Strength softening and stress relaxation of nanostructured materials

A composite model is proposed to rationalize the phenomena of strength softening with decreasing grain size for nanostructured materials, which is assumed to consist of a grain interior and an amorphous grain-boundary layer. The grain interior deforms elastically under external stresses, while the linear viscoelastic flow is responsible for the plastic deformation of the grain-boundary layer, whose stress "relaxation" follows Maxwell's equation. The results indicate that the strength of a nanostructured material decreases linearly with decreasing grain size, when the grain size is below a certain threshold. The model is compared with the experimental data from the published studies on nanostructured Cu and Ni. The relevant creep mechanisms for nanostructured materials are also discussed in light of model predictions.