The resistance of TiAl coupons to cyclic oxidation with temperature varying between room temperature and 1300 K in a flow of purified oxygen under atmospheric pressure has been significantly improved by preoxidation. The mass gain due to 20 cycle (400 h) oxidation was very small such as 5×10-3 kg m-2 and correspondingly the scale formed by the preoxidation thickened very slightly during the cyclic oxidation. The preoxidation was performed by heating the specimens, buried in a chromia powder pack and encapsulated in a silica tube under a vacuum of 1.3×10-3 Pa, at 1200 K for 100 ks. Excellent oxidation resistance obtained is attributable to the formation of an alumina-rich scale, virtually alumina scale, by the preoxidation under a very low oxygen partial pressure. The preoxidation in mixtures of chromia and metallic chromium powders resulted in the formation of thin alumina scales. However, they are not protective. Local growth of rutile mounds during the cyclic oxidation indicated that the scales have mechanically weak points which provided sites for easy access of oxygen to the substrate.
(Received February 18, 1994)
Keywords: oxidation resistance, titanium aluminide, high temperature, preoxidation, chromia powder, pack treatment, low oxygen pressure
† Present address: Mitsubishi Aluminum Co. Ltd., Shizuoka 410-11, Japan.
†† Present address: Koyo Precision Ind. Ltd., Osaka 582, Japan.