ROLE OF W, MO, NB AND SI ON OXIDATION OF TIAL IN AIR AT HIGH-TEMPERATURES

The influence of beneficial additional elements (W, Mo, Nb and Si) on the oxidation behavior of a Ti-34.5 mass%Al alloy has been compared and their roles were studied. The relative effectiveness was in the order W, Mo, Nb, Si (the former, the better) at 1173 K. After the analyses of rate law and comparison in the Arrhenius plots of parabolic rate constants, it was clarified that the oxidation resistant temperature was increased by 100 and 200 K for the Mo (Nb) and W containing alloys respectively in comparison with the binary alloy for which the temperature was around 1000 K. The Mo, Nb and W containing alloys did not show any internal oxidation nor breakaway oxidation which was observed in the binary alloy. The reason for the non-occurrence of internal oxidation in these ternary alloys was verified by the result that those elements cause reduction of oxygen solubility in the TiAl alloys very significantly. Owing to this effect, the Al2O3 layer next to the outermost layer may be stabilized and the morphology of the Al2O3 in the inner layer may become more impermeable.