Spallation of the oxide scale formed on TiAl alloys

The binary TiAl alloys and TiAl-based alloys were isothermally oxidized in air, and the oxidation behavior and scale spallation have been investigated. The mass gain due to oxidation of the TiAl-based alloys containing Nb, W or Ta is smaller than that of binary TiAl alloys, because of the suppression of TiO 2 growth by the so-called doping effect. The TîAl-based alloys show better scale adhesion than binary TiAl alloys. Although the scales formed on the alloys have similar structure regardless of the alloy composition, the scale/alloy interface of the TiAl-based alloys seems to have good contact compared with that of the binary alloys. The densities of the oxide scales formed on the alloys estimated from the mass gain and the scale thickness are found to be about a half value of the expected one for the completely densified oxides. This is attributable to the formation of the large amount of cavities and pores in the scale. The coefficient of the thermal expansion (CTE) of the alloys decreases with increase of the content of additional elements in the alloy. The better scale adhesion of the TiAl-based alloys may result from the structure of scale/alloy interface and low CTE value of the alloys.