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

The influences of beneficial additional elements (W, Mo, Nb and Si) on the oxidation behavior of Ti-34.5%Al alloy have been compared and the roles of those elements were studied. Relative effectiveness was in the order W greater-than-or-equal-to Mo = Nb greater-than-or-equal-to Si at 1173 K. After the analyses of rate law and comparison in the Arrhenius plots of parabolic rate constant, it was clarified that the oxidation resistant temperature was raised by 100 and 200 K for the Mo (Nb) and W added alloys respectively in comparison with the binary alloy for which the temperature was somewhere around 973 and 1023 K. The Mo, Nb and W added alloys did not show internal oxidation nor breakaway oxidation behavior which were 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 2nd from outer Al2O3 layer may be stabilized and the morphology of the Al2O3 in the inner layer may become more impermeable.