High Entropy Alloys V: Structures and Mechanical Properties I
Sponsored by: TMS Structural Materials Division, TMS Functional Materials Division, TMS: Alloy Phases Committee, TMS: Mechanical Behavior of Materials Committee
Program Organizers: Peter Liaw, University of Tennessee; Michael Gao, National Energy Technology Laboratory; Suveen Nigel Mathaudhu, University of California Riverside; Xie Xie, The University of Tennessee, Knoxville; Gongyao Wang, Alcoa Technical Center; E-Wen Huang, National Chiao Tung University

Wednesday 8:30 AM
March 1, 2017
Room: 32B
Location: San Diego Convention Ctr

Session Chair: Takeshi Egami, University of Tennessee; Jeffrey Hawk, National Energy Technology Laboratory

8:30 AM  Invited
Electronic and Lattice Heterogeneity in High-entropy Alloys: Takeshi Egami1; 1University of Tennessee
    High-entropy alloys (HEA) are stabilized by configurational entropy, but their properties, mechanical properties in particular, are controlled more by their electronic and lattice heterogeneity. They can pin dislocations and increase hardness. They also promote local melting upon irradiation, leading to self-healing effect. Calculations using the density functional theory show high atomic-level stresses in HEA due to charge transfer which mainly affects pressure, and atomic size mismatch which produces both pressure and shear stress. These atomic-level stresses result in local lattice distortions which can be observed by diffraction. We discuss the results of single-crystal neutron diffuse scattering studies of HEA which shows strong local lattice distortion. This work is supported by the Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division.

8:50 AM  Invited
Creep Strength, Deformation, and Fracture in Single Phase High Entropy Alloy: Jeffrey Hawk1; Kyle Rozman2; John Sears3; Paul Jablonski1; Michael Gao3; 1U.S. Department of Energy, National Energy Technology Laboratory; 2ORISE; 3AECOM
    The creep behavior of single phase high entropy alloy has been investigated. Tensile creep testing was performed in two ways: (1) setting the stress level and changing temperature (activation energy for creep) and (2) setting the temperature and changing the stress level (stress exponent for creep). High entropy alloys (HEA) have generated interest in recent years due to their unique positioning within the alloy world. At NETL the process of making large scale ingots employed induction melting, homogenization, and deformation processing to form a fully alloyed, single phase, wrought microstructures. Tensile mechanical behavior was determined as a function of temperature and then creep testing was performed based on these results in the manner described. The as-processed and creep tested microstructures were examined to characterize the deformation that occurs during creep, and the fractures surfaces were inspected to assess macroscopic failure modes.

9:10 AM  Invited
Hardening Mechanisms in High-entropy Alloys: Z. P. Lu1; 1University of Science and Technology Beijing
    High-entropy alloys (HEAs) have shown great potential to be utilized as engineering materials due to their high phase stability, large lattice distortion and complex chemical short-range ordering. The strengthening behavior and underlying mechanisms in these highly concentrated matrices attracted more and more attention since it is difficult to identify the conventional “solutes and solvents”. In this talk, representative examples regarding employment of different methodologies to strengthen HEAs will be presented: 1). Precipitation hardening in the fcc FeCoNiCrMn HEA matrix; effects of alloying additions and thermomechanical treatment on precipitation behavior and tensile properties will be discussed; 2). Solid-solution hardening in the bcc TaNbHfZrTi HEA matrix; effects of interstitial atoms on deformation behavior and damping properties will be discussed; and 3). Transformation mediated strengthening in a hcp matrix with an equal molar atomic ratio; effects of transformation-induced-plasticity and deformation behavior will be analyzed.

9:30 AM  Invited
Mechanical and Corrosion Properties of CoCrFeNiTi-based High-entropy Alloy Additive Manufactured Using Selective Electron Beam Melting: Tadashi Fujieda1; Hiroshi Shiratori2; Kosuke Kuwabara3; Mamoru Hirota1; Takahiko Kato4; Kenta Yamanaka2; Yuichiro Koizumi2; Akihiko Chiba2; Seiichi Watanabe5; 1Hitachi, Ltd.; 2Tohoku University; 3Hitachi Ltd.; 4Hitachi, Ltd.; Hokkaido University; 5Hokkaido University
    We could succeed in producing the high-density and homogeneous molded objects of CoCrFeNiTi-based high-entropy alloy by using selective electron beam melting (SEBM), which exhibited high tensile strength and high corrosion resistance in a 3.5% NaCl solution. However, the ductility was not sufficient because of excessive Ni3Ti intermetallic compound precipitates with basket-weave morphology. We solved Ni3Ti compounds into the matrix by solution treatment. As a result, the ductility markedly improved without decreasing the strength, which was ascribed to the presence of very fine single cubic ordering precipitations in the face-centered cubic matrix. Furthermore, corrosion resistance also greatly improved because of the uniform composition by the uniform precipitation of very fine particles. The solution-treated SEBM specimens exhibited both high strength and high pitting potential, which in combination are superior to the conventional alloys.

9:50 AM  
Nanomechanical Behavior and Nano-indentation Size Effects in High Entropy Alloys: Sanghita Mridha1; Hunter Oltman1; Sundeep Mukherjee1; 1University of North Texas
    Nano-mechanical behavior of several high entropy alloys was investigated in as-cast, rolled, annealed, and thin-film forms. Dislocation nucleation was studied by repeated indents at low load for the different processing conditions. Distinct displacement bursts were observed in the loading curve marked by incipient plasticity for all the samples. The as-cast and annealed samples showed pop-ins for 100% of the indents, while the rolled and thin-film samples showed much lower fraction of displacement bursts. This was explained by the high density of dislocations for the cold-worked and thin-film conditions. The strong depth dependence of hardness was explained by geometrically necessary dislocations. The nano-mechanical behavior and twinned microstructure was explained by low stacking fault energy for these high entropy alloys.

10:10 AM Break

10:30 AM  Invited
Short Range Order in a BCC V-Nb-Mo-Ta-W High Entropy Alloy: Hongru Du1; Jian Han1; Edwin Antillon2; Christopher Woodward2; David Srolovitz1; 1University of Pennsylvania; 2Air Force Research Laboratory
    Even though high entropy alloys are solid solutions, some degree of short range order (SRO) should be expected. Short range order will not only affect the alloy thermodynamics but other properties as well (e.g., the mean Peierls stress). We examine the equilibrium short range order in the equiatomic BCC V-Nb-Mo-Ta-W system using both atomistic Monte Carlo simulations and hybrid Monte Carlo-Molecular Dynamics and interatomic potentials developed by Zhou and co-workers. The short range order is described using the pairwise multicomponent SRO parameter of Coguerra, et al. (2010). We report the nearest and second nearest neighbor SRO for 300˚K ≤ T ≤ 2500˚K for all element pairs and attempt to rationalize it based upon existing models. From these data, we examine the alloy thermodynamic properties. One interesting finding is the existence of an effective Kauzmann temperature. Based on this, we will discuss the relationship between HEAs and glasses.

10:50 AM  
The Study of Fatigue Behavior in Refractory High Entropy Alloys: Shuying Chen1; Chien-Chang Juan2; Jien-Wei Yeh2; Karin Dahmen3; Peter Liaw1; 1University of Tennessee, Knoxville; 2National Tsing Hua University; 3University of Illinois at Urbana Champaign
    In our study, fatigue behavior was studied on refractory high-entropy alloys (HEAs) in the stress range from 800 MPa to 1,350 MPa during four-point-bending-cyclic loading experiments. The present fatigue investigation showed encouraging fatigue-resistance characteristics due to the prolonged fatigue lives at relatively high stresses. The current results indicate that the fatigue behavior of HEAs are generally better than many other conventional alloys, such as steels, titanium alloys, and advanced bulk metallic glasses with a fatigue endurance limit of between 360 and 512 MPa and a fatigue endurance limit to ultimate tensile strength ratio between 0.45 and 0.504. Besides, it was interesting to find that the noticeable grain growth could happen after long cyclic loading experiments, which may be a precursor to crack initiation.

11:10 AM  Invited
Microstructural Evolution and Mechanical Behavior of a Non-equiatomic High-entropy Alloy Reinforced by Nanoprecipitates: Zhiqiang Fu1; Benjamin MacDonald1; Baolong Zheng1; Weiping Chen2; Yaojun Lin3; Fei Chen3; Yizhang Zhou1; Lianmeng Zhang3; Enrique Lavernia1; 1University of California, Irvine; 2South China University of Technology; 3Wuhan University of Technology
    High entropy alloys are a type of novel metallic materials that have revolutionized the alloy design and exploring strategies, and can potentially be used as structural materials in extreme environments, such as at elevated temperatures and at cryogenic temperatures. This presentation reports a study on microstructural evolution and mechanical behavior of a non-equiatomic FeNiCoCuTi high-entropy alloy (HEA) containing nanoprecipitates produced by casting plus subsequent treatments. Microstructures and phase compositions of the as-cast, heat treated and rolled HEA samples were investigated via X-ray diffraction, scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy. The interface between the matrix and nanoprecipitates was also studied using high resolution transmission electron microscopy. In addition, the volume fractions of nanoprecipitates in the as-cast, heat treated and rolled HEA samples were estimated. Mechanical behavior of the material was assessed in terms of both tensile and compression tests at room temperature and at elevated temperatures.

11:30 AM  Invited
Atomic-level Disorder and Defect Dynamics in Concentrated Solid-solution Alloys: Yanwen Zhang1; Shijun Zhao2; Fredric Granberg3; Kai Nordlund3; Flyura Djurabekova3; William Weber4; 1Oak Ridge National Laboratory; University of Tennessee; 2Oak Ridge National Laboratory; 3University of Helsinki; 4University of Tennessee; Oak Ridge National Laboratory
    Performance enhancement of structural materials in extreme environments has been actively investigated for many decades. Recently developed single-phase concentrated solid solution alloys (CSAs) exhibit significant chemical disorders and unique site-to-site lattice distortions. While it has long been recognized that specific compositions of traditional alloys have enhanced radiation resistance, it remains unclear how the atomic-level alloying affects defect formation, damage accumulation, and microstructural evolution. Such knowledge gaps have been a roadblock to future-generation energy technology. CSAs with a simple crystal structure, but complex chemical disorder are ideal systems to understand how compositional complexity influences defect dynamics, and to fill the knowledge gaps with focus on electronic- and atomic-level interactions, mass and energy transfer processes, and radiation resistance performance. Recent advances of defect dynamics and irradiation performance in CSAs are reviewed, and intrinsic chemical effects on radiation performance are discussed. This work was supported by the Energy Dissipation to Defect Evolution Center (EDDE), an Energy Frontier Research Center funded by the U.S. DOE, BES, MSED.

11:50 AM  
Stability of Ordered Precipitates in Face Centered Cubic based High Entropy Alloys-Al 0.3 CoFeCrNi and Al 0.3 CuFeCrNi 2 and their Effect on Mechanicalproperties: Bharat Gwalani1; Vishal Soni1; J.Y. Hwang2; Deep Choudhuri1; Rajarshi Banerjee1; 1University of North Texas Denton; 2Institute of Advanced Composite Materials, Korea Institute of Science and Technology
    Adding a small amount of Al to the well-known high entropy alloy (HEA), Al0.1CoCrFeNi, to form Al0.3CoCrFeNi, leads to the precipitation of highly refined ordered L12 precipitates, which are stable at 550 C. However, the L12 precipitates (stoichiometry of (Ni,Cr)3(Al,Fe,Co)) are de-stabilized and replaced by coarser B2 precipitates on annealing at 700 C. Contrastingly, in the Co-free, Al0.3CuCrFeNi2 high entropy alloy, the L12 precipitates are stable up to ~900 oC and exhibit a stoichiometry of (Ni,Cu)3(Al,Fe,Cr). Both these alloys were studied in great detail using insitu-synchrotron experiments, high resolution transmission electron microscopy and atom probe tomography. The results lead to interesting insights into the stability of ordered phases in HEAs as a function of composition and temperature. The effect of precipitation of ordered phases on the mechanical properties like micro-hardness and tensile strength is also analyzed. Tremendous increase in strength is observed up on precipitation of fine scaled coherent precipitates.