Improvement in high temperature oxidation resistance of TiAl–Mn intermetallic compound by preoxidation in CO–CO2 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 pCO2/pCO , 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 pCO2 /pCO 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.