An internal variable approach to the grain size effect on the superplastic deformation 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 internal variable theory of structural superplasticity. A simple rheological model, including grain boundary sliding (GBS) and accommodating grain matrix plastic deformation (GMD), has been employed to interpret the plastic flow behavior of superplastic 7475 Al alloy at 516°C. A series of load relaxation and tensile tests have been carried out after obtaining various grain sizes ranging from 13 to 42 μm through proper thermomechanical treatment processes. The flow curves of log σ vs. log ε̇ have been found to be composite curves consisting of GBS and accommodating GMD by dislocation. The GBS appears to be a Newtonian viscous flow characterized by the power index value of Mg=1.0. A Hall–Petch type relation has been found between the grain size used in this study and an internal strength variable (σ*). The decrease in grain size appears to lower the friction stress for GBS (Σg) and to enhance the superplasticity due to GBS.