Effect of grain size from millimeters to nanometers on the flow stress and deformation kinetics of Ag

Data on the effects of grain size, d = 9 nm to 0.26 mm on the flow stress and plastic deformation kinetics of Ag at low homologous temperatures were evaluated. Two regimes were identified for this grain-size range: Regime I (d > 500 nm) and Regime II (d ≈ 10-500 nm). The effect of grain size on the flow stress in both regimes could be described by a Hall-Petch equation. KH-P was however significantly smaller and σi larger for Regime II compared to I. Dislocation activity occurred in both regimes; dislocation cells were observed in Regime I, but not reported for II. The available data suggest that the dislocation density model governs the grain-size dependence of the flow stress in Regime I and that dislocation pile-up applies in Regime II. The rate-controlling mechanism(s) in each of the two regimes are discussed.