Atomistic insights into the deformation mechanism of a CoCrNi medium entropy alloy under nanoindentation

Understanding the fundamental of contact-induced behavior in metals is critical for the studies of associated mechanical properties. Although nanocontact plasticity in the conventional pure metals or solutions have been well established, such theoretical frameworks may break down in the recently emerging medium entropy alloys (MEAs), owing to their unique disordered feature and inevitable chemical fluctuations. Here combined with crystal defect theories, we investigate the nucleation and evolution of dislocations in CoCrNi MEA during nanoindentation. Molecular dynamics simulations of nanoindentation are carried out to analyze the effects of composition inhomogeneity and temperature on the defect behavior. In contrast to the usual homogeneous nucleation criterion in pure metals, heterogeneous dislocation nucleation is preferred to occur at Cr-rich clusters in CoCrNi MEA at a lower indentation depth. Although the local composition fluctuation can facilitate the nucleation of dislocations, these partials are pinned significantly by the local atomic sites with a high level of CoCr chemical short-range order. Moreover, nanotwin and phase-transformation are promoted at low temperature, due to the low stacking fault energy and enhanced glide resistance. In particular, the formation and evolution of the distorted prismatic dislocation loop are visualized at high temperature. The new phenomena reported herein could provide a basis for tuning local composition and atomic arrangement to obtain excellent mechanical properties in medium or even high entropy alloys.