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

Abstract

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 Ti 7Al 5Si 12 and Al 12Si 3Ti 5, 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) m 2/s. A protective SiO 2-based scale and alumina-based scale were formed at the outermost surface during isothermal oxidation at 1273 K from the Ti 7Al 5Si 12 and Al 12Si 3Ti 5, 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 K p value of 0.7216 g 2/m 4 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.