Phase precipitation behavior and mechanical properties of TiVNbTaAlx refractory high-entropy alloys after annealing at medium temperatures

The thermal stability of the microstructure and the associated changes in mechanical properties are critical factors for the high-temperature performance of refractory high-entropy alloys (RHEAs). In this study, we fabricated a series of TiVNbTaAl_x (x = 0, 0.1, 0.2, 0.3) RHEAs with the single-phase body-centered cubic (BCC) structure and investigated the phase precipitation behavior of those alloys after annealing at 400–800°C for 100 hours. When annealing temperatures were below 600°C, no significant changes were observed in the alloys' microstructure. However, after annealing at 800°C, all alloys precipitated Ti-rich phases. Remarkably, in addition to the hexagonal close-packed (HCP) Ti-rich phase, we identified a rare face-centered cubic (FCC) Ti-rich phase precipitating from the BCC matrix—a phenomenon seldom observed in Ti-containing HEAs and Ti alloys under normal conditions. In the TiVNbTa base alloy, the precipitate was predominantly an acicular FCC phase, while in Al0.1 and Al0.2, the formation of the FCC phase was suppressed, and a small amount of granular HCP phase precipitated at the grain boundary. When Al content (x) increased to 0.3, numerous rod-like FCC phase and granular HCP phase formed both at the grain boundary and within the grains. Although the addition of Al increased the yield strength of the RHEAs through solid-solution strengthening, the alloys' thermal stability decreased, and the fraction of precipitate phase generally increased with higher Al content. Therefore, a significant reduction in compressive yield strength and plasticity occurred in Al0.3 alloy after annealing at 800°C. These novel findings are crucial for enhancing our understanding of phase precipitation behavior in RHEAs at elevated temperatures.