An internal variable approach to the grain size effect on the superplasticdeformation behavior of a 7475 Al alloy

The effect of grain size on the superplastic deformation behavior of a 7475 Al alloy has been studied within the framework of the recently proposed in ternal variable theory of structural superplasticity. A simple theological model, including grain boundary sliding (GBS) and accommodating grain matri x plastic deformation (GMD), has been employed to interpret the plastic flo w behavior of superplastic 7475 Al alloy at 516 degrees C. A series of load relaxation and tensile tests have been carried out after obtaining various grain sizes ranging from 13 to 42 mu m through proper thermomechanical tre atment processes. The flow curves of log sigma vs. log (epsilon) over dot h ave been found to be composite curves consisting of GBS and accommodating G MD by dislocation. The GBS appears to be a Newtonian viscous flow character ized by the power index value of M-g=1.0. A Hall-Fetch type relation has be en found between the grain size used in this study and an internal strength variable (sigma*). The decrease in grain size appears to lower the frictio n stress for GBS (Sigma(g)) and to enhance the superplasticity due to GBS. (C) 1999 Elsevier Science S.A. All rights reserved.