AN EVALUATION OF THE STRAIN CONTRIBUTED BY GRAIN-BOUNDARY SLIDING IN SUPERPLASTICITY

Grain boundary sliding is an important mode of deformation in superplasticity. Measurements of the contribution of sliding to the total strain generally give values of about 50-70% so that there is an apparent ''missing strain'' of about 30-50%. It is demonstrated that the experimental method used to measure the sliding contribution will lead to estimates in the range of about 45-90% even when all the deformation is by grain boundary sliding and the associated accommodation process. Since the problem of accommodation is less severe at the specimen surface, it is shown that estimates of the sliding contribution from surface marker lines will tend to lie at the lower end of this predicted range. It is concluded that grain boundary sliding accounts for essentially all the deformation under optimum superplastic conditions and there is no ''missing strain'' of about 30-50%.