Hot‐pressed 3 mol% Y2O3 partially stabilized ZrO2 was reacted with titanium at 1550°C/30 min. The interface was characterized by analytical transmission microscopy (transmission electron microscopy/energy‐dispersive spectroscopy). The lamellar and the spherical Ti2ZrO as well as the orthorhombic β′‐Ti were found to exist in the titanium side after cooling down to room temperature. The crystal structures of the lamellar and the spherical Ti2ZrO were orthorhombic and hexagonal, respectively. On heating, the dissolution of a large amount of zirconium and oxygen into titanium gave rise to the metastably supersaturated disordered α‐Ti(Zr, O) solid solution where two different Ti2ZrO phases subsequently precipitated, while the β‐Ti coexisting with α‐Ti at high temperatures was transformed to the orthorhombic β′‐Ti during cooling. The spherical hexagonal Ti2ZrO was an ordered structure, with Zr and O occupying substitutional and interstitial sites, respectively. The orientation relations between α‐Ti and the lamellae orthorhombic Ti2ZrO were determined to be [0001]α‐Ti//[110]Ti2ZrO and (1010)α‐Ti//(110)Ti2ZrO; meanwhile, those between the α‐Ti and the spherical hexagonal Ti2ZrO were [0001]α‐Ti//[0001]Ti2rO and (1010)α‐Ti//(1010)Ti2ZrO.

The interfaces between 3-mol%-yttria-partially-stabilized zirconia and commercially pure titanium after reaction at 1750°C were analyzed with a scanning electron microscope and an analytical transmission microscope. Zirconia was reduced to oxygen-deficient zirconia (ZrO 2−x ) with an O/Zr ratio as low as 1.53, causing the evolution of oxygen. Part of the oxygen could accumulate at grain boundaries, the remainder being dissolved in titanium as ␣-Ti (O). An ordered titanium suboxide (Ti 3 O) could be formed from a solid solution of ␣-Ti(O) during cooling. A fine crystalline ZrO 2−x phase (O/Zr ≈ 2) was also found along with ␣-Zr near the interface on the zirconia side. The ␣-Zr was twinned with one of the twin planes being indexed as {1012}. The yttria stabilizer was excluded from zirconia as the reaction was progressing, existing as oxygen-deficient yttria. Extensive dissolution of zirconia in titanium gave rise to the formation of ␣-Ti(Zr,O) solid solution. On cooling, lamellae of Ti 2 ZrO precipitated from ␣-Ti(Zr,O) with an orientation relationship of {110} Ti 2 ZrO //{100} ␣-Ti and 〈111〉 Ti 2 ZrO //〈011〉 ␣-Ti .

Zirconia–titanium (ZrO2–Ti) composites have been considered potential thermal barrier graded materials for applications in the aerospace industry. Powder mixtures of Ti and 3 mol% Y2O3 partially stabilized ZrO2 in various ratios were sintered at 1500°C for 1 h in argon. The microstructures of the as‐sintered composites were characterized by X‐ray diffraction and transmission electron microscopy/energy‐dispersive spectroscopy. Ti reacted with and was mutually soluble in ZrO2, resulting in the formation of α‐Ti(O, Zr), Ti2ZrO, and/or TiO. These oxygen‐containing phases extracted oxygen ions from ZrO2, whereby oxygen‐deficient ZrO2 was generated. For relatively small Ti/ZrO2 ratios, specimens with ≤30 mol% Ti, TiO were formed as oxygen could be sufficiently supplied by excess ZrO2. For the specimens with ≥50 mol% Ti, lamellar Ti2ZrO was precipitated in α‐Ti(Zr, O), with no TiO being found. Both m‐ZrO2−x and t‐ZrO2−x were found in specimens with ≤50 mol% Ti; however, only c‐ZrO2−x was formed in the specimen with 70 mol% Ti. As ZrO2 was gradually dissolved into Ti, yttria was retained in ZrO2 because of the very limited solubility of yttria in α‐Ti(O, Zr) or TiO. The concentration of retained yttria and the degree of oxygen deficiency in ZrO2 increased with the Ti content. The complete dissolution of ZrO2 into Ti was followed by the precipitation of Y2Ti2O7 in the specimen with 90 mol% Ti.