On the deformation behavior of CoCrNi medium entropy alloys: Unraveling mechanistic competition

Recently, CoCrNi medium entropy alloys (MEA) have been the subject of numerous investigations due to their unique mechanical properties such as an exceptionally high strength-ductility combination. The resulting superior toughness of CoCrNi MEAs is attributed to an interplay of multiple deformation mechanisms, such as twinning, and partial and perfect dislocation glide. The current understanding of MEA deformation mostly stems from an indirect analysis of the defect evolution in deformed microstructures, where the contributions of individual mechanisms are assessed from the relative concentrations of associated defect structures. Here, we propose that the mechanistic contributions to microstructural deformation are more properly reflected by the percentages of total strain accommodation. Using atomistic simulations, the mechanical response of nanocrystalline CoCrNi MEA under uniaxial tension is investigated as function of grain size and chemical short-range ordering (SRO). The contributions of deformation mechanisms are resolved directly from the amount of strain accommodation by leveraging continuum based kinematic metrics. It is found that during initial loading, deformation occurs by partial dislocation slip, in agreement with experimental observations. Under continued loading, the governing deformation mechanisms transition to twinning and perfect dislocation slip. Furthermore, the grain size that corresponds to the maximum strength is found to decrease in presence of SRO.