Scale mass gain, morphology and phase composition of air and steam oxidized electron beam melted and cast Ti-48Al-2Nb-0.7Cr-0.3Si alloys

Oxidation resistance of γ - TiAl alloy RNT650 exposed to air and steam at 650 °C for 1000 h is reported. The samples were prepared either by conventional casting (CM) or electron beam melting (EBM). The materials were analysed using XPS, XRD, SEM and STEM/EDS techniques. Mass gains in both alloys were comparable and relatively small for air oxidation, but for steam treatment the mass increase in the EBM alloy was up to 50% more than in the CD material. Mass gain showed a faster growth of the scale in the steam treated material. The oxidized materials developed thin nano-crystalline scales predominantly of aluminium and titanium oxides arranged in TiO2+Al2O3/TiO2/Al2O3 sublayers. Whiskers were detected sticking above the scale surfaces, with density much higher in steam treated specimens.