Microstructural, Phase, and Thermophysical Stability of CrMoNbV Refractory Multi-Principal Element Alloys

Abstract

Structural survivability of refractory multi-principal element alloys (RMPEAs) at high temperatures hinges on a combination of microstructure, phase, and thermophysical stabilities. In this study, we investigate the high-temperature stability of three compositions from the CrMoNbV system: Cr7Mo18Nb35V40, Cr25Mo25Nb25V25, and Cr40Mo45Nb10V5. The microstructure, high-temperature phase stability, coefficient of thermal expansion (CTE), and yield strength were evaluated up to 1300oC. Results show that all three as-cast compositions have dendritic microstructures with an average grain size of ~100µm and no preferred texture. High-temperature analysis of phase stability shows that Cr7Mo18Nb35V40 maintains a single-phase BCC structure from room temperature up to 1300oC, while Cr25Mo25Nb25V25 forms an undesirable C15 Laves phase and Cr40Mo45Nb10V5 exhibits BCC phase separation. The CTE measured was 9.58, 9.52 and 8.80 ppm/oC, and the yield strength at 1250oC was 503, 629 and 628MPa for Cr7Mo18Nb35V40, Cr25Mo25Nb25V25 and Cr40Mo45Nb10V5, respectively. The low density, CTE and high-temperature yield strength of the three alloys sets the CrMoNbV system above current superalloys. More importantly, the microstructural, phase, and thermophysical stabilities of the Cr7Mo18Nb35V40 alloy over the equiatomic and Cr-rich alloys highlight the importance of high-temperature survivability over yield strength as an RMPEA design principle.