Size-dependent atomic strain localization mechanism in Nb/amorphous CuNb nanolayered composites

Experiments have shown that crystalline–amorphous nanolayered composites show a strong size dependence in strength and plasticity. However, the underlying mechanism remains unknown. Here, the layer thickness (h)-dependent compressive strength and plastic deformation of Nb/amorphous CuNb nanolayered composites with h ranging from 2.8 to 20 nm have been studied by molecular dynamics simulations. It is found that the strength increases monotonically with the decrease of h, which can be well captured by the refined confined layer slip model. Furthermore, the shear banding-induced strain localization was analyzed in detail based on the evolution of the von Mises strain distribution with the applied strain and a strain localization parameter that represents the deviation of the specific atomic Mises strain from the average one of all atoms. The results show that the strain localization of the composite shows a significant size dependence and its magnitude increases with the decrease in the layer thickness. The prevention of shear band propagation in thicker layer samples is attributed to that the amorphous phase is capable of forming an atomic vortex to alleviate the strain concentration caused by dislocation absorption. The thicker amorphous layers accommodate enhanced homogeneous plasticity than the thinner ones by forming a larger vortex. Note that the above size dependence of strength and strain localization in the composite agrees well with existing experimental measurements and observations in the layer thickness range considered. As a result, the present work gives a deeper insight into the understanding of the size-dependent strengthening and strain localization mechanism in the amorphous/crystalline composites.