Mechanisms of plastic deformation in the vicinity of a low-angle grain boundary in CrCoNi medium-entropy alloy

This work investigates interactions of nanoindentation-induced dislocations (NIDs) with a low-angle grain boundary (LAGB) in a single-crystalline CrCoNi medium entropy alloy. Microstructures before and after µN-nanoindentation were examined using accurate electron channeling contrast imaging. Initially, the microstructure consists of subgrains separated by LAGBs with an average size of 100 µm. After nanoindentation on the (001) plane in the vicinity of a LAGB with a misorientation angle of 0.24° and consisting of dislocations spaced ~60 nm apart, NIDs formed pile-ups on {111} planes. Microstructural analyses for indents near the LAGB reveal that the LAGB accommodates plasticity by blocking the NIDs, activating a dislocation nucleation site in the adjacent subgrain/emission of dislocation from the LAGB, and inducing slight motions of its constituent dislocations. For indents between 0-420 nm from the LAGB, micromechanical analyses show the absence of pop-ins which implies plasticity occurs by the motion of pre-existing dislocations. For indents at >420 nm, pop-ins were observed and plasticity was initiated by homogeneous and heterogeneous nucleation of dislocations. The LAGB is, therefore, a strong obstacle to dislocation motion and we point out areas for future research using nanoindentation in combination with advanced characterization methods to study the interaction between dislocations and LAGBs.