BAINITE-LIKE TRANSFORMATION IN ZIRCONIA CERAMICS

The authors have found in ZrO2-Y2O3 ceramics, an isothermal propagation of martensitic transformation, tetragonal to monoclinic, which occurred by means of a similar mechanism to generate the lower bainite in steels. This isothermal propagation of monoclinic phase appears to be controlled by the oxygen-ion diffusion or the interaction between so-called Frenkel type defects and oxygen ions which existed in the tetragonal lattice (a distorted CaF2 structure) of ZrO2 containing a few mol percents of Y2O3. Accordingly, the behaviour of nucleation and growth (propagation) of monoclinic martensite can be described in the same way as the homogeneous nucleation theory which often appeared in precipitation phenomena. On the other hand, the typical athermal behaviour of tetragonal to monoclinic transformation was observed in ZrO2-12 mol%CeO2 ceramics, in which the "Bainite-like" mechanism cannot be expected because of the existence of only a small amount of thermodynamical oxygen vacancies in the tetragonal lattice. However, in 8 mol%CeO2 specimens the isothermal propagation has been reported by Shimada et al., suggesting that the transformation behaviour must be very much affected by the CeO2 content. A brief discussion was focussed to the origin of different mechanisms observed in ZrO2-base ceramics.