The effect of long term exposure in oxidising and corroding environments on the tensile properties of two gamma-TiAl alloys

The lightweight γ-TiAl alloys offer good mechanical properties for gas turbine applications at high temperature (up to 750 °C). In order to replace the nickel-base alloys currently in use, however, their mechanical properties must be retained after exposure in a high temperature oxidising and corroding atmosphere. A complete study of the oxidation and corrosion processes of the two alloys, γ-Ti-46.5Al-4(Cr,Nb,Ta,B) and γ-Ti-45Al-X(Nb,B,C) was performed at temperatures from 700 to 800 °C. Hot corrosion tests were carried out with salt mixtures (93.7% Na2SO4+2.7% NaCl (wt%) or 75% Na2SO4+25% NaCl (wt%)) or with pure NaCl. A strong influence of the exposure on the tensile properties of these two γ-TiAl alloys has been demonstrated. Microscopic investigation showed that the bulk microstructure was stable. The main probable causes of these property reductions included the oxidation and corrosion processes and particularly the effect of niobium in the absence of chromium that induced major internal corrosion and premature failure.