High-temperature oxidation of titanium

Abstract

Oxidation of titanium has been studied at reduced oxygen pressures (2 × 10 −4−2 × 10 −3 torr) in the temperature range 1000°–1500°C. The experimental work has included thermogravimetric studies of oxidation rates and metallographic, X-ray diffraction, and electron-microprobe studies on oxidized specimens. The initial oxidation is linear and involves adsorption-controlled dissolution of oxygen in the β-phase of titanium. At and above 1400°C, titanium evaporation also influences the reaction behavior. At 1300°–1400°C the reaction coefficient of oxygen (sticking coefficient) amounts to approximately 0.6–0.7. After the linear stage and saturation of the β-Ti phase, the oxidation involves growth of oxygen-stabilized α-Ti and formation of trace amounts of oxide. The growth of the α-phase follows parabolic oxidation kinetics and from the oxidation data the diffusion coefficient of oxygen in α-Ti is estimated to be D 0 = 8.9 exp (− 52500 RT ) . The oxide growth at and above 10 −3 torr O 2 and at temperatures below 1300°C, comprises formation of rutile (TiO 2). However, at lower pressures and at higher temperatures and after oxygen saturation of the α-phase, all the oxides of titanium (Ti 2O, TiO, Ti 2O 3, Ti 3O 5, TiO 2) are formed as reaction products depending on the oxygen pressure and elapsed time of reaction. On the basis of the value for oxygen diffusion coefficients in α-Ti, a general discussion of the oxidation behavior of Ti in the temperature range 400°–900°C is given. It is concluded that the parabolic oxidation of titanium in the temperature range 600°–900°C is mainly governed by dissolution of oxygen in the metal.