Atomistic mechanisms of twin-boundary-controlled crack propagation in nanotwinned CoCrFeMnNi high-entropy alloys

Abstract

Twin boundaries (TBs) are effective microstructural features for tuning the mechanical response of face-centred cubic high-entropy alloys (HEAs), but their role in crack-tip deformation and crack propagation remains insufficiently quantified at the atomic scale. Here, molecular dynamics simulations are used to isolate the effects of TB spacing and TB orientation on mode I crack propagation in equiatomic nanotwinned CoCrFeMnNi HEAs. The simulations compare single-crystal, single-TB and multiple-TB configurations and track the coupled evolution of stress–strain response, local atomic structure, crack-extension distance, energy release rate and dislocation density. TBs redistribute stress from the crack tip to the interface region and alter the dominant dislocation source during deformation. For the spacing series, the yield strength shows a non-monotonic dependence on TB spacing, with a minimum at d = 5.58 nm. At smaller spacings, dislocation motion is strongly confined by closely spaced TBs; at larger spacings, the reduced density of available dislocation sources increases the stress required for dislocation nucleation. Crack propagation is delayed in multiple-TB models because dislocation pile-up, TB migration and local non-FCC structural disorder dissipate deformation energy and reduce the crack-driving force. For the orientation series, TBs parallel or normal to the loading direction mainly act as barriers to dislocation motion, whereas inclined TBs promote slip along or between TBs and shift stress concentration away from the crack tip. These results establish an atomistic structure-property link between TB geometry, dislocation-source transition and crack resistance in nanotwinned CoCrFeMnNi alloys. Three independently randomised chemical configurations for seven representative models further show that the principal qualitative contrasts persist despite quantitative seed-to-seed scatter.