Grain boundary kinetics—II. In situ observations of the role of grain boundary dislocations in high-angle boundary migration

Abstract

The capillary-induced motion of near-Σ5 grain boundaries in Au was directly observed and video-recorded by means of hot-stage in situ transmission electron microscopy. The observed boundary motion differed from the boundary “displacement” caused by the stress-induced passage of secondary grain boundary dislocation (SGBD)-steps along the boundary which was studied in Part I of this work in the following respects. It occurred in a jerky fashion, no translation of one abutting crystal with respect to the other was observed (proving that the motion occurred by the growth of one crystal at the expense of the other), and quantative evidence proved that SGBDs were not involved. These results ruled out the SGBD-Step Model for the observed motion which was identified as common long-range boundary migration. In situ observations of the migration of more general high-angle boundaries in polycrystalline Al films revealed similar jerky motion and an apparent lack of any significant contribution to migration by observed SGBDs. Although it was impossible to rule out absolutely the SGBD-Step Model for these boundaries because all possible SGBDs could not be imaged, its applicability seems highly unlikely. On the basis of these results, it is suggested that the migration of high-angle boundaries, in general, occurs mainly by the local conservative shuffling of atoms across the boundary. These shuffles take place either at pure steps or at other favorable sites, the details of which depend upon the boundary type.