Effect of grain size from mm to nm on the flow stress and plastic deformation kinetics of Au at low homologous temperatures

Three grain size regimes were identified: Regime I (d > ∼0.5 μm), Regime II (d ≈ 10-500 nm) and Regime III (d < ∼10 nm). Grain size hardening in accord with the Hall-Petch (H-P) equation occurred in Regimes I and II, grain size softening with the flow stress proportional to d−1 occurred in III. The rate-controlling mechanism in Regime I was concluded to be the intersection of dislocations, that in II grain boundary shear promoted by the pile-up of dislocations and that in III grain boundary shear without pile-ups. The transition from I to II occurred when the dislocation cell size became larger than the grain size, that from II to III when the dislocation elastic interaction spacing became larger than the grain size. The H-P behavior in Regime I is in accord with the dislocation density model, that in II with the dislocation pile-up model. The general behavior of Au is similar to that reported previously for Cu and Ag.