Transformations of grain boundaries in deformed nanocrystalline materials

Theoretical models are suggested which describe transformations of grain boundaries in nanocrystalline materials under plastic deformation. We consider such transformations as decay of low-angle grain boundaries, bowing of high-angle grain boundaries, and emission of partial dislocations by grain boundaries in deformed nanocrystalline materials. In the framework of the suggested model description, lattice dislocations that form a low-angle tilt boundary glide under the action of the forces owing to external (applied) and internal stresses. The balance of the forces causes the critical shear stress at which the low-angle boundary decays. Such decay processes result in the formation of high-density ensembles of mobile lattice dislocations that are capable of inducing plastic flow localization (shear banding) in mechanically loaded nanocrystalline materials. High-angle grain boundaries are modeled as those containing grain boundary dislocations with small Burgers vectors. The movement of grain boundary dislocations under the shear stress action gives rise to bowing of high-angle boundaries. In certain ranges of parameters, grain boundary dislocations undergo splitting transformations followed by emission of partial dislocations from high-angle boundaries into adjacent grain interiors. The models account for experimental data reported in the literature.