Abstract:The introduction of gradient nano-grained (GNG) structure into metallic materials is recognized as an effective strategy to mitigate or eliminate the strength-ductility trade-off. The distribution of grain sizes within the gradient structure plays a critical role in modulating the strength-ductility synergy. In this study, the influences of the gradient grain-size distribution on the mechanical characteristics, strain and strain distributions, dislocation distribution, and grain boundary migration were investigated using molecular dynamics simulations. The results demonstrate that an optimal strength-ductility synergy is achieved when the gradient grain-size range lies within the transition region between the inverse Hall-Petch (IHP) and Hall-Petch (HP) regimes (9.6?19.2 nm). Further analysis indicates that within the IHP-HP transition zone (9.6?19.2 nm), the gradients of local strain and stress are the most pronounced, and the dislocation density reaches a maximum, leading to an optimal balance between strength and ductility. When the gradient grain-size range falls within the IHP softening regime, the plastic deformation is predominantly governed by grain boundary-mediated mechanisms. In contrast, when the gradient grain-size range is located in the HP strengthening regime, dislocation slip becomes the primary mode of plastic deformation.