The role of Cr in promoting protective alumina scale formation by gamma-based Ti-Al-Cr alloys .1. Compatibility with alumina and oxidation behavior in oxygen

The substitution of greater than 8-10 at.% Cr for Ti in Ti-Al reduces the level of Al needed for protective alumina scale formation during oxidation, the ''Cr effect''. To elucidate the mechanism of the Cr effect, the interaction of binary Ti-Al and ternary Ti-Al-Cr alloys with alumina was examined at 1000 degrees C by hot-pressed diffusion couples. The oxidation behavior of the same alloys was also examined al 1000 degrees C in dry oxygen. The dissolution of alumina in an alloy/alumina diffusion couple correlated with an inability of the alloy to form a protective alumina scale on exposure in dry oxygen because of internal attack. Both of these effects were associated with high oxygen permeability in the alloy. It was postulated that the Cr effect results from the Ti(Cr,Al)(2) Laves phase, which has a low oxygen permeability and is capable of alumina scale formation despite its relatively low Al content of 37-42 at.% Al. (C) 1997 Acta Metallurgica Inc.