Factors affecting superplastic ductility and elongation

Superplastic ductility, while known to be strongly related to the strain rate sensitive m, is difficult to predict and many times can be difficult to understand. Cavitation is most often indicated as the cause of a lower than expected elongation; however, this consideration has not always explained the variation in ductility observed, such as a significant scatter in elongations measured for the same material and under the same test conditions. The range of factors which may be involved are suggested here to include initial gradients, or inhomogeneities, in such factors as cavitation, temperature, grain size as well as geometry (i.e. area, stress, or stress state). A model is summarized which constructs the tensile test section of a number of elements, each allowing a different initial condition as well subsequent evolution. The model is applied to cavitation variations with supporting evidence from controlled experiment on the superplastic Pb-Sn alloy which had various patterns of pre-machined holes throughout the test section. Also, the potential effect of dynamic recrystallization is explored with the model, and which suggests that a substantial loss in ductility can result.