Phase equilibria in the ZrO2-YO1.5-TaO2.5 system at 1250 °C

The phase relationships in the ZrO2-YO1.5-TaO2.5 (ZYTO) system at 1250 degrees C-a temperature of interest for thermal barrier coatings applications-were investigated using precursor-derived powders. The system is bisected by a quasi-binary joining the tetragonal (t) ZrO2 and monoclinic (M') YTaO4 solid solutions. The mutual solubility limits for these phases differ significantly depending on whether the third phase in equilibrium with t and M' is fluorite or the orthorhombic (O) Zr6Ta2O17. More importantly, the tetragonal phase in equilibrium with fluorite is non-transformable to the monoclinic form upon cooling, whereas that in equilibrium with O, which has lower stabilizer content, is transformable. The behavior contrasts with that previously observed for the equilibrium at 1500 degrees C, wherein the t phase on both sides of the quasi-binary is non-transformable. The fluorite solid solution extends from the ZrO2-YO1.5 binary to the YO1.5-TaO2.5 binary at 1250 degrees C, as previously shown for 1500 degrees C.