Long-time Oxidation of Ti-3(Al,Si)C-2 Carbides at 400-800 degrees C

Abstract

Quarternay carbides of Ti(3)AlxSi(1-x)C(2) (x=0.3, 0.5, and 0.7) were oxidized at 400, 600, and 800 degrees C for 0.5-6 months in order to study their long-time oxidation behavior in air. When they were oxidized at 400-600 degrees C for 0.5-3 months, oxidation proceeded relatively slowly with moderate weight gains. However, further oxidation at 400-600 degrees C for 6 months resulted in the oxidation-induced microcracking of oxide scales due to large volume expansion and large stress induced owing to the formation of Al2O3, SiO2. TiO3, and TiO2 in oxide scales. However, at 800 degrees C, microcracking of oxide scales, which could lead to pulverization of Ti3AlxSi1-xC2, did not occur due to stress relaxation in oxide scales. Instead, at 800 degrees C, Ti(3)AlxSi(1-x)C(2) 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 Ti3AlxSi1-xC2 at 400-800 degrees C was considered to be poor. Factors that determined the oxidation rates of Ti3AlxSi1-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 Ti3AlxSi1-xC2 and the matrix grain size were apparently not dominant factors, because the basic oxidation mode of Ti3Al(0.3)Si(0.7)C(2), Ti3Al0.5Si0.5C2, and Ti3Al0.7Si0.3C2 was similar under the identical oxidation condition.