High Entropy Alloys IX: Structures and Modeling : Structures and Modeling III
Sponsored by: TMS Functional Materials Division, TMS Structural 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; E-Wen Huang, National Chiao Tung University; Srivatsan Tirumalai; Xie Xie, FCA US LLC; Gongyao Wang, Globus Medical

Tuesday 8:30 AM
March 16, 2021
Room: RM 9
Location: TMS2021 Virtual

Session Chair: Michael Gao, National Energy Technology Lab; Louis Santodonato, Advanced Research Systems


8:30 AM  Invited
Phase Stability of High Entropy Alloys: Effects of Pressure and Temperature: Michael Gao1; Xuesong Fan2; Sita Ram Aryal3; Lizhi Ouyang3; Peter Liaw2; Jeffrey Hawk1; David Alman1; 1National Energy Technology Laboratory; 2University of Tennessee; 3Tennessee State University
    Deformation induced phase transformations are reported in FCC- and BCC-based high entropy alloys, but their phase stability remain controversial and require further investigation. This talk presents ongoing computational modeling and experimental research on the phase stability of Co-Cr-Fe-Mn-Ni and HfTiZrMx (M=Nb, Ta, V; x = 0-1) HEA systems as a function of pressure and temperature. Select unary, binary, ternary, quaternary and quinary alloys are studied systematically. Computational modeling is done using density functional theory, molecular dynamics, Monte Carlo and CALPHAD. Gibb free energy of competing phases (for example, FCC vs HCP, and BCC vs HCP) as a function of pressure and temperature are predicted. Vibrational free energy and short-range order are considered. High pressure uniaxial and hydrostatic compression experiments are done using diamond anvil cells to generate high pressure up to ~40 GPa. Correlation between electronic structures and mechanical properties will also be discussed.

8:55 AM  Invited
Monte Carlo Study of the Entropy Hypothesis Associated with High-entropy Alloys: Louis Santodonato1; Peter Liaw2; 1Advanced Research Systems; 2University of Tennessee
    High-entropy alloy research is flourishing despite the considerable evidence against the underlying “entropy hypothesis”, which states that disordered solid-solution phases will tend to be stabilized, particularly in compositions with multiple elements in equimolar ratios, because of their high configurational entropy. This hypothesis is seemingly discredited because of the abundance of ordered phases and complex multiphase microstructures observed in room-temperature high-entropy alloys. The present Monte Carlo study, however, supports the validity of the entropy hypothesis, provided that we consider the stabilizing entropy effect acting on partially-ordered phases, derived from solid solutions. Using a model where the enthalpy is a function of the atomic configuration on a fixed lattice, we find that the configurational entropy of a high-temperature solid solution may evolve during cooling to produce a best-of-both-worlds atomic arrangement, such that a large enthalpy enhancement is obtained from a small entropy decrease.

9:20 AM  Invited
Core Effect of Local Atomic Configuration and Design Principles in AlxCoCrFeNi High-entropy Alloys: Yu-Chia Yang1; Zhenhai Xia1; 1University of North Texas
    High-entropy alloys (HEAs) are known to have four core effects leading to superior properties over traditional alloys. In this paper, we investigate an additional core effect, local atomic configuration, due to inherent variations of local chemical composition at the nanoscale. The stacking fault and twin formation energies of AlxCoCrFeNi HEAs, calculated with density functional theory methods, show large variations and even negative energies due to the local atomic configurations. A design principle is proposed to predict the mechanical properties of the HEAs. The effect of temperature on stacking fault energy is also determined, which is consistent with experimental results.

9:45 AM  
Atomistic Modeling of Screw Dislocations in Body-centered Cubic High-entropy Alloys: Sheng Yin1; Jun Ding1; Mark Asta1; Robert Ritchie1; 1Lawrence Berkeley National Laboratory
    Recently, much attention has been focused on refractory HEAs. However, unlike fcc-HEAs, there have been fewer investigations on bcc-HEAs, specifically on the dislocation core properties, mobility and possible effects of chemical short-range order (SRO). Here, using density-functional-theory combined with molecular dynamics, we investigate the distribution of dislocation core properties in MoNbTaW RHEAs alloys, how they are influenced by SRO and the mobility of screw/edge dislocations. The averaged core energies are found to be larger than those in the corresponding pure constituent bcc metals, and are relatively insensitive to the degree of SRO. However, the presence of SRO will induce diffuse antiphase boundaries and is shown to have an effect on narrowing the distribution of dislocation core energies and decreasing the spatial heterogeneity of core energies in the RHEA. It is argued that the energy landscape of the dislocations would heterogeneously influence dislocation motion, thereby affecting the mechanical behavior of HEAs.

10:05 AM  
Can We Control Lattice Distortions in Entropy-stabilized Oxides?: Keivan Esfarjani1; Jonathan Kaufman1; 1University of Virginia
    Lattice distortion in high entropy alloys is postulated to have major effects on these alloys and their properties. There are limited studies that look at the effect of lattice distortion on entropy-stabilized oxides. In this study, we explore the effect of bond strength, atomic size and temperature on distortions in the entropy-stabilized oxide, MgCoNiCuZnO5. This work uses molecular dynamics to identify the explicit distances that each atom and atom type distorts from its parent rocksalt crystal structure at different temperatures. This study shows that this material can be optimized to either increase or decrease the total lattice distortion in the system by changing the atomic composition or by replacing certain elements with alternative elements.