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.

The apparent activation volume as computed from the plot of the logarithm of strain rate vs. yield stress normalized by the respective temperature is shown in the Figure. This is the first time a mapping of the limiting mechanisms for plastic deformation in nanocrystalline Au films has been obtained from experimental data at different strain rates and temperatures. This paper provides a discussion of the deformation limiting mechanisms responsible for the bilinear trends shown in the Figure. The Figure present a strain rate-temperature superposition plot for the calculation of the apparent activation volume using yield stress as the flow parameter. The diagonal dashed line is provided for graphic reasons to indicate the separation of the two deformation regimes discussed in the paper.Figure optionsDownload high-quality image (130 K)Download as PowerPoint slide