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Direct observation of superplastic deformation in shear in a scanning electron microscope in Pb-62% Sn eutectic alloy at different microstructural levels were performed. The sliding of grains as an entity was observed. Such cooperative grain-boundary sliding proceeds by means of the sequential shear of grains along shear surfaces, and is accompanied by cooperative grain-boundary migration. The pattern of shear surfaces at the level of the entire deformed volume is close to one that is predicted by slip-lines field theory. The processes to accommodate deformation of grain-boundary sliding include the operation of grain-boundary migration, intragranular deformation, and grain rotation. A critical analysis of the proposed geometrical models for superplastic flow is given. The model of sequential shear of grains as a result of the movement of cellular dislocations is extended to the case of the two-phase material, having structure that is typical for superplastic eutectics.

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