Suppression of nanocrack generation in nanocrystalline materials under superplastic deformation

A theoretical model is suggested which describes the suppressing effect of grain boundary diffusion on the nanocrack generation in nanocrystalline materials under high strain rate superplastic deformation. In the framework of the model, grain boundary sliding intensively occurs in deformed nanocrystalline materials and causes the storage of grain boundary dislocations at triple junctions of grain boundaries. Relaxation of the stresses of these dislocations occurs via either the nanocrack generation or enhanced diffusional mass transfer in vicinities of triple junctions, depending on the material, structure and deformation parameters. If the former relaxation mechanism is dominant, nanocracks are intensively generated and converge giving rise to the brittle behavior of a nanocrystalline specimen. If the second relaxation mechanism is dominant, diffusion suppresses the nanocrack generation and thereby promotes superplastic deformation in a nanocrystalline material.