Effect of solid-solution strengthening on deformation mechanisms and strain hardening in medium-entropy V_1-xCr_xCoNi alloys

Abstract

High- and medium-entropy alloys (HEAs and MEAs) possess high solid-solution strength. Numerous investigations have been conducted on its impact on yield strength, however, there are limited reports regarding the relation between solid-solution strengthening and strain-hardening rate. In addition, no attempt has been made to account for the dislocation-mediated plasticity; most works focused on twinning- or transformation-induced plasticity (TWIP or TRIP). In this work we reveal the role of solid-solution strengthening on the strain-hardening rate via systematically investigating evolutions of deformation structures by controlling the Cr/V ratio in prototypical V_1-xCr_xCoNi alloys. Comparing the TWIP of CrCoNi with the dislocation slip of V_0.4Cr_0.6CoNi, the hardening rate of CrCoNi was superior to slip-band refinements of V_0.4Cr_0.6CoNi due to the dynamic Hall-Petch effect. However, as V content increased further to V_0.7Cr_0.3CoNi and VCoNi, their rate of slip-band refinement in V_0.7Cr_0.3CoNi and VCoNi with high solid-solution strength surpassed that of CrCoNi. Although it is generally accepted in conventional alloys that deformation twinning results in a higher strain-hardening rate than dislocation-mediated plasticity, we observed that the latter can be predominant in the former under an activated huge solid-solution strengthening effect. The high solid-solution strength lowered the cross-slip activation and consequently retarded the dislocation rearrangement rate, i.e., the dynamic recovery. This delay in the hardening rate decrease, therefore, increased the strain-hardening rate, results in an overall higher strain-hardening rate of V-rich alloys.