Effect of Al on oxidation behaviour of Ti–Si eutectic alloys

Abstract

Ti–Si eutectic alloys with various Al contents were fabricated using vacuum arc melting. The effect of Al alloying element additions on the isothermal oxidation behaviour of Ti–Si alloys was investigated at 800°C. The oxide scales formed at different oxidation stages were examined using X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with an energy dispersive X-ray analysis (EDX). The results show that Al additions appear to make the oxidation kinetics of Ti–Si eutectic alloys deviate significantly from a cubic rate expression to follow more closely parabolic kinetics. Addition of 4·9 wt-%Al significantly improves the oxidation resistance, while a lower Al content (1·8 wt-%) decreases the oxidation resistance. It was found that correct addition of aluminium increases the volume ratio of the continuous phase α-Ti in the as cast alloy, resulting in the simultaneous growth of TiO2, SiO2 and Al2O3. These oxide particles closely mix with each other to form a compact and strongly cohesive substrate for the further growth of oxide scale. It is this oxide layer that improves the oxidation resistance of the Ti–Si eutectic alloys.