Load relaxation behavior of superplastic Al alloys

Abstract

The high temperature deformation behavior of superplastic Al alloys, such as 8090 AI-Li and 7475 Al, has been studied experimentally within the framework of an internal variable theory of inelastic deformation. A simple rheological model including the grain boundary sliding (GBS) has been developed and used to interpret the plastic flow behavior of these alloys at high temperatures. A series of load relaxation tests has been carried out for fine grained specimens of superplastic Al alloys at various temperatures. The effect of grain size has also been examined. A Hall-Fetch type relation has been found between the grain size and an internal strength variable (sigma*), instead of generally used flow stress (sigma). Based on the experimental observation, a necessary condition for superplastic deformation has finally been proposed in terms of sigma* and the static friction stress for GBS (Sigma(g)) as Sigma(g) less than or equal to sigma less than or equal to sigma*.