X-ray microtomography study of cavity coalescence during superplastic deformation of an Al-Mg alloy

Coalescence between cavities induced during superplastic deformation of an Al- Mg alloy was studied, in particular by X-ray microtomography, which can provide three-dimensional images of the population of cavities through the volume of the material. From such images, the variation with strain of the number of cavities per unit volume was measured for various strain rates. For a given strain, the number of cavities increases with strain rate, but for a given strain rate, it is shown that significant interlinkage between cavities takes place in a large strain interval. Moreover, although the cav ity volume fraction increases with strain, it becomes difficult to describe the population of cavities by a mean volume since the largest cavity may c oncentrate more than 70% of the total volume of cavities, For such circumst ances, an interlinkage parameter was introduced as the ratio between the vo lume of the largest cavity and the total volume of cavities for given exper imental Conditions (strain and strain rate). This work illustrates the unce rtainty of some conclusions directly drawn from data obtained using only tw o-dimensional techniques, such as quantitative metallography performed on p olished sections. MST/4778.