High Entropy Alloys V: Alloy Development and Applications I
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

Tuesday 8:30 AM
February 28, 2017
Room: 32B
Location: San Diego Convention Ctr

Session Chair: Peter Liaw, The University of Tennessee, Knoxville; Michael Gao, National Energy Technology Laboratory

8:30 AM  Invited
A Critical Review of High Entropy Alloys and Related Concepts: Dan Miracle1; Oleg Senkov2; 1AF Research Laboratory; 2UES, Inc.
    It has been over 12 years since the first publications of high entropy alloys (HEAs) and the related concepts of multi-principle element alloys (MPEAs) and complex, concentrated alloys (CCAs). Here we present major results drawn from a critical review of this body of work. Major themes include thermodynamics; alloy taxonomy; microstructures; mechanical properties; applications and alloy design; future work; and major accomplishments. This presentation emphasizes a critical evaluation of mechanical properties and a description of the most significant accomplishments in the field. This presentation also includes a brief summary of the most important conclusions drawn from the other major themes.

8:50 AM  Invited
Formations, Thermodynamics and Elasticity of High-entropy Alloys: Michael Gao1; Jeffrey Hawk1; David Alman1; 1National Energy Technology Lab
    Identifying single-phase high-entropy alloys (HEAs) is of fundamental importance to understanding HEA formation and their intrinsic properties. However, to date single-phase HEAs are still very limited. In this talk we first present hundreds of new single-phase equi-molar HEAs in FCC, BCC and HCP structures with 4-10 principal components. These compositions are identified using an efficient searching strategy that combines phase diagram inspection, CALPHAD modeling, first-principles density functional theory calculations, and empirical rules. The atomic structures of disordered HEAs are modeled using hybrid Monte Carlo/molecular dynamics (MC/MD) simulations and special quasi-random structure (SQS) models. Then we calculate the entropy sources of model HEAs, which include configurational, vibrational, and electronic contributions. Positive excess entropy is predicted in FCC HEAs, while negative excess entropy is predicted in BCC HEAs. The formation enthalpies of hundreds of HEAs are also predicted. Finally we present the calculated elastic properties of HEAs.

9:10 AM  Invited
On the Damage Tolerance of the High-entropy Alloy CrMnFeCoNi in the Range Room Temperature to Liquid Nitrogen Temperatures: Bernd Gludovatz1; Keli Thurston1; Anton Hohenwarter2; Guillaume Laplanche3; Easo George3; Robert Ritchie1; 1Lawrence Berkeley National Laboratory; 2University of Leoben; 3Ruhr-University Bochum
    (Near-)equiatomic multi-component alloys are an intriguing new class of materials that can crystallize as single-phase solid solutions with simple crystal structures despite containing high concentrations of elements with very different crystal structures which makes them in-teresting from a fundamental scientific viewpoint. Additionally, they can display a good combination of mechanical properties making them attractive for a wide range of applica-tions. Here we examine the high-entropy alloy CrMnFeCoNi which exhibits an excellent combination of strength, ductility and fracture toughness at ambient to cryogenic tempera-tures, consistent with its deformation modes of planar dislocation slip and deformation-induced nano-twinning. We will show our work on specifically designed ‘hourglass-shaped’ samples containing gradients in cross-section to determine the stresses required for the onset of twinning in order to shed light on the micro-scale mechanisms underlying the excellent mechanical properties of these materials and will report results of the material’s fatigue-crack propagation behavior at RT and below.

9:30 AM  Invited
Phase Stability of the CrMnFeCoNi High-entropy Alloy: F. Fox1; G. Laplanche1; A. Hohenwarter2; A. Kostka1; F. Otto1; E. P. George1; 1Ruhr University Bochum; 2Montanuniversität Leoben
    The equiatomic alloy, CrMnFeCoNi, was generally believed to be one of a handful of high-entropy alloys in which high configurational entropy stabilizes a single FCC solid solution phase. However, recent studies have demonstrated the limits of entropic stabilization since it decomposes into different metallic and intermetallic phases after intermediate temperature anneals. Here, we review our recent results on the kinetics of phase decomposition and effects of grain size. The influence of phase transformations on mechanical properties will be reported and the role of potentially embríttling phases discussed. Our results should motivate careful evaluation of the long-term stability of other HEAs that are currently thought to be stable single-phase solid solutions and related effects on properties. Funding is acknowledged from the German Research Foundation (DFG) through projects GE 2736/1-1 (FF and EPG) and LA 3607/1-1 (GL) and from the Austrian Science Fund (FWF) through project P26729-N19 (AH).

9:50 AM  Invited
A Highly Fracture and Fatigue Resistant Al0.3CoCrFeNi High Entropy Alloy: Mohsen Seifi1; Yunzhu Shi2; Peter Liaw2; Mingwei Chen3; John Lewandowski1; 1Case Western Reserve University; 2The University of Tennessee; 3Tohoku University
    The fracture-toughness and fatigue-crack-growth behavior of the Al0.3CoCrFeNi high entropy alloy was determined. Microstructure examination revealed a single FCC phase structure. The notched and fatigue-precracked toughness values were significantly higher than that of those reported in the literature and comparable to a recent report on a single phase face-centered-cubic (fcc)-HEA that was deformation processed. Fatigue-crack-growth experiments in 3-point bending revealed high fatigue thresholds that decreased significantly with an increase in load ratio (R), while Paris law slopes exhibited metallic-like behavior at low R. Fatigue thresholds at all R were much higher than most conventional alloys. Additional experiments were conducted using compact-tension specimens to minimize load-point effects on the measurements. Fracture-surface examinations of the fatigue-crack-growth samples revealed ductile features at various regions of the fatigue curve, with some evidence of fatigue striations in the Paris law regime.

10:10 AM Break

10:30 AM  Invited
Novel Precious Metal High Entropy Alloys – Design, Structure and Mechanical Performance: Caitlin Healy1; Jörg Löffler2; Michael Ferry1; Kevin Laws1; 1University of New South Wales; 2ETH Zürich
    By utilising the extended solid solution range of a series of multi-component precious metal-based systems, a number of new high entropy, or compositionally complex alloys have been developed. The aim of this work is to provide further fundamental insights into phase evolution and composition-based solid solution strengthening effects in high entropy fcc systems. Results show that these alloys exhibit strength and hardness values considerably higher than those of their base metals and conventional precious metal alloys and often exhibit exceptional ductility. The specific microstructures, phase evolution upon heat treatment, mechanical properties and more are reported. In addition to their scientific importance, these exceptional new alloys have potential use in commercial jewellery, reflector and possible catalyst applications.

10:50 AM  Invited
Hexagonal Close-Packed High-entropy Alloys: The Effect of Entropy: Junwei Qiao1; Michael Gao2; Huijun Yang1; 1Taiyuan University of Technology; 2National Energy Technology Laboratory
    The formation of disordered solid solution in the hexagonal close-packed (hcp) structure in the GdHoLaTbY alloy and its mechanical properties were investigated in this study. The single hcp phase of the alloy in the as-cast state was confirmed by X-ray diffraction and scanning electron microscopy analyses. The compressive yield strength, fracture strength, and plastic strain of the alloy are 108 MPa, 880 MPa, and 21.8 %, respectively, and the Vickers hardness is 96 HV. The results show that the yield strength, fracture strength, and hardness of the alloy obey the rule of mixture, which indicates that there is no hardening effect from entropy. Although the high entropy of mixing stabilizes the solid solution against intermetallic compounds, lack of severe lattice distortion from elastic strain or electronic interactions between principal elements impacts little on the mechanical properties.

11:10 AM  
Design of Light-weight High-Entropy Alloys: Rui Feng1; Michael C. Gao2; Chanho Lee1; Michael Mathes1; Tingting Zuo3; Shuying Chen1; Jeffrey A. Hawk2; Yong Zhang3; Peter K. Liaw1; 1The University of Tennessee; 2National Energy Technology Laboratory/AECOM; 3University of Science and Technology, Beijing
    Owing to the demand from transportation and defense industries, light-weight high-entropy alloys (HEAs) have garnered widespread interest from scientists for the use as potential structural materials. Great efforts have been made to study the phase-formation rules of HEAs to accelerate the discovery process. Many proposed solid-solution phase-formation rules are assessed, based on a series of light-weight HEAs. The results indicate that these empirical rules work for most compositions but also fail for several alloys. Light-weight HEAs often involve the additions of Al and/or Ti in great amounts, resulting in large negative enthalpies for forming solid-solution phases and/or intermetallics. Accordingly, these empirical rules need to be modified with the new experimental data. In contrast, CALPHAD is demonstrated to be an effective approach to predict the phase formation in HEAs as a function of composition and temperature. Future perspectives on the design of light-weight HEAs are discussed in light of CALPHAD modeling.

11:30 AM  
The Design of Creep-resistant High Entropy Alloys for Elevated-temperature Applications: Haoyan Diao1; Chuan Zhang2; Fan Zhang2; Karin Dahmen3; Peter Liaw4; 1The University of Tennessee ; 2CompuTherm, LLC; 3University of Illinois at Urbana-Champaign; 4The University of Tennessee
    The creation and design of novel structural materials with enhanced creep-resistance has always been the goal of many scientists. The high-entropy alloy (HEA) concept has revolutionized alloy-design approaches, by employing the use of multi-principal elements in contrast to traditional alloys, based on one or two principal elements with small amounts of alloying elements to achieve desired properties. HEAs have shown to be suitable materials for elevated-temperature applications. The fundamental studies on the AlxCoCrFeNi for use in boilers and steam and gas turbines at 760 °C and 35 MPa has been performed. The creep behavior and related deformation mechanisms of both single-phase and NiAl-strengthening HEAs have been studied, using in-situ neutron diffraction during tensile tests at different temperatures. The thermodynamics and kinetics of NiAl precipitates have also been investigated, through an integrated approach, coupling modeling and experiments, to identify HEAs that outperform conventional alloys for high-temperature applications.

11:50 AM  
Local Texture in a Swaged CrMnFeCoNi High-entropy Alloy: Aurimas Pukenas1; Guillaume Laplanche2; Easo George2; Werner Skrotzki1; 1TU Dresden; 2Ruhr-Universität Bochum
    Local texture measurements with an X-ray microdiffractometer have been done on the rod cross-section of a room temperature swaged CrMnFeCoNi high-entropy alloy. The texture is a <100><111> double fibre texture with the intensity of the fibres changing in radial direction of the rods. The main fibre component is <111>. A detailed texture analysis shows that the fibre texture is of cyclic nature, i.e. it is mainly composed of the plane strain components {112}<111> and {110}<100> with the planes of these components alligned in tangential direction. The texture formation will be discussed with regard to deformation mode of the swaging process, deformation microstructure and stacking fault energy of the face-centred cubic multi-component alloy.