Stress-driven Rotations of Deformation-distorted Grain Boundaries in Nanocrystalline and Ultrafine-grained Materials

摘要

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 φ_eq associated with the energy minimum. Dependences of φ_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.