High Entropy Alloys V: Structures and Mechanical Properties 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

Wednesday 2:00 PM
March 1, 2017
Room: 32B
Location: San Diego Convention Ctr

Session Chair: Paul Jablonski, National Energy Technology Laboratory; Zhongwu Zhang, Harbin Engineering University

2:00 PM  Invited
Microstructural Response of High Entropy Alloy under Extreme Environments: H.S. Oh1; J.Y. Kim1; Eun Soo Park1; H.J. Chang2; C.C. Tasan3; D. Raabe4; 1Seoul National University; 2Korea Institute of Science and Technology; 3Massachussetts Institute of Technology; 4Max-Planck Institut fr Eisenforschung GmbH
    Recently, high entropy alloys (HEAs) have received lots of attention as a candidate material for extreme environments due to their superior properties. Irradiation (=high energy particle) causes various defects in materials, which can lead to understanding of initial resistance to defect formation in materials. And high pressure torsion (HPT), a type of severe plastic deformation experiment inducing severe plastic shear deformation to the materials, is a useful method to evaluate overall stability and safety of metallic materials in extreme environments. In the present study, we report the microstructural response of HEAs under extreme environments, which are electron-beam/ion irradiation as well as HPT. HEAs exhibit higher resistance to irradiation damage as well as better phase stability under HPT due to sluggish diffusion of constituent elements by severe lattice distortion. Indeed, this result would offer a guideline to design a new type of structural materials for next generation reactors under harsher environments.

2:20 PM  Invited
Effect of Process Changes in the Manufacture and Mechanical Properties of High Entropy Alloys: Paul Jablonski1; Michael Gao2; Jeffrey Hawk1; 1U.S. Department of Energy, National Energy Technology Laboratory; 2AECOM
    High entropy alloys (HEA) have generated interest in recent years due to their unique positioning within the alloy world. By incorporating a number of elements in high proportion they have high configurational entropy which leads to interesting and useful properties such as enhanced oxidation resistance and strength. Previously, the process of making large scale ingots employed induction melting to form a fully alloyed structure. The solidified ingot is then heat treated using a computationally optimized homogenization treatment followed by hot working. Two lots of material were manufactured. The first lot has been discussed previously. However, the second lot employed melt processing changes to improve alloy cleanliness. In addition, the deformation processing was modified slightly as well to alter mechanical properties. These process changes will be discussed with respect to alloy chemistry and tensile mechanical properties.

2:40 PM  
Mechanisms Underlying the Remarkable Strength and Toughness of CrCoNi-based Medium- and High-Entropy Alloys at Ambient to Cryogenic Temperatures: Bernd Gludovatz1; Qian Yu2; Easo George3; Robert Ritchie4; 1Lawrence Berkeley National Laboratory; 2Zhejiang University; 3Ruhr University; 4University of California Berkeley
    Equiatomic multi-component alloys, referred to as high-entropy alloys, have received significant attention in the materials science community of late as some of these alloys can crystallize as single-phase solid solutions with simple crystal structures despite containing high concentrations of elements with very different crystal structures. Additionally, they can display good combinations of mechanical properties making them attractive for a wide range of applications. Here we examine equiatomic medium- and high-entropy, face-centered-cubic CrCoNi-based alloys, which exhibit exceptional combinations of strength, ductility and fracture toughness at ambient to cryogenic temperatures, consistent with their high lattice friction and low stacking fault energy characteristics. We further use in situ transmission electron microscopy to identify in real time a synergy of deformation mechanisms including planar dislocation slip, rapid motion of partial dislocations, near-tip crack bridging and deformation-induced nano-twinning.

3:00 PM  Invited
Effects of Preparation Methods on the Microstructures and Properties of High Entropy Alloys: Zhongwu Zhang1; Mingxing Qiu1; 1Harbin Engineering University
    High-entropy alloys break through the traditional alloy design frame work which is based on one major alloy element. The microstructure and texture of an alloy are significantly influenced by the preparation methods. In this study, the effects of drop casting and directional solidification on the microstructure and properties of high entropy alloys are investigated. The evolution of microstructure morphology and phase formation in high entropy alloys prepared by drop casting and directional solidification are characterized. The microstructure morphologies induced by drop casting and directional solidification are compared and their effects on the physical and mechanical properties are discussed. This work was supported by the NSFC Funding (51371062 and U1460102), NSFHLJ (ZD201411), the Scientific Research Foundation for the Returned Overseas Chinese Scholars (Heilongjiang Province), the Project for Innovative Talents of Science and Technology of Harbin (2014RFXXJ006).

3:20 PM Break

3:40 PM  Invited
The Strengthening Mechanisms for a Family of High-entropy and Equiatomic Solid-solution Alloys: Zhenggang Wu1; Yanfei Gao1; Hongbin Bei1; 1Oak Ridge National Laboratory
     To understand the underlying strengthening mechanisms, a large class of equiatomic or near equiatomic alloys with random solid-solution FCC structures has been recently developed and their mechanical properties were investigated. These alloys exhibit an unusual temperature-dependent strength, an unusual hardening behavior with respect to the crystallographic orientation, and extraordinary ductility at cryogenic conditions. Our analysis suggests that the Labusch-type solution strengthening mechanism, rather than the lattice friction (or lattice resistance), governs the deformation behavior in equiatomic alloys. A qualitative agreement exists between the measured strengthening effect and the Labusch strengthening factor based on the lattice and modulus mismatches. Consequently, the Labusch strengthening factor provides a practical parameter to understand and design such compositionally complex but structurally simple alloys. Research supported by the US Department of Energy, BES, Materials Sciences and Engineering Division.

4:00 PM  Invited
Size Effects and Thermal Stability of High-entropy Alloys: Single Crystalline vs. Nanocrystalline: Yu Zou1; Jeffrey Wheeler1; Huan Ma1; Roksolana Kozak1; Soumyadipta Maiti1; Walter Steurer1; Ralph Spolenak1; 1ETH Zurich
    A vast majority of studies on high-entropy alloys (HEAs) are focused on their bulk behavior, but how they perform at small length scales – both sample sizes and grain sizes – are still not clear. Here, we study the size-dependent plasticity of two typical HEAs – fcc CrMnFeCoNi and bcc NbMoTaW at both room temperature and elevated temperatures. We show a strong size dependence of strength in fcc CrMnFeCoNi and minor size effect in bcc NbMoTaW at room temperature. More interestingly, we find that these bcc nanocrystalline HEAs exhibit excellent thermal stability for the high-temperature, long-duration conditions (1100 C for 3 days) and maintain their high yield strengths (above 5 GPa) up to 600 C. Nanostructured HEAs with remarkably high strength, good ductility, low strain-rate sensitivity, and enhanced thermal stability make them attractive as a new class of structural materials in microscale and nanoscale devices.

4:20 PM  
Irradiation Responses of High-entropy Alloys at Elevated Temperatures: Songqin Xia1; Michael Gao2; Tengfei Yang3; Peter Liaw4; Yong Zhang1; 1University of Science and Technology Beijing; 2National Energy Technology Laboratory; 3Peking University; 4The University of Tennessee
    In the present study, the microstructure evolutions of AlxCoCrFeNi high-entropy alloys (HEAs) with 3 MeV Au ions at elevated temperatures ranging from 250 to 650 ̊C were studied. Transmission electron microscopy (TEM) and high-resolution TEM analysis show that the single-phase solid solution exhibits more excellent phase stability than multi-phase structures, as well as the micro-mechanism for high irradiation resistance of single-phase HEAs are also analyzed. The high configurational entropy and low atomic diffusion of single-phase HEAs maybe become more important than the interfaces of multi-phase HEAs against irradiation. Moreover, there is no observable irradiation-induced voids for all the alloys at test irradiation temperatures, suggesting the great resistant to irradiation-induced void swelling, which gives promise that the single-phase HEAs maybe pave the way for a promising irradiation-tolerant materials.

4:40 PM  Invited
Strong Grain-size Effect on Deformation Twinning of an Al0.1CoCrFeNi High-entropy Alloy: Shiwei Wu1; G. Wang1; J. Yi1; Q. J. Zhai1; P. K. Liaw2; 1Shanghai University; 2The University of Tennessee
    An Al0.1CoCrFeNi high-entropy alloy (HEA) with a face-cater-cubic structure in the as-cast and the recrystallized states is investigated. The mean grain size of the as-cast HEA is fourteen times larger than that of the recrystallized HEA. The tension tests reveal that deformation twinning is the main mechanism dominating the plastic deformation of two HEAs. With decreasing the grain size, twin spacing increases, and twin thickness decreases, which results in a low twinning activity. The twinning activity of the recrystallized HEA is strongly inhibited by grain refinement, which degrades the promotion of twinning on the strain-hardening ability and the tensile ductility.

5:00 PM  Invited
Interatomic Potential Function Development for the FeNiCoCr High Entropy Alloy: J. Wei1; Y. Zhuang1; PJ Yu1; Alice Hu1; 1City University of Hong Kong
    As the high entropy alloy has become a more and more popular research field, numerous experimental research work has been published with different applications, such as tension, fatigue, and fracture behavior, irradiation damage, oxidation, corrosion…etc. Reliable molecular dynamics simulations are thus needed to benchmark with experimental results for larger-scale calculations, such as the slip of dislocations, grain-boundary and irradiation cascade. However, none of the current mixing method from the embedded atom method (EAM) or Lennard-Jones (LJ) potential can accurately estimate the lattice structure and stacking fault energy. Therefore an accurate potential function is urgently required. We choose the modified embedded atom method (MEAM) as the fitting form, which can be expressed as the summation of the embedding function and pair potential between any two atoms. We will construct a database for different alloy compositions and calculate the total energy by the density function theory (DFT). The actual fitting process would be completed by the General Utility Lattice Program (GULP) code package.