Interfacial reactions between Ti-6Al-4V alloy and zirconia mold during casting

The interface of Ti-6Al-4V casting and ZrO₂ mold with silica binder was investigated by using electron probe microanalyses (EPMA), X-ray diffraction (XRD), and analytical transmission electron microscope (TEM). The interfacial reactions were proceeded by the penetration of liquid titanium through open pores near the mold surface. The metal side consisted of an -phase layer on the top of the typical + two-phase substrate. In the ceramic side, zirconia was reduced by titanium to form oxygen-deficient zirconia ZrO_2–x and evolved a gaseous phase (presumably oxygen). The SiO₂ binder, dissolved in the ZrO₂ mold, could react with titanium to form Ti₅Si₃ in the metal side. Meanwhile, titanium could transform to titanium suboxides Ti_yO (y 2) and the lower phase boundary of cubic ZrO_2–x was shifted to ZrO_1.76. Some amount of the stabilizer CaO, dissolved in Ti along with ZrO₂, could react with Ti(O) to form Ca₃Ti₂O₇ and CaAl₄O₇ in the reaction zone.