The constitution of titanium-oxygen binary system was experimentally studied using microscopical and X-ray analysis, and measuring the melting temperature of these alloys. According to the investigation on the titanium-rich side of this system carried out by Bumps, Kessler and Hansen, it has been shown that there exist two peritectic reactions, Melt+α\leftrightharpoonsβ at 1700° and Melt+α\leftrightharpoonsTiO at 1770°, and the phase α containing about 10% oxygen shows a maximum melting temperature at about 1900° on the liquidus. The present investigation has been carried out mainly to clarify the equilibrium diagram on the more oxygen-rich part of this system. The results were as follows: The liquidus temperature of TiO (δ) phase decreases with the increase of oxygen contents, and it reaches a munimum at about 1750°, its oxygen content being 30%. On the side of Ti
2O
3 (ε) phase, the liquidus temperature rises again and it shows a peritectic point at 1810°, the reaction being Melt+Ti
2O
3 (ε)\leftrightharpoonsTiO (δ). The solid solubility of TiO (δ) phase on the oxygen-rich side decreases with the fall of temperature from 32.4% oxygen at 1800° to about 30% oxygen at room temperature. The melting temperature of Ti
2O
3 is 1830° and it has a perceftible solid solubility range. Between Ti
2O
3 (ε) phase and TiO
2 (η) phase, there exists an eutectic reaction showing Melt\leftrightharpoonsε+η at 1680° and 36.5% oxygen. The TiO
2 (η) phase decomposes at above its melting temperature of 1825° into a liquid containing less oxygen than 40% and a gaseous phase.
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