An analysis was made of the flow-stress rate-controlling mechanisms which were produced by grain sizes of 10-9 to 10-3m at 300K and strain-rates of 10-4 to 10-3/s. Three grain-size regimes were identified: I (10-6 to 10-3m), II (10-6 to 10-8m) and III (less than 10-8m). Grain-size hardening occurred in regimes I and II, and grain-size softening occurred in regime III. The intersection of pyramidal forest dislocations by basal dislocations was concluded to be the rate-controlling mechanism in regimes I and II, where the main effect of the grain size was to affect forest and glide dislocation densities. The absence of twinning, and a dislocation cell structure, constituted the difference

between regimes II and I. The grain-size softening which was observed in regime III was in closer accord with grain boundary shear than with grain boundary diffusion creep.

Mechanisms for Grain Size Hardening and Softening in Zn. H.Conrad, J.Narayan: Acta Materialia, 2002, 50[20], 5067-78

 Table 12

 Diffusivity of Al in Zr46.8Be27.5Ti8.2Cu7.5Ni10