Some insights into the high temperature phase stability of the BCC + B2 microstructure in aluminum containing refractory high entropy alloys

The present manuscript explores the empirical correlations between the nominal composition and the stability of BCC + B2 microstructures in refractory high entropy alloys (RHEAs). The intermediate to high-temperature microstructure within this alloy class is principally dictated by competition between the extension of the miscibility gap, underlying the BCC + B2 microstructure, to high-temperatures, and the solvus temperature of lower symmetry Al_xZr_y type ordered omega-like phases, which usually embrittle the alloy. Therefore, the best suited RHEA compositions will typically possess a high onset temperature for the miscibility gap in combination with a low solvus of the embrittling ordered Al_xZr_y phase. Based on a survey of different RHEAs, the present study identifies that compositions with Al:Zr < 0.5 appear to have a low solvus for the embrittling ordered Al_xZr_y phase, while a higher (group V/VI): (group IV) ratio generally appears to increase the extent of the miscibility gap to higher temperatures. These simple indicators may provide a valuable tool for designing RHEAs with more stable BCC + B2 microstructures.