Inelastic deformation of nanocrystalline Au thin films as a function of temperature and strain rate

The mechanical behavior of nanocrystalline Au thin films with average grain size of 64 nm was investigated at strain rates 10^?5–10 s^?1, and temperatures between 298 and 383 K. The yield strength was highly sensitive to both temperature and strain rate: at room temperature it increased by ～100% within the range of applied strain rates, while it decreased by as much as 50% in the given temperature range at each strain rate. The ductility and activation volume trends pointed to two distinct regimes of plastic deformation: namely, creep-driven and dislocation-mediated plasticity, with the transition occurring at increasing strain rate for increasing temperature. The activation volume for creep-influenced deformation increased monotonically from 6.4b³ to 29.5b³ between 298 and 383 K, signifying grain boundary (GB) diffusion processes and dislocation-mediated creep, respectively. Dislocation climb, as an accommodation mechanism for GB sliding, provided an explanation for the increased activation volume at higher temperatures. The activation volumes calculated at high strain rates decreased from 19.7b³ to 11.4b³ between 298 and 383 K. A model for thermally activated dislocation depinning was applied to explain this abnormal decreasing trend in the activation volume, resulting in activation energy of 1.2 eV.