Effect of niobium on the structure and high-temperature oxidation of TiAl-Ti5Si3 eutectic alloy

Structure and high-temperature oxidation of Ti-40Al-5Si and Ti-39Al-5Si-2Nb alloys at 900 °C in air are studied. The structure, chemical and phase composition of the as-cast and oxidised alloys are determined by using scanning electron microscopy, energy dispersive spectrometry and X-ray diffraction. Oxidation kinetics is measured by weighing of samples and by using thermogravimetric analysis. It is found that the addition of 2 at.% of niobium to the eutectic TiAl + Ti5Si3 alloy has no significant effect on the structure consisting of TiAl matrix and silicide fibers or lamellae. Niobium homogeneously dissolves in both eutectic phases. In contrast, niobium markedly influences oxidation kinetics, since it reduces oxidation rate about six times. This reduction results from the doping effect and from the activation of Al. Silicon also plays a role in the oxidation process but its effect on the Al activation is weaker than of Nb. In addition, silicon is shown to stabilize anatase modification of TiO2 in the scales.