Plastic deformation mechanism and defect patterning under nanoindentation in medium entropy alloy CoCrNi

In this work, molecular dynamics simulations are employed to study the mechanical response and plastic deformation evolution of single-crystal medium entropy alloy (MEA) CoCrNi under nanoindentation. The effects of lattice distortion (LD) and chemical short-range order (CSRO) on plasticity initiation and defect evolution including the patterning of dislocations, planar faults and prismatic dislocation loops (PDLs) are discussed with respect to anisotropy. The results show that CSRO increases Young's modulus and critical nucleation stress of Shockley partial (SP), while LD and thermal vibration have the opposite effect. In MEAs, a preference of SP nucleation at CoCr clusters is observed, and the SP bundles develop into a compact plastic zone with dense dislocation networks due to frequent dislocation interactions and large slip resistance by LD and CSRO. Twin boundaries (TBs) and geometrically necessary dislocations (GNDs) play the dominant role in the accommodation of local plasticity, while the propagation of PDLs, dependent on the slip propensity of loading orientation and the degree of slip resistance, assist in the relief of local stress. Once the PDL nucleation is suppressed, dislocation pileup is triggered by dense dislocation networks with limited dislocation propagation tendency, leading to a higher strength of the material. The above results elaborate the relation between local plasticity and defect patterning, which sheds light on the design of MPEAs with heterogeneous deformation behavior.