Effect of the nature of grain boundary regions on cavitation of a superplastically deformed aluminium alloy

Superplastic deformation of aluminium alloys induces cavity formation throughout the material, so that superplastic forming usually requires to be carried out under superimposed gas pressure to minimize strain-induced damage. This paper deals with the beneficial effects of heat treatment at high temperature for several hours before deformation on cavitation behaviour of a superplastically deformed 7475 alloy. Transmission electron microscopy observations show that several microstructural transformations are induced by superplastic deformation and affected by the heat treatment. At first, the generation of dispersoid free zones at the periphery of the grains is observed, the composition of which depends on the prior history of the specimen. Secondly, the formation of long thin fibres extending in the cavities in the as received specimens, these fibres being no longer present in the hear-treated conditions. A TEM characterization of the fibres is presented and a mechanism of their formation is discussed. Such a reduction of the cavitation level for a given strain is interesting in view of superplastic forming of aluminium alloys under atmospheric pressure.