High Entropy Alloys V: Thermal and Other Properties
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

Thursday 2:00 PM
March 2, 2017
Room: 32A
Location: San Diego Convention Ctr

Session Chair: Nobuhiro Tsuji, Kyoto University; Thanh Tran, NSWC Carderock

2:00 PM  Invited
Recrystallization and Grain Growth in High Entropy Alloys: Nokeun Park1; Tilak Bhattacharjee2; Yoshihiko Nakamura2; Xian Li2; Rajeshwar Eleti2; Yu Bai2; Akinobu Shibata2; Nobuhiro Tsuji2; 1Yeungnam University; 2Kyoto University
    Sluggish diffusion of atoms that is expected in multi-elementary high entropy alloys (HEAs) would significantly affect their recovery, recrystallization and grain growth behaviors all of which are controlled by diffusion. We have succeeded in fabricating fully recrystallized ultra fine grained (UFG) structures in several kinds go HEAs (CoCrFeMnNi, (CoCrFeNi)95Cu5, and HfNbTiTaZr) by heavy plastic deformation and subsequent annealing processes, and the recrystallization and grain growth behaviors will be introduced and discussed from a viewpoint of sluggish diffusion.

2:20 PM  Invited
Aluminum Diffusion in High Entropy Alloys: K. Michael Mathes1; Thanh Tran2; Peter Liaw1; 1University of Tennessee; 2Naval Surface Warfare Center - Carderock Division
    Sluggish diffusion in high entropy alloys (HEAs) has often been reported, but seldom proven by experimentation. The aim of the present study is to replicate some experimental results of Co, Cr, Fe and Ni family with an additional aluminum component to analyze diffusion rates as a function of temperature. To accomplish this goal two dissimilar and homogenized HEAs, a single phase Al0.7CoCrFeNi BCC alloy will be matched with another Al0.1CoCrFeNi FCC alloy in a diffusion couple. Two sets of samples will be annealed at 100°C, 200°C, 400°C, 800°C for periods of 4hr, 8hr, 16hr, 32hr and air cooled to room temperatures. The resulting samples will be analyzed for their final composition and structure. Secondly, a binary aluminum and nickel alloy with the same equal atomic aluminum composition will be tested in the same manner to determine the aluminum diffusion rate. The rate of aluminum diffusion in HEA will then be compared to that of the binary alloy to draw conclusions about the possibility of sluggish diffusion in multi-principle element alloys.

2:40 PM  Invited
Deformation Characteristics and Thermomechanical Processing of Complex Concentrated Alloys: Mageshwari Komarasamy1; Rajiv Mishra1; 1University of North Texas
    Complex concentrated alloys, encompass high entropy alloys (HEAs), provide a unique opportunity to study the fundamental dislocation science in a compositionally complex alloy systems. Microstructurally simple alloy systems were chosen to investigate the effect of severe lattice distortion on dislocation plasticity in HEAs. Alloys with varying chemical composition were synthesized to maximize the intrinsic lattice distortional strain. To understand the dislocation plasticity, both rate sensitivity of the flow stress and activation volume of dislocations were evaluated as a function of lattice distortional strain and temperature. Furthermore, thermomechanical processing techniques were employed to tailor the microstructural features such as grain size and twin density, and the properties were subsequently examined.

3:00 PM  Invited
Structural and Thermodynamic Properties of a Lightweight AlTiVCr High Entropy Alloy: Yong-Jie Hu1; Yong-Jie Qiu2; N Birbilis2; Zi-Kui Liu1; 1The Pennsylvania State University; 2Monash University
    A light-weight AlTiVCr high entropy alloy (HEA) has been investigated experimentally and computationally recently. First-principles calculations, coupled with the quasi harmonic approach based on the Debye-Grüneisen model, are employed to predict the enthalpy, entropy and Gibbs energy of formation of a fully disordered BCC structure as a function of temperature in comparison with those of an ordered B2 structure with atomic mixing in sublattice sites. It is found that the B2 structure has a more negative Gibbs energy of formation up to 900 K, indicating that the AlTiVCr HEA has a B2 structure with atomic mixing in sublattice sites at low temperatures. The predication is validated by the experimental characterization of the selected area diffraction patterns. In addition, the lattice parameter, thermal expansion coefficient, and elastic moduli of the AlTiVCr HEA are also predicted as a function of temperature.

3:20 PM  Invited
High-entropy Alloys Properties and Short- and Long-range Ordering Predicted via Electronic-Structure-based Thermodynamics: Duane Johnson1; Prashant Singh1; Andrei Smirnov1; 1Ames Laboratory/Iowa State University
    The relative global stability of N-component high-entropy alloys (HEA) in A1, A2, A3 phases are predicted from the formation enthapies using first-principles KKR electronic-structure methods combined with the coherent potential approximation (CPA) to handle chemical and magnetic disorder. In addition, the chemical short-range order and incipient long-range order, dictated by N(N–1)/2 chemical correlations in the HEA, are directly predicted by thermodynamic linear-response theory using the HEA solid-solution phase as a reference state. We exemplify the accurate predictions in various solid-solutions, including AlxCrFeCoNi(1-x), well studied experimentally. We discuss how to use these methods to narrow the design search space for improved HEAs. The method also helps facilitate development of empirical potentials for simulations to predict mechanical behavior to connect to experiment. Research supported by the U.S. DOE, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division. Ames Laboratory is operated for DOE by ISU under Contract DE-AC02- 07CH11358.

3:40 PM Break

4:00 PM  Invited
Dynamic Behavior and Grain Refinement of AlxCoCrFeNi High-entropy Alloy: Zezhou Li1; Shiteng Zhao1; Haoyan Diao2; Shima Sabbaghianrad3; Terence G. Langdon3; Peter K. Liaw2; Marc A. Meyers1; 1University of California,San Diego; 2The University of Tennessee, Knoxville; 3University of Southern California
    The mechanical behavior of single phase (fcc) Al0.3CoCrFeNi HEA and dual phase (fcc and bcc) Al0.5CoCrFeNi HEA was studied in the low and high-strain-rate regimes. The combination of multiple strengthening mechanisms of alloys such as solid solution hardening, forest dislocation hardening, as well as mechanical twinning lead to an excellent work hardening rate, which is significantly larger than that for Al and is retained in the dynamic regime. The resistance to shear localization of Al0.3CoCrFeNi HEA was evaluated by dynamically loading hat-shaped specimens to induce forced shear localization but no adiabatic shear band could be observed. It is therefore proposed that the excellent strain hardening ability gives rise to remarkable resistance to shear localization, which makes Al0.3CoCrFeNi HEA an excellent candidate for armor. The coarse grained AlxCoCrFeNi HEA were subjected to severe plastic deformation using high-pressure torsion for grain refinement to achieve higher strength.

4:20 PM  Invited
Stress State, Strain Rate and Temperature Sensitivity of Alx(CrCoFeNi)1-x High Entropy Alloys (HEAs): Omar Rodriguez1; Paul Allison1; Haoyan Diao2; Peter Liaw2; Neng Wang1; Lin Li1; 1University of Alabama; 2University of Tennessee
    High Entropy Alloys (HEAs) are equiatomic, multicomponent metallic systems, that presents exceptional microstructural stability. Recently, HEAs have been proposed as potential replacements for existing high temperature structural materials and coatings due to their reportedly favorable combinations of high melting point, high strength, high ductility, and high resistance to oxidation and/or corrosion. Although the potential applications of HEAs are promising, knowledge of their mechanical performance under extreme deformation conditions are not well understood. This work focuses on the study of the stress-state, strain rate and temperature effects on the plastic flow and deformed microstructural features of a series of Al0.3-0.7at.%(CrCoFeNi)1-x HEAs. The culminations of experimental results are intended to be the fundamental building blocks necessary to develop accurate constitutive material models. Interestingly, X-ray Diffraction (XRD) identified that the Al0.7 HEA consisted of a multi-phase BCC and FCC microstructure that exhibited a linear hardening at both quasi-static (10-3/s) and high strain rate (2000/s). While the Al0.3 HEA exhibited a FCC microstructure according to XRD analysis with sigmoidal plastic behavior suggesting multiple deformation mechanisms occurring in the material when loaded at both quasi-static (10-3/s) and high strain rate (2000/s).

4:40 PM  
Experimental Demonstration of Isotope-free Simultaneous Measurement of Self- and Inter-diffusion Coefficients: Esin Schulz1; Irina Belova2; Graeme Murch2; Yongho Sohn1; 1University of Central Florida; 2The University of Newcastle
    Experimental determination of a tracer or self-diffusion coefficient in alloys can be burdensome due to the use of isotopes and the number of experiments required to assess composition- and temperature-dependence. A new formalism recently developed by Belova et al., based on linear response theory combined with the Boltzmann–Matano method allows determination of tracer and interdiffusion coefficients simultaneously from a single, isotope-free sandwich-type diffusion experiment. In this study, experimental demonstration of this new formalism is carried out in the Cu-Ni system. Thin film of pure Cu was deposited on a selected alloy (e.g., Cu-50Ni), sandwiched between two other alloys (e.g., Cu-75Ni), and annealed isothermally at 900 and 1000°C. SEM/XEDS was employed to examine the interdiffusion zone and determine concentration profiles. The self- and inter-diffusion coefficients simultaneously determined using the experimental methodology based on the new formalism produced results consistent with previously reported values determined independently by radiotracer and interdiffusion experiments.

5:00 PM  
Application of a High Accuracy Diffusion Kinetics Formalism to High Entropy Alloys: Alan Allnatt1; Irina Belova2; Tumpa Paul2; Graeme Murch2; 1University of Western Ontario; 2The University of Newcastle
    In this paper, a new, lighter, version of the highly accurate Moleko, Allnatt and Allnatt formalism for describing both tracer (self) and collective diffusion kinetics in multicomponent random alloys is presented. Verification of the resulting expressions is performed by means of kinetic Monte Carlo simulation. Using this formalism the possible ranges of the self diffusion coefficient ratio of the highest to the lowest atomic component are examined for equiatomic (or near equiatomic) binary, ternary, quaternary and quinary alloys. Results show that kinetics arguments involving jump reversals alone are not enough to explain the sluggish diffusion observed of all atomic components in (equiatomic) high entropy alloys.

5:20 PM  
Uncovering Micro Mechanisms during Tensile Deformation for an Outstanding High Entropy Alloy via In Situ Neutron Diffraction: Biao Cai1; Bin Liu2; Yiqiang Wang1; Kun Yan1; Saurabh Kabra3; Peter Lee1; Yong Liu2; 1University of Manchester; 2Central South University; 3ISIS Facility
    Recently, high entropy alloy (HEA) concept has been studied as a means of achieving excellent properties of metallic alloys. We have developed an FeCoCrNiMo0.2 HEA alloy through powder metallurgy, which possesses a good combination of high strength and ductility. However, currently the knowledge at the microstructural scale concerning the underpinning micro mechanisms during deformation are limited, especially with respect to the interaction between local strain hardening and grain defects (e.g. dislocation substructures and twins). In order to determine these, we used neutron diffraction to quantify the evolution of the lattice strain in situ during tensile deformation at both ambient and reduced temperatures. The results in concert with ex situ electron microscopic characterizations provided a pathway for designing HEAs with enhanced strength and superior ductility.