Investigation of size effects and gradient structures on tensile behavior in polycrystalline CoCrNi medium-entropy alloy via molecular dynamics simulations

This study investigates the tensile behavior of polycrystalline CoCrNi medium-entropy alloy using molecular dynamics simulations, analyzing the effects of the size and gradient structure on mechanical properties. First, tensile simulations were conducted on three uniform models with different specimen dimensions (60 × 15 × 15, 40 × 15 × 15, 20 × 15 × 15 nm3) and grain sizes (2, 5, 8 nm). The results reveal that specimen dimensions exhibit negligible influence on the deformation behavior of the CoCrNi alloy, whereas the grain size significantly affects both the macroscopic stress–strain response (including flow stress and Young's modulus) and microstructural evolution characteristics (encompassing dislocation density and phase transformation). Subsequently, tensile tests were conducted on two structural models, namely, the symmetric gradient model and the unidirectional gradient model, with each model encompassing four distinct gradient rates. The results indicate a monotonic enhancement in both flow stress and Young's modulus with increasing gradient rates. Notably, under high-gradient-rate conditions, a substantial augmentation in the dislocation line length was detected, suggesting intensified dislocation nucleation and propagation activities. The symmetric structure relies on the coordinated action of phase transformation and twinning, whereas the unidirectional structure is governed by dislocation proliferation. These findings establish critical theoretical underpinnings for comprehending the mechanical responses of CoCrNi medium-entropy alloys.