Characterization and Modeling of NbNiTaTiW and NbNiTaTiW-Al Refractory High-Entropy Alloys
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NTRODUCTION
HIGH-ENTROPY alloys (HEAs) are a relativity new type of alloy system which often exhibit improved properties over traditional alloys.[1–5] HEAs are defined as multicomponent equi-molar or near-equi-molar alloys whose composition ranges from 5 to 35 at. pct for each element in the system.[6] HEAs are potentially suitable as replacement alloys over the traditional alloys currently being used in the aerospace, defense, energy, and automotive industries. HEAs are often cited as favoring the formation of either ordered or disordered solid solutions or a combination of both due to their large mixing entropy which reduces the overall Gibbs free energy[7,8]; however, Miracle et al.[9] and Senkov et al.[10] show that there are medium-entropy alloys that also form 1- or 2-phase solid solution.[11–13] Yang et al.[14] propose two parameters to predict the formation of solid solution in HEAs. The parameters, d and X, take into consideration the atomic radii difference and the ratio of DSmix/DHmix, respectively. In addition to solid solution phases, nano-precipitate phases can form as well and are difficult to detect due to their size.
NATHAN A. LEY, SKYE SEGOVIA, and MARCUS L. YOUNG are with the Department of Materials Science and Engineering, University of North Texas, Denton, TX 76203. Contact e-mail: [email protected] STE´PHANE GORSSE is with Univ. Bordeaux, CNRS, Bordeaux INP, ICMCB, UMR 5026, 33600 Pessac, France. Manuscript submitted November 26, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Most HEA studies focus on HEA systems, which contain elements such as Al, Co, Cr, Fe, Ni, Ti, and Mn, while only a few studies focus on HEA systems, Senkov containing refractory elements.[7,15,16] [10,17–19] presented research on RHEAs and some et al. of the properties of these RHEAs exhibit promising materials properties above 1000 C. Structural alloys for turbine applications are required to operate at high temperatures while maintaining microstructural and phase stability. Currently, Ni-based superalloys, like Inconel 718, Waspaloy, and ATI 718Plus, are used for these types of applications due to their hightemperature capabilities, processability, and relatively low costs; however, all of these alloys are limited due to the loss of stability at higher operating temperatures, which can lead to early failure or fracture of the component. A revolutionary approach to alloy development and processing is needed to overcome these issues and create substantial rather than incremental technological advances. Multicomponent alloys such as high-entropy alloys (HEAs) or complex concentrated alloys (CCAs) offer a unique solution to these problems.[20,21] Recently, Soni et al. have shown that, by altering the microstructure of RHEAs, the ductility of certain alloys can be improved.[22] However, more investigation into the mechanisms of precipitation as well as microstructural evolution is required to determine if RHEAs can be used as replacements over the traditional Ni-based superalloys in high-temperature applications. In t
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