Substitution solid solution hardening effect of vanadium in Ni-Co-V medium entropy alloys

The single-phase polycrystalline NiCoV medium-entropy alloy (MEA) has shown ∼1 GPa yield strength, surpassing most single face centered cubic alloys. This excellent strength has been theoretically devoted to the special solid solution hardening effect of V. However, the intrinsic reasons for this excellent strengthening effect of V in high-/medium-entropy alloys are still less experimentally explored. To this end, we here investigated the solid solution hardening effect of V in the NiCoV system. The results showed that the solid hardening mechanism changes from lattice distortion to atomic volume mismatch when the V content exceeds 6 at. %. Particularly, the increase in V content increases the Hall-Petch coefficient of NiCoV MEAs significantly. The combination of efficient grain boundary hardening and atomic volume mismatch hardening leads to the excellent yield strength of concentrated NiCoV.