In situ observations of grain boundary sliding and associated accommodation mechanisms in experimentally pure- and simple-sheared octachloropropane at 75-85% of its absolute melting temperature and at strain rates of 10-5-10-6 s-1 are discussed. Discontinuities in the strain, rotation and/or translation components of deformation across the grain boundary induce grain boundary sliding. The influence of crystallographic orientation of grains on grain boundary sliding is so strong that grains unfavorably oriented for basal slip contribute to deformation mainly by grain boundary sliding. Grain boundary diffusion and intragranular plastic deformation are observed to accommodate grain boundary sliding. Grain boundary diffusion also causes grain boundary migration (Types II and III) that is different from conventional grain boundary migration (Type I) in being non-conservative, and in the details of boundary movement with respect to material points within grains. Grain boundary openings filled with fluid (octachloropropane vapor) are strongly associated with grain boundary sliding. Openings develop preferentially along grain boundaries at low angles to the shortening direction. Once openings grow, they are closed by thrusting of sliding grains and by diffusion in faster strain-rate experiments, and entirely by diffusion in slower strain-rate experiments. An approximately steady openings ratio of 0.5-3% of the sample volume persists without the development of any large-scale fracture. All grain boundary openings disappear during static readjustment of the microstructure after deformation.
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