Unveiling the atomic-scale mechanisms of grain-size-dependent crack propagation and microstructural evolution in CoCrNi medium-entropy alloy

In this paper, we systematically investigate the size-dependent mechanical response and crack propagation behavior of face-centered cubic (FCC) CoCrNi medium-entropy alloys (MEAs) with pre-existing cracks via molecular dynamics (MD) simulations. The investigation focuses on the stress-strain response, yield strength evolution, and microstructural deformation mechanisms in specimens with different grain sizes. In contrast to the classical Hall-Petch and inverse Hall-Petch relations observed in crack-free counterparts, the pre-cracked samples exhibit an anomalous size dependence. Specifically, the yield stress decreases with grain refinement in the larger-grain regime, whereas it exhibits a slight recovery in the smaller-grain regime (inverse Hall-Petch range), accompanied by the restoration of plasticity governed by grain boundary (GB) activities. Based on fracture mechanics theorem, this anomaly is fundamentally attributed to the stress concentration near the crack-tip. Furthermore, a transition in the crack propagation mode from transgranular to mixed, and ultimately to intergranular, is observed as grain size decreases, attributed to the intragranular dislocation starvation and the formation of nanotwins. Also observed are the dislocation emission/absorption at GBs and the reversible transformation of nanolayered HCP phases (stacking faults), among other microstructural evolution phenomena. This work provides insights into the synergy between crack propagation and plastic deformation in CoCrNi MEAs across different grain sizes, unraveling the atomic-scale mechanisms behind their superior mechanical properties.