The high temperature oxidation behaviour of Ti-47Al-2Cr-0.2Si and Ti-48Al-2Cr-2Nb compared with Ti-48Al-2Cr

The oxidation behaviour of Ti-47Al-2Cr-0.2Si and Ti-48Al-2Cr-2Nb (at%), with a fine-grained near-gamma microstructure was studied at 700 and 800 degrees C in comparison with Ti-48Al-2Cr. Isothermal and cyclic oxidation tests were performed either in humidified synthetic air or in static laboratory air. Ti-48Al-2Cr-2Nb showed the lowest oxidation rate and the highest spallation resistance. For Ti-47Al-2Cr-O.2Si a low isothermal oxidation rate but pronounced susceptibility for oxide spallation was found. The niobium-free and silicon-free alloy Ti-48Al-2Cr showed a high isothermal oxidation rate; however, spallation was between the two extremes represented by Ti-48Al-2Cr-2Nb and Ti-47Al-2Cr-0.2Si. During oxidation a complex multiphased and multilayered scale was formed on the surface of the alloys. The oxidation of the alloys initially resulted in the formation of alpha-Al2O3, TiO2 (rutile), Ti2AlN, and TiN. After longer exposure times the outer scale of Ti-47Al-2Cr-0.2Si was dominated by fast growing TiO2 crystals and an Al-depletion layer was formed at the scale/metal interface. In the case of Ti-48Al-2Cr-2Nb, however, no Al-depletion zone was found at the scale/ metal interface after longer exposure times, but a narrow layer slightly enriched in Nb. The outer scale also consisted of a TiO2 layer but with a crystal density and crystal size significantly smaller than in the case of Ti-48Al-2Cr and Ti-47Al-2Cr-0.2Si. Two-stage oxidation experiments with isotope tracers were performed to study the oxidation mechanisms. Some attempts to explain the effect of niobium and silicon on the oxidation resistance of TiAl-based intermetallics are discussed. (C) 1997 Elsevier Science Limited.