In situ TEM measurement of strain rate-dependent activation volume of ultrafine-grained Au films

Understanding the rate-controlling plastic deformation mechanisms in ultrafine-grained metals is essential for improving their mechanical performance. Strain rate sensitivity and the associated activation volume are commonly used to probe these underlying mechanisms. In this study, we investigate the strain rate-dependent deformation behavior of ultrafine grained gold films using an in situ transmission electron microscopy (TEM) nanomechanical testing technique. This approach enables direct measurement of the strain rate sensitivity and physical activation volume across a wide range of strain rates (∼10 -5 to 1 s -1 ), while observing dynamic dislocation processes. The results reveal a marked decrease in strain rate sensitivity with increasing strain rate, accompanied by an increase in physical activation volume, suggesting a transition in the rate-limiting mechanisms. In situ TEM observations capture a transition from single-slip to multi-slip activity and the formation of dislocation junctions at higher strain rates. Integrated experimental and modeling analyses reveal that the rate-controlling mechanisms are governed not only by the external loading condition but also by the internal stress state, evidenced by transient dislocation dynamics and evolving dislocation structures observed through in situ TEM straining experiments.