It was recalled that the occurrence of brittle fracture in Ir was thought to be related to the energetics of the dislocation core and, in particular, to the extremely high unstable stacking energy. It was not possible to measure the unstable stacking energy experimentally, but first-principles calculations were used to predict this and the stacking-fault energy. These calculations suggested that, in spite of large differences in the stacking-fault energy and elastic constants, Au and Ir exhibited similar dissociation behaviours; with screw dislocations in both metals being dissociated to the extent of about 1nm. High-resolution transmission electron microscopy was used here to observe the arrangement of the atomic columns which surrounded dislocation cores. Deviations from perfect lattice sites were measured, and experimental observations were quantified by comparison with image simulations. In the case of screw dislocations, the displacement field of atomic columns relative to a perfect lattice was used to determine the extent of in-plane lattice distortion. Comparison with simulated displacement maps permitted the screw dissociation width to be estimated as being equal to 0.8nm. Direct comparison of simulated and experimentally obtained images was used to characterize the core structures of 60° dislocations. These were found to be dissociated by 1.25nm. Weak-beam observations of dissociated dislocations agreed well with high-resolution transmission electron microscopic measurements, and yielded a stacking-fault energy of 420mJ/m2. High-resolution and weak-beam measurements of the dissociation distance both agreed with the orientation-dependence of the stacking-fault width; as predicted by anisotropic elasticity.

High Resolution Transmission Electron Microscopy of Dislocation Core Dissociations in Gold and Iridium. T.J.Balk, K.J.Hemker: Philosophical Magazine A, 2001, 81[6], 1507-31