Grain boundary dislocation models for grain boundary sliding and superplastic flow

A simple model for 'pure' grain boundary sliding at an isolated boundary by motion of the intrinsic grain boundary dislocation network is described, and its predictions compared with literature results from bicrystals. The similarities and differences between such sliding and superplastic flow of polycrystals are examined. An existing model for superplastic flow based on the crystallography of secondary grain boundary dislocations is outlined, and extended to predict the kinetics of steady state deformation. The model correctly predicts the classical kinetics of superplastic flow. Experiments over a wide range of temperatures (60 degrees C-200 degrees C) on specimens of Zn-22%Al with widely varying grain size (0.28 mu m-3.5 mu m) are described. It was found that both the strain rate sensitivity index, m, and the grain size exponent, p, got smaller as the temperature was reduced. The sum 1/m+p remained approximately constant at similar to 4, as is predicted by the model for superplastic now described.