Mechanisms for grain size hardening and softening in Zn

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 (epsilon) over dot = 10(-4) - 10(-3) s(-1) was performed. Three grain size regimes were indicated: Regime 1, d approximate to 10(-6) - 10(-3) m, Regime 11, d 10(-6) - 10(-8) m and Regime 111, d < 10(-8) m. Grain size hardening occurred in Regimes I and Il 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 11, 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. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.