Effect of grain boundary character distribution (GBCD) on the cavitation behaviour during superplastic deformation of Al 7475

The formation of cavity stringers in superplastically deformed Al 7475 alloy was investigated in this work. After superplastic deformation, cavity stringers have a tendency to form intergranularly along grain boundaries that are parallel to the rolling direction of the material. The microstructure and microtexture showed that the material has anisotropic grain boundary character distribution (GBCD). In the as-received state, small precipitates were found to be distributed preferentially along the grain boundaries which are parallel to the rolling direction. After superplastic deformation, a low melting point eutectic phase was found along the grain boundaries, which are also mainly parallel to the rolling direction. At the test temperature of 516 degreesC, the eutectic phase is expected to be in the liquid state. The presence of the liquid phase reduces the grain boundary bonding strength. It is proposed that the formation of cavity stringers is closely related to the GBCD. Based on the anisotropic distribution of GBCD and extensive grain boundary sliding during superplastic deformation, a model is proposed to explain the formation of cavity stringers. (C) 2002 Elsevier Science B.V. All rights reserved.