Microstructural evaluation of the addition of Zr to create novel refractory high entropy alloys

In this study, W0.4MoNb1.3TaTiZrX (X = 0, 0.5, 0.7, and 1.0) alloys were designed using empirical design parameters to expand upon an existing refractory high entropy alloy with the purpose of examining the microstructural effects of Zr addition on the alloy system. The four designed alloys were vacuum arc cast and were then subjected hot isostatic pressing prior to a detailed metallurgical and hardness evaluation. The W0.4MoNb1.3TaTiZrX alloys formed a primary BCC solid solution matrix, with the W0.4MoNb1.3TaTiZr0 alloy being a single-phase BCC solid solution in both the as-cast condition and following hot isostatic pressing. The addition of Zr in the as-cast W0.4MoNb1.3TaTiZrX alloy resulted in an increasing volume fraction of a secondary interdendritic BCC phase, within which a eutectic/eutectoid Zr-rich HCP phase formed proportionally within this BCC region as the Zr content of the alloy increased. Following hot isostatic pressing, the secondary BCC and HCP phases transformed into a Zr/Ti rich alpha-HCP phase, leaving a BCC + HCP phase composition in the Zr0.5-1.0 alloys. The microhardness of the W0.4MoNb1.3TaTiZrX alloys also increased with increasing Zr content in both heat treatment conditions. The formation of secondary phases is attributed to insolubilities between alloying elements which empirical design parameters failed to predict.