Effect of nitridation on the oxidation behavior of TiAl-based intermetallic alloys

In the present work, gas nitridation of the TiAl based alloys in an ammonia atmosphere was carried out at 800, 860 and 940 °C for different time, respectively. The nitride layers were characterized by X-ray diffraction (XRD), electron probe micro-analyzer (EPMA) and scanning electron microscopy (SEM). The composite of the nitride layers (Ti2AlN as the inner layer and TiN as the outer layer) and the diffusing layer formed on the surface of the TiAl based alloys. The oxidation behavior of the nitrided alloys in air was studied between 800 and 1000 °C. The scales were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) analysis. Generally an oxide scale consisting of two layers, an outward- and an inward-growing layer, formed. The outward-growing part of the scale consisted of mainly of TiO2 (rutile), while the inward-growing part was composed mainly of α-Al2O3. Due to more α-Al2O3 formed in the outward-growing part, the non-nitrided alloys showed better oxidation resistance than the nitrided alloys. On the observation of the EDX measurement, the alloys nitrided at 940 °C for 50 h grew more α-Al2O3 with fine grains on the outer layer of its scale after 100 h exposure time at 1000 °C. Because of its significant transient oxidation at the initial period, it displayed worse resistance. It is concluded that the high-temperature nitrided alloys exhibit better oxidation resistance at 1000 °C.