Row of Dislocation loops as a Vacancy Source in Ultrahigh-Purity Aluminum Single Crystals with a Low Dislocation Density

The vacancy generation process in ultrahigh-purity aluminum single crystals with a low dislocation density was investigated by synchrotron radiation topography using a white X-ray beam. Some straight lines were observed in the topographs taken after temperature rose to 300℃ from room temperature, and they 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 the following 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. However, contribution of new vacancy generation mechanism, growth of row of interstitial type dislocation loop for thermal equilibrium vacancy concentration is less than several percent. Therefore, major vacancy source is small vacancy cluster or vacancy type dislocation loops grown after slow cooling during crystal growth.