Phase stability of zirconia solid solutions

Using three analytical methods, X-ray diffraction, EDX composition analysis, and thermal expansion, a zirconia rich part of ZrO2Sc2O3 phase diagram was re-examined. The result shows the tetragonal solubility is similar to that in Strickers phase diagram and cubic phase boundary is close to that in Thornbers or in Spiridonovs phase diagram. A diffusionless phase transformation from cubic (c) to tetragonal (t) was found in this system when a sample with a high temperature cubic phase was quenched through a cubic plus tetragonal region to room temperature. This observation has been used to explain the discrepancies among the previously reported ZrO2-Sc2O3 phase diagrams. Through careful sample preparation and composition selection, a cubic plus tetragonal (t') coexistence microstructure was found in the ZrO2-SC2O3 system. From a TEM micrograph observation, the habit plane of the t' phase in a cubic matrix was found and analyzed as (101) planes. Different tetragonal (t') precipitate morphologies in the ZrO2-Yb2O3, ZrO2-Sc2O3 and ZrO2-In2O3 systems were observed. The tetragonal precipitate habit planes in these three systems were analyzed as (101). Tetragonal precipitates were suggested to grow by a strain-induced coarsening mechanism. After a prolonged aging treatment, the tetragonal precipitates in the ZrO2-Sc2O3, and ZrO2-In2O3 systems were able to transform to monoclinic after thermal cooling; but those in the ZrO2-Yb2O3 system could not. The anisotropic thermal expansion properties of the tetragonal phases were found in the ZrO2-Sc2O3, ZrO2-In2O3, ZrO2-Yb2O3, and ZrO2-YTaO4 systems