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 ≤10 nm, the shear activation volume (6 b³), strain-rate sensitivity ($0.03$), as well as the Helmholtz (0.9 eV) and Gibbs free energy of activation (ΔG=0.2 eV) 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 ΔG∝Δτ3/2, where Δτ is a residual load, giving rise to a generalization of the Johnson-Samwer T2/3 scaling law of yielding in metallic glasses.