Abstract
Studies are described of vacancy clusters, twin boundary structures and vacancy migration at twin boundaries. Much of the work involves atomistic computer simulation techniques based on a short ranged, central, 'non-equilibrium' pair potential representing copper. This potential is matched to the experimental lattice parameter, the elastic constants and the vacancy formation and intrinsic stacking fault energies. A geometrical-mathematical method is adopted to study the structures of vacancy clusters and their transformations (growth, contraction and migration). The implications of collapse of these clusters are also studied. The relative values of cluster migration energies are estimated from the number of super nearest neighbours (atoms which lie at a distance less than the first nearest neighbour, from a migrating self interstitial atom). The structures of some low index twin boundaries e. g. (111), (113), (112) and (120), are investigated and their low energy configurations are established. The (112) and (120) boundaries contain some coalesced planes (planes formed from the combination of two neighbouring planes parallel to the interface). The (111) and (113) boundaries do not reveal this unusual behaviour. The effect is interpreted in terms of atomic density of the interface plane. The simple and important (111) twin boundary is selected to study vacancy migration. Three distinct paths are chosen, (a) in the boundary, (b) parallel and adjacent to the boundary, and (c) between the boundary and its adjacent plane. For each of these cases the migration process involves the simulation of a model in which a self interstitial atom is moved in steps between the two sites of a divacancy. At each step the model is allowed to relax with the motion of interstitial restricted to a plane normal to the axis of the divacancy. In this way it is possible to deduce the actual migration path. Reductions of 2-6% are found in the vacancy migration energy.