The significance of sub-surface depletion layer composition for the oxidation behavoiur of gamma -titanium aluminides

Contradictory theoretical explanations and experimental data are there on the stable oxide phase in the oxidation of gamma -TiAl. An explanation of the stable phase was sought in the present report which is intended to solve that contradiction. Three alloy compositions were used (at.%): Ti48Al, Ti50Al, and Ti48Al5Cr. Surface finished specimens were oxidized in Ar+20vol.%O sub 2 at 900 deg C. SEM images of the oxidized specimens show single-phase depletion layer beneath the alumina scale and the two-phase layer beneath the mixed alumina/titania scale. The single-phase depletion layer was observed in the ternary (Cr containing) alloy also. Electron probe micro-analysis of the two-phase depletion layer beneath the oxide revealed that the second phase ( alpha sub 2 -Ti sub 3 Al) contained large amount of oxygen. Oxidation behavior of gamma -TiAl based intermetallics in oxygen is explained by taking the ternary Z-phase into account.