Re-examination of “missing strain” during superplastic deformation of AA7475

Missing strain,” a discrepancy between the total macroscopic strain and the strain contributed by grain boundary sliding (GBS) during superplastic deformation, appears to exist in previous investigations. In this work, the contribution of GBS (ξGBS) and intragranular strain (ξIG) were simultaneously measured using scratch test conducted on AA7475 samples after deformation at 500 °C at initial strain rate of 10−3 s−1. The result shows that the missing strain results most probably from the neglect of the intragranular strain as well as the anisotropic GBS-induced underestimation of ξGBS. The calculation of ξGBS was re-examined, based on the fact that anisotropic shrinkage of samples along width and thickness during superplastic deformation was noted.