Dynamic grain growth in superplastic and non-superplastic aluminium alloys

The dynamic grain growth in two aluminium alloys is examined. These are Al-4.5Mg (AA5083+Cr), which is a well-known superplastic alloy, and Al-6Ni which, despite being processed to give a fine grain size by equi-channel angular extrusion, is not superplastic. In addition to a large difference in the strain rate sensitivity of these materials, there are differences in the development of crystallographic texture during tensile deformation. In the case of the Al-6Ni alloy there can be little doubt that the majority of deformation is due to normal slip processes. Despite this, it shows dynamic grain growth which is essentially the same as that found in the superplastic alloy. It is proposed that in both cases the grain growth is due to a direct geometrical effect of deformation on the metastable Zener pinned state, and this can be modelled using both vertex and Monte Carlo-Potts methods. Reasons for the difference in rate sensitivity of the two alloys are discussed, and it seems likely that the presence of solute, such as magnesium in aluminium alloys, is a significant factor in superplastic behaviour.