THE PERFORMANCE OF FERRITIC ODS ALLOYS AT HIGH-TEMPERATURES IN NITROGEN-CONTAINING ENVIRONMENTS

The effect of exposure to nitrogen-containing environments on two ferr itic ODS alloys viz. MA 956 and ODM 751 has been investigated for temp eratures ranging between 1200 and 1300 degrees C. A pilot study at 130 0 degrees C in oxygen-free nitrogen revealed catastrophic modification s to the entire microstructure of MA 956 with evidence of formation of coarse AlN particles. In order to probe into the exact mechanism of n itridation in these alloys, exposure tests in a flowing mixture of 2% oxygen in nitrogen were performed al 1200 degrees C. In addition to in ternal nitridation, internal oxidation was also seen. To reduce the ki netics even further, and to investigate the effect of both the Al cont ent and the sample thickness on the nitridation behaviour of these all oys, further tests were performed in stagnant air using samples with v arying thicknesses. It was found that when the overall Al content in t he matrix reached a critical level (between 1.5-1.7 wt%), breakaway ox idation occurred which accelerated the internal nitridation behaviour.