Mechanisms for grain size hardening and softening in Zn

Abstract An analysis of the rate-controlling mechanisms corresponding to effect of grain size d =10 −9 to 10 −3 m on the flow stress of Zn at 300K and ϵ = 10 -4 -10 -3 s −1 was performed. Three grain size regimes were indicated: Regime I, d ≈10 −6 –10 −3 m, Regime II, d ≈10 −6 –10 −8 m and Regime III, d −8 m. Grain size hardening occurred in Regimes I and II and grain size softening in Regime III. The intersection of pyramidal forest dislocations by basal dislocations was concluded to be the rate-controlling mechanism in both Regimes I and II, the major effect of the grain size being on the forest and gliding dislocation densities. The absence of twinning and a dislocation cell structure distinguished Regime II from I. The grain size softening observed in Regime III is in better accord with grain boundary shear than with grain boundary diffusion creep.