Mechanisms for grain size hardening and softening in Zn Original Research Article

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 ϵ̇=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<10−8 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.