EFFECT OF GRAIN-SIZE ON MECHANICAL-PROPERTIES OF NANOCRYSTALLINE MATERIALS

The possibility of a dislocation mechanism in the deformation process of nanocrystalline materials is reviewed and analyzed. The present the oretical calculation, by taking the anisotropic characteristic of crys tallographic symmetry and different choices of critical shear strength into account, results in a reasonable limit in grain size for applyin g dislocation pile-up theory to nanocrystalline materials. The deviati on from the Hall-Fetch relationship is rationalized in terms of a smal l number dislocation pile-up mechanism. A composite model is proposed to evaluate the strength of nanocrystalline materials. It is shown tha t this model can be used for interpreting the various cases observed i n Hall-Fetch studies. An analytical expression for assessing the creep rate of nanocrystalline materials by a diffusion mechanism, including triple line diffusion, is derived. It is predicted that the creep rat e due to triple line diffusion will exhibit a stronger grain size depe ndence than that due to grain boundary diffusion.