Strain-Rate-Dependent Deformation Behavior of Ti29Zr24Nb23Hf24 High Entropy Alloys at Elevated and Room Temperature
Abstract
We investigate the strain-rate-dependent mechanical behavior and deformation mechanisms of a refractory high entropy alloy, Ti29Zr24Nb23Hf24 (at.%), with a single-phase body-centered cubic (BCC) structure. High-temperature compression tests were conducted at temperatures from 700 to 1100C at strain rates ranging from 10-3 to 10 s-1. A sudden stress drop after yield point was observed at higher temperatures and lower strain rates with the Zener-Holloman parameter, lnZ, in the range of 17.2-20.7. Such a softening behavior can be related to the interaction of dislocations with short-range clustering. However, at higher strain rates or lower temperatures (lnZ>25.0), kink bands were activated being responsible for the continuous strengthening of the alloy in competition with the softening mechanism. Systematic TEM investigations reveal that dislocation walls formed along 110 planes and dislocation loops were observed at a low strain of 6% at a high strain rate of 1 s-1 and 800C. Kink band induced dynamic recrystallization is evident upon further straining. On the other hand, at low strain rate of 10-3 s-1 and 800C, discontinuous recrystallization mechanisms become dominant with arrays of dislocations forming in front of the bulged boundaries of parent grains. These sub-grain boundaries eventually turn into high-angle grain boundaries. We also investigate the deformation mechanism of the alloy under extremely high strain rate (103 s-1) at room temperature. The specimen exhibits extensive kink bands with arrays of dislocation walls. As further strained, multiple slip systems can be activated and the interaction of dislocation walls plays a vital role in the strain hardening of the alloy.