Quarternay carbides of Ti₃Al_xSi_1-xC₂ (x=0.3, 0.5, and 0.7) were oxidized at 400, 600, and 800 °C for 0.5-6 months in order to study their long-time oxidation behavior in air. When they were oxidized at 400-600 ^oC for 0.5-3 months, oxidation proceeded relatively slowly with moderate weight gains. However, further oxidation at 400-600 ^oC for 6 months resulted in the oxidation-induced microcracking of oxide scales due to large

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... expansion and large stress induced owing to the formation of Al<usb>2</sub>O₃, SiO₂. TiO₃, and TiO₂ in oxide scales. However, at 800 ^oC, microcracking of oxide scales, which could lead to pulverization of Ti₃Al_xSi_1-xC₂, did not occur due to stress relaxation in oxide scales. Instead, at 800 ^oC, Ti₃Al_xSi_1-xC₂ oxidized rapidly to form thick, somewhat porous oxide scales, which consisted primarily of an outer TiO₂ layer with some Al₂O₃, an intermediate Al₂O₃ layer with some TiO₂, and an inner TiO₂ layer with some (SiO₂+Al₂O₃). The overall longtime oxidation resistance of Ti₃Al_xSi_1-xC₂ at 400-800 °C was considered to be poor. Factors that determined the oxidation rates of Ti₃Al_xSi_1-xC₂ were; (1) How fast titanium oxidized to semi-protective titanium oxides, (2) How fast Al and Si oxidized to Al₂O₃ and SiO₂ barrier oxides, and (3) Whether oxidation-induced microcracking occurred in oxide scales or not. The ratio of Al/Si in Ti₃Al_xSi_1-xC₂ and the matrix grain size were apparently not dominant factors, because the basic oxidation mode of Ti₃Al_0.3Si_0.7C₂, Ti₃Al_0.5Si_0.5C₂, and Ti₃Al_0.7Si_0.3C₂ was similar under the identical oxidation condition.