Microstructure evolution and strain rate sensitivity of ductile Hf20Nb10Ti35Zr35 medium-entropy alloy after thermal cycling

The medium-entropy alloy Hf20Nb10Ti35Zr35 has a ductile BCC structure in the as-solution-treated state and could be age-hardening with fine precipitates. In this study, the thermal stress produced by thermal cycling was found to accelerate the precipitation of the alpha" and the evolution of omega -> alpha phase of Hf20Nb10Ti35Zr35. The alpha"-martensite phase of Hf20Nb10Ti35Zr35 tended to grow in the (001)beta plane, with the misorientation angle being concentrated around 52 degrees after 10 thermal cycles. Moreover, the misorientation angle tended to have a bimodal distribution after 50 thermal cycles. Nanomechanical testing was performed to measure the strain rate sensitivity (SRS) at room temperature. The result shows that SRS of Hf20Nb10Ti35Zr35 changed from a small positive value of 0.09247 to a small negative one of either- 0.02125 or- 0.0445. This was mainly attributable to the decrease in the volume of the beta phase or the increase of the alpha" + alpha composite phase. This demonstrates that thermal cycling treatment of the present alloy could induce more precipitation of the second phase for hardening and enhance uniform deformation behavior.