Superplastic deformation behaviour of friction stir processed 7075Al alloy

Commercial 7075Al rolled plates were subjected to friction stir processing (FSP) with different processing parameters, resulting in two fine-grained 7075Al alloys with a grain size of 3.8 and 7.5 mum. Heat treatment at 490 degreesC for 1 h showed that the fine grain microstructures were stable at high temperatures. Superplastic investigations in the temperature range of 420-530 degreesC and strain rate range of 1x10(-3)-1x10(-1) s(-1) demonstrated that a decrease in grain size resulted in significantly enhanced superplasticity and a shift to higher optimum strain rate and lower optimum deformation temperature. For the 3.8 mum 7075Al alloy, superplastic elongations of >1250% were obtained at 480 degreesC in the strain rate range of 3x10(-3)-3x10(-2) s(-1), whereas the 7.5 mum 7075Al alloy exhibited a maximum ductility of 1042% at 500 degreesC and 3x10(-3) s(-1). The analyses of the superplastic data for the two alloys revealed a stress exponent of 2, an inverse grain size dependence of 2, and an activation energy close to that for grain boundary self-diffusion. This indicates that grain boundary sliding is the main deformation mechanism for the FSP 7075Al. This was verified by SEM examinations on the surfaces of deformed specimens. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.