Concomitant Clustering and Ordering Leading to B2 + BCC Microstructures in Refractory High Entropy Alloys

The presence of substantial concentrations of multiple principal elements in high entropy alloys or complex concentrated alloys, inevitably results in competing for clustering (or phase separation) and ordering tendencies between the constituent elemental pairs. Such competition in turn can result in interesting phase transformation pathways involving spinodal decomposition coupled with chemical ordering. Recently, a microstructure resembling that of Ni-base superalloys, with discrete BCC precipitates in a B2 matrix, in a checkered fashion, has been reported in multiple refractory high entropy alloys. This paper examines the microstructural evolution and the phase transformation pathways involved in two such candidate alloys, Al0.5NbTa0.8Ti1.5V0.2Zr and Al0.5Mo0.5NbTa0.5TiZr. The formation of this checkered pattern B2 + BCC microstructure is rationalized using schematic free energy curves and phase modulus criteria. Further, the effect of such phase distribution on mechanical properties is investigated via room-temperature compression.