Stacking fault and transformation-induced plasticity in nanocrystalline high-entropy alloys

In this work, the plastic deformation in a model nanocrystalline high entropy alloy (HEA), CoNiCrFeMn, is studied by using molecular dynamics simulations. It is found that the plastic deformation of nanocrystalline CoNiCrFeMn HEAs is dominated by a partially reversible face-centered cubic (FCC) to hexagonal close-packed (HCP) transformation mediated by stacking faults and partial dislocations, which is dramatically different from the full dislocation and deformation twinning-dominated plasticity in conventional FCC metals. This mechanism is strongly associated with the metastable nature of CoNiCrFeMn. Furthermore, although the transformed HCP structures can hinder the migration of the subsequent partial dislocations, they can penetrate each other to form a complicated stacking fault network, which is consistent with the recent experimental observations. Nevertheless, the nanocrystalline CoNiCrFeMn HEAs still show the conventional Hall–Petch breakdown when the grain sizes are reduced below a critical value. It is hoped that this study provides an atomistic insight into the plasticity of metastable HEAs and sheds some light on the design of novel HEAs for ultrahigh strength and plasticity.