Improvement of oxidation behavior of a Ti-48Al-2Cr alloy by a nitridation treatment

The effect of a nitridation treatment for 10 hr at 800 degreesC on the oxidation resistance of a Ti-48Al-2Cr (at. 0%) alloy in air at 800 degreesC was evaluated. Results prove that nitridation decreases by about 40% the total mass gain of non-nitrided material, although the oxidation mechanism is the same for both materials. The oxidation can be divided into two stages. The formation of a nonprotective mixed alumina-rutile scale during the transient stage results in a high oxidation rate. A further decrease in the oxidation rate arises from the establishment of an external alumina-rich layer during the steady stage. The main difference between the scale developed on both materials is the continuous nature of the nitride layer present in the nitrided material during the entire exposure. The thin continuous nitride layer formed during the nitridation treatment acts beneficially as a diffusion barrier, preventing oxygen dissolution in the alpha (2)-Ti3Al phase during the transient stage. Furthermore, the oxygen gradient through the oxide scale is kept low, because no oxygen is removed at the scale-alloy interface.