Important features observed during high strain rate superplastic deformation were enumerated. Starting from the premise that the phenomenon of structural superplasticity in different classes of materials results when grain boundary sliding that develops to a mesoscopic scale (defined to be of the order of a grain diameter or more) controls the rate of flow, the particular case of high strain rate superplasticity was explained. The rate equation developed was validated using experimental results concerning 5 alloy systems in which an ultra-fine grain size was developed by thermomechanical processing and retained in a similar condition during superplastic deformation by fine, grain boundary pinning particles and 3 alloy composites in which the volume fraction of the reinforcing constituent was significant (15–25%). It was demonstrated that the analysis results in estimates for the externally measured strain rates that were within a factor of two, in addition to providing a physically meaningful free energy of activation for the rate controlling process. This approach explains superplastic flow in different classes of materials in terms of a single rate controlling mechanism of deformation, viz., mesoscopic grain boundary sliding, with the help of a few constants that have the same values for all systems. The system-dependent variables of threshold stress needed for the onset of mesoscopic boundary sliding and free energy of activation were obtained directly from superplasticity stress–strain rate data, without external inputs.

Mesoscopic Grain Boundary Sliding as the Rate Controlling Process for High Strain Rate Superplastic Deformation. K.A.Padmanabhan, M.R.Basariya: Materials Science and Engineering A, 2009, 527[1-2], 225-34