High Entropy Alloys VIII: Structures and Modeling I
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

Thursday 8:30 AM
February 27, 2020
Room: Mission Hills
Location: Marriott Marquis Hotel

Session Chair: Michael Gao, National Energy Technology Laboratory; James Morris, Ames Laboratory


8:30 AM  Invited
First-principles Methods of Calculating Stacking Fault Energies in Refractory BCC High-entropy Alloys: Chelsey Hargather1; Joshua Strother1; 1New Mexico Institute of Mining and Technology
    Due to their unusual structure and composition, BCC high entropy alloys (HEAs) are generally refractive and have favorable properties such as high service temperatures, strength, and ductility, making them desirable for high performance applications. In the present work, first-principles calculations using density functional theory were used to analyze stacking fault energies (SFE) in the AlNbTaTiV BCC HEA system. Special quasi-random structures and the GGA-PBE exchange correlation functional were used. “Twinning-sense” and “non-twinning-sense” stacking faults on the (112) plane, with ABCDEFA stacking, were investigated. Additionally, a method for predicting the error bars on calculations from the approximations to a random solution was introduced and analyzed. This work demonstrates a method of averaging and approximation that could improve the high throughput efficiency of database generation of HEA properties. Finally, the effect of BCC SFE values on deformation properties such as nano-twin nucleation and growth is discussed.

8:50 AM  Invited
Compositional Design and Deformation Behavior in Ni-based Concentrated/HEA Alloys: Ridwan Sakidja1; Wai-Yim Ching2; 1Missouri State University; 2University of Missouri-Kansas City
    We evaluated the correlation between the compositions and the deformation behavior for a variety of Ni-based concentrated/HEA alloys based on external materials databases and developed the EAM potentials for a selected number of multi-component systems. The source of external materials database used is primarily the reference from Data in Brief (2018, 2664-2678) by S.Gorsse, M.H.Nguyen, O.N.Senkov, and D.B.Miracle, in combination with the secondary references to obtain the stress-strain curves (if available). The MD simulations and the Machine Learning analysis on the databases are utilized to evaluate the dislocation mobilities and the deformation behavior in general. The roles of the local chemical bonding within the structures are also discussed.

9:10 AM  Invited
Emerging Computational Tools for Exploring the Refractory Compositionally Complex Alloys: Christopher Woodward1; Satish Rao2; Edwin Antillon2; Brahim Akdim2; Triplicane Parthasarathy2; Daniel Miracle1; Oleg Senkov2; 1Air Force Research Laboratory; 2UES Inc.
    We review a range of computational methods for predicting fundamental properties and mechanisms controlling material response in Refractory Compositionally Complex alloys. A spreadsheet model of expected density, atomic size difference, mixing enthalpy, melting temperatures, cost, and CALculation of PHAse Diagram predictions of equilibrium phases can be used to narrow the pallet of possible elements for a specific application. Electronic structure methods can also be used to predict mean field properties such as elastic constants, as well as insights into deformation mechanisms. Atomistic methods have been used to better understand deformation over a wide range of temperatures, the role of chemical short range order, and inform analytic solution strengthening models. Also, new approaches for predicting phase stability for wide range of compositionally complex alloys are in development.

9:30 AM  Invited
Ab Initio Phase Stabilities of High Entropy and Chemically Complex Alloys: Yuji Ikeda1; Prashanth Srinivasan2; Biswanath Dutta2; Jörg Neugebauer1; Blazej Grabowski3; Tatiana Kostiuchenko4; Alexander Shapeev4; Fritz Koermann2; 1Max-Planck-Institut für Eisenforschung GmbH; 2Tu Delft; 3University of Stuttgart; 4Skoltech
    Concentrated solid solutions alas high entropy alloys are often characterized by a large configurational entropy. However, as in any realistic alloy, other contributions such as lattice vibrations, magnetic fluctuations as well as chemical short-range order or defects are also often crucial in determining their phase stability and materials properties. Capturing these contributions with high accuracy from first-principles calculations is extremely challenging. In this talk I will give an overview over the recent developments and advances including applications of machine learning techniques. Examples include computations of full vibrational free energies including anharmonicity and short-range order calculations for selected multicomponent alloys. Based on this I will discuss routes to optimize the mechanical properties, e.g., via modifying stacking fault energies by tuning the composition or by interstitial alloying.

9:50 AM  
Deformation-induced Crystalline to Amorphous Phase Transformation in High-entropy Alloys: Dengke Chen1; Yin Zhang1; Hao Wang2; Xiaozhou Liao2; Ting Zhu1; 1Georgia Institute of Technology; 2The University of Sydney
    A rational design of high-performance high-entropy alloys (HEAs) is critically hinged on a deep understanding of the underlying deformation mechanisms. Here we report the in situ TEM observation and atomistic simulation of crystalline to amorphous phase transformation at the crack tip of an ultrafine-grained Cantor alloy. Due to the rugged energy landscape, dislocations are trapped near the crack tip. Interactions between densely tangled dislocations on different slip systems result in solid-state amorphization around the crack tip. Further crack propagation is impeded by the amorphous bridges in the crack wake, contributing to the enhanced fracture toughness of the Cantor alloy.

10:10 AM Break

10:30 AM  Invited
Small-scale Mechanical Behavior of Single Phase and Complex High Entropy Alloys: Saideep Muskeri1; Vahid Hasannaeimi1; Maryam Sadeghilaridjani1; Sundeep Mukherjee1; 1University of North Texas
    High entropy alloys have attracted widespread interest due to their exceptional properties at multiple length-scales. Small-scale mechanical behavior of several single-phase and complex multi-principal high entropy alloys was investigated. Strain rate sensitivity and strain gradient plasticity was measured for fundamental understanding of deformation mechanism. The nano-mechanical behavior and twinned microstructure was explained by low stacking fault energy. Phase-specific response to mechanical loading was evaluated for thorough understanding of the origin of simultaneous high strength and good ductility in these alloys. Exceptional strength was realized for nanocrystalline high entropy alloy compared to pure metals of similar grain sizes. Grain boundary mediated deformation mechanisms led to high strain rate sensitivity of flow stress.

10:50 AM  Invited
Stability of Al-Li-Ti-Sc-Mg High Entropy Alloys from Monte Carlo Simulations: James Morris1; Eva Zarkadoula2; M. Claudia Troparevsky2; Andreas Kulovits3; 1Ames Laboratory; 2Oak Ridge National Laboratory; 3Arconic Inc.
    Based on a recent approach for utilizing high throughput first-principles calculations in Monte Carlo simulations of medium- and high-entropy alloys, we examine the temperature- and composition-dependent stability of Al-Li-Ti-Sc-Mg alloys. The Al20Li20Mg10Sc20Ti30 alloy is reported to form a single-phase high entropy material despite strong Al-Ti and Al-Sc interactions, and the tendency of Li-Ti and Li-Sc to phase separate. Our calculations indicate that a single phase likely forms above ~600 K; below this temperature, there is the strong development of Li-Mg, Ti-Al, and Ti-Sc ordering. The calculations suggest that for Al alloys, a strong enthalpy of mixing does not preclude single-phase solid solutions.

11:10 AM  Invited
Atomistic Simulations of the fcc-to-hcp Phase Transformation in the Equiatomic CoCrFeMnNi Alloy under High Compression: Chin-Lung Kuo1; 1National Taiwan University
    We performed the MEAM-based molecular dynamic simulations to investigate the plastic deformation and phase transformation in the CoCrFeMnNi HEA under high compression. Our MD simulations revealed that the stress-induced phase transformations in the CoCrFeMnNi HEA are strongly crystal orientation-dependent. The [001] uniaxial compression can induce significant fcc-to-hcp phase transformation via successive emissions of partial dislocations from the extended stacking faults, twin boundaries and hcp-lamellas created during the early stage of deformation. As for the [110] and [111] uniaxial compressions, however, the hcp atoms can simply form the intrinsic/extrinsic stacking faults. Although the [001] uniaxial compression produced a much lower dislocation density than other systems, it eventually induced much more constituents transformed into the hcp atoms upon the end of phase transformation. Our simulations clearly indicate that the deformation twin boundaries and extended hcp-lamellas play a critical role in facilitating the fcc-to-hcp phase transformation in the CoCrFeMnNi HEA under high compression.

11:30 AM  Invited
Data-driven Design of High-entropy Alloys: Houlong Zhuang1; Wenjiang Huang1; Duo Wang1; 1Arizona State University
     Applying data-driven approaches to a variety of problems has progressively become one of the frontier research areas in the field of materials science and engineering. One main objective of these methods is to accelerate the discovery of new materials through cooperative applications of data, algorithms, and fundamentals of materials theory. High-entropy alloys (HEAs) represent an important category of metal alloys that have received intense attention over the past decade. In this talk, we will present the weaknesses and strengths of major computational approaches to design, discover, and characterize HEAs. We then focus on a data-driven approach by introducing the cross-industry process for data mining (CRISP-DM) method, which is a standard technique commonly used in the data scienceindustry. We will use examples to show how each step of this method is involved in the context of designing HEAs.

11:50 AM  Invited
The Stacking Fault Energies of FCC High-entropy Alloys: An ab initio Study: Zongrui Pei1; Jeffrey Hawk1; David Alman1; Michael Gao1; 1National Energy Technology Laboratory
    The stacking fault energies (SFEs) are one of the most important factors in determining the mechanical properties of metals and alloys. In this study, we calculate the SFEs of a number of FCC high-entropy alloys (HEAs) based on Co-Cr-Fe-Ni system w/o Mo using ab initio methods. Our results reveal fantastic effects of the elements on SFEs: (i) increasing the concentration of Ni increases the negative SFEs up to almost zero, while (ii) increasing the concentration of Mo dramatically decreases the already negative SFEs. The point (i) indicates that the HCP (hard phase) and FCC (soft phase) structures are almost equal in energy and phase stability at 0K, which attracts us to further calculate the phase transition pressure. In addition, the connection between our ab initio results and the mechanical properties of the HEAs are also extensively studied using multi-scale models.