A study was made of the twins which formed during recrystallization annealing. The results confirmed previous measurements which had shown that the number of annealing twins which formed in single-phase Al was extremely small. However, it was noted that their number could increase markedly at either a free surface, at large second-phase particles, or at both. It was pointed out that the details of the mechanism via which annealing twins formed in face-centered cubic metals were still not clear. However, it was suggested that the twins were formed at moving grain boundaries by the initiation of growth faults on {111} planes. It was expected that such a twin would nucleate at a high energy interface. In the interior of single-phase alloys, such a site was likely to be a grain boundary triple-point. A large second-phase particle, at which recrystallization nucleated, was also a likely site. A free surface, which had an energy that was about 3 times a that of a high-angle grain boundary was expected to act in a similar manner. The frequency of twinning at a free surface during  in situ  annealing in a scanning electron microscope showed that great care had to be taken in using the results of such experiments to predict the annealing behavior within the interior of a specimen. The presence of 2 free surfaces was expected to promote twinning to an even greater extent, and it was recalled that extensive twinning had been observed during  in situ  high-vacuum electron microscopic annealing.

F.J.Humphreys, M.Ferry: Scripta Materialia, 1996, 35[1], 99-105