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 α" and the evolution of ω → α phase of Hf₂₀Nb₁₀Ti₃₅Zr₃₅. The α''-martensite phase of Hf₂₀Nb₁₀Ti₃₅Zr₃₅ tended to grow in the (001)_β plane, with the misorientation angle being concentrated around 52° 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 β phase or the increase of the α" + α composite phase. This demonstrates that thermal cycling treatment of the present alloy could induce more precipitation of second phase for hardening and enhance uniform deformation behavior.