The oxidation behavior of nine titanium aluminide alloys has been studied in air under isothermal conditions at 870°C and under cyclic exposure conditions at 760°C and 870°C. The isothermal oxidation of engineering titanium aluminide alloys at 870°C reveals that the mass gain due to oxidation consists of three stages, that is; the first with a linear gain, the second with parabolic increase, and the third with linear gain. Increased oxidation resistance was observed with increasing Nb content (up to 10 at%) in both isothermal and cyclic oxidation tests. This resulted from suppression of the growth of TiO 2 and formation of a continuous and compact A1203 scale layer. However, even 8-10 at% Nb contents were not sufficient in cyclic oxidation at 870°C. Small addition of Si, B, C and Hf does not appear to influence the oxidation behavior. The addition of 1 at% Mo did not improve the oxidation resistance under both isothermal nor cyclic oxidation condition.
Titanium aluminide alloys have a potential as high-temperature structural material in aerospace and automobile applications because of their high-temperature strength and density which is almost as half as conventional superalloys. Many engineering titanium aluminide alloys have been developed during the last 15 years through alloy modification and microstructure controP. Usually, the engineering titanium aluminide alloys contain 45-48 at% AI and various alloying elements, and consist of 7"-TiAI phase and a small amount of a2-Ti3A1 phase. The alloys are often called gamma alloys. The microstructure of the gamma alloys is equiaxed, lamellar or duplex, with the duplex being a mixture of equiaxed and lamellar structures. The mechanical properties of the gamma alloys depend on their compositions and microstructures. Small additions of V, Cr, or Mn increase the ductility of the alloys 24, Nb, Mo, Ta, Hf and Sn play a role for substitutional solution strengthening s, and additions of W, B, C, and N are known to yield either dispersion or precipitation hardening". Interstitial impurities such as O, C, N, and B reduce the ductility in excessive addition 6. Boron, W, Si, and N are sometimes added for microstructure control 1. The service temperature of the alloys is thought to be up to about 800°C from the view point of creep resistance and oxidation behavior. The oxidation resistance of the titanium aluminide alloys is good enough up to about 700°C, but higher than this temperature, it is significantly inferior to conventional superalloys. In spite of the high aluminum content in the alloy, titanium aluminide does not form a protective AIEO 3 scale, but rather forms an oxide scale consisting of an outer layer of rapid growing TiO 2 and inner layer which is a mixture of TiO2 and AIzO3 7-9. The oxide scale is often prone to spallation during cooling to room temperature. Alloying elements such as Nb, W, Si, and Mo are known to be effective in improving the oxidation resistance of titanium aluminides, but V, Cr, or Mn are detrimental lcj-I4. As the engineering gamma titanium aluminide alloys have a variety of alloy compositions and microstructures, there may exist complex and synergistic effects on oxidation behavior, however, there have been limited systematic investigations of the oxidation behavior of engineering gamma titanium aluminide alloys ~5'~6. In this study, selected engineering gamma titanium aluminide alloys were subjected to isothermal as well as cyclic oxidation exposure tests in air to estimate and compare their oxidation resistance. The results can be used for further development of engineering gamma alloys with optimum relationship between mechanical properties and oxidation resistance.
EXPERIMENTAL PROCEDURE