MEASUREMENTS OF THE CHANGES IN MICROSTRUCTURE DURING SUPERPLASTIC DEFORMATION

In this paper, changes in the volume fraction of cavities, in the frac tal dimension of cavities, in the grain size and aspect ratio (the rat io of the major axis to the minor axis) of grains and in the dislocati on density during superplastic deformation have been measured quantita tively. The experimental results show that the volume fraction of the cavities, the fractal dimension of the cavities and the dislocation de nsity increase with increase in the degree of deformation. At the earl y stage of deformation, the increase in the volume fraction of the cav ities varies slightly with the increase in strain, but beyond a partic ular strain, the volume fraction of the cavities increases sharply. Th e stress state has a considerable influence on the growth rate of the volume fraction of the cavities: at the same strain, the volume fracti on of the cavities under the plane equi-tensile stress state is greate r than that under uniaxial tension. However, the volume fraction of th e cavities at fracture under the plane equi-tensile stress state is sm aller than that under the uniaxial tensile-stress state. The grain siz e after superplastic deformation increases generally, but grain fining after superplastic deformation is also discovered sometimes, due depe nding 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 grai ns tends to the increase with the increase of strain, but it increases only slowly. (C) 1997 Elsevier Science S.A.