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 fo und that both the stress sensitivity and the activation energy increased du ring the deformation. In addition, an abnormal stress increment, observed i n the superplastic deformation, was analyzed by a viscous flow model taking into account grain rotation. It was demonstrated that the viscous flow mod el would be effective to explain the stress increment during superplastic d eformation. 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 nucl eation. (C) 1999 Elsevier Science S.A. All rights reserved.