Effects of Nb on deformation-induced transformation and mechanical properties of HfNbxTa0.2TiZr high entropy alloys

Body-centered cubic (BCC) refractory HEAs which have shown interesting properties at elevated temperatures such as outstanding high-temperature strength, high oxidation and corrosion resistance, nevertheless, they are generally brittle at ambient temperature, which limits their practical uses. Application of the transformation-induced plasticity (TRIP) concept into these BCC HEAs like HfTaTiZr has confirmed to be an effective approach to addressing this critical challenge. To further reveal the underlying mechanisms and improve the overall properties, in this work, we attempted to reveal the interplay between the phase stability, strain-induced transformation and deformation in HfNbxTa0.2TiZr (x = 0, 0.15, 0.20 and 0.25) HEAs. It was found that proper addition of Nb could not only enhance the TRIP effect due to the reduced phase stability of the prior BCC phase, but also facilitate twinning in the product HCP (hexagonal close packing) phase at the late stage of deformation. As a result, the HfNb0.2Ta0.2TiZr HEA shows much enhanced mechanical properties, i.e., pronounced work hardening behavior and large uniform ductility of up to 26%, which surmounts most of refractory BCC HEAs reported previously. Our findings are important not only for understanding the deformation mechanism of the TRIP reinforced HEAs, but also for the future development of other high-performance HEAs.