Fundamental Considerations for the Development of Oxidation-Resistant Alloys and Coatings Based on g-TiAl

Alloys based on *g-TiAl are promising high-temperature materials that may replace conventional heat-resistant steels and superalloys in applications where high strength in combination with low density is required. However, an important hindrance to the use of *g-TiAl alloys at high temperatures is their relatively poor oxidation resistance and sensitivity against environmentally induced embrittlement. This material degradation is related to the poor protective properties of the mixed TiO2/Al2O3 surface scales which form on the surface during high-temperature exposure. Recently, it was shown that protective alumina scale formation on *g-TiAl can be obtained by small additions of Ag. This effect was found to be related to the formation of Z phase in the subscale depletion layer at the expense of *a2-Ti3Al. It was found that the beneficial effect of Ag can be suppressed if the alloys contain additional *a2-stabilizing elements, such as Nb, as is the case for most (semi)commercial, high-strength alloys. Therefore, recent efforts have concentrated on developing Ag-containing *g-TiAl alloys as oxidation-resistant coatings for high-strength titanium aluminides. Preliminary results using magnetron sputtering have shown that, due to the similarities in chemical and physical properties of the coating and base material, the Ag-containing material offers promising potential to be qualified as a coating material for reducing the oxidation-induced degradation of titanium aluminides.