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Publisher: Elsevier   (Total: 3161 journals)

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Showing 1 - 200 of 3161 Journals sorted alphabetically
A Practical Logic of Cognitive Systems     Full-text available via subscription   (Followers: 9)
AASRI Procedia     Open Access   (Followers: 15)
Academic Pediatrics     Hybrid Journal   (Followers: 35, SJR: 1.655, CiteScore: 2)
Academic Radiology     Hybrid Journal   (Followers: 24, SJR: 1.015, CiteScore: 2)
Accident Analysis & Prevention     Partially Free   (Followers: 96, SJR: 1.462, CiteScore: 3)
Accounting Forum     Hybrid Journal   (Followers: 27, SJR: 0.932, CiteScore: 2)
Accounting, Organizations and Society     Hybrid Journal   (Followers: 37, SJR: 1.771, CiteScore: 3)
Achievements in the Life Sciences     Open Access   (Followers: 5)
Acta Anaesthesiologica Taiwanica     Open Access   (Followers: 7)
Acta Astronautica     Hybrid Journal   (Followers: 421, SJR: 0.758, CiteScore: 2)
Acta Automatica Sinica     Full-text available via subscription   (Followers: 2)
Acta Biomaterialia     Hybrid Journal   (Followers: 28, SJR: 1.967, CiteScore: 7)
Acta Colombiana de Cuidado Intensivo     Full-text available via subscription   (Followers: 2)
Acta de Investigación Psicológica     Open Access   (Followers: 3)
Acta Ecologica Sinica     Open Access   (Followers: 10, SJR: 0.18, CiteScore: 1)
Acta Haematologica Polonica     Free   (Followers: 1, SJR: 0.128, CiteScore: 0)
Acta Histochemica     Hybrid Journal   (Followers: 3, SJR: 0.661, CiteScore: 2)
Acta Materialia     Hybrid Journal   (Followers: 276, SJR: 3.263, CiteScore: 6)
Acta Mathematica Scientia     Full-text available via subscription   (Followers: 5, SJR: 0.504, CiteScore: 1)
Acta Mechanica Solida Sinica     Full-text available via subscription   (Followers: 9, SJR: 0.542, CiteScore: 1)
Acta Oecologica     Hybrid Journal   (Followers: 12, SJR: 0.834, CiteScore: 2)
Acta Otorrinolaringologica (English Edition)     Full-text available via subscription  
Acta Otorrinolaringológica Española     Full-text available via subscription   (Followers: 3, SJR: 0.307, CiteScore: 0)
Acta Pharmaceutica Sinica B     Open Access   (Followers: 1, SJR: 1.793, CiteScore: 6)
Acta Poética     Open Access   (Followers: 4, SJR: 0.101, CiteScore: 0)
Acta Psychologica     Hybrid Journal   (Followers: 27, SJR: 1.331, CiteScore: 2)
Acta Sociológica     Open Access   (Followers: 1)
Acta Tropica     Hybrid Journal   (Followers: 6, SJR: 1.052, CiteScore: 2)
Acta Urológica Portuguesa     Open Access  
Actas Dermo-Sifiliograficas     Full-text available via subscription   (Followers: 3, SJR: 0.374, CiteScore: 1)
Actas Dermo-Sifiliográficas (English Edition)     Full-text available via subscription   (Followers: 2)
Actas Urológicas Españolas     Full-text available via subscription   (Followers: 3, SJR: 0.344, CiteScore: 1)
Actas Urológicas Españolas (English Edition)     Full-text available via subscription   (Followers: 1)
Actualites Pharmaceutiques     Full-text available via subscription   (Followers: 6, SJR: 0.19, CiteScore: 0)
Actualites Pharmaceutiques Hospitalieres     Full-text available via subscription   (Followers: 3)
Acupuncture and Related Therapies     Hybrid Journal   (Followers: 8)
Acute Pain     Full-text available via subscription   (Followers: 14, SJR: 2.671, CiteScore: 5)
Ad Hoc Networks     Hybrid Journal   (Followers: 11, SJR: 0.53, CiteScore: 4)
Addictive Behaviors     Hybrid Journal   (Followers: 17, SJR: 1.29, CiteScore: 3)
Addictive Behaviors Reports     Open Access   (Followers: 8, SJR: 0.755, CiteScore: 2)
Additive Manufacturing     Hybrid Journal   (Followers: 11, SJR: 2.611, CiteScore: 8)
Additives for Polymers     Full-text available via subscription   (Followers: 23)
Advanced Drug Delivery Reviews     Hybrid Journal   (Followers: 167, SJR: 4.09, CiteScore: 13)
Advanced Engineering Informatics     Hybrid Journal   (Followers: 12, SJR: 1.167, CiteScore: 4)
Advanced Powder Technology     Hybrid Journal   (Followers: 17, SJR: 0.694, CiteScore: 3)
Advances in Accounting     Hybrid Journal   (Followers: 8, SJR: 0.277, CiteScore: 1)
Advances in Agronomy     Full-text available via subscription   (Followers: 15, SJR: 2.384, CiteScore: 5)
Advances in Anesthesia     Full-text available via subscription   (Followers: 28, SJR: 0.126, CiteScore: 0)
Advances in Antiviral Drug Design     Full-text available via subscription   (Followers: 2)
Advances in Applied Mathematics     Full-text available via subscription   (Followers: 10, SJR: 0.992, CiteScore: 1)
Advances in Applied Mechanics     Full-text available via subscription   (Followers: 11, SJR: 1.551, CiteScore: 4)
Advances in Applied Microbiology     Full-text available via subscription   (Followers: 24, SJR: 2.089, CiteScore: 5)
Advances In Atomic, Molecular, and Optical Physics     Full-text available via subscription   (Followers: 14, SJR: 0.572, CiteScore: 2)
Advances in Biological Regulation     Hybrid Journal   (Followers: 4, SJR: 2.61, CiteScore: 7)
Advances in Botanical Research     Full-text available via subscription   (Followers: 2, SJR: 0.686, CiteScore: 2)
Advances in Cancer Research     Full-text available via subscription   (Followers: 33, SJR: 3.043, CiteScore: 6)
Advances in Carbohydrate Chemistry and Biochemistry     Full-text available via subscription   (Followers: 9, SJR: 1.453, CiteScore: 2)
Advances in Catalysis     Full-text available via subscription   (Followers: 5, SJR: 1.992, CiteScore: 5)
Advances in Cell Aging and Gerontology     Full-text available via subscription   (Followers: 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: 28, SJR: 0.156, CiteScore: 1)
Advances in Child Development and Behavior     Full-text available via subscription   (Followers: 10, SJR: 0.713, CiteScore: 1)
Advances in Chronic Kidney Disease     Full-text available via subscription   (Followers: 10, SJR: 1.316, CiteScore: 2)
Advances in Clinical Chemistry     Full-text available via subscription   (Followers: 26, SJR: 1.562, CiteScore: 3)
Advances in Colloid and Interface Science     Full-text available via subscription   (Followers: 20, SJR: 1.977, CiteScore: 8)
Advances in Computers     Full-text available via subscription   (Followers: 14, SJR: 0.205, CiteScore: 1)
Advances in Dermatology     Full-text available via subscription   (Followers: 15)
Advances in Developmental Biology     Full-text available via subscription   (Followers: 12)
Advances in Digestive Medicine     Open Access   (Followers: 9)
Advances in DNA Sequence-Specific Agents     Full-text available via subscription   (Followers: 7)
Advances in Drug Research     Full-text available via subscription   (Followers: 25)
Advances in Ecological Research     Full-text available via subscription   (Followers: 44, SJR: 2.524, CiteScore: 4)
Advances in Engineering Software     Hybrid Journal   (Followers: 29, SJR: 1.159, CiteScore: 4)
Advances in Experimental Biology     Full-text available via subscription   (Followers: 8)
Advances in Experimental Social Psychology     Full-text available via subscription   (Followers: 46, SJR: 5.39, CiteScore: 8)
Advances in Exploration Geophysics     Full-text available via subscription   (Followers: 1)
Advances in Fluorine Science     Full-text available via subscription   (Followers: 9)
Advances in Food and Nutrition Research     Full-text available via subscription   (Followers: 60, SJR: 0.591, CiteScore: 2)
Advances in Fuel Cells     Full-text available via subscription   (Followers: 17)
Advances in Genetics     Full-text available via subscription   (Followers: 19, SJR: 1.354, CiteScore: 4)
Advances in Genome Biology     Full-text available via subscription   (Followers: 10, SJR: 12.74, CiteScore: 13)
Advances in Geophysics     Full-text available via subscription   (Followers: 6, SJR: 1.193, CiteScore: 3)
Advances in Heat Transfer     Full-text available via subscription   (Followers: 24, SJR: 0.368, CiteScore: 1)
Advances in Heterocyclic Chemistry     Full-text available via subscription   (Followers: 12, SJR: 0.749, CiteScore: 3)
Advances in Human Factors/Ergonomics     Full-text available via subscription   (Followers: 23)
Advances in Imaging and Electron Physics     Full-text available via subscription   (Followers: 2, SJR: 0.193, CiteScore: 0)
Advances in Immunology     Full-text available via subscription   (Followers: 36, SJR: 4.433, CiteScore: 6)
Advances in Inorganic Chemistry     Full-text available via subscription   (Followers: 10, SJR: 1.163, CiteScore: 2)
Advances in Insect Physiology     Full-text available via subscription   (Followers: 2, SJR: 1.938, CiteScore: 3)
Advances in Integrative Medicine     Hybrid Journal   (Followers: 6, SJR: 0.176, CiteScore: 0)
Advances in Intl. Accounting     Full-text available via subscription   (Followers: 3)
Advances in Life Course Research     Hybrid Journal   (Followers: 8, SJR: 0.682, CiteScore: 2)
Advances in Lipobiology     Full-text available via subscription   (Followers: 1)
Advances in Magnetic and Optical Resonance     Full-text available via subscription   (Followers: 8)
Advances in Marine Biology     Full-text available via subscription   (Followers: 18, SJR: 0.88, CiteScore: 2)
Advances in Mathematics     Full-text available via subscription   (Followers: 11, SJR: 3.027, CiteScore: 2)
Advances in Medical Sciences     Hybrid Journal   (Followers: 7, SJR: 0.694, CiteScore: 2)
Advances in Medicinal Chemistry     Full-text available via subscription   (Followers: 5)
Advances in Microbial Physiology     Full-text available via subscription   (Followers: 4, SJR: 1.158, CiteScore: 3)
Advances in Molecular and Cell Biology     Full-text available via subscription   (Followers: 23)
Advances in Molecular and Cellular Endocrinology     Full-text available via subscription   (Followers: 8)
Advances in Molecular Toxicology     Full-text available via subscription   (Followers: 7, SJR: 0.182, CiteScore: 0)
Advances in Nanoporous Materials     Full-text available via subscription   (Followers: 4)
Advances in Oncobiology     Full-text available via subscription   (Followers: 2)
Advances in Organ Biology     Full-text available via subscription   (Followers: 2)
Advances in Organometallic Chemistry     Full-text available via subscription   (Followers: 17, SJR: 1.875, CiteScore: 4)
Advances in Parallel Computing     Full-text available via subscription   (Followers: 7, SJR: 0.174, CiteScore: 0)
Advances in Parasitology     Full-text available via subscription   (Followers: 5, SJR: 1.579, CiteScore: 4)
Advances in Pediatrics     Full-text available via subscription   (Followers: 25, SJR: 0.461, CiteScore: 1)
Advances in Pharmaceutical Sciences     Full-text available via subscription   (Followers: 12)
Advances in Pharmacology     Full-text available via subscription   (Followers: 16, SJR: 1.536, CiteScore: 3)
Advances in Physical Organic Chemistry     Full-text available via subscription   (Followers: 8, SJR: 0.574, CiteScore: 1)
Advances in Phytomedicine     Full-text available via subscription  
Advances in Planar Lipid Bilayers and Liposomes     Full-text available via subscription   (Followers: 3, SJR: 0.109, CiteScore: 1)
Advances in Plant Biochemistry and Molecular Biology     Full-text available via subscription   (Followers: 10)
Advances in Plant Pathology     Full-text available via subscription   (Followers: 5)
Advances in Porous Media     Full-text available via subscription   (Followers: 5)
Advances in Protein Chemistry     Full-text available via subscription   (Followers: 19)
Advances in Protein Chemistry and Structural Biology     Full-text available via subscription   (Followers: 20, SJR: 0.791, CiteScore: 2)
Advances in Psychology     Full-text available via subscription   (Followers: 65)
Advances in Quantum Chemistry     Full-text available via subscription   (Followers: 6, SJR: 0.371, CiteScore: 1)
Advances in Radiation Oncology     Open Access   (Followers: 1, SJR: 0.263, CiteScore: 1)
Advances in Small Animal Medicine and Surgery     Hybrid Journal   (Followers: 3, SJR: 0.101, CiteScore: 0)
Advances in Space Biology and Medicine     Full-text available via subscription   (Followers: 6)
Advances in Space Research     Full-text available via subscription   (Followers: 403, SJR: 0.569, CiteScore: 2)
Advances in Structural Biology     Full-text available via subscription   (Followers: 5)
Advances in Surgery     Full-text available via subscription   (Followers: 12, SJR: 0.555, CiteScore: 2)
Advances in the Study of Behavior     Full-text available via subscription   (Followers: 34, SJR: 2.208, CiteScore: 4)
Advances in Veterinary Medicine     Full-text available via subscription   (Followers: 18)
Advances in Veterinary Science and Comparative Medicine     Full-text available via subscription   (Followers: 14)
Advances in Virus Research     Full-text available via subscription   (Followers: 5, SJR: 2.262, CiteScore: 5)
Advances in Water Resources     Hybrid Journal   (Followers: 47, SJR: 1.551, CiteScore: 3)
Aeolian Research     Hybrid Journal   (Followers: 6, SJR: 1.117, CiteScore: 3)
Aerospace Science and Technology     Hybrid Journal   (Followers: 358, SJR: 0.796, CiteScore: 3)
AEU - Intl. J. of Electronics and Communications     Hybrid Journal   (Followers: 8, SJR: 0.42, CiteScore: 2)
African J. of Emergency Medicine     Open Access   (Followers: 6, SJR: 0.296, CiteScore: 0)
Ageing Research Reviews     Hybrid Journal   (Followers: 11, SJR: 3.671, CiteScore: 9)
Aggression and Violent Behavior     Hybrid Journal   (Followers: 463, SJR: 1.238, CiteScore: 3)
Agri Gene     Hybrid Journal   (Followers: 1, SJR: 0.13, CiteScore: 0)
Agricultural and Forest Meteorology     Hybrid Journal   (Followers: 17, SJR: 1.818, CiteScore: 5)
Agricultural Systems     Hybrid Journal   (Followers: 31, SJR: 1.156, CiteScore: 4)
Agricultural Water Management     Hybrid Journal   (Followers: 42, SJR: 1.272, CiteScore: 3)
Agriculture and Agricultural Science Procedia     Open Access   (Followers: 4)
Agriculture and Natural Resources     Open Access   (Followers: 3)
Agriculture, Ecosystems & Environment     Hybrid Journal   (Followers: 57, SJR: 1.747, CiteScore: 4)
Ain Shams Engineering J.     Open Access   (Followers: 5, SJR: 0.589, CiteScore: 3)
Air Medical J.     Hybrid Journal   (Followers: 6, SJR: 0.26, CiteScore: 0)
AKCE Intl. J. of Graphs and Combinatorics     Open Access   (SJR: 0.19, CiteScore: 0)
Alcohol     Hybrid Journal   (Followers: 12, SJR: 1.153, CiteScore: 3)
Alcoholism and Drug Addiction     Open Access   (Followers: 11)
Alergologia Polska : Polish J. of Allergology     Full-text available via subscription   (Followers: 1)
Alexandria Engineering J.     Open Access   (Followers: 1, SJR: 0.604, CiteScore: 3)
Alexandria J. of Medicine     Open Access   (Followers: 1, SJR: 0.191, CiteScore: 1)
Algal Research     Partially Free   (Followers: 10, SJR: 1.142, CiteScore: 4)
Alkaloids: Chemical and Biological Perspectives     Full-text available via subscription   (Followers: 2)
Allergologia et Immunopathologia     Full-text available via subscription   (Followers: 1, SJR: 0.504, CiteScore: 1)
Allergology Intl.     Open Access   (Followers: 5, SJR: 1.148, CiteScore: 2)
Alpha Omegan     Full-text available via subscription   (SJR: 3.521, CiteScore: 6)
ALTER - European J. of Disability Research / Revue Européenne de Recherche sur le Handicap     Full-text available via subscription   (Followers: 10, SJR: 0.201, CiteScore: 1)
Alzheimer's & Dementia     Hybrid Journal   (Followers: 52, SJR: 4.66, CiteScore: 10)
Alzheimer's & Dementia: Diagnosis, Assessment & Disease Monitoring     Open Access   (Followers: 4, SJR: 1.796, CiteScore: 4)
Alzheimer's & Dementia: Translational Research & Clinical Interventions     Open Access   (Followers: 4, SJR: 1.108, CiteScore: 3)
Ambulatory Pediatrics     Hybrid Journal   (Followers: 6)
American Heart J.     Hybrid Journal   (Followers: 56, SJR: 3.267, CiteScore: 4)
American J. of Cardiology     Hybrid Journal   (Followers: 59, SJR: 1.93, CiteScore: 3)
American J. of Emergency Medicine     Hybrid Journal   (Followers: 44, SJR: 0.604, CiteScore: 1)
American J. of Geriatric Pharmacotherapy     Full-text available via subscription   (Followers: 11)
American J. of Geriatric Psychiatry     Hybrid Journal   (Followers: 13, SJR: 1.524, CiteScore: 3)
American J. of Human Genetics     Hybrid Journal   (Followers: 34, SJR: 7.45, CiteScore: 8)
American J. of Infection Control     Hybrid Journal   (Followers: 28, SJR: 1.062, CiteScore: 2)
American J. of Kidney Diseases     Hybrid Journal   (Followers: 35, SJR: 2.973, CiteScore: 4)
American J. of Medicine     Hybrid Journal   (Followers: 48)
American J. of Medicine Supplements     Full-text available via subscription   (Followers: 3, SJR: 1.967, CiteScore: 2)
American J. of Obstetrics and Gynecology     Hybrid Journal   (Followers: 225, SJR: 2.7, CiteScore: 4)
American J. of Ophthalmology     Hybrid Journal   (Followers: 66, SJR: 3.184, CiteScore: 4)
American J. of Ophthalmology Case Reports     Open Access   (Followers: 5, SJR: 0.265, CiteScore: 0)
American J. of Orthodontics and Dentofacial Orthopedics     Full-text available via subscription   (Followers: 6, SJR: 1.289, CiteScore: 1)
American J. of Otolaryngology     Hybrid Journal   (Followers: 25, SJR: 0.59, CiteScore: 1)
American J. of Pathology     Hybrid Journal   (Followers: 28, SJR: 2.139, CiteScore: 4)
American J. of Preventive Medicine     Hybrid Journal   (Followers: 29, SJR: 2.164, CiteScore: 4)
American J. of Surgery     Hybrid Journal   (Followers: 38, SJR: 1.141, CiteScore: 2)
American J. of the Medical Sciences     Hybrid Journal   (Followers: 12, SJR: 0.767, CiteScore: 1)
Ampersand : An Intl. J. of General and Applied Linguistics     Open Access   (Followers: 7)
Anaerobe     Hybrid Journal   (Followers: 4, SJR: 1.144, CiteScore: 3)
Anaesthesia & Intensive Care Medicine     Full-text available via subscription   (Followers: 63, SJR: 0.138, CiteScore: 0)
Anaesthesia Critical Care & Pain Medicine     Full-text available via subscription   (Followers: 19, SJR: 0.411, CiteScore: 1)
Anales de Cirugia Vascular     Full-text available via subscription   (Followers: 1)
Anales de Pediatría     Full-text available via subscription   (Followers: 3, SJR: 0.277, CiteScore: 0)
Anales de Pediatría (English Edition)     Full-text available via subscription  
Anales de Pediatría Continuada     Full-text available via subscription  
Analytic Methods in Accident Research     Hybrid Journal   (Followers: 5, SJR: 4.849, CiteScore: 10)
Analytica Chimica Acta     Hybrid Journal   (Followers: 43, SJR: 1.512, CiteScore: 5)
Analytical Biochemistry     Hybrid Journal   (Followers: 188, SJR: 0.633, CiteScore: 2)
Analytical Chemistry Research     Open Access   (Followers: 12, SJR: 0.411, CiteScore: 2)
Analytical Spectroscopy Library     Full-text available via subscription   (Followers: 13)
Anesthésie & Réanimation     Full-text available via subscription   (Followers: 2)
Anesthesiology Clinics     Full-text available via subscription   (Followers: 23, SJR: 0.683, CiteScore: 2)
Angiología     Full-text available via subscription   (SJR: 0.121, CiteScore: 0)
Angiologia e Cirurgia Vascular     Open Access   (Followers: 1, SJR: 0.111, CiteScore: 0)
Animal Behaviour     Hybrid Journal   (Followers: 205, SJR: 1.58, CiteScore: 3)

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Journal Cover
Acta Materialia
Journal Prestige (SJR): 3.263
Citation Impact (citeScore): 6
Number of Followers: 276  
 
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 1359-6454
Published by Elsevier Homepage  [3161 journals]
  • Editors for Acta Materialia
    • Abstract: Publication date: 15 April 2019Source: Acta Materialia, Volume 168Author(s):
       
  • Formation, stability and ultrahigh strength of novel nanostructured alloys
           by partial crystallization of high-entropy
           (Fe0.25Co0.25Ni0.25Cr0.125Mo0.125)86‒89B11‒14 amorphous phase
    • Abstract: Publication date: Available online 18 March 2019Source: Acta MaterialiaAuthor(s): F. Wang, A. Inoue, F.L. Kong, S.L. Zhu, E. Shalaan, F. Al-Marzouki, W.J. Botta, C.S. Kiminami, Yu.P. Ivanov, A.L. Greer Heating-induced crystallization of high-entropy (HE) (Fe0.25Co0.25Ni0.25Cr0.125Mo0.125)89‒86B11‒14 amorphous (am) alloys is examined to develop new structural materials with low B contents. The crystallization of 11B alloy occurs in three stages: first nanoscale bcc precipitates form in the amorphous matrix, second nanoscale fcc precipitates form, and the residual amorphous phase disappears in the third stage which yields borides in addition to the bcc and fcc phases. Crystallization of 14B alloy is the same, except that the order of appearance of bcc and fcc is reversed. The bcc and fcc particle diameters are 5‒15 nm and remain almost unchanged up to ∼960 K. On annealing, ultrahigh hardness of 1500‒1550 (unprecedented for boride-free structures) is attained just before the third crystallization stage. This hardening and the thermal stability of the novel [am + bcc + fcc] structures are remarkable at such low boron content and encouraging for development as ultrahigh-strength alloys. The results are interpreted in terms of the nature and extent of partitioning of elemental components between the bcc/fcc phases and the amorphous matrix, and the size and defect structures of the bcc and fcc precipitates. The magnetic flux density at RT increases by precipitation of bcc and decreases by appearance of fcc. Slower quenching of the 11B alloy shows a pseudo-polymorphic crystallization that may be characteristic of multicomponent HE systems.Graphical abstractImage 1
       
  • Basal slip mediated tension twin variant selection in magnesium WE43 alloy
    • Abstract: Publication date: Available online 18 March 2019Source: Acta MaterialiaAuthor(s): Dikai Guan, Brad Wynne, Junheng Gao, Yuhe Huang, W. Mark Rainforth Tension twinning nucleation and evolution in Mg WE43 alloy over a large sampling area was investigated using a quasi-in-situ EBSD/SEM method during interrupted compression testing. The results showed tension twins with both high and low macroscopic Schmid factor (MSF) were activated under a compressive stress of 100 MPa with a strain rate of 10-1 s-1. Basal slip in most grains dominated at this stress, so nucleation of twin variants required little interaction with non-basal slip, which was different from other studies that reported prismatic slip and/or tension twinning were required to activate some low MSF tension twin variants. The geometric compatibility factor (m') was demonstrated to be an important factor to determine tension twin variant selection assisted by basal slip. The analysis indicated m' played a critical role over MSF in tension twin variant selection during twin nucleation stage, and final twin variant types were insensitive to increasing stress, but they inherited twin variant types determined at twin nucleation stage. Moreover, which specific grain boundary of a grain with hard orientation for basal slip would nucleate which twin variant could be also validated by m' and largely depended on two factors: (a) high value of m' with 1st or 2nd rank between the tension twinning of nucleated twin variant and basal slip in adjoining grains; and (b) intensive basal slip activity in the neighbouring grains before twin nucleation.Graphical abstractImage 1
       
  • Phase-field study on the growth of magnesium silicide occasioned by
           reactive diffusion on the surface of Si-foams
    • Abstract: Publication date: Available online 17 March 2019Source: Acta MaterialiaAuthor(s): Fei Wang, Patrick Altschuh, Alexander M. Matz, Johann Heimann, Bettina S. Matz, Britta Nestler, Norbert Jost Magnesium silicide has been widely exploited in thermoelectric and photovoltaic devices. In contrast to the fabrication of the magnesium silicide phase on flat Si-substrates in literature, we here concentrate on the growth of the Mg2Si phase on the surface of Si-foams by solid-state phase transformation. Based on the reactive diffusion mechanism, which is responsible for the growth of magnesium silicide, we adopt a grand-potential-based phase-field model to investigate the microstructural evolution during the solid-state phase transformation. The presently developed phase-field concept is capable to model the solid-state phase transformation between three stoichiometric phases, Mg2Si, diamond and Mg-hcp phases. The simulated microstructures are scrutinized via a skeleton algorithm. The simulation results reveal that the thickness distribution of the Mg2Si phase follows the one of the foam-ligaments and that the average thickness of the magnesium silicide phase strongly depends upon the surface-volume ratio of the Si-foam rather than the porosity. In addition, it has been found that for a constant porosity, the mean value for the thickness of the magnesium silicide is different when the thickness distribution of the foam-strut is different. The relationship between the local thickness of the magnesium silicide phase and the foam-strut is analyzed based on the skeleton of the microstructure.Graphical abstractImage 1
       
  • Electrocaloric Fatigue of Lead Magnesium Niobate Mediated by an
           Electric-Field-Induced Phase Transformation
    • Abstract: Publication date: Available online 16 March 2019Source: Acta MaterialiaAuthor(s): Andraž Bradeško, Lovro Fulanović, Marko Vrabelj, Mojca Otoničar, Hana Uršič, Alexandra Henriques, Ching-Chang Chung, Jacob L. Jones, Barbara Malič, Zdravko Kutnjak, Tadej Rojac Electrocaloric fatigue, i.e., the degradation of the electrocaloric temperature change of an active material under continuous electric-field cycling, has not been addressed in detail so far, despite the elevated electric fields expected for EC cooling devices. Here, we investigate the electrocaloric fatigue mechanism of a prototype relaxor material, i.e., Pb(Mg1/3Nb2/3)O3, by directly measuring its temperature response under device-relevant electric-field conditions. We show that after a critical number of field cycles the temperature of the sample begins to increase dramatically, leading to a significant degradation of the cooling properties. The degradation of cooling properties is investigated using a combination of multiscale characterization techniques, revealing that the origin of the degradation is the increased grain boundary conductance caused by an unexpected electric-field-induced phase transformation to a ferroelectric phase. We further show that this transformation and thus the fatigue can be regulated by careful control of the temperature and electric-field conditions. By revealing a previously unexplored fatigue mechanism, this study provides the first guidelines for the integration of high-performance relaxors into cooling devices.Graphical abstractImage 1
       
  • Ab initio thermodynamics of complex alloys: the case of Al- and Mn-doped
           ferritic steels
    • Abstract: Publication date: Available online 15 March 2019Source: Acta MaterialiaAuthor(s): Rémy Besson, Jérôme Dequeker, Ludovic Thuinet, Alexandre Legris In the context of Al- and Mn-doped ferritic steels, we progressively elaborate an atomic-scale energy model to reproduce the thermodynamic behavior of quaternary Fe-Al-Mn-C on a bcc lattice. This model is built on physical concepts: DFT calculations, pair Hamiltonians, non-configurational thermal effects, these elements being combined in a reasoned way to lead to a mastered and predictive formulation. In particular, this approach allows to explore the correlation between ordering in substitutional ternary Fe-Al-Mn and interstitial carbon, which brings new elements to the metallurgy of carbon in ferritic steels.Graphical abstractImage 1
       
  • Tailoring the anisotropic mechanical properties of hexagonal M7X3 (M=Fe,
           Cr, W, Mo; X=C, B) by multialloying
    • Abstract: Publication date: Available online 15 March 2019Source: Acta MaterialiaAuthor(s): XiaoYu Chong, MingYu Hu, Peng Wu, Quan Shan, Ye Hua Jiang, Zu Lai Li, Jing Feng As the main strengthening phases in high-chromium cast irons (HCCIs), the elastic and ductile-brittle properties of M7C3 carbides are critical for the wear-resistance and application of HCCIs. The M7C3 carbides are characterized to be Cr3.87Fe3.04C3.09 and hexagonal system (P63mc) in Fe-25.81 wt% Cr-4.45 wt% C alloy. Based on the elemental ratio and distribution, the crystals are built by a non-dilute ordered model. Mulialloying of Fe, Cr, W, Mo and B is adopted to design the mechanical properties of M7C3 carbides. Results from first-principles calculations and nanoindentation show that the W+B and W+Mo doping can increase the ductility but not significantly decrease the mechanical modulus of Cr4Fe3C3, and Mo+B and Mo+W+B doping can improve the hardness of Cr4Fe3C3 in HCCIs with finite decrease of ductility, which are all effective strategy to balance the ductility and strength of Cr4Fe3C3 and enhance the wear-resistance of HCCIs. The relationship between intrinsic hardness (HV) and Pugh ratio (B/G) are fitted as HV=29.4 GPa-7.6 GPa×B/G, from which the maximum HV and B/G are 29.4 GPa and 3.87 by multialloying strategy, respectively. The bulk, shear, Young’s modulus and hardness are largest during 0.61 -0.63 electrons/Å3 range of the effective density of valence electrons, while B/G and Poisson's ratio (σ) are smallest. Considering that M7C3 carbide is rod-like monocrystal with strong orientation in HCCIs, the calculated and experimental Young’s modulus from nanoindentation along non-[0001] direction is smaller than other directions, which provides guidance to achieve high wear-resistance of HCCIs by directional solidification. The elastic anisotropy is determined by the different atomic arrangement and chemical bonding along different crystallographic orientation. The decrease of mechanical modulus is attributed to the C-Mo and C-W bonds in M7C3 multicomponent carbides weaker than C-Fe and C-Cr bonds in Cr4Fe3C3.Graphical abstractImage 1
       
  • Machine-learning phase prediction of high-entropy alloys
    • Abstract: Publication date: Available online 15 March 2019Source: Acta MaterialiaAuthor(s): Wenjiang Huang, Pedro Martin, Houlong L. Zhuang High-entropy alloys (HEAs) have been receiving intensive attention due to their unusual properties that largely depend on the selection among three phases: solid solution (SS), intermetallic compound (IM), and mixed SS and IM (SS+IM). Accurate phase prediction is therefore crucial for guiding the selection of a combination of elements to form a HEA with desirable properties. It is widely accepted that the phase selection is correlated with elemental features such as valence electron concentration and the formation enthalpy, leading to a set of parametric phase-selection rules [1]. Previous studies on predicting the phase selection employed density functional theory (DFT) calculations to obtain some correlated parameters. But DFT calculations are time consuming and exhibit uncertainties in terms of treating the d orbitals of transition-metal atoms that are often components of HEAs. Here we employ machine learning (ML) algorithms to efficiently explore phase selection rules using a comprehensive experimental dataset consisting of 401 different HEAs including 174 SS, 54 IM, and 173 SS+IM phases. We adopt three different ML algorithms: K-nearest neighbors (KNN), support vector machine (SVM), and artificial neural network (ANN). To avoid overfitting, we divide the whole dataset into four nearly equal portions to perform a cross validation. For the classification of the three phases at the same time, the testing accuracy values from the KNN, SVM and ANN calculations achieve 68.3%, 64.3% and 74.3%, respectively. We then focus on the classification of two of the three phases using SVM and ANN. We find that the testing accuracy values using ANN in classifying the SS and IM phases, the IM and SS+IM phases, and the SS and SS+IM phases, are 86.7%, 94.3%, and 78.9%, respectively, which are higher than the corresponding testing accuracy values using SVM. As such, the trained ANN model performs the best among the three ML algorithms and is useful for predicting the phases of new HEAs. Our work provides an alternative route of computational design of HEAs, which is also applicable to accelerate the discovery of other metal alloys for modern engineering applications.Graphical abstractImage 1
       
  • Dispersoid Composition in Zirconium Containing Al-Zn-Mg-Cu (AA7010)
           Aluminium Alloy
    • Abstract: Publication date: Available online 13 March 2019Source: Acta MaterialiaAuthor(s): A.M. Cassell, J.D. Robson, C.P. Race, A. Eggeman, T. Hashimoto, M. Besel Zirconium (Zr) is used in modern aluminum alloys to form dispersoids that control grain structure. The interaction of these dispersoids with the alloying elements used to strengthen aluminum remains poorly understood. We have used high resolution imaging and composition analysis via electron microscopy to study the Zr-rich dispersoids in AA7010, a commercial Al-Zn-Mg-Cu alloy, addressing this knowledge gap.We show that the dispersoids are not of the ideal Al3Zr stoichiometry, and contain Zn up to approximately 15 at%. Copper also concentrates in the dispersoids up to approximately 6 at%. Atomistic simulation was used to predict the partitioning, demonstrating favourable substitution of Zn and Cu onto the Al sublattice in the dispersoid phase, consistent with the measurements. We have also observed larger, facetted dispersoids, which are not of a phase observed in the binary Al–Zr system. Instead, we have found a previously unreported dispersoid structure (tI10 Ni4Mo structure type), which we propose is stabilized by the presence of Zn.We have calculated the total loss in Zn and Cu due to partitioning into the dispersoids. We show this is too small to directly have a detrimental effect on age hardening, but may have a secondary effect in promoting heterogeneous nucleation for undesirable quench induced precipitation.Graphical abstractImage 1
       
  • Structure and Morphology of Crystalline Nuclei arising in a Crystallizing
           Liquid Metallic Film
    • Abstract: Publication date: Available online 13 March 2019Source: Acta MaterialiaAuthor(s): Bulat N. Galimzyanov, Dinar T. Yarullin, Anatolii V. Mokshin Control of the crystallization process at the microscopic level makes it possible to generate the nanocrystalline samples with the desired structural and morphological properties, that is of great importance for modern industry. In the present work, we study the influence of supercooling on the structure and morphology of the crystalline nuclei arising and growing within a liquid metallic film. The cluster analysis allows us to compute the diffraction patterns and to evaluate the morphological characteristics (the linear sizes of the homogeneous part and the thickness of the surface layer) of the crystalline nuclei emergent in the system at different levels of supercooling. We find that the liquid metallic film at the temperatures corresponded to low supercooling levels crystallizes into a monocrystal, whereas a polycrystalline structure forms at deep supercooling levels. We find that the temperature dependence of critical size of the crystalline nuclei contains two distinguishable regimes with the crossover temperature T/Tg≈1.15 (Tg is the glass transition temperature), which appears due to the specific geometry of the system.Graphical abstractImage 1
       
  • Influence of electronic and metallographic structures on hydrogen
           redistribution in La(Fe,Si)13-based magnetocaloric compounds
    • Abstract: Publication date: Available online 12 March 2019Source: Acta MaterialiaAuthor(s): Asaya Fujita Hydrogen redistribution between the two magnetic states that coexist at transition temperature, which is called the splitting phenomenon, was investigated in La(Fe,Si)13-based magnetocaloric compounds. Asymmetric splitting into two portions with different volumes and H concentrations was observed on analysis of thermomagnetization curves, and this asymmetric growth was determined, using first-principle calculations, to be correlated with the electronic band structure. Excellent entropy change was observed in (Ce,La)(Fe,Mn,Si)13H specimens, on which full hydrogenation was performed to inhibit the splitting phenomenon. The splitting phenomenon was found to be incompletely suppressed in these specimens after a short duration of homogenization annealing, and it gradually disappeared with increasing annealing duration. Magneto-optical observation with a magnetic transfer film shows that first heterogeneous nucleation appears at the spot-like sites; this is followed by the instant growth of a cloud-shaped droplet. The pinning of the droplet boundary by the spot-like nucleation site was also confirmed. Comparison of the metallographic structure revealed that the spot-like heterogeneous nucleation sites correspond to the grain-boundary triple points formed by imperfect grains, which would be swallowed by other stable grains during a ripening action. The droplet boundary, which is central to the hydrogen diffusion process, runs inside of crystalline grains, and in consequence, the main diffusion path of hydrogen was considered to be the body diffusion.Graphical abstractImage 1
       
  • Understanding the mechanism of thermal-stable high-performance
           piezoelectricity
    • Abstract: Publication date: Available online 12 March 2019Source: Acta MaterialiaAuthor(s): Minxia Fang, Shailendra Rajput, Zhonghua Dai, Yuanchao Ji, Yanshuang Hao, Xiaobing Ren Lead zirconate titanate (PZT) ceramics have been widely used because of their large piezoelectric response and good temperature-stability in the vicinity of morphotropic phase boundary (MPB). However, the understanding on the mechanism of temperature-stable piezoelectricity in PZT is still blur and thus needs to explore properly since the urgently-needed Pb-free systems designed by the same MPB method commonly show poor temperature stability. In this paper, we investigate the property-microstructure relationship for the thermal-stable piezoelectricity through comparative studies of the phase diagram, in-situ piezoelectricity and microstructure evolution in both Pb-based and Pb-free systems. Our results show that the piezoelectric thermal-stability is closely related to the verticality of MPB and the doping elements. Tilted MPB shows worse thermal-stability as compared with that of the vertical MPB. While acceptor doped commercial PZTs show better thermal-stability when compared with that of the donor doped PZT. Moreover, our in-situ TEM reveals that it is the thermal-stable fine microstructure (i.e., no degeneration) that corresponds to the thermal-stable high-performance piezoelectricity. Our work provides an in-depth understanding of the relationship between the phase diagram, microstructure and thermal-stable piezoelectricity, which shall benefit the designing of new lead-free temperature-stable piezoelectric materials.Graphical abstractImage 1
       
  • Machine learning assisted design of high entropy alloys with desired
           property
    • Abstract: Publication date: Available online 12 March 2019Source: Acta MaterialiaAuthor(s): Cheng Wen, Yan Zhang, Changxin Wang, Dezhen Xue, Yang Bai, Stoichko Antonov, Lanhong Dai, Turab Lookman, Yanjing Su We formulate a materials design strategy combining a machine learning (ML) surrogate model with experimental design algorithms to search for high entropy alloys (HEAs) with large hardness in a model Al-Co-Cr-Cu-Fe-Ni system. We fabricated several alloys with hardness 10% higher than the best value in the original training dataset via only seven experiments. We find that a strategy using both the compositions and descriptors based on a knowledge of the properties of HEAs, outperforms that merely based on the compositions alone. This strategy offers a recipe to rapidly optimize multi-component systems, such as bulk metallic glasses and superalloys, towards desired properties.Graphical abstractIn the present study, we proposed a data-driven approach combining machine learning, experimental design and feedback from experiment to accelerate the search for multi-component alloys with target properties. We demonstrated the efficiency of our approach by rapidly obtaining several alloys with hardness 10% higher than the best value in the original training dataset via only seven experiments. In Iteration Loop I, a machine learning surrogate model is trained to learn the property-composition, relationship, yi=f(ci), with associated uncertainties. The model is applied to the vast unexplored space and a utility function is employed to recommend the most informative candidate for the next experiment, which balances the exploitation and exploration. Feedback from experimental synthesis and characterization allows the subsequent iterative improvement of the surrogate model. Iteration Loop II is essentially same as Iteration Loop I, except that a features pool was introduced to the Iteration Loop I and a surrogate model is trained from composition (ci) and the preselected physical features (pi), yi=f(ci,pi). We found that the approach using both the composition and the descriptors based on domain knowledge can more effectively accelerate material optimization compared to the approach using only the compositions.Image 1
       
  • Tunable tensile ductility of metallic glasses with partially rejuvenated
           amorphous structures
    • Abstract: Publication date: Available online 12 March 2019Source: Acta MaterialiaAuthor(s): L. Zhao, K.C. Chan, S.H. Chen, S.D. Feng, D.X. Han, G. Wang We report that the tensile ductility of metallic glass (MGs) is tunable by introducing gradient rejuvenated amorphous structures (GRASs) using large-scale atomistic simulations. The results reveal that the ductile GRASs promote the formation and propagation of new shear bands in the interior unrejuvenated region by suppressing the catastrophic propagation of individual shear bands across the GRASs, thus resulting in a more dispersed plastic shearing throughout the sample. It is also demonstrated that increasing both the volume fraction and degree of structural disordering of GRASs can improve the tensile ductility of MGs and lead to a brittle-to-ductile transition of the deformation mode, although at the expense of some strength. Moreover, the critical volume fraction of GRASs required for switching the transition is found to depend on the specific degree of structural disordering. The observed structural state-dependent transition of the deformation mode is further understood from a mechanical perspective by considering the competition between the macroscopic yield strength and the critical stress of the material required for shear delocalization, based on which a criterion is developed to predict the critical transition boundary in MGs with GRASs across a wide range of structural states. The findings provide a detailed atomistic understanding of the relationship between the structural state and mechanical properties in MGs with partially rejuvenated amorphous structures, which may offer useful insights for designing and processing MGs with a sought-after combination of ductility and strength.Graphical abstractImage 1
       
  • Attaining high magnetic performance in as-sintered multi-main-phase
           Nd-La-Ce-Fe-B magnets: Toward skipping the post-sinter annealing treatment
           
    • Abstract: Publication date: Available online 11 March 2019Source: Acta MaterialiaAuthor(s): Jiaying Jin, Yongsheng Liu, Baixing Peng, Guohua Bai, Mi Yan Two-step post-sinter annealing (PSA) treatment, due to its universal applicability, has been one of the most effective strategies for enhancing the coercivity of Nd-Fe-B sintered magnets. Here we report a peculiar phenomenon that the as-sintered multi-main-phase (MMP) Nd-La-Ce-Fe-B magnet exhibits the highest reported to date coercivity of 13.0 kOe upon 25 wt.% La-Ce substituting for Nd, combined with Br = 13.12 kG, and (BH)max = 41.67 MGOe. Further annealing cannot enhance the coercivity whereas the remanence and energy product are drastically lowered, i.e. to 11.0 kOe, 12.57 kG, and 38.71 MGOe after two-step PSA, being remarkably different from the conventional Nd-Fe-B. Microstructural analysis reveals that the as-sintered Nd-La-Ce-Fe-B magnet with coexisting REFe2 (Fd3¯m) and RE-rich (Fm3¯m) intergranular phases possesses continuous GB layer isolating adjacent ferromagnetic grains. Hence the beneficial role of PSA on modifying the microstructure and strengthening the coercivity is tiny. Additionally, high-temperature PSA destroys the initial chemical heterogeneity of MMP magnets, leading to the formation of REs homogeneously distributed grains and resultant reduction of the intrinsic magnetic properties. As evidenced by in-situ Lorentz TEM characterization, PSA sample with reduced chemical gradient exhibits quick domain wall motion, compared to the as-sintered one with basically unchanged magnetic domain structure upon the same applied field. The proof-of-principle micromagnetic simulation further confirms that retaining the inhomogeneous La/Ce/Nd distribution is essential to suppress the magnetic dilution effect. These findings demonstrate the realistic prospect of skipping PSA treatment in MMP magnets, which can not only reduce the production process and cost, but also delight the scenario for designing high-performance 2:14:1-type sintered magnets based on abundant La/Ce.Graphical abstractThe as-sintered MMP magnet exhibits a highest reported to date coercivity of 13.0 kOe at 25 wt.% La-Ce substitution level, with strengthened remanence of 13.12 kG, and maximum energy product of 41.67 MGOe (a-b). The preferable magnetic performances are attributed to two main features, first is the coexisting REFe2 (Fd3¯m) and RE-rich (Fm 3¯ m) intergranular phases to form continuous GB layer, as evidenced by the smooth and clear interface between RE2Fe14B/RE2Fe14B, RE2Fe14B/REFe2, and RE2Fe14B/fcc-REOx phases (c), second is the retained chemical gradient to form core-shell morphology within an individual grain and intergrain REs deviations (d). The former feature weakens the intergranular exchange coupling, and the latter strengthens the intrinsic magnetic properties, as revealed by the reverse magnetic domain structure across the GB layer and the stable domain walls upon the applied field (e).Image 1
       
  • In-situ XRD study of actuation mechanisms in BiFeO3-K0.5Bi0.5TiO3-PbTiO3
           ceramics
    • Abstract: Publication date: 15 April 2019Source: Acta Materialia, Volume 168Author(s): Yizhe Li, Ying Chen, Zhenbo Zhang, Annette Kleppe, David A. Hall In the present study, we report a nonergodic relaxor ferroelectric composition for high temperature piezoelectric applications, 0.57BiFeO3-0.21K0.5Bi0.5TiO3-0.22PbTiO3, which exhibits Tm around 420 °C. By combining the results of in-situ synchrotron XRD and strain measurements using digital image correlation, a pseudocubic nonergodic relaxor to rhombohedral ferroelectric transformation is identified, accompanied by a volume strain close to zero. A methodology is developed to determine the crystallographic parameters of the transformed rhombohedral ferroelectric phase in a strain-free state, using the invariant intersection for diffraction stress analysis. The phase transformation process was analyzed by methods combining peak profile fitting and full pattern refinement; the results obtained illustrate the strain arising from the phase transformation, together with intrinsic/extrinsic contributions and anisotropy in the field-induced strain. The study reveals unusual microscopic strain behavior, distinguished from that of normal rhombohedral ferroelectrics, showing the combined properties of ergodic and normal ferroelectric materials and leading to a dominant intrinsic lattice strain together with a weaker extrinsic domain switching effect. The elastic coupling between different grain families is also reflected in their similar strain orientation distribution (SOD) functions.Graphical abstractImage 1
       
  • Study on complex precipitation kinetics in Cr- and Cu-added nitriding
           steels by atom probe tomography
    • Abstract: Publication date: Available online 8 March 2019Source: Acta MaterialiaAuthor(s): Jun Takahashi, Kazuto Kawakami, Kaoru Kawasaki Cr- and Cu-added steel sheets enable hardening of the surface region (the diffusion layer) by nitriding and simultaneously strengthening of the inner region due to thermally activated Cu precipitation during nitriding. Atom probe tomography (APT) analysis revealed that precipitate pairs, each composed of a Cu particle and CrN particle, were generated in the nitrided surface region. The influence of the complex precipitation of Cu and CrN on the precipitation kinetics and hardening properties was investigated by experimentally varying the initial state of Cu atoms in the steel by pre-aging before nitriding. It was found that Cu particles and CrN particles acted as heterogeneous nucleation sites for CrN and Cu, respectively, and the complex precipitation adversely affected the hardening of the nitrided surface region depending on the initial state of Cu. In addition, it was found that there were two types of precipitate pair forms (type-A/B), which were considered to be determined by the relation between the {001}-facet-size of the Cu particle and the size of the CrN nucleus in heterogeneous nucleation.Graphical abstractImage 1
       
  • A Transmission Electron Microscopy study of the neutron-irradiation
           response of Ti-based MAX phases at high temperatures
    • Abstract: Publication date: Available online 8 March 2019Source: Acta MaterialiaAuthor(s): Matheus A. Tunes, Robert W. Harrison, Stephen E. Donnelly, Philip D. Edmondson Mn+1AXn phases, or simply MAX phases, are unique nanolayered materials that have been attracting the attention of the nuclear materials community worldwide due to the recent reports of superior radiation resistance compared to conventional ceramics. However, the knowledge and understanding of their response to neutron irradiation is fairly limited, in particular at high temperatures where MAX phases are expected to have high thermodynamic phase stability. In this paper, a complete and extensive study of neutron-irradiation effects at high temperatures on Ti-based MAX phases is presented. The MAX phases Ti3SiC2 and Ti2AlC were irradiated at 1273 K in the High-Flux Isotope Reactor located at the Oak Ridge National Laboratory up to 10 displacement per atom (dpa). Post-irradiation characterisation was performed within a Transmission Electron Microscope on both irradiated and pristine samples. Upon increasing the dose from 2 to 10 dpa, the areal density of black-spots in the Ti2AlC was observed to significantly increase while in the Ti3SiC2, disordered dislocation networks were observed. Regarding the Ti3SiC2, black-spot damage was observed to be concentrated within secondary phases, but absent in the matrix. Dislocation lines and loops were observed at both 2 and 10 dpa. The dislocation loops were identified to be of type. At 2 dpa, stacking faults were observed in both materials, but were absent at 10 dpa. Cavities have also been observed, although no relationship with between size and dose was obtained. Finally, at 10 dpa, both MAX phases exhibited evidences of phase decomposition and irradiation-induced segregation. The presented results shed light on a very complex chain of radiation-induced defects in neutron-induced microstructures in both materials at high temperatures, and provide information that will enable better design of more radiation tolerant materials in the future.Graphical abstractImage 1
       
  • Initiation and stagnation of room temperature grain coarsening in
           cyclically strained gold films
    • Abstract: Publication date: Available online 7 March 2019Source: Acta MaterialiaAuthor(s): Oleksandr Glushko, Gerhard Dehm Despite the large number of experiments demonstrating that grains in a metallic material can grow at room temperature due to applied mechanical load, the mechanisms and the driving forces responsible for mechanically induced grain coarsening are still not understood. Here we present a systematic study of room temperature grain coarsening induced by cyclic strain in thin polymer-supported gold films. By means of detailed electron backscatter diffraction analysis we were able to capture both the growth of individual grains and the evolution of the whole microstructure on the basis of statistical data over thousands of grains. The experimental data are reported for three film thicknesses with slightly different microstructures and three different amplitudes of cyclic mechanical loading. Although different kinds of grain size evolution with increasing cycle number are observed depending on film thickness and strain amplitude, a single model based on a thermodynamic driving force is shown to be capable to explain initiation and stagnation of grain coarsening in all cases. The main implication of the model is that the grains having lower individual yield stress are coarsening preferentially. Besides, it is demonstrated that the existence of local shear stresses imposed on a grain boundary is not a necessary requirement for room-temperature grain coarsening. (2.2).Graphical abstractImage 1
       
  • Microstructural optimization through heat treatment for enhancing the
           fracture toughness and fatigue crack growth resistance of selective laser
           melted Ti-6Al-4V alloy
    • Abstract: Publication date: Available online 6 March 2019Source: Acta MaterialiaAuthor(s): Punit Kumar, Upadrasta Ramamurty The yield strength (σy) of Ti-6Al-4V alloy, additively manufactured via selective laser melting (SLM) of powder beds, can exceed 1000 MPa while possessing a mode I fracture toughness (KIc) of ∼50 MPam. The possibility of enhancing KIc as well as fatigue crack growth resistance, without a significant penalty on σy, via a judicious heat treatment process that transforms martensitic α’, which is present in the as-SLM microstructure due to rapid cooling of the molten metal, into an α/β phase mixture is examined. It was demonstrated that duplex annealing at temperatures below the β transus temperature of the alloy would lead to such a microstructure while retaining the mesostructure, whose nature depends on the process parameter combinations utilized. Near-doubling of the fracture toughness with only a ∼20% reduction in σy was noted upon heat treatment. While the strength becomes isotropic after heat treatment, significant anisotropy in the fracture toughness of the heat-treated alloy with columnar prior β structure was noted. While the steady state fatigue crack growth (FCG) rates are comparable to corresponding values of the same alloy, but manufactured using conventional means, the threshold for fatigue crack initiation (ΔK0) increases by 34% to 56%. The enhancement in ΔK0 was found to be a result of the transition in the near-threshold crack growth, from trans-to inter-granular and caused by the α/β basket weave microstructure, which imparts a high crack path tortuosity. Overall, this study demonstrates that post-processing heat-treatment can improve the damage tolerance of SLM Ti64 by increasing both KIc and ΔK0.Graphical abstractImage 1
       
  • Magnetic coercivity control by heat treatment in Heusler Ni-Mn-Ga(-B)
           single crystals
    • Abstract: Publication date: Available online 5 March 2019Source: Acta MaterialiaAuthor(s): Ladislav Straka, Ladislav Fekete, Michal Rameš, Eduard Belas, Oleg Heczko The combination of enlarged magnetic coercivity and magnetic shape memory (MSM) functionality is essential for novel magnetomechanical effects in MSM alloys. We found that increasing the density of thermal antiphase boundaries (APBs) provides a method to increase the magnetic coercivity without deteriorating the MSM functionality. APB density was controlled by different heat treatments in Ni-Mn-Ga(-B) MSM single crystals with five-layered modulated martensite structure. Slow cooling ∼1 K/min of Ni-Mn-Ga through the B2’–L210−1 transition resulted in a low density (< 1/micrometer) of APBs observed by magnetic force microscopy and low coercivity
       
  • IN-PLANE and out-of-plane deformation at the SUB-GRAIN scale in
           polycrystalline materials assessed by confocal microscopy
    • Abstract: Publication date: Available online 5 March 2019Source: Acta MaterialiaAuthor(s): H. Liu, J, N. Vanderesse, J.-C. Stinville, T.M. Pollock, P. Bocher, D. Texier High-resolution digital image correlation (HR-DIC) techniques have become essential in material mechanics to assess strain measurements at the scale of the elementary mechanisms responsible of the deformation in polycrystalline materials. The purpose of this study is to demonstrate the use of laser scanning confocal microscopy (LSCM) coupled with DIC techniques to deepen knowledge on the deformation process of polycrystalline nickel-based superalloy at room temperature. The LSCM technique is capable of detecting both in-plane and out-of-plane strain localization within slip bands at the sub-grain level. The LSCM observations are consistent with previous in-situ scanning electron microscopy (SEM) studies: The onset of crystal plasticity occurs primarily near Σ3 twin boundaries with bulk locations in the elastic domain (macroscopic stress as low as 80% of the 0.2 % offset yield strength (Y.S.0.2%)). This intense irreversible strain localization occurs with either a high Schmid factor (μ> 0.43) or a significant elastic modulus difference between the pair of twins (ΔΕ> 100 GPa). In the plastic deformation domain, transgranular slip activity following slip systems with the highest Schmid factor is mostly responsible for the deformation at the grain level, thus leading to strain percolation. The simultaneous in-plane and out-of-plane deformation assessment via the HR-LSCM-DIC technique was found to be essential for the identification of active slip systems. Finally, the HR-LSCM-DIC technique enabled the quantification of the real glide amplitude involved in the three-dimensional shearing process at the grain level that solely in-plane measurements cannot provide.Graphical abstractImage 1
       
  • Tailoring the strength and ductility of T91 steel by partial tempering
           treatment
    • Abstract: Publication date: Available online 4 March 2019Source: Acta MaterialiaAuthor(s): Z. Shang, Jie Ding, C. Fan, M. Song, Jin Li, Q. Li, S. Xue, K.T. Hartwig, X. Zhang T91 ferritic/martensitic steel (modified 9Cr-1Mo steel) with high strength and high ductility is considered as one of the promising structural materials for various industrial applications. Recent studies show that equal channel angular pressing can improve the yield strength of T91 steel to 1.6 GPa but degrade the uniform elongation to ∼ 1%. Here, we report that by using a modified quenching and tempering treatment, the T91 steel can achieve a high yield strength of 1.4 GPa and a uniform elongation of ∼ 5.5%. Extensive microstructure analyses show the formation of transition carbides and ultra-fine martensite within the bainite grains. This study provides an alternative approach to obtain a combined enhancement of strength and ductility for T91 steel.Graphical abstractImage 1
       
  • Apparent phase stability and domain distribution of PMN-30PT single
           crystals with nanograted Au/MnOx electrodes
    • Abstract: Publication date: Available online 4 March 2019Source: Acta MaterialiaAuthor(s): Min Gao, Chengtao Luo, Wei-Yi Chang, Chung Ming Leung, Jian Tian, Jiefang Li, Xiaoning Jiang, D. Viehland X-ray diffraction (XRD) reciprocal space mapping (RSM) was used to investigate how nanograted electrodes affect the nanodomain distribution and average crystal structure in near-surface regions of poled Pb(Mg1/3Nb2/3)O3-30%PbTiO3 (PMN-30PT) single crystals. The RSM scans revealed a transverse broadening along the (H00) direction, which was quite different from that of either the rhombohedral (R) or monoclinic A (MA) phase. This broadening provides evidence of a non-uniform distribution of tilt angles between neighboring nanodomains, induced by a gradient in the applied electric field (E). Investigations of the front and back surfaces of the crystals revealed significant differences in the RSM scans. Gradients in the domain distribution and apparent symmetry may extend throughout the crystal thickness. The data evidence changes in the nanodomain distribution that results in adaptations of the average symmetry.Graphical abstractImage 1
       
  • Interactive contraction nanotwins-stacking faults strengthening mechanism
           of Mg alloys
    • Abstract: Publication date: Available online 4 March 2019Source: Acta MaterialiaAuthor(s): Yong Sun, Bingcheng Ge, Hui Fu, Qun Zu, Xiaozhi Tang, Jianyu Huang, Qiuming Peng Light-weight Mg alloys with higher strength are especially desirable for the applications in transportation, aerospace, electronic components and implants owing to their high stiffness, abundant raw materials and environmental friendliness. Nevertheless, the majority of traditional strengthening approaches involving grain refining and precipitation strengthening could effectively prohibit dislocation movement as well as compromise ductility invariably. Here we report a novel strategy for simultaneously achieving a high specific yield strength (182 ± 8 kNmKg−1) and a good elongation (21 ± 2%) in the Mg-13 wt% Li at room temperature, based on the formation of a hierarchical contraction nanotwins-stacking faults (CTWSFs) structure by cryorolling followed by ultrahigh pressure. Both of them are the highest values reported so far, even compared to commercial Al/Ti alloys and steel. The formation process and strengthening mechanism have been clarified by ex-situ transmission electron microscopy observation and molecule dynamics simulations. It demonstrates that this unique nanoscale interactive coherent interface structure is effective to prohibit dislocation motion in Mg crystal, analogous to twin boundaries. Those new results provide insights towards designing alternative and more innovative HCP-type structural materials with higher mechanical properties.Graphical abstractImage 1
       
  • Notch strengthening in nanoscale metallic glasses
    • Abstract: Publication date: Available online 3 March 2019Source: Acta MaterialiaAuthor(s): Zhendong Sha, Yun Teng, Leong Hien Poh, Qingxiang Pei, Guichuan Xing, Huajian Gao The study of structural flaws is of paramount importance in engineering applications. However, the effect of flaws on both the macroscale and nanoscale metallic glasses (MGs) is a debatable topic, drawing from conflicting reports on notch strengthening, notch weakening, and notch insensitivity for MGs. In the present study, molecular dynamics simulations have been performed on nanoscale notched MGs to systematically investigate the influence of notch geometrical features on their fracture strengths and failure mechanisms. It is found that the symmetric double-edge notched MGs always induce a notch strengthening effect, which becomes more pronounced with increasing notch depth or sharpness. It is reasoned that the constrained growth of the plastic zone is responsible for the notch strengthening behavior. Accompanied by this notch strengthening, the deformation mode shifts from shear banding to homogeneous deformation within the un-notched ligament, when the un-notched ligament length is less than twice the shear band width. Furthermore, our simulations reveal that notch strengthening effect vanishes for asymmetric double-edge notched MGs and single-edge notched MGs. The present study provides significant insights into the deformation and failure mechanisms of nano-sized notched MGs, as well as useful guidelines for the design of MGs-based nanostructures.Graphical abstractImage 1
       
  • The assessment of local lattice strains in alloys using total scattering
    • Abstract: Publication date: Available online 2 March 2019Source: Acta MaterialiaAuthor(s): L.R. Owen, H.J. Stone, H.Y. Playford The highly-strained lattice hypothesis in high-entropy alloys (HEAs) has led to an interest in local distortions created in substitutional solid-solution alloys. In this work, the use of total scattering for the assessment and analysis of local lattice strains in alloys is considered. Using two theoretical models, the variation in the width of peaks in the pair distribution function (PDF) with changes in composition, ordering and atomic radius is presented. Key practical considerations for the successful analysis of local lattice strains using this technique are discussed, with particular reference to sample preparation, instrumental and data processing effects. Further, the mitigation of errors in local-strain measurements caused by differences in the scattering length of constituent atoms is presented. This is concluded with a proposed methodology for the analysis of local strains using this technique.Graphical abstractImage 1
       
  • Dramatically reduced lattice thermal conductivity of Mg2Si thermoelectric
           material from nanotwinning
    • Abstract: Publication date: Available online 1 March 2019Source: Acta MaterialiaAuthor(s): Guodong Li, Jiangang He, Qi An, Sergey I. Morozov, Shiqiang Hao, Pengcheng Zhai, Qingjie Zhang, William A. Goddard, G. Jeffrey Snyder Tuning phonon transport to reduce the lattice thermal conductivity (κL) is crucial for advancing thermoelectrics (TEs). Traditional strategies on κL reduction focus on introducing scattering sources such as point defects, dislocations, and grain boundaries, that may degrade the electrical conductivity and Seebeck coefficient. We suggest here, a novel twin boundary (TB) strategy that can decrease the κL of Mg2Si by ∼90%, but which may not degrade the electrical properties significantly. We validate this suggestion using density functional theory (DFT). We attribute the mechanism of TB induced κL reduction to (i) the lower phonon velocities and larger Grüneisen parameter, (ii) “rattling” of the Mg−Mg pair induced soft acoustic and optical modes, (iii) shorter phonon lifetime and higher phonon scattering rate. We predict that the size of nanotwinned structure should be controlled between 3 nm and 100 nm in the Mg2Si matrix for the most effective κL reduction. These results should be applicable for other TE or non TE energy materials with desired low thermal conductivity, suggesting rational designs of high-performance Mg2Si TE materials with low κL for the energy conversion applications.Graphical abstractImage 1
       
  • Microstructure and formation mechanisms of δ-hydrides in variable grain
           size Zircaloy-4 studied by electron backscatter diffraction
    • Abstract: Publication date: Available online 1 March 2019Source: Acta MaterialiaAuthor(s): Siyang Wang, Finn Giuliani, T. Ben Britton Microstructure and crystallography of δ phase hydrides in as-received fine grain and ‘blocky alpha’ large grain Zircaloy-4 (average grain size ∼11 μm and>200 μm, respectively) were examined using electron backscatter diffraction. Results suggest that the matrix-hydride orientation relationship is {0001}α {111}δ;α δ for all the cases studied. The habit plane of intragranular hydrides and some intergranular hydrides has been found to be {101¯7} of the surrounding matrix. The morphology of intergranular hydrides can vary depending upon the angle between the grain boundary and the hydride habit plane. The misfit strain between α-Zr and δ-hydride is accommodated mainly by high density of dislocations and twin structures in the hydrides, and a mechanism of twin formation in the hydrides has been proposed. The growth of hydrides across grain boundaries is achieved through an auto-catalytic manner similar to the growth pattern of intragranular hydrides. Easy collective shear along makes it possible for hydride nucleation at any grain boundaries, while the process seems to favour grain boundaries with low (80°) c-axis misorientation angles. Moreover, the angle between the grain boundary and the adjacent basal planes does not influence the propensity for hydride nucleation.Graphical abstractImage 1
       
  • Quantitative Linkage between the Stress at Dislocation Channel – Grain
           Boundary Interaction Sites and Irradiation Assisted Stress Corrosion Crack
           Initiation
    • Abstract: Publication date: Available online 28 February 2019Source: Acta MaterialiaAuthor(s): D.C. Johnson, B. Kuhr, D. Farkas, G.S. Was Localized deformation has emerged as a key factor in the crack initiation process for irradiated steels, as cracks are observed to nucleate preferentially at these sites. Using high resolution electron backscatter diffraction (HREBSD), the local stress tensor surrounding the dislocation channel-grain boundary interaction sites was quantified and coupled with fully determined grain boundary plane orientation information to determine, for the first time, the relationship between grain boundary normal stress and intergranular crack initiation in irradiated austenitic stainless steel. A Fe-13Cr-15Ni alloy was strained in simulated boiling water reactor, normal water chemistry after quantifying the residual stress tensor at discontinuous dislocation channel – grain boundary interaction sites where grain boundaries were determined to be well oriented with respect to the loading axis. Local stresses at the grain boundary were observed to reach magnitudes greater than 1.5 GPa at a distance of 200 nm from the intersection between the dislocation channel and the grain boundary. A pseudo-threshold stress of 0.9 GPa was measured, below which no cracking was observed. As the stress acting normal to the grain boundary increased above this value, the susceptibility to cracking increased with the cracking fraction reaching 100% at the high end of the stress range. This study shows for the first time that not only does intersection between discontinuous dislocation channels and grain boundaries result in peak local stresses, but the magnitude of the local tensile stress drives the crack initiation process.Graphical abstractImage 1
       
  • Corrigendum to “Relationship between electromechanical properties and
           phase diagram in the Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 lead-free
           piezoceramic” [Acta Mater. 80 (2014) 48–55]
    • Abstract: Publication date: Available online 28 February 2019Source: Acta MaterialiaAuthor(s): Matias Acosta, Nikola Novak, Wook Jo, Jürgen Rödel
       
  • Corrigendum to “Crystal orientation dependence of the stress-induced
           martensitic transformation in zirconia-based shape memory ceramics”
           [Acta Mater. 116 (2016) 124–135]
    • Abstract: Publication date: Available online 27 February 2019Source: Acta MaterialiaAuthor(s): Xiao Mei Zeng, Alan Lai, Chee Lip Gan, Christopher A. Schuh
       
  • Role of local stresses on co-zone twin-twin junction formation in HCP
           magnesium
    • Abstract: Publication date: Available online 26 February 2019Source: Acta MaterialiaAuthor(s): M. Arul Kumar, M. Gong, I.J. Beyerlein, J. Wang, C.N. Tomé Twin-twin interactions control the formation of twin-twin junctions (TTJ) in hexagonal metals when multiple twin variants are activated in a grain. In this work, we employ a combination of two computational techniques, a 3D full-field crystal plasticity model (CP) and large-scale molecular dynamics (MD), to study the TTJ formation associated with two non-parallel {101¯2} twins in Mg. The local intra-granular stresses generated by discrete twins are computed using a spatially resolved CP model. Atomic-scale knowledge regarding formation processes and local stresses is revealed by MD. The combined analyses suggest that the twin junction forms by the migration of the boundaries of both, the impinging and impinged twin, taking place in the immediate vicinity of the contact point. It is further shown that local stress fields that are generated after initial contact promote thickening of the impinging twin, and may facilitate nucleation of a new twin on the opposite boundary of the recipient twin. Calculations of the strain energies suggest that formation of the co-zone twin-twin junction is energetically favorable but detwinning of the TTJ upon load reversal or under cyclic loading is not.Graphical abstractImage 1
       
  • Influence of microstructural features on the plastic deformation behavior
           of metallic nanoglasses
    • Abstract: Publication date: Available online 26 February 2019Source: Acta MaterialiaAuthor(s): Omar Adjaoud, Karsten Albe We investigate the influence of microstructural properties on the plastic deformation behavior of Cu64 Zr36 nanoglasses by means of molecular dynamics simulations. Two different setups are used to prepare nanoglasses. One sample type is a nanoglass obtained by cold-compaction of chemically homogenous and inhomogenous nanoparticles. The second type is generated by assembling pre-shaped polyhedral cuts from the bulk phase. A detailed analysis of both types of microstructures shows that the volume fraction of interfaces in the particle-derived nanoglasses is significantly higher than in the bulk-derived nanoglasses with the same average grain size. The simulations also reveal a clearly distinct plastic response on uniaxial loading: The particle derived samples do not show a stress drop upon yielding, very little strain localization and no strain softening, whereas the bulk-derived samples exhibit a stress drop, strain softening and large strain localization upon loading. These findings are explained in terms of the different glass-glass interfaces present in both structure types. Our results therefore show that the macroscopic deformation behavior of metallic nanoglasses is intimately linked to the structure and topology of the glass-glass interfaces which in turn depend on the processing route.Graphical abstractImage 1
       
  • Quantum mechanics basis of quality control in hard metals
    • Abstract: Publication date: Available online 26 February 2019Source: Acta MaterialiaAuthor(s): Ruiwen Xie, Raquel Lizárraga, David Linder, Ziyong Hou, Valter Ström, Martina Lattemann, Erik Holmström, Wei Li, Levente Vitos Non-destructive and reliable quality control methods are a key aspect to designing, developing and manufacturing new materials for industrial applications and new technologies. The measurement of the magnetic saturation is one of such methods and it is conventionally employed in the cemented carbides industry. We present a general quantum mechanics based relation between the magnetic saturation and the components of the binder phase of cemented carbides, which can be directly employed as a quality control. To illustrate our results, we calculate the magnetic saturation of a binder phase, 85Ni15Fe binary alloy, using ab-initio methods and compare the theoretical predictions to the magnetic saturation measurements. We also analyse interface and segregation effects on the magnetic saturation by studying the electronic structure of the binder phase. The excellent agreement between calculations and measurements demonstrates the applicability of our method to any binder phase. Since the magnetic saturation is employed to ensure the quality of cemented carbides, the present method allows us to explore new materials for alternative binder phases efficiently.Graphical abstractImage 1
       
  • Atomistic modeling of dislocation cross-slip in nickel using free-end
           nudged elastic band method
    • Abstract: Publication date: Available online 25 February 2019Source: Acta MaterialiaAuthor(s): Dengke Chen, Luke L. Costello, Clint B. Geller, Ting Zhu, David L. McDowell Cross-slip of screw dislocations plays an important role in the plastic deformation of face-centered cubic (FCC) metals and alloys. Here we use the free-end nudged elastic band (FENEB) method to determine the atomistic reaction pathways and energy barriers of cross-slip in an FCC single crystal of Ni. We focus on the cross-slip process mediated by an array of pinning obstacles of vacancy clusters in the form of stacking fault tetrahedra. We also study a competing process of screw glide by direct cutting of pinning obstacles on the original slip plane. The activation energies of both cross-slip and obstacle-cutting are determined for different stresses, obstacle spacings and sizes. Using FENEB-calculated energy barriers, we construct dislocation mechanism maps to reveal the effects of resolved shear stress, obstacle spacing and size on the rate-controlling dislocation process for plastic deformation. We further evaluate the activation volumes of cross-slip and obstacle-cutting. The latter result emphasizes the notion of finite strength of the atomically sized pinning obstacles to dislocations and also validates the Nabarro scaling law of the linear dependence of activation volume on obstacle spacing.Graphical abstractImage 1
       
  • Mobility of Dislocations in Aluminum: Faceting and Asymmetry during
           Nanoscale Dislocation Shear Loop Expansion
    • Abstract: Publication date: Available online 25 February 2019Source: Acta MaterialiaAuthor(s): Khanh Dang, Darshan Bamney, Kanis Bootsita, Laurent Capolungo, Douglas E. Spearot Dislocation loop expansion in Al is studied with a hierarchical multiscale approach via atomistic and discrete dislocation dynamics (DDD) simulations. First, mobility laws for straight screw, 30o, 60o, and edge dislocations are calculated using molecular dynamics (MD) simulations. Each atomistic mobility law is described by an empirical piecewise function with a character angle dependent power law exponent to capture the nonlinear damping regime. For the dislocation velocity range considered in this work, the mobilities of screw and 60o dislocations are lower than edge and 30o dislocations. The mobilities of mixed dislocations, such as 30o and 60o, are found to depend on the character of the leading Shockley partial. Second, MD simulations of dislocation loop expansion under a constant external Schmid stress are performed. The expanded dislocation loop is found to facet in the 60o and screw segments due to low mobilities. The dislocation loop is also found to have reflection asymmetry about the edge axis due to mixed dislocation mobility asymmetry. Finally, the nonlinear atomistic mobility laws are implemented into DDD simulations to illustrate that atomistic mobility laws for straight dislocations can describe faceting and asymmetric behaviors observed during dislocation loop expansion in higher length scale simulations.Graphical abstractImage 1
       
  • In Situ Investigation of Stress-Induced Martensitic Transformation in
           Granular Shape Memory Ceramic Packings
    • Abstract: Publication date: Available online 22 February 2019Source: Acta MaterialiaAuthor(s): Hunter A. Rauch, Yan Chen, Ke An, Hang Z. Yu Stress-induced martensitic transformation can occur in granular shape memory materials, when individual particles experience high stresses and transform from a high-symmetry austenite phase to a low-symmetry martensite phase. This involves a highly heterogeneous distribution of the driving force and very low mechanical constraint for martensite nucleation, so the transformation behavior can be dramatically different from the well-documented case of monolithic solids. In this work, we investigate the stress-induced martensitic transformation in granular shape memory ceramic packings, which consist of single-crystal micro-particles of ZrO2-12at%CeO2 and ZrO2-15at%CeO2. Through in situ neutron diffraction, we study how the phase fraction, lattice strain, and integral peak broadness evolve during external loading, unloading, and subsequent heating. Several peculiar features are discovered, including (i) a continuous mode of transformation with a wide range of transformation loads, (ii) co-evolution of the packing structure, contact deformation, and martensitic transformation, and (iii) a strong correlation between the peak broadening and the transformed phase fraction. In addition, we show the first direct evidence of reversible stress-induced martensitic transformation in granular materials. We finally discuss the mechanism for martensite nucleation and growth in granular packings and show how that leads to the observed transformation characteristics.Graphical abstractImage 1
       
  • Doping α-Al2O3 to reduce its hydrogen permeability: thermodynamic
           assessment of hydrogen defects and solubility from first principles
    • Abstract: Publication date: Available online 21 February 2019Source: Acta MaterialiaAuthor(s): Vrindaa Somjit, Bilge Yildiz This paper assesses the role of doping on the hydrogen permeability and electronic properties of α-Al2O3. Formation energies of intrinsic and extrinsic defects in α-Al2O3 were calculated using density functional theory. Using these energies as input, a thermodynamic model was utilized to identify the equilibrium defect concentrations (barring hydrogen defects) in undoped and doped α-Al2O3 under aluminization conditions of 1100 K, over a range of pO2 at a fixed doping level of 1 ppm. Defect concentrations calculated at 1100 K under pO2-rich conditions were used as input to establish hydrogen and electronic defect concentrations under functional conditions of 300 K, over a range of pH2. The effect of dopants on the fraction of free hydrogen interstitials, which has implications on diffusivity, and the overall hydrogen solubility, was found to be substantial and distinct. Relative to the undoped case, Mg-doping increased the concentration of free hydrogen interstitials, the primary diffusing species, by 107 times, whereas Ti-, Si-, Fe-, Cr-doping eliminated it to negligible amounts. Comparing the impact on total hydrogen solubility, Mg-doping increased it by 104 times; Fe- and Cr-doping increased it negligibly by ∼1.5 times. In contrast, Ti- and Si- doping decreased it to nearly 1/3 that of the undoped case. Analyzing the role of isolated defect concentrations and binding energies of defect complexes helps elucidate these effects. Effect of dopant concentrations of 10 and 100 ppm was also investigated, with the conclusion that doping with Si and Ti at 1 ppm is the best strategy to reduce hydrogen diffusivity and solubility by the greatest amount. The findings aid in the design of effective hydrogen permeation barrier layers for use in hydrogen storage and transport infrastructure as well as in the understanding of defect states in Al2O3 used in electronic devices, such as resistive switching.Graphical abstractImage 1
       
  • Mechanism of Hardening and Damage Initiation in Oxygen Embrittlement of
           Body-Centred-Cubic Niobium
    • Abstract: Publication date: Available online 21 February 2019Source: Acta MaterialiaAuthor(s): Ping-Jiong Yang, Qing-Jie Li, Tomohito Tsuru, Shigenobu Ogata, Jie-Wen Zhang, Hong-Wei Sheng, Zhi-Wei Shan, Gang Sha, Wei-Zhong Han, Ju Li, Evan Ma Body-centred-cubic metallic materials, such as niobium (Nb) and other refractory metals, are prone to embrittlement due to low levels of oxygen solutes. The mechanisms responsible for the oxygen-induced rampant hardening and damage are unclear. Here we illustrate that screw dislocations moving through a random repulsive force field imposed by impurity oxygen interstitials readily form cross-kinks and emit excess vacancies in Nb. The vacancies bind strongly with oxygen and screw dislocation in a three-body fashion, rendering dislocation motion difficult and hence pronounced dislocation storage and hardening. While self-interstitials anneal out fast during plastic flow, the vacancy-oxygen complexes are stable against passing dislocations. The debris in fact amplify the random force field, facilitating the generation of even more defects in a self-reinforcing loop. This leads to unusually high strain hardening rates and fast breeding of nano-cavities that underlie damage and failure.Graphical abstractImage 1
       
  • Engineering Strain and Conductivity of MoO3 by Ion Implantation
    • Abstract: Publication date: Available online 21 February 2019Source: Acta MaterialiaAuthor(s): Daniela R. Pereira, Carlos Díaz-Guerra, Marco Peres, Sérgio Magalhães, João G. Correia, José G. Marques, Ana G. Silva, Eduardo Alves, Katharina Lorenz α-MoO3 lamellar crystals are implanted with 170 keV oxygen ions at room temperature and with fluences between 1×1012 cm-2 and 1×1017 cm-2, in order to modify the electrical and structural properties of the crystals. A controllable and significant increase of the electrical conductivity, over several orders of magnitude, is observed after implantation at high fluences. Based on high resolution X-ray diffraction (HRXRD) and micro-Raman spectroscopy measurements, this effect is attributed to the formation of donor-type defect complexes and new phases more conductive than the α-MoO3 orthorhombic phase. A significant expansion of the b lattice parameter, increasing with fluence, is observed as a response to the defects created by implantation. Strain build-up occurs in several steps and in distinct depth regions within the implanted layer. Contrary to the typical values reported for other implanted oxide materials, an unusually high maximum perpendicular deformation of ∼3% is verified.Graphical abstractImage 1
       
  • Multiscale Structure of Super Insulation Nano-Fumed Silicas Studied by
           SAXS, Tomography and Porosimetry
    • Abstract: Publication date: Available online 20 February 2019Source: Acta MaterialiaAuthor(s): Belynda Benane, Guilhem P. Baeza, Bruno Chal, Lucian Roiban, Sylvain Meille, Christian Olagnon, Bernard Yrieix, Geneviève Foray We focus on describing the multi-scale structure of a fumed silica characterized by remarkably low thermal conductivity (ca. 2-5 mW.m-1.K-1) when used as a core material in vacuum insulating panels. While such powders are known to be highly polydisperse at different lengthscales (hardly quantifiable), we propose to adapt a recent methodology based on small-angle X-ray scattering experiments with the aim of providing simple criteria for characterizing the morphology of these nanostructured silicas. Combining this technique with transmission electron microscopy, electron-tomography and mercury intrusion porosimetry then allows assigning the origin of the super-insulation to the low dimensionality of the silica aggregates at lengthscales smaller than 500 nm. Remarkably, by using independently these three techniques, we always find the compacity of the aggregates (radius of ca. 40 nm) to be equal to 0.29 ± 0.01. This study proposes therefore a robust methodology, potentially of a great interest for industrial applications.Graphical abstractImage 1
       
  • Fracture of notched ductile bulk metallic glass bars subjected to
           tension-torsion: experiments and simulations
    • Abstract: Publication date: Available online 20 February 2019Source: Acta MaterialiaAuthor(s): Devaraj Raut, R.L. Narayan, Yoshihiko Yokoyama, Parag Tandaiya, Upadrasta Ramamurty In bulk metallic glasses (BMGs) that are susceptible to shear band mediated plasticity, crack initiation and growth is a strain-controlled process associated with a unique material specific length scale. In notched specimens subjected to pure modes I and II, and mixed mode I/II loading conditions, this length scale determines the extent to which a crack can grow within a dominant shear band before propagating catastrophically. While a similar fracture mechanism is expected in the anti-plane shear mode (mode III) dominant loading condition, there are few studies that have investigated this aspect. In this paper, an attempt is made to understand the crack growth processes and fracture mechanisms under mode III and mixed mode I/III loading conditions by conducting pure torsion and combined tension-torsion experiments on a Zr-based BMG that exhibits significant room temperature plasticity. Specimens with either high or low notch acuity are tested to assess the effects of plastic constraint on the fracture process. Detailed finite element simulations were performed to study the evolution of stress and strain fields within each specimen before the onset of fracture. These are correlated with the fractographic features to determine the fracture criterion and mechanism. Results indicate that the length scale associated with fracture and the mechanism of crack growth are both sensitive to the application of tensile loads. The fracture toughness of BMGs under different modes of loading are evaluated and compared.Graphical abstractImage 1
       
  • In-situ and ex-situ synchrotron X-ray diffraction studies of
           microstructural length scale controlled dealloying
    • Abstract: Publication date: Available online 19 February 2019Source: Acta MaterialiaAuthor(s): T. Song, M. Yan, N.A.S. Webster, M.J. Styles, J.A. Kimpton, M. Qian This paper reports the critical role of microstructural length scale in dealloying for the fabrication of bimodal or monolithic functional nanoporous metal structures and the underlying mechanisms and kinetics. Two dual-phased (Al2Cu-AlCu) precursor alloys Al65Cu35 and Al55Cu45 (at.%) were selected to demonstrate the concept. Microstructural observations revealed that the two constituent phases Al2Cu and AlCu in each alloy can undergo either sequential dealloying, which leads to bimodal nanoporous Cu, or simultaneous dealloying, which results in monolithic nanoporous Cu. In-situ and ex-situ synchrotron X-ray diffraction (XRD), focused ion beam scanning electron microscopy (FIB-SEM), and potentiodynamic polarization scans were used to identify the detailed phase evolution processes and kinetics. It is concluded that microstructural length scale plays a decisive role in regulating the dealloying pathways. Sequential dealloying of Al2Cu and AlCu occurs when both phases are micrometer-scaled, while simultaneous dealloying takes over when both phases are nanoscaled. The nanosize effect of Al2Cu and AlCu can override their intrinsic difference in electrochemical potential at the micro- or macro-scale, and the advantage of tetragonal Al2Cu over monoclinic AlCu in crystallographic transition to face-centered-cubic (FCC) Cu by dealloying. The high-resolution in-situ synchrotron XRD data revealed a two-stage kinetic process for dealloying of Al2Cu to Cu. The Avrami-Erofe′ev kinetic model provides an excellent description of each stage. The rationales and implications are discussed.Graphical abstractImage 1
       
  • Long term evolution of microstructure and stress around tin whiskers
           investigated using scanning Laue microdiffraction
    • Abstract: Publication date: Available online 18 February 2019Source: Acta MaterialiaAuthor(s): Johan Hektor, Jean-Sébastien Micha, Stephen A. Hall, Srinivasan Iyengar, Matti Ristinmaa Scanning Laue microdiffraction was used to study the evolution of the microstructure and the stress field around two tin whiskers during ageing for up to 21 months. In the heterogeneous stress fields obtained, localised ridges of high compressive stress leading to the root of the whiskers were found. Due to the evolution of the intermetallic compound in the interface between the copper substrate and the tin coating, the stress field was also evolving with time. The temporal evolution of the stress field indicates that the regions supplying material to the whisker root is changing with time, highlighting that whisker growth is a highly dynamical process. During the experimental campaign, a new surface feature appeared in a grain boundary within the scanned area of the sample. The new feature had a twinning relationship with one of the neighbouring grains, a similar twin relation was also seen for one of the two larger whiskers. It is suggested that tin atoms diffuse out from the ridges of high compressive stress to the nearby, less compressed grain boundaries along which diffusion towards the root of the whisker occurs. The observations made from the Laue diffraction measurements also suggest that whiskers form in regions where the gradient in hydrostatic stress is large and that they grow to relax compressive stresses.Graphical abstractImage 1
       
  • Modern Data Analytics Approach to Predict Creep of High-Temperature Alloys
    • Abstract: Publication date: Available online 18 February 2019Source: Acta MaterialiaAuthor(s): D. Shin, Y. Yamamoto, M.P. Brady, S. Lee, J.A. Haynes A breakthrough in alloy design often requires comprehensive understanding in complex multi-component/multi-phase systems to generate novel material hypotheses. We introduce a modern data analytics workflow that leverages high-quality experimental data augmented with advanced features obtained from high-fidelity models. Herein, we use an example of a consistently-measured creep dataset of developmental high-temperature alloy combined with scientific alloy features populated from a high-throughput computational thermodynamic approach. Extensive correlation analyses provide ranking insights for most impactful alloy features for creep resistance, evaluated from a large set of candidate features suggested by domain experts. We also show that we can accurately train machine learning models by integrating high-ranking features obtained from correlation analyses. The demonstrated approach can be extended beyond incorporating thermodynamic features, with input from domain experts used to compile lists of features from other alloy physics, such as diffusion kinetics and microstructure evolution.Graphical abstractImage 1
       
  • Modeling Solution Hardening in BCC Refractory Complex Concentrated Alloys:
           NbTiZr, Nb1.5TiZr0.5 and Nb0.5TiZr1.5
    • Abstract: Publication date: Available online 18 February 2019Source: Acta MaterialiaAuthor(s): S.I. Rao, B. Akdim, E. Antillon, C. Woodward, T.A. Parthasarathy, O.N. Senkov Large scale, atomistic simulations of the core structure and mobility of ½[111] screw, edge and mixed dislocations in ternary multicomponent alloys (e.g. High Entropy alloys), NbTiZr, Nb1.5TiZr0.5 and Nb0.5TiZr1.5, are presented. The core structure of ½[111] screw dislocations continuously varies from compact to 3-fold with decreasing Nb content. The screw dislocation core structures in NbTiZr and Nb1.5TiZr0.5 are calculated using Embedded Atom Potentials (Johnson-Zhou) and compared with first-principles calculations of the screw dislocation in a quasi-random structure. In both simulations the dislocation core spreads on different (110) glide planes as the composition varies along the dislocation line in stoichiometric NbTiZr. The Nb-rich composition Nb1.5TiZr0.5 shows a compact core with very little core structure variation along the dislocation line in both First Principles and atomistic simulations. The screw dislocation deposits interstitial and vacancy dipole debris as it moves under stress. Average solute-dislocation core interaction energies in NbTiZr, Nb1.5TiZr0.5 and Nb0.5TiZr1.5 are derived from the average interatomic potential derived for each of the three systems. The interaction energies are used to determine the critical stress for the motion of ½[111] screw dislocations in the three systems as a function of temperature using the Suzuki model of kink migration controlled mobility developed for concentrated BCC random alloys. This analysis shows that the relatively high barrier for kink migration caused by fluctuations in solute concentration along the screw dislocation line and the dipole dragging stress associated with the screw dislocation motion results in a shallow fall-off of critical stress with temperature in these alloys as compared to simple BCC metals. Finally, the screw dislocation to edge and mixed dislocation critical stress ratio in NbTiZr are shown to be low, ∼ 2 at 5K, in contrast to simple BCC metals, where it could be as high as 100-1000.Graphical abstractFig.: A comparison of Suzuki model results, without (a) and with (b) interstitials at an applied strain rate of 0.001/s with experimental yield stress data as a function of temperature in stoichiometric NbTiZr.Image 1
       
  • The effects of solid solution and oxide dispersion alloying on the
           viscoelastic behavior of Au alloy thin films
    • Abstract: Publication date: Available online 18 February 2019Source: Acta MaterialiaAuthor(s): K. Mongkolsuttirat, J.R. Smyth, M. McLean, W.L. Brown, R.P. Vinci The effects of solid solution and oxide dispersion alloying on viscoelastic relaxation behavior of Au alloy films were investigated by using gas pressure bulge testing in the temperature range of 20 to 80 °C. Three sets of 500 nm thick films were fabricated for the study: nominally pure Au, Au-V solid solution alloys with V concentration ranging from 0.89 to 5.4 at% V, and a Au-V2O5 nanoparticle dispersion alloy with a V concentration of 5.4 at%. The residual stress, unrelaxed plane strain modulus, and normalized time-dependent effective modulus were determined as a function of alloy type and temperature. Both alloying approaches resulted in a refined grain size that was thermally stable. As expected, the fractional modulus decay measured after 3 hours was greater at higher temperatures for all cases. Solid solution strengthening and dispersion strengthening were both effective at reducing the 3-hour modulus decay, and the Au-V2O5 films showed the largest resistance to relaxation at all temperatures studied. Activation energy analysis reveals a value of approximately 0.11 eV, matching the Peierls barrier height, for pure Au and the two alloys. The results are consistent with a viscoelastic stress relaxation mechanism based on reversible dislocation bowing, weak solid solution hardening through elastic solute/dislocation interactions, and strong dislocation pinning associated with oxide nanoparticles.Graphical abstractImage 1
       
  • Promoting the columnar to equiaxed transition and grain refinement of
           titanium alloys during additive manufacturing
    • Abstract: Publication date: Available online 16 February 2019Source: Acta MaterialiaAuthor(s): M.J. Bermingham, D.H. StJohn, J. Krynen, S. Tedman-Jones, M.S. Dargusch Preventing columnar grain formation during additive manufacturing has become a significant challenge. Columnar grains are generally regarded as unfavourable as their presence can impart solidification defects and mechanical property anisotropy, however, the thermal conditions experienced during additive manufacturing make columnar grains difficult to avoid. In this work the thermal conditions during solidification (cooling rate, temperature gradients) are characterised during wire based additive manufacturing. For the selection of deposition conditions that favour equiaxed grain formation, the role of alloy constitution is explored in three classical alloy design regimes: an alloy containing no grain refiners (Ti-6Al-4V); an alloy only containing grain refining solutes (Ti-3Al-8V-6Cr-4Mo-4Zr); and an alloy containing both grain refining solute and nucleant particles (Ti-3Al-8V-6Cr-4Mo-4Zr + La2O3). Substantial refinement and equiaxed grain formation is achieved in the latter case which is attributed to β-Ti nucleation on La2O3. However, the thermal environment is dynamic during additive manufacturing and equiaxed grain formation is only achievable when temperature gradients decrease sufficiently to permit constitutional supercooling.Graphical abstractImage 1
       
  • Local non-equilibrium effect on the growth kinetics of crystals
    • Abstract: Publication date: Available online 15 February 2019Source: Acta MaterialiaAuthor(s): P.K. Galenko, V. Ankudinov A phase field model for small and large driving forces on solidification and melting of a pure metal or binary alloy is formulated. A traveling wave solution of the phase field equation predicts the non-linear behavior in the velocity of the crystal/liquid interface at the large driving force. This non-linearity has the dependence of velocity with saturation or exhibiting the velocity with maximum at a fixed undercooling/superheating. The predicted velocity is compared with the molecular dynamics simulation data for pure Fe that confirms a crucial role of local non-equilibrium in the form of relaxation of gradient flow in the quantitative description of the crystal growth kinetics.Graphical abstractImage 1
       
  • Stress-induced formation of TCP phases during high temperature low cycle
           fatigue loading of the single-crystal Ni-base superalloy ERBO/1
    • Abstract: Publication date: Available online 15 February 2019Source: Acta MaterialiaAuthor(s): C. Meid, M. Eggeler, P. Watermeyer, A. Kostka, T. Hammerschmidt, R. Drautz, G. Eggeler, M. Bartsch The microstructural evolution in the single crystal Ni-base superalloy ERBO/1 (CMSX 4 type) is investigated after load controlled low cycle fatigue (LCF) at 950°C (load-ratio: 0.6, tensile stress range: 420 to 740 MPa, test frequency: 0.25 Hz, fatigue rupture life: about 1000 - 3000 cycles). Bulk topologically close packed (TCP) phase particles precipitated and were analyzed by three-dimensional focus ion beam slice and view imaging and analytical transmission electron microscopy. The particles did not precipitate homogenously but at locations with enhanced levels of local stresses/strains, such as isolated γ-channels subjected to cross channel stresses, shear bands and in front of micro cracks. The influence of stress/strain is furthermore apparent in the spatial arrangement and the shape of the TCP phase particles. Only μ-phase TCP particles were found by electron diffraction. Results of a structure-map analysis suggest that most of these TCP particles observed after LCF testing would not precipitate in thermodynamic equilibrium. In order to rationalize this effect the atomic volume was analyzed that transition-metal (TM) elements take in unary fcc and in unary μ-phase crystal structures and found that all TM elements except Zr and V take a larger volume in a unary μ phase than in a unary fcc phase. This trend is in line with the observed localized precipitation of TCP phases that are rich in Ni and other late TM elements. The experimental and theoretical findings suggest consistently that formation of TCP particles in LCF tests is considerably influenced by the local tensile stress/strain states.Graphical abstractImage 1
       
  • Comparison between Stress-Strain Plots obtained from Indentation
           
    • Abstract: Publication date: Available online 14 February 2019Source: Acta MaterialiaAuthor(s): J.E. Campbell, R.P. Thompson, J. Dean, T.W. Clyne This paper is focused on comparisons between stress-strain plots from conventional uniaxial (tensile or compressive) testing and those obtained from indentation experiments, via iterative FEM modeling of the process in which the plasticity is represented using a constitutive law. Both Ludwik-Hollomon and Voce equations are used in the current work. Advantages of a spherical indenter shape, and of using the residual indent profile as the main experimental outcome, are highlighted. It is shown via detailed study of two different materials, with low and high work hardening rates, that the methodology (here termed indentation plastometry) can be used to obtain (nominal) tensile stress-strain curves, which incorporate the onset of necking and the ultimate tensile strength. High levels of fidelity are observed between these and corresponding plots obtained by conventional tensile testing. It is noted that, while there is also excellent consistency with the outcomes of uniaxial compression tests, the latter inevitably involve some experimental complications that are best avoided. It is concluded that indentation plastometry has the potential to become a mainstream testing methodology in the near future.Graphical abstractImage 1
       
  • Structure-Property Relationships in the Lead-free Piezoceramic System
           K0.5Bi0.5TiO3 - BiMg0.5Ti0.5O3
    • Abstract: Publication date: Available online 14 February 2019Source: Acta MaterialiaAuthor(s): Aurang Zeb, David A. Hall, Zabeada Aslam, Jennifer Forrester, Jing-Feng Li, Yizhe L. Li, Chiu C. Tang, Ge Wang, Fangyuan Zhu, Steven J. Milne Distinctive structure-property relationships are revealed in the relaxor ferroelectric ceramic solid solution, (1-x)K0.5Bi0.5TiO3 - xBiMg0.5Ti0.5O3: 0.02 < x < 0.08. The constructed phase diagram and results of in-situ synchrotron X-ray diffraction provide explanations for temperature and electric field dependent anomalies in dielectric, ferroelectric and electromechanical properties. At room temperature a mixed phase tetragonal and pseudocubic phase field occurs for compositions 0> x ≤ 0.07. As temperature rises to ≥ 150 °C, the ferroelectric tetragonal relaxor phase changes to a pseudocubic ergodic relaxor phase; this change in length scale of polar order is responsible for an inflection in relative permittivity - temperature plots. The transition is reversed by a sufficient electric field, thereby explaining the constricted form of polarisation-electric field loops measured at>150°C. It is also responsible for a change in slope of the strain-electric field (S-E) plots which are relatively linear in the ferroelectric regime i.e. at temperatures up to 150 °C, giving unipolar strains of 0.11 % at 20 °C and 0.14% at 150 °C (50 kV cm-1 field). The additional contribution from the effect of the field-induced pseudocubic to tetragonal transition, generates strains of ∼ 0.2 % at 185 °C. Unusual for a piezoelectric solid solution, the maximum strains and charge coefficients (d33 =150 pC N-1, 20 °C) do not coincide with a morphotropic or polymorphic phase boundary.Graphical abstractImage 1
       
  • Analysis of the Partial Molar Excess Entropy of Dilute Hydrogen in Liquid
           Metals and Its Change at the Solid-Liquid Transition
    • Abstract: Publication date: Available online 14 February 2019Source: Acta MaterialiaAuthor(s): Andrew H. Caldwell, Antoine Allanore A systematic change in the partial molar enthalpy of mixing (Δh¯Hmix) and partial molar excess entropy (Δs¯Hex) for dilute hydrogen-metal systems at the solid-liquid transition is reported. Expressions for Δh¯Hmixand Δs¯Hexare derived from the Fowler model of hydrogen solubility, and the change in Δs¯Hexat melting is bounded. The theoretical bound is in agreement with measured data. A connection is made between the change in Δs¯Hexand short range order in the metal-hydrogen system.Graphical abstractImage 1
       
  • Interplay between the Effects of Deformation Mechanisms and Dynamic
           Recrystallization on the Failure of Mg-3Al-1Zn
    • Abstract: Publication date: Available online 13 February 2019Source: Acta MaterialiaAuthor(s): M.W. Vaughan, W. Nasim, E. Dogan, J.S. Herrington, G. Proust, A.A. Benzerga, I. Karaman This work focuses on the relationship between deformation mechanisms, dynamic recrystallization (DRX), and failure mechanisms in Mg-3Al-1Zn under tension at relatively low temperatures (25-200°C). The loading orientation was selected to favor either prismatic slip, basal slip, or extension twinning as the primary active deformation mechanism upon yielding. The tensile response was accurately simulated at various temperatures using visco-plastic self-consistent crystal plasticity modeling coupled with optimization under constraints. The relative deformation mechanism activities were predicted to gain insight into the role of microstructure evolution and DRX in failure. Extensive microstructural investigation was performed to identify characteristics of failure and categorize them according to slip, twin, and DRX activities, in an attempt to ultimately provide insights for achieving better formability at low temperatures. Failure processes were categorized according to four primary microstructural features observed during deformation prior to failure: (1) extension twinning without DRX, (2) extension twinning with DRX, (3) contraction twinning followed by double twinning, and (4) abundant DRX and high microcrack density along DRX regions. Prismatic slip was found to promote homogeneous DRX via the continuous DRX mechanism, resulting in ductile fracture. Similarly, basal slip allows for high elongation to failure but low yield strengths while promoting discontinuous DRX, whereas extension twinning contributed to quasi-brittle failure at low temperatures and minimal DRX activity. As the yield strength becomes orientation-independent at 150°C, with increased elongation, transition from twinning-to slip-dominant microstructures, and higher DRX activity, the findings indicate that the flow anisotropy in Mg-3Al-1Zn can be mitigated beginning at 150°C.Graphical abstractImage 1
       
  • Quantitative phase field modeling of solute trapping and continuous growth
           kinetics in quasi-rapid solidification
    • Abstract: Publication date: Available online 12 February 2019Source: Acta MaterialiaAuthor(s): Tatu Pinomaa, Nikolas Provatas Solute trapping is an important phenomenon in rapid solidification of alloys, for which the continuous growth model (CGM) of Aziz et al. [1] is a popular sharp interface theory. By modulating the so-called anti-trapping current and using asymptotic analysis, we show how to quantitatively map the thin interface behavior of an ideal dilute binary alloy phase field model onto the CGM kinetics. We present the parametrizations that allow our phase field model to map onto the sharp interface kinetics of the CGM, both in terms of partition coefficient k(V) and kinetic undercooling. We also show that the mapping is convergent for different interface widths, both in transient and steady state simulations. Finally we present the effect that solute trapping can have on cellular growth in directional solidification. The presented treatment for solute trapping can be easily implemented in different phase field models, and is expected to be an important feature in future studies of quantitative phase field modeling in quasi-rapid solidification regimes, such as those relevant to metal additive manufacturing.Graphical abstractImage 1
       
  • Grain Growth and Solid-State Dewetting of Bi-Crystal Ni-Fe Thin Films on
           Sapphire
    • Abstract: Publication date: Available online 12 February 2019Source: Acta MaterialiaAuthor(s): Amit Sharma, Aakash Kumar, Nimrod Gazit, David J. Srolovitz, Eugen Rabkin We studied the solid-state dewetting behavior of thin Ni80Fe20 films deposited on basal plane oriented sapphire substrate and annealed in the range of temperatures of 1023-1323 K. All studied films exhibited strong texture and maze bicrystal microstructure, with only two grains misoriented by 60° around the common axis present in the film. The morphology of partially dewetted films changed from the one typical for polycrystalline thin films to the one typical for single crystalline heteroepitaxial films with increasing temperatures and annealing times. This change of dewetting behavior was associated with the fast grain growth in the films. The films of pure Ni of identical thickness, annealed under identical conditions exhibited significantly slower grain growth and lower thermal stability. Both the high-resolution X-ray diffraction and the cross-sectional high-resolution transmission electron microscopy observations revealed the phase separation of the Ni80Fe20 films into two parallel layers of the face-centered cubic (adjacent to the substrate) and hexagonal close-packed (on the top of the film) phases of similar compositions. Our density functional theory (DFT) calculations indicated that this phase separation is driven by the decrease of the film surface and interface energy, leading to the thermodynamically equilibrium thickness of the metastable hexagonal close-packed phase. This phase exhibits higher surface anisotropy than its stable face-centered cubic counterpart and is instrumental in accelerating the grain growth in the film via suppression of grain boundary grooving.Graphical abstractImage 1
       
  • Materials Informatics: From the Atomic-Level to the Continuum
    • Abstract: Publication date: Available online 30 January 2019Source: Acta MaterialiaAuthor(s): J.M. Rickman, T. Lookman, S.V. Kalinin In recent years materials informatics, which is the application of data science to problems in materials science and engineering, has emerged as a powerful tool for materials discovery and design. This relatively new field is already having a significant impact on the interpretation of data for a variety of materials systems, including those used in thermoelectrics, ferroelectrics, battery anodes and cathodes, hydrogen storage materials, polymer dielectrics, etc. Its practitioners employ the methods of multivariate statistics and machine learning in conjunction with standard computational tools (e.g., density-functional theory) to, for example, visualize and dimensionally reduce large data sets, identify patterns in hyperspectral data, parse microstructural images of polycrystals, characterize vortex structures in ferroelectrics, design batteries and, in general, establish correlations to extract important physics and infer structure-property-processing relationships. In this Overview, we critically examine the role of informatics in several important materials subfields, highlighting significant contributions to date and identifying known shortcomings. We specifically focus attention on the difference between the correlative approach of classical data science and the causative approach of physical sciences. From this perspective, we also outline some potential opportunities and challenges for informatics in the materials realm in this era of big data.Graphical abstractImage 1
       
  • Origin of an unusual systematic variation in the heteroepitaxy of Ag on Ni
           – the roles of twinning and step alignment
    • Abstract: Publication date: Available online 30 January 2019Source: Acta MaterialiaAuthor(s): P. Wynblatt, D. Chatain, A.D. Rollett, U. DahmenABSTRACTA systematic variation in the orientation relationship (OR) of Ag films grown on Ni substrates previously discovered by a combinatorial approach is analyzed using concepts of grain boundaries, surface science and phase transformations. On roughly half of all Ni substrate orientations, Ag adopts a “special” OR that varies systematically from a twin OR, which develops on substrates lying along the (111)-(210) line of the standard stereographic triangle (SST), to the so-called oct-cube OR, which arises exclusively on (100) substrates. On the other half of Ni substrate orientations, Ag adopts the standard cube-on-cube OR. The special ORs are modeled by a) linear interpolation, b) 1D edge-to-edge matching and c) 2D transformation strains, using the twin relationship as a reference. The 1D model shows that the systematic variation in the special ORs can be understood in terms of a growth mechanism by attachment at steps and a lattice rotation due to the difference between the step heights in the substrate and the film. The 2D model explains why this mechanism does not apply to substrates on which Ag displays the cube-on-cube OR.Graphical abstractImage 1
       
  • Dislocation Dynamics in a Nickel-Based Superalloy via In-Situ Transmission
           Scanning Electron Microscopy
    • Abstract: Publication date: Available online 29 January 2019Source: Acta MaterialiaAuthor(s): J.C. Stinville, Eric R. Yao, Patrick G. Callahan, Jungho Shin, Fulin Wang, McLean P. Echlin, Tresa M. Pollock, Daniel S. Gianola Micro-tensile specimens of nickel based-superalloy oligocrystals were tested in-situ in an SEM in transmission mode (TSEM) enabling observation of dislocations. The dynamics of dislocation motion during tensile loading were captured and correlated with the measured intermittencies during plastic flow recorded by high load- and temporal-resolution sensors. This investigation in particular focused on the dislocation behavior near twin boundaries with different slip configurations. A multiplicity of deformation mechanisms at the dislocation scale were observed within individual slip bands, including precipitate shearing, dislocation decorrelation and APB-coupled shearing. These processes affect strain localization near twin boundaries and provide new defect-level insights on strain localization and fatigue crack initiation in these alloys.Graphical abstractImage 1
       
  • New tessellation-based procedure to design perfectly hyperuniform
           disordered dispersions for materials discovery
    • Abstract: Publication date: Available online 29 January 2019Source: Acta MaterialiaAuthor(s): J. Kim, S. Torquato Disordered hyperuniform dispersions are exotic amorphous two-phase materials characterized by an anomalous suppression of long-wavelength volume-fraction fluctuations, endowing them with novel physical properties. While such unusual materials have received considerable attention, a stumbling block has been an inability to create large samples that are truly hyperuniform due to current computational and experimental limitations. To overcome such barriers, we introduce a new, simple construction procedure that guarantees perfect hyperuniformity for very large sample sizes. It involves tessellating space into cells and then inserting a particle into each cell such that the local-cell particle packing fractions are identical to the global packing fraction. We analytically prove that such dispersions are perfectly hyperuniform in the infinite-sample-size limit. Our methodology enables a remarkable mapping that converts a very large nonhyperuniform disordered dispersion into a perfectly hyperuniform one, which we numerically demonstrate in two and three dimensions. A similar analysis also establishes the hyperuniformity of the famous Hashin-Shtrikman multiscale dispersions, which possess optimal transport and elastic properties. Our hyperuniform designs can be readily fabricated using modern photolithographic and 3D printing technologies. The exploration of the enormous class of hyperuniform dispersions that can be designed and tuned by our tessellation-based methodology paves the way for accelerating the discovery of novel hyperuniform materials.Graphical abstractImage 1
       
  • Morphological Evolution of Transformation Products and Eutectoid
           Transformation(s) in a Hyper-Eutectoid Ti-12 at% Cu Alloy
    • Abstract: Publication date: Available online 29 January 2019Source: Acta MaterialiaAuthor(s): Harish Donthula, B. Vishwanadh, T. Alam, T. Borkar, R.J. Contieri, R. Banerjee, R. Tewari, G.K. Dey, S. Banerjee Evidences of both sluggish eutectoid and active eutectoid (not suppressible under rapid cooling) transformations have been found for the first time in a single hyper-eutectoid Ti-Cu alloy. Both of these types of eutectoid reactions have been investigated in detail, at different length scales, by coupling scanning electron microscopy (SEM),transmission electron microscopy (TEM) and atom probe tomography (APT). The unique three phase crystallographic relationship between the parent β (bcc) and the two product phases, α (hcp) and Ti2Cu, has been established. The extent of partitioning of the solute (Cu) between the two product phases has been determined by APT and is rationalised in terms of thermodynamic considerations. Based on the observed lattice site correspondence and the extent of solute partitioning, a possible mechanism of active eutectoid transformation is proposed.Graphical abstractImage 1
       
  • Boundary micro-cracking in metastable Fe45Mn35Co10Cr10 high-entropy alloys
    • Abstract: Publication date: Available online 25 January 2019Source: Acta MaterialiaAuthor(s): Shaolou Wei, Jinwoo Kim, Cemal Cem Tasan Mechanically-induced martensitic transformation can be a double-edged sword: depending on composition and processing history it can either lead to various beneficial mechanical effects (e.g. transformation-induced plasticity, transformation-toughening), or induce local brittleness and damage nucleation. While several corresponding guidelines are presented in steels research, controlling microstructure metastability has not drawn sufficient attention in the quick-emerging field of high-entropy alloys. In the present work, we investigated the damage mechanisms of a mechanically metastable Fe45Mn35Co10Cr10 high-entropy alloy under uniaxial tensile loading. Our integrated in-situ scanning electron microscopy/electron backscatter diffraction experiments revealed a two-fold effect of the highly localized strain, induced by asynchronously transformed martensite, leading to boundary damage nucleation and dissimilarly oriented martensitic variant formation. The latter suppresses slip transfer between adjacent grains, further expediting the growth of the nucleated damage incidents. Based on these experimental observations and corresponding theoretical calculations, we discuss the underlying mechanisms and propose a sequence of micro-events that create the observed phenomena.Graphical abstractImage 1
       
  • Sintering processes in direct ink write additive manufacturing: A
           mesoscopic modeling approach
    • Abstract: Publication date: Available online 10 January 2019Source: Acta MaterialiaAuthor(s): Fadi Abdeljawad, Dan S. Bolintineanu, Adam Cook, Harlan Brown-Shaklee, Christopher DiAntonio, Daniel Kammler, Allen Roach Direct ink write (DIW) is an emerging additive manufacturing technique that allows for the fabrication of arbitrary complex geometries required in many technologies. DIW of metallic or ceramic materials involves a sintering step, which greatly influences many of the microstructural features of the printed object. Herein, we explore solid-state sintering in DIW through a mesoscopic modeling framework that is capable of capturing bulk and interface thermodynamics and accounting for various mass transport mechanisms. Simulation results of idealized geometries identify regimes in materials parameter space, where densification rates are enhanced. With the aid of several statistical and topological descriptors, the role of particle size distribution (PSD) on the microstructural evolution is explored and quantified. More specifically, it is found that a bi-dispersed PSD enhances pore shrinkage kinetics. However, bi-dispersity yields microstructures with pores that are highly eccentric, an effect that could be detrimental to the mechanical properties of the printed material. On the whole, our modeling approach provides a capability to explore the phase space of DIW process parameters and determine ones that lead to optimal microstructures.Graphical abstractImage 1
       
  • Revealing the factors influencing grain boundary segregation of P, As in
           Si: Insights from first-principles
    • Abstract: Publication date: Available online 12 February 2019Source: Acta MaterialiaAuthor(s): Dongdong Zhao, Yanjun Li Phosphorous (P) and arsenic (As) segregation at grain boundaries (GBs) usually deteriorate the electrical performance of n-type doped Si-wafers. In order to probe the factors influencing P and As segregation behaviors along Si GBs, a systematic investigation upon the interactions between P, As atoms and a series of coincidence site lattice Si GBs, including Σ3 {111}, Σ9 {221}, Σ27 {552}, Σ3 {112}, was carried out via first-principles calculations. It is revealed that the segregation behaviors of P, As along different GBs are different, which is dependent on both GB characteristics, i.e. the intrinsic lattice distortion, number of dangling/extra bonds, deep levels in the bandgap of density of states, and also the impurity properties. It reveals that smaller P atoms tend to segregate to the compressed atomic sites in GBs, by which the intrinsic GB lattice distortion is reduced. However, GB lattice distortion hardly induce As segregation owing to the similar atomic size between As and Si atoms. Calculations also indicate that under-coordinated core sites with dangling bond along GBs are remarkably attractive for P and As segregation, as a result of the nature of group-V elements to be three-coordinated rather than four-coordinated. Furthermore, GBs having deep levels in the bandgap of density of states, produced either by dangling or extra bonds, show a strikingly high attractive strength for both P and As segregation. The present work is supposed to provide important insights on substitutional impurity segregation along GBs in multi-crystalline Si in the atomic and electronic level.Graphical abstractImage 1
       
  • Formation Mechanism of Abnormally Large Grains in a Polycrystalline
           Nickel-based Superalloy during Heat Treatment Processing
    • Abstract: Publication date: Available online 11 February 2019Source: Acta MaterialiaAuthor(s): Xin Wang, Zaiwang Huang, Biao Cai, Ning Zhou, Oxana Magdysyuk, Yanfei Gao, Shesh Srivatsa, Liming Tan, Liang Jiang Controlling the final grain size in a uniform and controlled manner in powder metallurgy nickel-based superalloys is important since many mechanical properties are closely related to it. However, it has been widely documented that powder metallurgy superalloys are prone to suffer from growth of abnormally large grains (ALGs) during supersolvus heat treatment, which is harmful to in-service mechanical performance. The underlying mechanisms behind the formation of ALGs are not yet fully understood. In this research, ALGs were intentionally created using spherical indentation applied to a polycrystalline nickel-based superalloy at room temperature, establishing a deformation gradient underneath the indentation impression, which was quantitatively determined using finite element modelling, electron backscatter diffraction (EBSD) and synchrotron diffraction. Subsequent supersolvus heat treatment leads to the formation of ALGs in a narrow strain range, which also coincides with the contour of residual plastic strain in a range of about 2% to 10%. The formation mechanisms can be attributed to: (1) nucleation sites available for recrystallization are limited, (2) gradient distribution of stored energy across grain boundary. The proposed mechanisms were validated by the phase-field simulation. This research provides a deeper insight in understanding the formation of ALGs in polycrystalline nickel-based superalloy components during heat treatment, when subsurface plastic deformation caused by (mis)handling before super-solvus heat treatment occurs.Graphical abstractImage 1
       
  • Crystallographic orientation dependent maximum layer thickness of cubic
           AlN in CrN/AlN multilayers
    • Abstract: Publication date: Available online 10 February 2019Source: Acta MaterialiaAuthor(s): Zhuo Chen, David Holec, Matthias Bartosik, Paul H. Mayrhofer, Zaoli Zhang Metastable rock-salt (face centered cubic, c-) AlN can be grown in CrN/AlN multilayers when the AlN layer is thin enough. Exceeding a certain critical thickness, the thermodynamically stable wurtzite (w) structure grows. In this work, a bilayer-period-gradient (21 repeated blocks, each consisting of 10 bilayers with AlN layer-thicknesses ranging from 1.0 nm to 10.0 nm), ∼2.0-μm-thick, reactively magnetron sputtered multilayer was characterized in detail with a spherical aberration-corrected transmission electron microscope (TEM). The studies are complemented by DFT (density functional theory) calculations.The high resolution TEM (HRTEM) studies reveal that the growth-orientation is not as effective as the and growth-orientations in stabilizing the metastable c-AlN. The critical thickness for the c-AlN layers (before the thermodynamically stable w-AlN forms) is around ∼2.0 nm for the growth-orientation but reaches as high as 4.1 nm for both and growth-orientations. Contrary to the orientation, in both and orientations several unusually highly mismatched c-CrN/w-AlN interface structures form as soon as w-AlN is present. DFT studies suggest that the larger critical thickness of the AlN layers in and orientation is allowed by the lower surface energy and higher cubic/wurtzite interfacial energy. The combination of HRTEM and DFT studies allows answering open questions on the impact of crystallographic orientations and interface structures, and also provides a better understanding on the growth mechanisms of c-AlN, necessary for the outstanding mechanical properties of AlN-containing multilayers.Graphical abstractImage 1
       
  • Tilt strain glass in Sr and Nb co-doped LaAlO3 ceramics
    • Abstract: Publication date: Available online 10 February 2019Source: Acta MaterialiaAuthor(s): Yuanchao Ji, Pei Zhang, Liqiang He, Dong Wang, Hanyu Luo, Kazuhiro Otsuka, Yunzhi Wang, Xiaobing Ren Strain glass, a glassy state of lattice strain, has been found in Ti50-xNi50+x alloys and later in many metallic ferroelastic/martensitic systems, where shear or shuffle serves as a primary order parameter (POP). Another class of non-metallic ferroelastic ceramics are also known to widely exist and commonly possess a polyhedral tilt as the POP. So far, it is unclear whether a “tilt” strain glass exists. Here, we report a finding of a tilt strain glass in La1-xSrxAl0.95Nb0.05O3-δ ceramics. With increasing Sr2+ dopants, the ferroelastic transition from cubic to rhombohedral phases is gradually suppressed. At a critical concentration (xc ∼10%), a strain glass transition emerges, characterized by five sets of evidence: (I) an invariance of average structure; (II) frequency dependence of elastic moduli at a strain glass transition temperature Tg; (III) non-ergodicity; (IV) formation of rhombohedral nanodomains; (V) a gradual increase of tilt angle upon cooling. Surprisingly, the established phase diagram shows an increase of Tg with increasing dopants (a positive correlation), which is different from previous strain glass phase diagrams. The positive and negative correlations can be explained as a balance between two factors of strain glass transition: a global transition factor producing a negative contribution competes with a local field one producing a positive contribution. Our discovery of strain glass in ceramics may also bring novel properties as in metals.Graphical abstractImage 1
       
  • Simultaneous enhancement of stress- and strain-controlled fatigue
           properties in 316L stainless steel with gradient nanostructure
    • Abstract: Publication date: Available online 10 February 2019Source: Acta MaterialiaAuthor(s): Y.B. Lei, Z.B. Wang, J.L. Xu, K. Lu A gradient nanostructured (GNS) surface layer with full austenitic phase was synthesized on AISI 316L stainless steel by surface mechanical rolling treatment at ∼280 °C. The mean grain size is ∼45 nm at the top surface and increases gradually with depth. Deformation-induced martensite (DIM) transformation was suppressed and the microstructural refinement was dominated by dislocation activities and twinning during the formation of the GNS layer. Axial tension-compression fatigue tests showed that fatigue strength and life are simultaneously enhanced in the GNS samples relative to the corresponding coarse-grained counterparts in both stress- and strain-controlled tests. This is very different from fatigue behavior of conventional nanostructured materials, i.e. an enhanced stress-controlled fatigue strength with a decreased strain-controlled fatigue life. Besides contributions from the enhanced mechanical properties and the suppressed surface defects formation, analyses of fatigue mechanisms demonstrated that the promoted formation of DIM during cyclic strain plays a crucial role in enhancing fatigue properties of the GNS samples in strain-controlled tests.Graphical abstractImage 1
       
  • Thermodynamics of grain boundary segregation, interfacial spinodal and
           their relevance for nucleation during solid-solid phase transitions
    • Abstract: Publication date: Available online 8 February 2019Source: Acta MaterialiaAuthor(s): A. Kwiatkowski da Silva, R. Darvishi Kamachali, D. Ponge, B. Gault, J. Neugebauer, D. Raabe Grain boundary segregation, embrittlement and phase nucleation are interconnected phenomena that are often treated separately, which is partly due to limitations of the current models to predict grain boundary segregation in non-ideal solid solutions. Here, a simple model is introduced to predict grain boundary segregation in solid solutions by coupling available bulk thermodynamic data with a mean-field description of the grain boundary character. The model is confronted with experimental results obtained in Fe-Mn alloys analysed by atom probe tomography. This model successfully predicts a first order transition or a discontinuous jump in the composition of the grain boundary which kinetically implies the formation of spinodal Mn fluctuations that tend to grow further with time within the segregated region. The increase in solute concentration at the grain boundary leads to an increase of the enthalpy of the boundary and to its embrittlement at lower temperatures. Once austenite is formed, the amount of segregated solute Mn on the grain boundaries is drastically reduced and the toughness of the grain boundary is increased.Graphical abstractImage 1
       
  • Linking microstructure and local mechanical properties in SiC-SiC fiber
           composite using micromechanical testing
    • Abstract: Publication date: Available online 6 February 2019Source: Acta MaterialiaAuthor(s): Y. Zayachuk, P. Karamched, C. Deck, P. Hosemann, D.E.J. Armstrong Local mechanical properties of SiC-SiC fiber-reinforced composite – matrix, fiber and interphases – were evaluated using nanoindentation and microcantilever fracture testing. The fracture toughness was found to be ∼4.25 MPa*m1/2 in the matrix, ∼2 MPa*m1/2 in the fibers and ∼0.8 MPa*m1/2 at the interphases. Nanoindentation hardness was found to vary from ∼17 GPa in the center of the fibers to ∼40 GPa in the matrix. Values obtained with micromechanical testing were found to be in good agreement with the available data on bulk mechanical properties. The mechanical property variations in the different components of the composite can be explained by the variations in the microstructure. The matrix has complex hierarchical microstructure with elongated grains, often featuring twinning, growing radially from the fibers in predominantly direction and forming sets of concentric rings around them. The fibers contain equiaxed grains with carbon precipitates at the grain boundaries. It was found that in the matrix fracture is transgranular, while in the fibers it can be both trans- and intergranular; at the interphases the fracture occurs at the carbon-fiber boundary. The differences in mechanical properties between the matrix and the fibers are attributed to the presence of carbon inclusions in the fibers, which reduce both hardness and fracture toughness.Graphical abstractImage 1
       
  • Tuning the microstructure and metastability of β-Ti for simultaneous
           enhancement of strength and ductility of Ti-based bulk metallic glass
           composites
    • Abstract: Publication date: Available online 5 February 2019Source: Acta MaterialiaAuthor(s): L. Zhang, R.L. Narayan, H.M. Fu, U. Ramamurty, W.R. Li, Y. Li, H.F. Zhang A parametric experimental study on the role played by the size, metastability, and volume fraction of the dendritic β-Ti phase on the tensile properties of amorphous matrix composites is conducted. Towards this end, several bulk metallic glass composites (BMGCs) with varying compositions were synthesized, processed under different cooling rates and tensile tested. Results show that the stress induced martensitic transformation, from β to α″, of the dendritic Ti phase enhances the resistance to shear band propagation and, in turn, imparts significant strain hardening capability to the composite. This transformation was found to be favored in BMGCs in which the size of the dendrites is relatively coarse and Co content is ∼1 at.%. Furthermore, a volume fraction of the dendritic phase between 34% and 45% was found to result in optimum combination of strength and ductility. The utility of these microstructural design principles learned from this study was demonstrated by design, synthesis, and testing of a BMGC containing transformable β-Ti with a volume fraction of ∼38% that simultaneously exhibits high strength and ductility.Graphical abstractImage 1
       
  • Particle deformation and microstructure evolution during cold spray of
           individual Al-Cu alloy powder particles
    • Abstract: Publication date: Available online 1 February 2019Source: Acta MaterialiaAuthor(s): Tian Liu, Jeremy D. Leazer, Luke N. Brewer This paper examines the single particle impact process for a series of Al-Cu alloy powder particles with 2-5 wt% copper, performed using a low-pressure cold spray system with helium as the carrier gas. The cold spray deposition process is fundamentally controlled by the deformation processes, which occur during single particle impacts. Single particle depositions on steel substrates were produced for each composition over a range of gas temperatures (225-325 ⁰C). Cross sections from single, impacted particles were produced using focused ion beam methods. The deformation microstructure in individual cold sprayed particles was studied using precession electron diffraction (PED) and transmission Kikuchi diffraction (TKD) techniques. For Al-Cu alloy particles, the amount of deformation estimated using a compression ratio did not show a significant difference with varying copper alloy additions. The single particles experienced large deformation upon impact, and ultrafine grains were formed at the particle/substrate interface via dynamic recrystallization. The particles were bonded to the steel substrate via a thin amorphous layer at the interface.Graphical abstractImage 1
       
  • On the effect of Re addition on microstructural evolution of a CoNi-based
           superalloy
    • Abstract: Publication date: Available online 1 February 2019Source: Acta MaterialiaAuthor(s): P. Pandey, A.K. Sawant, B. Nithin, Z. Peng, S.K. Makineni, B. Gault, K. Chattopadhyay In this study, the effect of rhenium (Re) addition on microstructural evolution of a new low-density Co-Ni-Al-Mo-Nb based superalloy is presented. Addition of Re significantly influences the γ′ precipitate morphology, the γ/γ′ lattice misfit and the γ/γ′ microstructural stability during long term aging. An addition of 2 at.% Re to a Co-30Ni-10Al-5Mo-2Nb (all in at.%) alloy, aged at 900 °C for 50 h, reduces the γ/γ′ lattice misfit by ∼ 40% (from +0.32% to +0.19%, measured at room temperature) and hence alters the γ′ morphology from cuboidal to round-cornered cuboidal precipitates. The composition profiles across the γ/γ′ interface by atom probe tomography (APT) reveals Re partitions to the γ phase (KRe=0.34) and also results in the partitioning reversal of Mo to the γ phase (KMo=0.90) from the γ′ precipitate. An inhomogeneous distribution of Gibbsian interfacial excess for the solute Re (ΓRe, ranging from 0.8 to 9.6 atom.nm−2) has been observed at the γ/γ′ interface. A coarsening study at 900 °C (up to 1000 h) suggests that the coarsening of γ′ precipitates occurs solely by an evaporation–condensation (EC) mechanism. This is contrary to that observed in the Co-30Ni-10Al-5Mo-2Nb alloy as well as in some of the Ni-Al based and high mass density Co-Al-W based superalloys, where γ′ precipitates coarsen by coagulation/coalescence mechanism with extensive alignment of γ′ along directions as a sign of microstructural instability. The γ′ coarsening rate exponent (Kr) and γ/γ′ interfacial energy are estimated to be 1.41 × 10−27 m3/s and 8.4 mJ/m2, which are comparable and lower than Co-Al-W based superalloys.Graphical abstractImage 1
       
  • The role of elastic and plastic anisotropy in intergranular spall failure
    • Abstract: Publication date: Available online 31 January 2019Source: Acta MaterialiaAuthor(s): Thao Nguyen, Darby J. Luscher, Justin W. Wilkerson Recent mesoscale experimental observations of dynamic ductile failure [1, 2] have demonstrated a strong relationship between grain boundary (GB) misorientation and the likelihood of failure initiation along said GB. This correlation has been attributed to inherent GB weakness of particular misorientation. Here we discuss the role played by mechanics, i.e. elastic and plastic anisotropy, on the experimental observation [1, 2]. We make use of a recently developed framework for modeling dislocation-based crystal plasticity and ductile failure of single crystals under dynamic loading (CPD-FE) [3]. Polycrystals are studied at the mesoscale level through the explicit resolution of individual grains, i.e. resolving each individual grain’s size, shape, and orientation. In our simulations, failure naturally localizes along the GBs with no necessity for ad hoc rules governing damage nucleation. We carry out a few thousand mesoscale calculations, systematically varying the misorientation angles of the GB in the computational microstructure. Despite the fact that we neglect the possibility of variations in inherent GB weakness, our simulations agree favorably with the experimental observations, implying that stress concentration generated by elastic and plastic anisotropies across GBs is a dominant governing factor in this phenomenon. Lastly, we find that misorientation angle is an insufficient GB descriptor to predict the likelihood of intergranular spall failure, which is better understood through the consideration of additional GB degrees of freedom.Graphical abstractImage 1
       
  • Directional and oscillating residual stress on the mesoscale in additively
           manufactured Ti-6Al-4V
    • Abstract: Publication date: Available online 30 January 2019Source: Acta MaterialiaAuthor(s): M. Strantza, B. Vrancken, M.B. Prime, C. Truman, M. Rombouts, D.W. Brown, P. Guillaume, D. Van Hemelrijck In additive manufacturing of metals as compared to conventional processing, the directional track-by-track and layer-by-layer nature of the fabrication process can lead to residual stresses that locally are both directionally and spatially heterogeneous. Much of the existing literature has focused either on the macroscale residual stress inside the entire part, or on the microscale residual stresses that are created around a scan vector, thereby neglecting the intermediate length-scale of the different layers. The objective of this research is to investigate such mesoscale residual stress distributions across several layers in Ti-6Al-4V components produced by laser metal deposition process. The incremental centre hole drilling and incremental slitting methods provide measurements with excellent spatial resolution within and across the layer length scale. In this work, the two methods complemented each other to quantify both mesoscale and directional variations of residual stress that correlate with the deposition pattern. Our findings also provide strong evidence that an oscillatory residual stress variation persists even after thermal cycling that occurs during deposition of subsequent layers.Graphical abstractImage 1
       
 
 
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