Phase inversion in a two-phase, BCC+B2, refractory high entropy alloy

A phenomenon of phase inversion, presumably the first ever experimental evidence in metallic alloys, is shown in a refractory high entropy alloy (RHEA), Al0.5NbTa0.8Ti1.5V0.2Zr. Phase inversion in crystalline solid systems is driven by the differences in elastic modulus of the two phases. Quenching from a high temperature single phase field, the RHEA exhibits a co-continuous mixture of a disordered BCC and an ordered B2 phase, that upon isothermal annealing at 600 degrees C develops via spinodal decomposition into a continuous 82 matrix with discrete cuboidal BCC precipitates aligned along the < 001 > directions. Longer term annealing at 600 degrees C results in the development of necking constrictions along the B2 channels, eventually pinching-off these channels and making the BCC phase continuous with discrete 82 precipitates. This inversion process can be related to the simultaneous operation of two processes: (i) spheroidization of the initially discrete cuboidal BCC precipitates driven by a reduction in the total interface energy and (ii) an increase in the stiffness of the B2 phase, relative to the BCC phase, due to chemical composition changes during annealing, forcing the B2 regions to become discrete driven by the reduction in the total elastic strain energy. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.