Nanostructured metals are a promising class of high-strength and radiation-tolerant materials. In particular, the large volume fraction of grain boundaries (GB) in nanocrystalline (NC) and ultrafine-grained (UFG) metals serve as obstacles to dislocation glide and sinks for radiation damage. However, there remains a lack of quantitative characterization of the deformation mechanisms that control the mechanical properties of NC and UFG metals which hinders material design towards exception properties. In this work, the plastic deformation mechanisms that govern the mechanical properties of NC and UFG metal thin films are investigated through in situ transmission electron microscopy (TEM) nanomechanical experiments. This technique allows for the simultaneous observation of the active deformation mechanisms and quantification of the mechanical properties during monotonic and stress-relaxation experiments. Experiments were performed on NC Al and UFG Au specimens with different microstructures (grain sizes, thickness, texture) and irradiation levels. Observed deformation mechanisms include dislocation nucleation at GBs, dislocation pinning/de-pinning on radiation defects and GB migration. In irradiated films, the GB migration leads to defect-free zones which then provide avenues for unimpeded dislocation glide. This work implies that the deleterious effects of irradiation can be reduced by an evolving network of migrating GBs under stress.
