On the physical nature of optimal superplastic flow

Grain/interphase boundary sliding in high temperature creep leads to boundary embrittlement but its dominance under certain conditions can also cause superplastic flow. Thus, there is a need to distinguish between the types of boundary sliding present in the two phenomena. In creep, sliding is present on a microscopic scale at different boundaries. Steric hindrance renders this form of sliding rather ineffective. In contrast, during superplastic flow two or more boundaries connect to form the plane interface needed for mesoscopic/cooperative boundary sliding. Such a plane interface connects with other plane interfaces to form a continuous network in three dimensions of deforming grain/interphase boundaries. Transmission electron microscopic evidence for plane interface formation due to mesoscopic sliding is available. The long range threshold stress, σo, needed to cause mesoscopic sliding can be calculated and the difference between the applied stress and σo is available for microscopic sliding at a boundary. The rate equation developed to describe this process is regarded as controlling the rate of optimal superplastic flow. Diffusion and/or dislocation motion needed for facilitating/accommodating mesoscopic sliding will have no effect on the rate equation so long as they do not contribute independently to the external strain rate.