Improvement in Oxidation Resistance of TiAl by Preoxidation at 1200K in a Pack Mixture of Cr₂O₃ and Cr Powders

Abstract

The cyclic-oxidation resistance of a TiAl coupon at 1300K was significantly improved by the preoxidation at 1200K for 100ks in a pack of a mixture of 70mass% Cr₂O₃ and 30mass% metallic Cr powders. This is attributable to the formation of a thin dense scale of about 1μm thickness consisting mainly of α-Al₂O₃. The scale contains small amounts of TiO₂ and Cr₂O₃, and a very small amount of metallic Cr on its surface. The scale became thicker very slightly after ten-cycle oxidation (720ks) with no significant structural changes. Other combinations of temperature (1100, 1200, and 1300K) and pack composition (100, 70, and 30% Cr₂O₃ with balance of metallic Cr) lead to the formation of poorly protective scales. There is a general trend; when temperature is low or the amount of Cr₂O₃ in the pack is small, thin defective scales very rich in Al are formed, while when temperature is high or the Cr₂O₃ content is large, thick porous scales consisting of TiO₂ and Al₂O₃ are developed. The above can be explained in terms of the rate of oxygen supply.