Fundamental considerations for the development of oxidation-resistant alloys and coatings based on γ-TiAl

Abstract

Alloys based on γ-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 γ-TiAl alloys at high temperatures is their relatively poor oxidation resistance and sensitivity against environmentally induced embrittle-ment. This material degradation is related to the poor protective properries of the mixed TiO 2 /Al 2 O 3 suface scales which form on the surface during high-temperature exposure. Recently, it was shown that protective alumina scale formation on γ-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 α 2 -Ti 3 Al. It was found that the beneficial effect of Ag can be suppressed if the alloys contain additional α 2 -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 γ-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.