TiAl基合金の高温酸化挙動に及ぼすH2O, CO2およびN2の影響

Abstract

TiAl-based alloys (Ti-48Al-2Cr-2Nb, Ti-48Al -2Cr-2Fe, Ti-48Al-2Cr-2W and Ti-48Al-2Cr-1W-1Ta in mol%) were oxidized isothermally at 1123 K for 360 ks in a flow of O2, O2-H2O, H2O, O2-CO2, CO2, O2-N2 or N2 and the influence of H2O, CO2 and N2 in the oxidizing atmosphere on the oxidation behavior has been studied. The oxidation of Nb, W or Ta containing alloys in O2 results in the formation of continuous Al2O3 layers in the scales and thus in superior oxidation resistance. On the other hand, the alloy with Fe shows large mass gains due to oxidation without forming a continuous Al2O3 layer. The presence of H2O in the atmosphere accelerates the oxidation of all the alloys, and this influence is prominent for the Fe-containing alloy. Since H2O affects more significantly TiO2 than Al2O3, the observed influence of H2O is attributable to the anisotropic and enhanced growth of TiO2. While the oxidation behavior of the alloy with Fe is affected by the presence of CO2, the other alloys are not susceptible to it. Although N2 is less influential than H2O and CO2, the alloys with Nb, W or Ta show larger mass gains in O2-N2 than in O2. The influence of CO2 or N2 is attributable to the formation of carbides or nitrides respectively in the scales that impede the continuity of the protective Al2O3 layers. The influence of these gas species is smaller for the alloys forming protective Al2O3 layers in the scales and vice versa.