Nonequilibrium grain boundaries in bulk nanostructured metals and their recovery under the influences of heating and cyclic deformation. Review

Bulk nanostructured, or ultrafine-grained (UFG) metals and alloys produced by severe plastic deformation (SPD) methods have grain boundaries (GBs), which are in a specific, non-equilibrium state associated with extrinsic grain boundary dislocations (EGBDs) introduced into the boundaries during deformation. In the present review, the origin of this state is analyzed basing on the results of studies of large plastic deformations, according to which during straining the GBs accumulate mesodefects consisting of Rybin disclinations at triple junctions and tangential EGBD arrays and inherit them after removing the load. The main experimental evidences of the nonequilibrium character of GBs in as-prepared nanostructured materials are presented. Main ideas and results of the structural model of UFG metals based on the physics of dislocations and disclinations are overviewed. The latest works on molecular dynamics simulations carried out to explore the detailed atomic structure of disclinations and nonequilibrium GBs are considered and their results are compared to the predictions of the structural model. Mechanisms and kinetics of diffusion controlled recovery of nonequilirium GBs are described. An alternative mechanism of athermal relaxation of nonequilirium GB structure under the action of oscillating stresses has been most recently discovered and elucidated experimentally and by means of computer simulations. The main results of these studies are shortly reviewed too.