Exceptional thermal stability and hot deformation behavior of a powder metallurgy ultra-fine-grained MoNbTaTiV refractory high-entropy alloy

This work investigates the microstructure evolution and hot deformation behavior of a powder metallurgy (P/M) ultra-fine-grained MoNbTaTiV refractory high-entropy alloy (RHEA), following vacuum heat treatments (VHT). VHT temperature ranged from 1373 to 1573 K for durations of 1–8 h. Isothermal compression tests were conducted at 1573 K with strain rates of 0.005 or 0.0005 s ⁻¹. Scanning electron microscopy and transmission electron microscopy images were used to analyze the effect of VHT temperature and time on matrix grain size and precipitated phase evolution. Results showed that both the precipitated phase and matrix grain size exhibited excellent thermal stability in the P/M ultra-fine-grained MoNbTaTiV RHEA. After an 8 h heat treatment at 1573 K, the average sizes of the matrix grain and precipitated phase increased from 180 and 580 nm to 415 and 942 nm, respectively. Precipitated phase size exhibited a strong linear relationship with the cube root of VHT time at a given temperature. Significantly larger precipitated phase sizes were observed at 1573 K compared to 1373 and 1473 K, indicating a degraded pinning effect and a reduction in the matrix grain growth exponent from 10 to 6. However, the matrix grain size remained in the submicron range due to inherent lattice distortion, sluggish diffusion, and precipitated phase pinning effects in high-entropy alloys. Consequently, excellent thermal stability of the microstructure was observed across various VHT conditions, resulting in a constant low-level hot deformation resistance in the studied alloy.