Stress-driven migration, convergence and splitting transformations of grain boundaries in nanomaterials

Abstract Convergence and splitting transformations of grain boundaries (GBs) migrating under stress in nanocrystalline and ultrafine-grained materials are theoretically described. With the disclination model of GB junctions, the elastic interaction between migrating GBs that mediate plastic deformation and their response to the external stress are examined. A special attention is devoted to convergence of migrating GBs with their immobile counterparts as well as to their following splitting transformations. Equilibrium migration distances, energy and critical stress characteristics for migration of GBs and their various transformations are calculated. In particular, it is theoretically revealed that a GB migrating under a comparatively high shear stress tends to converge with its immobile counterpart and then splits into two new GBs that migrate in opposite directions. We estimated the critical stress for convergence and splitting transformations of GBs and the saturation grain size in metals (Cu and Ni) processed by severe plastic deformation. Our estimates are consistent with the corresponding experimental data reported in the literature.