Dynamic observations of dislocation generation at grain boundaries in ice

Dynamic in situ X-ray topographic deformation studies have been performed on polycrystalline ice. Based on these observations, a new and dominant mechanism for dislocation nucleation, which is related to stress concentrations observed in the vicinity of grain boundaries, is proposed. It was found that the areas near grain boundaries always deform before the grain interiors. Lattice dislocations were nucleated continuously at large-angle grain boundaries, driven by internal stresses that are higher than the external stress. The dislocations, once generated, glide on the basal plane as semi-hexagonal loops. The shape of these loops is a result of a balance of the stresses present. The dislocation generation mechanism at grain boundaries was found to depend strongly on the basal plane orientation relative to both the loading direction and the grain boundary plane.