Novel atomic-scale mechanism of incipient plasticity in a chemically complex CrCoNi medium-entropy alloy associated with inhomogeneity in local chemical environment

Abstract Understanding the incipient plastic mechanism in metals is critical for their associated mechanical properties. While heterogeneous dislocation nucleation from pre-existing defects constitutes the most prevalent onset mechanism of plasticity in the conventional solutions, such a scenario may break down in the recently emerging chemically-disordered high/medium entropy alloys (HEAs/MEAs), owing to their unique multiple-component feature and the inevitable inhomogeneity in local atomic environments. Here, classical molecular dynamics simulations and first-principles density functional theory calculations are carried out to study the atomic-scale mechanisms governing the incipient plasticity in a prototypical chemically complex face-centered cubic (fcc) CrCoNi MEA. Dislocation nucleation is found to occur preferentially at an energetically unstable defect cluster with body-centered cubic like (bcc-like) atomic environment as a precursor, after certain deformation before plasticity, which is in contrast with the usual mechanism of heterogeneous dislocation nucleation in the conventional solute solution metals. The minimum energy pathway of dislocation nucleation from a bcc precursor is discussed to rationalize the usual phenomenon. First-principles athermal quasi-static compression test validates the mechanism suggested by atomistic simulations. Further electronic structure analysis suggests that the local bcc-like defect cluster is related to the localized electronic behaviors of Cr atoms and the weak Cr-Cr bonding, which promote the dislocation nucleation and therefore the incipient plasticity of CrCoNi MEA. The atomic and electronic insights reported here highlight the significant role of local chemical order in determining the mechanical property, and shed light on the strategy of optimizing mechanical performance via tailoring composition and local atomic arrangement in the generic highly concentrated solutions.