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 degreesC in vacuum. The coupon samples were oxidized at 900 degreesC 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-50-2Ag 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 alpha-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 gamma-phase had partitioned to the Laves and/or Ll(2) phase and Ag to the precipitates of Ag in gamma-phase or the Ll(2) 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. (C) 2003 Published by Elsevier Ltd.