THE DIRECT OBSERVATION OF COOPERATIVE GRAIN-BOUNDARY SLIDING AND MIGRATION DURING SUPERPLASTIC DEFORMATION OF LEAD-TIN EUTECTIC IN SHEAR

Direct observation of superplastic deformation in shear in a scanning electron microscope in Pb-62% Sn eutectic alloy at different microstru ctural levels were performed. The sliding of grains as an entity was o bserved. Such cooperative grain-boundary sliding proceeds by means of the sequential shear of grains along shear surfaces, and is accompanie d by cooperative grain-boundary migration. The pattern of shear surfac es at the level of the entire deformed volume is close to one that is predicted by slip-lines field theory. The processes to accommodate def ormation of grain-boundary sliding include the operation of grain-boun dary migration, intragranular deformation, and grain rotation. A criti cal analysis of the proposed geometrical models for superplastic flow is given. The model of sequential shear of grains as a result of the m ovement of cellular dislocations is extended to the case of the two-ph ase material, having structure that is typical for superplastic eutect ics.