HEA 2023: Processing, Microstructure, and Properties of HEAs I
Program Organizers: Andrew Detor, DARPA/DSO; Amy Clarke, Los Alamos National Laboratory

Monday 9:00 AM
November 13, 2023
Room: Three Rivers
Location: Omni William Penn

Session Chair: Adam Pilchak, Pratt & Whitney; Ahmed Halkoum, Royal Melbourne Institute of Technology

9:00 AM Introductory Comments

9:05 AM  Invited
Phase Stability and Tensile Properties of Refractory High-entropy Alloys: Easo George¹; ¹University of Tennessee and Ruhr University Bochum
    We investigated the phase stability of the VNbMoTaW and TiZrNbHfTa quinaries and their equiatomic quaternary, ternary, and binary subsets after long-term anneals at 1200-800 °C for 3-300 days. The propensity for forming single-phase solid solutions increased with increasing annealing temperature (vibrational entropy) but there was no correlation with increasing configurational entropy. There was good correlation between the number of BCC constituent elements and single BCC solid solutions, consistent with the Hume-Rothery crystal-structure criterion for extensive solid solubility. We also investigated the mechanical behavior of several pseudo-binary subsystems of the Ti-Zr-Hf-Nb-Ta alloy in which pairs of elements were varied while the concentrations of the remaining three constituent elements were kept constant. Only compositions resulting in singe-phase BCC solid solutions at room temperature were considered. The roles of shear modulus, melting point, and volume misfit on their room-temperature tensile properties (strength and ductility) will be discussed.

9:35 AM
Microstructure and Deformation Behavior of a MoReW Medium Entropy Alloy: Oleg Senkov¹; Satish Rao¹; Glenn Balbus²; Robert Wheeler³; ¹MRL Materials Resources LLC; ²Air Force Research Laboratory; ³UES, Inc.
    Microstructure and mechanical properties of an equiatomic MoReW alloy, which has demanding combination of strength and ductility in the temperature range of 25°C to 1200°C, are reported. The alloy has a weak temperature dependence of yield stress resulting in remarkably good yield stress retention at high temperatures, and it shows strong strain hardening. Extensive deformation twinning occurred at the beginning of deformation and activation of other deformation modes at later deformation stages in this temperature range. To understand and explain the observed behaviors of the alloy, a detailed analysis of strengthening mechanisms associated with screw and edge dislocation glide, as well as with twin nucleation and propagation, has been conducted and reasonable qualitative models of yielding, strain hardening and ductility of the studied MoReW alloy have been proposed.

9:55 AM
Thermo-mechanical Processing of Refractory Multi Principal Element Alloys: Nathan Peterson¹; Benjamin Ellyson¹; Adira Balzac¹; Nelson De Campos Neto¹; Kester Clarke¹; Amy Clarke¹; ¹Colorado School of Mines
    Refractory multi-principal element alloys (RMPEAs) have been identified as having potential for new alloy and microstructural designs for ultra-high temperature applications (≥1473 K, or 1200 °C), due to their high melting points compared to traditional Ni-base superalloys. Since large-scale production of RMPEAs will inevitably require the use of thermo-mechanical processing to control final microstructural conditions, it is important to ellucidate processing-structure-property relationships to identify potential manufacturing-relevant processing windows. Here we study two unique systems: an off-equiatomic NbTiZr-based RMPEA, Nb₂₆Ti₂₃Zr₈V₂₀Mo₂₃ and a ternary system, MoNbV. An evaluation of the strength and workability was performed up to 1300 °C using a Gleeble 3500 thermo-mechanical simulator at various strain rates (10^-3 to 10 s^-1). In the off-equiatomic NbTiZrVMo alloy, discontinuous and continuous dynamic recrystallization was observed at 1000 °C and 1200 °C, respectively, under quasi-static strain rates. In MoNbV, only discontinuous dynamic recrystallization was observed at 1300 °C under quasi-static strain rates.

10:15 AM
ULTIMATE: Advanced Manufacturing of Refractory Complex Concentrated Alloy-based Composites: Xingbo Liu¹; Xin Chen²; Fei Wang²; Bai Cui²; Michael Gao³; Shanshan Hu¹; Dongsheng Li⁴; ¹West Virginia University; ²University of Nebraska-Lincoln; ³National Energy Technology Lab; ⁴Advanced Manufacturing LLC
    To develop new refractory alloys and composites that can be used as blade materials for turbine engine operated at 1300C, refractory complex concentrated alloys (RCCAs) were designed and experimentally fabricated and tested. A large number of new compositions of the single-phase RCCAs were designed based on the high throughput computational design. Experimental verification of these RCCAs was conducted by arc melting, spark plasma sintering, and laser power bed fusion. Mechanical properties, including yield strength, ductility, hardness, and creep properties, were measured at both room temperature and high temperatures. Optimized RCCA compositions with a combination of excellent mechanical properties were selected based on the experimental and simulation results. The mechanical properties of RCCAs were enhanced by the second phase of high entropy carbides.

10:35 AM Break

10:55 AM
Tensile Creep Behavior of a Nb45Ta25Ti15Hf15 Refractory High-entropy Alloy: Gianmarco Sahragard-Monfared¹; Calvin Belcher²; Mingwei Zhang³; Cheng Zhang²; Andrew Minor³; Diran Apelian²; Enrique Lavernia²; Jeffery Gibeling¹; ¹University of California, Davis; ²University of California, Irvine; ³Lawrence Berkeley National Laboratory
    The tensile creep behavior of a plasma arc-melted and cold-rolled Nb45Ta25Ti15Hf15 refractory high entropy alloy was investigated at temperatures ranging from 1123-1223 K and constant true stresses ranging from 50-300 MPa. A stress exponent of 1.2 was observed at low applied stresses, while at high applied stresses, the exponent was 5.7. TEM analysis on creep samples interrupted at steady-state deformation revealed the control of long straight screw dislocation motion, contrary to the conventional power-law creep interpretation. The creep data was better described by an exponential equation, indicating thermally activated glide of screw dislocations. Samples that crept at lower applied stresses and longer time were affected by phase decomposition and Hf-oxide formation at grain boundaries, reducing creep ductility and life. Under higher applied stresses, Nb45Ta25Ti15Hf15 exhibited superior creep resistance and excellent creep ductility compared to TiNbHfZrTa (Senkov alloy) and FCC MPEAs.

11:15 AM
Thermomechanical Processing Maps and Microstructure Characterization of Cr-containing Refractory Complex Concentrated Alloys: Nelson Delfino De Campos Neto¹; John Rotella²; Todd Butler²; Samuel Kuhr²; Matthew Snyder³; Nathan Peterson¹; Benjamin Ellyson¹; Francisco Coury⁴; Kester Clarke¹; Amy Clarke¹; ¹Colorado School Of Mines; ²Air Force Research Laboratory; ³ARCTOS Technology Solutions; ⁴Federal University of São Carlos
    Refractory complex concentrated alloys (RCCAs) are a new class of metallic alloys under development for high-temperature performance. Cr-containing RCCAs can present a good combination of mechanical properties, suitable oxidation resistance, and perhaps even be compatible with coatings. Unlike single-phase, body centered cubic (BCC) RCCAs, these alloys tend to contain multiple phases, depending upon the constituent elements. For example, CrNb, CrNbTi, and CrNbTaTi RCCAs containing BCC + Laves (C15) phases have revealed promising mechanical properties and oxidation behaviors up to 1200 °C. But, the reduced ductility at room temperature and the sluggish diffusion leading to microstructural inhomogeneities still need to be overcome to produce consistent microstructures in bulk. Significant opportunity exists to tailor the microstructures and properties of multiphase Cr-containing RCCAs by thermomechanical processing. Here we report our recent progress in the development of thermomechanical processing maps and microstructural characterization of Cr-containing RCCAs for high-temperature applications.

11:35 AM
Quasi-static Mechanical Response and Microstructure Analysis of Tantalum-tungsten Alloys: Charles Smith¹; Nathan Peterson¹; Joseph McKeown²; Sharon Torres²; Kester Clarke¹; Amy Clarke¹; ¹Colorado School of Mines; ²Lawrence Livermore National Laboratory
    Ta-W alloys serve as structural alloys for high-temperature and corrosive environments. Renewed interest in refractory alloys for performance in extreme environments, including the development of refractory multi-principal element alloys (RMPEAs), is driving new investigations into the mechanical response of refractory alloys like Ta-W during quasi-static deformation to understand the microstructural response and inform alloy-microstructure-property design. Toward this aim, quasi-static tension testing has been performed at room and elevated temperatures on wrought Ta-10W (wt.%), along with microstructural characterization after deformation. Baselining the microstructure and mechanical response to quasi-static mechanical testing is needed to better understand the deformation mechanisms in Ta-W alloys, and to provide insights into the development of novel RMPEAs for aerospace, defense, and nuclear applications.

11:55 AM
Effect of Al and Ti Addition on the Strain Hardening Behavior of Non-equiatomic CoCrFeNi High Entropy Alloy: Bushra Harun¹; E-Wen Huang²; An-Chou Yeh²; Suresh Neelakantan¹; Jayant Jain¹; ¹Indian Institute of Technology Delhi; ²National Tsing Hua University
    Near-equiatomic multicomponent systems have proven effective in achieving remarkable mechanical properties. This inspired researchers to comprehend the reason for their exceptional behavior and to analyze processes that would further enhance their properties. Precipitation strengthening is one such viable process. This study examined four precipitation hardenable high entropy alloy systems, AlxCo1.5CrFeNi1.5Tiy, where x = 0, 0.2, 0.3, 0.5, and x + y = 0.5 in molar ratios. Their strain hardening behavior, considering tensile testing, was analyzed in solutionized and peak-aged conditions using TEM, highlighting the microstructural influence of Al and Ti additions and precipitates formed in these systems. The results indicate increased Ti content and precipitation of different phases (B2 and L12) lead to enhanced strain hardening. However, Al addition provides improved ductility. The formation of complex dislocation substructures like Taylor lattices and stacking faults is observed in these systems.