Isothermal Oxidation Behavior of Ti-Si Eutectic Alloys with Al and Nb Addition

The oxidation behavior of Ti-Si eutectic alloys with various Al and Nb contents was investigated at 850°C in atmosphere. The oxide layer of various oxidation time were examined using scanning electron microscopy (SEM) equipped with an energy-dispersive x-ray analysis (EDX), x-ray powder diffraction (XRD), with the aid of optical microscopy (OM) techniques. The oxidation kinetics follows a parabolic rate law. The oxidation rate decreases gradually as the oxidation proceeds. The results of oxidation kinetics and oxide scales formed in the test temperature indicate that addition of 4.9 wt% Al and 8 wt% Nb imparts excellent oxidation resistance, while a higher Al content (6.9 wt%) and Nb (8 wt%) result in worse oxidation resistance. In all the alloys, the scale was composed of a layer of coarse grain titanium dioxide (TiO₂, rutile), alumina (Al₂O₃), and silicon dioxide (SiO₂), which mixed with each other to form a compact and strongly cohesive substrate for the further growth of oxide scale. Correct addition of Al and Nb increases the volume ratio of the continuous phase α-Ti(Al, Si, Nb) in the as-cast alloy, therefore providing more substrate for the growth of rutile-type TiO₂ and leading to the formation of a closed-packed structure in the scale. The quaternary Ti-Si eutectic alloys having good oxidation resistance was attributed to the formation of the multiplex oxide layer.