Change in stress, stress sensitivity and activation energy during superplastic deformation of silicon nitride

Stress sensitivity and activation energy for superplastic deformation were investigated for silicon nitride consisting of rod-shaped grains. It was found that both the stress sensitivity and the activation energy increased during the deformation. In addition, an abnormal stress increment, observed in the superplastic deformation, was analyzed by a viscous flow model taking into account grain rotation. It was demonstrated that the viscous flow model would be effective to explain the stress increment during superplastic deformation. The principle deformation mechanism is considered to be the non-Newtonian viscous flow of grain boundary glassy phase. At the last stage (more than 200% elongation), however, superplastic behavior is considered to be mainly controlled by the solution-precipitation with 2-dimensional nucleation.