Abstract
Many models have been suggested for the phase transformation superplasticity. Most of them provide only qualitative pictures of the mechanism. The internal stress theory by Greenwood et al. gives a quantitative explanation of experimental results, but neglects the compatibility of strain and the balance of stress. In the present study, we have proposed the equation between the transformation plastic strain εtp and the applied stress by using continuum mechanics which satisfies the compatibility of strain and the balance of stress.
Main results obtained are as follows.
(1) The equation proposed shows a qualitative good agreement with experimental results that the transformation plastic strain increases linearly in the low stress range and rapidly in the high stress range, and that the transformation plastic strain increases as heating rate decreases. These results suggest that the characteristics of the deformation through a phase transformation approaches those of normal creep, because the contribution of internal stress decreases relatively in the high stress range or low heating rate.
(2) The eigen strain due to volume change in a newly transformed region arises to relieve the applied stress and creates internal stress. The matrix is deformed plastically by internal stress and applied stress. It seems that the internal stress decreases immediately due to mechanisms proposed up to the present, for instance, stress relaxation by plastic deformation. It is, therefore, suggested that the phase transformation superplasticity results from the repeating preferential occurrence and decrease of internal stress.
