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Publisher: Elsevier   (Total: 3042 journals)

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Showing 1 - 200 of 3042 Journals sorted alphabetically
AASRI Procedia     Open Access   (Followers: 15)
Academic Pediatrics     Hybrid Journal   (Followers: 19, SJR: 1.402, h-index: 51)
Academic Radiology     Hybrid Journal   (Followers: 16, SJR: 1.008, h-index: 75)
Accident Analysis & Prevention     Partially Free   (Followers: 81, SJR: 1.109, h-index: 94)
Accounting Forum     Hybrid Journal   (Followers: 23, SJR: 0.612, h-index: 27)
Accounting, Organizations and Society     Hybrid Journal   (Followers: 27, SJR: 2.515, h-index: 90)
Achievements in the Life Sciences     Open Access   (Followers: 4)
Acta Anaesthesiologica Taiwanica     Open Access   (Followers: 5, SJR: 0.338, h-index: 19)
Acta Astronautica     Hybrid Journal   (Followers: 325, SJR: 0.726, h-index: 43)
Acta Automatica Sinica     Full-text available via subscription   (Followers: 3)
Acta Biomaterialia     Hybrid Journal   (Followers: 25, SJR: 2.02, h-index: 104)
Acta Colombiana de Cuidado Intensivo     Full-text available via subscription  
Acta de Investigación Psicológica     Open Access   (Followers: 2)
Acta Ecologica Sinica     Open Access   (Followers: 8, SJR: 0.172, h-index: 29)
Acta Haematologica Polonica     Free   (SJR: 0.123, h-index: 8)
Acta Histochemica     Hybrid Journal   (Followers: 3, SJR: 0.604, h-index: 38)
Acta Materialia     Hybrid Journal   (Followers: 204, 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: 9, SJR: 0.915, h-index: 53)
Acta Otorrinolaringologica (English Edition)     Full-text available via subscription   (Followers: 1)
Acta Otorrinolaringológica Española     Full-text available via subscription   (Followers: 3, SJR: 0.311, h-index: 16)
Acta Pharmaceutica Sinica B     Open Access   (Followers: 2)
Acta Poética     Open Access   (Followers: 4)
Acta Psychologica     Hybrid Journal   (Followers: 22, SJR: 1.365, h-index: 73)
Acta Sociológica     Open Access  
Acta Tropica     Hybrid Journal   (Followers: 5, SJR: 1.059, h-index: 77)
Acta Urológica Portuguesa     Open Access  
Actas Dermo-Sifiliograficas     Full-text available via subscription   (Followers: 4)
Actas Dermo-Sifiliográficas (English Edition)     Full-text available via subscription   (Followers: 3)
Actas Urológicas Españolas     Full-text available via subscription   (Followers: 4, SJR: 0.383, h-index: 19)
Actas Urológicas Españolas (English Edition)     Full-text available via subscription   (Followers: 2)
Actualites Pharmaceutiques     Full-text available via subscription   (Followers: 5, SJR: 0.141, h-index: 3)
Actualites Pharmaceutiques Hospitalieres     Full-text available via subscription   (Followers: 4, SJR: 0.112, h-index: 2)
Acupuncture and Related Therapies     Hybrid Journal   (Followers: 3)
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: 5)
Additive Manufacturing     Hybrid Journal   (Followers: 7, SJR: 1.039, h-index: 5)
Additives for Polymers     Full-text available via subscription   (Followers: 20)
Advanced Drug Delivery Reviews     Hybrid Journal   (Followers: 123, 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: 9, SJR: 0.299, h-index: 15)
Advances in Agronomy     Full-text available via subscription   (Followers: 15, SJR: 2.071, h-index: 82)
Advances in Anesthesia     Full-text available via subscription   (Followers: 25, SJR: 0.169, h-index: 4)
Advances in Antiviral Drug Design     Full-text available via subscription   (Followers: 3)
Advances in Applied Mathematics     Full-text available via subscription   (Followers: 6, 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: 21, SJR: 1.286, h-index: 49)
Advances In Atomic, Molecular, and Optical Physics     Full-text available via subscription   (Followers: 16, 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: 3, SJR: 0.619, h-index: 48)
Advances in Cancer Research     Full-text available via subscription   (Followers: 25, SJR: 2.215, h-index: 78)
Advances in Carbohydrate Chemistry and Biochemistry     Full-text available via subscription   (Followers: 9, SJR: 0.9, h-index: 30)
Advances in Catalysis     Full-text available via subscription   (Followers: 5, SJR: 2.139, h-index: 42)
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: 24, 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: 8, 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: 18, SJR: 2.314, h-index: 130)
Advances in Computers     Full-text available via subscription   (Followers: 16, SJR: 0.223, h-index: 22)
Advances in Developmental Biology     Full-text available via subscription   (Followers: 11)
Advances in Digestive Medicine     Open Access   (Followers: 4)
Advances in DNA Sequence-Specific Agents     Full-text available via subscription   (Followers: 5)
Advances in Drug Research     Full-text available via subscription   (Followers: 22)
Advances in Ecological Research     Full-text available via subscription   (Followers: 39, SJR: 3.25, h-index: 43)
Advances in Engineering Software     Hybrid Journal   (Followers: 25, SJR: 0.486, h-index: 10)
Advances in Experimental Biology     Full-text available via subscription   (Followers: 7)
Advances in Experimental Social Psychology     Full-text available via subscription   (Followers: 40, SJR: 5.465, h-index: 64)
Advances in Exploration Geophysics     Full-text available via subscription   (Followers: 3)
Advances in Fluorine Science     Full-text available via subscription   (Followers: 8)
Advances in Food and Nutrition Research     Full-text available via subscription   (Followers: 45, SJR: 0.674, h-index: 38)
Advances in Fuel Cells     Full-text available via subscription   (Followers: 14)
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: 12)
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: 20, SJR: 0.906, h-index: 24)
Advances in Heterocyclic Chemistry     Full-text available via subscription   (Followers: 8, SJR: 0.497, h-index: 31)
Advances in Human Factors/Ergonomics     Full-text available via subscription   (Followers: 24)
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: 34, SJR: 4.152, h-index: 85)
Advances in Inorganic Chemistry     Full-text available via subscription   (Followers: 9, SJR: 1.132, h-index: 42)
Advances in Insect Physiology     Full-text available via subscription   (Followers: 3, SJR: 1.274, h-index: 27)
Advances in Integrative Medicine     Hybrid Journal   (Followers: 4)
Advances in Intl. Accounting     Full-text available via subscription   (Followers: 4)
Advances in Life Course Research     Hybrid Journal   (Followers: 8, SJR: 0.764, h-index: 15)
Advances in Lipobiology     Full-text available via subscription   (Followers: 2)
Advances in Magnetic and Optical Resonance     Full-text available via subscription   (Followers: 9)
Advances in Marine Biology     Full-text available via subscription   (Followers: 16, 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: 5, 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: 22)
Advances in Molecular and Cellular Endocrinology     Full-text available via subscription   (Followers: 10)
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: 4)
Advances in Oncobiology     Full-text available via subscription   (Followers: 3)
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: 7, SJR: 0.148, h-index: 11)
Advances in Parasitology     Full-text available via subscription   (Followers: 7, SJR: 2.37, h-index: 73)
Advances in Pediatrics     Full-text available via subscription   (Followers: 21, SJR: 0.4, h-index: 28)
Advances in Pharmaceutical Sciences     Full-text available via subscription   (Followers: 13)
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: 7, 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: 8)
Advances in Plant Pathology     Full-text available via subscription   (Followers: 5)
Advances in Porous Media     Full-text available via subscription   (Followers: 4)
Advances in Protein Chemistry     Full-text available via subscription   (Followers: 18)
Advances in Protein Chemistry and Structural Biology     Full-text available via subscription   (Followers: 19, SJR: 1.5, h-index: 62)
Advances in Psychology     Full-text available via subscription   (Followers: 58)
Advances in Quantum Chemistry     Full-text available via subscription   (Followers: 5, SJR: 0.478, h-index: 32)
Advances in Radiation Oncology     Open Access  
Advances in Small Animal Medicine and Surgery     Hybrid Journal   (Followers: 2, SJR: 0.1, h-index: 2)
Advances in Space Research     Full-text available via subscription   (Followers: 338, SJR: 0.606, h-index: 65)
Advances in Structural Biology     Full-text available via subscription   (Followers: 8)
Advances in Surgery     Full-text available via subscription   (Followers: 6, 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: 15)
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: 43, SJR: 2.408, h-index: 94)
Aeolian Research     Hybrid Journal   (Followers: 5, SJR: 0.973, h-index: 22)
Aerospace Science and Technology     Hybrid Journal   (Followers: 307, 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: 5, SJR: 0.344, h-index: 6)
Ageing Research Reviews     Hybrid Journal   (Followers: 7, SJR: 3.289, h-index: 78)
Aggression and Violent Behavior     Hybrid Journal   (Followers: 422, 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: 30, SJR: 1.275, h-index: 74)
Agricultural Water Management     Hybrid Journal   (Followers: 38, SJR: 1.546, h-index: 79)
Agriculture and Agricultural Science Procedia     Open Access  
Agriculture and Natural Resources     Open Access   (Followers: 1)
Agriculture, Ecosystems & Environment     Hybrid Journal   (Followers: 50, 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: 10, SJR: 0.922, h-index: 66)
Alcoholism and Drug Addiction     Open Access   (Followers: 6)
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  
Algal Research     Partially Free   (Followers: 8, SJR: 2.05, h-index: 20)
Alkaloids: Chemical and Biological Perspectives     Full-text available via subscription   (Followers: 3)
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)
ALTER - European J. of Disability Research / Revue Européenne de Recherche sur le Handicap     Full-text available via subscription   (Followers: 6, SJR: 0.158, h-index: 9)
Alzheimer's & Dementia     Hybrid Journal   (Followers: 46, SJR: 4.289, h-index: 64)
Alzheimer's & Dementia: Diagnosis, Assessment & Disease Monitoring     Open Access   (Followers: 5)
Alzheimer's & Dementia: Translational Research & Clinical Interventions     Open Access   (Followers: 3)
American Heart J.     Hybrid Journal   (Followers: 47, SJR: 3.157, h-index: 153)
American J. of Cardiology     Hybrid Journal   (Followers: 44, SJR: 2.063, h-index: 186)
American J. of Emergency Medicine     Hybrid Journal   (Followers: 34, SJR: 0.574, h-index: 65)
American J. of Geriatric Pharmacotherapy     Full-text available via subscription   (Followers: 6, SJR: 1.091, h-index: 45)
American J. of Geriatric Psychiatry     Hybrid Journal   (Followers: 15, SJR: 1.653, h-index: 93)
American J. of Human Genetics     Hybrid Journal   (Followers: 30, SJR: 8.769, h-index: 256)
American J. of Infection Control     Hybrid Journal   (Followers: 24, 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: 44, 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: 179, SJR: 2.255, h-index: 171)
American J. of Ophthalmology     Hybrid Journal   (Followers: 54, SJR: 2.803, h-index: 148)
American J. of Ophthalmology Case Reports     Open Access   (Followers: 2)
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: 23, SJR: 0.59, h-index: 45)
American J. of Pathology     Hybrid Journal   (Followers: 23, SJR: 2.653, h-index: 228)
American J. of Preventive Medicine     Hybrid Journal   (Followers: 21, SJR: 2.764, h-index: 154)
American J. of Surgery     Hybrid Journal   (Followers: 33, 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: 5)
Anaerobe     Hybrid Journal   (Followers: 4, SJR: 1.066, h-index: 51)
Anaesthesia & Intensive Care Medicine     Full-text available via subscription   (Followers: 53, SJR: 0.124, h-index: 9)
Anaesthesia Critical Care & Pain Medicine     Full-text available via subscription   (Followers: 5)
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: 2, SJR: 2.577, h-index: 7)
Analytica Chimica Acta     Hybrid Journal   (Followers: 38, SJR: 1.548, h-index: 152)
Analytical Biochemistry     Hybrid Journal   (Followers: 160, SJR: 0.725, h-index: 154)
Analytical Chemistry Research     Open Access   (Followers: 8, SJR: 0.18, h-index: 2)
Analytical Spectroscopy Library     Full-text available via subscription   (Followers: 10)
Anesthésie & Réanimation     Full-text available via subscription  
Anesthesiology Clinics     Full-text available via subscription   (Followers: 21, SJR: 0.421, h-index: 40)
Angiología     Full-text available via subscription   (SJR: 0.124, h-index: 9)
Angiologia e Cirurgia Vascular     Open Access  
Animal Behaviour     Hybrid Journal   (Followers: 152, SJR: 1.907, h-index: 126)
Animal Feed Science and Technology     Hybrid Journal   (Followers: 5, SJR: 1.151, h-index: 83)
Animal Reproduction Science     Hybrid Journal   (Followers: 5, SJR: 0.711, h-index: 78)
Annales d'Endocrinologie     Full-text available via subscription   (SJR: 0.394, h-index: 30)
Annales d'Urologie     Full-text available via subscription  
Annales de Cardiologie et d'Angéiologie     Full-text available via subscription   (SJR: 0.177, h-index: 13)
Annales de Chirurgie de la Main et du Membre Supérieur     Full-text available via subscription  
Annales de Chirurgie Plastique Esthétique     Full-text available via subscription   (Followers: 2, SJR: 0.354, h-index: 22)
Annales de Chirurgie Vasculaire     Full-text available via subscription   (Followers: 1)

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Journal Cover Acta Materialia
  [SJR: 3.683]   [H-I: 202]   [204 followers]  Follow
    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 1359-6454
   Published by Elsevier Homepage  [3042 journals]
  • Comparison of densification kinetics of a TiAl powder by spark plasma
           sintering and hot pressing
    • Authors: Zofia Trzaska; Guillaume Bonnefont; Gilbert Fantozzi; Jean-Philippe Monchoux
      Pages: 1 - 13
      Abstract: Publication date: 15 August 2017
      Source:Acta Materialia, Volume 135
      Author(s): Zofia Trzaska, Guillaume Bonnefont, Gilbert Fantozzi, Jean-Philippe Monchoux
      Densification kinetics by spark plasma sintering (SPS) and hot pressing (HP) have been compared, under isothermal conditions and with heating rates of 20 °C/min. Careful calibration of sample temperature has been carried out to obtain comparable results. In all cases, densification kinetics did not exhibit significant differences, ruling out any influence of the SPS current. The stress exponent n and the activation energy Q of the Norton law describing deformation at high temperature of the powder particles have been determined by isothermal experiments at different stresses and temperatures, respectively. The values obtained, n = 1.9 ± 0.3 and Q = 308 ± 20 kJ/mol for SPS, n = 1.5 ± 0.3 and Q = 276 ± 40 kJ/mol for HP, come close in both techniques. Using these values, anisothermal densification kinetics at heating rates of 20°C/min and 100 °C/min, typical of the SPS, could be analytically reproduced, using literature models. The activation parameters suggest that SPS densification kinetics occurs by dislocation climb controlled by Al bulk diffusion, that is, by classical metallurgical mechanisms.
      Graphical abstract image

      PubDate: 2017-06-13T12:04:53Z
      DOI: 10.1016/j.actamat.2017.06.004
      Issue No: Vol. 135 (2017)
       
  • Individual effect of recrystallisation nucleation sites on texture
           weakening in a magnesium alloy: Part 1- double twins
    • Authors: Dikai Guan; W. Mark Rainforth; Junheng Gao; Joanne Sharp; Brad Wynne; Le Ma
      Pages: 14 - 24
      Abstract: Publication date: 15 August 2017
      Source:Acta Materialia, Volume 135
      Author(s): Dikai Guan, W. Mark Rainforth, Junheng Gao, Joanne Sharp, Brad Wynne, Le Ma
      Recrystallised grain nucleation, grain growth and corresponding texture evolution in a cold-rolled rare earth containing WE43 Mg alloy during annealing at 490 °C was fully tracked using a quasi-in-situ electron backscatter diffraction method. The results show nucleation sites, such as double twins, can weaken the deformed texture and for the first time provide direct evidence that recrystallised grains originating from double twins can form the rare earth texture during annealing. Precipitation and recrystallisation occurred concurrently during most of the annealing period, with precipitates forming preferentially along prior grain and twin boundaries. These precipitates effectively retard the recrystallisation due to particle pinning leading to an excessively long time for the completion of recrystallisation. A large portion of recrystallised grains were observed to have 〈0001〉 poles tilted 20–45° away from the normal direction. The RE texture emerges during the nucleation of recrystallised grains and is maintained during subsequent uniform grain growth, which results in a stable RE texture being developed as recrystallisation progresses. The uniform grain growth could be attributed to solute drag suppressing the grain boundary mobility of those grains that had recrystallised with a basal texture and precipitate pinning restricting potential orientated grain growth.
      Graphical abstract image

      PubDate: 2017-06-13T12:04:53Z
      DOI: 10.1016/j.actamat.2017.06.015
      Issue No: Vol. 135 (2017)
       
  • Vacancy mediated alloying strengthening effects on γ/γ′ interface of
           Ni-based single crystal superalloys: A first-principles study
    • Authors: Wenyue Zhao; Zhimei Sun; Shengkai Gong
      Pages: 25 - 34
      Abstract: Publication date: 15 August 2017
      Source:Acta Materialia, Volume 135
      Author(s): Wenyue Zhao, Zhimei Sun, Shengkai Gong
      The vacancy effects on the site preferences of alloying elements Re, Mo, Ta and Cr at γ/γ′ interface are studied using first-principles density functional theory. The alloying strengthening mechanism mediated by vacancy is also evaluated via calculating the Griffith work of fracture for γ/γ′ interface as well as the energy barrier of vacancy migration at the interface. The calculated results show that a Ni vacancy on (001)γ′ plane does not change the most preferable substitution position of the alloying elements at corner-point Ni site on (001)γ except Cr. These alloying elements in γ′ enhance the interfacial bonding strength of γ/γ′ interface even with the presence of a neighboring vacancy, among which Re exhibits the best strengthening effect. The improvement in the γ/γ′ interfacial strength is ascribed to the increased chemical bonding strength between the alloying atom and the nearest-neighbor host atom. Furthermore, a single Ni vacancy has higher migration energy barriers around the alloying solutes especially Re solute substituting on (001)γ at γ/γ′ interface, although the alloying solutes at these preferable sites have no contribution to the interfacial bonding strength.
      Graphical abstract image

      PubDate: 2017-06-13T12:04:53Z
      DOI: 10.1016/j.actamat.2017.05.074
      Issue No: Vol. 135 (2017)
       
  • Interconfigurational and intraconfigurational transitions of Yb2+ and Yb3+
           ions in hydroxyapatite: A cathodoluminescence study
    • Authors: Luz A. Zavala; Paloma Fernández; Ekaterina Novitskaya; Jorge N. Díaz; Manuel Herrera; Olivia A. Graeve
      Pages: 35 - 43
      Abstract: Publication date: 15 August 2017
      Source:Acta Materialia, Volume 135
      Author(s): Luz A. Zavala, Paloma Fernández, Ekaterina Novitskaya, Jorge N. Díaz, Manuel Herrera, Olivia A. Graeve
      We present a cathodoluminescence (CL) study of the interconfigurational transitions of Yb2+ and intraconfigurational transitions of Yb3+ dopant ions in hydroxyapatite (HAp) powders. Our results demonstrate that strong electric fields present in the HAp lattice induce manifold levels in the electronic configuration of the ytterbium ions, due to splitting of the 4f 135d 1 orbitals. CL spectra of Yb2+ display a series of sharp peaks centered at 3.27, 2.98, 2.85, 2.53, 2.27, 2.09 and 1.63 eV, corresponding to transitions between multiple levels produced by a trigonal distortion of the regular octahedral crystal field of the Yb2+ 4f 135d 1 configuration. CL spectra of Yb3+ display four emissions centered at 1.17, 1.21, 1.24 and 1.27 eV, generated by intraconfigurational transitions between the 2F5/2 and 2F7/2 states of Yb3+ ions. Two types of samples were synthesized at different pH values, resulting in variations of the valence of the ytterbium ions, such that the ratios of Yb2+/Yb3+ were 0.31 and 0.55 in calcined powders synthesized at pH values of 6 and 4, respectively, as determined from X-ray photoelectron spectroscopy. Infrared CL images of the two samples show an inhomogeneous spatial distribution of the Yb3+ dopant in the powders. Thermal treatment of the samples, at 873 K in an oxygen atmosphere, result in quenching of the Yb2+ luminescence due to oxidation of the Yb2+ into Yb3+.
      Graphical abstract image

      PubDate: 2017-06-13T12:04:53Z
      DOI: 10.1016/j.actamat.2017.06.003
      Issue No: Vol. 135 (2017)
       
  • Near-isotropic elastic properties and high shear deformation resistance:
           Making low symmetry and open structured YbAlB14, LuAlB14 and ScMgB14
           superhard
    • Authors: Yanchun Zhou; Fu-Zhi Dai; Huimin Xiang; Zhihai Feng
      Pages: 44 - 53
      Abstract: Publication date: 15 August 2017
      Source:Acta Materialia, Volume 135
      Author(s): Yanchun Zhou, Fu-Zhi Dai, Huimin Xiang, Zhihai Feng
      Superhard materials are usually found in high symmetry and high bond density compounds like diamond and cubic BN. Herein, using a combination of first-principles calculations and the microscopic and macroscopic models for hardness prediction, three new possible superhard boron-rich metal borides with low symmetry and open structure are predicted. The predicted hardness values are 37.9 GPa, 37.5 GPa and 41.3 GPa for YbAlB14, LuAlB14 and ScMgB14, respectively, from the macroscopic model and 45.1 GPa, 50.8 GPa and 50.4 GPa, respectively, from the microscopic model. The reliability of this work is supported by comparing the measured hardness of AlMgB14 and Y0.62Al0.71B14 with the calculated Vickers hardness of AlMgB14 and YAlB14. The metal elements are found to influence the anisotropy of BB bonds and elastic properties beside transferring valence electrons to the boron framework and stabilizing the crystal structure. From elastic property angle, it is the near-isotropic elastic modulus and high shear deformation resistance underpinned by the nearly isotropic BB bonding that warrant these low symmetry and open structured ternary borides superhard.
      Graphical abstract image

      PubDate: 2017-06-18T16:58:00Z
      DOI: 10.1016/j.actamat.2017.06.016
      Issue No: Vol. 135 (2017)
       
  • Irradiation-induced damage evolution in concentrated Ni-based alloys
    • Authors: Gihan Velişa; Mohammad W. Ullah; Haizhou Xue; Ke Jin; Miguel L. Crespillo; Hongbin Bei; William J. Weber; Yanwen Zhang
      Pages: 54 - 60
      Abstract: Publication date: 15 August 2017
      Source:Acta Materialia, Volume 135
      Author(s): Gihan Velişa, Mohammad W. Ullah, Haizhou Xue, Ke Jin, Miguel L. Crespillo, Hongbin Bei, William J. Weber, Yanwen Zhang
      Understanding the effects of chemical complexity from the number, type and concentration of alloying elements in single-phase concentred solid-solution alloys (SP-CSAs) on defect dynamics and microstructure evolution is pivotal for developing next-generation radiation-tolerant structural alloys. A specially chosen set of SP-CSAs with different chemical complexity (Ni80Fe20, Ni80Cr20 and Ni40Fe40Cr20) are investigated using 1.5 MeV Mn ions over a wide fluence range, from 2 × 1013 to 1 × 1016 ions cm−2 at room temperature. Based on an integrated study of Rutherford backscattering spectroscopy in channeling geometry and molecular dynamics simulations, the results demonstrate that Ni40Fe40Cr20 is more radiation tolerant than Ni80Fe20, Ni80Cr20 and elemental Ni in the low fluence regime. While chemical complexity of this set of SP-CSAs is clearly demonstrated to affect defect evolution through suppressed defect production and enhanced recombination at early stages, the effect of the mixed ferro- and anti-ferromagnetic interactions is not the only controlling factor responsible for the improved radiation performance. The observed strong alloying effect on defect evolution is attributed to the altered defect migration mobilities of defect clusters in these alloys, an intrinsic characteristic of the complex energy landscapes in CSAs.
      Graphical abstract image

      PubDate: 2017-06-18T16:58:00Z
      DOI: 10.1016/j.actamat.2017.06.002
      Issue No: Vol. 135 (2017)
       
  • Reaction of amorphous/crystalline SiOC/Fe interfaces by thermal annealing
    • Authors: Qing Su; Mikhail Zhernenkov; Hepeng Ding; Lloyd Price; Daniel Haskel; Erik Benjamin Watkins; Jaroslaw Majewski; Lin Shao; Michael J. Demkowicz; Michael Nastasi
      Pages: 61 - 67
      Abstract: Publication date: 15 August 2017
      Source:Acta Materialia, Volume 135
      Author(s): Qing Su, Mikhail Zhernenkov, Hepeng Ding, Lloyd Price, Daniel Haskel, Erik Benjamin Watkins, Jaroslaw Majewski, Lin Shao, Michael J. Demkowicz, Michael Nastasi
      The development of revolutionary new alloys and composites is crucial to meeting materials requirements for next generation nuclear reactors. The newly developed amorphous silicon oxycarbide (SiOC) and crystalline Fe composite system has shown radiation tolerance over a wide range of temperatures. To advance understanding of this new composite, we investigate the structure and thermal stability of the interface between amorphous SiOC and crystalline Fe by combining various experimental techniques and simulation methods. We show that the SiOC/Fe interface is thermally stable up to at least 400 °C. When the annealing temperature reaches 600 °C, an intermixed region forms at this interface. This region appears to be a crystalline phase that forms an incoherent interface with the Fe layer. Density functional theory (DFT) Molecular dynamics (MD) is performed on the homogeneous SiFeOC phase to study the early stages of formation of the intermixed layer. Both experimental and simulation results suggest this phase has the fayalite crystal structure. The physical processes involved in the formation of the intermixed region are discussed.
      Graphical abstract image

      PubDate: 2017-06-18T16:58:00Z
      DOI: 10.1016/j.actamat.2017.06.020
      Issue No: Vol. 135 (2017)
       
  • Magnetization reversal of exchange-coupled and exchange-decoupled Nd-Fe-B
           magnets observed by magneto-optical Kerr effect microscopy
    • Authors: M. Soderžnik; H. Sepehri-Amin; T.T. Sasaki; T. Ohkubo; Y. Takada; T. Sato; Y. Kaneko; A. Kato; T. Schrefl; K. Hono
      Pages: 68 - 76
      Abstract: Publication date: 15 August 2017
      Source:Acta Materialia, Volume 135
      Author(s): M. Soderžnik, H. Sepehri-Amin, T.T. Sasaki, T. Ohkubo, Y. Takada, T. Sato, Y. Kaneko, A. Kato, T. Schrefl, K. Hono
      We observed the magnetization reversals of exchange-coupled and exchange-decoupled Nd-Fe-B sintered magnets with coercivities of 1.16 and 1.80 T, respectively, by magneto-optical Kerr effect (MOKE) microscopy. The cascade propagation of reversed magnetic domain throughout multiple grains was observed in the standard sintered magnet, in which Nd2Fe14B grains are believed to be exchange-coupled through ferromagnetic grain boundary phase. In the exchange-decoupled Nd-rich Ga-doped sintered magnet, the magnetization reversal is suppressed by thick nonmagnetic grain boundary phase. The EBSD revealed 11° misalignment of 2:14:1 grains in the standard sintered magnet, while higher misalignment of 18° was found in the exchange-decoupled Nd-rich Ga-doped magnet. Micromagnetic simulations incorporating such microstructural features reproduced experimentally observed demagnetization curves fairly well including the enhancement of the coercivity and the deterioration of the squareness in the exchange decoupled sintered magnet.
      Graphical abstract image

      PubDate: 2017-06-18T16:58:00Z
      DOI: 10.1016/j.actamat.2017.05.006
      Issue No: Vol. 135 (2017)
       
  • Lattice strain evolution and load partitioning during creep of a Ni-based
           superalloy single crystal with rafted γ′ microstructure
    • Authors: James Coakley; Dong Ma; Matthew Frost; David Dye; David N. Seidman; David C. Dunand; Howard J. Stone
      Pages: 77 - 87
      Abstract: Publication date: 15 August 2017
      Source:Acta Materialia, Volume 135
      Author(s): James Coakley, Dong Ma, Matthew Frost, David Dye, David N. Seidman, David C. Dunand, Howard J. Stone
      In-situ neutron diffraction measurements were performed on monocrystalline samples of the Ni-based superalloy CMSX-4 during N-type γ′ raft formation under the tensile creep conditions of 1150 °C/100 MPa, and subsequently on a rafted sample under the low temperature/high stress creep conditions of 715 °C/825 MPa. During 1150 °C/100 MPa creep, the γ′ volume fraction decreased from ∼70% to ∼50%, the lattice parameter misfit was partly relieved, and the load was transferred from the creeping γ matrix to the γ′ precipitates. On cooling back to room temperature, a fine distribution of γ′ precipitates formed in the γ channels, and these precipitates were present in the 715 °C/825 MPa creep regime. Under low temperature/high stress creep, the alloy with rafted γ′ microstructure exhibited superior creep strength to the cuboidal γ′ microstructure produced following a standard heat-treatment. A lengthy creep incubation period was observed, believed to be associated with {111} 〈 110 〉 dislocations hindering propagation of {111} 〈 112 〉 dislocations. Following the creep incubation period, extensive macroscopic creep strain accumulated during primary creep as the γ phase yielded. Finally, the diffraction data suggest a loss of precipitate/matrix coherency in the (0k0) interfaces as creep strain accumulated.
      Graphical abstract image

      PubDate: 2017-06-18T16:58:00Z
      DOI: 10.1016/j.actamat.2017.06.021
      Issue No: Vol. 135 (2017)
       
  • First-principles prediction of the stacking fault energy of gold at finite
           temperature
    • Authors: Xiaoqing Li; Stephan Schönecker
      Pages: 88 - 95
      Abstract: Publication date: 15 August 2017
      Source:Acta Materialia, Volume 135
      Author(s): Xiaoqing Li, Stephan Schönecker
      The intrinsic stacking fault energy (ISFE) γ is a material parameter fundamental to the discussion of plastic deformation mechanisms in metals. Here, we scrutinize the temperature dependence of the ISFE of Au through accurate first-principles derived Helmholtz free energies employing both the super cell approach and the axial Ising model (AIM). A significant decrease of the ISFE with temperature, − ( 36 – 39 ) % from 0 to 890 K depending on the treatment of thermal expansion, is revealed, which matches the estimate based on the experimental temperature coefficient d γ / d T closely. We make evident that this decrease predominantly originates from the excess vibrational entropy at the stacking fault layer, although the contribution arising from the static lattice expansion compensates it by approximately 60%. Electronic excitations are found to be of minor importance for the ISFE change with temperature. We show that the Debye model in combination with the AIM captures the correct sign but significantly underestimates the magnitude of the vibrational contribution to γ ( T ) . The hexagonal close-packed (hcp) and double hcp structures are established as metastable phases of Au. Our results demonstrate that quantitative agreement with experiments can be obtained if all relevant temperature-induced excitations are considered in first-principles modeling and that the temperature dependence of the ISFE is substantial enough to be taken into account in crystal plasticity modeling.
      Graphical abstract image

      PubDate: 2017-06-18T16:58:00Z
      DOI: 10.1016/j.actamat.2017.06.009
      Issue No: Vol. 135 (2017)
       
  • Influence of hydrogen on dislocation self-organization in Ni
    • Authors: Shuai Wang; Akihide Nagao; Kaveh Edalati; Zenji Horita; Ian M. Robertson
      Pages: 96 - 102
      Abstract: Publication date: 15 August 2017
      Source:Acta Materialia, Volume 135
      Author(s): Shuai Wang, Akihide Nagao, Kaveh Edalati, Zenji Horita, Ian M. Robertson
      The influence of hydrogen on the evolution of the deformation microstructure in Ni at the same specific macroscopic strain has been determined for the first time. The evolution of the microstructure was accelerated by hydrogen as evidenced by the formation of smaller dislocation cells and dense dislocation walls compared to only dislocation cells in the absence of hydrogen. This enhanced evolution is described in terms of the hydrogen-enhanced localized plasticity mechanism.
      Graphical abstract image

      PubDate: 2017-06-18T16:58:00Z
      DOI: 10.1016/j.actamat.2017.05.073
      Issue No: Vol. 135 (2017)
       
  • Nonstoichiometric [012] dislocation in strontium titanate
    • Authors: Yuho Furushima; Yuki Arakawa; Atsutomo Nakamura; Eita Tochigi; Katsuyuki Matsunaga
      Pages: 103 - 111
      Abstract: Publication date: 15 August 2017
      Source:Acta Materialia, Volume 135
      Author(s): Yuho Furushima, Yuki Arakawa, Atsutomo Nakamura, Eita Tochigi, Katsuyuki Matsunaga
      Dislocations are one-dimensional lattice defects in crystalline materials, and unique atomic configurations at their core can affect functional properties of materials. Here we report on a [012] dislocation having a nonstoichiometric core structure in strontium titanate (SrTiO3). TEM and STEM observations showed that a (001) low-angle tilt grain boundary consists of two types of dislocations. One is a [001] edge dislocation that compensates a tilt angle of the boundary as found in previous studies. The other is a [012] dislocation with both edge and screw component. This dislocation is considered to be formed to compensate a slight twist angle of the boundary. It is interesting that the [012] dislocations have a specific core structure with the dissociation into three partial dislocations of b  = 1/2 [011], [001] and 1/2 [011]. At stacking faults formed with the dissociation, two (001) Ti-O layers were located to be neighbors across the stacking fault plane due to missing of a (001) Sr-O layer. In addition, the energy of the stacking fault was estimated to be 0.4 J/m2 from the separation distances between partial dislocations. It is suggested that the [012] dislocation is formed due to presence of the nonstoichiometric stacking fault structure with low energy.
      Graphical abstract image

      PubDate: 2017-06-18T16:58:00Z
      DOI: 10.1016/j.actamat.2017.06.017
      Issue No: Vol. 135 (2017)
       
  • Micro-plasticity of medium Mn austenitic steel: Perfect dislocation
           plasticity and deformation twinning
    • Authors: Eun Jung Seo; Jin Kyung Kim; Lawrence Cho; Javad Mola; Chang Yeol Oh; Bruno C. De Cooman
      Pages: 112 - 123
      Abstract: Publication date: 15 August 2017
      Source:Acta Materialia, Volume 135
      Author(s): Eun Jung Seo, Jin Kyung Kim, Lawrence Cho, Javad Mola, Chang Yeol Oh, Bruno C. De Cooman
      The micro-scale plastic deformation behavior of an austenitic Fe-1.2%C-7.0%Mn (in wt%) steel was studied by means of nano-indentation and in situ compression of micro-pillars with selected crystallographic orientations. Transmission electron microscopy analysis reveals that the partial dislocation mediated twinning is preferred in a [001]-oriented single grain under compression. Twinning leads to large strain bursts during the nano-indentation and the micro-pillar compression. Perfect dislocation slip is the dominant deformation mechanism in a [111]-oriented single grain under compression. The observations offer strong support for the hypothesis that deformation twinning is a plasticity enhancing mechanism activated during the deformation of medium Mn steel.
      Graphical abstract image

      PubDate: 2017-06-24T02:23:53Z
      DOI: 10.1016/j.actamat.2017.06.014
      Issue No: Vol. 135 (2017)
       
  • Study on the effects of polarization on local morphological change of
           nickel at active three-phase-boundary using patterned nickel-film
           electrode in solid oxide fuel cell anode
    • Authors: Zhenjun Jiao; Naoki Shikazono
      Pages: 124 - 131
      Abstract: Publication date: 15 August 2017
      Source:Acta Materialia, Volume 135
      Author(s): Zhenjun Jiao, Naoki Shikazono
      In this study, patterned nickel-film electrode was used to investigate the effects of polarization on local morphological changes of nickel at three-phase-boundary in solid oxide fuel cell anode. Compared to conventional porous composite nickel-yttria-stabilized zirconia composite cermet anode, patterned nickel-film anode facilitates the direct observations at nickel-yttria-stabilized zirconia interface and three-phase-boundary so that the local morphological changes of nickel can be qualitatively characterized by scanning electron microscopy. The phenomena of enhanced nickel wettability and nickel spreading on yttria-stabilized zirconia surface at active three-phase-boundary under polarization were observed and explained by a model based on the modification of local nickel surface tension. The nickel-film anode performances and local nickel morphological changes at three-phase-boundary before and after operations with different conditions were analyzed. The influences of operation time and anode overpotential variation on local morphological change of nickel were investigated.

      PubDate: 2017-06-24T02:23:53Z
      DOI: 10.1016/j.actamat.2017.05.051
      Issue No: Vol. 135 (2017)
       
  • Equivalent energy level hybridization approach for high-performance
           metamaterials design
    • Authors: Chuanbao Liu; Yang Bai; Liqiao Jing; Yihao Yang; Hongsheng Chen; Ji Zhou; Qian Zhao; Lijie Qiao
      Pages: 144 - 149
      Abstract: Publication date: 15 August 2017
      Source:Acta Materialia, Volume 135
      Author(s): Chuanbao Liu, Yang Bai, Liqiao Jing, Yihao Yang, Hongsheng Chen, Ji Zhou, Qian Zhao, Lijie Qiao
      Metamaterials have microscopic physical properties highly analogous to real materials, since their characteristic frequencies behave as equivalent energy levels, providing an efficient route to deal with stacked meta-atoms with strong interactions. Based on the hybridization of equivalent energy levels theory, in this paper, we designed transmissive multilayer metasurfaces to convert the handedness of a circularly polarized wave with high efficiency in a broad bandwidth. In stacked metasurfaces, the equivalent energy levels gradually split with an increasing layer number (i.e. meta-atoms). Their hybridization introduces trapped modes with near-unity transmission, contributes to a near-π transmission phase difference and promotes the split modes overlapping to form a broad bandwidth. Through theoretical calculations, numerical simulations and experimental measurements, a four-layer metasurface was shown to exhibit a high transmission conversion coefficient (80–98%) of a circularly polarized wave covering the whole X-band. Furthermore, a multifunctional meta-lens with broad bandwidth was designed and fabricated based on the Pancharatnam-Berry phase to converge or diverge incident Gaussian beams with different handedness at a high efficiency rate.
      Graphical abstract image

      PubDate: 2017-06-24T02:23:53Z
      DOI: 10.1016/j.actamat.2017.06.032
      Issue No: Vol. 135 (2017)
       
  • Mapping the world of complex concentrated alloys
    • Authors: Stéphane Gorsse; Daniel B. Miracle; Oleg N. Senkov
      Pages: 177 - 187
      Abstract: Publication date: 15 August 2017
      Source:Acta Materialia, Volume 135
      Author(s): Stéphane Gorsse, Daniel B. Miracle, Oleg N. Senkov
      This work explores the mechanical properties of high entropy alloys (HEAs) and complex concentrated alloys (CCAs) by comparing them with commercially available engineering alloys including industry-standard aerospace alloys. To reach this goal we have developed a materials database covering the main mechanical properties of HEAs and CCAs from the published literature. The database is used to represent various property spaces enabling an assessment of their performance for light weight structures and high-temperature structural applications. In addition, we illustrate the effects of alloying and of specific elements on the room temperature mechanical properties of HEAs and CCAs. With densities between titanium alloys and steels or nickel alloys, the best CCAs exceed commercial alloys in uniaxial loading and beam bending at room temperature. Where use temperature or cost excludes commercial alloys based on Mg, Al or Ti, the best CCAs also offer attractive specific yield strength in panel bending and specific stiffness for all loading conditions at room temperature. Many CCAs have superior structural properties at elevated temperatures.
      Graphical abstract image

      PubDate: 2017-06-24T02:23:53Z
      DOI: 10.1016/j.actamat.2017.06.027
      Issue No: Vol. 135 (2017)
       
  • Discrete dislocation plasticity analysis of the effect of interfacial
           diffusion on the creep response of Ni single-crystal superalloys
    • Authors: Siamak S. Shishvan; Robert M. McMeeking; Tresa M. Pollock; Vikram S. Deshpande
      Pages: 188 - 200
      Abstract: Publication date: 15 August 2017
      Source:Acta Materialia, Volume 135
      Author(s): Siamak S. Shishvan, Robert M. McMeeking, Tresa M. Pollock, Vikram S. Deshpande
      Discrete dislocation plasticity (DDP) analysis of the high temperature creep deformation of a single crystal Ni superalloy comprising Ni3Al precipitates ( γ ′ ) in a Ni matrix (γ) is presented. The γ ′ precipitates remain elastic but can also deform due to the stress-driven inter-diffusion of the Al within the Ni on the γ / γ ′ interface while plastic deformation of the γ phase occurs by a combination of dislocation glide and dislocation climb coupled to the diffusion of vacancies. At relatively low applied uniaxial tensile stresses, the creep strain rates are very low in the absence of interfacial diffusion. This is due to the stress-induced pile up of dislocations at γ / γ ′ interfaces that serves to inhibit further nucleation and suppresses continued plastic flow in the γ phase. When interfacial diffusion is permitted, the creep rates not only increase but the superalloy also exhibits distinct secondary and tertiary creep regimes. While this change in behaviour is a result of interfacial diffusion, the contribution of the average γ ′ strain to the deformation of the superalloy is small. Rather, the diffusional deformation at the interface results in the development of a wavy interface which relaxes the back-stresses of dislocations piled-up at the γ / γ ′ interfaces. This permits continued dislocation activity within the γ phase with dislocations arranging themselves into low energy cell-structures in the γ phase via dislocation climb. The formation of these structures results in an increase in the creep strain rate and the onset of the tertiary creep regime. At high applied stresses, the high initial dislocation density within the γ phase results in the continued climb motion of dislocations and an evolving spatial distribution of vacancies within the superalloy. Thus, creep deformation occurs even in the absence of interfacial diffusion although the creep rates are significantly increased when interfacial diffusion is present. The DDP analysis presented here demonstrates the critical role of interfacial diffusion in controlling the creep rates of Ni superalloys and suggests that interface engineering to reduce interfacial diffusion rates will aid in improving the creep performance of these alloys.
      Graphical abstract image

      PubDate: 2017-06-24T02:23:53Z
      DOI: 10.1016/j.actamat.2017.06.026
      Issue No: Vol. 135 (2017)
       
  • A new approach to the mechanics of fatigue crack growth in metals:
           Correlation of mean stress (stress ratio) effects using the change in
           net-section strain energy
    • Authors: K.S. Ravi Chandran
      Pages: 201 - 214
      Abstract: Publication date: 15 August 2017
      Source:Acta Materialia, Volume 135
      Author(s): K.S. Ravi Chandran
      An unsolved problem in the mechanics of fatigue crack growth is why mean stress or stress ratio in a fatigue cycle has a profound effect on growth rate when compared in terms of stress intensity factor range (ΔK). Specifically, there has not been any fracture-mechanically-consistent theory to explain the effects of mean stress on fatigue crack growth. In this work, a new and generalized driving force parameter for fatigue crack growth, which effectively incorporates the mean stress or stress ratio effect, is developed from energy principles of solid mechanics. A successful explanation of the effect of mean stress on the growth rate is provided. The driving force is the accumulated change in net-section strain energy, which develops as a function of increasing crack length and decreasing net-section size in fatigue. A generalized mechanics analysis, showing how to account for the stress amplitude and the maximum stress of a fatigue cycle in the change in net-section strain energy, is presented. Equivalently, the new crack growth parameter can also be interpreted as the cumulative work done by cyclic loading at the crack length at which the crack growth data is determined. Several experimental data, including some historically significant fatigue crack growth data, generated on aluminum, steel and titanium alloys, are used to demonstrate the success of the correlation. As a further validation of the proposed concept, it is shown that the empirical Walker parameter (ΔK0.5Kmax0.5), which had some success in correlating stress ratio effect, is in fact related to the change in net-section strain energy.
      Graphical abstract image

      PubDate: 2017-06-24T02:23:53Z
      DOI: 10.1016/j.actamat.2017.06.013
      Issue No: Vol. 135 (2017)
       
  • Two modes of grain boundary pinning by coherent precipitates
    • Authors: Nan Wang; Yanzhou Ji; Yongbiao Wang; Youhai Wen; Long-Qing Chen
      Pages: 226 - 232
      Abstract: Publication date: 15 August 2017
      Source:Acta Materialia, Volume 135
      Author(s): Nan Wang, Yanzhou Ji, Yongbiao Wang, Youhai Wen, Long-Qing Chen
      We propose a two-mechanism theory to estimate the pinning effect of coherent precipitates on grain-boundary (GB) migration in grain growth, taking into account the important effect of elastic misfit strain at the coherent interface. Depending on the relative importance of the elastic and the GB contributions to the total free energy, Zener type stabilization or a novel elastic energy induced stabilization may occur. It is found that the pinning is most effective in the crossover region between these two mechanisms. A phase-field-crystal model is used to numerically validate the theory. Relevant experiments and potential impacts on alloy design are also discussed.
      Graphical abstract image

      PubDate: 2017-06-24T02:23:53Z
      DOI: 10.1016/j.actamat.2017.06.031
      Issue No: Vol. 135 (2017)
       
  • Colossal thermopower, spin states and delocalization effects in single
           layered La2−xSrxCoO4
    • Authors: Abdul Ahad; D.K. Shukla; F. Rahman; S. Majid; Tarachand; G.S. Okram; A.K. Sinha; D.M. Phase
      Pages: 233 - 243
      Abstract: Publication date: 15 August 2017
      Source:Acta Materialia, Volume 135
      Author(s): Abdul Ahad, D.K. Shukla, F. Rahman, S. Majid, Tarachand, G.S. Okram, A.K. Sinha, D.M. Phase
      Hole-doped La2CoO4 compounds have been widely studied due to their multifaceted behavior such as charge ordering, spin ordering and spin blockade. Hole doping in La2CoO4 causes appearance of the Co3+ along with the Co2+, which makes it an interesting model system for study of the spin-state transitions. In hole-doped samples, the spin-states of the Co3+ ions at large have been reported as the low or intermediate state. We report here a study on the Sr doping in La2CoO4. The Sr doping has been found to stabilize the crystal structure. By combining structural, electronic structure and electrical transport results, it is observed that the high-spin state is present in all the La2−x Sr x CoO4 (x = 0.5, 0.8, 0.9 and 1.0) samples. The high-spin state has been found to be concomitant with the delocalization of the e g electrons. In addition, in these materials, a colossal thermopower has been observed for first time in the cryogenic temperatures.
      Graphical abstract image

      PubDate: 2017-06-24T02:23:53Z
      DOI: 10.1016/j.actamat.2017.06.030
      Issue No: Vol. 135 (2017)
       
  • A novel type of Co–Ti–Cr-base γ/γ′ superalloys
           with low mass density
    • Authors: Christopher H. Zenk; Ivan Povstugar; Rui Li; Fernando Rinaldi; Steffen Neumeier; Dierk Raabe; Mathias Göken
      Pages: 244 - 251
      Abstract: Publication date: 15 August 2017
      Source:Acta Materialia, Volume 135
      Author(s): Christopher H. Zenk, Ivan Povstugar, Rui Li, Fernando Rinaldi, Steffen Neumeier, Dierk Raabe, Mathias Göken
      A γ ′ strengthened Co–Ti–Cr superalloy is presented with a mass density ∼ 14 % below that of typical Co–Al–W-based alloys. The lattice misfit is sufficiently low to form coherent cuboidal γ ′ precipitates. Atom probe tomography shows that Cr partitions to the γ phase, but increases the γ ′ volume fraction compared to a binary Co-Ti alloy to more than 60 %. The solubility of Cr in the γ ′ phase is significantly higher than expected from previously published values. The γ ′ solvus temperature is above 1100 °C. The yield strength shows a distinct increase above 600 °C surpassing that of Co–9Al–8W (at.%) and conventional Co-base superalloys, even more so when it is normalized by the mass density.
      Graphical abstract image

      PubDate: 2017-06-24T02:23:53Z
      DOI: 10.1016/j.actamat.2017.06.024
      Issue No: Vol. 135 (2017)
       
  • Particle deposition and deformation from high speed impaction of Ag
           nanoparticles
    • Authors: T.V. Chitrakar; J.W. Keto; M.F. Becker; D. Kovar
      Pages: 252 - 262
      Abstract: Publication date: 15 August 2017
      Source:Acta Materialia, Volume 135
      Author(s): T.V. Chitrakar, J.W. Keto, M.F. Becker, D. Kovar
      The impaction of a single Ag nanoparticle onto an (001) Ag substrate was studied as a function of particle diameter (2–9 nm) and impaction velocity (10–1500 m/sec) using molecular dynamics simulations. The final crystallographic structures were observed to transition from a polycrystalline to an epitaxial morphology as impaction velocity was increased and the velocity required to achieve epitaxy increased with particle size. To understand how the crystallographic structures evolved to their final state, the deformation mechanisms were then studied over a range of time scales, beginning immediately upon impaction. The observed mechanisms included disordering of the atoms and the initiation and propagation of partial dislocations. Deformation increased with impaction velocity due to increases in the degree of disordering and the partial dislocation density. At longer time scales, relaxation of the disordered particles produced epitaxial morphologies, whereas polycrystalline morphologies were observed following incomplete disordering. These results suggest that the microstructures of thick films produced by high speed impaction of nanoparticle aerosols are strongly influenced by processing parameters.
      Graphical abstract image

      PubDate: 2017-06-24T02:23:53Z
      DOI: 10.1016/j.actamat.2017.05.062
      Issue No: Vol. 135 (2017)
       
  • Site-specific atomic-scale characterisation of retained austenite in a
           strip cast TRIP steel
    • Authors: Z.P. Xiong; A.A. Saleh; R.K.W. Marceau; A.S. Taylor; N.E. Stanford; A.G. Kostryzhev; E.V. Pereloma
      Pages: 1 - 15
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): Z.P. Xiong, A.A. Saleh, R.K.W. Marceau, A.S. Taylor, N.E. Stanford, A.G. Kostryzhev, E.V. Pereloma
      Knowledge of carbon content in retained austenite (RA) with different neighbouring phases is essential to understand the chemical stability of RA, which is useful for microstructure tuning of transformation-induced plasticity (TRIP) steels. The present study investigates different morphologies and chemical compositions of RA by correlating electron backscattering diffraction, transmission electron microscopy and atom probe tomography. The effect of neighbouring phases, such as polygonal ferrite, bainitic ferrite lath, ferrite in granular bainite and carbides, on the carbon content in the RA is investigated. The results reveal that the film RA morphology does not always have a higher carbon content than the blocky RA; as coarse RA sometimes displays a higher carbon content than the fine RA films or islands depending on the neighbouring phases. The diffusion of carbon and manganese between austenite and ferrite in bainitic ferrite/granular bainite has been explained according to either diffusionless and/or diffusional mechanism of bainitic ferrite formation followed by tempering.
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      PubDate: 2017-06-04T11:40:05Z
      DOI: 10.1016/j.actamat.2017.05.060
      Issue No: Vol. 134 (2017)
       
  • Non-spherical voids and lattice reorientation patterning in a shock-loaded
           Al single crystal
    • Authors: Chuanshi Hong; Søren Fæster; Niels Hansen; Xiaoxu Huang; Rozaliya I. Barabash
      Pages: 16 - 30
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): Chuanshi Hong, Søren Fæster, Niels Hansen, Xiaoxu Huang, Rozaliya I. Barabash
      An Al single crystal shock loaded in the <1 2 3> direction and captured at incipient spallation was examined by combining X-ray tomography, electron backscatter diffraction on a scanning electron microscope, and transmission electron microscopy (TEM). Octahedral voids with {1 1 1} faces were a characteristic feature in the spall region. Regular patterns of lattice reorientation were found around individual voids, with lattice rotation being up to 25–30°. Each reorientation pattern consists of a number of reoriented zones. The direction of lattice rotation varies systematically from one zone to another. Four groups of reorientation patterns were identified morphologically in the same metallographic section, which result from different sectioning positions relative to the voids and thus provide equivalently a “serial sectioning” investigation of the deformed volume around the voids. An analysis of the observed reorientation patterns based on active slip systems rationalizes the key features observed and suggests that the systematic reorientation patterns result from the dominance of a single slip system in each individual zone. Microstructures revealed by TEM in the spall region show formation of dislocation cells and extended dislocation boundaries, illustrating the importance of plastic deformation during void growth.
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      PubDate: 2017-06-04T11:40:05Z
      DOI: 10.1016/j.actamat.2017.05.035
      Issue No: Vol. 134 (2017)
       
  • Using complimentary microscopy methods to examine Ni-Mn-Si-precipitates in
           highly-irradiated reactor pressure vessel steels
    • Authors: P.D. Edmondson; C.M. Parish; R.K. Nanstad
      Pages: 31 - 39
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): P.D. Edmondson, C.M. Parish, R.K. Nanstad
      Nano-scale Ni-Mn-Si-rich precipitates formed in a reactor pressure vessel steel under high neutron fluence have been characterized using highly complimentary atom probe tomography (APT) and scanning transmission electron microscopy with energy dispersive spectroscopy (STEM-EDS) combined with STEM-EDS modeling. Using these techniques in a synergistic manner to overcome the well-known trajectory aberrations in APT data, the average upper limit Fe concentration within the precipitates was found to be ∼6 at.%. Using this knowledge, accurate compositions of the precipitates was determined and it was found that the spread of precipitate compositions was large, but mostly centered around the Γ2-and G-phases. The use of STEM-EDS also allowed for larger areas to be examined, and segregation of minor solutes was observed to occur on grain boundaries, along with Ni-Mn-Si-rich precipitates that were smaller in size than those in the matrix. Solute segregation at the grain boundaries is proposed to occur through a radiation induced segregation or radiation enhanced diffusion mechanism due to the presence of a denuded zone about the grain boundary. It is also proposed that the reduced precipitate size at the grain boundaries is due to the structure of the grain boundary. The lack of Ni-Mn-Si precipitates observed in larger Mo-rich precipitates is also discussed, and the absence of the minor solutes required to form the Ni-Mn-Si precipitates results in the lack of nucleation. This is in contrast to cementite phases in which Ni-Mn-Si precipitates have been observed to have formed. It was also determined through this work that the exclusion of all the Fe ions during atom probe analysis is a reasonable approximation.
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      PubDate: 2017-06-04T11:40:05Z
      DOI: 10.1016/j.actamat.2017.05.043
      Issue No: Vol. 134 (2017)
       
  • Correlation of the high and very high cycle fatigue response of ferrite
           based steels with strain rate-temperature conditions
    • Authors: Noushin Torabian; Véronique Favier; Justin Dirrenberger; Frédéric Adamski; Saeed Ziaei-Rad; Nicolas Ranc
      Pages: 40 - 52
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): Noushin Torabian, Véronique Favier, Justin Dirrenberger, Frédéric Adamski, Saeed Ziaei-Rad, Nicolas Ranc
      The discrepancies observed between conventional and ultrasonic fatigue testing are assessed through the mechanisms of dislocation mobility in BCC metals. The existence of a transition condition between thermally-activated and athermal regimes for screw dislocation mobility is studied under fatigue loading based on infrared thermography and microstructural characterization, here in the case of DP600 dual-phase steel. Evidence is obtained regarding the microstructural sources of crack initiation, which is found to be consistent with the existence of a transition in the modes of deformation. From the analysis of the experimental data gathered in this work, guidelines are given regarding the comparison and interpretation of S-N curves obtained from conventional and ultrasonic fatigue testing. The inevitable temperature increases under ultrasonic fatigue at high stress amplitudes along with the rate dependent deformation behavior of ferrite, as a BCC structure, were found as the key parameters explaining the observed fatigue behavior and thermal response under low and ultrasonic frequencies. A transition map was produced using the experimental results for DP600 steel as well as data available in the literature for other ferrite based steels, showing the correlation between thermally-activated screw dislocation movement and the absence of failure in very high cycle fatigue.
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      PubDate: 2017-06-04T11:40:05Z
      DOI: 10.1016/j.actamat.2017.05.064
      Issue No: Vol. 134 (2017)
       
  • Indentation Schmid factor and incipient plasticity by nanoindentation
           pop-in tests in hexagonal close-packed single crystals
    • Authors: Wei Zhang; Yanfei Gao; Yuzhi Xia; Hongbin Bei
      Pages: 53 - 65
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): Wei Zhang, Yanfei Gao, Yuzhi Xia, Hongbin Bei
      For hexagonal close-packed (HCP) Hf, Mg and Ti single crystals, indentation Schmid factors, defined as the ratio of maximum resolved shear stress to the maximum contact pressure, were calculated for three typical families of slip systems, including basal 〈 a 〉 , prismatic 〈 a 〉 , and second pyramidal 〈 c + a 〉 slip systems. The dependence of indentation Schmid factors on the crystallographic orientation shows the same hard/soft directions as in the uniaxial tests, but with much reduced variations of magnitudes. The axial ratio, c / a , leads to noticeable effects on the indentation Schmid factors only when the indentation direction deviates from the c -axis by more than about 30°, as shown by the comparison among these three crystals. These predictions are compared to nanoindentation pop-in tests on Mg single crystals in literature and Hf single crystals conducted in this work. The elastic-plastic transition under spherical nanoindentation can be quantitatively measured by a sudden displacement burst (or pop-in) in the load-displacement curve. For defect-free single crystals, the first pop-in results from homogeneous dislocation nucleation, and correspondingly the maximum resolved shear stress (as calculated from pop-in load and indentation Schmid factor) reaches the theoretical strength of the material. Pop-in loads were found to be low for 〈 10 1 ¯ 2 〉 indentation and high for orientations close to the c -axis. While previous experiments in NiAl showed excellent agreement between indentation Schmid factor and pop-in measurements, Mg and Hf works here do not have such an agreement. It is suggested that each of the three sets of slip systems may govern a different range of indentation directions, or stress components other than resolved shear stress dictate the incipient plasticity.
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      PubDate: 2017-06-04T11:40:05Z
      DOI: 10.1016/j.actamat.2017.05.058
      Issue No: Vol. 134 (2017)
       
  • Epitaxial growth during the rapid solidification of plasma-sprayed molten
           TiO2 splat
    • Authors: Shu-Wei Yao; Tao Liu; Chang-Jiu Li; Guan-Jun Yang; Cheng-Xin Li
      Pages: 66 - 80
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): Shu-Wei Yao, Tao Liu, Chang-Jiu Li, Guan-Jun Yang, Cheng-Xin Li
      In this study, epitaxial growth during the rapid solidification of plasma-sprayed molten TiO2 droplet was studied. The crystallographic structure of the TiO2 splats deposited on rutile and α-Al2O3 substrates at 150, 300 and 500 °C was characterized by high resolution transmission electron microscopy and electron back scattering diffraction. The results reveal that homo-epitaxial and hetero-epitaxial TiO2 splats can be formed at the deposition temperature of 500 °C. Crystal orientation is another key factor influencing the epitaxial growth process. It is easier to form an epitaxial TiO2 splat with the 〈001〉 orientation in the crystal growth direction. Based on the experimental results, a competition mechanism between heterogeneous nucleation and epitaxial growth was proposed to understand the phenomena of epitaxy during the rapid solidification process. The effect of undercooling degree, crystal orientation and deposition temperature on the epitaxial growth of TiO2 splat was examined. The simulation results are in close agreement with the experimental observations.
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      PubDate: 2017-06-04T11:40:05Z
      DOI: 10.1016/j.actamat.2017.05.052
      Issue No: Vol. 134 (2017)
       
  • Ab initio study of beryllium surfaces with different hydrogen coverages
    • Authors: D.V. Bachurin; P.V. Vladimirov
      Pages: 81 - 92
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): D.V. Bachurin, P.V. Vladimirov
      The effect of hydrogen concentration on principal hexagonal close-packed beryllium surfaces (basal, prismatic type I and II, pyramidal type I and II) at 0 K was studied with the help of ab initio methods. The configurations with critical hydrogen coverage were revealed. Relaxation of outermost atomic layers at critical coverage drastically differs from that found for clean beryllium surfaces. The presence of hydrogen atoms significantly changes the energy of all studied beryllium surfaces except for prismatic type II plane. Influence of hydrogen concentration on equilibrium shape of hydrogen covered voids was investigated by means of construction of Wulff polyhedra. In addition we generalized Gibbs-Wulff approach for accounting of different hydrogen coverages at crystallographic non-equivalent surfaces and temperature effect. The results of the construction were compared with classical Gibbs-Wulff approach as well as the available experimental data on ion or neutron irradiation at elevated temperatures.
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      PubDate: 2017-06-04T11:40:05Z
      DOI: 10.1016/j.actamat.2017.05.031
      Issue No: Vol. 134 (2017)
       
  • Microstructural and crystallographic characteristics of modulated
           martensite, non-modulated martensite, and pre-martensitic tweed austenite
           in Ni-Mn-Ga alloys
    • Authors: Le Zhou; Matthew M. Schneider; Anit Giri; Kyu Cho; Yongho Sohn
      Pages: 93 - 103
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): Le Zhou, Matthew M. Schneider, Anit Giri, Kyu Cho, Yongho Sohn
      A combinatorial approach using diffusion couples and TEM analyses was carried out to investigate the composition-dependent martensitic transformation in NiMnGa alloys. The compositions cover a large portion of the off-stoichiometric Ni2MnGa compositions and some Mn-rich compositions. Crystallographic variations of the martensitic phase, including non-modulated (NM) martensite, modulated (5M or 7M) martensite, and austenitic phase were identified in the diffusion couples and investigated with respect to their microstructure and crystallography. The 5M and 7M martensitic structures were only found near the interphase boundary between austenite and martensite, while the NM martensitic structures were found mostly away from the interphase boundary. The tetragonality ratio (c/a) for NM martensite generally increases with e/a ratio, but was also dependent on the composition. The habit plane and martensitic microstructure that consists of twinned variants with differing orientations were documented using electron diffraction. The pre-martensitic state was observed in the austenitic phase that was located near the interphase boundary between austenite and martensite, with distinctive tweed microstructure and a strain field originating from the local lattice distortions. The combinatorial approach proves to be efficient and systematic in studying the composition-dependent martensitic transformation in NiMnGa alloys and can be potentially applied to other shape memory alloys.
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      PubDate: 2017-06-08T12:03:00Z
      DOI: 10.1016/j.actamat.2017.05.050
      Issue No: Vol. 134 (2017)
       
  • Influence of nanoscale structural heterogeneity on shear banding in
           metallic glasses
    • Authors: Pengyang Zhao; Ju Li; Jinwoo Hwang; Yunzhi Wang
      Pages: 104 - 115
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): Pengyang Zhao, Ju Li, Jinwoo Hwang, Yunzhi Wang
      Structural heterogeneity at the nanoscale such as medium-range order (MRO) in metallic glasses (MGs) has been suggested by theories and simulations for many years and recently been observed by experiments. However, its correlation with plastic deformation is still poorly understood and investigation into this correlation has just begun. In this study, we first propose a correlation between the properties of shear transformation zone (STZ) and the local structural information at the MRO length scale. As revealed by experiments, the MRO exhibits different types of atomic order and symmetry in MGs depending strongly on the processing history and service condition. We then carry out STZ dynamics simulations to document effects of volume fraction and symmetry of dissimilar STZs (derived from experimental characterization of the MRO) on shear banding dynamics and stress-strain behavior. In particular, a “precipitation-hardening” effect is observed and “strain frustration” is introduced to explain and better understand the influence of nanoscale structural heterogeneity on mechanical behavior of MGs. Our approach intends to provide new fundamental insights into the relationship between the mechanical properties of MGs and their MRO-level structural heterogeneities.
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      PubDate: 2017-06-08T12:03:00Z
      DOI: 10.1016/j.actamat.2017.05.057
      Issue No: Vol. 134 (2017)
       
  • Effect of tube processing methods on microstructure, mechanical properties
           and irradiation response of 14YWT nanostructured ferritic alloys
    • Authors: E. Aydogan; S.A. Maloy; O. Anderoglu; C. Sun; J.G. Gigax; L. Shao; F.A. Garner; I.E. Anderson; J.J. Lewandowski
      Pages: 116 - 127
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): E. Aydogan, S.A. Maloy, O. Anderoglu, C. Sun, J.G. Gigax, L. Shao, F.A. Garner, I.E. Anderson, J.J. Lewandowski
      In this research, innovative thermal spray deposition (Process I) and conventional hot extrusion processing (Process II) methods have been used to produce thin walled tubing (∼0.5 mm wall thickness) out of 14YWT, a nanostructured ferritic alloy. The effects of processing methods on the microstructure, mechanical properties and irradiation response have been investigated by using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and, micro- and nano-hardness techniques. It has been found that these two processes have a significant effect on the microstructure and mechanical properties of the as-fabricated 14YWT tubes. Even though both processing methods yield the formation of various size Y-Ti-O particles, the conventional hot extrusion method results in a microstructure with smaller, homogenously distributed nano-oxides (NOs, Y-Ti-O particles < 5 nm) with higher density. Therefore, Process II tubes exhibit twice the hardness of Process I tubes. It has also been found that these two tremendously different initial microstructures strongly affect irradiation response in these tubes under extremely high dose ion irradiations up to 1100 peak dpa at 450 °C. The finer, denser and homogenously distributed NOs in the Process II tube result in a reduction in swelling by two orders of magnitude. On the other hand, inhomogeneity of the initial microstructure in the Process I tube leads to large variations in both swelling and irradiation induced hardening. Moreover, hardening mechanisms before and after irradiation were measured and compared with detailed calculations. This study clearly indicates the crucial effect of initial microstructure on radiation response of 14YWT alloys.
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      PubDate: 2017-06-08T12:03:00Z
      DOI: 10.1016/j.actamat.2017.05.053
      Issue No: Vol. 134 (2017)
       
  • Improvement of low-cycle fatigue resistance in TWIP steel by regulating
           the grain size and distribution
    • Authors: C.W. Shao; P. Zhang; Y.K. Zhu; Z.J. Zhang; J.C. Pang; Z.F. Zhang
      Pages: 128 - 142
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): C.W. Shao, P. Zhang, Y.K. Zhu, Z.J. Zhang, J.C. Pang, Z.F. Zhang
      Compared to stress controlled high-cycle fatigue, enhancing strain controlled low-cycle fatigue (LCF) properties of material is much more difficult and less reported. In this study, we introduced two strategies and technologies to improve the LCF performance of Fe-Mn-C twinning induced plasticity (TWIP) steel. One is grain refining without introducing residual stress by traditional cold rolling and following recrystallization annealing (FG sample); the other is to introduce a linear gradient in grain size into TWIP steel by the original processing technology (GS sample). It is found that GS samples exhibit a higher cyclic hardening ability and cyclic saturation stress than that of as-received coarse grain (CG) samples and FG samples, which invalids the rule of mixture. Based on the dependence of the fatigue life (N f) on the total strain amplitude (Δε/2), GS shows the longest life at high strain amplitudes, while FG shows the longest life at low strain amplitudes. Judging from the aspect of stress (Δσ/2-N f curve), i.e. the Basquin curve, GS exhibits a better LCF performance than both FG and CG. The excellent fatigue properties of GS are believed to originate from the large generation of geometrically necessary dislocations (GNDs) and the formation of hard core and soft shell structure during cyclic loading. The significant influences of grain size and distribution on fatigue damage mechanisms may provide new and important implications for the optimized microstructural design of the high fatigue performance material.
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      PubDate: 2017-06-08T12:03:00Z
      DOI: 10.1016/j.actamat.2017.05.004
      Issue No: Vol. 134 (2017)
       
  • On the 650 °C thermostability of 9–12Cr heat resistant steels
           containing different precipitates
    • Authors: Hai Wang; Wei Yan; Sybrand van Zwaag; Quanqiang Shi; Wei Wang; Ke Yang; Yiyin Shan
      Pages: 143 - 154
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): Hai Wang, Wei Yan, Sybrand van Zwaag, Quanqiang Shi, Wei Wang, Ke Yang, Yiyin Shan
      The thermal stability of the microstructure, in particular that of the precipitates, has been known to be a critical factor for the creep strength of 9–12Cr martensitic heat-resistant steels. In the present study, four martensitic heat-resistant steels were designed to have different precipitate types and morphologies. Steel NS1 (nitride strengthened, NS) has a low density of fine nitrides and Steel NS2 is strengthened by a high density of fine nitrides. Steel CNS (carbide and nitride strengthened, CNS) contains large size carbides and fine carbonitrides and Steel CS (carbide strengthened, CS) mainly possesses large size carbides. Both aging and creep tests at 650 °C were employed to evaluate the stability of the microstructure and properties of the steels during high-temperature exposure as a function of the precipitate morphology. It was found that Steel NS1 and Steel NS2 displayed a good creep performance, but they already recrystallized after a relatively short-term aging treatment. On the other hand, Steel CNS and Steel CS gave poor creep strength, but they could still maintain the tempered martensitic microstructure during long-term aging. The relation between microstructural stability and precipitate morphology/density was linked to the dislocation-precipitate interaction.
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      PubDate: 2017-06-08T12:03:00Z
      DOI: 10.1016/j.actamat.2017.05.069
      Issue No: Vol. 134 (2017)
       
  • Anomalous influence of grain size on the global structure, ferroelectric
           and piezoelectric response of Na0.5Bi0.5TiO3
    • Authors: Dipak Kumar Khatua; Tarang Mehrotra; Anupam Mishra; Bhaskar Majumdar; Anatoliy Senyshyn; Rajeev Ranjan
      Pages: 177 - 187
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): Dipak Kumar Khatua, Tarang Mehrotra, Anupam Mishra, Bhaskar Majumdar, Anatoliy Senyshyn, Rajeev Ranjan
      Na0.5Bi0.5TiO3 (NBT) based lead-free piezoelectrics have received significant attention in the last few years for their anomalous high-field electrostrain response. The positional disorder on the nano/meso scale in these systems have a profound effect on their perceived global structure and dielectric properties. Here we have carried out a systematic analysis of the grain size dependent structural, ferroelectric, piezoelectric and high temperature electrical conduction behaviour of NBT. We show that the global monoclinic lattice distortion collapses even while the grains are unusually large (∼2.5 μm). We also found that the polarization switching ability and piezoelectric response of bulk NBT ceramics reduce drastically when grains are submicron in size. We use a powder poling technique to prove that the significant reduction of piezoelectricity in the submicron grain ceramic is not due to suppression of ferroelectricity within the grains itself, but rather due to inability of the grains to transform collectively to a long range rhombohedral ferroelectric state, either because of the clamping effect and/or enhanced incoherence of the grain boundaries. We also noted a qualitative change in the electrical conduction behaviour as a function of grain size and comment on a possible correlation between the grain size, global structure, piezoelectric response and oxygen ion conductivity of this lead-free compound.
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      PubDate: 2017-06-13T12:04:53Z
      DOI: 10.1016/j.actamat.2017.05.068
      Issue No: Vol. 134 (2017)
       
  • Atomistic simulations of dislocation behavior in a model FCC
           multicomponent concentrated solid solution alloy
    • Authors: S.I. Rao; C. Woodward; T.A. Parthasarathy; O. Senkov
      Pages: 188 - 194
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): S.I. Rao, C. Woodward, T.A. Parthasarathy, O. Senkov
      In this work, molecular statics and molecular dynamics simulations of a/2<110> dislocation behavior for a model FCC Co30Fe16.67Ni36.67Ti16.67 alloy are discussed. It is shown that the single FCC phase is elastically stable in this alloy. Local stacking fault energies for the FCC alloy are determined as a function of average composition. The core structure of a/2<110> screw and edge dislocations in the FCC Co30Fe16.67Ni36.67Ti16.67 alloy is shown to be planar with significant variations in the Shockley partial splitting along the dislocation line (factor of ∼3) due to concentration fluctuations. The correlation lengths for dislocation line fluctuations in this alloy are determined and discussed. The critical stress to move both a/2<110> screw and edge dislocations at 0 K in the model FCC Co30Fe16.67Ni36.67Ti16.67 alloy is of the order of 0.0025–0.005μ, where μ is the (111) shear modulus, and is significantly higher than that of pure FCC Ni. Molecular dynamics simulation results on the critical stress to move a/2<110> screw and edge dislocations in the model FCC concentrated solid solution alloy show that it decreases with increasing temperature, similar to solid-solution strengthened FCC metals. These molecular dynamics simulation results are in reasonable agreement with experimental tensile yield strength data for an analogous FCC concentrated solid solution alloy. It is also shown that local fluctuations in the concentration of solutes has a strong effect on the effective cross-slip activation energy of screw dislocations in the random alloy.
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      PubDate: 2017-06-13T12:04:53Z
      DOI: 10.1016/j.actamat.2017.05.071
      Issue No: Vol. 134 (2017)
       
  • Crucial role of octahedral untilting R3m/P4mm morphotropic phase boundary
           in highly piezoelectric perovskite oxide
    • Authors: Kang Yan; Shuai Ren; Minxia Fang; Xiaobing Ren
      Pages: 195 - 202
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): Kang Yan, Shuai Ren, Minxia Fang, Xiaobing Ren
      Large piezoelectricity is usually found at morphotropic phase boundary (MPB), a composition-induced phase boundary between two ferroelectric phases. However, only a small fraction of MPBs (e.g., in PZT and PMN-PT) show large piezoelectricity. It remains a long-standing puzzle why not all MPBs yield large piezoelectricity. Here, by a comparative study of two MPB systems of BaZr0.2Ti0.8O3-PbTiO3 (BZT-PT) and Bi0.5Na0.5TiO3-PbTiO3 (BNT-PT), we find that the symmetry relation between the two ferroelectric phases at MPB plays a crucial role in piezoelectric activity. BZT-PT ceramic, having an R3m/P4mm MPB, exhibits strong piezoelectricity of d 33 = 500 pC/N, whereas BNT-PT ceramic, having an R3c/P4mm MPB, exhibits much weaker piezoelectricity of d 33 = 150 pC/N. The sharp difference between the two types of R/T MPBs is shown to arise from a significant difference in free energy barrier associated with oxygen octahedral tilting at MPB. An oxygen octahedral tilting/untilting transition at R3c/P4mm MPB leads to a high free energy barrier and results in low piezoelectric activity, whereas R3m/P4mm MPB with octahedral untilting produces large piezoelectricity due to low free energy barrier at such MPB. This feature show quite general in most of present MPBs systems and may become a dependable guideline for designing highly piezoelectric Pb-free materials.
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      PubDate: 2017-06-13T12:04:53Z
      DOI: 10.1016/j.actamat.2017.05.066
      Issue No: Vol. 134 (2017)
       
  • Modal analysis of dislocation vibration and reaction attempt frequency
    • Authors: Cameron Sobie; Laurent Capolungo; David L. McDowell; Enrique Martinez
      Pages: 203 - 210
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): Cameron Sobie, Laurent Capolungo, David L. McDowell, Enrique Martinez
      Transition state theory is a fundamental approach for temporal coarse-graining. It estimates the reaction rate for a transition processes by quantifying the activation free energy and attempt frequency for the unit process. To calculate the transition rate of a gliding dislocation, the attempt frequency is often obtained from line tension estimates of dislocation vibrations, a highly simplified model of dislocation behavior. This work revisits the calculation of attempt frequency for a dislocation bypassing an obstacle, in this case a self-interstitial atom (SIA) loop. First, a direct calculation of the vibrational characteristics of a finite pinned dislocation segment is compared to line tension estimates before moving to the more complex case of dislocation-obstacle bypass. The entropic factor associated with the attempt frequency is calculated for a finite dislocation segment and for an infinite glide dislocation interacting with an SIA loop. It is found to be dislocation length independent for three cases of dislocation-self interstitial atom (SIA) loop interactions.
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      PubDate: 2017-06-13T12:04:53Z
      DOI: 10.1016/j.actamat.2017.02.005
      Issue No: Vol. 134 (2017)
       
  • High strength NiTiHf shape memory alloys with tailorable properties
    • Authors: S.M. Saghaian; H.E. Karaca; H. Tobe; A.S. Turabi; S. Saedi; S.E. Saghaian; Y.I. Chumlyakov; R.D. Noebe
      Pages: 211 - 220
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): S.M. Saghaian, H.E. Karaca, H. Tobe, A.S. Turabi, S. Saedi, S.E. Saghaian, Y.I. Chumlyakov, R.D. Noebe
      Microstructure of NiTiHf shape memory alloys can be engineered to have high strength and operate at high stress levels for a large temperature window. Nanoprecipitation is well-known method to improve the strength of materials but it can be employed to NiTiHf alloys to substantially alter their phase transformation characteristics (martensite morphology, transformation strain, hysteresis and stress). The martensitic transformation of Ni-rich Ni51.2Ti28.8Hf20 was severely suppressed in the solution treated condition (900 °C-3h/water quench) and after aging at low temperatures, while the transformation temperatures were greater than 100 °C after 650 °C-3h aging. Generation of nanosize precipitates (∼20 nm in size) after 3 h aging at 450 °C and 550 °C improved the strength of the material, resulting in a near perfect dimensional stability during isobaric thermal cycling at stress levels of greater than 1500 MPa, with work output of 20–30 J cm−3. Superelastic behavior with 4% recoverable strain was demonstrated at low temperatures (−20 to 40 °C) after aging at 450 °C-3h and at elevated temperatures (120–160 °C) after aging at 550 °C-3h, with stresses reaching 2 GPa without the onset of plastic deformation. A clear relationship between thermal treatments, microstructure, mechanical and shape memory properties will be shown.
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      PubDate: 2017-06-13T12:04:53Z
      DOI: 10.1016/j.actamat.2017.05.065
      Issue No: Vol. 134 (2017)
       
  • Effect of cooling rate after solution treatment on subsequent phase
           separation during aging of Fe-Cr alloys: A small-angle neutron scattering
           study
    • Authors: Xin Xu; Joakim Odqvist; Magnus Hörnqvist Colliander; Stephen King; Mattias Thuvander; Axel Steuwer; Peter Hedström
      Pages: 221 - 229
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): Xin Xu, Joakim Odqvist, Magnus Hörnqvist Colliander, Stephen King, Mattias Thuvander, Axel Steuwer, Peter Hedström
      The effect of cooling rate after solution treatment on the initial structure of concentrated binary Fe-Cr alloys and the effect of the initial structure on phase separation during subsequent aging has been investigated. The nano-scale compositional fluctuations in the bulk of the alloys are studied using small-angle neutron scattering and the results are compared with simulations using the Cahn-Hilliard-Cook (CHC) model. The alloys investigated represent different mechanisms of phase separation and at higher Cr content, when spinodal decomposition (SD) is favored, the initial Cr compositional fluctuations due to slow cooling after solution treatment reduce the kinetics of phase decomposition, whereas, at lower Cr composition when nucleation and growth is favored, the kinetics of phase decomposition is more rapid. Regardless of the nominal Cr composition of the alloy, the phase decomposition after extended aging up to 300 h at 748 K is always larger for the more non-random initial structure. The CHC modeling of the cooling process and subsequent initial aging (below 10 h) is in reasonable qualitative agreement with the experimental results for the Fe-40 wt.% Cr alloy decomposing via SD. However, the modeling approach must be refined for accurate quantitative modeling of the full SD process, including coarsening.
      Graphical abstract image

      PubDate: 2017-06-13T12:04:53Z
      DOI: 10.1016/j.actamat.2017.06.001
      Issue No: Vol. 134 (2017)
       
  • A 2-D nucleation-growth model of spheroidal graphite
    • Authors: Jacques Lacaze; Jacques Bourdie; Manuel Jesus Castro-Román
      Pages: 230 - 235
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): Jacques Lacaze, Jacques Bourdie, Manuel Jesus Castro-Román
      Analysis of recent experimental investigations, in particular by transmission electron microscopy, suggests spheroidal graphite grows by 2-D nucleation of new graphite layers at the outer surface of the nodules. These layers spread over the surface along the prismatic direction of graphite which is the energetically preferred growth direction of graphite when the apparent growth direction of the nodules is along the basal direction of graphite. 2-D nucleation-growth models first developed for precipitation of pure substances are then adapted to graphite growth from the liquid in spheroidal graphite cast irons. Lateral extension of the new graphite layers is controlled by carbon diffusion in the liquid. This allows describing quantitatively previous experimental results giving strong support to this approach.
      Graphical abstract image

      PubDate: 2017-06-13T12:04:53Z
      DOI: 10.1016/j.actamat.2017.05.032
      Issue No: Vol. 134 (2017)
       
  • Giant and reversible room-temperature magnetocaloric effect in Ti-doped
           Ni-Co-Mn-Sn magnetic shape memory alloys
    • Authors: Y.H. Qu; D.Y. Cong; X.M. Sun; Z.H. Nie; W.Y. Gui; R.G. Li; Y. Ren; Y.D. Wang
      Pages: 236 - 248
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): Y.H. Qu, D.Y. Cong, X.M. Sun, Z.H. Nie, W.Y. Gui, R.G. Li, Y. Ren, Y.D. Wang
      The practical applications of magnetocaloric materials for magnetic refrigeration require not only large magnitude but also reversibility, as well as room-temperature operating temperature, of the magnetocaloric effect. However, it is a complex problem to simultaneously meet these demands in the Heusler-type Ni-(Co)-Mn-X (X = In, Sn, Sb, Ga) magnetic shape memory alloys which are a promising candidate for magnetocaloric materials, owing to the interdependence of magnetostructural transformation parameters. Here, through synergic tuning of magnetostructural transformation parameters via alloying with Ti in Ni42-x Ti x Co9Mn39Sn10 alloys, we achieved a large reversible room-temperature magnetocaloric effect. By alloying with a proper amount of Ti, the martensitic transformation temperature was brought down to room temperature and the sensitivity of transformation temperature to magnetic field change was greatly enhanced with the transformation entropy change still remaining a large value, while the thermal hysteresis and the transformation interval only slightly increased. Thus, the field required to induce the complete and reversible transformation was significantly reduced. As a result, a large reversible room-temperature magnetic entropy change ΔS m of 18.7 J kg−1 K−1 under 5 T was achieved in the Ni41Ti1Co9Mn39Sn10 alloy. This is the first report on reversible ΔS m in Ni-Mn-Sn-based Heusler alloys and the ΔS m we achieved represents the highest reversible ΔS m under 5 T reported heretofore in Ni-Mn-based Heusler alloys. Furthermore, the Ni41Ti1Co9Mn39Sn10 alloy shows good compressive properties and stable martensitic transformation during thermal cycling, beneficial for potential magnetocaloric applications. This study is instructive for the development of high-performance magnetocaloric materials for room-temperature magnetic refrigeration.
      Graphical abstract image

      PubDate: 2017-06-13T12:04:53Z
      DOI: 10.1016/j.actamat.2017.06.010
      Issue No: Vol. 134 (2017)
       
  • Mechanism of damage buildup in ion bombarded ZnO
    • Authors: A. Turos; P. Jóźwik; M. Wójcik; J. Gaca; R. Ratajczak; A. Stonert
      Pages: 249 - 256
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): A. Turos, P. Jóźwik, M. Wójcik, J. Gaca, R. Ratajczak, A. Stonert
      Ion beam induced defect processes in wide bandgap compound semiconductors are rather complex because of the formation of different types of defects and their transformations. In this paper, we report results of the study on the mechanism of defect formation, migration and agglomeration in 300 keV Ar-ion bombarded ZnO single crystals. RBS/c analysis performed with the use of the unique McChasy code allowed determination of depth distributions for different defect types separately. Complementary HRXRD analysis was applied to study lattice deformation due to ion bombardment. It has been observed that migration of simple defects and their agglomeration lead to the formation of two types of dislocation loops: basal loops located at the depth corresponding to the range of incident Ar ions and prismatic loops located beyond the ion range. The stress induced by defects in the bombarded layer has been identified as the driving force for defect migration and loop formation. As soon as the critical stress is attained bombarded layer undergoes plastic deformation resulting in the formation of a dense dislocation tangle.
      Graphical abstract image

      PubDate: 2017-06-13T12:04:53Z
      DOI: 10.1016/j.actamat.2017.06.005
      Issue No: Vol. 134 (2017)
       
  • In-situ studies on martensitic transformation and high-temperature shape
           memory in small volume zirconia
    • Authors: Xiao Mei Zeng; Zehui Du; Nobumichi Tamura; Qing Liu; Christopher A. Schuh; Chee Lip Gan
      Pages: 257 - 266
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): Xiao Mei Zeng, Zehui Du, Nobumichi Tamura, Qing Liu, Christopher A. Schuh, Chee Lip Gan
      Recently, the shape memory effect with significant recoverable shape deformation has been discovered in small volume single-crystal zirconia. The application potential for such shape memory ceramics has spurred in-depth exploration of the martensitic transformation crystallography and high temperature shape memory effect. In this work, the martensitic transformation of micron-sized zirconia has been studied in both a pristine as-produced condition and after micromechanical compression, using synchrotron scanning micro X-ray diffraction (μXRD). The pristine zirconia was found to transform into the monoclinic phase via a different crystallographic path than the compressed zirconia, resulting in distinct monoclinic variant preferences. The characteristic martensitic transformation temperatures were also found to be higher after uniaxial compression than in the pristine condition. Such observations provide insight on the differences between stress-induced and thermally-induced martensitic transformation. Moreover, we have observed a full cycle of the shape memory effect with large recoverable strain (7%) in micron-sized zirconia at a temperature of 400 °C. Our findings provide an important guideline to tailor the high-temperature shape memory properties of zirconia for further scientific research and engineering applications.
      Graphical abstract image

      PubDate: 2017-06-13T12:04:53Z
      DOI: 10.1016/j.actamat.2017.06.006
      Issue No: Vol. 134 (2017)
       
  • On the twinning shear of {101¯2} twins in magnesium – Experimental
           determination and formal description
    • Authors: Konstantin D. Molodov; Talal Al-Samman; Dmitri A. Molodov; Sandra Korte-Kerzel
      Pages: 267 - 273
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): Konstantin D. Molodov, Talal Al-Samman, Dmitri A. Molodov, Sandra Korte-Kerzel
      Specifically oriented Mg single crystals, aligned for c-axis extension, were deformed in channel-die plane strain compression in order to form { 10 1 ¯ 2 } extension twins. The amount of twinning shear for { 10 1 ¯ 2 } twins was determined directly from the deflection of marker lines to be s = 0.126 , in excellent agreement with the theoretical value. The twinning shear was discussed with respect to the shear-coupled grain boundary migration by analyzing the formal dislocation content of { 10 1 ¯ 2 } twin boundaries. The respective boundary migration – shear coupling factor is proposed to result from a combination of two elementary coupling modes, 〈 10 1 ¯ 0 〉 and [ 0001 ] .
      Graphical abstract image

      PubDate: 2017-06-18T16:58:00Z
      DOI: 10.1016/j.actamat.2017.05.041
      Issue No: Vol. 134 (2017)
       
  • Theoretical investigation of solid solution states of Ti1−xVxH2
    • Authors: Noriko Otani; Akihide Kuwabara; Takafumi Ogawa; Junko Matsuda; Atsuto Seko; Isao Tanaka; Etsuo Akiba
      Pages: 274 - 282
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): Noriko Otani, Akihide Kuwabara, Takafumi Ogawa, Junko Matsuda, Atsuto Seko, Isao Tanaka, Etsuo Akiba
      The solution states of metal atoms in Ti1−x V x H2, the stability of ordered ground-state structures, and the effect of metal-atom configurations on the bonding strength of H atoms are investigated by combining the cluster expansion method with first-principles calculations. Formation energies of all possible configurations of metal atoms in CaF2-type structures with up to sixteen metal atoms calculated using an optimized set of clusters are reported. Nine ground-state structures with different compositions are identified, most of which are found to have layered structures containing V bilayers. Analysis of the component clusters suggests that, while pair clusters contribute to formation of the layered structures, the higher-body clusters contribute more to stabilization of V bilayers than Ti bilayers. Based on calculations of short-range order parameters for all configurations of composition x = 1/2, low-energy structures including the layered structures appear to have a tendency for phase separation at the atomic scale. Order-disorder transition temperatures of the ground-state structures are estimated from Monte Carlo simulations to be less than 460 K. Comparison of lattice constants with experimental values as well as the low transition temperatures suggests that disordered solutions will be predominantly formed under typical synthesis conditions. In addition, we examine the stability of H atoms in different coordination environments and find that the metal-atom ordering does not greatly affect the local bonding between H and surrounding metal atoms.
      Graphical abstract image

      PubDate: 2017-06-18T16:58:00Z
      DOI: 10.1016/j.actamat.2017.04.073
      Issue No: Vol. 134 (2017)
       
  • Orientation dependent spheroidization response and macro-zone formation
           during sub β-transus processing of Ti-6Al-4V alloy
    • Authors: Shibayan Roy; Satyam Suwas
      Pages: 283 - 301
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): Shibayan Roy, Satyam Suwas
      In the present study, sub β-transus static annealing was carried out on a (α+β)-warm-rolled (700 °C, 90% thickness reduction) Ti-6Al-4V alloy at 900 °C for various durations (15, 30, 45, 60, 75 and 90 min). The key observation was that the spheroidization response of the constituent α-colonies from the starting warm-rolled material differs significantly during subsequent (α+β)-annealing treatment. While some of the α-colonies underwent an early morphological conversion from lamellar to equiaxed, some others stayed stable for prolonged durations. This unique phenomenon has been examined, for the first time, from an orientation perspective by coupling slip activation, boundary formation, and interfacial energy anisotropy of individual α-colonies with their response to static spheroidization. Orientation of the α-colonies in reference to the loading directions dictate the nature of slip activation (single versus multiple slip; basal or prism <a> slip plus pyramidal 〈c+a〉 slip) during prior (α+β)-rolling. During subsequent static (α+β)-annealing, this factor translates into the relative ease of boundary splitting and thermal grooving for them. Formation of longitudinal boundaries, then set the order of spheroidization for diffusion based coarsening processes during long term annealing. Anisotropy in interfacial energy, owing to the loss of coherency during prior deformation (warm-rolling), creates further orientation dependency in the spheroidization sequence. An immediate consequence of this orientation dependent spheroidization response is realized in the formation of the macro-zones for two-phase Titanium alloys after secondary thermomechanical processing. Some remedial processing strategies are highlighted therein.
      Graphical abstract image

      PubDate: 2017-06-18T16:58:00Z
      DOI: 10.1016/j.actamat.2017.04.071
      Issue No: Vol. 134 (2017)
       
  • The austenite/lath martensite interface in steels: Structure, athermal
           motion, and in-situ transformation strain revealed by simulation and
           theory
    • Authors: F. Maresca; W.A. Curtin
      Pages: 302 - 323
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): F. Maresca, W.A. Curtin
      The austenite/martensite (fcc/bcc) interface is prevalent across many new classes of high-strength steels, and yet both its fundamental structure and its mechanism of motion remain uncertain in spite of decades of research. Here, atomistic simulations are used to create an fcc-bcc iron interface having a structure and motion that match the major experimental observations on dislocated lath martensite. The simulated interface reveals a defect structure and a mechanism of glissile and athermal propagation that differ in important respects from longstanding assumptions. The atomistically-observed interface defects provide a basis for a parameter-free predictive crystallographic double-shear theory of lath martensite. Predictions of the theory match simulations well and yield very good agreement with experiments on Fe-Ni-Mn and Fe-C. The theory shows that the fcc/bcc lattice parameter ratio is the dominant factor for controlling the “shape” deformation – the overall, in-situ strain associated with the martensitic transformation – which is related to macroscopic toughening, and quantitatively rationalizes many experimental observations. This new understanding about the nature of this special interface provides fundamental insights needed for guiding design of emerging high-strength steels.
      Graphical abstract image

      PubDate: 2017-06-18T16:58:00Z
      DOI: 10.1016/j.actamat.2017.05.044
      Issue No: Vol. 134 (2017)
       
  • Multi-physics modeling of single/multiple-track defect mechanisms in
           electron beam selective melting
    • Authors: Wentao Yan; Wenjun Ge; Ya Qian; Stephen Lin; Bin Zhou; Wing Kam Liu; Feng Lin; Gregory J. Wagner
      Pages: 324 - 333
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): Wentao Yan, Wenjun Ge, Ya Qian, Stephen Lin, Bin Zhou, Wing Kam Liu, Feng Lin, Gregory J. Wagner
      Metallic powder bed-based additive manufacturing technologies have many promising attributes. The single track acts as one fundamental building unit, which largely influences the final product quality such as the surface roughness and dimensional accuracy. A high-fidelity powder-scale model is developed to predict the detailed formation processes of single/multiple-track defects, including the balling effect, single track nonuniformity and inter-track voids. These processes are difficult to observe in experiments; previous studies have proposed different or even conflicting explanations. Our study clarifies the underlying formation mechanisms, reveals the influence of key factors, and guides the improvement of fabrication quality of single tracks. Additionally, the manufacturing processes of multiple tracks along S/Z-shaped scan paths with various hatching distance are simulated to further understand the defects in complex structures. The simulations demonstrate that the hatching distance should be no larger than the width of the remelted region within the substrate rather than the width of the melted region within the powder layer. Thus, single track simulations can provide valuable insight for complex structures.
      Graphical abstract image

      PubDate: 2017-06-18T16:58:00Z
      DOI: 10.1016/j.actamat.2017.05.061
      Issue No: Vol. 134 (2017)
       
  • Stacking fault energies of face-centered cubic concentrated solid solution
           alloys
    • Authors: Shijun Zhao; G. Malcolm Stocks; Yanwen Zhang
      Pages: 334 - 345
      Abstract: Publication date: 1 August 2017
      Source:Acta Materialia, Volume 134
      Author(s): Shijun Zhao, G. Malcolm Stocks, Yanwen Zhang
      We report the stacking fault energy (SFE) for a series of face-centered cubic (fcc) equiatomic concentrated solid solution alloys (CSAs) derived as subsystems from the NiCoFeCrMn and NiCoFeCrPd high entropy alloys based on ab initio calculations. At low temperatures, these CSAs display very low even negative SFEs, indicating that hexagonal close-pack (hcp) is more energy favorable than fcc structure. The temperature dependence of SFE for some CSAs is studied. With increasing temperature, a hcp-to-fcc transition is revealed for those CSAs with negative SFEs, which can be attributed to the role of intrinsic vibrational entropy. The analysis of the vibrational modes suggests that the vibrational entropy arises from the high frequency states in the hcp structure that originate from local vibrational mode. Our results underscore the importance of vibrational entropy in determining the temperature dependence of SFE for CSAs.
      Graphical abstract image

      PubDate: 2017-06-24T02:23:53Z
      DOI: 10.1016/j.actamat.2017.05.001
      Issue No: Vol. 134 (2017)
       
 
 
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