Mechanisms for grain size hardneing and softening in Zn

摘要

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 < 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 the be 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.