The effect of Si3N4 additions on the oxidation resistance of TiAl alloys

Abstract

The oxidation kinetics of TiAl alloys with and without 3 and 5 wt.% additions of Si3N4 particles were studied at 1173 and 1273 K in 1 atm of air. The Si3N4 dispersions were unstable in the matrix phase, so that some of them reacted with titanium during sintering to form Ti5Si3 and dissolved nitrogen. The oxide scale formed on TiAl-Si3N4 alloys consisted of an outer TiO2, an intermediate (Al2O3+TiO2), and an inner (TiO2+Al2O3) mixed layers. The enhanced alumina-forming tendency, the presence of discrete SiO2 particles below the outer TiO2 layer, and the improved scale adhesion by Si3N4 dispersions were attributable mainly to the increased oxidation resistance compared to the Si3N4-free TiAl alloys. Marker experiments showed that, for TiAl-Si3N4 alloys, the primary mode of scale growth was the outward diffusion of titanium ions for the outer scale and the inward transport of oxygen ions for the inner scale.