Oxidation behavior of Ti-46Al-8Nb alloy with boron and carbon addition under thermal cycling conditions

Oxidation behavior of Ti-46Al-8Nb (in at.%) alloy with boron and carbon addition under thermal cycling conditions was investigated. Oxidation of Ti-46Al-8Nb, Ti-46Al-8Nb-1B and Ti-46Al-8Nb-1B-0.25C (in at.%) alloys was carried out at 1073 K in laboratory air for 42 cycles (1 cycle, 24 h), 1008 h in total. The mass loss rates of Ti-46Al-8Nb and Ti-46Al-8Nb-1B measured during the oxidation were similar. The best oxidation resistance was found for Ti-46Al-8Nb-1B-0.25C alloy with the smallest mass change. XRD and SEM-EDS investigations showed that in all cases, the oxide scales compositions were substantially similar. The scale consisted of an outer layer built of Al2O3 with the presence of some amounts of TiO2, an intermediate layer of the scale consisting of TiO2, an inner layer composed of oxides and nitrides. Additionally, niobium rich particles at the scale/substrate interface were present. The oxidation mechanism of Ti-46Al-8Nb was studied via two-stage isothermal oxidation (24 h in 16O2 followed by 24 h in 18O2) at 1073 K combined with secondary neutral mass spectroscopy (SNMS). These results indicate that the oxidation mechanism depends on a mixed diffusion process, consisting of outward transport of cations and simultaneous oxygen inward transport.