Molecular dynamics study on partitioned plasticity and strain-transfer mechanisms in gradient nanotwinned Ni

In this paper, the mechanical response and microscopic deformation mechanism of gradient nanotwinned Ni are investigated using molecular dynamics simulations. By constructing seven comparative models (M1–M7), the effects of grain boundaries, twin boundaries, grain size gradient, and twin spacing gradient on strength and plastic deformation are analysed. The results show that well-organised nanotwinned structures increase the peak strength of Ni, indicating that continuous twin-boundary structures are an important source of strengthening. However, the peak strength of gradient nanotwinned M6 is close to that of uniform nanotwinned M3/M4, showing no obvious additional peak strengthening under the present loading condition. CNA and DXA analyses reveal that the uniqueness of M6 lies in twin spacing gradient-induced partitioned plasticity and dislocation regulation rather than further peak strengthening. Regions with different twin spacings exhibit dislocation confinement, plasticity coordination, and plasticity release, while dislocations are blocked, deflected, and rearranged at twin boundaries, forming heterogeneous strain transfer paths. Compared with M7, the special response of gradient nanotwinned structures originates from the multi-level interface constraint generated by the twin spacing gradient. This study provides an atomic-scale basis for understanding plastic deformation and strain transfer in gradient nanotwinned metals.