Vacancy generation mechanism at high temperatures in ultrahigh-purity aluminum single crystals with a low dislocation density

Abstract

The vacancy generation mechanism at high temperatures in ultrahigh-purity (7-N) aluminum single crystals with a low dislocation density was investigated by synchrotron radiation topography using a white X-ray beam. The specimens were prepared in vacuum using a strain-annealing method from zone-refined aluminum (1×10 5 residual resistance ratio). The dislocation density of the specimens was decreased to 1×10 3 cm −2 by cyclic annealing. Straight lines were observed in the topographs taken after a temperature rise from room temperature to 300 °C, and were confirmed to be rows of successive small interstitial-type dislocation loops grown as vacancy sources. It was concluded that the thermal generation mechanism of vacancies in ultrahigh-purity aluminum single crystals with a low dislocation density consists of two steps. First, small interstitial loops are heterogeneously formed in the crystal lattice. Second, these convert to lengthened loops with the development of screw components and finally grow into rows of dislocation loops emitting vacancies into the lattice through parallel helical dislocations.