OXIDATION BEHAVIOR OF AN FE-38NI-13CO-4.7NB-1.5TI-0.4SI SUPERALLOY ATHIGH-TEMPERATURE IN AR-H2O ATMOSPHERES

The oxidation of an Fe-38Ni-13Co-4.7Nb-1.5Ti-0.4Si superalloy (Incoloy 909 type alloy), was investigated at temperatures between 1000 K and 1400 K in Ar-(1, 10%) H2O atmosphere using metallographic, electron pr obe microanalysis, and X-ray diffraction techniques. The oxide scales consist of an external scale and an internal scale which has an interg ranular scale (above 1200 K) and an intragranular scale. The oxide pha ses in each scale are identified as alpha-Fe2O3 (below 1200 K) or FeO (above 1300 K) and CoO . Fe2O3 and FeO . Nb2O5, respectively. The morp hologies, the oxide phases and the oxidation rates do not depend on th e partial pressure of H2O in the range between one and ten per cent in Al gas. The rate constants for the intergranular-scale formation in t his alloy are about one-tenth as large as those in Fe-36% Ni alloy rep orted previously. At all the temperatures the scales grow according to a parabolic, ate law and the apparent activation energies for the pro cesses ape estimated.