Shear shuffling governs plastic flow in nanocrystalline metals: An analysis of thermal activation parameters

From strain rate- and temperature-dependent deformation studies on nanocrystalline PdAu alloys with grain sizes $\leq$ 10nm, the shear activation volume $6b^3$, strain rate sensitivity 0.03 as well as the Helmholtz 0.9eV and Gibbs free energy of activation 0.2eV have been extracted. The close similarity to values found for metallic glasses indicates that grain boundary mediated shear shuffling dominates plasticity at the low end of the nanoscale. More fundamentally, we find that the energy barrier height exhibits universal scaling behavior $\Delta G \propto \Delta\tau^{3/2}$, where $\Delta\tau$ is a residual load, giving rise to a generalization of the Johnson-Samwer $T^{2/3}$ scaling law of yielding in metallic glasses.