Improvement in high temperature oxidation resistance of TiAl–Mn intermetallic compound by preoxidation in CO–CO 2 gas mixture

Abstract

The oxidation behaviour of (Ti–44·9Al)–1·6Mn (at.-%) preoxidised at high temperatures in a CO–CO₂ gas mixture with a very low equilibrium partial pressure of oxygen was investigated by a cyclic oxidation test at 1000°C in static air. The TiAl–Mn specimens were preoxidised in the temperature range 900–1000°C for various times (10–50 h) in CO–CO₂ gas mixtures with various ratios of partial pressure p_CO2/p_CO, and were subsequently examined by X-ray diffractometry, scanning electron microscopy, and energy dispersive X-ray spectroscopy. In the cyclic oxidation test, the TiAl–Mn preoxidised at 975°C for 20 h in CO–CO₂ with a p_CO2 /p_CO of 3 : 7 showed excellent oxidation resistance up to 600 h. This is attributed to the formation of a continuous flat Al₂ O₃ rich layer beneath the outer mixed oxide layer generated during the CO–CO₂ preoxidation. Furthermore, the parabolic rate constant for the early stages of cyclic oxidation was determined to be 1·71 × 10^-11 kg² m^-4 s^-1 for the above specimen.