Dislocation-disclination models of grain boundary migration in ultrathin nanocrystalline films

The dislocation-disclination models describing the athermal migration of grain boundaries in stretched ultrathin nanocrystalline films have been proposed. The cases where the grain boundary emerges on a free surface of the film or is located in its central region have been considered. The changes in the total energy of the system due to the migration of the grain boundary have been calculated, the critical stresses of the onset of migration and the transition from a stable migration to an unstable migration have been determined, and the equilibrium positions of the grain boundary have been found. The dependences of the calculated parameters on the length, the misorientation angle, the position, and the orientation of the grain boundary in the film, as well as on the film thickness, have been investigated. It has been shown that the critical stresses responsible for the onset of migration of the grain boundary and its transition to an unstable regime decrease with a decrease in the thickness of the film. The critical stresses determining the transition from the stable migration to the unstable migration decrease with an increase in the grain size. The closer is the grain boundary to the surface of the film, the more pronounced is the tendency of the grain boundary toward migration.