INTERACTION OF CRYSTAL DISLOCATIONS WITH GRAIN-BOUNDARIES

Grain boundaries in polycrystalline aluminium have been examined using transmission electron microscopy (TEM) in order to determine the nature of grain boundary dislocations and the interactions that occur when crystal dislocations enter the boundary region. Both annealed and creep specimens have been studied. In annealed material less than half of the grain boundaries were observed to contain dislocations or other fine structure and these were of both coincidence and noncoincidence type. Evidence from coincidence boundaries was obtained which established 1. (1) that crystal dislocations dissociate on entering these boundaries to form perfect dislocations with Burgers vectors of the DSC lattice; 2. (2) that these dislocations are mobile in the boundary; 3. (3) that their mobility depends on the orientation relationship of the adjoining grains and on the boundary plane; 4. (4) mobility is enhanced by boundary migration. Separation of the dissociation products effectively eliminates the long-range strain field of the original crystal dislocation and it is suggested that such dissociation will alter the effectiveness of a boundary as a barrier to further approaching dislocations with consequent effects on the mechanical properties of the material. Evidence is also presented to show that mobility and dissociation of grain boundary dislocations allows dislocations that enter from one side of the boundary to exit from the other with a changed Burgers vector and on a slip plane which is inclined to the first. © 1979.