Interpretation of superplastic flow in terms of a threshold stress

In several recent experiments on the Zn-22% Al eutectoid and the Pb-62% Sn eutectic, a sigmoidal relationship between stress and strain rate is noted and the mechanical behaviour has been divided into three regions: low-stress region (region I), intermediatestress region (the superplastic region or region II), and high-stress region (region III). In region II, the stress exponent,n, is ≃ 2 and the apparent activation energy,Q, is close to grain-boundary diffusion,Q gb, but in both regions I and III the stress exponent and the activation energy increase (n > 2 andQ >Q gb). Analysis of the experimental data of the two superplastic alloys suggests that the transition in behaviour between region II and region I may not necessarily reflect a change in deformation process but can arise from the presence of a threshold stress which decreases strongly with increasing temperature. Based on consideration of various possible threshold stress processes during superplastic flow, it seems most likely that a threshold stress which depends strongly on temperature may result from impurity atom segregation at boundaries and their interaction with boundary dislocations.