kHz). The S-N curves show longer fatigue lives for the coated microbeams by at least a factor of 5 compared to the uncoated ones. This beneficial effect is demonstrated to be related to the delay in oxygen-assisted void formation, and therefore in void-assisted fatigue crack nucleation and growth in Ni. The improvement in fatigue life is limited by the fatigue degradation of the Au coating, which is also controlled by the formation of nanosized voids. Once a fatigue crack in the coating reaches the interface, delamination occurs, leading to exposure of the underlying Ni to air and fast, “uncoated-like”, fatigue degradation thereafter. This study highlights that thin, noble metallic coatings can significantly improve the fatigue lives of metallic microbeams whose very high cycle fatigue behavior is sensitive to the environment and controlled by void formation.Graphical abstractImage 1
       
  • Facile Growth of High Aspect Ratio c-axis GaN Nanowires and their
           Application as Flexible p-n NiO/GaN Piezoelectric Nanogenerators
    • Abstract: Publication date: Available online 19 September 2018Source: Acta MaterialiaAuthor(s): Muhammad Ali Johar, Aadil Waseem, Mostafa Afifi Hassan, Jin-Ho Kang, Jun-Seok Ha, June Key Lee, Sang-wan Ryu Piezoelectric nanogenerators (PNGs) have attracted great interest as energy sources to power-up smart clothing, micro/nano systems, and portable electronic gadgets. Due to non-centrosymmetric crystal structure, bio-compatibility, and mechanical robustness of GaN, it is a promising candidate to fabricate PNGs. In this study, c-axis GaN nanowires were grown by MOCVD, then were embedded inside polydimethylsiloxane and flipped on to the flexible substrate, followed by the deposition of p-type NiO to form heterojunction. The fabrication of GaN nanowires based heterojunction PNG on flexible substrate is the first report to the best of our knowledge. The piezoelectric properties of PNGs were investigated as a function of the GaN nanowire length. A maximum piezoelectric output potential of 20.8 V and current of 253 nA were measured. The stability of the device was also evaluated and found stable even after 20,000 cycles. This high piezoelectric output was attributed to the suppression of free carrier screening and junction screening. Moreover, the underlying reasons for the high stability are the malleability of the device and high aspect ratio of the GaN nanowires. The design and stability of our device make it a promising candidate for applications in self-powered systems for environment monitoring and low power electronics.Graphical abstractImage 1
       
  • Good comprehensive performance of Laves phase Hf1-x Ta x Fe2 as negative
           thermal expansion materials
    • Abstract: Publication date: Available online 19 September 2018Source: Acta MaterialiaAuthor(s): L.F. Li, P. Tong, Y.M. Zou, W. Tong, W.B. Jiang, Y. Jiang, X.K. Zhang, J.C. Lin, M. Wang, C. Yang, X.B. Zhu, W.H. Song, Y.P. Sun Negative thermal expansion (NTE) materials can compensate for the normal positive thermal expansion (PTE) of most materials, and thus have great potential applications. Itinerant magnetic Laves phase compounds Hf1-xTaxFe2 with x ∼ 0.16 - 0.22 exhibit an abrupt volume shrink as large as ΔV/V ∼ 1% at the ferromagnetic (FM) to antiferromagnetic phase transition. Here we report that by reducing the Ta concentration the sharp volume change was gradually modified to a continuous one and moved to room temperature. NTE was optimized in x = 0.13, showing a linear NTE coefficient as large as -16.3 ppm/K over a broad window of 105 K (222 K - 327 K). As revealed by Electron Spin Resonance, the broadened NTE window is closely coupled with the asynchronous FM orderings of Fe moments at 6h and 2a Fe sites. In addition to good mechanical properties (i.e., Young’s modulus, compressive strength and Vickers hardness), their thermal and electrical conductivities are superior to other metallic NTE materials, suggesting their wide applications as PTE compensators.Graphical abstractImage 1
       
  • Correlating Defects Density in Metallic Glasses with the Distribution of
           Inherent Structures in Potential Energy Landscape
    • Abstract: Publication date: Available online 18 September 2018Source: Acta MaterialiaAuthor(s): Chaoyi Liu, Pengfei Guan, Yue Fan The structural evolutions of potential energy landscape (PEL) are investigated in a metallic glass modeling system with different cooling histories spanning more than five orders of magnitude. The local minima in the PEL are observed to be spatially more separated in the samples with lower fictive temperature Tfic than in the samples with higher Tfic. An Arrhenius scaling between the local minima density and Tfic is observed, which directly links the distribution of inherent structures in the PEL to the population of shear transformation zones (STZ) in amorphous solids. Moreover, very interestingly, the Arrhenius scaling breaks at 1.3∼1.4 Tg, above which an invariant PEL structure with a saturated density of local minima is achieved. The hereby obtained critical temperature coincides with the experimentally observed dynamics crossover temperature, which suggests a profound connection between the PEL structure and dynamics in glassy system.Graphical abstractImage 1
       
  • Transition from the twinning induced plasticity to the γ-ε
           transformation induced plasticity in a high manganese steel
    • Abstract: Publication date: Available online 17 September 2018Source: Acta MaterialiaAuthor(s): Q. Xie, Z. Pei, J. Liang, D. Yu, Z. Zhao, P. Yang, R. Li, M. Eisenbach, K. An Neutron-diffraction investigation on the deformation of a Fe-18Mn-3Si-0.6C-0.4Al steel reveals that the twinning is mainly dominant at a stress below 900 MPa, above which the twinning behavior is largely outweighed by phase transformation from the face-centered-cubic (FCC) γ-austenite to the hexagonal-close-packed (HCP) ε-martensite. In the deformed grains, distribution of ε-martensite is parallel with the twin boundary. Both the well-known {111}γ//{0001}ε relationship and an additional {111}γ//{11-21}ε orientation relation were identified. After the phase transformation, both phases deformed further with the lattice rotating around an axis perpendicular to the tensile direction. Transition from the twinning dominant behavior to the phase transformation induced plasticity effect is explained by the grain orientation dependence of the effective stacking fault energy (ESFE). An asymmetric and inverse relationship between the width of stacking faults (SF) and the ESFE is obtained from the density functional theory (DFT). The tensile stress always increases proportionally with the SF width in the twinning favorable grains and thus a decreasing ESFE. The replacement of twinning activities by phase transformation could be due to both the nucleation site of ε-martensite at the SF of the twin boundary and the decrease of the ESFE in grains which originally favors the twinning.Graphical abstractImage 1
       
  • Correlation between imposed deformation and transformation lattice strain
           on α variant selection in a metastable β-Ti alloy under isothermal
           compression
    • Abstract: Publication date: Available online 17 September 2018Source: Acta MaterialiaAuthor(s): Ke Hua, Yudong Zhang, Weimin Gan, Hongchao Kou, Jinshan Li, Claude Esling sMany phase transformations produce crystallographic variants. Under a mechanical constraint, certain variants could be selected. Although efforts have been made on resolving the selection rule, the transformation lattice deformation associated selection mechanism has not been well addressed. Thus in the present work, α variant selection of the β to α transformation in a metastable β-Ti alloy under compression was studied. Results show that the selection of the α variants is strongly affected by the imposed strain and the applied load with dependence on the local crystal perfection of the β grains. In the slightly deformed β grains, 2 Burger orientation relationship (BOR) variants forming ‘cross-shaped’ clusters and interrelated by a 90° rotation around the α axis are selected and form in large quantities (group I variants). Such variants consume the maximum deformation work by the applied load. This energy consumption is rooted from the maximum strain contribution of the selected variants to the macroscopic deformation and the maximum shear stress from the external load resolved on their {11¯2¯}ββ systems for transformation. In the heavily deformed β grains occupied by dislocation slip bands, several numbers (2 to 4) of BOR variants are selected but form in much less quantities (group II variants). The selection energy criterion is still obeyed by the group II variants but with restriction from the local deformation. The present work provides clear information on the interweaving of the imposed compression with the internal lattice deformation and its impact on the β to α transformation variant selection.Graphical abstractImage 1
       
  • Solute Interaction Effects on Grain Boundary Segregation in Ternary Alloys
    • Abstract: Publication date: Available online 17 September 2018Source: Acta MaterialiaAuthor(s): Wenting Xing, Arvind R. Kalidindi, Dor Amram, Christopher A. Schuh In polycrystalline materials, the segregation of elements to grain boundaries (GBs) can be harmful, e.g., embrittling the GB, or helpful, e.g., strengthening the GB or stabilizing nanocrystalline states, depending on the alloying elements present. While the GB segregation tendency in binary alloys can be reasonably estimated, the effect of additional elements on this equilibrium remains largely speculative, making it difficult to design the GB chemistry of ternary alloys. Using a lattice-model framework and Monte Carlo simulations, we develop a first-order theoretical framework with which the GB segregation behavior in ternary alloys can be estimated based on the thermodynamic properties of the constituent binary systems. We then experimentally study GB segregation in the Pt-Au-Pd system as a case study. In line with our predictions, this system exhibits induced co-segregation of Pd to GBs at 400 ºC, driven by the presence of Au.Graphical abstractImage 1
       
  • Nanocrystalline Ag-W alloys lose stability upon solute desegregation from
           grain boundaries
    • Abstract: Publication date: Available online 17 September 2018Source: Acta MaterialiaAuthor(s): Z.B. Jiao, C.A. Schuh Alloying has proven an enabling strategy to stabilize nanocrystalline materials against grain growth, especially in cases where the solute segregates to grain boundaries and lowers their energy. Among such materials reported to date, most all are stable up to some temperature at which second phases precipitate, depleting solute from the boundaries. Here in contrast we present a system that loses stability by thermal desegregation of solute back into solution in the grains. Specifically, we explore minor additions of W (0, 0.3, 1.3, and 1.9 at.%) on the grain structure, grain boundary segregation, and thermal stability of nanocrystalline Ag using transmission electron microscopy and atom probe tomography. W is shown to segregate at grain boundaries in electrodeposited nanocrystalline Ag, pushing the onset temperature for grain growth from ∼ 200 °C up to ∼300 ºC. Upon such heating we observe the dissolution of W off the grain boundaries and back into the FCC host lattice, at a temperature in line with thermodynamic expectations on the basis of the segregation isotherm.Graphical abstractImage 1
       
  • Amorphous Nickel nanophases inducing ferromagnetism in equiatomic Ni-Ti
           alloy
    • Abstract: Publication date: Available online 15 September 2018Source: Acta MaterialiaAuthor(s): M.R. Chellali, S.H. Nandam, S. Li, M.H. Fawey, E. Moreno-Pineda, L. Velasco, T. Boll, L. Pastewka, R. Kruk, P. Gumbsch, H. Hahn Ni50Ti50 nanometer-sized amorphous particles are prepared using inert-gas condensation followed by in situ compaction. Elemental segregation of Ni and Ti is observed in the consolidated nanostructured material. Amorphous, nearly pure Nickel (96%) nanophases form within the amorphous Ni50Ti50 alloy. Combining atom probe tomography and scanning transmission electron microscopy with computer modelling, we explore the formation process of such amorphous nanophase structure. It is shown that the Ni rich amorphous phase in the consolidated nanostructured material is responsible for the ferromagnetic behavior of the sample whereas the rapidly quenched amorphous and crystalline samples with the same chemical composition (Ni50Ti50) were found to be paramagnetic. Due to the high cooling rate obtained using the inert gas condensation technique, an exceptional control over the crystallization processes is possible, promoting the formation of various amorphous phases, which are not obtained by standard rapid quenching techniques. Our findings demonstrate the potential of amorphous metallic nanostructures as advanced technological materials, and useful magnetic compounds.Graphical abstractImage 1
       
  • Coercivity and its thermal stability of Nd-Fe-B hot-deformed magnets
           enhanced by the eutectic grain boundary diffusion process
    • Abstract: Publication date: Available online 15 September 2018Source: Acta MaterialiaAuthor(s): J. Li, Lihua Liu, H. Sepehri-Amin, Xin Tang, T. Ohkubo, N. Sakuma, T. Shoji, A. Kato, T. Schrefl, K. Hono Eutectic grain boundary diffusion process was applied to Nd-Fe-B hot-deformed magnet using Nd60Tb20Cu20 alloy, which resulted in a large coercivity enhancement from 0.87 T to 2.57 T with a relatively small decrease in remanent magnetization from 1.50 T to 1.38 T. Improved temperature coefficient of coercivity from -0.493 %°C-1 to -0.328 %/°C-1 led to a high coercivity of 1.47 T at 150°C. The partial formation of Tb-rich shell on the surface of platelet shaped Nd2Fe14B grains while maintaining their ultra-fine grain size is the reasons for the substantial enhancement of the coercivity. Micromagnetic simulations suggested that a higher coercivity can be obtained when Tb-rich shell covers the c-plane surface interface of the grains than that covering the side surface interfaces. Improvement of the thermal stability of coercivity was found to be due to the exchange decoupling of Nd2Fe14B grains and the formation of (Nd,Tb)2Fe14B shell. In the frame of Kronmüller equation and based on the micromagnetic simulations, the improvement of the thermal stability of coercivity is attributed to the decrease of Neff and increase of α induced by exchange decoupling of grains, as well as the additional decrease of Neff induced by the formation of high-Ha shell.Graphical abstractImage 1
       
  • Low-temperature intrinsic plasticity in silicon at small scales
    • Abstract: Publication date: Available online 15 September 2018Source: Acta MaterialiaAuthor(s): A. Merabet, M. Texier, C. Tromas, S. Brochard, L. Pizzagalli, L. Thilly, J. Rabier, A. Talneau, Y.-M. Le Vaillant, O. Thomas, J. Godet The mechanical properties of materials usually depend on the size of the considered object. Silicon, for instance, undergoes between the macroscopic and nanometer scales, a brittle-to-ductile transition at room temperature. Although essential for the constantly developing Si-based nanotechnologies, the origin of this remarkable behavior change remains undetermined for several years. The observation of the mechanisms responsible for plastic deformation in nano-objects is indeed highly challenging at the microscopic scale. One needs controlling the deformation while identifying induced individual plastic events at the smallest scale during the first stages of plasticity. This work describes nano-compression experiments on 100nm-diameter Si nanopillars followed by post-mortem analysis of the deformed specimens through SEM and atomic resolution TEM imaging. The observed plastic deformation disrupts the usual description of low-temperature undissociated-dislocations-mediated plasticity and is comforted by molecular dynamics calculations. These results shed a new light on the transition between ductile and brittle regimes in silicon by introducing the missing link between plasticity and fracture.Graphical abstractImage 1
       
  • Magnetization, magnetic anisotropy and magnetocaloric effect of the
           Tb0.2Gd0.8 single crystal in high magnetic fields up to 14 T in region of
           a phase transition
    • Abstract: Publication date: Available online 15 September 2018Source: Acta MaterialiaAuthor(s): S.A. Nikitin, T.I. Ivanova, A.I. Zvonov, Yu.S. Koshkid'ko, J. Ćwik, K. Rogacki The Tb0.2Gd0.8 single crystal magnetization, magnetic anisotropy and adiabatic temperature change ΔT dependencies on magnetic field and temperature have been studied in high magnetic fields (up to 14 T). In this work, we analyze experimental data within the framework of thermodynamic Landau theory for second order magnetic phase transitions to describe the relationship between the magnetization, adiabatic temperature change ΔT and magnetic field in the region of a phase transition. Moreover, it is proved that the Landau-Ginzburg equations are applicable in the case of high magnetic fields. Furthermore it is discovered that the adiabatic temperature change is proportional to the magnetic field with the relation ΔT ∼ (μ0H)2/5 in the region of high magnetic fields.Graphical abstractImage 1
       
  • Microstructure evolution and deformation mechanisms during high rate and
           cryogenic sliding of copper
    • Abstract: Publication date: Available online 15 September 2018Source: Acta MaterialiaAuthor(s): Xiang Chen, Reinhard Schneider, Peter Gumbsch, Christian Greiner Frictional sliding induces a distinct discontinuity in the microstructure between a subsurface layer and the underlying bulk material, which strongly influences the tribological performance. Here, the strain rate and temperature dependence of such tribologically induced microstructure evolution was systematically investigated during reciprocating sliding of copper. It was found that an increase in strain rate and a decrease in temperature each result in a transition in the dominating deformation mechanism from dislocation slip to twinning-mediated plasticity at the very beginning of sliding. A sequence of deformation mechanisms was revealed under high rate and/or cryogenic sliding (strain rate ∼ 104 s-1; liquid nitrogen temperature): First, nanoscale dislocation trace lines form beneath the surface during the first forward pass; Second, partial dislocation nucleation from the sliding surface accompanied by nano-twinning and abundant stacking faults in the backward pass; Third, formation of a nanocrystalline layer upon further sliding. Sliding induced surface roughening is found to assist partial dislocation nucleation from the surface during high rate and cryogenic sliding. Our results suggest that the sliding surface can act as an effective source of dislocations to initiate and accommodate associated plastic deformation, which may be explored to model the microstructure evolution during sliding.Graphical abstractImage 1
       
  • Anisotropic plastic deformation of single crystals of the MAX phase
           compound Ti3SiC2 investigated by micropillar compression
    • Abstract: Publication date: Available online 15 September 2018Source: Acta MaterialiaAuthor(s): Masaya Higashi, Shogo Momono, Kyosuke Kishida, Norihiko L. Okamoto, Haruyuki Inui The anisotropic deformation behavior of single crystals of Ti3SiC2 has been investigated at room temperature as a function of loading axis orientation and specimen size by micropillar compression tests. Basal slip is found to be only the operative slip system at room temperature. The a/3[12¯10] basal dislocation responsible for basal slip dissociates into two partial dislocations of the Shockley-type and glides between Ti1 and Si atomic layers, as confirmed both experimentally and theoretically. The CRSS value for basal slip increases with the decrease in the specimen size, following an inverse power-law relationship with a very large power-law exponent of about 1.0, which is almost the upper limit of those reported for FCC metals. When the loading axis is parallel to the basal plane, kink band formation occurs only after the occurrence of basal-plane delamination and the subsequent activation of a-dislocations gliding on the basal plane. Kink bands consists of a/3[12¯10] basal edge dislocations of the same sign, each of which had glided between Ti1 and Si atomic layers. These results indicates that the kink formation can simply be interpreted by the classical dislocation-based kink-formation model.Graphical abstractImage 1
       
  • Ductility Enhancement in Mg-0.2%Ce Alloys
    • Abstract: Publication date: Available online 15 September 2018Source: Acta MaterialiaAuthor(s): R.K. Sabat, A.P. Brahme, R.K. Mishra, K. Inal, S. Suwas Ductility of Mg alloys can be enhanced by alloying, controlling the grain size and randomizing the texture. In this study, Mg-0.2%Ce alloys were processed using rolling, multi-axial forging (MAF) and equal channel angular pressing (ECAP) to fabricate three different textured samples from the same alloy. The samples were further annealed to produce similar grain size without altering texture. Rolled sample had a strong basal {0001} texture, the MAF sample developed a weak {011¯2¯} [2¯31¯2] texture component in addition to the basal texture and the ECAP sample exhibited a strong non-basal {1¯21¯3} [2¯111¯] texture component. The tensile properties, texture evolution and relative slip/twin activities in the samples were investigated experimentally and numerically. The tensile yield strength, ultimate strength and uniform elongation of the rolled, MAF and ECAP samples were 110MPa, 250MPa, 17%; 60MPa, 200MPa, 30% and 55MPa, 250MPa and 40%, respectively. The non-basal texture components in ECAP of MAF and samples favored the formation of extension twins and pyr. slip during tensile loading. Full field crystal plasticity finite element modelling (CPFEM) using the initial texture of the materials as input provided insights into the activation of different deformation modes and observed differences in hardening mechanisms as well as strain localization and premature failure of the rolled samples. CPFEM analysis confirms that Ce addition reduces the relative values of the critical resolved shear stress (CRSS) for the slip and twinning systems which, in turn, allows for texture modification during material fabrication. These, combined with the ability to control grain size in Mg-Ce alloy with excess Ce, provide options for ductility enhancement in Mg alloys.Graphical abstractImage 1
       
  • Unveiling the Role of Super-Jogs and Dislocation Induced Atomic-Shuffling
           on Controlling Plasticity in Magnesium
    • Abstract: Publication date: Available online 15 September 2018Source: Acta MaterialiaAuthor(s): Kinshuk Srivastava, Satish I. Rao, Jaafar A. El-Awady Magnesium (Mg) alloys are promising metals for many lightweight structural applications. However, the fundamental deformation mechanism in Mg that lead to poor ductility, formability and anomalous thermal-hardening response remain elusive. Here, atomistic simulations were utilized to unveil the origins of these mechanisms. We show that pyramidal 〈c+a〉 screw dislocations glide in hexagonal-close packed crystals by a fundamentally new mechanism that involves atomic shuffling and kink pair formation on the trailing partial. Due to local fluctuations in the stresses in the dislocation core, stable super-jogs subsequently form due to inhomogeneities in the shuffling process along the dislocation length as well as due to kink pair collisions. The screw dislocation then moves while dragging these super-jogs, sometimes leaving debris behind (e.g. vacancies or interstitials) as well as long faulted loops on the basal plane that are aligned along the basal plane intersection with the pyramidal-II plane. These dragged out super-jogs dissociate athermally on the basal plane and exert a strong drag-effect on dislocation glide, resulting in dramatic work-hardening at small strains between 0-600K, which correlates with the experimentally observed low-ductility during c-axis compression of Mg. Furthermore, this new mechanism also accounts for the experimentally reported anomalous thermal-hardening as a result of the increase in the number of super-jogs per dislocation unit-length due to thermal activation.Graphical abstractImage 1
       
  • Quantifying and Connecting Atomic and Crystallographic Grain Boundary
           Structure using Local Environment Representation and Dimensionality
           Reduction Techniques
    • Abstract: Publication date: Available online 14 September 2018Source: Acta MaterialiaAuthor(s): Jonathan L. Priedeman, Conrad W. Rosenbrock, Oliver K. Johnson, Eric R. Homer In this work, the relationship between grain boundary crystallography and grain boundary atomic structure is examined, using [100] - symmetric tilt grain boundaries in nickel. The structural unit model is used as a benchmark to evaluate the atomic structure description capacities of an emerging structural descriptor, the local environment representation, which itself is a refinement of the Smooth Overlap of Atomic Positions (SOAP) descriptor. We show that the local environment representation encodes both the information of the structural unit model and additional information, such as distortion in the structural units and the arrangement of the structural units at the interface. The use of the local environment representation permits the use of a visualization tool known as SPRING to represent structural similarities between grain boundaries. With the SPRING representation, we produce objective evidence of a relationship between crystallography and atomic structure, at least for [100] - symmetric tilt grain boundaries.Graphical abstractImage 1
       
  • Atomistic underpinnings for growth direction and pattern formation of hcp
           magnesium alloy dendrite
    • Abstract: Publication date: Available online 11 September 2018Source: Acta MaterialiaAuthor(s): Jinglian Du, Ang Zhang, Zhipeng Guo, Manhong Yang, Mei Li, Feng Liu, Shoumei Xiong The three-dimensional (3D) growth pattern, preferred growth directions and the underlying growth mechanism of magnesium alloy dendrite are investigated via 3D experimental characterization and multiscale mathematical simulations. It is found that the formation of the dendritic microstructure is associated with the magnitude of surface energy anisotropy. The results based on synchrotron X-ray tomography and electroback scattered diffraction techniques show that typical 3D morphology of the α-Mg dendrite exhibits an 18-primary-branch pattern, with six along the basal direction, and the other twelve along the nonbasal direction. The underlying mechanism is investigated by performing relevant atomistic calculations at the ground state and the elevated temperatures in light of density functional theory (DFT) and quasi-harmonic approximation (QHA). The results indicate that the preferred growth direction for the α-Mg dendrite growth is rather than , and the anisotropic surface energy decreases as the temperature increases. Subsequent analysis further confirms that the preferred growth directions of the α-Mg dendrite at different temperatures correspond consistently to those orientations with higher surface energy anisotropy, i.e., the and . Accordingly, the 3D phase-field simulations are performed to investigate the growth behavior of the α-Mg dendrite, with the anisotropic strength determining via DFT-based calculations.Graphical abstractImage 1
       
  • Shearing of γ’ particles in Co-base and Co-Ni-base superalloys
    • Abstract: Publication date: Available online 11 September 2018Source: Acta MaterialiaAuthor(s): L. Feng, D. Lv, R.K. Rhein, J.G. Goiri, M.S. Titus, A. Van der Ven, T.M. Pollock, Y. Wang Shearing mechanisms of the primary strengthening phase in cobalt-base and cobalt-nickel-base superalloys, γ′ (L12), are investigated at the elementary defect level by using a combination of generalized-stacking-fault energy (GSF) calculations and phase field simulations. The GSF energy surfaces of the γ and γ′ phases, as determined from available experimental data and ab initio calculations, are used in the phase field simulations. Sophisticated deformation pathways leading to various planar defects including antiphase boundaries (APB), remnant superlattice intrinsic stacking faults (SISF), APB-SISF-APB ribbons and SISF islands are predicted as a function of alloy composition and particle shapes. The predicted stacking fault configurations for both alloys are consistent with recent transmission electron microscopy observations. Effects of dislocation line tension difference in γ and γ′ phases and planar defect energy variation due to segregation at the planar defects are discussed. The detailed dislocation core structures, effects of dislocation line tension differences on deformation mechanisms, and unique deformation mechanisms uncovered, which include stacking fault ribbon shearing, antiphase boundary shearing, and mixed modes, could be used to improve constitutive microstructure-property relationships in advanced crystal plasticity modeling and to assist in alloy design.Graphical abstractImage 1
       
  • Geometry and kinetics of glide of screw dislocations in tungsten between
           95K and 573K
    • Abstract: Publication date: Available online 11 September 2018Source: Acta MaterialiaAuthor(s): D. Caillard In situ straining experiments were carried out in pure tungsten in order to study the geometry and kinetics of glide of dislocations as a function of stress and temperature. For tensile axes different from , screw dislocations move by a combination of steady and jerky motion in planes which cannot be identified unambiguously. For a tensile axis close to , however, screw dislocations have a much jerkier motion with jumps over large distances, in {112} planes exactly but only in the twinning direction, and in {123} planes. This involves a strong violation of the Schmid law, in agreement with slip line observations reported in the 60’s by different authors. Jerky {112} slip is combined with a more classical steady motion in planes close to {110}, but the proportion of steady motion decreases rapidly to zero at decreasing temperature, which leads to a gradual transition between the two mechanisms. These results account for the specific mechanical properties measured in case of a straining axis. They also bring new elements to understand the discrepancy between theoretical and experimental stress values at low temperatures. Indeed, like in Fe, local stress values as a function of temperature and dislocation velocity do not obey to the classical rules of thermodynamics, in agreement with possible quantum effects.Graphical abstractImage 1
       
  • Multiscale characterization of irradiation behaviour of ion-irradiated
           SiC/SiC composites
    • Abstract: Publication date: Available online 11 September 2018Source: Acta MaterialiaAuthor(s): S. Agarwal, G. Duscher, Y. Zhao, M.L. Crespillo, Y. Katoh, W.J. Weber The irradiation tolerance of SiC/SiC composites was studied using 10 MeV Au ion irradiations at 350 ºC, for surface doses between 1-50 displacements per atom (dpa). Atomic force microscopy and optical profilometry revealed irradiation-induced axial and radial shrinkage of SiC-fibers. At 50 dpa, net fiber shrinkage reached 2.8±0.3 %. We conclude that the primary cause of SiC-fiber shrinkage in SiC/SiC composites is the irradiation-induced loss of pre-existing carbon packets, which had occupied 2-3% fiber volume in unirradiated state. A compelling evidence of the carbon packet loss was revealed using a combination of state-of-art conventional transmission electron microscopy (TEM), high resolution TEM, energy-filtered TEM and electron energy loss spectroscopy. The carbon packet volume fraction decreased with increasing dose, reaching near-complete loss after 50 dpa. Carbon packet loss was further confirmed using Raman spectroscopy where the carbon D and G peaks disappeared after irradiation. In contrast, irradiation-induced swelling of 1±0.5 % was observed in the matrix after 50 dpa. The study also shows that up to 50 dpa, the multilayer pyrolytic-carbon (PyC) interface in the composite is highly irradiation tolerant as it maintained its morphology, graphitic nature and showed no signs of amorphization. Additionally, Raman spectroscopy revealed a saturation of TEM invisible disorder at 1 dpa for both ultra-fine grains of the fiber and the larger SiC-matrix grains. However, TEM visible extended defect formation such as dislocation loops were only detected in the larger matrix grains, thereby revealing a potential role of grain size on defect accumulation in SiC.Graphical abstractImage 1
       
  • The effect of molybdenum on interphase precipitation and microstructures
           in microalloyed steels containing titanium and vanadium
    • Abstract: Publication date: Available online 10 September 2018Source: Acta MaterialiaAuthor(s): P. Gong, X.G. Liu, A. Rijkenberg, W.M. Rainforth Despite much research into steels strengthened through interphase precitation, there remains much that is not clear, such as the role of a range of elements, particularly Mo, in the interphase precipitation process. Four steels were manufactured with identical composition, but with variations in Ti, V, Mo and N content to investigate the effect of composition on interphase precipitation. Alloys were rapidly cooled from the single austenite phase field and isothermally transformed at 630°C and 650°C for 90min. The addition of Mo was found to significantly reduce the austenite to ferrite transformation kinetics, particularly for the V steel. Interphase precipitation was observed in all alloys at both transformation temperatures. For the Ti bearing steel, the two types of precipitate were observed throughout the sample, namely TiC (finer) and Ti2C (coarser), while for the V bearing steels, VC (finer) and V4C3 (coarser) were observed. Where Mo was present in the alloy, it was found dissolved in all carbide types. The (Ti,Mo)C and (V,Mo)C formed by classical planer interphase precipitation (PIP) while the (Ti,Mo)2C and (V,Mo)4C3, that had a much wider row spacing, formed through curved interphase precipitation (CIP). Each adopted one variant of the Baker-Nutting orientation relationship. The Ti-microalloyed steels exhibited the smallest precipitates of all the steels, which were approximately the same size irrespective of whether Mo was present in the alloy and irrespective of the transformation temperature. However, the addition of Mo to the V bearing steels resulted in a significant increase in precipitate volume fraction and a reduction in precipitate size. The mechanisms of interphase precipitation leading to the coincident production of two different precipitate types is considered and the role of Mo on the interphase precipitation process is discussed. The resultant effect on strength is considered.Graphical abstractImage 1
       
  • Enhanced high-temperature tensile property by gradient twin structure of
           duplex high-Nb-containing TiAl alloy
    • Abstract: Publication date: Available online 7 September 2018Source: Acta MaterialiaAuthor(s): Jie Ding, Minghe Zhang, Yongfeng Liang, Yang Ren, Chengli Dong, Junpin Lin A pre-deformation process was employed for a TiAl alloy via high-temperature torsion, in which the stability of the constituent phases was tailored, resulting in enhanced hardening capability and ductility via a gradient microstructure. A sample with a pre-torsion of 360° exhibited a yield strength of 475 MPa and an ultimate tensile strength of 592 MPa, with a tensile ductility of 47% at 850 °C. The tensile properties were significantly enhanced compared with the as-forged sample, which exhibited values of 395 MPa, 494 MPa, and 4.6%, respectively. The physical mechanisms for the significant enhancement of the mechanical property of the TiAl alloys were studied in-depth via of transmission electron microscopy, electron-backscattered diffraction, and high-energy X-ray diffraction techniques. The high strength is mainly attributed to the twin structure formed during torsion, while high fracture elongation correlates to the recrystallization of the γ phase at twin-twin sections and the load partitioning regulated by a hierarchical microstructure. When the tensile micro-strains along the loading direction and transverse direction in the γ phase of the pre-deformed TiAl alloy, a higher mechanical performance was obtained. Moreover, the fracture mode of the pre-torsional tensile sample is a combination of pores and cleavage facets which resulted from the microstructure after torsion. The gradient twin structure approach in this study provides a strategy for developing TiAl alloys with exceptionally high-temperature tensile property, and the results of the micromechanical behavior-microstructure-property relationship may improve the understanding of the plastic deformation of TiAl alloys.Graphical abstractImage 1
       
  • The Mechanisms of Ductile Rupture
    • Abstract: Publication date: Available online 6 September 2018Source: Acta MaterialiaAuthor(s): Philip J. Noell, Jay D. Carroll, Brad L. Boyce One of the most confounding controversies in the ductile fracture community is the large discrepancy between predicted and experimentally observed strain-to-failure values during shear-dominant loading. Currently proposed solutions focus on better accounting for how the deviatoric stress state influences void growth or on measuring strain at the microscale rather than the macroscale. While these approaches are useful, they do not address the root of the problem: the only rupture micromechanisms that are generally considered are void nucleation, growth, and coalescence (for tensile-dominated loading), and shear-localization and void coalescence (for shear-dominated loading). Current phenomenological models have thus focused on predicting the competition between these mechanisms based on the stress state and the strain-hardening capacity of the material. However, in the present study, we demonstrate that there are at least five other failure mechanisms. Because these have long been ignored, little is known about how all seven mechanisms interact with one another or the factors that control their competition between. These questions are addressed by characterizing the fracture process in three high-purity face-centered cubic (FCC) metals of medium-to-high stacking fault energy: copper, nickel, and aluminum. These data demonstrate that, for a given stress state and material, several mechanisms frequently work together in a sequential manner to cause fracture. The selection of a failure mechanism is significantly affected by the plasticity-induced microstructural evolution that occurs before tearing begins, which can create or eliminate sites for void nucleation. At the macroscale, failure mechanisms that do not involve cracking or pore growth were observed to facilitate subsequent void growth and coalescence processes. While the focus of this study is on damage accumulation in pure metals, these results are also applicable to understanding failure in engineering alloys.Graphical abstractImage 1
       
  • On the prediction of low-cost high entropy alloys using new thermodynamic
           multi-objective criteria
    • Abstract: Publication date: Available online 6 September 2018Source: Acta MaterialiaAuthor(s): A.E. Gheribi, A.D. Pelton, E. Bélisle, S. Le Digabel, J.P. Harvey In an attempt to identify new low-cost metallic materials with interesting thermo-physical properties from the Fe-Cr-Mn-Ni-V-Ti-Al- (Co,Mo) system, we present here an original tool for the design of first-generation High Entropy Alloys (HEAs). The composition of potential HEAs is calculated under a set of non-smooth and non-linear constraints and multi-objective functions linked to the single phase start temperature, the room-temperature driving force for phase assemblage evolution and the solidification range. These are all new thermodynamic criteria for the design of HEAs. This tool links a constrained Gibbs energy minimization algorithm that uses accurate thermodynamic databases to a optimization algorithm implemented for solving “blackbox” objective functions and constraints. As a result of this work, we have identified entire sets of new FCC and BCC first-generation HEAs potentially suitable for future industrial applications.
       
  • In situ STEM/SEM Study of the Coarsening of Nanoporous Gold
    • Abstract: Publication date: Available online 4 September 2018Source: Acta MaterialiaAuthor(s): A.A. El-Zoka, J.Y. Howe, R.C. Newman, D.D. Perovic Nanoporous gold (NPG), formed by the chemical or electrochemical dealloying of binary or ternary solid solutions, has strong prospects as a catalyst, sensor substrate, membrane, actuator, and other applications. In view of the wide range of thermochemical conditions expected, understanding the evolution of NPG during thermal coarsening is necessary to assess and improve the prospects for its functionality. The addition of Pt to NPG (NPG-Pt) was shown in recent years to have an effect of inhibtion of coarsening, both during dealloying and during oxidizing post-treatment. In this study, the direct observation of the coarsening of nanoligaments by simultaneous transmitted electron (TE) and secondary electron (SE) imaging reveals the complex nature of the coarsening process. As a result of adding Pt, alterations in the volume fraction of ligaments are shown to have a strong impact on the operating coarsening mechanism and kinetics in a low-pressure hydrogen environment. Pt was also shown to increase the temperature for the onset of thermal coarsening. The important roles of surface diffusion and the possibility of coalescence-controlled coarsening kinetics are discussed in detail. Reproducible observations of ligament collapse and void annihilation serve to expand the current understanding of thermal coarsening of NPG.Graphical abstractImage 1
       
  • Hashin-Shtrikman bounds with eigenfields in terms of texture coefficients
           for polycrystalline materials
    • Abstract: Publication date: Available online 3 September 2018Source: Acta MaterialiaAuthor(s): Mauricio Lobos Fernández, Thomas Böhlke The Hashin-Shtrikman bounds accounting for eigenfields are represented in terms of tensorial texture coefficients for arbitrarily anisotropic materials and arbitrarily textured polycrystals. This requires a short review of the Hashin-Shtrikman bounds with eigenfields, an investigation of the polarization field determined by the stationarity condition and, finally, the analysis of the resulting expressions of the Hashin-Shtrikman bound of the effective potential. The resulting expressions are given naturally in terms of symmetric second-order tensors and minor and major symmetric fourth-order tensors. These properties induce, based on the tensorial Fourier expansion of the crystallite orientation distribution function, a dependency of all Hashin-Shtrikman properties in terms of solely the second- and the fourth-order texture coefficients. This is a new result, which is not self-evident, since an alternative formulation of the polarization field would alter the implied algebraic properties of the Hashin-Shtrikman functional. The results obtained by the polarization field, determined through the stationarity condition of the Hashin-Shtrikman functional, are discussed and demonstrated with an example for linear thermoelasticity in which bounds for elastic and thermoelastic properties are illustrated.Graphical abstract(Left) Set of texture coefficients of second- and fourth-order for polycrystals of hexagonal materials with macroscopic hexagonal symmetry. The blue region depictes the region described by the Frobenius norm of the textures coefficients bounded by unity. The orange region describes the region of all possible texture coefficients of second- and fourth-order. The texture coefficients for the single crystal are marked by the blue point, while the red point marks the texture coefficients for a uniform distribution.(Right) Properties-closure for a linear thermoelastic polycrystal of a hexagonal material with macroscopic hexagonal symmetry. The bounds of Voigt, Reuss and Hashin-Shtrikman have been evaluated with the representations presented in this work using the set of all possible texture coefficients depicted in orange in the left graphic. The bounds for the effective stiffness component C_1111 and for the effective thermal expansion coefficient beta_11 have been investigated. The properties-closure for these properties based on the Voigt and Reuss region (first-order bounds) delivers the green region. The properties-closure based on the Hashin-Shtrikman bounds (second-order bounds) delivers the red region. A propertiesprofile with accepted tolerances is depicted by the black point. The Hashin-Shtrikman bounds help material scientists in order to check if aimed properties-profile are reachable or not by polycrystals of considered materials.Image 1
       
  • Modeling of eutectic growth kinetics with thermodynamic couplings
    • Abstract: Publication date: Available online 30 August 2018Source: Acta MaterialiaAuthor(s): O. Senninger, Ch-A. Gandin, G. Guillemot An original model of directional eutectic growth for lamellar eutectics is presented. This model is based on the analysis of thermodynamic equilibrium at the solidification front. Contrary to previous ones, this model does not use linearization hypothesis on the alloy phase diagram and takes into account variations of densities between phases. Moreover, curvature effects are considered on thermodynamic equilibrium at the solidification front. Its numerical implementation is made possible through a coupling with thermodynamic software based on the CALPHAD approach. This new numerical model is applied to the Al-Al2Cu eutectic structure and compared to the Jackson-Hunt theory. Large differences between results of the present model and this theory are observed at high growth rate. In particular, curvature effects on phases concentrations can no more be modeled using a linear approximation, as in the Gibbs-Thomson relation. A good agreement of the numerical model with reported experimental studies is observed for a large range of growth velocities.Graphical abstractImage 1
       
  • Cluster hardening in Al-3Mg triggered by small Cu additions
    • Abstract: Publication date: Available online 30 August 2018Source: Acta MaterialiaAuthor(s): S. Medrano, H. Zhao, F. De Geuser, B. Gault, L.T. Stephenson, A. Deschamps, D. Ponge, D. Raabe, C.W. Sinclair The aging response of two Al-3Mg alloys with Cu addition
       
  • In-situ Investigation of Hf6Ta2O17 Anisotropic Thermal Expansion and
           Topotactic, Peritectic Transformation
    • Abstract: Publication date: Available online 23 August 2018Source: Acta MaterialiaAuthor(s): Scott J. McCormack, Richard J. Weber, Waltraud M. Kriven The anisotropic coefficients of thermal and the peritectic transformation of orthorhombic-Hf6Ta2O17 to tetragonal-HfO2 plus liquid at 2250 ˚C have been studied by in-situ X-ray powder diffraction from room temperature to complete melting (∼2450 ˚C) in air, using a quadrupole lamp furnace (QLF) and a conical nozzle levitator (CNL) equipped with a CO2 laser. The topotactic, peritectic transformation has been fully described by extracting the orientation relationship, lattice variant deformation and a motif of cations that relates the two structures at the transformation temperature. The calculation of these two important parameters and identification of the motif is facilitated by a knowledge of the anisotropic coefficients of thermal expansion as a function of temperature. Symmetry decomposition has been performed to show that the orthorhombic-Hf6Ta2O17 and tetragonal-HfO2 structures are simply related by polyhedral rotations.Graphical abstractImage 1
       
  • Radiation-induced bcc-fcc phase transformation in a Fe-3%Ni alloy
    • Abstract: Publication date: Available online 22 August 2018Source: Acta MaterialiaAuthor(s): L.T. Belkacemi, E. Meslin, B. Décamps, B. Radiguet, J. Henry The issue of neutron irradiation embrittlement of Reactor Pressure Vessel steels must be considered for Nuclear Power Plant life extension. This phenomenon partly arises from the existing interactions between dislocations and nanometric clusters composed of Cu, P, Si, Mn and Ni. The latter alloying element, playing a key role in the evolution of solute enriched clusters under irradiation, is the focus of this publication. To assess the effect of Ni on microstructure evolution under irradiation, particle accelerator based experiments were conducted. An under-saturated Fe3at.%Ni alloy was irradiated with self-ions, at 673 K, up to ∼1.2 dpa. Then, the microstructural damage was characterized, at the atomic scale, by conventional Transmission Electron Microscopy, Scanning Transmission Electron Microscopy coupled to Energy Dispersive X-ray Spectroscopy and Electron Energy Loss Spectroscopy, while chemical features were investigated by Atom Probe Tomography. Informations obtained by combining these coupled techniques provide evidence for the formation of a FCC phase, containing 25 at.%Ni, which can be either the disordered γ phase or the ordered L12 type Fe3Ni phase. The metastable or stable state of this FCC phase is discussed in the light of what is known from the literature. It is the first time that this BCC-FCC phase transformation is observed in an under-saturated α-FeNi alloy and this likely occurred via a Radiation Induced Precipitation (RIP) mechanism. Ni atom segregation is observed on cavities, dislocation lines and dislocation loops. The latter constitute nuclei for precipitates, leading to the formation of an additional segregation site for Ni: the precipitate FCC - matrix BCC nearly coherent interface. Similar mechanisms are argued to be operating also in high Ni RPV steels under neutron irradiation.Graphical abstractImage 1
       
  • Electron radiation-induced material diffusion and nanocrystallization in
           nanostructured amorphous CoFeB thin film
    • Abstract: Publication date: Available online 18 June 2018Source: Acta MaterialiaAuthor(s): Binghai Liu, Taiebeh Tahmasebi, Kenny Ong, Hanwei Teo, Zhiqiang Mo, Jeffrey Lam, Pik Kee Tan, Yuzhe Zhao, Zhili Dong, Dimitri Houssameddine, Jacob Wang, Junming Xue, Zhihong Mai Transmission electron microscopy (TEM) is widely used for physical characterization of CoFeB -based magnetic tunneling junctions (MTJ) with its atomic-scale resolution. However, highly energetic electron radiation during TEM analysis may cause phase and microstructure modification of CoFeB and its associated MTJ layers. It is the intention of this work to address the issues of the electron-beam sensitivity of CoFeB material. With in-situ TEM, we investigated the electron beam radiation-induced material diffusion and the nanocrystallization behaviors in nanostructured amorphous CowFexByOz/Co60Fe20B20/SiO2 thin films. It was found that electron radiation with different electron dose led to massive diffusion of Co, Fe, B and O atoms across the whole thin film layers, which directly resulted in the modification of the phase and composition of the thin film layers, i.e. the oxidation of Co, Fe, B with O diffusion and the formation of pure Si phase from SiO2. Two stages of material diffusion were observed. While Stage-I material diffusion proceeded with a high diffusion speed, Stage-II had a relatively low diffusion rate accompanying with the nanocrystallization at the bottom of the CoFeB layer. A detailed kinetic study by in-situ TEM revealed the electron-beam radiation induced massive diffusion was a non-thermal process, and the underlying driving force arose from radiation-enhanced diffusion (RED) effects. Nanocrystallization during Stage-II electron-radiation experiment showed unique phase transformation phenomena, repeated nanocrystallization, amorphization, and nanocrystallization processes in the sequence before a stable grain growth could be achieved. A detailed TEM analysis revealed that RED-enhanced B diffusion was responsible for such unique repeated phase transformation processes. B diffusion and the associated structure distortion and the local short-range re-ordering may also account for the phase transformation from fcc-CoxFe23-xB6 to B-rich orthorhombic- CoxFe3-xB phase.
       
 
 
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