Correlation of high temperature oxidation with tensile properties for Ti-48Al-2Cr-2Nb and Ti-48Al-2Cr-2Fe alloys

Ti-48Al-2Cr-2Nb and Ti-48Al-2Cr-2Fe (in mol%) were oxidized at 1173 K in a simulated combustion gas for 1800 and 10.8 ks, respectively. Then, their tensile properties (total elongation, yield strength, and ultimate tensile strength) were measured at 1073 K in air at a strain rate of 10−4 s−1. Acoustic emission was also measured during the tensile deformation. The oxide scale on 2Cr2Nb fails before the substrate in a form of cracking, fracture and spallation. The fracture mode is cleavage along the direction nearly normal to the loading direction. The scale on 2Cr2Fe fails from the beginning of the loading, because of its porous and loose structure. The tensile properties are degraded rapidly for the first 216 ks oxidation, and then a little less for 2Cr2Nb. They decrease linearly with the oxidation time of up to 10.8 ks for 2Cr2Fe. The degradation is attributable to the crack propagation in the substrate enhanced by (a) the formation of brittle oxide scales and Al-depletion layers (for 2Cr2Nb), (b) roughening of the substrate surface, and (c) composition change in the subsurface area. The degradation after long term oxidation for 2Cr2Nb can be explained in terms of (d) solution of gaseous species into the substrate, though this should be confirmed by further studies.