Ti-Al系開発合金の耐酸化性

Abstract

The oxidation behavior of nine titanium aluminide alloys has been studied in air, with isothermal conditions at 870°C up to 200 h and 2 h-cyclic conditions both at 760°C and 870°C up to 1000 h. The isothermal oxidation curves for the alloys consist of three stages, that is, the first stage with a linear mass gain, the second with a parabolic increase and the third with a linear mass gain. The alloys with higher Nb contents show smaller mass gains and smaller parabolic rate constants in the second stage. The oxide scales formed on the alloys are basically similar to those on the binary TiAl, however, the high Nb alloys tend to have more continuous and more compact Al2O3 layers in the scales. The enrichment of nitrogen as well as Nb at scale/matrix interface was observed by EPMA analysis. The nitride such as TiN seems to be continuous on the high Nb alloys. The improved oxidation resistance of the Nb-containing alloys is attributable to suppression of TiO2 growth by doping effect and probably also to the nitride layer as a diffusion barrier. However, even 8-10 at%Nb contents are not sufficient for long term cyclic oxidation at 870°C, resulting in a layered scale structure. Small additions of C, B, Si and Hf do not appear to influence the oxidation behavior. The addition of 1 at%Mo did not show any improvement in oxidation resistance under both isothermal nor cyclic oxidation conditions. The addition of W may stabilize the oxidation behavior to be protective.