A review of theoretical models describing the strengthening and softening mechanisms in nanocrystalline materials under superplastic deformation is given. In the framework of these models, the strengthening occurs due to the effects of triple junctions of grain boundaries as obstacles for grain boundary sliding. The local migration of triple interface junctions (caused by grain boundary sliding) and the emission of lattice dislocations bring about softening of a nanocrystalline material. The flow stress is found as a function of the total plastic strain, and the results agree well with experimental data from nanocrystalline materials exhibiting superplasticity, reported in the literature.
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