Interfacial defects, such as grain boundary dislocations, play an important role in the creep behavior of alumina. In the present work, interfacial defects are analyzed in detail using a Volterra approach without a reference to a near-coincidence description. We investigate disconnections (boundary steps with dislocation character) in a diffusion-bonded alumina bicrystal, with a misorientation close to the rhombohedral twin, by conventional and atomic resolution electron microscopy. The bicrystal contains two arrays of parallel disconnections with Burgers vectors that have alternating equal and opposite twist components, so there is no long-range stress field. This configuration is discussed in terms of the stability of different grain boundary disconnection arrangements. The complex core structure of the defects is revealed by high resolution electron microscopy using exit wave reconstruction. It is shown that the defects are dissociated into two partials that delimit grain boundary segments with alternating structure.
The interaction between a glissile cluster of self-interstitial atoms and a vacancy has been studied in f -Fe by atomistic modelling and elasticity theory. It was found that vacancies can annihilate only with the cluster edge. Vacancies inside the cluster glide prism do not annihilate with interstitials but affect the cluster dynamic properties. Depending on the cluster size and ambient temperature, these vacancies reduce or even prevent cluster motion. Qualitative differences in the results of atomistic and elasticity theory approaches were found, thereby demonstrating the need for the atomistic approach.
Recent computer simulation studies have shown that small clusters of selfinterstitial atoms (SIAs) formed in displacement cascades consist of crowdions and are highly mobile in the crowdion direction. In the present work, we use molecular dynamics to investigate whether small perfect vacancy loops formed in cascades in alpha-Fe and Cu are also mobile. Loops containing more than about 30 vacancies in Fe are found to produce atomic displacements during annealing, due to thermally activated movement in the direction of their Burgers vector that is qualitatively similar to the mechanism of SIA cluster motion. Although vacancy clusters are slightly less mobile than SIA clusters under the same conditions, their mobility is significantly higher than that of the monovacancy. The motion of vacancy loops in Cu does not occur because they transform into sessile configurations similar to stacking-fault tetrahedra. These results point to the possibly important contribution of vacancy loop mobility to the difference in radiation damage between bcc and fcc metals, and between fcc metals with different stacking-fault energies.
A (1012) twin boundary in deformed alpha-Ti has been observed by highresolution electron microscopy. The observations are in agreement with a model calculated using a many-body potential. A twinning dislocation is predicted by the topological theory and characterized directly in the experimental image with the circuit mapping method. It is shown that the core defect is spread along the twin plane as predicted by the atomistic model. Finally, the observation of a large step associated with a dislocation with Burgers vector b10/9 points to the mechanism of matrix dislocation decomposition in the twin boundary.
The interfacial structure of  twins obtained using atomic-scale computer simulation by Serra, Pond and Bacon in 1991 is shown to be qualitatively consistent with high-resolution transmission electron micrographs of such twins in Ti obtained by Kasukabe, Yamada, Peng and Bursil in 1993. In addition, images of two twinning dislocations are analysed using a circuit mapping method to determine their Burgers vectors and to demonstrate that they correspond to defects studied previously using computer simulation by Serra, Bacon and Pond in 1988 and by Serra, Pond and Bacon in 1991.