Effects of strain gradient on the mechanical behaviors of nanocrystalline materials

For the purpose of evaluating the effects of strain gradient on the mechanical behavior of nanocrystalline (NC) materials, a new composite constitutive model comprised of grain interior (GI) regarded as an ordered crystal phase and plastically softer grain boundary affected zone (GBAZ) phase with respect to strain gradient has been developed. Due to their dissimilar properties and mismatch between the two phases, dislocation-controlling mechanism based on the statistically stored dislocations (SSDs) and geometrically necessary dislocations (GNDs) was analyzed and extended to NC regime to consider the different influences of two parts in the composite model, respectively. A stress-strain relation for strain gradient plasticity was firstly built to predict the effect of grain size on the flow stress. To describe the strain strength quantitatively, a strain-hardened law determined from strain gradient and a nanostructure characteristic length parameter which differed from that of classical strain gradient theory were introduced in detail. It was shown that the strain gradient in NC materials contributes directly to the mechanical behaviors of this sort of materials and cannot be neglected.