Optimal grain size distribution in gradient nano-grained nickel

Gradient nano-grained (GNG) metals are a unique class of metallic materials by manipulating the gradient size distribution, but the optimal grain size distribution producing the best strength–ductility synergy is still unclear. Here, the mechanical behavior and deformation mechanism of GNG Ni with different spatial distribution of grain size varying in the inverse Hall-Petch regime are investigated by molecular dynamics method. The results show that the GNG structure has the optimal strength–ductility synergy when the distribution of grain size gradient satisfies a linear relationship (i.e. the gradient rate n = 1). The strengthening and toughening mechanisms of the optimal GNG structure are further illustrated. It is found that the stress gradient and strain gradient in the GNG Ni with gradient rate n = 1 are the largest among different gradient structures. The optimal strength-ductility synergy in GNG structures with linear distribution of grain size is attributed to the remarkable enhancement of dislocations and the diminution of GB activities. These results would provide an insight into the mechanical performance tuning of GNG structures experimentally.