Measurements of the changes in microstructure during superplastic deformation

In this paper, changes in the volume fraction of cavities, in the fractal dimension of cavities, in the grain size and aspect ratio (the ratio of the major axis to the minor axis) of grains and in the dislocation density during superplastic deformation have been measured quantitatively. The experimental results show that the volume fraction of the cavities, the fractal dimension of the cavities and the dislocation density increase with increase in the degree of deformation. At the early stage of deformation, the increase in the volume fraction of the cavities varies slightly with the increase in strain, but beyond a particular strain, the volume fraction of the cavities increases sharply. The stress state has a considerable influence on the growth rate of the volume fraction of the cavities: at the same strain, the volume fraction of the cavities under the plane equi-tensile stress state is greater than that under uniaxial tension. However, the volume fraction of the cavities at fracture under the plane equi-tensile stress state is smaller than that under the uniaxial tensile-stress state. The grain size after superplastic deformation increases generally, but grain fining after superplastic deformation is also discovered sometimes, due depending upon the type or commercial state of the metal: of course, it is related to the stress state in addition. The aspect ratio of the grains tends to the increase with the increase of strain. but it increases only slowly.