Surface-governed deformation mechanisms in sub-20-nm metallic crystals stimulated by electrical loading

Metal nanocrystals have been shown to present two sorts of antithetical mechanical behaviors during mechanical tests, that is, “smaller is stronger” and “smaller is weaker”. Dislocation-starvation induced strengthening and surface diffusion induced softening are revealed as two main mechanisms that govern such behaviors. However, the competition between these two mechanisms, as well as their relationship with temperature, is still unclear. To probe into this issue, we here perform an investigation on sub-20-nm Au and Co nanocrystals using an electrical loading-coupled transmission electron microscopy, as the electrical loading can create an adjustable local heating effect, providing variable conditions for testing temperature-related competitions between displacive and diffusional events. It is found that surface diffusion is much more sensitive to temperature than dislocation activities. Apparent transitions of deformation mechanisms are observed when surface diffusion is energetically activated or inhibited. Surface curvature acts as a stimulus or mediator to atom diffusion, generating double-edged impacts on the nucleation of dislocation slip. At elevated temperatures (i.e. high electrical loading), the metal nanocrystals present liquid-like behaviors. Their surfaces can consecutively reshape when surface diffusion dominates; otherwise, shape transformations take place in a collapse-and-reconstruction manner.