THE OXIDATION BEHAVIOR OF NIOBIUM CONTAINING GAMMA-TIAL BASED INTERMETALLICS IN AIR AND ARGON OXYGEN/

The oxidation behaviour of gamma-TiAl based alloys with different Nb c ontents (2-10 At. %) was investigated in air and in argon-20 % oxygen at 900 degrees C using thermogravimetric analysis. The oxide scales we re characterized by a combination of optical microscopy, SEM/EDX and X -ray diffraction analyses. Although in all studied cases the presence of niobium improves the oxidation resistance of gamma-TiAl, the oxidat ion kinetics, scale morphology and composition in air differed strongl y from that in argon-oxygen. In air the oxidation resistance increases with increasing niobium-content. In Ar/O-2 the niobium dependence is far more complex because internal oxidation occurs which is favoured b y the presence of niobium. SNMS analysis revealed that the differences in behaviour in the two atmospheres are related to the formation of T i-rich nitride at the scale/alloy interface during air oxidation. The positive effect of niobium on the oxidation resistance of gamma-TiAl i s mainly caused by a decrease of the transport processes in the hetero geneous TiO2/Al2O3-surface scale. Nitride formation and/or niobium enr ichment at the scale/alloy interface also affect the oxidation behavio ur, however these factors are believed to be the result of the decreas ed transport processes rather than the main reason for the niobium eff ect.