Effects of strain accumulation on the superplastic deformation behavior of7075 AL alloy

The superplastic deformation behavior of a fine-grained 7075 Al alloy has b een investigated within the framework of an internal variable theory for in elastic deformation, The theory takes the dislocation glide process within and across the grain boundaries (grain matrix deformation (GMD)) as the maj or accommodation mechanism for the grain boundary sliding (GBS), The flow c urves mere obtained by performing a series of load relaxation tests at the various prestrain values to examine the effects of accumulated strain on th e superplastic deformation behavior. The most significant result obtained i n this study is that the grain boundary characteristics change gradually wi th the strain accumulation from an initially Newtonian viscous flow signifi ed with the power index value of M-g = 1 to a non-Newtonian flow with the v alue of M-g = 0.5 commonly observed in the various microduplex alloys such as Ti-6Al-4V, The variation of GBS characteristics with the prestrain is th en examined by observing the microstructural evolution with the strain thro ugh the use of transmission electron microscopy (TEM).