Dislocation/boundary interactions in polycrystalline samples of the Ih phase during creep were studied  in situ  by means of synchrotron X-ray topography. The basal slip system with the highest Schmid factor was found to be the most active one in polycrystalline samples, whereas the grain boundary orientation relative to the loading direction seemed to be unimportant. The grain boundaries acted as effective sources of lattice dislocations and as strong obstacles to dislocation motion. The observations revealed pile-up formation upon loading, and pile-up relaxation after unloading. Non-basal segments of lattice dislocations could be generated from the grain boundaries. However, they did not noticeably decrease stress concentrations or contribute significantly to the overall plastic deformation. It was found that dislocations could be generated both from the intersections of grain boundaries with free surfaces, and from the interiors of grain boundaries. This indicated that a previously postulated dislocation generation mechanism was not a surface artefact. Other evidence suggested that, because of the effect of image forces, the analysis of etch pits and whiskers on ice surfaces could not reveal the dislocation behavior in the bulk.

F.Liu, I.Baker, M.Dudley: Philosophical Magazine A, 1995, 71[1], 15-42