Kinematical Analysis of 748 Grains in TZP Ceramics during Superplastic Deformation

Abstract

This paper describes motion of 748 grains during superplastic deformation of TZP (Tetragonal Zirconia Polycrystal) ceramics. In the same way as the latest paper of the present authors, the same TZP specimen was 4 times further deformed at 1673K (=1400°C) in air under tensile loading. The increment of true plastic strain was set to be about 2%, and the final true plastic strain reached 25.3%. After each deformation, a position vector of the same grain was read out from a FE-SEM photograph, and thereby its displacement vector was determined. By changing the increment of true plastic strain in calculation, various displacement vector maps were obtained in consideration of the previous data also. At an increment of about 25% of true plastic strain; most of the grains seem to move along stationary streamlines similar to laminar flow. When we look at the grain motion at increments of about 5%, several tens of grains move to the same direction instantaneously, and therefore a kind of domain is formed at the first loading. In a subsequent deformation, however, the domains gradually disappear and each grain seems to move randomly, indicating that grain motion becomes spatially uniform. At increments of about 2%, convergence and divergence in grain motion gradually diminish with increasing the true plastic strain and this tendency corresponds to the observation at increments of about 5%, although each grain moves in a zigzag way gradually turning to the loading direction. From these results, the grain motion has a similarity to diffusion phenomenon because the observed time period makes a profound effect on the motion. As for time evolution of grain structure, by the first 15% of true plastic strain, local deformation resistance (viz. constraint by neighbor grains) is gradually homogenized, and consequently spatially uniform grain motion is attained leading to superplasticity in TZP ceramics.