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

Abstract

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 stackingfault energy are evaluated to determine the governing mechanism. Special consideration isgiven to the anomalous temperature dependence of the strain rate sensitivity of the flowstress expressed by the apparent activation volume . It is shown that the anomalous temperature dependence ofυ is consistent with the mechanism of grain boundary shear promoted by the pile-up ofdislocations, which gives for the shear rate , where the subscript c refers to the stress concentrationτc* resulting from the pile-up of dislocations at the grain boundaries.Furthermore, the experimental values of the Helmholtz free energyΔFo,c*, the trueactivation volume υc* and the pre-exponential are in accord with theoretical predictions,i.e. ΔFo,c*≈ΔFb, the activation energy for grain boundary diffusion,υc*≈1−10b,where b is the Burgers vector and . Although it is well established that the stacking fault energyγSF has a significant influence on the plastic deformation kinetics offcc metals with grain size in the micron range, no clear effect ofγSF was detected for the submicron grain size range considered in the present analysis.