Grain boundary sliding during superplastic stretching of AA7475 cylindrical cups

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Topographic and microstructural examinations were performed at the deformed surface and also in the bulk superplastically stretched 7475 aluminum alloy cups. Sliding of grain groups at shear surfaces spaced at approximately four grain diameters was observed at strain level, ∈=0.6. The spacing between shear surfaces of such a cooperative grain boundary sliding (CGBS) decreases when the strain increases; it is approximately three grain diameters at ∈=0.85. The pattern of the CGBS surfaces is consistent with that predicted using slip line field theory. Long range correlated migration of sliding grain boundaries, the formation of dispersoid free zones and fibres have also been observed. These processes can be rationalized in terms of CGBS.

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