BENEFICIAL AND DETRIMENTAL EFFECTS OF NITROGEN ON THE OXIDATION BEHAVIOR OF TIAL-BASED INTERMETALLICS

The isothermal oxidation of binary and ternary Nb-containing titanium aluminides was studied at 900 degrees C in air and Ar/O-2. It was foun d that the presence of nitrogen in the atmosphere is in some cases ben eficial, in others detrimental. For an interpretation of the effects o f nitrogen it is necessary to consider the various oxidation stages in which the mechanisms of titanium aluminide oxidation fundamentally di ffer. For high-Al alloys, which tend to form protective alumina based scales in the early oxidation stages, nitrogen always seems to be detr imental because it promotes the formation of a more rapidly growing mi xed alumina/titania scale. After this shift to mixed oxide scales, nit rogen can be detrimental as well as beneficial. It is proposed that th e last mentioned case prevails if a near-continuous Ti-rich nitride la yer is stable at the oxide/alloy interface, thereby preventing interna l oxidation of aluminium. This beneficial effect of nitrogen was found under all studied conditions for the Nb-containing alloys but only in some cases for the binary alloys.