Surface marker behaviour in superplastic and non-superplastic tension of Al-4.5% Mg

These results tend to confirm the findings of the internal marker experiment on Ti-6Al-4V and the conclusions drawn from retention of aligned grain structures, that relative grain translation may not be significant in superplastic deformation. This does not mean that there is no grain boundary shear or even "sliding' in the more general sense of those terms, and indeed this was observed. However, it occurred even when the rate sensitivity was less than 0.3, which would be the least lower bound for any reasonable definition of superplasticity. In fact, marker displacement can be readily observed at much lower rate sensitivity, say in conventional room temperature tensile testing of aluminium. In those cases, displacement occurs at slip bands and other intragranular features. To give the behaviour observed here, it is simply necessary for the inhomogeneity of deformation to occur at greater length scales and become associated with the grain boundaries. The presence of a free surface naturally allows a greater expression of inhomogeneous deformation, whether intragranular or intergranular.