Oxidation behavior of TiAl(Cr, Ag) at 900 °C in water vapor

Ti-(48,52)Al-5Cr-xAg (x=0, 2, 3 at.%) alloys were made by Ar-arc melting and casting, followed by a homogenization treatment at 1100 °C in vacuum. The coupon samples were oxidized at 900 °C in room air and air with 30.8 vol.% H2O for 340 h. Ti-52Al-5Cr was unable to resist the oxidation in either “dry” or “wet” air. Ti-52Al-5Cr-2Ag was an alumina former in room air, but in wet air it formed non-protective oxides (directionally grown TiO2 and the mixture of TiO2 and Al2O3). Water vapor showed little effect on the oxidation resistance of Ti-(48,52)Al-5Cr-3Ag, which formed continuous α-Al2O3 scales in both atmospheres, with no sub-layers composed of the Laves phase observed beneath the Al2O3 scales. As compared with their as-cast alloys, Cr in γ-phase had partitioned to the Laves and/or L12 phase and Ag to the precipitates of Ag in γ-phase or the L12 phase after oxidation exposures, which was believed to have prevented Cr fluxing to the oxide/alloy interface to form the Laves phase sub-layers. The anisotropic diffusion of hydrogen in TiO2 was considered to be the main reason for the formation of directionally grown TiO2, which had deteriorated the oxidation resistance.