EFFECT OF NITROGEN ON THE OXIDATION BEHAVIOR OF TI3AL-BASED INTERMETALLIC ALLOYS

The effect of nitrogen on the oxidation behavior of Ti-25Al, T-24Al-15 Nb, and Ti-25Al-11Nb (at. pct) has been examined in this study. The ga ses employed were nitrogen and oxygen-nitrogen and oxygen-argon mixtur es in various proportions at a total pressure of 1 atmosphere. The exp eriments were carried out in the temperature range of 1100 to 1300 K b y thermogravimetry. The suitability of employing the parabolic rate la w as the basis of interpretation of weight gain vs time data has been discussed. Oxidation resistance of Nb-containing alloys was superior t o that of binary Ti-25Al, irrespective of the gas composition employed . The nitridation rates of alloys, with or without Nb, were more than an order of magnitude lower than those for,oxidation. The scales were characterized by X-ray diffraction (XRD) and scanning electron microsc opy with energy dispersive X-ray (EDX) analysis. The scales formed in all the conditions mostly consisted of TiO2 and Al2O3. However, TiN wa s observed in scales of Nb-containing alloys in all nitrogen bearing g ases and seemed to primarily account for improved oxidation resistance of the preceding in comparison to alloys without Nb. Nitrogen pretrea tment was provided for some samples before oxidation for further eluci dation of the role of nitrogen.