STRESS-DRIVEN ROTATIONS OF DEFORMATION-DISTORTED GRAIN BOUNDARIES IN NANOCRYSTALLINE AND ULTRAFINE-GRAINED MATERIALS

A special mechanism/mode of plastic deformation occurring through stress-driven rotations of deformation-distorted grain boundaries (GBs) in nanowires, micropillars, thin films and subsurface areas of bulk solids with nanocrystalline and ultrafine-grained structures is theoretically described. The suggested approach serves as a generalization of the theoretical model [Bobylev and Ovid'ko, Phys. Rev. Lett. 109, 175501 (2012)] describing stress-driven rotations of regular (non-distorted) low-angle tilt boundaries composed of periodically arranged lattice dislocations in crystals. In the exemplary case of nickel specimens with nanocrystalline and ultrafine-grained structures, it is found that the rotations of deformation-distorted GBs are energetically favorable processes in wide range of GB parameters. Each energetically favorable GB rotation is specified by its equilibrium rotation angle phi(eq) associated with the energy minimum. Dependences of phi(eq) on applied stress, GB misorientation and other geometric characteristics of a rotating GB are calculated which show the trends in realization of stress-driven rotations of deformation-distorted GBs in nanocrystalline and ultrafine-grained solids. Also, combined splitting and rotations of deformation-distorted GBs are theoretically described as energetically favorable processes in wide ranges of parameters characterizing GB configuration. These processes result in formation of nanoscale grains in nanocrystalline and ultrafine-grained solids. Our theory is consistent with the corresponding experimental data reported in the literature.