High Entropy Alloys V: Structures and Modeling II
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: 32B
Location: San Diego Convention Ctr

Session Chair: Karin Dahmen, University of Illinois at Urbana Champaign; James Morris, The University of Tennessee, Knoxville

2:00 PM  Invited
A Computational Investigation on Diffusion in High-entropy Alloys: Chuan Zhang1; Fan Zhang1; Shuanglin Chen1; Weisheng Cao1; Jun Zhu1; Haoyan Diao2; Peter Liaw2; 1CompuTherm LLC; 2University of Tennessee
    As one of the core effects on the high-temperature structural stability, the “sluggish diffusion effect” in high-entropy alloy (HEA) has attracted more attention. Experimental investigations on the diffusion kinetics have been carried out in a few HEA systems. However, the mechanisms behind this effect remain unclear. For better understanding the diffusion kinetics of the HEAs, the computational-thermodynamics approach is employed in this study. Using the self-consistent mobility database developed in the present work, we simulate the concentration profiles of both Al-Co-Cr-Fe-Ni and Co-Cr-Fe-Mn-Ni HEAs to compare with experimental data in the literature. It shows that our simulated results can well describe the experimental measurements in the literature, which validate the reliability of our current mobility database. Taking advantage of the computational thermodynamics, the diffusivities of alloying elements vs. composition and/or temperature are then calculated, which provide us an overview picture of the diffusion kinetics within the Al-Co-Cr-Fe-Ni and Co-Cr-Fe-Mn-Ni systems.

2:20 PM  Invited
Modeling Slips in Slowly Deformed High Entropy Alloys and Comparison to Experiments: Karin Dahmen1; XJ Gu2; Li Shu1; Aya Nawano1; Shuying Chen3; Peter Liaw3; Jonathan Uhl4; Wendelin Wright2; Jien-Wei Yeh5; 1University of Illinois at Urbana Champaign; 2Bucknell University; 3The University of Tennessee, Knoxville; 4Retired; 5National Tsing Hua University
    In a certain temperature and strain rate regime, high entropy alloys deform with sudden slips [1]. We discuss an analytical model model that predicts the statistics and the dynamics of the their time-series properties for different experimental conditions and compare the results to experiments. The results can be used for materials testing and the design of alloys with smooth deformation properties. Reference: [1] Robert Carroll, Chi Lee, Che-Wei Tsai, Jien-Wei Yeh, James Antonaglia, Braden Brinkman, Michael LeBlanc, Xie Xie, Shuying Chen, Peter K. Liaw, and Karin A. Dahmen, Experiments and Model for Serration Statistics in Low-Entropy, Medium-Entropy, and High-Entropy Alloys, Scientific Reports 5, Article number: 16997 (2015), doi:10.1038/srep16997 and references therein. Acknowledgments: We gratefully acknowledge NSF grants DMR-1005209, DMS-1069224 and DOE project FE0011194 with project manager Jessica Mullen.

2:40 PM  Invited
Modeling Fundamental Properties of High Entropy Alloys: James Morris1; 1Oak Ridge National Laboratory
    Ab initio calculations, combined with experiment and longer length scale modeling, provides critical information on the stabilization of particular phases and microstructures. This talk will focus on approaches for connecting these issues for high entropy alloys (HEAs). These alloys pose new questions concerning the ability to predict single phase materials in alloys with a large number of components, particularly metastable phases. High-throughput calculations provide important predictive information as to which compositions may form single-phase solid solutions, including those where Hume-Rothery considerations have been shown to be inadequate. Effective Monte Carlo models of Al-containing HEAs show a complex set of phase transformations. These same materials exhibit unusual mechanical properties, including the unusual combination of increased strength and ductility when cooled. The materials challenge traditional considerations of solid-solution hardening, and atomistic simulations may provide critical insight into these processes. Work has been supported by the US DOE Office of Science, BES-MSED.

3:00 PM  
Using a Large Scale Modelling Technique for Selection of HEAs Containing Atypical Elements: Rob Snell1; Iain Todd1; Russell Goodall1; 1University of Sheffield
    The vast majority of HEAs use d-block transition elements and rarely feature elements from certain groups, such as precious metals. A modelling technique was developed for finding HEAs with unusual compositions chosen from a wider variety of elements. The technique uses a program that can rapidly scan across large numbers of compositions and alloy systems. Filtering through the results, using both simple and complex selection criteria, compositions could be selected that had a high probability of forming a HEA. By way of an example the program was used to find novel HEAs containing silver. It was shown that even small quantities silver of silver added to existing HEAs corrupted the structure, but using the screening technique silver HEAs were found with high concentrations.

3:20 PM  Invited
Atomistic Modeling of Solid-solution Structures of High Entropy Alloys: Guofeng Wang1; Zhenyu Liu1; Yinkai Lei1; 1University of Pittsburgh
    To computationally address the structural complexities of high entropy alloys (HEAs), we have recently used atomistic modeling methods to predict the solid-solution stability of CoCrFeNi and AlCoCrFeNi HEAs. In our atomistic simulations, the interatomic interactions were described with the modified embedded atom method (MEAM). Using the Monte Carlo (MC) simulations based on the developed MEAM potentials, we sampled the thermodynamically equilibrium structures of the CoCrFeNi alloy and further predicted that the CoCrFeNi alloy could form an fcc solid solution phase with high configurational entropy of 1.329R at 1373 K. Furthermore, we studied the stability of fcc and bcc solid-solution phases of AlxCoCrFeNi HEAs with varying Al content x at 1300 K using the atomistic simulation methods. We predicted that the AlxCoCrFeNi HEAs would form a single fcc solid-solution phase when x<0.44 and a single bcc solid solution phase when x>1.75. Our theoretical results are quite consistent with experimental observations.

3:40 PM Break

4:00 PM  Invited
Predicted Properties of NiFeCrCo Based HEAs from First Principles: Douglas Irving1; Changning Niu1; Alex Zaddach1; Adedapo Oni1; James LeBeau1; Carl Koch1; 1North Carolina State University
    In this talk we will present results from our integrated program that combines first principles simulation together with experimental synthesis and characterization in an effort to design HEAs with low stacking fault energies. The systems of interest for this work are the NiFeCrCo based HEAs. The magnetic properties of these alloys and the tendency for local ordering in these systems will be discussed. The predicted properties of non-stoichiometric alloys will also be presented. Finally, we will also discuss defects in these HEAs. The results of this project were made possible through support from the National Science Foundation via grant DMR-1104930.

4:20 PM  Invited
The Serrations of TiZrTM1TM2 (TM=Hf, Mo, Ta, V and W) High Entropy Alloys: An Integrated First-principles Calculation and Finite-elements Method Study: William Yi Wang1; FengBo Han1; Yi Dong Wu2; Deye Lin3; Bin Tang1; Jun Wang1; Shun-Li Shang4; Yi Wang4; HongChao Kou1; Xi-Dong Hui2; Karin Dahmen5; Peter Liaw6; JinShan Li1; Zi-Kui Liu4; 1Northwestern Polytechnical University; 2University of Science and Technology Beijing; 3Institute of Applied Physics and Computational Mathematics; 4The Pennsylvania State University; 5University of Illinois at Urbana Champaign; 6The University of Tennessee
    Refractory high entropy alloys (RHEAs) possess attractive mechanical properties, e.g., high yield strength and fracture toughness. The fundamental understandings of the atomic-structure-dominated deformation behavior of HEAs are critical to expand the alloy-development strategies. In the current work, the local atomic-arrangement-dominated yield strength of TiZrTM1TM2 (TM= Hf, Mo, Ta, V and W) BCC RHEAs is predicted by the integrated first-principles calculations and finite-elements method. The cluster-plus-glue-atom model is utilized to generate ordered and disordered configurations of those BCC equiatomic refractory RHEAs. The bonding charge density conveniently captures the electron redistributions caused by the elastically-distorted crystal lattice, providing an insight into the nature of the loosely-bonded weak spots and the strongly bonded clusters. Therefore, the deviations of the predicted yield stresses among those configurations suggest that the plastic deformation is localized between the clusters, which contribute to the formation of serrations in the stress-strain curve.

4:40 PM  Invited
Understanding and Designing High-entropy Alloys using a Cluster-plus-Glue-Atom Model: Qing Wang1; Xiaona Li1; Chuang Dong1; Peter K. Liaw2; 1Dalian University of Technology; 2The University of Tennessee
    High-entropy alloys (HEAs), also called multi-principal-element solid solution alloys, generally exhibit high-temperature structural stabilities and excellent properties. Similar to the traditional solid solutions, the stabilities and properties of HEAs were also determined largely by chemical short-range interactions between elements. In recent years, we have developed a structural model, i.e., the cluster-plus-glue-atom model, for the description of chemical short-range orders in solid solution alloys. Thereof, the composition and structure characteristics of HEAs in Al-TM (TM: transition metals) systems were investigated using this new structural tool, where Al is regarded as the main solute that interacts with the TM multiple elements as a whole. The relevant cluster-based local structural units and composition formulas were then extracted, and a HEA alloy of [Al-M14]Al1 (M= Co1/5Cr1/5Fe2/5Ni1/5) was designed to possess prominent tensile properties, which is attributed to a superalloy-like microstructure of cuboidal B2 nanoprecipitates coherently embedded in the BCC matrix.

5:00 PM  Invited
A Multifaceted Approach to Analyze the Serration Behavior in High Entropy Alloys and Other Material Systems: Jamieson Brechtl1; Xie Xie1; Shuying Chen1; Haoyan Diao1; Yunzhu Shi1; Tengfei Yang1; Bilin Chen1; Karin Dahmen2; Peter Liaw1; Steven Zinkle1; 1University of Tennessee; 2University of Illinois at Urbana-Champaign
    An example of a complex process of great engineering significance is the serrated flow that occurs in many materials during mechanical deformation. Here a serrated flow is associated with the inhomogeneous deformation in an alloy and is represented by fluctuations in the stress-strain curve. For this study, a multifaceted approach was used to examine this behavior in high entropy alloys and other material systems exposed to different strain rates, temperatures, and irradiation conditions. Analytical methods used include the refined composite multiscale entropy analysis and multifractal analysis techniques. The cumulative distribution functions of stress drops were analyzed for any universal scaling behavior based on the Mean Field Theory. Results of the analysis found that the serrated flow was affected by factors such as test temperature, strain rates and irradiation-displacement damage. A link between the changes in the stress-drop behavior and the microstructure of the crystalline alloys will also be discussed.

5:20 PM  
New Deformation Twinning Mechanism in Equimolar Multi-component Alloys with Low Stacking Fault Energy: Qingjie Li1; Evan Ma1; 1Johns Hopkins University
    Single-phase FCC equimolar multi-component alloys (EMAs) such as CrMnFeCoNi and NiCrCo often exhibit an unusual combination of high tensile strength and ductility. Such an extraordinary strength-ductility synergy is believed to be closely related to the hierarchical deformation twinning (DT) microstructures developed in these alloys, however it is unclear how the profuse deformation twins form. Using molecular dynamics simulations with realistic EAM potentials, we demonstrate a new DT mechanism that takes advantage of the intersection between faults. These intersections are of glide-like symmetry in lieu of the screw symmetry in the conventional pole mechanism. The translational component of this glide-like symmetry plays the role of promoter to mediate DT, constituting the source of twinning dislocations. This new mechanism is especially prolific at high driving stresses, explaining the enhanced occurrence of hierarchical twin structures in EMAs at cryogenic deformation temperatures.

5:40 PM  
Fatigue Behavior of High-entropy Alloys: Peiyong Chen1; Bilin Chen1; Michael Hemphill1; Zhi Tang1; Tao Yuan2; Gongyao Wang1; Che-Wei Tsai3; Andrew Chuang1; Carl D Lundin1; Jien-Wei Yeh3; Mohsen Seifi4; Dongyue Li5; John J Lewandowski4; Karin A Dahmen6; Peter K Liaw1; 1University of Tennessee Knoxville; 2Ohio University; 3National Tsing Hua University; 4Case Western Reserve University; 5State Key Laboratory for Advanced Metals and Materials; 6University of Illinois at Urbana-Champaign
    Fatigue behavior is an important consideration in the selection of any important application of high-entropy alloys (HEA’s) for use in the aerospace industry or other important fields of engineering, when structural considerations are of importance in the design of high-quality components. Research has been performed on HEA’s, Al0.5CoCrCuFeNi, Al0.2CrFeNiTi0.2, and AlCrFeNi2Cu, to study the fatigue-endurance limits, fatigue ratio, and fatigue crack growth behaviors. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) have been used to study the mechanisms of fatigue behavior. This study revealed that nanotwinning was the main deformation mechanism before crack initiation and during crack propogation. Statistical models have been developed to predict the fatigue life. Compared with conventional structural alloys, HEAs exhibit improved fatigue properties especially with regard to the fatigue-endurance limit and fatigue ratio.