A study on severely cold-rolled and intermediate temperature aged HfNbTiZr refractory high-entropy alloy

A single-phase body-centered cubic (BCC) HfNbTiZr refractory high-entropy alloy (RHEA) was fabricated by vacuum arc melting. The effect of aging at 450 degrees C on 80% cold-rolled HfNbTiZr specimens (abbr. as CRA) as well as that on 80% cold-rolled and then recrystallized at 1000 degrees C for 1 h (abbr. as CRRA) have been investigated. A significant increase of hardness after aging at 450 degrees C was presented in both CRA and CRRA specimens. The hardness of CRA specimens increased dramatically within a shorter time than that of CRRA specimens, indicating that the cold-rolled process can significantly enhance the age hardening effect. During aging at 450 degrees C, BCC and hexagonal closed-packed (HCP) precipitates formed in the BCC matrix. The particle-like Nb-rich BCC precipitates were mainly within the grains and needle-like HfTiZr HCP precipitates mostly located near the grain boundaries. In CRA specimens aged for 500 h, BCC precipitates could grow up to 30-90 nm in radius and HCP precipitates to 300-1600 nm in length and 40-140 nm in width. The results of tensile test revealed that the presence of the precipitates can induce brittleness in HfNbTiZr alloy.