Plastic deformation kinetics in nanocrystalline FCC metals based on the pile-up of dislocations

Experimental data on the effect of grain size in the nanocrystalline range (d = 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 v = kT partial derivative In is an element of/partial derivative sigma. It is shown that the anomalous temperature dependence of nu is consistent with the mechanism of grain boundary shear promoted by the pile-up of dislocations, which gives for the shear rate gamma = gamma(o,c) exp-[(Delta F-o,c(*) - nu(*)(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 nu(*)(c) and the pre-exponential gamma(o,c) are in accord with theoretical predictions, i.e. Delta F-o,c(*) approximate to F-b, the activation energy for grain boundary diffusion, nu(*)(c) approximate to 1-10b, where b is the Burgers vector and, gamma(o,c) = 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.