Mechanisms of scale formation on new alumina forming gamma-TiAl alloys studied by MCs+-SIMS

Two Ag-containing gamma-titanium aluminides, Ti-48Al-2Ag and Ti-50Al-2Ag (c oncentrations in at.%), have been studied in respect to their long-term oxi dation behaviour in air at 800 degrees C. The exposures up to 6000 hours sh owed that the Ag additions promote formation of long-term protective alumin a surface scales on gamma -TiAl alloys. Extensive analyses of the oxidation products using optical metallography, SEM/EDX, XRD, EPMA and SIMS revealed that Ag stabilises the Z-phase (Ti5Al3O2) in the sub-scale depletion layer thereby preventing formation of alpha(2)-Ti3Al as well as Ti-rich nitrides , which are responsible for the destruction of alumina surface scales in co mmon gamma-TiAl alloys. The best results were obtained for Alloy Ti-50Al-2A g; even after 6,000h exposure the thickness of the alumina based scale on t his alloy was only around 0.5 to 1.5 mu m. The results indicate that for op timum oxidation resistance careful adjustment of the Al and Ag content is n ecessary because otherwise massive Ag precipitation occurs at the Z-phase-a lloy interface which leads to localized destruction of the protective scale .