Distinct nucleation and propagation of prismatic dislocation loop arrays in Ni and medium-entropy CrCoNi alloy: Insights from molecular dynamics simulations

This paper utilizes molecular dynamics simulations to investigate the mechanisms of nucleation and propagation of prismatic dislocation loop (PDL) arrays in single-crystal medium-entropy CrCoNi alloys and pure Ni, with a particular focus on understanding the different size-dependent plasticity. To explore this phenomenon, we varied the size of PDLs. Our findings suggest that the anticipated size effects (i.e., smaller is stronger) are observed in both the elastic and plastic stages of Ni samples, but they are less pronounced in CrCoNi samples. Furthermore, it is observed that PDLs in CrCoNi alloys are often trapped in stable pileups, which results in a surprisingly significant strengthening effect. In contrast, PDLs in similar Ni configurations glide easily and exit from the free bottom. The observed differences between the behavior of CrCoNi and Ni can be attributed to the dislocationlength-dependent lattice friction in CrCoNi, as well as a significant stress gradient resulting from localized loading during nanoindentation. A simple dislocation mechanics model is used to rationalize the abnormal size effect in CrCoNi. These findings provide an insightful understanding of the strengthening mechanisms due to complex dislocations in multi-principal element alloys.