The influence of Nb and Hf on the oxidation behaviour of γ-TiAl alloys

摘要

The isothermal oxidation behaviour of two high strength TiAl alloys containing 8 at.%Nb and 4 at.%Nb-4 at.%Hf, has been studied at 850°C. Thermo-gravimetric analyses indicated that both of these alloys exhibited significantly improved oxidation resistance compared to both a binary TiAl alloy and a TiAl alloy containing only 2 at.%Nb. A detailed investigation of the scale morphology and composition following oxidation exposure has been carried out. The improvement in the oxidation behaviour of the Nb and Nb-Hf containing alloys was attributed to a valency doping effect, via the incorporation of Nb~(5+) ions into the TiO_2 sub-scale. The extensive internal oxidation observed in the Nb-Hf alloy was attributed to the diffusion of oxygen through the outer scale and the formation of HfO_2 precipitates in the matrix.