The morphology of planar vacancy aggregates in copper

The morphology of small planar aggregates has been investigated by computer simulation. Clusters of as few as six vacancies have been found to collapse to form loops or stacking fault tetrahedra. This is consistent with the theory that vacancy loops are nucleated directly in individual neutron cascades. Larger triangular clusters collapse and dissociate to form stacking fault tetrahedra by the Silcox-Hirsch mechanism. Hexagonal clusters show a more complex rhombohedral relaxation with extensive dissociation of the Frank dislocation on (111) planes. This may explain the fine structure observed recently in electron microscope images of vacancy loops in copper.