Strengthening and softening mechanisms in nanocrystalline materials under superplastic deformation

A theoretical model is suggested which describes the strengthening and softening mechanisms in nanocrystalline materials under high-strain-rate superplastic deformation. In the framework of the model, the strengthening occurs due to the effects of triple junctions of grain boundaries (GBs) as obstacles for GB sliding. Deformation-induced migration of GBs and their triple junctions gives rise to local softening in nanocrystalline samples under superplastic deformation. With representations of the model, dependences of the yield stress on parameters characterizing the geometry of triple junctions and plastic strain degree are revealed. The results of the model account for experimental data for nanocrystalline materials exhibiting superplasticity, reported in the literature.