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Publisher: Elsevier   (Total: 3118 journals)

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Showing 1 - 200 of 3118 Journals sorted alphabetically
A Practical Logic of Cognitive Systems     Full-text available via subscription   (Followers: 7)
AASRI Procedia     Open Access   (Followers: 15)
Academic Pediatrics     Hybrid Journal   (Followers: 25, SJR: 1.402, h-index: 51)
Academic Radiology     Hybrid Journal   (Followers: 22, SJR: 1.008, h-index: 75)
Accident Analysis & Prevention     Partially Free   (Followers: 89, SJR: 1.109, h-index: 94)
Accounting Forum     Hybrid Journal   (Followers: 25, SJR: 0.612, h-index: 27)
Accounting, Organizations and Society     Hybrid Journal   (Followers: 30, 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: 371, SJR: 0.726, h-index: 43)
Acta Automatica Sinica     Full-text available via subscription   (Followers: 3)
Acta Biomaterialia     Hybrid Journal   (Followers: 27, SJR: 2.02, h-index: 104)
Acta Colombiana de Cuidado Intensivo     Full-text available via subscription   (Followers: 1)
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   (Followers: 1, SJR: 0.123, h-index: 8)
Acta Histochemica     Hybrid Journal   (Followers: 3, SJR: 0.604, h-index: 38)
Acta Materialia     Hybrid Journal   (Followers: 235, SJR: 3.683, h-index: 202)
Acta Mathematica Scientia     Full-text available via subscription   (Followers: 5, SJR: 0.615, h-index: 21)
Acta Mechanica Solida Sinica     Full-text available via subscription   (Followers: 9, SJR: 0.442, h-index: 21)
Acta Oecologica     Hybrid Journal   (Followers: 10, SJR: 0.915, h-index: 53)
Acta Otorrinolaringologica (English Edition)     Full-text available via subscription   (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: 25, SJR: 1.365, h-index: 73)
Acta Sociológica     Open Access  
Acta Tropica     Hybrid Journal   (Followers: 6, SJR: 1.059, h-index: 77)
Acta Urológica Portuguesa     Open Access  
Actas Dermo-Sifiliograficas     Full-text available via subscription   (Followers: 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: 5)
Acute Pain     Full-text available via subscription   (Followers: 13)
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: 6)
Additive Manufacturing     Hybrid Journal   (Followers: 7, SJR: 1.039, h-index: 5)
Additives for Polymers     Full-text available via subscription   (Followers: 22)
Advanced Cement Based Materials     Full-text available via subscription   (Followers: 3)
Advanced Drug Delivery Reviews     Hybrid Journal   (Followers: 136, 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: 17, 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: 27, 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: 11, SJR: 0.801, h-index: 26)
Advances in Applied Microbiology     Full-text available via subscription   (Followers: 22, SJR: 1.286, h-index: 49)
Advances In Atomic, Molecular, and Optical Physics     Full-text available via subscription   (Followers: 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: 26, 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 Cell Aging and Gerontology     Full-text available via subscription   (Followers: 4)
Advances in Cellular and Molecular Biology of Membranes and Organelles     Full-text available via subscription   (Followers: 13)
Advances in Chemical Engineering     Full-text available via subscription   (Followers: 26, 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: 9, SJR: 1.268, h-index: 45)
Advances in Clinical Chemistry     Full-text available via subscription   (Followers: 29, 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 Dermatology     Full-text available via subscription   (Followers: 12)
Advances in Developmental Biology     Full-text available via subscription   (Followers: 12)
Advances in Digestive Medicine     Open Access   (Followers: 7)
Advances in DNA Sequence-Specific Agents     Full-text available via subscription   (Followers: 6)
Advances in Drug Research     Full-text available via subscription   (Followers: 22)
Advances in Ecological Research     Full-text available via subscription   (Followers: 46, SJR: 3.25, h-index: 43)
Advances in Engineering Software     Hybrid Journal   (Followers: 26, SJR: 0.486, h-index: 10)
Advances in Experimental Biology     Full-text available via subscription   (Followers: 8)
Advances in Experimental Social Psychology     Full-text available via subscription   (Followers: 44, 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: 52, SJR: 0.674, h-index: 38)
Advances in Fuel Cells     Full-text available via subscription   (Followers: 16)
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: 11)
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: 22, SJR: 0.906, h-index: 24)
Advances in Heterocyclic Chemistry     Full-text available via subscription   (Followers: 9, SJR: 0.497, h-index: 31)
Advances in Human Factors/Ergonomics     Full-text available via subscription   (Followers: 26)
Advances in Imaging and Electron Physics     Full-text available via subscription   (Followers: 2, SJR: 0.396, h-index: 27)
Advances in Immunology     Full-text available via subscription   (Followers: 36, SJR: 4.152, h-index: 85)
Advances in Inorganic Chemistry     Full-text available via subscription   (Followers: 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: 6)
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: 6, SJR: 0.489, h-index: 25)
Advances in Medicinal Chemistry     Full-text available via subscription   (Followers: 5)
Advances in Microbial Physiology     Full-text available via subscription   (Followers: 5, SJR: 1.44, h-index: 51)
Advances in Molecular and Cell Biology     Full-text available via subscription   (Followers: 23)
Advances in Molecular and Cellular Endocrinology     Full-text available via subscription   (Followers: 10)
Advances in Molecular Toxicology     Full-text available via subscription   (Followers: 9, 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 Organ Biology     Full-text available via subscription   (Followers: 2)
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: 24, 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: 8, SJR: 0.384, h-index: 26)
Advances in Phytomedicine     Full-text available via subscription  
Advances in Planar Lipid Bilayers and Liposomes     Full-text available via subscription   (Followers: 3, SJR: 0.248, h-index: 11)
Advances in Plant Biochemistry and Molecular Biology     Full-text available via subscription   (Followers: 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: 20, SJR: 1.5, h-index: 62)
Advances in Psychology     Full-text available via subscription   (Followers: 62)
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: 3, SJR: 0.1, h-index: 2)
Advances in Space Biology and Medicine     Full-text available via subscription   (Followers: 5)
Advances in Space Research     Full-text available via subscription   (Followers: 366, 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: 8, SJR: 0.823, h-index: 27)
Advances in the Study of Behavior     Full-text available via subscription   (Followers: 31, SJR: 1.321, h-index: 56)
Advances in Veterinary Medicine     Full-text available via subscription   (Followers: 16)
Advances in Veterinary Science and Comparative Medicine     Full-text available via subscription   (Followers: 13)
Advances in Virus Research     Full-text available via subscription   (Followers: 6, SJR: 1.878, h-index: 68)
Advances in Water Resources     Hybrid Journal   (Followers: 45, 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: 333, SJR: 0.816, h-index: 49)
AEU - Intl. J. of Electronics and Communications     Hybrid Journal   (Followers: 8, SJR: 0.318, h-index: 36)
African J. of Emergency Medicine     Open Access   (Followers: 5, SJR: 0.344, h-index: 6)
Ageing Research Reviews     Hybrid Journal   (Followers: 9, SJR: 3.289, h-index: 78)
Aggression and Violent Behavior     Hybrid Journal   (Followers: 445, SJR: 1.385, h-index: 72)
Agri Gene     Hybrid Journal  
Agricultural and Forest Meteorology     Hybrid Journal   (Followers: 15, SJR: 2.18, h-index: 116)
Agricultural Systems     Hybrid Journal   (Followers: 31, SJR: 1.275, h-index: 74)
Agricultural Water Management     Hybrid Journal   (Followers: 41, SJR: 1.546, h-index: 79)
Agriculture and Agricultural Science Procedia     Open Access  
Agriculture and Natural Resources     Open Access   (Followers: 3)
Agriculture, Ecosystems & Environment     Hybrid Journal   (Followers: 56, SJR: 1.879, h-index: 120)
Ain Shams Engineering J.     Open Access   (Followers: 5, SJR: 0.434, h-index: 14)
Air Medical J.     Hybrid Journal   (Followers: 5, SJR: 0.234, h-index: 18)
AKCE Intl. J. of Graphs and Combinatorics     Open Access   (SJR: 0.285, h-index: 3)
Alcohol     Hybrid Journal   (Followers: 11, SJR: 0.922, h-index: 66)
Alcoholism and Drug Addiction     Open Access   (Followers: 8)
Alergologia Polska : Polish J. of Allergology     Full-text available via subscription   (Followers: 1)
Alexandria Engineering J.     Open Access   (Followers: 1, SJR: 0.436, h-index: 12)
Alexandria J. of Medicine     Open Access   (Followers: 1)
Algal Research     Partially Free   (Followers: 9, SJR: 2.05, h-index: 20)
Alkaloids: Chemical and Biological Perspectives     Full-text available via subscription   (Followers: 3)
Allergologia et Immunopathologia     Full-text available via subscription   (Followers: 1, SJR: 0.46, h-index: 29)
Allergology Intl.     Open Access   (Followers: 4, SJR: 0.776, h-index: 35)
Alpha Omegan     Full-text available via subscription   (SJR: 0.121, h-index: 9)
ALTER - European J. of Disability Research / Revue Européenne de Recherche sur le Handicap     Full-text available via subscription   (Followers: 9, SJR: 0.158, h-index: 9)
Alzheimer's & Dementia     Hybrid Journal   (Followers: 48, SJR: 4.289, h-index: 64)
Alzheimer's & Dementia: Diagnosis, Assessment & Disease Monitoring     Open Access   (Followers: 4)
Alzheimer's & Dementia: Translational Research & Clinical Interventions     Open Access   (Followers: 4)
Ambulatory Pediatrics     Hybrid Journal   (Followers: 5)
American Heart J.     Hybrid Journal   (Followers: 49, SJR: 3.157, h-index: 153)
American J. of Cardiology     Hybrid Journal   (Followers: 48, SJR: 2.063, h-index: 186)
American J. of Emergency Medicine     Hybrid Journal   (Followers: 41, SJR: 0.574, h-index: 65)
American J. of Geriatric Pharmacotherapy     Full-text available via subscription   (Followers: 9, SJR: 1.091, h-index: 45)
American J. of Geriatric Psychiatry     Hybrid Journal   (Followers: 14, SJR: 1.653, h-index: 93)
American J. of Human Genetics     Hybrid Journal   (Followers: 32, SJR: 8.769, h-index: 256)
American J. of Infection Control     Hybrid Journal   (Followers: 26, SJR: 1.259, h-index: 81)
American J. of Kidney Diseases     Hybrid Journal   (Followers: 31, SJR: 2.313, h-index: 172)
American J. of Medicine     Hybrid Journal   (Followers: 46, 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: 201, SJR: 2.255, h-index: 171)
American J. of Ophthalmology     Hybrid Journal   (Followers: 60, SJR: 2.803, h-index: 148)
American J. of Ophthalmology Case Reports     Open Access   (Followers: 6)
American J. of Orthodontics and Dentofacial Orthopedics     Full-text available via subscription   (Followers: 6, SJR: 1.249, h-index: 88)
American J. of Otolaryngology     Hybrid Journal   (Followers: 24, SJR: 0.59, h-index: 45)
American J. of Pathology     Hybrid Journal   (Followers: 27, SJR: 2.653, h-index: 228)
American J. of Preventive Medicine     Hybrid Journal   (Followers: 26, SJR: 2.764, h-index: 154)
American J. of Surgery     Hybrid Journal   (Followers: 35, SJR: 1.286, h-index: 125)
American J. of the Medical Sciences     Hybrid Journal   (Followers: 12, SJR: 0.653, h-index: 70)
Ampersand : An Intl. J. of General and Applied Linguistics     Open Access   (Followers: 6)
Anaerobe     Hybrid Journal   (Followers: 4, SJR: 1.066, h-index: 51)
Anaesthesia & Intensive Care Medicine     Full-text available via subscription   (Followers: 59, SJR: 0.124, h-index: 9)
Anaesthesia Critical Care & Pain Medicine     Full-text available via subscription   (Followers: 13)
Anales de Cirugia Vascular     Full-text available via subscription  
Anales de Pediatría     Full-text available via subscription   (Followers: 2, SJR: 0.209, h-index: 27)
Anales de Pediatría (English Edition)     Full-text available via subscription  
Anales de Pediatría Continuada     Full-text available via subscription   (SJR: 0.104, h-index: 3)
Analytic Methods in Accident Research     Hybrid Journal   (Followers: 4, SJR: 2.577, h-index: 7)
Analytica Chimica Acta     Hybrid Journal   (Followers: 36, SJR: 1.548, h-index: 152)
Analytical Biochemistry     Hybrid Journal   (Followers: 164, 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: 12)
Anesthésie & Réanimation     Full-text available via subscription   (Followers: 1)
Anesthesiology Clinics     Full-text available via subscription   (Followers: 22, SJR: 0.421, h-index: 40)
Angiología     Full-text available via subscription   (SJR: 0.124, h-index: 9)
Angiologia e Cirurgia Vascular     Open Access  

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Journal Cover Acta Materialia
  [SJR: 3.683]   [H-I: 202]   [235 followers]  Follow
    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 1359-6454
   Published by Elsevier Homepage  [3118 journals]
  • Lattice-alignment mechanism of SiGe on Sapphire
    • Authors: Hyun Jung Kim; Adam Duzik; Sang H. Choi
      Pages: 1 - 7
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): Hyun Jung Kim, Adam Duzik, Sang H. Choi
      Heteroepitaxy of silicon germanium (SiGe) prepared on a sapphire substrate (Al2O3) requires scrupulous attention to growth conditions. Previous work was used a substrate temperature of 890°C to grow a SiGe (111) film on the trigonal sapphire (0001) substrate without twin defects. Although the growth conditions were effective for the formation of single crystal film, how the formation of SiGe at the interface of sapphire was not experimentally defined with the order of atomic arrangement. This work presents high resolution transmission electron microscope (TEM) images of the SiGe/Al2O3 interface to show the SiGe/Al2O3 interface bonding for heteroepitaxy mechanism. The first two monolayers of the SiGe are Si-rich as this match with the surface oxygen lattice of the Al2O3 substrate. After the Ge composition increases, the monolayer spacing also increased while maintaining the cubic crystal structure. These results highlight the importance of a cleanliness of sapphire substrate, the Al2O3 termination for SiGe growth, and the cubic structure deformation of SiGe for heteroepitaxy. From the essential understanding of the SiGe/Al2O3 interface and growth mechanism, both low temperature SiGe heteroepitaxy and the III-V or II-VI semiconductor epitaxy are possible.
      Graphical abstract image

      PubDate: 2017-12-13T08:05:17Z
      DOI: 10.1016/j.actamat.2017.11.031
      Issue No: Vol. 145 (2017)
       
  • Metastable phase transformation and deformation twinning induced
           hardening-stiffening mechanism in compression of silicon nanoparticles
    • Authors: Yu Hong; Ning Zhang; Mohsen Asle Zaeem
      Pages: 8 - 18
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): Yu Hong, Ning Zhang, Mohsen Asle Zaeem
      The compressive mechanical responses of silicon nanoparticles with respect to crystallographic orientations are investigated by atomistic simulations. Superelastic and abrupt hardening-stiffening behaviors are revealed in [110]-, [111]- and [112]-oriented nanoparticles. The obtained hardness values of these particles are in good agreement with the experimental results. In particular, [111]-oriented particle is extremely hard since its hardness (∼33.7 GPa) is almost three times greater than that of the bulk silicon (∼12 GPa). To understand the underlying deformation mechanisms, metastable phase transformation is detected in these particles. Deformation twinning of the metastable phase accounts for the early hardening-stiffening behavior observed in [110]-oriented particle. The twin phase then coalescences and undergoes compression to resist further deformation, and leads to the subsequent re-hardening and re-stiffening events. The same metastable phase is also detected to form in [111]- and [112]-oriented particles. The compression of such metastable phase is responsible for their hardening-stiffening behavior. In contrast, the crystal lattice of diamond cubic silicon is merely elastically deformed when compressing along [100] direction. Throughout the simulations, no perfect tetragonal β-tin silicon phase formed due to the deconfinement status of nanoparticle comparing to the bulk silicon. A size effect on hardness of silicon nanoparticles, i.e., “smaller is harder”, is also revealed.
      Graphical abstract image

      PubDate: 2017-12-13T08:05:17Z
      DOI: 10.1016/j.actamat.2017.11.034
      Issue No: Vol. 145 (2017)
       
  • Sample-size-dependent surface dislocation nucleation in nanoscale crystals
    • Authors: Qing-Jie Li; Bin Xu; Shotaro Hara; Ju Li; Evan Ma
      Pages: 19 - 29
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): Qing-Jie Li, Bin Xu, Shotaro Hara, Ju Li, Evan Ma
      The finite-temperature mechanical strength of nanoscale pristine metals at laboratory strain rates may be controlled by surface dislocation nucleation, which was hypothesized to be only weakly dependent on the sample size. Previous studies on surface dislocation nucleation investigated factors such as surface steps, oxidation layers and surface diffusion, while the role of surface stresses and sample size remains unclear. Here we perform systematic atomistic calculations on the activation free energy barriers of surface dislocation nucleation in sub-50 nm nanowires. The results demonstrate that surface stresses significantly influence the activation processes of surface dislocation nucleation. This renders the strength strongly dependent on sample size; whether it is “smaller is stronger” or “smaller is weaker” depends on the combined effects of surface stress and applied axial stress, which can be universally explained in terms of the local maximum resolved shear stress. A linear relation between the activation entropy and activation enthalpy (Meyer-Neldel compensation rule) was found to work well across a range of stresses and sample sizes.
      Graphical abstract image

      PubDate: 2017-12-13T08:05:17Z
      DOI: 10.1016/j.actamat.2017.11.048
      Issue No: Vol. 145 (2017)
       
  • Atomic and electronic basis for solutes strengthened (010) anti-phase
           boundary of L12 Co3(Al, TM): A comprehensive first-principles study
    • Authors: William Yi Wang; Fei Xue; Ying Zhang; Shun-Li Shang; Yi Wang; Kristopher A. Darling; Laszlo J. Kecskes; Jinshan Li; Xidong Hui; Qiang Feng; Zi-Kui Liu
      Pages: 30 - 40
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): William Yi Wang, Fei Xue, Ying Zhang, Shun-Li Shang, Yi Wang, Kristopher A. Darling, Laszlo J. Kecskes, Jinshan Li, Xidong Hui, Qiang Feng, Zi-Kui Liu
      The crystallographic and electronic structures of (010) APB of L12 Co3Al0.75TM0.25 are studied by high-resolution transmission electron microscopy and first-principles calculations. Effects of solute atoms (TM = Cr, Hf, Mo, Ni, Re, Ru, Ta, Ti, W and Y) on the formation energy, lattice parameters/distortion, magnetism, and bonding strength of the (010) APB in Co3Al0.75TM0.25 are obtained from first-principles calculations. Comparing to the equilibrium volume of Co3Al, it is found that the volume change of the Co3Al0.75TM0.25 with and without the presence of APB increases linearly with the volume of the corresponding FCC elements, indicating the contribution of the solute atoms on lattice distortion of bulk and (010) APB. Particularly, the strong dependence of the APB energy on the composition is comprehensively discussed together with the available experimental and theoretical data in the literature. The negative (010) APB energy indicates that the formation of (010) APB could stabilize the ordered L12 (or the FCC-lattice) Co3Al, and the local L12 → D022 phase transformation can occur. The physical natures of lattice distortions caused by the fault layers of APB and the solute atoms are characterized by bonding charge density. It is found that the solute atoms, occupying Al site of L12 phase and its (010) APB, increase the local bonding strength along (010) through the electron redistribution during forming the chemical bonds with Co, revealing an intrinsic solid-solution strengthening mechanism. This work provides an insight into the atomic and electronic basis for solid-solution strengthening mechanism of L12 Co3Al0.75TM0.25.
      Graphical abstract image

      PubDate: 2017-12-13T08:05:17Z
      DOI: 10.1016/j.actamat.2017.10.041
      Issue No: Vol. 145 (2017)
       
  • Prediction of ceramic fracture with normal distribution pertinent to grain
           size
    • Authors: Chunguo Zhang; Xiaozhi Hu; Tim Sercombe; Qingbin Li; Zhimin Wu; Pengmin Lu
      Pages: 41 - 48
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): Chunguo Zhang, Xiaozhi Hu, Tim Sercombe, Qingbin Li, Zhimin Wu, Pengmin Lu
      Fracture of brittle ceramics initiated from shallow surface cracks comparable to their average grain sizes (G) can fluctuate significantly. Such fluctuations can contain crucial information on the inherent relations between the average grain size G and bulk ceramic properties such as the tensile strength f t and fracture toughness K IC. It was proposed in this study that the characteristic crack a*ch = 0.25(K IC/f t)2 = constant × G, inspired by observations of strength distributions with different a*ch/G ratios. It was found that normal distributions with the smallest standard deviation exist around a*ch = (2.5–3.5) × G, based on quasi-brittle fracture results of four different ceramics with G from 2 to 20 μm and shallow surface cracks from 100 nm to 650 μm. Using the average value of the relative characteristic crack a*ch/G ≈ 3, the mean and standard deviation (σ) were determined by normal distributions for both the tensile strength ft and fracture toughness K IC. Quasi-brittle fracture of those fine-grained ceramics based on the mean values and standard deviations was thus predicted. The upper and lower bounds with 96% reliability (±2σ) specified by the normal distributions covered nearly all experimental data ranging from the strength-controlled to toughness-controlled asymptotic limits, and quasi-brittle fracture between the two. With the knowledge of the average grain size G, the tensile strength f t becomes the sole parameter required to describe the entire fracture range.
      Graphical abstract image

      PubDate: 2017-12-13T08:05:17Z
      DOI: 10.1016/j.actamat.2017.11.041
      Issue No: Vol. 145 (2017)
       
  • The Third Law of Thermodynamics: Phase equilibria and phase diagrams at
           low temperatures
    • Authors: David E. Laughlin; William A. Soffa
      Pages: 49 - 61
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): David E. Laughlin, William A. Soffa
      Great progress has been made over the recent decades in the application of computational thermodynamics (Calphad) and theoretical methodologies (CVM) including so-called first principles approaches to modeling thermodynamic properties and the calculation of phase diagrams of materials. The aim of this paper is to call attention to considerations of the THIRD LAW OF THERMODYNAMICS when evaluating these results when applied to low temperature phase equilibria. In this effort we call attention to the essential content of the modern version of this third principle of thermodynamics using an historical and pedagogical approach. An appreciation of the constraints of the THIRD LAW is shown to be valuable in projecting possible low temperature phase fields and boundaries and predicting thermodynamically consistent phase diagram configurations as T→0 K. The ideas of Simon regarding aspects or subsystems are shown to be of paramount importance in assessing the thermodynamic properties of materials at low temperatures.

      PubDate: 2017-12-13T08:05:17Z
      DOI: 10.1016/j.actamat.2017.11.037
      Issue No: Vol. 145 (2017)
       
  • Microstructural effects on effective piezoelectric responses of textured
           PMN-PT ceramics
    • Authors: Chen Ming; Tiannan Yang; Kun Luan; Lei Chen; Liang Wang; Jiangtao Zeng; Yongxiang Li; Wenqing Zhang; Long-Qing Chen
      Pages: 62 - 70
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): Chen Ming, Tiannan Yang, Kun Luan, Lei Chen, Liang Wang, Jiangtao Zeng, Yongxiang Li, Wenqing Zhang, Long-Qing Chen
      The effective piezoelectric properties of [001]c fiber textured Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) ceramics were studied by phase-field modeling. The effects of microstructures such as texture, grain shape, grain boundaries, residual pores and heterogeneous growth templates were investigated. It was found that the degree of texture plays a dominant role in determining the properties. The pores, heterogeneous templates and grain boundaries reduce the properties significantly at high degrees of texture with the effect diminishing at decreasing degrees of texture. The presence of heterogeneous templates leads to a more significant reduction in the properties than pores although the piezoelectric coefficients of pores are zero. The shape of grains has a weak effect at all degrees of texture. By utilizing the experimentally measured microstructural parameters in the calculations and comparing the computed properties with the corresponding measurements, we showed that the low performance of sintered textured PMN-PT ceramics ( d 33 ∼1000 pC/N) relative to single crystals ( d 33 ∼2800 pC/N) is mainly due to the insufficiently high degree of texture even with Lotgering factors up to 0.9, while the influences of other microstructures are weak.
      Graphical abstract image

      PubDate: 2017-12-13T08:05:17Z
      DOI: 10.1016/j.actamat.2017.11.043
      Issue No: Vol. 145 (2017)
       
  • Composition and automated crystal orientation mapping of rapid
           solidification products in hypoeutectic Al-4 at.%Cu alloys
    • Authors: K.W. Zweiacker; Can Liu; M.A. Gordillo; J.T. McKeown; G.H. Campbell; J.M.K. Wiezorek
      Pages: 71 - 83
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): K.W. Zweiacker, Can Liu, M.A. Gordillo, J.T. McKeown, G.H. Campbell, J.M.K. Wiezorek
      Rapid solidification can produce metastable phases and unusual microstructure modifications in multi-component alloys during additive manufacturing or laser beam welding. Composition and phase mapping by transmission electron microscopy have been used here to characterize the morphologically distinct zones developing in hypoeutectic Al-4 at.% Cu alloy after pulsed laser melting for different crystal growth rate regimes. Deviations of the compositions of the alloy phases from equilibrium predictions and unique orientation relationships between the solidification transformation products have been determined. Specifically, for the columnar growth zone at solidification rates of 0.8 m s − 1 < v < v a = 1.8 m s − 1 , two distinct orientation relationships were established between the concomitantly forming non-equilibrium phases, supersaturated α-Al solid solution and the discontinuously distributed α-Al2Cu-based θ′-phase, which can be described as {110}θ ∥ {001}α, [001]θ ∥ [110]α and {001}θ ∥ {001}α, [100]θ ∥ [100]α. These orientation relationships permit formation of coherent interphase interfaces with low interfacial free energy. This endows a kinetic advantage to the thermodynamically less stable θ′-Al2Cu phase relative to the more stable equilibrium θ-Al2Cu phase during formation of the morphologically modified eutectic of the columnar growth zone grains, since repeated nucleation is required to establish the discontinuous distribution of θ′-Al2Cu phase.
      Graphical abstract image

      PubDate: 2017-12-13T08:05:17Z
      DOI: 10.1016/j.actamat.2017.11.040
      Issue No: Vol. 145 (2017)
       
  • Investigating nano-precipitation in a V-containing HSLA steel using small
           angle neutron scattering
    • Authors: Y.Q. Wang; S.J. Clark; V. Janik; R.K. Heenan; D. Alba Venero; K. Yan; D.G. McCartney; S. Sridhar; P.D. Lee
      Pages: 84 - 96
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): Y.Q. Wang, S.J. Clark, V. Janik, R.K. Heenan, D. Alba Venero, K. Yan, D.G. McCartney, S. Sridhar, P.D. Lee
      Interphase precipitation (IPP) of nanoscale carbides in a vanadium-containing high-strength low-alloy steel has been investigated. Small angle neutron scattering (SANS) and transmission electron microscopy (TEM) were employed to characterize the precipitates and their size distributions in Fe-0.047C-0.2V-1.6Mn (in wt.%) alloy samples which had been austenitized, isothermally transformed at 700 °C for between 3 and 600 min and water quenched. TEM confirms that, following heat treatment, rows of vanadium-containing nanoscale interphase precipitates were present. Model-independent analysis of the nuclear SANS signal and model fitting calculations, using oblate spheroid and disc-shapes, were performed. The major axis diameter increased from 18 nm after 3 min to 35 nm after 600 min. Precipitate volume percent increased from 0.09 to 0.22 vol% over the same period and number density fell from 2 × 1021 to 5 × 1020 m−3. A limited number of measurements of precipitate maximum diameters from TEM images showed the mean value increased from 8 nm after 5 min to 28 nm after 600 min which is in reasonable agreement with the SANS data.
      Graphical abstract image

      PubDate: 2017-12-13T08:05:17Z
      DOI: 10.1016/j.actamat.2017.11.032
      Issue No: Vol. 145 (2017)
       
  • First-principles modeling of superlattice intrinsic stacking fault
           energies in Ni3Al based alloys
    • Authors: A. Breidi; J. Allen; A. Mottura
      Pages: 97 - 108
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): A. Breidi, J. Allen, A. Mottura
      High-throughput quantum mechanics based simulations have been carried out to establish the change in lattice parameter and superlattice intrinsic stacking fault (SISF) formation energies in Ni3Al-based alloys using the axial Ising model. We had direct access to the variation in SISF energies due to finite compositional change of the added ternary transition metal (TM) element through constructing large supercells, which was equally necessary to account for chemical disorder. We find that most added TM ternaries induce an important quasi-linear increase in the SISF energy as a function of alloying composition x. The most pronounced increase corresponds to Fe addition, while Co addition decreases the SISF energy monotonically. Our results shed light on the role played by TM elements on strengthening L12 Ni3Al precipitates against stacking fault shear. The data are of high importance for designing new Ni-based superalloys based on computational approaches.
      Graphical abstract image

      PubDate: 2017-12-13T08:05:17Z
      DOI: 10.1016/j.actamat.2017.11.042
      Issue No: Vol. 145 (2017)
       
  • Strain-induced indium clustering in non-polar a-plane InGaN quantum wells
    • Authors: Ja Kyung Lee; Bumsu Park; Kyung Song; Woo Young Jung; Dmitry Tyutyunnikov; Tiannan Yang; Christoph T. Koch; Chan Gyung Park; Peter A. van Aken; Young-Min Kim; Jong Kyu Kim; Junhyeok Bang; Long-Qing Chen; Sang Ho Oh
      Pages: 109 - 122
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): Ja Kyung Lee, Bumsu Park, Kyung Song, Woo Young Jung, Dmitry Tyutyunnikov, Tiannan Yang, Christoph T. Koch, Chan Gyung Park, Peter A. van Aken, Young-Min Kim, Jong Kyu Kim, Junhyeok Bang, Long-Qing Chen, Sang Ho Oh
      In conventional light-emitting diodes the epitaxial strain and related piezoelectric polarization arising along the polar [0001] growth direction of the InGaN/GaN quantum wells (QWs) induce internal fields which adversely affect the radiative recombination of electron-hole pairs therein. Growing the quantum wells along a nonpolar orientation can, in principle, avoid this problem but seems to face with another problem associated with indium clustering. In this study, we present experimental evidence that supports the inhomogeneous distribution of indium in non-polar a-plane InGaN QWs by using dark-field inline electron holography as well as atom probe tomography measurements and discuss the possible origin by density functional theory calculation. A model non-polar a-plane QW structure with 10 nm-thick In0.1Ga0.9N double QWs was investigated and compared with the polar c-plane QWs with the same QW structure. Unlike the random distribution in the polar QWs, the indium atoms in the non-polar QW exhibit inhomogeneous distribution and show a tendency of periodic clustering. We suggest the dipole interaction energy and the strain energy associated with indium substitution could have a substantial influence on the local composition of strained InGaN QWs and, particularly, triggers In clustering in the non-polar a-plane QW structure. Accompanying phase field modeling rationalizes that In clustering can also modify the in-plane polarization through piezoelectric effects, preventing the electrostatic potential from diverging along the in-plane polar direction.
      Graphical abstract image

      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.11.039
      Issue No: Vol. 145 (2017)
       
  • Nano-phase separation sintering in nanostructure-stable vs. bulk-stable
           alloys
    • Authors: Mansoo Park; Tongjai Chookajorn; Christopher A. Schuh
      Pages: 123 - 133
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): Mansoo Park, Tongjai Chookajorn, Christopher A. Schuh
      Accelerated sintering through nanoscale-phase separation is explored in the Cr-Ni binary system. When the processing requirements of both supersaturation and nanocrystallinity are met in the initial powders, Cr-Ni alloys show an onset of sintering at a low temperature and a rapid rate of densification. Independent characterization techniques, namely x-ray diffraction, thermomechanical analysis, and electron microscopy, confirm that Ni-rich phases develop upon sintering in a way that enhances consolidation. However, this system is shown to be unique in that the Ni addition facilitates rapid nano-phase separation sintering but does not promote stability of a nanoscale grain structure; the nanostructure is a transient feature of the system and the fully consolidated material is microcrystalline with a phase-separated structure. A thermodynamic stability analysis explains this structural evolution, and clarifies the roles of alloying elements that stabilize nanostructure and those that accelerate sintering in nanostructured systems. This work may broaden the applicability of nano-phase separation sintering and inform alloy design based on sinterability.
      Graphical abstract image

      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.11.030
      Issue No: Vol. 145 (2017)
       
  • High-quality TiN/AlN thin film heterostructures on c-sapphire
    • Authors: A. Moatti; J. Narayan
      Pages: 134 - 141
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): A. Moatti, J. Narayan
      We have developed TiN/AlN/c-sapphire epitaxial heterostructures and compared it with TiN/c-sapphire epitaxial heterostructures, needed for GaN-based LEDs and lasers. AlN is used as a buffer layer to provide a high misfit strain and facilitate the 2D growth on sapphire. The large misfit strain between sapphire and AlN makes this substrate a great candidate for GaN-based devices because it guarantees a full relaxation of AlN thin films through domain matching epitaxy paradigm. TiN can also act as an excellent contact and bottom electrode for Ⅲ-Ⅴ nitrides. Also, the introduction of TiN as a buffer layer decreases the critical thickness beyond which dislocations can grow in GaN thin films due to higher misfit strain compared to sapphire, which also improves the quality of potential GaN thin films. The selected-area-electron-diffraction patterns, scanning transmission electron microscopy, and transmission Kikuchi diffractions along with atomic arrangement simulations revealed that films are epitaxial with the following relationships: TiN<101>‖AlN[ 1 ¯ 2 1 ¯ 0]‖sapphire[01 1 ¯ 0] (in-plane), and TiN<111>‖AlN[0001]‖sapphire[0001] (out-of-plane). This is equivalent to a 30° rotation of Al basal plane in AlN with respect to that in sapphire. In TiN/c-sapphire epitaxial platforms, there is a 30° rotation: TiN<101>‖sapphire[01 1 ¯ 0] (in-plane), and TiN<111>‖sapphire[0001] (out-of-plane). It is shown that these heterostructures are fully relaxed in terms of misfit strains and only thermal strain stays as unrelaxed. The domain matching epitaxy paradigm is used to rationalize the epitaxial growth. The details of dislocations nucleation and glide in these heterostructures were studied and the results also discussed to elucidate the mechanism of strain relaxation.
      Graphical abstract image

      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.11.044
      Issue No: Vol. 145 (2017)
       
  • Phase, hardness, and deformation slip behavior in mixed HfxTa1-xC
    • Authors: Chase J. Smith; Xiao-Xiang Yu; Qianying Guo; Christopher R. Weinberger; Gregory B. Thompson
      Pages: 142 - 153
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): Chase J. Smith, Xiao-Xiang Yu, Qianying Guo, Christopher R. Weinberger, Gregory B. Thompson
      A series of HfxTa1-xC atomic compositions, where x = 0.0, 0.13, 0.25, 0.50, 0.75, 0.83, and 1.0, were computationally and experimentally studied in terms of their phase, hardness, and dislocation emission behavior. Using an ab initio evolutionary algorithm, the B1 rocksalt structure was found to be the 0 K ground state phase for the binary carbides whereas the ternary compounds were a mixture of metal-site ordered monoclinic, trigonal, and cubic structures. However, all experimental carbides were found to exhibit a B1 structure, with two B1 phases forming for Hf-rich ternary compositions. A modest rise in the elastic constant computationally predicted hardness was found with experimental mean values showing a similar trend from nanoindentation tests. With the presence of Ta, even in small amounts, in the Hf-rich ternary compositions, <110>{111} slip was observed which is a change from the reported <110>{110} slip for HfC. This change is explained by the promotion of an intrinsic stacking fault in the {111} planes.
      Graphical abstract image

      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.11.038
      Issue No: Vol. 145 (2017)
       
  • Chemistry and three-dimensional morphology of martensite-austenite
           constituent in the bainite structure of low-carbon low-alloy steels
    • Authors: N. Takayama; G. Miyamoto; T. Furuhara
      Pages: 154 - 164
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): N. Takayama, G. Miyamoto, T. Furuhara
      The martensite-austenite constituent (MA) originates from carbon-enriched and untransformed austenite surrounded by bainite and is known to deteriorate the toughness of bainitic steel. The relationship between MA and bainite formation in low-carbon low-alloy steel was investigated with respect to the effects of cooling rate and Nb addition, as well as the relationship between the MA distribution and surrounding bainite. Nb addition enhanced the formation of MA over a wider range of cooling rates, and the fraction of MA became maximum at a cooling rate of 5 K/s. A microstructure consisting of carbide-free coarse bainite with few high angle boundaries is formed near the prior austenite grain boundary, and fine bainite with a high density of carbide and high angle boundaries is formed within the austenite grains. MA tends to be located in the coarse bainite region. Three-dimensional observations of the bainite and MA indicated MA is elongated along bainitic ferrites with the same growth direction or the same habit plane, while MA is blocky when the surrounding bainitic ferrites have different growth directions. Thus, the shape of MA is strongly affected by the crystallography of the surrounding bainite structure. The carbon content in austenite revealed that the carbon content of MA is close to the T0 composition at the transformation temperature, which suggests that MA formation is caused by a reduction of the driving force for transformation by carbon enrichment, which results in incomplete bainite transformation. Nb addition further inhibits the decomposition of austenite and promotes MA formation.
      Graphical abstract image

      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.11.036
      Issue No: Vol. 145 (2017)
       
  • Elasticity and strength of silica aerogels: A molecular dynamics study on
           large volumes
    • Authors: William Gonçalves; Julien Morthomas; Patrice Chantrenne; Michel Perez; Geneviève Foray; Christophe L. Martin
      Pages: 165 - 174
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): William Gonçalves, Julien Morthomas, Patrice Chantrenne, Michel Perez, Geneviève Foray, Christophe L. Martin
      The nanoporous structure and the mechanical properties of silica aerogels are studied by molecular dynamics simulations on large samples. For the first time, atomistic simulations are able to reproduce a pore size distribution, centered on 10 nm, comparable to experimental values. Using an unprecedented combination for silica aerogels of large volumes, large strains, and relatively small strain-rates, direct evidence of their peculiar mechanical behavior at the nanoscale is provided, from elasticity to fracture. The surface stress that silica ligaments experience produces a significant tension-compression asymmetry and an unusual discontinuity in the Poisson effect. The ductility of highly porous silica aerogels arises from an interplay between surface stress and a significant amount of reorganization in ligaments. Taking advantage of the large volumes accessible here, the very heterogeneous nature of low-density silica aerogels is reproduced, with an impact both on their elasticity and on their strength. In particular, a clear dependence between tensile strength and sample volume is uncovered, which opens perspectives for the elaboration of multi-scale models.
      Graphical abstract image

      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.12.005
      Issue No: Vol. 145 (2017)
       
  • Coupling eutectic nucleation mechanism investigated by phase field crystal
           model
    • Authors: Can Guo; Jincheng Wang; Junjie Li; Zhijun Wang; Yunhao Huang; Jiwei Gu; Xin Lin
      Pages: 175 - 185
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): Can Guo, Jincheng Wang, Junjie Li, Zhijun Wang, Yunhao Huang, Jiwei Gu, Xin Lin
      Eutectic solidification has been investigated for decades; however, our understanding of the mechanism of eutectic nucleation remains very limited. In this work, we investigated the regular eutectic solidification process by an atomistic simulation method, and a new eutectic nucleation scenario is proposed. We found that, near the eutectic composition, the nucleation of a metastable triangle phase is more energy favorable for diffusion-controlled binary eutectic systems, and the solidification often passes through a transient state. With the proceeding of solidification, alternated A-rich and B-rich domains will be generated at the interface between the transient phase and liquid phase by pseudospinodal decomposition. When the compositions of alternated domains are close to the respective equilibrium values, eutectic phases will be formed inside the domains simultaneously, which further develops into coupling eutectic colonies. Furthermore, the construction of solute interdiffusion field and the formation of interface curvature are explained self-consistently with our eutectic nucleation mechanism.
      Graphical abstract image

      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.12.012
      Issue No: Vol. 145 (2017)
       
  • On the origin of anomalous eutectic growth from undercooled melts: Why
           re-melting is not a plausible explanation
    • Authors: Andrew M. Mullis; Caroline R. Clopet
      Pages: 186 - 195
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): Andrew M. Mullis, Caroline R. Clopet
      Ag-Cu melt at the equilibrium eutectic composition has been undercooled using a melt fluxing technique. The resulting samples have been sectioned through the nucleation point and the spatial distribution of anomalous eutectic has been quantified. This is shown to be highly inhomogeneous, with the volume of anomalous material decreasing approximately linearly with distance from the nucleation site. In samples at low undercooling the volume fraction of anomalous eutectic near the nucleation site is around an order of magnitude greater than the calculated recalescence solid fraction. As such, any model for the origin of the anomalous eutectic invoking partial remelting of this initial solid is shown to be infeasible. An alternative model, based on a kinetic shift of the eutectic point during rapid solidification is proposed.
      Graphical abstract image

      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.12.016
      Issue No: Vol. 145 (2017)
       
  • Effect of ferrite-to-austenite phase transformation path on the interface
           crystallographic character distributions in a duplex stainless steel
    • Authors: N. Haghdadi; P. Cizek; P.D. Hodgson; V. Tari; G.S. Rohrer; H. Beladi
      Pages: 196 - 209
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): N. Haghdadi, P. Cizek, P.D. Hodgson, V. Tari, G.S. Rohrer, H. Beladi
      The effect of the ferrite to austenite phase transformation route on the microstructure and interface plane character distributions was studied in a duplex stainless steel. Two markedly different austenite morphologies (i.e., equiaxed and Widmanstätten) were produced through diffusional (slow cooling) and semi-shear (air-cooling) transformations, respectively. Both austenite morphologies had textures similar to the as-received condition, which was attributed to a “texture memory” effect. The air-cooled microstructure displayed a significantly higher content of Kurdjumov-Sachs (K-S) and Nishiyama-Wassermann (N-W) interfaces (39%) compared with the slow-cooled one (16%), due to the change in the austenite nucleation and growth mechanism during the phase transformation. A five-parameter analysis of different interfaces revealed that for K-S/N-W orientation relationships, ferrite and austenite terminated on (110) and (111) planes, respectively, regardless of the transformation route. The population of these planes, however, increased as the transformation rate increased. A higher fraction of Σ3 boundaries was observed in the equiaxed austenite morphology compared with its Widmanstätten counterpart, which was mainly attributed to the different kinetics and the growth mode of austenite plates during the phase transformation. Σ9 boundaries were mostly formed where two Σ3 boundaries met and were largely of tilt character because of geometric constraints. The intervariant boundary plane distributions of both austenite microstructures displayed more frequent {111} orientations than other planes for a majority of the boundaries. This trend was markedly stronger for Widmanstätten austenite.
      Graphical abstract image

      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.11.057
      Issue No: Vol. 145 (2017)
       
  • Solid state dewetting of thin plasmonic films under focused cw-laser
           irradiation
    • Authors: William M. Abbott; Simon Corbett; Graeme Cunningham; Amanda Petford-Long; Sheng Zhang; John F. Donegan; David McCloskey
      Pages: 210 - 219
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): William M. Abbott, Simon Corbett, Graeme Cunningham, Amanda Petford-Long, Sheng Zhang, John F. Donegan, David McCloskey
      Elevated temperatures and large thermal gradients are a significant source of component failure in microelectronics, and are the limiting factor in heat-assisted magnetic recording (HAMR). We have investigated the effect of solid-state dewetting in Au thin films, as a function of local temperature, film thickness, and substrate adhesion. In this work, a localized temperature rise is induced in thin (≤50 nm) polycrystalline Au films on SiO2 substrates via focused continuous-wave laser irradiation at 488 nm. The magnitude and distribution of the total temperature rise is measured using CCD-based thermoreflectance. This also allows a sensitive measurement of the temperature at which dewetting occurs, showing that for thin Au films without adhesion layers, rapid dewetting can occur at temperatures as low as 50 °C, which corresponds with an absorbed laser power of 4 mW. The time decay of the reflected light from the illuminating laser is used to monitor locally the dynamics of solid state dewetting. TEM diffraction analysis shows significant changes in the microstructure and crystallographic texture of the films as far as 10 μm away from the illuminated area. The use of a thin metallic adhesion layer (such as Ti or Cr) is shown to significantly improve the adhesion of the Au to the substrate, inhibit grain growth, reduce the tendency towards dewetting, and to allow the film to develop a pseudo-biaxial texture.
      Graphical abstract image

      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.12.030
      Issue No: Vol. 145 (2017)
       
  • Coexistence of rhombohedral and orthorhombic phases in ultrathin BiFeO3
           films driven by interfacial oxygen octahedral coupling
    • Authors: M.J. Han; Y.J. Wang; D.S. Ma; Y.L. Zhu; Y.L. Tang; Y. Liu; N.B. Zhang; J.Y. Ma; X.L. Ma
      Pages: 220 - 226
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): M.J. Han, Y.J. Wang, D.S. Ma, Y.L. Zhu, Y.L. Tang, Y. Liu, N.B. Zhang, J.Y. Ma, X.L. Ma
      Coexistence of two phases creates a morphotropic phase boundary in perovskite oxides, which can provide large piezoelectric response, generating it a well suited system for probe-based memories and actuator applications. The coexistence of two phases in thin films is proposed to be induced by epitaxial constraints from substrates or chemical compositional modifications by substitution. In this work, we found a new formation mechanism of two-phase coexistence driven by interfacial oxygen octahedral coupling (OOC) in oxide heterostructures. We fabricated a series of BiFeO3 (BFO) ultrathin films on various orthorhombic substrates exerting from tensile to compressive strains by Pulsed Laser Deposition (PLD) techniques. Aberration-corrected transmission electron microscopy demonstrates that the lattice rotation and oxygen octahedral rotation (OOR) patterns transfer from these substrates to BFO films in about 3 unit cells while an orthorhombic (Pnma) phase forms at the interface due to OOC. This Pnma phase is non-polar, which differs from polar phases of Ima2 or Pmc2 1 when a large tensile strain is imposed onto BFO. First-principles calculations reproduce these experimental results perfectly. This phase transition occurs when BFO films are under both tensile and compressive strains suggesting that OOC alone can induce phase transition in ultrathin BFO films. Such coexistence of two phases may have many potential applications in the field of electronics, such as ferroelectric sensors and actuators.
      Graphical abstract image

      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.12.038
      Issue No: Vol. 145 (2017)
       
  • Thermodynamic and structural evolution of Dy2Ti2O7 pyrochlore after swift
           heavy ion irradiation
    • Authors: Cheng-Kai Chung; Jacob Shamblin; Eric C. O'Quinn; Anna Shelyug; Igor Gussev; Maik Lang; Alexandra Navrotsky
      Pages: 227 - 234
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): Cheng-Kai Chung, Jacob Shamblin, Eric C. O'Quinn, Anna Shelyug, Igor Gussev, Maik Lang, Alexandra Navrotsky
      Utilizing a combined approach of high temperature calorimetry and neutron total scattering, we obtained critical insights into the energetics and annealing of radiation damage in swift heavy ion irradiated Dy2Ti2O7 pyrochlore. Oxide melt solution calorimetry reveals that the radiation amorphized Dy2Ti2O7 is destabilized by 243 kJ/mol compared to fully ordered, crystalline pyrochlore. Differential scanning calorimetry of the amorphized sample shows a rapid exothermic event starting at 1063 K, which, based on neutron structural analysis, is related to recrystallization. The heat release on annealing to 1473 K, –137 kJ/mol, is only about half of the total energetic difference between the ordered crystalline and amorphized samples, despite the apparent recovery of long-range pyrochlore-like ordering. Detailed neutron structural analysis confirms the persistence of residual damage in the 1473 K annealed sample. This metastability is attributed to local disorder in the form of weberite-like short-range domains in the recrystallized material. The annealing of radiation damage appears to be a complex, multistep process with decoupled short- and long-range damage recovery. Heating well above the initial recrystallization temperature does not erase all the damage, which may have important implications for the use of pyrochlores as nuclear waste forms.
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      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.12.044
      Issue No: Vol. 145 (2017)
       
  • Segregation of Mg, Cu and their effects on the strength of Al Σ5
           (210)[001] symmetrical tilt grain boundary
    • Authors: Dongdong Zhao; Ole Martin Løvvik; Knut Marthinsen; Yanjun Li
      Pages: 235 - 246
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): Dongdong Zhao, Ole Martin Løvvik, Knut Marthinsen, Yanjun Li
      Alien elemental segregation can pronouncedly change the grain boundary properties. Systematic first-principles calculations were performed to investigate the Mg and Cu segregation behavior at Σ5 (210)[001] symmetrical tilt grain boundary (STGB) in Al. The mechanical properties of Mg or Cu containing Σ5 (210)[001] STGBs were probed by combining a canonical Griffith fracture model with an ab-initio tensile test method. It is found that both Mg and Cu have a large driving force to segregate to Al grain boundaries, with Mg preferentially segregating at symmetric substitutional core sites and Cu at interstitial hollow sites at the grain boundary. Interestingly, Al Σ5 (210)[001] is shown to possess a stronger sink strength of Cu impurities than Mg. Both Mg and Cu segregation leads to a grain boundary expansion and a significant decrease of the grain boundary energy. Calculations show that Mg segregation leads to embrittlement of the STGB, contrary to the cohesion enhancing effect of Cu solutes on Al grain boundaries. The Mg induced embrittlement is due to a combination of “structural effect” ‒ (grain boundary expansion) and “chemical effect” ‒ (charge density depletion). The strengthening effect of Cu solutes lies in the creation of new Cu-Al bonds across the grain boundary, which is considered as a strong contribution to the grain boundary cohesion, thereby increasing its resistance against intergranular cleavage. This work demonstrates how a fundamental theoretical understanding on the atomic and electronic level can rationalize mechanical properties of alloys at the macroscopic scale.
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      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.12.023
      Issue No: Vol. 145 (2017)
       
  • On the grain boundary strengthening effect of boron in
           γ/γ′ Cobalt-base superalloys
    • Authors: M. Kolb; L.P. Freund; F. Fischer; I. Povstugar; S.K. Makineni; B. Gault; D. Raabe; J. Müller; E. Spiecker; S. Neumeier; M. Göken
      Pages: 247 - 254
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): M. Kolb, L.P. Freund, F. Fischer, I. Povstugar, S.K. Makineni, B. Gault, D. Raabe, J. Müller, E. Spiecker, S. Neumeier, M. Göken
      Boron is an essential solute element for improving the grain boundary strength in several high temperature metallic alloys especially in Ni- and Co-base superalloys although the detailed strengthening mechanisms are still not well understood. In superalloys, boron leads to the formation of borides and precipitate depleted zones around the grain boundaries and alters the bond strength among the grains directly. In this paper, we explore in detail the role of the boron content in ternary γ/γ′ Co-9Al-9W alloys. Local as well as bulk mechanical properties were evaluated using nanoindentation and compression testing and correlated to near-atomic scale microstructure and compositions obtained from electron microscopy and atom probe tomography. The alloy variant with low B content (0.005 at.% B) reveals an increase in yield strength at room temperature and 600 °C and atom probe tomography investigations show that solute B segregates to the grain boundaries. However, in the bulk B exclusively partitions to the γ′ phase. Additionally, the γ′/γ′ grain boundaries are depleted in W and Al with the concentration locally shifted towards the γ composition forming a very thin γ layer at the γ′/γ′ grain boundaries, which supports dislocation mobility in the γ′/γ′ grain boundary region during deformation. Higher content of B (0.04 at.% B) promotes formation of W-rich borides at the grain boundaries that leads to undesirable precipitate depleted zones adjacent to these borides that decrease the strength of the alloy drastically. However, it was also found that a subsequent annealing heat treatment eliminates these detrimental zones by re-precipitating γ′ and thus elevating the strength of the alloy. This result shows that, if a precipitate depleted zone can be avoided, B significantly improves the mechanical properties of polycrystalline Co-base superalloys by strengthening the γ′ phase and by improving grain boundary cohesion.
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      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.12.020
      Issue No: Vol. 145 (2017)
       
  • Investigating the thermal stability of irradiation-induced damage in a
           zirconium alloy with novel in situ techniques
    • Authors: M. Topping; T. Ungár; C.P. Race; A. Harte; A. Garner; F. Baxter; S. Dumbill; P. Frankel; M. Preuss
      Pages: 255 - 263
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): M. Topping, T. Ungár, C.P. Race, A. Harte, A. Garner, F. Baxter, S. Dumbill, P. Frankel, M. Preuss
      Zr alloys exhibit irradiation-induced growth and hardening which is associated with the defects and dislocation loops that form during irradiation. In this study, state-of-the-art in-situ synchrotron X-ray diffraction (SXRD) and transmission electron microscopy (TEM) techniques were used to investigate the stability of dislocation loops in two proton-irradiated Zr-Fe binary alloys in real time. Complementary data from both techniques show rapid annealing of a-loops occurs between 300 °C and 450 °C. Line profile analysis was performed on the SXRD patterns using the convoluted multiple whole profile analysis tool, to calculate the change in a-loop line density as a function of post-irradiation heat treatment temperature and time. At temperatures below 300 °C, no significant decrease in a-loop density was detected when held for 1 h at temperature. From this SXRD experiment, we calculate the effective activation energy for the annealing process as 0.46 eV. On-axis in-situ STEM imaging was used to directly observe a-loop mobility during heating cycles and confirm that a-loops begin to glide in the trace of the basal plane at ∼200 °C in a thin foil specimen. Such a-loop gliding events, leading to annihilation at the foil's surfaces, became more frequent between 300 and 450 °C.
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      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.11.051
      Issue No: Vol. 145 (2017)
       
  • Understanding the high temperature reversed yield asymmetry in a Mg-rare
           earth alloy by slip trace analysis
    • Authors: C.M. Cepeda-Jiménez; C. Prado-Martínez; M.T. Pérez-Prado
      Pages: 264 - 277
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): C.M. Cepeda-Jiménez, C. Prado-Martínez, M.T. Pérez-Prado
      In this work electron backscattered diffraction-assisted slip trace analysis was utilized to investigate the origin of the high temperature reversed yield asymmetry in a Mg-rare earth alloy. This material was tested at room and high temperature, in tension and compression, both in the as-extruded condition, in which the alloying elements are in solid solution, as well as following two aging treatments which gave rise to different precipitation levels. A statistically relevant number of slip traces were analyzed after each test in order to estimate the influence of the testing conditions and the precipitation state on the relative activity of basal, prismatic and pyramidal <c+a> systems. In tension, basal slip was found to be the dominant deformation mechanism under all the testing conditions investigated. In compression, twinning predominated always at room temperature and was replaced by basal slip in the aged samples at high temperature. A pronounced reversed yield asymmetry, i.e., the presence of a higher yield stress in compression than in tension, was observed at high temperature in the aged alloy samples. Careful microstructural examination of the spatial distribution of slip traces by high resolution scanning electron microscopy (SEM) revealed increased intra- and intergranular basal slip localization in those specimens under tensile deformation at high temperature. The high temperature reversed yield asymmetry observed in the aged samples was thus attributed to the suppression of twinning in compression and to enhanced softening in tension due to basal slip localization.
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      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.12.021
      Issue No: Vol. 145 (2017)
       
  • Meridian crack test strength of plasma-sprayed amorphous and
           nanocrystalline ceramic microparticles
    • Authors: Václav Pejchal; Marta Fornabaio; Goran Žagar; Grégory Riesen; Romain G. Martin; Jan Medřický; Tomáš Chráska; Andreas Mortensen
      Pages: 278 - 289
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): Václav Pejchal, Marta Fornabaio, Goran Žagar, Grégory Riesen, Romain G. Martin, Jan Medřický, Tomáš Chráska, Andreas Mortensen
      We implement the meridian crack test method to measure the surface and subsurface flaw-controlled strength of spherical near-eutectic plasma-sprayed alumina-zirconia-silica particles of average diameter near 30 μm. The particles are tested in two states, namely as-sprayed amorphous, or nanocrystalline as obtained after a supplementary annealing step. The test consists in uniaxial compression testing of individual particles between a pair of elasto-plastic steel platens, the hardness of which is tailored to achieve relative contact radii (defined as the ratio between the radius of the projected particle-platen contact area to the particle radius) above 0.6 at the moment of particle failure. Results show that nanocrystalline particles exhibit characteristic Weibull strength 1490 MPa, which is approximately 30% higher than for amorphous particles while the Weibull modulus is, relative to the test precision, comparable, being equal to 6.0 and 7.8 for nanocrystalline and amorphous particles, respectively. This result is an indication that the flaw size distribution is not significantly affected by the annealing step, the strength increase resulting from an increase in fracture toughness upon nanocrystallisation. This conclusion is consistent with fractographic observations. The principal strength-limiting defects identified for both amorphous and nanocrystalline particles were micropores formed during plasma spraying.
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      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.12.031
      Issue No: Vol. 145 (2017)
       
  • Simultaneous X-ray diffraction, crystallography and fluorescence mapping
           using the Maia detector
    • Authors: Henry J. Kirkwood; Martin D. de Jonge; Ondrej Muránsky; Felix Hofmann; Daryl L. Howard; Chris G. Ryan; Grant van Riessen; Matthew R. Rowles; Anna M. Paradowska; Brian Abbey
      Pages: 1 - 10
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Henry J. Kirkwood, Martin D. de Jonge, Ondrej Muránsky, Felix Hofmann, Daryl L. Howard, Chris G. Ryan, Grant van Riessen, Matthew R. Rowles, Anna M. Paradowska, Brian Abbey
      Interactions between neighboring grains influence the macroscale behavior of polycrystalline materials, particularly their deformation behavior, damage initiation and propagation mechanisms. However, mapping all of the critical material properties normally requires that several independent measurements are performed. Here we report the first grain mapping of a polycrystalline foil using a pixelated energy-dispersive X-ray area detector, simultaneously measuring X-ray fluorescence and diffraction with the Maia detector in order to determine grain orientation and estimate lattice strain. These results demonstrate the potential of the next generation of X-ray area detectors for materials characterization. By scanning the incident X-ray energy we investigate these detectors as a complete solution for simultaneously mapping the crystallographic and chemical properties of the sample. The extension of these techniques to broadband X-ray sources is also discussed.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.025
      Issue No: Vol. 144 (2017)
       
  • Anomalous hardening in magnesium driven by a size-dependent transition in
           deformation modes
    • Authors: Gi-Dong Sim; Gyuseok Kim; Steven Lavenstein; Mohamed H. Hamza; Haidong Fan; Jaafar A. El-Awady
      Pages: 11 - 20
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Gi-Dong Sim, Gyuseok Kim, Steven Lavenstein, Mohamed H. Hamza, Haidong Fan, Jaafar A. El-Awady
      Here, we report a comprehensive study that combines in situ scanning electron microscopy experiments and atomistic simulations to quantify the effect of crystal size on the transformation in deformation modes in a-axis oriented Mg single crystals at room temperature. The experimental results indicate that the deformation is dominated by the nucleation and propagation of tensile twins. The stress required for twin propagation was found to increase with decreasing sample size, showing a typical “smaller is stronger” behavior. Furthermore, an anomalous increase in strain hardening is first reported for microcrystals having diameters larger than ∼18 μm, which is induced by twin-twin and dislocation-twin interactions. The hardening rate gradually decreases toward the bulk response as the microcrystal size increases. Below 18 μm, deformation is dominated by the nucleation and propagation of a single tensile twin followed by basal slip activity in the twinned crystal, leading to no apparent hardening. In addition, molecular dynamics simulations indicate a transition from twinning mediated plasticity to dislocation mediated plasticity for crystal sizes below a few hundred nanometers in size. A deformation mechanism map for twin oriented Mg single crystals, ranging from the nano-scale to bulk scale is proposed based on the current simulations and experiments. The current predicted size-affected deformation mechanism of twin oriented Mg single crystals can lead to better understanding of the competition between dislocations plasticity and twinning plasticity.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.033
      Issue No: Vol. 144 (2017)
       
  • Solubility in Zr-Nb alloys from first-principles
    • Authors: Maeva Cottura; Emmanuel Clouet
      Pages: 21 - 30
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Maeva Cottura, Emmanuel Clouet
      The thermodynamic properties of the Zr-Nb alloy are investigated at temperatures below 890 K with ab initio calculations. The solution energies of the bcc Nb-rich and hcp Zr-rich solid solutions obtained within the framework of density functional theory are in good agreement with experimental data, although insufficient for a quantitative description of the miscibility gap, for which non configurational entropy has to be accounted for. Whereas electronic free energies can be neglected, we show, using the harmonic approximation and the density functional perturbation theory, that both solution free energies are strongly modified by the contribution related to atomic vibrations. Considering this vibrational free energy leads to a good description of the phase diagram.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.035
      Issue No: Vol. 144 (2017)
       
  • Synergistic effect of ultrasonic melt treatment and fast cooling on the
           refinement of primary Si in a hypereutectic Al–Si alloy
    • Authors: Jae-Gil Jung; Tae-Young Ahn; Young-Hee Cho; Su-Hyeon Kim; Jung-Moo Lee
      Pages: 31 - 40
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Jae-Gil Jung, Tae-Young Ahn, Young-Hee Cho, Su-Hyeon Kim, Jung-Moo Lee
      A mechanism describing the synergistic effect of ultrasonic melt treatment (UST) and subsequent fast cooling on the refinement of primary Si particles in a hypereutectic Al−Si alloy was investigated by examining inoculant particles via high-resolution transmission electron microscopy, serial sectioning, and melt filtration. The application of UST activated non-wetting MgAl2O4 particles with diameters of ∼0.5 μm to nucleate the primary Si phase. The cavitation-enhanced wetting of MgAl2O4 particles caused the formation of the AlP phase at the MgAl2O4 interface, further improving the nucleation potential. The cavitation-enhanced wetting and dispersion of inclusions (such as MgAl2O4) also resulted in the refinement and de-agglomeration of AlP particles. The UST-induced changes to the inoculant particles ultimately increased their number density, and the observed effects became more pronounced after increasing the degree of undercooling up to 20 K, leading to enhanced refinement of primary Si particles at higher cooling rates (up to 102 K s−1).
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.039
      Issue No: Vol. 144 (2017)
       
  • Twinning mechanism at three-grain tri-junction during directional
           solidification of multi-crystalline silicon
    • Authors: T. Jain; H.K. Lin; C.W. Lan
      Pages: 41 - 50
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): T. Jain, H.K. Lin, C.W. Lan
      We propose a model to explain the formation mechanism of twin grains at the three-grain tri-junction (3GTJ) on the growth interface during directional solidification of multi-crystalline silicon. We also attempt to confirm its validity by comparing with the experimental results. This model is an extension of the previous model for the two-dimensional (2D) nucleation at the grain boundaries (GBs). It is found that the energy barriers for faceting and twinning nucleus at the 3GTJ are much smaller than that at GBs. As a result, a higher twinning probability can be obtained at a much lower undercooling. Two types of tri-junctions are considered according to the experiments and the dominant factors which decide the twinning probability on each facet at the 3GTJ are further discussed.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.042
      Issue No: Vol. 144 (2017)
       
  • Strain accumulation during microstructurally small fatigue crack
           propagation in bcc Fe-Cr ferritic stainless steel
    • Authors: E. Malitckii; H. Remes; P. Lehto; Y. Yagodzinskyy; S. Bossuyt; H. Hänninen
      Pages: 51 - 59
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): E. Malitckii, H. Remes, P. Lehto, Y. Yagodzinskyy, S. Bossuyt, H. Hänninen
      Strain accumulation was studied by digital image correlation technique (DIC) during microstructurally small fatigue crack propagation in polycrystalline 18%Cr ferritic stainless steel. Load-controlled fatigue testing was performed with R-ratio of 0.1 and frequency 10 Hz. The maximum applied stress was well below the yield stress of the studied material. The effect of the observed strain field on crack growth rate variation is discussed. Fracture surfaces were studied by scanning electron microscopy (SEM) evidencing the connection between the mechanism of the fatigue crack growth, accumulated strain and crack growth rate. Detailed study of fracture surface morphology was carried out by atomic force microscopy (AFM). Results indicate two processes of material damage accumulation and failure during cyclic loading: 1) local shear strain zones form successively ahead of the crack tip, and 2) fatigue crack growth occurs by both single- and multiple-slip mechanisms. The place and intensity of shear strain localization zones vary during the crack growth that is related closely to the local variation of crack growth rate.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.038
      Issue No: Vol. 144 (2017)
       
  • Similar local order in disordered fluorite and aperiodic pyrochlore
           structures
    • Authors: Jacob Shamblin; Cameron L. Tracy; Raul I. Palomares; Eric C. O'Quinn; Rodney C. Ewing; Joerg Neuefeind; Mikhail Feygenson; Jason Behrens; Christina Trautmann; Maik Lang
      Pages: 60 - 67
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Jacob Shamblin, Cameron L. Tracy, Raul I. Palomares, Eric C. O'Quinn, Rodney C. Ewing, Joerg Neuefeind, Mikhail Feygenson, Jason Behrens, Christina Trautmann, Maik Lang
      A major challenge to understanding the response of materials to extreme environments (e.g., nuclear fuels/waste forms and fusion materials) is to unravel the processes by which a material can incorporate atomic-scale disorder, and at the same time, remain crystalline. While it has long been known that all condensed matter, even liquids and glasses, possess short-range order, the relation between fully-ordered, disordered, and aperiodic structures over multiple length scales is not well understood. For example, when defects are introduced (via pressure or irradiation) into materials adopting the pyrochlore structure, these complex oxides either disorder over specific crystallographic sites, remaining crystalline, or become aperiodic. Here we present neutron total scattering results characterizing the irradiation response of two pyrochlores, one that is known to disorder (Er2Sn2O7) and the other to amorphize (Dy2Sn2O7) under ion irradiation. The results demonstrate that in both cases, the local pyrochlore structure is transformed into similar short range configurations that are best fit by the orthorhombic weberite structure, even though the two compositions have distinctly different structures, aperiodic vs. disordered-crystalline, at longer length scales. Thus, a material's resistance to amorphization may not depend primarily on local defect formation energies, but rather on the structure's compatibility with meso-scale modulations of the local order in a way that maintains long-range periodicity.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.044
      Issue No: Vol. 144 (2017)
       
  • Dislocation interactions at reduced strain rates in atomistic simulations
           of nanocrystalline Al
    • Authors: Maxime Dupraz; Zhen Sun; C. Brandl; Helena Van Swygenhoven
      Pages: 68 - 79
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Maxime Dupraz, Zhen Sun, C. Brandl, Helena Van Swygenhoven
      Molecular dynamics simulations of transient stress drops have been carried out in different regimes on a nanocrystalline Aluminum sample with average grain size of 12 nm. Besides confirming the interpretation of experimental results obtained during in situ X-ray diffraction, the creep simulations performed at 2 or 3 orders of magnitude lower strain rates than usual reveal deformation mechanisms that have not been observed previously. First of all, it is evidenced that the misfit dislocations available at the GB assist the propagation of a lattice dislocation on a plane with low resolved shear stress. Furthermore, it is shown that the interaction of two dislocations gliding on parallel slip planes can result in the emission of a vacancy in the grain interior. Finally, the importance of the Schmid factor in the activation of slip in nanocrystalline structures is discussed.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.043
      Issue No: Vol. 144 (2017)
       
  • Microstructure and mechanical properties of a precipitation-strengthened
           Al-Zr-Sc-Er-Si alloy with a very small Sc content
    • Authors: Anthony De Luca; David C. Dunand; David N. Seidman
      Pages: 80 - 91
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Anthony De Luca, David C. Dunand, David N. Seidman
      The precipitation hardening behavior of an Al-0.08Zr-0.014Sc-0.008Er-0.10Si (at.%) alloy was investigated utilizing microhardness, electrical conductivity, atom-probe tomography (APT), and compressive creep-measurements. This new composition, with a Sc:Zr atomic ratio of less than 1:5 represents a significant reduction of the alloy's cost when compared to the more usual Al-0.06Sc-0.02Zr based alloys with typical Sc:Zr atomic ratios of 3:1. To study the precipitation behavior of this low-Sc alloy, isothermal aging experiments between 350 and 425 °C for a duration of up to 6 months were performed. The low concentration of Sc, compensated by the high Zr concentration, permits the alloy to achieve a higher peak microhardness than the corresponding Sc-richer, Zr-leaner alloys. The low-Sc alloy also shows better over aging resistance, as anticipated from the smaller diffusivity of Zr when compared to Sc, leading to slower coarsening kinetics. Atom-probe tomography demonstrates that the high microhardness is due to the formation of a high number density of nano-precipitates, ∼1023 m−3 for peak aging conditions, with a mean radius of 1.9 nm, thus yielding a high volume fraction (0.35%) of nano-precipitates. Like alloys with much higher Sc and Er concentrations, the (Al,Si)3(Sc,Zr,Er) nano-precipitates still exhibit a core-shell structure with a concentration of Zr in the shell of up to 25 at.%, and a Sc- and Er-enriched core. Compressive creep experiments at 300 °C demonstrate that the new alloy, with only 0.014 at% Sc, is as creep resistant as a binary Al-0.08Sc at.% alloy, displaying a threshold stress of 17.5 ± 0.6 MPa at peak aged condition.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.040
      Issue No: Vol. 144 (2017)
       
  • Determining the strengths of HCP slip systems using harmonic analyses of
           lattice strain distributions
    • Authors: Paul R. Dawson; Donald E. Boyce; Jun-Sang Park; Euan Wielewski; Matthew P. Miller
      Pages: 92 - 106
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Paul R. Dawson, Donald E. Boyce, Jun-Sang Park, Euan Wielewski, Matthew P. Miller
      A robust methodology is presented to extract slip system strengths from lattice strain distributions for polycrystalline samples obtained from high-energy x-ray diffraction (HEXD) experiments with in situ loading. The methodology consists of matching the evolution of coefficients of a harmonic expansion of the distributions from simulation to the coefficients derived from measurements. Simulation results are generated via finite element simulations of virtual polycrystals that are subjected to the loading history applied in the HEXD experiments. Advantages of the methodology include: (1) its ability to utilize extensive data sets generated by HEXD experiments; (2) its ability to capture trends in distributions that may be noisy (both measured and simulated); and (3) its sensitivity to the ratios of the family strengths. The approach is used to evaluate the slip system strengths of Ti-6Al-4V using samples having relatively equiaxed grains. These strength estimates are compared to values in the literature.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.032
      Issue No: Vol. 144 (2017)
       
  • Transmission electron microscopy characterization of dislocation structure
           in a face-centered cubic high-entropy alloy Al0.1CoCrFeNi
    • Authors: X.D. Xu; P. Liu; Z. Tang; A. Hirata; S.X. Song; T.G. Nieh; P.K. Liaw; C.T. Liu; M.W. Chen
      Pages: 107 - 115
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): X.D. Xu, P. Liu, Z. Tang, A. Hirata, S.X. Song, T.G. Nieh, P.K. Liaw, C.T. Liu, M.W. Chen
      Structural characterization of dislocations and dislocation reactions in a face-centered cubic high entropy alloy was conducted using the state-of-the-art spherical aberration corrected transmission electron microscopy. We experimentally measured the stacking fault energy of the high entropy alloy from the atomic images and diffraction contrast of dislocation cores. The low stacking fault energy results in widely dissociated dislocations and extensive dislocation reactions, which leads to the formation of immobile Lomer and Lomer-Cottrell dislocation locks. These dislocation locks act as both dislocation barriers and sources and are responsible for the significant work hardening with a large hardening rate in the alloy. Based on the atomic-scale characterization and classical dislocation theory, a simple equation was derived to describe the work hardening behavior of the high entropy alloy in the early-stage of plastic deformation.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.050
      Issue No: Vol. 144 (2017)
       
  • Unveiling the mechanisms of cold sintering of ZnO at 250 °C by varying
           applied stress and characterizing grain boundaries by Kelvin Probe Force
           Microscopy
    • Authors: J. Gonzalez-Julian; K. Neuhaus; M. Bernemann; J. Pereira da Silva; A. Laptev; M. Bram; O. Guillon
      Pages: 116 - 128
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): J. Gonzalez-Julian, K. Neuhaus, M. Bernemann, J. Pereira da Silva, A. Laptev, M. Bram, O. Guillon
      The sintering behavior of nanocrystalline ZnO was investigated at only 250 °C. Densification was achieved by the combined effect of uniaxial pressure and the addition of water both in a Field Assisted Sintering Technology/Spark Plasma Sintering apparatus and a hand press with a heater holder. The final pure ZnO materials present high densities (>90% theoretical density) with nano-grain sizes. By measuring the shrinkage rate as a function of applied stress it was possible to identify the stress exponent related to the densification process. A value larger than one points to non-linear relationship going beyond single solid-state diffusion or liquid phase sintering. Only a low amount of water (1.7 wt%) was needed since the process is dictated by the adsorption on the surface of the ZnO particles. Part of the adsorbed water dissociates into H+ and OH− ions, which diffuse into the ZnO crystal structure, generating grain boundaries/interfaces with high defect chemistry. As characterized by Kelvin Probe Force Microscopy, and supported by impedance spectroscopy, this highly defective grain boundary area presents much higher surface energy than the bulk. This highly defective grain boundary area with high potential reduces the activation energy of the atomic diffusion, leading to sinter the compound at low temperature.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.055
      Issue No: Vol. 144 (2017)
       
  • Composition design of high entropy alloys using the valence electron
           concentration to balance strength and ductility
    • Authors: Ruirun Chen; Gang Qin; Huiting Zheng; Liang Wang; Yanqing Su; YuLung Chiu; Hongsheng Ding; Jingjie Guo; Hengzhi Fu
      Pages: 129 - 137
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Ruirun Chen, Gang Qin, Huiting Zheng, Liang Wang, Yanqing Su, YuLung Chiu, Hongsheng Ding, Jingjie Guo, Hengzhi Fu
      The valence electron concentration (VEC) is an important physical factor for phase formation. A high VEC is conducive to forming an FCC phase and improving an alloy's ductility, while a low VEC is beneficial in forming a BCC phase that improves an alloy's strength. This is demonstrated for two HEAs, CoCrCuFeNi (FCC) and AlCoCrFeNi (BCC), that were designed as matrix alloys, where Ni and Mo are alloyed. The microstructure, phase evolution, and the mechanical properties for (AlCoCrFeNi)100-x Ni x and (CoCrCuFeNi)100-x Mo x were systematically investigated. As the phase structure for the (AlCoCrFeNi)100-x Ni x high entropy alloy (HEA) transformed from a BCC to an FCC crystal structure as the Ni content increased from 0 at.% to 16 at.%, the FCC volume fraction increased from 0% to 85%, its compressive fracture strain increased from 25% to 40%, its VEC increased from 7.2 to 7.6. As the phase structure for the (CoCrCuFeNi)100-x Mo x HEA transformed from FCC to BCC as the Mo content increased from 0 at.% to 16 at.%, the BCC volume fraction increased from 0% to 65%, its compressive yield strength increased from 260 MPa to 928 MPa, its VEC decreased from 8.8 to 8.3. Selecting an element based upon an alloy's VEC is a practical method for designing compositions for HEAs that balance strength and ductility. According to the needs of practical applications, balancing both strength and plasticity requires the following criteria for selecting an element for incorporation into an HEA system: matrix strength is improved by selecting an element with a VEC lower than the average VEC of the matrix, while ductility is improved by selecting another element with a VEC higher than the average VEC for the matrix.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.058
      Issue No: Vol. 144 (2017)
       
  • Microstructural deformation in fatigued nanotwinned copper alloys
    • Authors: Nathan M. Heckman; Matthew F. Berwind; Christoph Eberl; Andrea M. Hodge
      Pages: 138 - 144
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Nathan M. Heckman, Matthew F. Berwind, Christoph Eberl, Andrea M. Hodge
      In this study, the uniaxial tension-tension fatigue behavior of fully nanotwinned magnetron sputtered Cu-6wt%Al, Cu-2wt%Al, and Cu-10 wt%Ni is presented. These alloys have average twin thicknesses ranging from 4 to 8 nm, average grain widths from 90 to 180 nm, and tensile strengths from 1 to 1.5 GPa. In the high cycle regime (103 to 107 cycles), the nanotwinned alloys exhibit fatigue strengths ranging from 210 to 370 MPa, which is higher than previously observed in nanotwinned Cu (fatigue strengths between 80 and 200 MPa). Fatigue strengths are normalized by tensile strength for Cu alloys with different microstructures to study the correlation between tensile and fatigue properties. Post-mortem analysis of the materials reveals a newly observed deformation mechanism, where localized detwinning leads to intergranular fracture between columnar grains. Overall, materials displaying detwinning as a deformation mechanism show lower normalized fatigue strengths in comparison to materials that deform with slip band like behavior.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.057
      Issue No: Vol. 144 (2017)
       
  • Strain-engineered allotrope-like bismuth nanowires for enhanced
           thermoelectric performance
    • Authors: Jeongmin Kim; Min-Wook Oh; Gwansik Kim; Je-Hyeong Bahk; Jae Yong Song; Seong Gi Jeon; Dong Won Chun; Jee-Hwan Bae; Wooyoung Shim; Wooyoung Lee
      Pages: 145 - 153
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Jeongmin Kim, Min-Wook Oh, Gwansik Kim, Je-Hyeong Bahk, Jae Yong Song, Seong Gi Jeon, Dong Won Chun, Jee-Hwan Bae, Wooyoung Shim, Wooyoung Lee
      Allotropy is a fundamental concept that has been frequently studied since the mid-1800s. Although the bulk allotropy of elemental solids is fairly well understood, it remains challenging to reliably produce an allotrope at the nanoscale that has a different crystal structure and accompanies a change in physical properties for specific applications. Here, we demonstrate a "heterostructure" approach to produce allotrope-like bismuth nanowires, where it utilizes the lattice constant difference between bismuth and tellurium in core/shell structure. We find that the resultant strain of [100]-grown Bi nanowires increases the atomic linear density along the c-axis that has been predicted from theoretical considerations, enabling us to establish a design rule for strain-induced allotropic transformation. With our >400-nm-diameter nanowires, we measure a thermoelectric figure of merit ZT of 0.5 at room temperature with reduced thermal conductivity and enhanced Seebeck coefficient, which are primarily a result of the rough interface and the reduced band overlap according to our density-functional calculations.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.062
      Issue No: Vol. 144 (2017)
       
  • High frequency in situ fatigue response of Ni-base superalloy René-N5
           microcrystals
    • Authors: Steven Lavenstein; Bryan Crawford; Gi-Dong Sim; Paul A. Shade; Christopher Woodward; Jaafar A. El-Awady
      Pages: 154 - 163
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Steven Lavenstein, Bryan Crawford, Gi-Dong Sim, Paul A. Shade, Christopher Woodward, Jaafar A. El-Awady
      A novel in situ scanning electron microscope (SEM), high frequency fatigue testing methodology is developed using a combination of laser milling, focused ion beam fabrication and nanoindentation. This methodology is used to investigate crack initiation, propagation, fracture, fatigue life, and the mechanical response of microcantilever samples of a Ni-based superalloy (René-N5) under different cyclic strain amplitudes. The crack initiation and propagation in the microcantilever is monitored by observing changes in the beam's dynamic stiffness and continuous SEM imaging. The dynamic stiffness response of the micro-beams exhibits a transition from softening to hardening at a critical strain amplitude of 7 × 10 − 3 . Theoretical analysis indicates that this transition corresponds to the stress required to shear γ ′ precipitates. SEM imaging reveals the evolution of significant extrusions, intrusions, and slip traces during cyclic loading above this critical strain amplitude. Below this strain amplitude, very little surface roughening is observed. In addition, the measured dynamic stiffness is observed to exhibit two regimes of decrease after crack initiation. These two regimes correspond to short and large crack propagation. Finally, an overall increase in fatigue life is observed when comparing to bulk scale experiments on nickel-base superalloys. It is proposed that this is an inherent size effect in the small-volume, single crystal specimens tested.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.049
      Issue No: Vol. 144 (2017)
       
  • Hydrogen-modified dislocation structures in a cyclically deformed
           ferritic-pearlitic low carbon steel
    • Authors: Shuai Wang; Akihide Nagao; Petros Sofronis; Ian M. Robertson
      Pages: 164 - 176
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Shuai Wang, Akihide Nagao, Petros Sofronis, Ian M. Robertson
      The fatigue-crack growth rate of a ferritic-pearlitic low carbon steel was faster when the tests were conducted in high-pressure H2 gas environments than in air. The predominant fracture feature changed from ductile fatigue striations with some “quasi-cleavage-like” regions when the test was conducted in air to mixed “quasi-cleavage” and “flat” facets when tested in a H2 gas environment. The microstructure beneath the fracture surfaces produced in air was sub-grains, and over a distance of 15 μm from the fracture surface, the dimensions of the sub-grains increased. With hydrogen, dense dislocation bands and refined dislocation cells existed beneath the “quasi-cleavage” and “flat” fracture surfaces. The cell size increased with distance from the fracture surface. The decrease in the dimensions of the key microstructural features as the fracture surface is approached is attributed to the propagation of the crack through an already deformed matrix. The differences in evolved dislocation structure are explained in terms of the hydrogen-enhanced localized plasticity mechanism, and the hydrogen-modified dislocation structure establishes the local conditions that promote the fracture mode transition from ductile fatigue striations to a mixture of “quasi-cleavage” and “flat” features, which directly leads to enhanced fatigue-crack growth.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.034
      Issue No: Vol. 144 (2017)
       
  • Alternative misfit dislocations pattern in semi-coherent FCC {100}
           interfaces
    • Authors: Shuai Shao; Firas Akasheh; Jian Wang; Yue Liu
      Pages: 177 - 186
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Shuai Shao, Firas Akasheh, Jian Wang, Yue Liu
      The character of interface misfit dislocations is determined according to interface crystallography and minimization of interface energy, which includes coherent interface energy and dislocation line energy. The core energy of dislocations is generally ignored in such analysis. In this work, we demonstrate that the core energy of misfit dislocations is dependent on the mechanical and thermal loading condition, and ultimately determines the nature of interface misfit dislocation patterns (MDP). Employing atomistic simulations with empirical interatomic potentials, we show the transformation of conventional MDP consisting of a/2<110> dislocation into an alternative MDP consisting of mixed a<100> and a/2<110> dislocations under elevated temperatures and/or normal-to-interface tensile stresses. Although a<100> type dislocations typically have greater line energy in bulk, molecular statics/dynamics calculations show that a<100> type misfit dislocations are preferred over a/2<110> type under elevated temperatures and/or normal-to-interface tensile stresses due to their reduced core energy. In addition, we found that the a<100> dislocations possess significantly reduced vacancy formation energies compared to the a/2<110> dislocations. The potential application of this unique property of the alternative dislocation pattern for nanoscale multilayered composite as a functional material is discussed.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.052
      Issue No: Vol. 144 (2017)
       
  • Surface termination analysis of stoichiometric metal hexaborides: Insights
           from first-principles and XPS measurements
    • Authors: K.M. Schmidt; O. Jaime; J.T. Cahill; D. Edwards; S.T. Misture; O.A. Graeve; V.R. Vasquez
      Pages: 187 - 201
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): K.M. Schmidt, O. Jaime, J.T. Cahill, D. Edwards, S.T. Misture, O.A. Graeve, V.R. Vasquez
      We present a modeling framework and discuss the energetics and structural features of the surface terminations of Ca, Ba, Sr and La hexaborides using density functional theory analysis in combination with X-ray photoelectron spectroscopy. There is significant uncertainty in the literature about the nature of the surface terminations in metal hexaborides in terms of metal versus boron terminations. We show from electronic structure calculations that segregated regions of metal and boron-terminations produce the lowest energies for di-cations of CaB6, SrB6 and BaB6, while trivalent LaB6 minimizes the surface energy by arranging the metal ions in parallel rows on the surface. XPS measurements show that CaB6 and SrB6 have surfaces that are close to stoichiometric for the compound, while BaB6 has surfaces that are Ba-rich. Energetic barriers are calculated for transitions between each of the surface geometries considered. There is a substantial increase in the activation energy for the lanthanum migrations compared to the divalent cations. We also find that the boron octahedra units in these materials tend to contract or expand from their bulk values depending on the proximity to regions of high metal concentrations. These materials have many attractive features, such as low work functions, high hardness, low thermal expansion coefficients, and high melting points, among many other properties of interest for industrial applications. Promising uses of these materials also include catalytic applications for chemical dissociation reactions of various molecules such as hydrogen, water and carbon monoxide, for example, thus, the interest in determining relevant surface properties.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.045
      Issue No: Vol. 144 (2017)
       
  • Strong and tough metal/ceramic micro-laminates
    • Authors: Claudio Ferraro; Sylvain Meille; Julien Réthoré; Na Ni; Jerome Chevalier; Eduardo Saiz
      Pages: 202 - 215
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Claudio Ferraro, Sylvain Meille, Julien Réthoré, Na Ni, Jerome Chevalier, Eduardo Saiz
      There is a growing interest in the development of composites with complex structures designed to generate enhanced mechanical properties. The challenge is how to implement these structures in practical materials with the required degree of control. Here we show how freeze casting of ceramic preforms combined with metal infiltration can be used to fabricate Al2O3/Al-4wt% Mg micro-laminated composites. By manipulating the solid content of the suspension and the morphology of the ceramic particles (from platelets to round particles) it is possible to access a range of structures with layer thickness varying between 1 and 30 μm and metallic contents between 66 and 86 vol%. The mechanical response of the materials is characterized by combining bending tests with observation of crack propagation in two and three dimensions using different imaging techniques. These composites are able to combine high strength and toughness. They exhibit a rising R-curve behaviour although different structures generate different toughening mechanisms. Composites fabricated with Al2O3 particles exhibit the highest fracture resistance approaching 60 MPa m1/2, while laminates prepared from Al2O3 platelets exhibit higher strengths (above 700 MPa) while retaining fracture resistance up to ∼40 MPa m1/2. The results provide new insights on the effect of structure on the mechanical properties in metal-ceramic composites as well as on the design of appropriate testing procedures.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.059
      Issue No: Vol. 144 (2017)
       
  • Tailoring the mechanical properties of sputter deposited nanotwinned
           nickel-molybdenum-tungsten films
    • Authors: Gi-Dong Sim; Jessica A. Krogstad; Kelvin Y. Xie; Suman Dasgupta; Gianna M. Valentino; Timothy P. Weihs; Kevin J. Hemker
      Pages: 216 - 225
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Gi-Dong Sim, Jessica A. Krogstad, Kelvin Y. Xie, Suman Dasgupta, Gianna M. Valentino, Timothy P. Weihs, Kevin J. Hemker
      Advanced metallic alloys are attractive in microelectromechanical systems (MEMS) applications that require high density, electrical and thermal conductivity, strength, and dimensional stability. Here we report the mechanical behavior of direct current (DC) magnetron sputter deposited Nickel (Ni)-Molybdenum (Mo)-Tungsten (W) films annealed at various temperatures. The films deposit as single-phase nanotwinned solid solutions and possess ultra-high tensile strengths of approximately 3 GPa, but negligible ductility. Subsequent heat treatments resulted in grain growth and nucleation of Mo-rich precipitates. While films annealed at 600 °C or 800 °C for 1 h still showed brittle behavior, films annealed at 1,000 °C for 1 h were found to exhibit strength greater than 1.2 GPa and near 10% tensile ductility. In addition to the excellent mechanical properties, alloy films further exhibit remarkably improved dimensional stability – a lower coefficient of thermal expansion and greater microstructural stability. An excellent balance between mechanical properties and dimensional stability make sputter deposited Ni-Mo-W alloys promising structural materials for MEMS applications.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.065
      Issue No: Vol. 144 (2017)
       
  • Editors for Acta Materialia
    • Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144


      PubDate: 2017-12-27T09:11:53Z
       
  • Acta Awards 2018
    • Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144


      PubDate: 2017-12-27T09:11:53Z
       
 
 
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