Strain rate dependent deformation behavior of BCC-structured Ti₂₉Zr₂₄Nb₂₃Hf₂₄ high entropy alloy at elevated temperatures

The mechanical behavior and deformation mechanisms of a body-centered cubic (BCC) Ti₂₉Zr₂₄Nb₂₃Hf₂₄ (at%) high entropy alloy (HEA) was investigated in temperatures and strain rates from 700° to 1100 °C and 10⁻³ to 10 s⁻¹, respectively. The HEA exhibits a substantial increase in yield stress with increasing strain rate. The strain rate sensitivity (SRS) coefficient is ~3 times that of BCC alloy Nb-1Zr and even ~3.5 times that of pure Nb. This high SRS is attributed to the high Peierls stress of the HEA, which is about twice the Peierls stress of pure Nb. On the other hand, the flow stress exhibits a tendency from strain softening to strain hardening with the increase of strain rate especially at the relatively low temperatures. This behavior is explained by a change in dislocation motion from climbing to multiple slip with the increase of strain rate. Taking the specimen subjected to 800 ºC for example, dislocation walls formed at the early stage of deformation and at low strain rate of 10⁻³ s⁻¹. In this case there is sufficient time to activate dislocations climb, which results in discontinuous dynamic recrystallization, and strain softening. However, when the strain rate amounts to 1 s⁻¹, thermally activated processes such as dislocation climb are too sluggish. As a consequence, multiple slip systems are activated, and the dislocation interactions lead to the evolution of deformation bands, leading to strain hardening.