The individual effects of niobium and silicon on the oxidation behaviour of Ti3Al based alloys

The oxidation kinetics of the Ti3Al based alloys with Nb (0 to 20 at%) or Si (0 to 15 at%) addition was carefully measured at 800 and 900°C, respectively. The oxide scale was characterised by high temperature in-situ XRD, SEM, EPMA and conventional XRD. The weight gains of the alloys were greatly reduced by Nb or Si addition. The improvement of the oxidation resistance in the Ti3Al alloys by Nb addition is attributed to the formation of a compact scale. The formation of a compact scale is favoured not only by doping effect but also by the TiN layer which barriers the outward Al diffusion into the scale to form porous intermixed titania/alumina. Si addition promotes the formation of a compact TiO2 layer in the oxide scale and the formation of SiO2, thus the oxidation resistance of the Ti3Al based alloys is improved. The oxidation behaviour of the alloys with Si addition is a combinative function of the α2-Ti3Al matrix and the Ti5Si3 silicide phase.