HIGH-TEMPERATURE OXIDATION OF CATHODICALLY HYDROGEN-CHARGED 2-PHASE (TI3AL, TIAL) TITANIUM ALUMINIDES

Ti-42Al, Ti-45Al, and Ti-50Al (at. pct) titanium aluminides, which wer e cathodically hydrogen charged in a 5 pet H2SO4 solution for charging times between 1.8 ks (0.5 hours) and 14.4 ks (4 hours), were oxidized in a static air under atmospheric pressure at temperatures between 11 70 K (897 degrees C) and 1350 K (1077 degrees C). All the hydrogen-cha rged alloys, as well as alloys without hydrogen charging, followed par abolic oxidation kinetics. The weight gains of the alloys after hydrog en charging for normally less than 3.6 ks (1 hour) were 20 to 30 pct l ess than those without hydrogen charging. In the alloys charged with h ydrogen for more than 7.2 ks (2 hours), the weight gains increased wit h increasing the charging time. The activation energies of oxidation i ndicated that the oxidation-controlling factor would change after a ch arging time of 7.2 ks (2 hours) in all the alloys. The decrease in the activation energies with charging time was more drastic in the Ti-50A l alloy, which suggested that hydrogen damage, such as cracking, was m ore severe in the Ti-50Al alloy than in the Ti-42Al or Ti-45Al alloys. The formation of cracks during hydrogen charging provides titanium-di ffusion paths and accelerates formation of rutile (TiO2) scale on the surface of the alloys. The TiO2 on the alloys after hydrogen charging formed at a comparatively lower temperature than that on the alloys wi thout charging.