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Publisher: Elsevier   (Total: 3175 journals)

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Showing 1 - 200 of 3175 Journals sorted alphabetically
A Practical Logic of Cognitive Systems     Full-text available via subscription   (Followers: 9)
AASRI Procedia     Open Access   (Followers: 14)
Academic Pediatrics     Hybrid Journal   (Followers: 28, SJR: 1.402, h-index: 51)
Academic Radiology     Hybrid Journal   (Followers: 22, SJR: 1.008, h-index: 75)
Accident Analysis & Prevention     Partially Free   (Followers: 90, SJR: 1.109, h-index: 94)
Accounting Forum     Hybrid Journal   (Followers: 25, SJR: 0.612, h-index: 27)
Accounting, Organizations and Society     Hybrid Journal   (Followers: 33, SJR: 2.515, h-index: 90)
Achievements in the Life Sciences     Open Access   (Followers: 5)
Acta Anaesthesiologica Taiwanica     Open Access   (Followers: 6, SJR: 0.338, h-index: 19)
Acta Astronautica     Hybrid Journal   (Followers: 376, SJR: 0.726, h-index: 43)
Acta Automatica Sinica     Full-text available via subscription   (Followers: 2)
Acta Biomaterialia     Hybrid Journal   (Followers: 27, SJR: 2.02, h-index: 104)
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: 8, SJR: 0.172, h-index: 29)
Acta Haematologica Polonica     Free   (Followers: 1, SJR: 0.123, h-index: 8)
Acta Histochemica     Hybrid Journal   (Followers: 3, SJR: 0.604, h-index: 38)
Acta Materialia     Hybrid Journal   (Followers: 236, SJR: 3.683, h-index: 202)
Acta Mathematica Scientia     Full-text available via subscription   (Followers: 5, SJR: 0.615, h-index: 21)
Acta Mechanica Solida Sinica     Full-text available via subscription   (Followers: 9, SJR: 0.442, h-index: 21)
Acta Oecologica     Hybrid Journal   (Followers: 10, SJR: 0.915, h-index: 53)
Acta Otorrinolaringologica (English Edition)     Full-text available via subscription  
Acta Otorrinolaringológica Española     Full-text available via subscription   (Followers: 2, SJR: 0.311, h-index: 16)
Acta Pharmaceutica Sinica B     Open Access   (Followers: 1)
Acta Poética     Open Access   (Followers: 4)
Acta Psychologica     Hybrid Journal   (Followers: 25, SJR: 1.365, h-index: 73)
Acta Sociológica     Open Access  
Acta Tropica     Hybrid Journal   (Followers: 6, SJR: 1.059, h-index: 77)
Acta Urológica Portuguesa     Open Access  
Actas Dermo-Sifiliograficas     Full-text available via subscription   (Followers: 3)
Actas Dermo-Sifiliográficas (English Edition)     Full-text available via subscription   (Followers: 2)
Actas Urológicas Españolas     Full-text available via subscription   (Followers: 3, SJR: 0.383, h-index: 19)
Actas Urológicas Españolas (English Edition)     Full-text available via subscription   (Followers: 1)
Actualites Pharmaceutiques     Full-text available via subscription   (Followers: 6, SJR: 0.141, h-index: 3)
Actualites Pharmaceutiques Hospitalieres     Full-text available via subscription   (Followers: 3, SJR: 0.112, h-index: 2)
Acupuncture and Related Therapies     Hybrid Journal   (Followers: 6)
Acute Pain     Full-text available via subscription   (Followers: 14)
Ad Hoc Networks     Hybrid Journal   (Followers: 11, SJR: 0.967, h-index: 57)
Addictive Behaviors     Hybrid Journal   (Followers: 15, SJR: 1.514, h-index: 92)
Addictive Behaviors Reports     Open Access   (Followers: 7)
Additive Manufacturing     Hybrid Journal   (Followers: 9, SJR: 1.039, h-index: 5)
Additives for Polymers     Full-text available via subscription   (Followers: 22)
Advanced Cement Based Materials     Full-text available via subscription   (Followers: 3)
Advanced Drug Delivery Reviews     Hybrid Journal   (Followers: 132, SJR: 5.2, h-index: 222)
Advanced Engineering Informatics     Hybrid Journal   (Followers: 11, SJR: 1.265, h-index: 53)
Advanced Powder Technology     Hybrid Journal   (Followers: 16, SJR: 0.739, h-index: 33)
Advances in Accounting     Hybrid Journal   (Followers: 8, SJR: 0.299, h-index: 15)
Advances in Agronomy     Full-text available via subscription   (Followers: 12, SJR: 2.071, h-index: 82)
Advances in Anesthesia     Full-text available via subscription   (Followers: 27, SJR: 0.169, h-index: 4)
Advances in Antiviral Drug Design     Full-text available via subscription   (Followers: 2)
Advances in Applied Mathematics     Full-text available via subscription   (Followers: 10, SJR: 1.054, h-index: 35)
Advances in Applied Mechanics     Full-text available via subscription   (Followers: 10, SJR: 0.801, h-index: 26)
Advances in Applied Microbiology     Full-text available via subscription   (Followers: 22, SJR: 1.286, h-index: 49)
Advances In Atomic, Molecular, and Optical Physics     Full-text available via subscription   (Followers: 14, SJR: 3.31, h-index: 42)
Advances in Biological Regulation     Hybrid Journal   (Followers: 4, SJR: 2.277, h-index: 43)
Advances in Botanical Research     Full-text available via subscription   (Followers: 2, SJR: 0.619, h-index: 48)
Advances in Cancer Research     Full-text available via subscription   (Followers: 28, SJR: 2.215, h-index: 78)
Advances in Carbohydrate Chemistry and Biochemistry     Full-text available via subscription   (Followers: 7, SJR: 0.9, h-index: 30)
Advances in Catalysis     Full-text available via subscription   (Followers: 5, SJR: 2.139, h-index: 42)
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: 0.183, h-index: 23)
Advances in Child Development and Behavior     Full-text available via subscription   (Followers: 10, SJR: 0.665, h-index: 29)
Advances in Chronic Kidney Disease     Full-text available via subscription   (Followers: 10, SJR: 1.268, h-index: 45)
Advances in Clinical Chemistry     Full-text available via subscription   (Followers: 28, SJR: 0.938, h-index: 33)
Advances in Colloid and Interface Science     Full-text available via subscription   (Followers: 19, SJR: 2.314, h-index: 130)
Advances in Computers     Full-text available via subscription   (Followers: 14, SJR: 0.223, h-index: 22)
Advances in Dermatology     Full-text available via subscription   (Followers: 14)
Advances in Developmental Biology     Full-text available via subscription   (Followers: 10)
Advances in Digestive Medicine     Open Access   (Followers: 8)
Advances in DNA Sequence-Specific Agents     Full-text available via subscription   (Followers: 5)
Advances in Drug Research     Full-text available via subscription   (Followers: 21)
Advances in Ecological Research     Full-text available via subscription   (Followers: 42, SJR: 3.25, h-index: 43)
Advances in Engineering Software     Hybrid Journal   (Followers: 27, SJR: 0.486, h-index: 10)
Advances in Experimental Biology     Full-text available via subscription   (Followers: 6)
Advances in Experimental Social Psychology     Full-text available via subscription   (Followers: 42, SJR: 5.465, h-index: 64)
Advances in Exploration Geophysics     Full-text available via subscription   (Followers: 1)
Advances in Fluorine Science     Full-text available via subscription   (Followers: 7)
Advances in Food and Nutrition Research     Full-text available via subscription   (Followers: 53, SJR: 0.674, h-index: 38)
Advances in Fuel Cells     Full-text available via subscription   (Followers: 15)
Advances in Genetics     Full-text available via subscription   (Followers: 15, SJR: 2.558, h-index: 54)
Advances in Genome Biology     Full-text available via subscription   (Followers: 7)
Advances in Geophysics     Full-text available via subscription   (Followers: 6, SJR: 2.325, h-index: 20)
Advances in Heat Transfer     Full-text available via subscription   (Followers: 21, SJR: 0.906, h-index: 24)
Advances in Heterocyclic Chemistry     Full-text available via subscription   (Followers: 9, SJR: 0.497, h-index: 31)
Advances in Human Factors/Ergonomics     Full-text available via subscription   (Followers: 23)
Advances in Imaging and Electron Physics     Full-text available via subscription   (Followers: 2, SJR: 0.396, h-index: 27)
Advances in Immunology     Full-text available via subscription   (Followers: 36, SJR: 4.152, h-index: 85)
Advances in Inorganic Chemistry     Full-text available via subscription   (Followers: 8, SJR: 1.132, h-index: 42)
Advances in Insect Physiology     Full-text available via subscription   (Followers: 2, SJR: 1.274, h-index: 27)
Advances in Integrative Medicine     Hybrid Journal   (Followers: 6)
Advances in Intl. Accounting     Full-text available via subscription   (Followers: 3)
Advances in Life Course Research     Hybrid Journal   (Followers: 8, SJR: 0.764, h-index: 15)
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: 14, SJR: 1.645, h-index: 45)
Advances in Mathematics     Full-text available via subscription   (Followers: 10, SJR: 3.261, h-index: 65)
Advances in Medical Sciences     Hybrid Journal   (Followers: 6, SJR: 0.489, h-index: 25)
Advances in Medicinal Chemistry     Full-text available via subscription   (Followers: 5)
Advances in Microbial Physiology     Full-text available via subscription   (Followers: 4, SJR: 1.44, h-index: 51)
Advances in Molecular and Cell Biology     Full-text available via subscription   (Followers: 21)
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: 0.324, h-index: 8)
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: 15, SJR: 2.885, h-index: 45)
Advances in Parallel Computing     Full-text available via subscription   (Followers: 6, SJR: 0.148, h-index: 11)
Advances in Parasitology     Full-text available via subscription   (Followers: 5, SJR: 2.37, h-index: 73)
Advances in Pediatrics     Full-text available via subscription   (Followers: 24, SJR: 0.4, h-index: 28)
Advances in Pharmaceutical Sciences     Full-text available via subscription   (Followers: 10)
Advances in Pharmacology     Full-text available via subscription   (Followers: 15, SJR: 1.718, h-index: 58)
Advances in Physical Organic Chemistry     Full-text available via subscription   (Followers: 8, SJR: 0.384, h-index: 26)
Advances in Phytomedicine     Full-text available via subscription  
Advances in Planar Lipid Bilayers and Liposomes     Full-text available via subscription   (Followers: 3, SJR: 0.248, h-index: 11)
Advances in Plant Biochemistry and Molecular Biology     Full-text available via subscription   (Followers: 7)
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: 17)
Advances in Protein Chemistry and Structural Biology     Full-text available via subscription   (Followers: 18, SJR: 1.5, h-index: 62)
Advances in Psychology     Full-text available via subscription   (Followers: 59)
Advances in Quantum Chemistry     Full-text available via subscription   (Followers: 6, SJR: 0.478, h-index: 32)
Advances in Radiation Oncology     Open Access  
Advances in Small Animal Medicine and Surgery     Hybrid Journal   (Followers: 3, SJR: 0.1, h-index: 2)
Advances in Space Biology and Medicine     Full-text available via subscription   (Followers: 5)
Advances in Space Research     Full-text available via subscription   (Followers: 375, SJR: 0.606, h-index: 65)
Advances in Structural Biology     Full-text available via subscription   (Followers: 5)
Advances in Surgery     Full-text available via subscription   (Followers: 9, SJR: 0.823, h-index: 27)
Advances in the Study of Behavior     Full-text available via subscription   (Followers: 29, SJR: 1.321, h-index: 56)
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: 1.878, h-index: 68)
Advances in Water Resources     Hybrid Journal   (Followers: 46, SJR: 2.408, h-index: 94)
Aeolian Research     Hybrid Journal   (Followers: 6, SJR: 0.973, h-index: 22)
Aerospace Science and Technology     Hybrid Journal   (Followers: 333, SJR: 0.816, h-index: 49)
AEU - Intl. J. of Electronics and Communications     Hybrid Journal   (Followers: 8, SJR: 0.318, h-index: 36)
African J. of Emergency Medicine     Open Access   (Followers: 6, SJR: 0.344, h-index: 6)
Ageing Research Reviews     Hybrid Journal   (Followers: 9, SJR: 3.289, h-index: 78)
Aggression and Violent Behavior     Hybrid Journal   (Followers: 429, SJR: 1.385, h-index: 72)
Agri Gene     Hybrid Journal  
Agricultural and Forest Meteorology     Hybrid Journal   (Followers: 15, SJR: 2.18, h-index: 116)
Agricultural Systems     Hybrid Journal   (Followers: 31, SJR: 1.275, h-index: 74)
Agricultural Water Management     Hybrid Journal   (Followers: 43, SJR: 1.546, h-index: 79)
Agriculture and Agricultural Science Procedia     Open Access   (Followers: 1)
Agriculture and Natural Resources     Open Access   (Followers: 2)
Agriculture, Ecosystems & Environment     Hybrid Journal   (Followers: 56, SJR: 1.879, h-index: 120)
Ain Shams Engineering J.     Open Access   (Followers: 5, SJR: 0.434, h-index: 14)
Air Medical J.     Hybrid Journal   (Followers: 5, SJR: 0.234, h-index: 18)
AKCE Intl. J. of Graphs and Combinatorics     Open Access   (SJR: 0.285, h-index: 3)
Alcohol     Hybrid Journal   (Followers: 11, SJR: 0.922, h-index: 66)
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: 0.436, h-index: 12)
Alexandria J. of Medicine     Open Access   (Followers: 1)
Algal Research     Partially Free   (Followers: 9, SJR: 2.05, h-index: 20)
Alkaloids: Chemical and Biological Perspectives     Full-text available via subscription   (Followers: 2)
Allergologia et Immunopathologia     Full-text available via subscription   (Followers: 1, SJR: 0.46, h-index: 29)
Allergology Intl.     Open Access   (Followers: 5, SJR: 0.776, h-index: 35)
Alpha Omegan     Full-text available via subscription   (SJR: 0.121, h-index: 9)
ALTER - European J. of Disability Research / Revue Européenne de Recherche sur le Handicap     Full-text available via subscription   (Followers: 9, SJR: 0.158, h-index: 9)
Alzheimer's & Dementia     Hybrid Journal   (Followers: 48, SJR: 4.289, h-index: 64)
Alzheimer's & Dementia: Diagnosis, Assessment & Disease Monitoring     Open Access   (Followers: 4)
Alzheimer's & Dementia: Translational Research & Clinical Interventions     Open Access   (Followers: 4)
Ambulatory Pediatrics     Hybrid Journal   (Followers: 6)
American Heart J.     Hybrid Journal   (Followers: 50, SJR: 3.157, h-index: 153)
American J. of Cardiology     Hybrid Journal   (Followers: 50, SJR: 2.063, h-index: 186)
American J. of Emergency Medicine     Hybrid Journal   (Followers: 42, SJR: 0.574, h-index: 65)
American J. of Geriatric Pharmacotherapy     Full-text available via subscription   (Followers: 10, SJR: 1.091, h-index: 45)
American J. of Geriatric Psychiatry     Hybrid Journal   (Followers: 14, SJR: 1.653, h-index: 93)
American J. of Human Genetics     Hybrid Journal   (Followers: 31, SJR: 8.769, h-index: 256)
American J. of Infection Control     Hybrid Journal   (Followers: 26, SJR: 1.259, h-index: 81)
American J. of Kidney Diseases     Hybrid Journal   (Followers: 32, SJR: 2.313, h-index: 172)
American J. of Medicine     Hybrid Journal   (Followers: 42, SJR: 2.023, h-index: 189)
American J. of Medicine Supplements     Full-text available via subscription   (Followers: 3)
American J. of Obstetrics and Gynecology     Hybrid Journal   (Followers: 189, SJR: 2.255, h-index: 171)
American J. of Ophthalmology     Hybrid Journal   (Followers: 62, SJR: 2.803, h-index: 148)
American J. of Ophthalmology Case Reports     Open Access   (Followers: 6)
American J. of Orthodontics and Dentofacial Orthopedics     Full-text available via subscription   (Followers: 6, SJR: 1.249, h-index: 88)
American J. of Otolaryngology     Hybrid Journal   (Followers: 25, SJR: 0.59, h-index: 45)
American J. of Pathology     Hybrid Journal   (Followers: 27, SJR: 2.653, h-index: 228)
American J. of Preventive Medicine     Hybrid Journal   (Followers: 27, SJR: 2.764, h-index: 154)
American J. of Surgery     Hybrid Journal   (Followers: 37, SJR: 1.286, h-index: 125)
American J. of the Medical Sciences     Hybrid Journal   (Followers: 12, SJR: 0.653, h-index: 70)
Ampersand : An Intl. J. of General and Applied Linguistics     Open Access   (Followers: 6)
Anaerobe     Hybrid Journal   (Followers: 4, SJR: 1.066, h-index: 51)
Anaesthesia & Intensive Care Medicine     Full-text available via subscription   (Followers: 61, SJR: 0.124, h-index: 9)
Anaesthesia Critical Care & Pain Medicine     Full-text available via subscription   (Followers: 14)
Anales de Cirugia Vascular     Full-text available via subscription  
Anales de Pediatría     Full-text available via subscription   (Followers: 2, SJR: 0.209, h-index: 27)
Anales de Pediatría (English Edition)     Full-text available via subscription  
Anales de Pediatría Continuada     Full-text available via subscription   (SJR: 0.104, h-index: 3)
Analytic Methods in Accident Research     Hybrid Journal   (Followers: 4, SJR: 2.577, h-index: 7)
Analytica Chimica Acta     Hybrid Journal   (Followers: 39, SJR: 1.548, h-index: 152)
Analytical Biochemistry     Hybrid Journal   (Followers: 164, SJR: 0.725, h-index: 154)
Analytical Chemistry Research     Open Access   (Followers: 10, SJR: 0.18, h-index: 2)
Analytical Spectroscopy Library     Full-text available via subscription   (Followers: 11)
Anesthésie & Réanimation     Full-text available via subscription   (Followers: 1)
Anesthesiology Clinics     Full-text available via subscription   (Followers: 22, SJR: 0.421, h-index: 40)
Angiología     Full-text available via subscription   (SJR: 0.124, h-index: 9)
Angiologia e Cirurgia Vascular     Open Access   (Followers: 1)

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Journal Cover Acta Materialia
  [SJR: 3.683]   [H-I: 202]   [236 followers]  Follow
    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 1359-6454
   Published by Elsevier Homepage  [3175 journals]
  • The effects of temperature and strain rate in fcc and bcc metals during
           extreme deformation rates
    • Authors: Mohammadreza Yaghoobi; George Z. Voyiadjis
      Pages: 1 - 10
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): Mohammadreza Yaghoobi, George Z. Voyiadjis
      The present work investigates the effects of temperature and strain rate in fcc and bcc metallic samples subjected to the extreme strain rates, higher than 10 8 s − 1 , using large scale atomistic simulation. In addition to the stress-strain curve, the microstructural information of the samples is studied to capture the underlying mechanisms of temperature and rate effects. The results show that as the strain rate increases, the material strength increases in a way that the underlying mechanisms of strength depend on the material crystal structure. However, the results show that the material response dependency on temperature is much smaller than that of the strain rate in the region of extreme deformation rates. In other words, the change in temperature does not lead to a noticeable change in the material response. In the case of fcc metals subjected to the extreme strain rates, the results show that the work hardening mechanism, i.e., interaction of dislocation with one another, is the governing mechanism of material strength, and the contribution of thermal activation mechanism is negligible. In the case of bcc metals subjected during the extreme rate deformations, both work hardening and phonon drag mechanisms are important, while the former one is dominant. Again, the thermal activation mechanisms can be neglected in the regime of high strain rates. Finally, a multi-scale model is incorporated to capture the observed response of material in the regime of extreme strain rates.
      Graphical abstract image

      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.029
      Issue No: Vol. 151 (2018)
       
  • Inherent toughness and fracture mechanisms of refractory transition-metal
           nitrides via density-functional molecular dynamics
    • Authors: D.G. Sangiovanni
      Pages: 11 - 20
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): D.G. Sangiovanni
      Hard refractory transition-metal nitrides possess unique combinations of outstanding mechanical and physical properties, but are typically brittle. Recent experimental results demonstrated that single-crystal NaCl-structure (B1) V0.5Mo0.5N pseudobinary solid solutions are both hard (∼20 GPa) and ductile; that is, they exhibit toughness, which is unusual for ceramics. However, key atomic-scale mechanisms underlying this inherent toughness are unknown. Here, I carry out density-functional ab initio molecular dynamics (AIMD) simulations at room temperature to identify atomistic processes and associated changes in the electronic structure which control strength, plasticity, and fracture in V0.5Mo0.5N, as well as reference B1 TiN, subject to <001> and <110> tensile deformation. AIMD simulations reveal that V0.5Mo0.5N is considerably tougher than TiN owing to its ability to (i) isotropically redistribute mechanical stresses within the elastic regime, (ii) dissipate the accumulated strain energy by activating local structural transformations beyond the yield point. In direct contrast, TiN breaks in brittle manner when applied stresses reach its tensile strength. Charge transfer maps show that the adaptive mechanical response of V0.5Mo0.5N originates from highly populated d-d metallic-states, which allow for counterbalancing the destabilization induced via tensile deformation by enabling formation of new chemical bonds. The high ionic character and electron-localization in TiN precludes the possibility of modifying bonding geometries to accommodate the accumulated stresses, thus suddenly causing material's fracture for relatively low strain values.
      Graphical abstract image

      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.038
      Issue No: Vol. 151 (2018)
       
  • Synthesis and stabilization of a new phase regime in a Mo-Si-B based alloy
           by laser-based additive manufacturing
    • Authors: S.K. Makineni; A.R. Kini; E.A. Jägle; H. Springer; D. Raabe; B. Gault
      Pages: 31 - 40
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): S.K. Makineni, A.R. Kini, E.A. Jägle, H. Springer, D. Raabe, B. Gault
      Mo-Si-B alloys are potential creep resistant materials for accessing harsh loading scenarios beyond Ni-based superalloys due to their excellent mechanical performance at ultra-high temperatures (> 1200 °C). Here, we report on the fabrication through laser additive manufacturing of a Mo rich Mo-Si-B alloy with and without dispersion of oxide (La2O3) particles. The major phase in the solidified material is dendritic α-Mo. The inter-dendritic regions contain a mixture of the Mo5Si3 (T1) + Mo5SiB2 (T2) phases, and not the expected equilibrium Mo3Si + Mo5SiB2 (T2) phases. This combination of phases is shown to yield improved high temperature creep resistance but was only accessible through by addition of Nb, W or Ti that substitute Mo in the intermetallic phases. Whereas here it is attributed to the large undercooling in the small melt pool produced during laser processing. We show that this phase mixture, upon annealing, is stable at 1200 °C for 200 h. We also demonstrate successful dispersion of oxide particles mainly in the inter-dendritic regions leading to a high indentation fracture toughness of ∼18 MPa√m at room temperature. Toughening originates from crack trapping in the ductile α-Mo and the formation of micro-cracks and crack deflection in the vicinity of oxide particles.
      Graphical abstract image

      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.037
      Issue No: Vol. 151 (2018)
       
  • Simultaneously achieved large reversible elastocaloric and magnetocaloric
           effects and their coupling in a magnetic shape memory alloy
    • Authors: Y.H. Qu; D.Y. Cong; S.H. Li; W.Y. Gui; Z.H. Nie; M.H. Zhang; Y. Ren; Y.D. Wang
      Pages: 41 - 55
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): Y.H. Qu, D.Y. Cong, S.H. Li, W.Y. Gui, Z.H. Nie, M.H. Zhang, Y. Ren, Y.D. Wang
      Large reversible caloric effects covering a broad temperature region are essential for high-efficiency and environment-friendly solid-state caloric refrigeration that can potentially replace the traditional vapor-compression-based cooling technology. Here, we report the simultaneously achieved large reversible magnetocaloric and elastocaloric effects in a Ni43Co6Mn40Sn11 magnetic shape memory alloy. A reversible near-room-temperature magnetic entropy change ΔS m of as high as 19.3 J kg−1 K−1 under 5 T was experimentally obtained and the corresponding adiabatic temperature change ΔT ad was estimated to be 7.7 K. Meanwhile, a large reversible elastocaloric effect with a directly measured ΔT ad up to 7.1 K was attained. The elastocaloric effect exhibits high cyclic stability with no apparent degradation during 380 cycles of loading and unloading. Furthermore, we propose and demonstrate the utilization of the multicaloric approach under the coupled uniaxial stress and magnetic field to enlarge the refrigeration temperature region of reversible caloric effects. By combining the reversible magnetocaloric and elastocaloric effects and the reversible multicaloric effect under the coupling of uniaxial stress and magnetic field in the hysteresis region, large reversible caloric effects covering a broad temperature region from 257 K to 383 K can be obtained. This study may pave the way for designing advanced caloric materials with cyclically stable and reversible large caloric effects and wide refrigeration temperature region for solid-state refrigeration.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.031
      Issue No: Vol. 151 (2018)
       
  • Solute strengthening of basal slip in Mg alloys
    • Authors: A. Tehranchi; B. Yin; W.A. Curtin
      Pages: 56 - 66
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): A. Tehranchi, B. Yin, W.A. Curtin
      Strengthening of basal slip could enhance formability of magnesium by decreasing the ratio of pyramidal to basal critical resolved shear stress (P/B CRSS ratio). Here solute strengthening of basal slip is predicted for a wide range of solutes as a function of temperature and solute concentration at experimental strain rates using a recently-developed parameter-free model. The model is simplified by approximating the solute/dislocation interaction energy as the sum of a misfit strain contribution and a stacking fault interaction term, with the dislocation stress field estimated using a Peierls-Nabarro-type model of the basal edge dislocation. The approach is validated against DFT results for both Al and Zn solutes, and the predicted strengths agree well with experiments. The model is then applied to predict basal strengthening of many other solutes versus temperature, with key parameters tabulated for general future use. Comparisons to experimental data on 0.5 at% Dy and 1.0 at% Y versus temperature show moderate agreement, and the predicted effects of deviatoric misfit strains are shown to be small. An analytic formula is developed to predict strengthening as a function of solute misfit volume and stacking fault energies, enabling rapid assessment for many solutes and their combinations because. the model naturally extends to multiple solutes. Predictions are made for a range of existing ternary and higher alloys. Overall, the analysis and models here provide an accurate and easy formulation for estimating basal solute strengthening in dilute multicomponent Mg alloys and thus estimating the P/B CRSS ratio.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.02.056
      Issue No: Vol. 151 (2018)
       
  • ω phase acts as a switch between dislocation channeling and joint
           twinning- and transformation-induced plasticity in a metastable β
           titanium alloy
    • Authors: M.J. Lai; T. Li; D. Raabe
      Pages: 67 - 77
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): M.J. Lai, T. Li, D. Raabe
      We have investigated the twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) as well as the influence of ω phase on these two phenomena in a metastable β-type Ti–25Nb–0.7Ta–2Zr (at.%) alloy. We set off with two starting states: one is ω-free and the other one contains a high number density (3.20 ± 0.78 × 1024 m−3) of nanometer-sized (∼1.23 nm) ω particles. Deformation experiments demonstrate that the plastic deformation of the ω-free alloy is mediated by stress-induced β → α" martensitic transformation, {332} twinning and dislocation slip, where the former two induce joint TRIP and TWIP effects and the latter one carries the majority of the plastic strain. In the ω-enriched alloy, the ω particles fully suppress the TWIP and TRIP effects and promote localization of dislocation plasticity into specific ω-devoid channels. Atom probe tomography analysis reveals that the elemental partitioning between β and ω results in only subtle enrichment of solutes in the β matrix, which cannot sufficiently stabilize the matrix to prevent martensitic transformation and twinning. A new mechanism based on the shear modulus difference between β and ω is proposed to explain the suppression of TRIP and TWIP effects by ω particles.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.053
      Issue No: Vol. 151 (2018)
       
  • Interfacial segregation and fracture in Mg-based binary alloys:
           Experimental and first-principles perspective
    • Authors: T. Tsuru; H. Somekawa; D.C. Chrzan
      Pages: 78 - 86
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): T. Tsuru, H. Somekawa, D.C. Chrzan
      We investigated the effect of solute elements on interfacial segregation and fracture in Mg alloys by first-principles density functional theory calculations in conjunction with both experiment and interfacial fracture mechanics. Based on the assumption of brittle fracture in Mg alloys, the interfacial separation caused by segregated solutes in Mg can be efficiently described by the energy-based criterion of fracture, which is in good agreement with the fracture toughness obtained by experimental tests of Mg–M binary alloys. The electronic interaction—that is, the change in the electronic state between the interface and surface—mainly influences the ideal work of separation regardless of the type of interface. We found that IIIB (d 1) and IVB (d 2) solutes, such as Zr, show distinctive hybridization between the p band of Mg and the d band of the solute, which characterizes the strong fracture toughness of Zr-doped Mg alloys in both the calculations and experiments.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.061
      Issue No: Vol. 151 (2018)
       
  • Tuning the microstructure and mechanical properties of magnetron sputtered
           Cu-Cr thin films: The optimal Cr addition
    • Authors: X.G. Li; L.F. Cao; J.Y. Zhang; J. Li; J.T. Zhao; X.B. Feng; Y.Q. Wang; K. Wu; P. Zhang; G. Liu; J. Sun
      Pages: 87 - 99
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): X.G. Li, L.F. Cao, J.Y. Zhang, J. Li, J.T. Zhao, X.B. Feng, Y.Q. Wang, K. Wu, P. Zhang, G. Liu, J. Sun
      Grain boundary (GB) engineering via alloying opens a pathway to design the interfaces in alloys for tuning their mechanical properties, thus it is quite critical to determine the optimum addition of alloying element. Here, we prepared immiscible Cu-Cr alloyed thin films by non-equilibrium magnetron sputtering deposition to study Cr alloying effects on the microstructure and mechanical properties of Cu thin films. It is found that Cr doping can notably refine the grains and promote the formation of fully nanotwinned columnar grains at low Cr volume concentrations (≤∼3.7 at.%) associated with the average GB Cr concentrations ≤ ∼15 at.%, beyond which the formation of nanotwins is significantly suppressed. The role of Cr atoms/clusters played in the twinning process is rationalized in terms of dislocation rebound-promotion mechanism. Importantly, a maximum hardness is discovered at the optimum Cr addition of ∼2.0 at.%. The Cr concentration-dependent hardness of Cu-Cr alloyed films was quantified by a combined strengthening model. The achievement of the maximum hardness was related to the greatest GB solute segregation-induced strength contribution. It is further uncovered that the Cu-Cr system exhibits strain rate sensitivity (SRS, m) monotonically reduced with increasing the Cr concentration, ranging from a positive m of 0.0314 (for pure Cu) to a negative m of −0.0245, which is attributed to a competition between the dislocation-boundary interactions (increasing m) and the dislocation-solute atoms interactions (decreasing m). These findings provide valuable insights into tuning the composition of alloyed thin films to achieve optimized mechanical performance.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.044
      Issue No: Vol. 151 (2018)
       
  • Grain boundary complexions and the strength of nanocrystalline metals:
           Dislocation emission and propagation
    • Authors: Vladyslav Turlo; Timothy J. Rupert
      Pages: 100 - 111
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): Vladyslav Turlo, Timothy J. Rupert
      Grain boundary complexions have been observed to affect the mechanical behavior of nanocrystalline metals, improving both strength and ductility. While an explanation for the improved ductility exists, the observed effect on strength remains unexplained. In this work, we use atomistic simulations to explore the influence of ordered and disordered complexions on two deformation mechanisms which are essential for nanocrystalline plasticity, namely dislocation emission and propagation. Both ordered and disordered grain boundary complexions in Cu-Zr are characterized by excess free volume and promote dislocation emission by reducing the critical emission stress. Alternatively, these complexions are characterized by strong dislocation pinning regions that increase the flow stress required for dislocation propagation. Such pinning regions are caused by ledges and solute atoms at the grain-complexion interfaces and may be dependent on the complexion state as well as the atomic size mismatch between the matrix and solute elements. The trends observed in our simulations of dislocation propagation align with the available experimental data, suggesting that dislocation propagation is the rate-limiting mechanism behind plasticity in nanocrystalline Cu-Zr alloys.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.055
      Issue No: Vol. 151 (2018)
       
  • Configuration of dislocations in low-angle kink boundaries formed in a
           single crystalline long-period stacking ordered Mg-Zn-Y alloy
    • Authors: Tsubasa Matsumoto; Michiaki Yamasaki; Koji Hagihara; Yoshihito Kawamura
      Pages: 112 - 124
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): Tsubasa Matsumoto, Michiaki Yamasaki, Koji Hagihara, Yoshihito Kawamura
      The dislocation configuration around low-angle kink boundaries formed in bent single crystalline long-period stacking ordered (LPSO) Mg85Zn6Y9 alloys was investigated. Intragranular misorientation axis (IGMA) analysis through SEM/EBSD observation and g•b analysis through TEM observation were conducted. Various kink bands possessing different lattice rotation axes ranging from < 1 1 ¯ 00 > to < 1 2 ¯ 10 >, i.e. along < 1 1 ¯ 00 >, <uvt0>, and < 1 2 ¯ 10 > were revealed using IGMA analysis. TEM observation with g•b analysis unveiled that the < 1 1 ¯ 00 >-rotation kink boundary observed using IGMA analysis consisted of basal <a> edge dislocations with one Burgers vector, while the < 1 2 ¯ 10 >/<uvt0>-rotation kink boundaries were comprised of basal <a> edge dislocations with more than one Burgers vector. Low-angle kink boundaries were regarded as low-angle tilt boundaries consisting of perfect basal <a> dislocations, regardless of their rotation axes. The formation of various kink bands with <uvt0>-combination-rotation axes ranging from < 1 1 ¯ 00 > to < 1 2 ¯ 10 > suggests that the statistically stored basal <a> dislocations may turn into geometrically necessary basal <a> edge dislocations during the initial stage of low-angle kink boundary formation. When a kink boundary is considered as a disclination, IGMA analysis capable of determining the lattice rotation axis and the angle is effective in evaluating the Frank vector of the kink boundary.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.034
      Issue No: Vol. 151 (2018)
       
  • Synthesis, theoretical and experimental characterisation of thin film
           Cu2Sn1-xGexS3 ternary alloys (x = 0 to 1): Homogeneous intermixing of Sn
           and Ge
    • Authors: Erika V.C. Robert; René Gunder; Jessica de Wild; Conrad Spindler; Finn Babbe; Hossam Elanzeery; Brahime El Adib; Robert Treharne; Henrique P.C. Miranda; Ludger Wirtz; Susan Schorr; Phillip J. Dale
      Pages: 125 - 136
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): Erika V.C. Robert, René Gunder, Jessica de Wild, Conrad Spindler, Finn Babbe, Hossam Elanzeery, Brahime El Adib, Robert Treharne, Henrique P.C. Miranda, Ludger Wirtz, Susan Schorr, Phillip J. Dale
      Cu2Sn1-x Ge x S3 is a p-type semiconductor alloy currently investigated for use as an absorber layer in thin film solar cells. The aim of this study is to investigate the properties of this alloy in thin film form in order to establish relationships between group IV composition and structural, vibrational and opto-electronic properties. Seven single phase Cu2Sn1-x Ge x S3 films are prepared from x = 0 to 1, showing a uniform distribution of Ge and Sn laterally and in depth. The films all show a monoclinic crystal structure. The lattice parameters are extracted using Le Bail refinement and show a linear decrease with increasing Ge content. Using density-functional theory with hybrid functionals, we calculate the Raman active phonon frequencies of Cu2SnS3 and Cu2GeS3. For the alloyed compounds, we use a virtual atom approximation. The shift of the main Raman peak from x = 0 to x = 1 can be explained as being half due to the change in atomic masses and half being due to the different bond strength. The bandgaps of the alloys are extracted from photoluminescence measurements and increase linearly from about 0.90 to 1.56 eV with increasing Ge. The net acceptor density of all films is around 1018 cm−3. These analyses have established that the alloy forms a solid solution over the entire composition range meaning that intentional band gap grading should be possible for future absorber layers. The linear variation of the unit cell parameters and the band gap with group IV content allows composition determination by scattering or optical measurements. Further research is required to reduce the doping density by two orders of magnitude in order to improve the current collection within a solar cell device structure.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.043
      Issue No: Vol. 151 (2018)
       
  • γ+γ′ microstructures in the Co-Ta-V and Co-Nb-V ternary
           systems
    • Authors: Fernando L. Reyes Tirado; Jacques Perrin Toinin; David C. Dunand
      Pages: 137 - 148
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): Fernando L. Reyes Tirado, Jacques Perrin Toinin, David C. Dunand
      The Co-Ta-V and Co-Nb-V ternary systems are investigated in a search for L12-ordered γ′ precipitation, based on recent computational predictions of stable γ+γ′ microstructures. Four alloys with nominal (at. %) composition Co-6Ta-6V, Co-5.4Ta-6.6V-xNi (x = 0 and 10), and Co-6Nb-6V are arc-melted, homogenized at 1250 °C, and aged at 900 °C for 2, 16 and 64 h. Nanometric, cuboidal γ′ precipitates within a fcc-γ matrix are discovered in the Co-Ta-V system after aging for 2 h, and in the Co-Nb-V system after cooling from homogenization. The compositions of these two new γ′-phases, as measured via atom probe tomography, are Co3(Ta0.76V0.24) and Co3(Nb0.65V0.35), respectively. Upon aging at 900 °C, the γ′ precipitates coarsen, dissolve and transform to lamellar C36-Co3(Ta,V) and needle-shape D019-Co3(Nb,V), measured as Co3(Nb0.81V0.19) by APT, respectively. This shows that these ternary γ′ phases are metastable and points to the need for their stabilization via additional alloying elements.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.057
      Issue No: Vol. 151 (2018)
       
  • Deformation mechanisms of nil temperature ductile polycrystalline B2
           intermetallic compound YAg
    • Authors: Rolf Schaarschuch; Carl-Georg Oertel; Guanghui Cao; Jens Freudenberger; Heinz-Günther Brokmeier; Werner Skrotzki
      Pages: 149 - 158
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): Rolf Schaarschuch, Carl-Georg Oertel, Guanghui Cao, Jens Freudenberger, Heinz-Günther Brokmeier, Werner Skrotzki
      The most ductile rare earth intermetallic compound, YAg, was subjected to an thermal activation analysis at low temperatures down to 4 K. Evaluation of the activation parameters and their dependence on stress and temperature yields strong indication for forest dislocation cutting as the rate-controlling deformation mechanism, similar to face-centered cubic metals. Surprisingly, nil temperature ductility was observed. Together with results of a detailed TEM analysis of the active slip systems it is concluded that, despite of violating the von Mises criterion for the plastic deformation of polycrystalline materials, a low elastic anisotropy and/or low Peierls stress is responsible for the appreciable ductility at low temperatures. This finding may help to search for other ductile systems in the broad class of intermetallic compounds.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.064
      Issue No: Vol. 151 (2018)
       
  • Effect of alloying elements on defect evolution in Ni-20X binary alloys
    • Authors: Tai-ni Yang; Chenyang Lu; Gihan Velisa; Ke Jin; Pengyuan Xiu; Miguel L. Crespillo; Yanwen Zhang; Hongbin Bei; Lumin Wang
      Pages: 159 - 168
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): Tai-ni Yang, Chenyang Lu, Gihan Velisa, Ke Jin, Pengyuan Xiu, Miguel L. Crespillo, Yanwen Zhang, Hongbin Bei, Lumin Wang
      The effect of alloying elements on radiation-induced microstructural evolution in Ni and Ni-20X (X = Fe, Cr, Mn and Pd) binary alloys was investigated using ion irradiation and cross-sectional transmission electron microscopy. The three-dimensional migration mode is identified to be the dominating migration mechanism for interstitial clusters in these binary alloys, contrary to the one-dimensional mode that dominates in the well-studied dilute alloys. The results reveal that: (1) the average size of defect clusters decreases as the solute atomic volume size factor increases. Smaller void size in Ni-20Cr is attributed to faster vacancy mobility in the near surface region, and weaker vacancy binding energy beyond the irradiation peak than Ni-20Fe. The smaller voids observed in Ni-20Mn and Ni-20Pd beyond the damage peak are due to the stronger Mn/Pd-vacancy binding effect of largely oversized solute atoms. (2) Oversized solutes can act as strong trapping sites for interstitials. The larger the solute atomic volume factor, the stronger the trapping force. This leads to a more significantly sluggish interstitial migration and smaller dislocation loop size. The average dislocation loop size in Ni-20Fe was four times larger than Ni-20Pd (atomic volume factor being 10.6% and 41.3%) but an order of magnitude lower in density. The smaller dislocation loop size in Ni-20Cr is attributed to stronger interstitial binding of Cr-Ni. Overall, the alloying effect on defects is more significant in concentrated binary alloys than in dilute binary alloys, due to the concentration difference of alloying atoms and the interstitial dominant migration mechanisms in the main irradiated region.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.054
      Issue No: Vol. 151 (2018)
       
  • Transient dynamics of powder spattering in laser powder bed fusion
           additive manufacturing process revealed by in-situ high-speed high-energy
           x-ray imaging
    • Authors: Qilin Guo; Cang Zhao; Luis I. Escano; Zachary Young; Lianghua Xiong; Kamel Fezzaa; Wes Everhart; Ben Brown; Tao Sun; Lianyi Chen
      Pages: 169 - 180
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): Qilin Guo, Cang Zhao, Luis I. Escano, Zachary Young, Lianghua Xiong, Kamel Fezzaa, Wes Everhart, Ben Brown, Tao Sun, Lianyi Chen
      Powder spattering is a major cause of defect formation and quality uncertainty in the laser powder bed fusion (LPBF) additive manufacturing (AM) process. It is very difficult to investigate this with either conventional characterization tools or modeling and simulation. The detailed dynamics of powder spattering in the LPBF process is still not fully understood. Here, we report insights into the transient dynamics of powder spattering in the LPBF process that was observed with in-situ high-speed high-energy x-ray imaging. Powder motion dynamics, as a function of time, environment pressure, and location, is presented. The moving speed, acceleration, and driving force of powder motion that are induced by metal vapor jet/plume and argon gas flow are quantified. A schematic map showing the dynamics and mechanisms of powder motion during the LPBF process as functions of time and pressure is constructed. Potential ways to mitigate powder spattering during the LPBF process are discussed and proposed, based on the revealed powder motion dynamics and mechanisms.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.036
      Issue No: Vol. 151 (2018)
       
  • Grain boundary engineering to control the discontinuous precipitation in
           multicomponent U10Mo alloy
    • Authors: Arun Devaraj; Libor Kovarik; Elizabeth Kautz; Bruce Arey; Saumyadeep Jana; Curt Lavender; Vineet Joshi
      Pages: 181 - 190
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): Arun Devaraj, Libor Kovarik, Elizabeth Kautz, Bruce Arey, Saumyadeep Jana, Curt Lavender, Vineet Joshi
      We demonstrate here that locally stabilized structure and compositional segregation at grain boundaries in a complex multicomponent alloy can be modified using high temperature homogenization treatment to influence the kinetics of phase transformations initiating from grain boundaries during subsequent low temperature annealing. Using aberration-corrected scanning transmission electron microscopy and atom probe tomography of a model multicomponent metallic alloy —uranium-10 wt% molybdenum (U-10Mo) a nuclear fuel, that is highly relevant to worldwide nuclear non-proliferation efforts, we demonstrate the ability to change the structure and compositional segregation at grain boundary, which then controls the subsequent discontinuous precipitation kinetics during sub-eutectoid annealing. A change in grain boundary from one characterized by segregation of Mo and impurities at grain boundary to a phase boundary with a distinct U2MoSi2C wetting phase precipitates introducing Ni and Al rich interphase complexions caused a pronounced reduction in area fraction of subsequent discontinuous precipitation. The broader implication of this work is in highlighting the role of grain boundary structure and composition in metallic alloys on dictating the fate of grain boundary initiated phase transformations like discontinuous precipitation or cellular transformation. This work highlights a new pathway to tune the grain boundary structure and composition to tailor the final microstructure of multicomponent metallic alloys.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.039
      Issue No: Vol. 151 (2018)
       
  • GeV ion irradiation of NiFe and NiCo: Insights from MD simulations and
           experiments
    • Authors: Aleksi A. Leino; German D. Samolyuk; Ritesh Sachan; Fredric Granberg; William J. Weber; Hongbin Bei; Jie Liu; Pengfei Zhai; Yanwen Zhang
      Pages: 191 - 200
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): Aleksi A. Leino, German D. Samolyuk, Ritesh Sachan, Fredric Granberg, William J. Weber, Hongbin Bei, Jie Liu, Pengfei Zhai, Yanwen Zhang
      Concentrated solid solution alloys have attracted rapidly increasing attention due to their potential for designing materials with high tolerance to radiation damage. To tackle the effects of chemical complexity in defect dynamics and radiation response, we present a computational study on swift heavy ion induced effects in Ni and equiatomic Ni -based alloys (Ni50Fe50, Ni50Co50) using two-temperature molecular dynamics simulations (2T-MD). The electronic heat conductivity in the two-temperature equations is parameterized from the results of first principles electronic structure calculations. A bismuth ion (1.542 GeV) is selected and single impact simulations performed in each target. We study the heat flow in the electronic subsystem and show that alloying Ni with Co or Fe reduces the heat dissipation from the impact by the electronic subsystem. Simulation results suggest no melting or residual damage in pure Ni while a cylindrical region melts along the ion propagation path in the alloys. In Ni50Co50 the damage consists of a dislocation loop structure (d = 2 nm) and isolated point defects, while in Ni50Fe50, a defect cluster (d = 4 nm) along the ion path is, in addition, formed. The simulation results are supported by atomic-level structural and defect characterizations in bismuth-irradiated Ni and Ni50Fe50. The significance of the 2T-MD model is demonstrated by comparing the results to those obtained with an instantaneous energy deposition model without consideration of e-ph interactions in pure Ni and by showing that it leads to a different qualitative behavior.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.058
      Issue No: Vol. 151 (2018)
       
  • Compositional effect on microstructure and properties of NbTiZr-based
           complex concentrated alloys
    • Authors: O.N. Senkov; S. Rao; K.J. Chaput; C. Woodward
      Pages: 201 - 215
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): O.N. Senkov, S. Rao, K.J. Chaput, C. Woodward
      Phase composition, microstructure and mechanical properties of six complex concentrated alloys, NbTiZr, NbTiZrV, NbTiZrVMo, NbTiZrVTa, NbTiZrVCr, and NbTiZrVAl0.24, are reported. The alloy density was in the range from 6.34 g/cm3 to 8.43 g/cm3. After homogenization annealing at 1400 °C or 1200 °C, only the ternary NbTiZr alloy had a single-phase BCC crystal structure. NbTiZrV and NbTiZrVTa produced predominantly single-phase BCC structure with clusters of fine V-rich precipitates inside NbTiZrV grains and fine, Zr-rich grain boundary precipitates in NbTiZrVTa. The other three alloys contained BCC and Laves phases, and NbTiZrVMo also contained a second BCC phase. After compression deformation at 1000 °C, noticeable changes in the phase composition were found in NbTiZrV, NbTiZrVTa and NbTiZrVAl0.24, while the phase composition of other alloys retained unchanged. NbTiZr, NbTiZrV and NbTiZrVTa showed good ductility and noticeable work hardening rate at room temperature, while the ductility of other alloys rapidly decreased with increasing the amount of Laves phase. The RT yield stress varied from 975 MPa for NbTiZ to 1510 MPa for NbTiZrVMo. At 1000 °C, all the alloys showed excellent compression ductility; however only NbTiZrVCr and NbTiZrVMo were stronger than NbTiZr. The results indicate that increasing the composition complexity of refractory near-equiatomic alloys by increasing the number of the alloying elements does not necessary result in a simpler phase structure and/or better mechanical properties. The type of the alloying elements seems to be more important than their number. Additions of V, Al or Ta decrease while additions of Cr or Mo increase the 1000 °C strength of NbTiZr.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.065
      Issue No: Vol. 151 (2018)
       
  • Understanding the oxidation behavior of a ZrB2–MoSi2 composite at
           ultra-high temperatures
    • Authors: Laura Silvestroni; Kerstin Stricker; Diletta Sciti; Hans-Joachim Kleebe
      Pages: 216 - 228
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): Laura Silvestroni, Kerstin Stricker, Diletta Sciti, Hans-Joachim Kleebe
      This basic research investigates the microstructure evolution of a composite based on ZrB2-MoSi2 from the as-sintered features to the changes occurring upon oxidation at ultra-high temperatures, 1650 and 1800 °C, in a bottom-up loading furnace for 15 min. Scanning and transmission electron microscopy evidenced the formation of a matrix typified by ZrB2-cores surrounded by (Zr,Mo)B2-rims with dispersed MoSi2 particles and SiO2 glass trapped at the triple junctions. The oxidation at 1650 °C induced the migration of silica to the surface, which formed a continuous and protective scale. Below this scale, the matrix evolved into ZrO2 grains encasing MoB nano-inclusions, as a result of the oxygen and boron oxide partial pressures established in the subscales. Underneath, a MoSi2-depleted boride region, but substituted by SiO2 and MoB was found. The same phases were observed upon oxidation at 1800 °C, but a thicker and more turbulent oxidized layer formed as a consequence of the rapid evolution of MoO3, SiO and B2O3 gases from the scales beneath the outermost silica-layer. According to the observed phases and the calculated phase stability diagrams, the partial pressures gradient within the oxide layer were defined and the effect of Mo-doping in boride matrices on the oxidation behavior was compared to that of other transition metals to establish a criterion design for the realization of ceramics with improved oxidation resistance.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.042
      Issue No: Vol. 151 (2018)
       
  • Topological model of type II deformation twinning
    • Authors: R.C. Pond; J.P. Hirth
      Pages: 229 - 242
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): R.C. Pond, J.P. Hirth
      A model for the formation and growth of type II twins is described using the topological theory of interfacial defects and interface structures. A type I twin forms and grows to macroscopic dimensions by the generation and expansion of disconnection loops, provided these defects are sufficiently mobile. However, if their mobility is limited, they accumulate into a tilt wall, which, after accommodational relaxation, forms the type II conjugate twin. Thus, whether the type I or type II conjugate twin forms is the outcome of competitive mechanisms, depending primarily on disconnection mobility. The plausibility of this model is discussed with reference to experimental observations of twinning in α − U . Disconnection mobility is shown to be limited by atomic shuffling in the cases of " { 17 6 ¯ } " and " { 1 7 ¯ 2 } " type II twins, as compared with the higher mobility expected for the active disconnections in { 1 3 ¯ 0 } compound twins. In the topological model, the twinning shears are identical to those predicted by the classical model of deformation twinning. However, while type I twins are formed directly by shear due to the motion of disconnections on their glide planes, the mechanism of type II twinning is different, involving not only shear by disconnection motion but also accommodational relaxation. This understanding prompts a reassessment of the physical significance of the twinning elements of the classical geometrical approach.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.014
      Issue No: Vol. 151 (2018)
       
  • Thickness dependent response of domain wall motion in declamped {001}
           Pb(Zr0·3Ti0.7)O3 thin films
    • Authors: Lyndsey M. Denis; Giovanni Esteves; Julian Walker; Jacob L. Jones; Susan Trolier-McKinstry
      Pages: 243 - 252
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): Lyndsey M. Denis, Giovanni Esteves, Julian Walker, Jacob L. Jones, Susan Trolier-McKinstry
      Scaling effects were investigated in tetragonal {001} textured Pb(Zr0·3Ti0.7)O3 thin films doped with 2 mol% Nb over a thickness range of 0.27 μm–1.11 μm. Scaling effects refer to the size-induced degradation of properties at length scales exceeding those associated with the ferroelectric stability limit. The irreversible Rayleigh coefficient was found to be thickness-dependent, indicating suppression of the extrinsic contributions to the relative permittivity for all clamped films. Both defects in the seed layer and substrate clamping contributed to the observed thickness dependence. The influence of the seed layer on dielectric properties was accounted for using a capacitor in series model. After the films were partially declamped from the substrate, the irreversible contributions increased up to 23% in Nb-doped films and became more frequency dependent (by up to 29%). The suppressed frequency dependence in the clamped films was attributed to the pinning of irreversible domain walls active at lower frequencies. Both the seed layer and substrate clamping contributed to the pinning of irreversible domain walls.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.046
      Issue No: Vol. 151 (2018)
       
  • An ideal amplitude window against electric fatigue in BaTiO3-based
           lead-free piezoelectric materials
    • Authors: Zhongming Fan; Jurij Koruza; Jürgen Rödel; Xiaoli Tan
      Pages: 253 - 259
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): Zhongming Fan, Jurij Koruza, Jürgen Rödel, Xiaoli Tan
      Electric fatigue has been a vexing issue for Pb(Zr,Ti)O3 ceramics, the material-of-choice for piezoelectric technologies, where higher field amplitudes always lead to a more severe property degradation. Thus, piezoelectric devices must be driven under low electric fields to ensure performance reliability, which results in a low efficiency. In the past decade, the intensive worldwide research on lead-free compositions has identified a few ceramics with piezoelectric properties comparable to those of lead-containing ones. However, their resistance to electric fatigue has not been well studied. In this work, we report an abnormal amplitude dependence of electric fatigue in lead-free piezoelectrics: A BaTiO3-based ceramic suffers fatigue degradation when the field amplitude is low, but exhibits an amplitude window at higher fields with essentially no fatigue. Furthermore, electric-field in-situ transmission electron microscopy (TEM) experiments up to 105 cycles are conducted to clearly reveal that the degradation at low fields is due to the unique single-domain state. We, therefore, have identified an ideal amplitude window with performance at full potential and, at the same time, extremely high reliability for a lead-free piezoelectric ceramic that is promising to replace Pb(Zr,Ti)O3.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.067
      Issue No: Vol. 151 (2018)
       
  • Altered ageing behaviour of a nanostructured Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr
           alloy processed by high pressure torsion
    • Authors: W.T. Sun; X.G. Qiao; M.Y. Zheng; C. Xu; S. Kamado; X.J. Zhao; H.W. Chen; N. Gao; M.J. Starink
      Pages: 260 - 270
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): W.T. Sun, X.G. Qiao, M.Y. Zheng, C. Xu, S. Kamado, X.J. Zhao, H.W. Chen, N. Gao, M.J. Starink
      In this study, the ageing behaviour of a nanostructured Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr (wt.%) alloy produced by solution treatment followed by high pressure torsion (HPT) was systematically investigated using hardness testing, high resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), elemental mapping, X-ray diffraction (XRD) and XRD line broadening analysis. The HPT-deformed alloy exhibits an ageing response that produces a higher peak-aged hardness at lower temperature and shorter ageing time as compared to the same alloy aged after conventional thermomechanical processing. The HAADF-STEM and elemental mapping reveal extensive segregation of solute atoms along grain boundaries during ageing. A model is developed which shows that the main structures causing hardening for peak-aged samples are the grain boundaries and the segregation of solute atoms formed along grain boundaries. The metastable β′ phase precipitates, which form on ageing of conventionally processed Mg-Gd-Y-Zn-Zr alloy samples, do not form in the present aged samples, and instead equilibrium β-Mg5(RE,Zn) phase forms on over-ageing. This altered precipitation behaviour is attributed to the high defect density (e.g. grain boundaries, dislocations and vacancies) introduced by HPT, leading to enhanced diffusion of solutes. The present processing produces an alloy that has a hardness of ∼145 HV. A model of strengthening indicates that whilst grain boundary strengthening provides the largest contribution to strengthening, it is the additional solid solution hardening, cluster hardening, and dislocation hardening that provide the main factors that caused the hardness to surpass that of other bulk processed Mg alloys studied to date.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.04.003
      Issue No: Vol. 151 (2018)
       
  • A study of deformation and strain induced in bulk by the oxide layers
           formation on a Fe-Cr-Al alloy in high-temperature liquid Pb-Bi eutectic
    • Authors: M.P. Popovic; K. Chen; H. Shen; C.V. Stan; D.L. Olmsted; N. Tamura; M. Asta; M.D. Abad; P. Hosemann
      Pages: 301 - 309
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): M.P. Popovic, K. Chen, H. Shen, C.V. Stan, D.L. Olmsted, N. Tamura, M. Asta, M.D. Abad, P. Hosemann
      At elevated temperatures, heavy liquid metals and their alloys are known to create a highly corrosive environment that causes irreversible degradation of most iron-based materials. It has been found that an appropriate concentration of oxygen in the liquid alloy can significantly reduce this issue by creating a passivating oxide scale that controls diffusion, especially if Al is present in Fe-based materials (by Al-oxide formation). However, the increase of the temperature and of oxygen content in liquid phase leads to the increase of oxygen diffusion into bulk, and to promotion of the internal Al oxidation. This can cause a strain in bulk near the oxide layer, due either to mismatch between the thermal expansion coefficients of the oxides and bulk material, or to misfit of the crystal lattices (bulk vs. oxides). This work investigates the strain induced into proximal bulk of a Fe-Cr-Al alloy by oxide layers formation in liquid lead-bismuth eutectic utilizing synchrotron X-ray Laue microdiffraction. It is found that internal oxidation is the most likely cause for the strain in the metal rather than thermal expansion mismatch as a two-layer problem.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.041
      Issue No: Vol. 151 (2018)
       
  • Combined strengthening from nanotwins and nanoprecipitates in an
           iron-based superalloy
    • Authors: B.B. Zhang; F.K. Yan; M.J. Zhao; N.R. Tao; K. Lu
      Pages: 310 - 320
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): B.B. Zhang, F.K. Yan, M.J. Zhao, N.R. Tao, K. Lu
      A bulk nanostructured solutionized iron-based superalloy consisting of nanotwins and dislocation structures was prepared by means of dynamic plastic deformation at a high strain rate. The nanotwins constitute ∼51% in volume with the average twin/matrix lamella thickness of ∼20 nm. The sample exhibits a yield strength as high as 1130 MPa but with nearly zero uniform elongation. Subsequent peak-aging treatment at 700 °C for 1 h results in the precipitation of nano-sized gamma prime phases with a random distribution in the nanotwins and deformed matrix. The nanotwins still remain and keep the corresponding strengthening after aging. Those dispersed nanoprecipitates further raise the strength of the aged sample to 1420 MPa. Meanwhile, the uniform elongation increases to ∼3% due to the intrinsic plasticity of nanotwins with a reduced dislocation density after aging annealing. This kind of combined strengthening from both nanotwins and precipitates provides a new strategy to further raise the strength of nanotwinned materials or precipitation-strengthened materials.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.04.001
      Issue No: Vol. 151 (2018)
       
  • Compressive and fatigue behavior of functionally graded Ti-6Al-4V meshes
           fabricated by electron beam melting
    • Authors: S. Zhao; S.J. Li; S.G. Wang; W.T. Hou; Y. Li; L.C. Zhang; Y.L. Hao; R. Yang; R.D.K. Misra; L.E. Murr
      Pages: 1 - 15
      Abstract: Publication date: 15 May 2018
      Source:Acta Materialia, Volume 150
      Author(s): S. Zhao, S.J. Li, S.G. Wang, W.T. Hou, Y. Li, L.C. Zhang, Y.L. Hao, R. Yang, R.D.K. Misra, L.E. Murr
      In recent years, cellular metallic materials have attracted significant interest for biomedical applications. However, mutually opposing requirements of porous architecture and mechanical strength in conjunction with the high energy absorption capability have restricted their use. Here, we illustrate that electron beam melting can fabricate functionally graded Ti-6Al-4V alloy interconnected mesh structures with a combination of low density (0.5–2 g/cm3), high fatigue strength (∼70 MPa) and energy absorption (∼50 MJ/mg), which is superior to the ordinary uniform cellular structures. The underlying fundamental mechanisms governing the compressive and fatigue behavior of the graded cellular structures are elucidated via in situ tomography experiments and digital volume correlation analyses. It is underscored that during cyclic deformation, the stress can be continuously redistributed because of inhomogeneous mechanical properties and crack formation in constituent meshes, thereby resulting in variation of cyclic ratcheting rate for the graded cellular structures.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.02.060
      Issue No: Vol. 150 (2018)
       
  • Determination of the structure of α-β interfaces in metastable
           β-Ti alloys
    • Authors: Yufeng Zheng; Robert E.A. Williams; Gopal B. Viswanathan; William A.T. Clark; Hamish L. Fraser
      Pages: 25 - 39
      Abstract: Publication date: 15 May 2018
      Source:Acta Materialia, Volume 150
      Author(s): Yufeng Zheng, Robert E.A. Williams, Gopal B. Viswanathan, William A.T. Clark, Hamish L. Fraser
      The crystallography and structure of the interfaces formed between refined hexagonal close packed (hcp) α precipitates and the body centered cubic (bcc) β matrix in a solution-treated and quenched metastable β titanium alloy, Ti-5Al-5Mo-5V-3Cr (wt%, Ti-5553), were investigated after heating to 600 °C and subsequently aging for 0 and 30 min, respectively. The two phases adopted the classical Burgers orientation, and the observed interfacial structures were analyzed using a topological model, i.e. they contained disconnections, as well as β crystal lattice dislocations acting as a lattice invariant deformation (LID). The structure of the α/β interface was observed along two mutually perpendicular directions, in high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Viewed along the [ 0001 ] α : [ 110 ] β directions, an array of disconnections was observed to accommodate the misfit between the α and β crystals. While the spacing of these disconnections varied, the step height associated with them was always twice the { 112 } β interplanar spacing, and their Burgers vector b was [ − 0.019 0.019 0.034 ] β . When the interface was viewed along the [ 1 ¯ 1 ¯ 20 ] α : [ 1 ¯ 11 ] β direction, in the sample aged for 0 min at 600 °C the misfit between the two lattices along [ 0001 ] α : [ 110 ] β was so small that no misfit or lattice invariant dislocations were observed within the field of view, in contrast with the interface structure reported in other more conventional titanium alloys. After aging the sample for 30 min, however, the interfacial structure equilibrated, and crystal lattice dislocations were observed on the terrace planes, accommodating the LID, with b =  1 2 [ 1 ¯ 1 ¯ 1 ¯ ] β . Growth of the α precipitates is shown to occur principally by the emission of disconnections from the tip of the existing α precipitates into the β matrix, and its resulting conversion to α. These results, combined with analysis using the topological model, show that the nucleation and growth of the hcp α phase from the bcc β matrix in this alloy can be described in terms of the motion of disconnections, accompanied by the displacive diffusion necessary to accommodate the differences in composition between the α and β phases.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.003
      Issue No: Vol. 150 (2018)
       
  • Experimental evaluation of effective surface energy for cleavage
           microcrack propagation across grain boundary in steels
    • Authors: Itsuki Kawata; Hiroaki Nakai; Shuji Aihara
      Pages: 40 - 52
      Abstract: Publication date: 15 May 2018
      Source:Acta Materialia, Volume 150
      Author(s): Itsuki Kawata, Hiroaki Nakai, Shuji Aihara
      Fracture tests and finite element analysis were conducted to evaluate the temperature dependence of the effective surface energy γ mm corresponding to microcrack propagation across the grain boundary of steels. Steels containing coarse cementite particles were prepared to enable γ mm evaluation. The critical fracture stress of microcrack propagation across the grain boundary is considered larger than the critical fracture stress of microcrack propagation across the interface between a cementite particle and a ferrite grain in these steels because of the large cementite particles; therefore, local fracture stress estimated by finite element analysis and fracture initiation site obtained by fractography reflect the critical fracture stress of microcrack propagation across the grain boundary. Thus, the accuracy of γ mm calculated from local fracture stress is observed to be enhanced by employing the steels prepared for the present study. In addition, the effect of nickel on γ mm was studied using steels containing different concentrations of nickel. Values of γ mm were evaluated considering both the direction of a neighboring facet relative to the crack initiation facet and the mixed mode stress intensity factors. The authors proposed a new temperature dependence of γ mm . Additionally, the negligible effect of nickel addition on γ mm was observed. The results of the present study can be used effectively to develop a probabilistic cleavage fracture model based on microstructural information.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.02.057
      Issue No: Vol. 150 (2018)
       
  • Ab initio simulation of hydrogen-induced decohesion in
           cementite-containing microstructures
    • Authors: Eunan J. McEniry; Tilmann Hickel; Jörg Neugebauer
      Pages: 53 - 58
      Abstract: Publication date: 15 May 2018
      Source:Acta Materialia, Volume 150
      Author(s): Eunan J. McEniry, Tilmann Hickel, Jörg Neugebauer
      In high-strength carbon steels suitable for use in the automotive industry, hydrogen embrittlement (HE) is a potential barrier to the widespread application of these materials. The behaviour of hydrogen within the most prevalent carbide, namely cementite, has been investigated via ab initio simulation. In order to examine possible decohesion effects of hydrogen on carbon steels, the binding and diffusion of hydrogen at the interface between ferrite and cementite has been examined. In order to understand the effect of hydrogen on the mechanical properties of carbon steels, simulated ab initio tensile tests have been performed on the ferrite-cementite bicrystal. The results of the tensile tests can be combined with thermodynamic considerations in order to obtain the expected hydrogen concentrations at such ferrite-cementite phase boundaries. We find that the effect of hydrogen on the cohesion of the phase boundary may be significant, even when the bulk hydrogen concentration is low.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.005
      Issue No: Vol. 150 (2018)
       
  • The effect of deformation twins on the quasi-cleavage crack propagation in
           twinning-induced plasticity steels
    • Authors: Z.C. Luo; R.D. Liu; X. Wang; M.X. Huang
      Pages: 59 - 68
      Abstract: Publication date: 15 May 2018
      Source:Acta Materialia, Volume 150
      Author(s): Z.C. Luo, R.D. Liu, X. Wang, M.X. Huang
      The present work investigates the crack propagation behaviour in a twinning-induced plasticity (TWIP) steel. At room temperature, the dominant fracture mechanism of TWIP steel is quasi-cleavage, which is assisted by the deformation twins since such fracture mode is circumvented at 573 K as deformation twins are prohibited. The quasi-cleavage fracture mechanism is responsible for the high notch sensitivity and low essential work of fracture (EWF) of TWIP steel at room temperature. Furthermore, the crack propagation process of TWIP steel at 298 K has been characterized quantitatively by in-situ digital image correlation (DIC) technique and J resistance curve (J-R curve). The results show that the cracking process of TWIP steel at 298 K is stable, even though cleavage patches are observed on the fracture surface. A physical model based on the dislocation behaviour at the crack tip has been proposed to explain qualitatively the mechanism responsible for the stable cleavage cracking in TWIP steels.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.004
      Issue No: Vol. 150 (2018)
       
  • A quantitative connection between shear band mediated plasticity and
           fracture initiation toughness of metallic glasses
    • Authors: R.L. Narayan; Devaraj Raut; U. Ramamurty
      Pages: 69 - 77
      Abstract: Publication date: 15 May 2018
      Source:Acta Materialia, Volume 150
      Author(s): R.L. Narayan, Devaraj Raut, U. Ramamurty
      While it is well recognized, albeit qualitatively, that shear band mediated plasticity ahead of crack or notch tips is the raison d'être for the high fracture toughness of 'ductile' bulk metallic glasses (BMGs), quantitative connection between those two material properties is yet to be established. In an attempt to study this, we examine if mode I fracture initiation toughness, K Ic, of a number of BMGs can be related to the shear band number, N i, which is a discretized measure of plasticity in MGs, around spherical indentation impressions that are made to a fracture mechanism based predetermined indentation strain. Results show that K Ic scales with (N i)3/2. Then, the relation between the shear band density in the notch tip plastic zone, N n, and K Ic is examined, which shows that a power law: K Ic ∝ (N n)1/2, captures the data reported in literature for a number of BMGs. This result confirms that it is indeed the notch tip plasticity that determines K Ic of BMGs. The power law exponent of 0.5 is rationalized by recourse to elasto-plastic fracture mechanics. Possible connections between N i and N n, ways of enhancing the latter so as to increase K Ic, and the central role played by the relative density of MGs in determining both elastic, plastic, and fracture responses are discussed.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.011
      Issue No: Vol. 150 (2018)
       
  • Phase-field simulations of pearlitic divergence in Fe-C-Mn steels
    • Authors: L.T. Mushongera; P.G. Kubendran Amos; B. Nestler; K. Ankit
      Pages: 78 - 87
      Abstract: Publication date: 15 May 2018
      Source:Acta Materialia, Volume 150
      Author(s): L.T. Mushongera, P.G. Kubendran Amos, B. Nestler, K. Ankit
      Anomalous divergence of the pearlitic lamellae is typically observed during isothermal eutectoid transformation when the steel composition falls in the three-phase (austenite+ferrite+cementite) region. Here, the transformation progresses such that the composition of austenite changes continuously in the region ahead of the growing pearlite thus necessitating volume diffusion of Mn and resulting in a temporal increase in the interlamellar spacing as the growth kinetics becomes sluggish. In the present work, we develop a CALPHAD-informed multicomponent multiphase-field model to simulate the morphological evolution of pearlite in ternary Fe-2.46at.%C-3.50at.%Mn and Fe-2.51at.%C-5.40at.%Mn steels. Our phase-field simulations of lamellar growth capture the complex physics of multicomponent diffusion while provides in-depth insights into the mechanism of pearlitic divergence. Numerical investigations suggest that a temporal decrease in the driving force which is otherwise necessary to support manganese diffusion from the ferrite to cementite leads to lamellar divergence. Present investigations while showcasing the capabilities of our numerical approach enable the prediction of divergent pearlitic microstructure for a range of compositions and heat treatment cycles.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.02.059
      Issue No: Vol. 150 (2018)
       
  • Influence of deformation and annealing twinning on the microstructure and
           texture evolution of face-centered cubic high-entropy alloys
    • Authors: Christian Haase; Luis A. Barrales-Mora
      Pages: 88 - 103
      Abstract: Publication date: 15 May 2018
      Source:Acta Materialia, Volume 150
      Author(s): Christian Haase, Luis A. Barrales-Mora
      The influence of the physical mechanisms activated during deformation and annealing on the microstructure and texture evolution as well as on the mechanical properties in the equiatomic CoCrFeMnNi high-entropy alloy (HEA) were investigated. A combination of cold rolling and annealing was used to investigate the HEA in the deformed, recovered, partially recrystallized, and fully recrystallized states. Detailed microstructure and texture analysis was performed by electron backscatter diffraction, transmission electron microscopy, and X-ray diffraction. The mechanical properties were evaluated using uniaxial tensile testing. A specific focus of this investigation was put on studying the influence of deformation and annealing twinning on the material behavior. It was substantiated that during cold rolling deformation, twinning facilitates the transition from the Cu-type to the Brass-type texture, whereas annealing-twinning leads to a strong modification of the texture formed during recrystallization. The formation of specific twin orientations and the randomization of the recrystallization texture were proven by experiments as well as by cellular automaton simulations. During tension of the cold-rolled and annealed material high work-hardenability was observed. We attribute this behavior primarily to the dominance of planar dislocation slip and reduced tendency for dynamic recovery, since deformation twinning was observed to activate only in the non-recrystallized grains and thus, contributed minimally to the overall plasticity. The correlation between deformation/annealing twinning and the microstructure evolution, texture development, and mechanical properties was discussed in detail.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.02.048
      Issue No: Vol. 150 (2018)
       
  • Size dependent plasticity and damage response in multiphase body centered
           cubic high entropy alloys
    • Authors: Indranil Basu; Václav Ocelík; Jeff Th M. De Hosson
      Pages: 104 - 116
      Abstract: Publication date: 15 May 2018
      Source:Acta Materialia, Volume 150
      Author(s): Indranil Basu, Václav Ocelík, Jeff Th M. De Hosson
      The paper concentrates on the prediction of damage behavior in BCC HEAs and its subsequent dependence on intrinsic length scales. Size dependent plastic response is investigated using correlative nanomechanical testing, high resolution microscopy and phase analysis. Hot forged multiphase Al0.7CoCrFeNi HEA, comprising both face centered cubic (FCC) and body centered cubic (BCC) structured phases were used. Phase characterization inside the BCC grains indicated spinodal decomposition leading to a periodic arrangement of ordered B2 and disordered A2 phases. Room temperature nanoindentation response in the BCC grain revealed appearance of repeated strain bursts occurring in a random and stochastic manner. The observed plastic behavior was correlated to dislocation hardening mechanisms arising from a combination of the spinodally modulated microstructure and order hardening effects. Interestingly, in contrast to the commonly observed FCC to BCC transformations, certain indents revealed a structural transition from BCC to FCC. The stress induced microstructural evolution is discussed in terms of inherent compositional gradients and relevant defect-interphase interactions.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.03.015
      Issue No: Vol. 150 (2018)
       
  • Effect of multicomponent alloying with Ni, Mn and Mo on phase stability of
           bcc Fe-Cr alloys
    • Authors: A.V. Ponomareva; A.V. Ruban; B.O. Mukhamedov; I.A. Abrikosov
      Pages: 117 - 129
      Abstract: Publication date: 15 May 2018
      Source:Acta Materialia, Volume 150
      Author(s): A.V. Ponomareva, A.V. Ruban, B.O. Mukhamedov, I.A. Abrikosov
      Fe-Cr system attracts lot of attention in condensed matter physics due to its technological importance and extraordinary physics related to a non-trivial interplay between magnetic and chemical interactions. However, the effect of multicomponent alloying on the properties of Fe-Cr alloys are less studied. We have calculated the mixing enthalpy, magnetic moments, effective chemical, strain-induced and magnetic exchange interactions to investigate the alloying effect of Ni, Mn, Mo on the phase stability of the ferromagnetic bcc Fe−Cr system at zero K. We demonstrate that the alloying reduces the stability of Fe-Cr alloys and expands the region of spinodal decomposition. At the same time, the mixing enthalpy in ternary Fe100-≿-05Cr≿Ni05 alloys indicates a stability of solid solution phase up to 6 at. % Cr. In Fe100-≿-07Cr≿Ni05Mn01Mo01 alloys, we did not find any alloy composition that has negative enthalpy of formation. Analyzing magnetic and electronic properties of the alloys and investigating magnetic, chemical and strain-induced interactions in the studied systems, we provide physically transparent picture of the main factors leading to the destabilization of the Fe-Cr solid solutions by the multicomponent alloying with Ni, Mn, Mo.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.02.007
      Issue No: Vol. 150 (2018)
       
  • Fracture behavior of heterogeneous nanostructured 316L austenitic
           stainless steel with nanotwin bundles
    • Authors: L. Xiong; Z.S. You; S.D. Qu; L. Lu
      Pages: 130 - 138
      Abstract: Publication date: 15 May 2018
      Source:Acta Materialia, Volume 150
      Author(s): L. Xiong, Z.S. You, S.D. Qu, L. Lu
      The fracture behavior was investigated of a bulk nanostructured 316L austenitic stainless steel with embedded nanotwin bundles incorporated by dynamic plastic deformation. The nanotwin bundles were demonstrated to be critical to strengthen and toughen the as-deformed samples with mixed microstructures of nano-grains and nano-twins. With increment in strength, the fracture toughness decreases due to the generation of increasingly more nano-grains. Additional controlled thermal annealing that makes the nano-grains recover or recrystallize leads to reduced strengths but more remarkably improved fracture toughness. The enhanced strength−fracture toughness synergy can be attributed to the nanotwin bundles that constrict the damage development in the matrix of either nano-grains or recrystallized grains, and that resist crack propagation via acting as ductile crack bridging ligaments.
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      PubDate: 2018-04-15T21:54:33Z
      DOI: 10.1016/j.actamat.2018.02.065
      Issue No: Vol. 150 (2018)
       
  • The linear relationship between diffusivity and crystallization kinetics
           in a deeply supercooled liquid Ni50Ti50 alloy
    • Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): Simin An, Yang Li, Jiahao Li, Shuai Zhao, Baixin Liu, Pengfei Guan
      The NiTi amorphous alloys are heated to melting (just above T g) using pulsed laser irradiation to manufacture NiTi shape memory alloys (SMAs). It has been found that the crystallization of these deeply supercooled Ni50Ti50 liquids is unexpectedly fast. Despite the technological importance, the origin of this fast crystallization behavior, however, still remains elusive. In this work, we investigate the crystallization process of deeply supercooled Ni50Ti50 liquids (700–825 K) via molecular dynamics simulation. Through the extensive simulations, we reveal the ultrafast crystallization behavior and, more importantly, establish the strong non-Arrhenius temperature dependence of the kinetic coefficient u kin for the crystal growth. The linear relationship between u kin and the diffusion coefficient D obtained from this work provides compelling evidence that the fast crystallization behavior at large supercoolings is rooted in the atomic diffusivity rather than the viscosity, which is the consequence of the breakdown in the Stokes–Einstein relation.
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      PubDate: 2018-04-23T08:53:02Z
       
  • Formation of nanocrystalline tobermorite in calcium silicate binders with
           low C/S ratio
    • Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): P. Paradiso, R.L. Santos, R.B. Horta, J.N.C. Lopes, P.J. Ferreira, R. Colaço
      Pastes of calcium silicate binders with low CaO/SiO2 (C/S) ratio with an overall C/S molar ratio of 1.1 were produced by mixing these materials with water or with an aqueous solution of Na2SiO3. Both types of pastes (non-activated and Na2SiO3 activated) were analysed using X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM). The observations show that the hydration process of this novel type of cements results in the formation of tobermorite nanocrystals embedded in the amorphous calcium-silicate-hydrate (C-S-H) matrix, both in the case of water and of water + Na2SiO3 hydration. Selected area electron diffraction (SAED) patterns obtained in the HR-TEM observations enabled a full characterization of the tobermorite crystalline structure (type, unit cell, space group, lattice angles and parameters). Molecular dynamics (MD) simulations of the identified tobermorite structure were performed at various temperatures, showing its viability and thereby supporting the experimental results. To the best of the authors' knowledge, this report is the first to describe the spontaneous formation of crystalline tobermorite in the form of nanodomains dispersed in the C-S-H phase upon the hydration of calcium-silicate binders.
      Graphical abstract image

      PubDate: 2018-04-23T08:53:02Z
       
  • Fatigue deformation in a polycrystalline nickel base superalloy at
           intermediate and high temperature: Competing failure modes
    • Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): Jean Charles Stinville, Etienne Martin, Mallikarjun Karadge, Shak Ismonov, Monica Soare, Tim Hanlon, Sairam Sundaram, McLean P. Echlin, Patrick G. Callahan, William C. Lenthe, V.M. Miller, Jiashi Miao, Andrew E. Wessman, Rebecca Finlay, Adrian Loghin, Judson Marte, Tresa M. Pollock
      The microstructural configurations that favor early strain localization and fatigue crack initiation at intermediate and high temperature (400 °C–650 °C) have been investigated using novel experimental techniques, including high resolution digital image correlation and transmission scanning electron microscopy. Cyclic fatigue experiments in the high and low cycle fatigue regimes have been performed on a René 88DT polycrystalline nickel-base superalloy at temperatures up to 650 °C and compared to previous fatigue results obtained from tests in the very high cycle fatigue regime. Competing failure modes are observed along with an inversion in the temperature fatigue life dependence of fatigue strength from the low to high cycle fatigue regime. Oxidation-assisted processes are dominant at high applied stresses while cyclic plastic localization and accumulation govern fracture at low applied stresses. In addition, a second competing mode exists in the high and very high cycle fatigue regime from non-metallic inclusions as compared to internal intrinsic initiation sites. The grain-scale features that exhibit strain localization and crack initiation were investigated in detail. Transmission electron microscopy (TEM), transmission scanning electron microscopy (TSEM) and electron channeling contrast imaging have been conducted on samples removed from targeted regions with microstructural configurations that favor crack initiation to characterize the associated dislocation sub-structure and its evolution with temperature. Plasticity is observed to be less localized during cyclic loading at high temperature compared to room temperature. The microstructural features that drive initiation across the temperature range investigated are: twin-parent grains pairs that are at the upper end of the size distribution, are oriented for near maximum elastic modulus mismatch, and have high stresses along planes parallel to the twin boundaries.
      Graphical abstract image

      PubDate: 2018-04-23T08:53:02Z
       
  • Kinetics of Zr-Al intermetallic compound formation during ultra-high
           vacuum magnetron sputter-deposition of Zr/Al2O3(0001) thin films
    • Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): Koichi Tanaka, Joshua Fankhauser, Hicham Zaid, Angel Aleman, Masaki Sato, Dian Yu, Abbas Ebnonnasir, Chao Li, Makoto Kobashi, Mark S. Goorsky, Suneel Kodambaka
      Zr layers are grown on Al2O3(0001) substrates by ultra-high vacuum dc magnetron sputter-deposition in pure Ar discharges at temperatures T s between 873 K and 1173 K. We investigate the T s-dependent evolution of microstructure and composition across the Zr/Al2O3(0001) interfaces using X-ray diffraction, transmission electron microscopy, and X-ray energy dispersive spectroscopy. We obtain fully-dense, close-packed hexagonal structured Zr(0001) thin films with smooth surfaces at 873 K ≤ T s ≤ 973 K and increasingly porous, 0002-textured layers with highly corrugated surfaces at T s > 1023 K. At the Zr/Al2O3 interfaces, we observe thin crystalline films of Zr-Al intermetallic compounds, whose thicknesses increase from 10 ± 4 nm at T s = 873 K to 116 ± 4 nm at T s = 1173 K. The interfacial layers are composed of hexagonal Ni2In-structured Zr2Al phase at T s = 973 K and both orthorhombic-structured Zr2Al3 and hexagonal Mn5Al3-structured Zr5Al3 phases at T s > 973 K. The thicknesses of both the intermetallic phases increase with increasing T s, however, the Zr2Al3 layer grows faster than the Zr-rich phase at higher T s. We attribute the formation of Zr-rich Zr2Al, rather than the thermodynamically more stable Al-rich Zr2Al3 layers, to the presence of dissolved oxygen and epitaxial registry with Al2O3(0001). Our results indicate that reaction-diffusion kinetics rather than thermodynamics govern the interfacial layer composition and we suggest that the Zr/Al2O3(0001) interfacial stability is controlled by the supply of Al from the Al2O3(0001) substrate during sputter-deposition of Zr.
      Graphical abstract image

      PubDate: 2018-04-23T08:53:02Z
       
  • Efficient exploration of the High Entropy Alloy composition-phase space
    • Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): A. Abu-Odeh, E. Galvan, T. Kirk, H. Mao, Q. Chen, P. Mason, R. Malak, R. Arróyave
      High Entropy Alloys (HEAs), Multi-principal Component Alloys (MCA), or Compositionally Complex Alloys (CCAs) are alloys that contain multiple principal alloying elements. While many HEAs have been shown to have unique properties, their discovery has been largely done through costly and time-consuming trial-and-error approaches, with only an infinitesimally small fraction of the entire possible composition space having been explored. In this work, the exploration of the HEA composition space is framed as a Continuous Constraint Satisfaction Problem (CCSP) and solved using a novel Constraint Satisfaction Algorithm (CSA) for the rapid and robust exploration of alloy thermodynamic spaces. The algorithm is used to discover regions in the HEA Composition-Temperature space that satisfy desired phase constitution requirements. The algorithm is demonstrated against a new (TCHEA1) CALPHAD HEA thermodynamic database. The database is first validated by comparing phase stability predictions against experiments and then the CSA is deployed and tested against design tasks consisting of identifying not only single phase solid solution regions in ternary, quaternary and quinary composition spaces but also the identification of regions that are likely to yield precipitation-strengthened HEAs.

      PubDate: 2018-04-23T08:53:02Z
       
  • Secondary twin variant selection in four types of double twins in titanium
    • Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): Shun Xu, Mingyu Gong, Yanyao Jiang, Christophe Schuman, Jean-Sébastien Lecomte, Jian Wang
      We experimentally characterized four types of double twins in pure titanium and discussed the selection of secondary twin variants based on the apparent Schmid factor (a-SF), the displacement gradient accommodation (DGA), the modified DGA (m-DGA), and the nucleation via dislocation dissociation (NDD). The four types of double twins are classified into two families according to their zone axes: co-family double twins and non-family double twins. Co-family double twins share the zone axis <10 1 ¯ 0>. Non-family double twins have different zone axes with one along < 11 2 ¯ 0 > and the other along <10 1 ¯ 0>. Experimental observations reveal that the co-family double twins have one secondary twin variant prevailed over others and the non-family double twins have two secondary twin variants preferred than others. The a-SFs associated with secondary twins are always positive once the primary twin is activated, thus predicting no obvious preference for the selection of secondary twin variants. The m-DGA and NDD correctly predict the selection of secondary twin variants, implying that the preferred secondary twin variant would, to the greatest extent, relax plastic deformation associated with the primary twinning and the nucleation of the secondary twin variant is facilitated by the accumulation of available gliding dislocations at the primary twin boundary.
      Graphical abstract image

      PubDate: 2018-04-23T08:53:02Z
       
  • Cleavage cracking of ductile-metal substrates induced by brittle coating
           fracture
    • Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): Tao Guo, Yanmeng Chen, Ruohan Cao, Xiaolu Pang, Jianying He, Lijie Qiao
      Brittle coatings are often used to protect underlying ductile substrates from damage. Recent experimental observations show that the fracture of a brittle coating can cause the micro-cracking of the ductile metal substrates, threatening the safety and reliability of engineering structures. The cracking mode of the substrates is unclear and the corresponding mechanism remains poorly understood. In the present work, by performing room-temperature uniaxial tension experiments with a 10−4 s−1 straining rate, we have observed cleavage cracking in ductile-metal substrates (pure iron, AISI 1020 steel and brass) coated with WC-10%Co-4%Cr. Theoretical analysis reveals that the cleavage cracking is the result of brittle coating and its fracture, which synergically inhibit the local plastic deformation of the underlying substrate via two mechanisms. One is to restrain the dislocation nucleation and mobility near the interface of the substrate. The other is to bring a local high strain rate loading to the substrate, due to fast crack propagation in the brittle coating. The coupling of two effects leads to the nucleation of cleavage crack in the normally ductile substrates and causes significant loss in their ductility. This detrimental effect will be much more pronounced with the increase in the coating thickness. The findings shed new light on the failure mechanisms of brittle coating-metal substrate and provide guidelines in the material design of such systems.
      Graphical abstract image

      PubDate: 2018-04-23T08:53:02Z
       
  • Shapes and velocity relaxation of dislocation avalanches in Au and Nb
           microcrystals
    • Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): G. Sparks, R. Maaß
      We capture the spatiotemporal velocity dynamics of dislocation avalanches in face-centered cubic (FCC) gold and body-centered cubic (BCC) niobium crystals by compression testing of cylindrical microcrystals. In niobium, avalanche peak-velocities are over one order of magnitude lower, have distinctly rougher avalanche shapes, and relax more slowly to zero velocity than in gold. The avalanche dynamics (including averaged avalanche shapes) can be described reasonably well by mean-field predictions for avalanches near the depinning transition in the case of gold, but not for niobium. A detailed analysis shows consistent deviations (non-trivial exponents) from the predicted functional forms for both gold and niobium if the exponent that describes the velocity decay of the shape function is treated as a free parameter. While the avalanche size statistics and related scaling exponents are similar, these noticeable differences in the dislocation-avalanche dynamics demonstrate material-specific variations not predicted by “universal” behavior. The marked difference in avalanche shapes is discussed in terms of edge- and screw-dislocation mobility of an operating spiral arm source.
      Graphical abstract image

      PubDate: 2018-04-23T08:53:02Z
       
  • Revisiting building block ordering of long-period stacking ordered
           structures in Mg–Y–Al alloys
    • Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): H. Zhang, C.Q. Liu, Y.M. Zhu, H.W. Chen, L. Bourgeois, J.F. Nie
      Long-period stacking-ordered (LPSO) structures in Mg alloys have received considerable attention in the past decade due to their unique crystallographic features, but their precise structures are still not fully established. In this work, the 18R LPSO structure in two Mg–Y–Al alloys, in both cast and homogenized conditions, is systematically investigated using atomic-scale high-angle annular dark-field scanning transmission electron microscopy and density functional theory. Our observations reveal that there exist at least four types of building clusters—three of them being metastable and hitherto unreported—in the 18R structure in the as-cast condition. Each of the four types of building clusters comprises three Al–Y cubes that are connected by one Mg–Y cube. After annealing at 530–550 °C for 47–48 h, the three metastable building clusters transform to a stable configuration that is characterized by the Al6Y8 L12-type building cluster. These transformations are validated by first-principles calculations. Based on the experimental observations and computational results, the evolution of the building clusters in the 18R structure is elaborated.
      Graphical abstract image

      PubDate: 2018-04-23T08:53:02Z
       
  • Extremely hard amorphous-crystalline hybrid steel surface produced by
           deformation induced cementite amorphization
    • Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): Wei Guo, Yifei Meng, Xie Zhang, Vikram Bedekar, Hongbin Bei, Scott Hyde, Qianying Guo, Gregory B. Thompson, Rajiv Shivpuri, Jian-min Zuo, Jonathan D. Poplawsky
      Amorphous and nanograined (NG) steels are two categories of strong steels. However, over the past decade, their application has been hindered by their limited plasticity, the addition of expensive alloying elements, and processing challenges associated with producing bulk materials. Here, we report that the surface of a carburized Fe-Mn-Si martensitic steel with extremely low elemental alloying additions can be economically fabricated into an amorphous-nanocrystalline hybrid structure. Atom probe tomography and nanobeam diffraction of a hard turned steel surface together with molecular dynamics (MD) simulations reveal that the original cementite surface structure experiences a size-dependent amorphization and phase transformation during heavy plastic deformation. MD simulations further show that the martensite-cementite interface serves as a nucleation site for cementite amorphization, and that cementite can become disordered if further strained when the cementite particles are relatively small. These graded structures exhibit a surface hardness of ∼16.2 GPa, which exceeds the value of ∼8.8 GPa for the original nanocrystalline martensitic steel and most nanocrystalline steels reported before. This practical and cost-efficient approach for producing a hard surface with retained bulk ductility and toughness can provide expanded opportunities for producing an amorphous-crystalline hybrid structure in steels and other alloy systems.
      Graphical abstract image

      PubDate: 2018-04-23T08:53:02Z
       
  • 3D multi-layer grain structure simulation of powder bed fusion additive
           manufacturing
    • Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): Johannes A. Koepf, Martin R. Gotterbarm, Matthias Markl, Carolin Körner
      In powder bed fusion (PBF) additive manufacturing, powder layers are locally melted with a laser or an electron beam to build a component. Hatching strategies and beam parameters as beam power, scan velocity and line offset significantly affect the grain structure of the manufactured part. While experiments reveal the result of specific parameter combinations, the precise impact of distinct parameters on the resulting grain structure is widely unknown. This knowledge is necessary for a reliable prediction of the microstructure and consequently the mechanical properties of the manufactured part. We introduce the adaption of a three-dimensional model for the prediction of dendritic growth for use with PBF. The heat input is calculated using an analytical solution of the transient heat conduction equation. Massively parallel processing on a high-performance cluster computer allows the computation of the grain structure on the scale of small parts within reasonable times. The model is validated by accurately reproducing experimental grain structures of Inconel 718 test specimens manufactured by selective electron beam melting. The grain selection zone within the first layers as well as the subsequent microstructure in several millimeters build height is modeled in unprecedented level of detail. This model represents the cutting-edge of grain structure simulation in PBF and enables a reliable numerical prediction of appropriate beam parameters for arbitrary applications.
      Graphical abstract image

      PubDate: 2018-04-23T08:53:02Z
       
  • The influence of nanoparticles on dendritic grain growth in Mg alloys
    • Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): Enyu Guo, Sansan Shuai, Daniil Kazantsev, Shyamprasad Karagadde, A.B. Phillion, Tao Jing, Wenzhen Li, Peter D. Lee
      Melt processing offers a cost effective method for producing metal matrix nanocomposite (MMNC) components; however, the influence of nanoparticles on the evolving microstructure during solidification is still not well understood. In this study, the effect of SiC nanoparticles on α-Mg dendrite evolution in a Mg-25Zn-7Al (wt.%) alloy was investigated through 4D (three dimensions plus time) synchrotron tomographic quantification of solidification experiments conducted at different cooling rates with and without nanoparticles. Key features of the solidifying primary α-Mg dendritic grains were quantified, including grain morphology, size distribution, and dendrite tip velocity. To obtain the high-contrast tomography dataset necessary for structure quantification, a new image reconstruction and processing methodology was implemented. The results reveal that the addition of nanoparticles increases grain nucleation whilst restricting dendritic growth and altering the dendritic grain growth morphology. Using LGK model calculations, it is shown that these changes in solidification microstructure occur as a result of nanoparticle-induced restriction in Zn's effective diffusivity ahead of the dendrite tips, reducing tip velocity. The results both suggest the key phenomena required to be simulated when numerically modelling solidifying Mg-based MMNC and provide the data required to validate those models.
      Graphical abstract image

      PubDate: 2018-04-23T08:53:02Z
       
  • Effects of an oxygen potential gradient and water vapor on mass transfer
           in polycrystalline alumina at high temperatures
    • Authors: Tsuneaki Matsudaira; Satoshi Kitaoka Naoya Shibata Yuichi Ikuhara Miyuki Takeuchi
      Abstract: Publication date: 1 June 2018
      Source:Acta Materialia, Volume 151
      Author(s): Tsuneaki Matsudaira, Satoshi Kitaoka, Naoya Shibata, Yuichi Ikuhara, Miyuki Takeuchi, Takafumi Ogawa
      The mass transfer mechanisms in polycrystalline alumina wafers serving as models for anti-oxidation films were investigated by evaluating oxygen permeability at 1873 K using 18O2 as a tracer. The grain boundary (GB) diffusion coefficients for oxygen in the vicinity of the higher oxygen partial pressure (PO2(hi)) surface were directly determined from the secondary ion mass spectrometry line profile for each GB, obtained from cross-sections of the exposed wafer. An especially interesting result was that, under dry conditions, the oxygen GB diffusivity obtained with an applied oxygen potential gradient (dμO) was significantly lower than that associated with oxygen self-diffusion in the absence of a dμO. Moreover, the contribution of electronic conductivity near the PO2(hi) surface was increased upon applying a dμO. The addition of H2O to the PO2(hi) environment in conjunction with a dμO accelerated oxygen GB diffusion, while the presence of D2O retarded the diffusivity compared to that observed under dry conditions. However, in the absence of a dμO, the oxygen GB diffusivity was essentially equal to that under dry conditions. The dissociative adsorption reactions of H2O or D2O over the entire PO2(hi) surface appear to greatly affect the diffusivity of oxygen and aluminum on the surface and along the GBs in the vicinity of the surface.
      Graphical abstract image

      PubDate: 2018-04-15T21:54:33Z
       
 
 
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