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 degreesC for different time . respectively. The nitride layers were characterized by X-ray diffraction (XRD), electron probe micro-analyzer (EPMA) and scanning electron microscop y (SEM). The composite of the nitride layers (Ti,AIN as the inner layer and TiN as the outer laver) and the diffusing layer formed on the surface of t he TiAl based alloys. The oxidation behavior of the nitrided alloys in air was studied between 800 and 1000 degreesC. 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 tw o layers, an outward- and an inward-growing layer, formed. The outward-grow ing part of the scale consisted of mainly of TiO2 (rutile), while the inwar d-growing part was composed mainly of alpha -Al2O3. Due to more alpha -Al2O 3 formed in the outward-growing part., the non-nitrided alloys showed bette r oxidation resistance than the nitrided alloys, On the observation of the EDX measurement, the alloys nitrided at 940 degreesC for 50 h grew more alp ha -Al2O3 with fine grains on the outer layer of its scale after 100 h expo sure time at 1000 degreesC. 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 degreesC. (C) 2001 Elsevier Science Ltd. All rights reserved.