Reduction of large vacancy clusters in nearly perfect aluminum single crysatals

Large vacancy clusters in aluminum single crystals with low dislocation density, which show up as black dots in X-ray topographs, generate new dislocations and stacking faults during heat treatments. In order to obtain dislocation-free or low-dislocation-density metal single crystals, it is necessary to suppress the formation of large vacancy clusters. To this end, our starting material was either (1) ultrahigh-purity aluminum, intended to minimize the number of nucleation sites for large vacancy clusters, or (2) a dilute alloy of Zn in Al, intended to block the migration of excess vacancies by binding them with zinc atoms during slow cooling. Single crystals of the Al-Zn dilute alloy failed to improve the perfection unless a large vacancy cluster was formed. Upon cyclic annealing, however, the number density of black dots in X-ray topographs of ultrahigh-purity aluminum crystals decreased rapidly and significantly. It was thus confirmed that using a high-purity starting material was effective in suppressing the formation of large vacancy clusters.