The evolution and deterioration of the B2 phase and corresponding mechanical properties in the BCC/B2 refractory high-entropy superalloys

Abstract

Refractory high-entropy superalloys (RHESAs) with a coherent BCC/B2 microstructure, analogous to the γ/γ′ structure in Ni-based superalloys, are regarded as promising candidates to surpass Ni-based superalloys due to their high melting points and exceptional creep resistance. However, the inherent brittleness and high-temperature instability of the B2 phase limit their practical application. This study focuses on the Al_9.26Nb_17.92Ta_18.4Ti_43.47Zr_10.95 (at.%) RHESA prepared via arc melting, followed by homogenization and aging treatments for various durations. The phase evolution pathways, elemental diffusion mechanisms, and mechanical property changes during aging were systematically investigated using transmission electron microscopy (TEM) and room-temperature compression testing. The results demonstrate that in the ST (solution-treated) state, the B2 phase exists as nanodomains smaller than 5 nm, formed through spinodal decomposition and the ordering reaction. During aging for 5–60 h, the B2 phase grows into an interconnected network structure. The 60h-aged sample exhibits a ∼25 % increase in strength without compromising ductility. After 150 h of aging, the B2 phase degenerates radially from elongated precursors to form Al-Zr intermetallic compounds, accompanied by the expulsion of Ti, Nb and Ta element. While both strength and ductility remain largely unchanged, a significant reduction in the strain hardening rate is observed. This study reveals the evolution mechanism of the B2 phase and the formation pathway of the Al–Zr intermetallic phase. For the first time, the intermediate process of the direct deterioration of the B2 phase into the Al–Zr intermetallic compound was directly observed using TEM, and meanwhile, a novel Ta-rich cubic phase was identified during the evolution process. Those provides a theoretical foundation for further optimizing the high-temperature stability of RHEAs.