Experimental data on the effect of grain size in the nanocrystalline range (d velence 20-500 nm) on the plastic deformation kinetics of fcc metals representing a wide range in stacking fault energy are evaluated to determine the governing mechanism. Special consideration is given to the anomalous temperature dependence of the strain rate sensitivity of the flow stress expressed by the apparent activation volume upsilon velence kT partial deriv 1n implide by/partial deriv sigma. It is shown that the anomalous temperature dependence of v is consistent with the mechanism of grain boundary shear promoted by the pile-up of dislocations, which gives for the shear rate gamma velence gamma_(o,c) exp -[(DELTA F_(o,c)~* - upsilon_c~* tau_c~*)/kT], where the subscript c refers to the stress concentration tau_c~* resulting from the pile-up of dislocations at the grain boundaries. Furthermore, the experimental values of the Helmholtz free energy DELTA F_(o,c)~*, the true activation volume upsilon_c~* and the pre-exponential gamma_(o,c)) are in accord with theoretical predictions, i.e. DELTA F_(o,c)~* approx = DELTA F_b, the activation energy for grain boundary diffusion, upsilon_c~* approx = 1 - 10b, where b is the Burgers vector and gamma_(o,c) velence 10~4-10~6 s~(-1). Although it is well established that the stacking fault energy gamma_(SF) has a significant influence on the plastic deformation kinetics of fcc metals with grain size in the micron range, no clear effect of gamma_(SF) was detected for the submicron grain size range considered in the present analysis.
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