摘要
Refractory high-entropy superalloys (RHESAs), or refractory high-entropy alloys (RHEAs) with superalloy-like A2+B2 microstructure, have significant potential for next-generation high-temperature applications due to their superior strength at elevated temperatures. The stability of the B2 phase is crucial for maintaining the integrity of the superalloy-like microstructure. However, the conditions for B2 stability remain unclear. Here, we conduct a comprehensive investigation into the stability of the B2 phase in the Al-Nb-Ta-Ti-Zr system, the most representative RHESA system. Three key compositions covering the potential A2+B2 region were selected as model alloys for in-depth analysis. Surprisingly, our findings reveal that, despite the seemly presence of the B2 phase on a mesoscopic scale, it is not the stable equilibrium phase in these alloys above 700°C. Two primary factors contribute to the misperception of the B2 phase: quench-induced nanoscale B2 and the B82 phase. Nanoscale interconnected A2+B2 structures were discovered within the mesoscopic "B2" phases, and they are formed during quenching via spinodal decomposition. Additionally, the presence of (Al-Zr)-rich B82 phases at lower temperatures complicates the identification of the B2 phase. Our experiments and simulations demonstrate that the structural similarities between B82 and B2 structures can lead to misidentification when using selected area diffraction pattern (SADP)-based strategies. We aim to raise awareness within the community regarding the challenges associated with accurately identifying the B2 phase in the Al-Nb-Ta-Ti-Zr system, as well as other RHEA systems, and provide potential solutions to address these challenges.
•There is no stable equilibrium B2 phase in the present alloys above 700°C.•Mesoscopic "B2" phases all contain a nanoscale interconnected A2 +B2 structures.•Interconnected structures are formed during quenching via spinodal decomposition.•The B82 phase is easily misidentified as the B2 phase when analyzing by TEM SADP.•Practical solutions to these two challenges are recommended in this work.