Liquid-phase siliconizing by Al–Si alloys at the surface of a TiAl-based alloy and improvement in oxidation resistance Original Research Article

A simple and inexpensive method to modify the surface of a TiAl-based alloy by liquid-phase siliconizing by Al–Si alloys, has been proposed. The Al–12.87wt%Si alloy and the Al–10wt%Si alloy resulted in the main siliconizing products of Ti7Al5Si12 and Al12Si3Ti5, respectively. The increase of the thickness of the silicide coatings obtained by the Al–10wt%Si alloy obeyed the parabolic law, and the parabolic rate constant (k) can be described as k=5.266 × 10−7 exp(−115 379/RT) m2/s. A protective SiO2-based scale and alumina-based scale were formed at the outermost surface during isothermal oxidation at 1273 K from the Ti7Al5Si12 and Al12Si3Ti5, respectively. In particular, a remarkable effect on the improvement in the oxidation resistance at 1273 K was achieved by the siliconizing condition of 1053 K/20 min + 953 K/20 min using Al–10wt%Si alloy, with a low Kp value of 0.7216 g2/m4 h, only about 1/1050 of that of the bare alloy. The results of cyclic oxidation test of 1173 K/100 h followed by 1273 K/100 h showed that the alumina-based scale had a good adherence with the inner partially-oxidized silicide coating. The diffusion reactions between the silicide coating and the TiAl substrate were also discussed.