Thursday 8:30 AM

March 2, 2017

Room: 32B

Location: San Diego Convention Ctr

Chemical substitution disorder can preserve global translational and orientational symmetry even while chemical bonding creates short-range chemical order. We explore the competition between entropy and chemical order using a combination of analytic cluster variation method approximations and full first principles computer simulations, applied to body centered cubic high entropy alloy families. Our finding reveal limits of single phase BCC stability against phase separation among refractory HEA-formers and against B2 (CsCl-like) ordering in transition metal aluminides.

Recently it has been discovered that high entropy contributions to Gibbs free energy can stabilize a new oxide phase with rock salt structure, (Mg, Co, Ni, Cu, Zn)O, with cations randomly distributed over the cation sites.1 Understanding the fundamental properties of this entropy-stabilized oxide (ESO) is of great interest because it opens up possibilities for designing similar materials with well-defined properties for specific applications. Using electronic structure methods within the DFT, the properties of three ESOs (with addition of Li and Sc to the cation-sublattice) are investigated, to see if the empirical parameters may exist that would predict stability of ESOs. Results indicate that lattice constants and, for certain compositions, the Bader charges are transferable from binary and ternary oxides to ESO. The electronic structure analysis of Cu d-states reveals a Jahn-Teller distortion involving O atom displacements. 1Rost, C. M. et al. Entropy-stabilized oxides. Nat. Commun. 6, 8485 (2015).

"High-entropy alloys" are an interesting new class of multi-component alloys. Equimolar multi-component alloys (EMAs) CrMnFeCoNi or NiCrCo are representatives of such alloys with single-phase FCC crystal structure. They are believed to be characterized by a low stacking fault energy (SFE). However, as of yet, the SFE in these alloys is not well understood. Using molecular dynamics simulations with realistic EAM potentials, we determine the magnitude of the SFE and analyse the SFE variations from location to location, as well as the temperature dependence of the average SFE and its distribution. We also show variations of the separation between partial dislocations and found that this dissociation distance depends not only on local SFE, but also on the lattice friction stress which is large and variable in these highly-concentrated solutions. Our results shed light on the complex and unconventional behavior of SFE in EMAs.

The material-design strategy of combining multiple elements in near-equimolar ratios has spearheaded the emergence of high-entropy alloys (HEAs), an exciting class of materials with exceptional engineering properties. Elastic properties are important criteria for engineering material design. The elastic constants of a material provide a complete description of the response of the material to external stresses in the elastic limit, which provides fundamental insight into the nature of bonding in the material, and is known to correlate with many mechanical properties. We employed first-principles approaches to study the elastic properties of prototype HEA systems, such as Al0.3CrFeCoNi. Computed elastic constant results from Special Quasi-random Structures (SQS) and Coherent Potential Approximations (CPA) will be compared with experimental results. We will also evaluate the validity of a previously developed statistical learning model of elastic constants for HEA systems. The effects of HEA compositions on the elastic properties and phase stability will be discussed.

High-entropy alloys (HEAs) containing Molybdenum (Mo) are known for their high-temperature strength and superior corrosion resistance. Molybdenum is known to segregate to inter-dendritic regions in the microstructure of HEAs. We investigate the phase stability of Mo-based quinary{Ta-W-Ti-Zr}_{1-x}-Mo_{x} (x = 0 - 40, x in atomic %) using the semi-relativistic Korringa-Kohn-Rostoker (KKR) electronic-structure code in combination with the screened Coherent Potential Approximation (CPA). Calculated properties (e.g., lattice constants, bulk modulus, and formation enthalpies) are compared with the classical molecular dynamics (MD) simulations based on Embedded Atomic Model (EAM) potential. Through analysis of MD results, we explore the short-range order (SRO) effects from assessed pair probabilities. Initial results show that weak Mo-driven short-range order decreases with increase in Mo content. These results provide insights on the ordering behavior in Mo-based refractory high-entropy alloys.

HEAs have unveiled and unique mechanical responses and need constitutive equations based on deformation mechanisms and microstructural features, instead of simple stress-strain numeric data. In this presentation, even if many factors governing properties of HEAs are still unclear, we propose constitutive models based on microstructural features, such as grain size, dislocation density, twin probability, friction stress, and texture, and deformation mechanisms, such as dislocation glide, twinning, and diffusion. Experimental works for mechanical behaviors, e.g. indentation, static tensile testing, and dynamic shock testing, and for microstructural characterizations using synchrotron XRD, SEM-EBSD, TEM, and 3D atom probe tomography, are performed in order to correlate microstructural features with mechanical responses, and finally to develop the mesoscopic mechanism-based constitutive model. The proposed model is verified by investigating mechanical responses and by quantitative comparisons of predicted and experimental features of microstructure.

High-entropy alloys (HEA) represent a new class of materials with interesting strength and high-temperature properties. Yield stress of a model FCC multicomponent material (Co30-Fe16.67-Ni36.67-Ti16.67) is investigated using pairwise (EAM) potentials. Previous work has shown that a random chemical distribution gives rise to large variations in the dislocation core structure and local pinning due to variations in composition. We extend this work by applying a hybrid Molecular-dynamics/Monte-Carlo simulation to mimic alloy annealing at various temperatures. Even at large annealing temperatures significant compositional short-range order (CSRO) emerges which can regularize the large variability of the stacking fault energies seen in the purely random configurations. We explore the distribution of hard and soft regions along the dislocation line due to concentration fluctuations and ordering in order to inform development of high-concentration solid-solution models.

Phase prediction from simulation can greatly assist the development of high entropy alloys (HEAs). The Monte Carlo algorithm provides a general method to find the stable phases of a material. An efficient Monte Carlo (MC) method using ab-initio energies is proposed for studies of phase formation in HEAs. We apply this MC method for multiple HEAs, e.g., HfNbTaTiZr and NiFeCrCoMn, and obtain stable phase predictions. Further density functional theory (DFT) calculations are performed to give insight into the prediction of phase stability of the HEAs.

High Entropy Alloys (HEAs) is a new class of alloys which are composed of 5 or more elements with equal or near equal atomic weight with promising mechanical behavior such as higher specific yield strength. In this study a Continuum Dislocation Dynamics (CDD) framework is used to study the deformation behavior of High Entropy Alloys. In this framework, the dislocations are divided into three types, a positive, a negative and an immobile, and coupled evolution equations are employed to describe the complex interactions between the different types of dislocations. Furthermore, the effect of the lattice distortion is considered by introducing stochastic terms in the evolution equations. The effect of temperature, grain size, and residual dislocation density is investigated.

The phase stability and dislocation structure of a multicomponent equimolar HEA is studied by a variance constrained semi-grandcanonical Monte-Carlo method in combination with molecular dynamics simulations. We report simulations of a random solid solution of CuNiCoFe, which is stabilized by configurational entropy at elevated temperatures. A detailed analysis of the chemical and structural order at lower temperature, however, provides evidence for the fact that HEAs evolve into multiphase alloy systems at lower temperature. We further study the motion of an initially straight dislocation line under the action of external shear stress. The dislocation line is developing a kinked structure and is locally pinned. Our simulations reveal that local disorder has a major impact on the dislocation motion. By analyzing the temperature and stress dependence of the depinning process of the dislocation line, we find significantly increased local Peierls barriers, which are one possible reason for the observed strength of HEAs.