Microstructural, phase, and thermophysical stability of CrMoNbV refractory multi-principal element alloys

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: Cr ₇Mo ₁₈Nb ₃₅V ₄₀, Cr ₂₅Mo ₂₅Nb ₂₅V ₂₅, and Cr ₄₀Mo ₄₅Nb ₁₀V ₅. The microstructure, high-temperature phase stability, coefficient of thermal expansion (CTE), and yield strength were evaluated up to 1300 °C. Results show that all three as-cast compositions have dendritic microstructures with an average grain size of ∼100 µm and no preferred texture. High-temperature analysis of phase stability shows that Cr ₇Mo ₁₈Nb ₃₅V ₄₀ maintains a single-phase BCC structure from room temperature up to 1300 °C, while Cr ₂₅Mo ₂₅Nb ₂₅V ₂₅ forms an undesirable C15 Laves phase and Cr ₄₀Mo ₄₅Nb ₁₀V ₅ exhibits BCC phase separation. The CTE measured was 9.58, 9.52 and 8.80 ppm/°C, and the yield strength at 1250 °C was 503, 629 and 628 MPa for Cr ₇Mo ₁₈Nb ₃₅V ₄₀, Cr ₂₅Mo ₂₅Nb ₂₅V ₂₅ and Cr ₄₀Mo ₄₅Nb ₁₀V ₅, 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 Cr ₇Mo ₁₈Nb ₃₅V ₄₀ alloy over the equiatomic and Cr-rich alloys highlight the importance of high-temperature survivability over yield strength as an RMPEA design principle.