High Temperature Oxidation of Nb-Ti-Al Alloys

The oxidation behavior of four Nb-Ti-Al alloys was studied in air and in oxygen between 700 and 900 C. The oxidation kinetics are parabolic. The alloy of smaller Nb content oxidizes at a rate similar to Cr2O3-former alloys. The effect of Mo additions on oxidation kinetics is complex; the oxidation kinetics of the Mo-containing alloy are quite different in pure oxygen and in air. Additional tests in synthetic N2+O2 and Ar+O2 mixtures permitted the analysis of the effect of nitrogen. The external oxide scales are poorly adherent, porous, and cracked depending on oxidation conditions. The diffusional component of the oxidation kinetics can be related to the diffusion of oxygen through these porous or cracked scales. However, the oxide formed during oxidation indicates that the interfacial reactions associated with the oxide scale growth must play a quite important role in the oxidation of alloys of high Nb content. (Author)