EFFECT OF THE NATURE OF GRAIN-BOUNDARY REGIONS ON CAVITATION OF A SUPERPLASTICALLY DEFORMED ALUMINUM-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 t reatment at high temperature for several hours before deformation on c avitation behaviour of a superplastically deformed 7475 alloy. Transmi ssion electron microscopy observations show that several microstructur al transformations are induced by superplastic deformation and affecte d by the heat treatment. At first, the generation of dispersoid free z ones at the periphery of the grains is observed, the composition of wh ich depends on the prior history of the specimen. Secondly, the format ion of long thin fibres extending in the cavities in the as received s pecimens, these fibres being no longer present in the hear-treated con ditions. A TEM characterization of the fibres is presented and a mecha nism of their formation is discussed. Such a reduction of the cavitati on level for a given strain is interesting in view of superplastic for ming of aluminium alloys under atmospheric pressure.