A theoretical model was proposed to describe the physical mechanisms of hardening and softening of nanocrystalline materials during superplastic deformation. According to this model, triple interface junctions were obstacles to glide motion of grain boundary dislocations, which were carriers of grain boundary glide deformation. Transformations of an ensemble of grain boundary dislocations that occurred at triple interface junctions bring about the formation of partial dislocations and the local migration of triple junctions. The energy characteristics of these transformations were considered. Pileups of partial dislocations at triple junctions cause hardening and initiate intragrain lattice sliding. When the Burgers vectors of partial dislocations reach a critical value, lattice dislocations were emitted and glide into adjacent grains, thereby smoothing the hardening effect. 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 was found as a function of the total plastic strain, and the result agrees well with experimental data.

Grain Boundary Sliding and Lattice Dislocation Emission in Nanocrystalline Materials under Plastic Deformation. M.Y.Gutkin, I.A.Ovidko, N.V.Skiba: Physics of the Solid State, 2005, 47[9], 1662-74