Dislocation-grain boundary interactions in ice crystals

Dislocation-grain-boundary (GB) interactions in polycrystalline ice I_h during creep have been studied in situ using synchrotron X-ray topography. The basal slip system with the highest Schmid factor was found to be the most active in polycrystalline ice whereas the GB orientation relative to the loading direction seemed unimportant. GBs act both 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 can be generated from GBs in ice. However, they neither noticeably decrease stress concentrations nor contribute significantly to the overall plastic deformation. It was found that dislocations can be generated from both free-surface-GB intersections and from the interiors of GBs, indicating that the dislocation generation mechanism presented in our 1993 paper is not a surface artefact. Evidence is also presented that, because of the effect of image forces, analyses of etch pits and whiskers obtained on ice surfaces cannot reveal dislocation behaviour in the bulk. The generality of these observations is discussed.