It was found that vacancy-type loops appeared at 230C, in nearly perfect crystals, even during slow cooling from 300C; as revealed by synchrotron radiation topography using white radiation. The temperature of the specimen was cycled between 300 and 230C, at a cooling rate of 2000C/h and a heating rate of 1000C/h. Almost all of the vacancy loops appeared at the same place during each cooling cycle, and some loops were formed along dislocation lines which had been observed during the preceding stage. The formation of these loops could not be explained in terms of homogeneous nucleation theory. It was therefore concluded that the loops which grew at fairly high temperatures nucleated heterogeneously (probably at inclusions). Almost all of the vacancy loops that remained in a nearly perfect crystal were formed by heterogeneous nucleation during cooling.

K.Mizuno, A.Tamiya, K.Ono, M.Iwami, E.Hashimoto, T.Kino: Journal of Crystal Growth, 1996, 166, 146-50