Microstructural evolution and high-temperature oxidation mechanisms of a titanium aluminide based alloy

Abstract

Oxidation resistance of titanium aluminide (TiAl) based alloys is a fundamental aspect for the high-temperature structural applications such as in the advanced hypersonic aircraft engines and gas turbines. The aim of this study was to identify oxidation kinetics and mechanisms through detailed microstructural characterization of a newly-developed Ti-44Al-4Nb-1.5Cr-0.5Mo-0.1B-0.1Y alloy via focused ion beam (FIB), transmission electron microscopy (TEM), X-ray diffraction (XRD), electron probe microanalysis (EPMA), scanning transmission electron microscopy (STEM), along with density functional theory (DFT) calculations. The alloy consisting mainly of γ-TiAl/α2-Ti3Al lamellar structure exhibited a superior oxidation resistance at 700 °C, and followed parabolic oxidation kinetics at 800 °C and 900 °C. The observed multi-layered scale structure consisted of TiO2, Al2O3-rich, Al2O3+TiO2, H-Ti2AlN+Al2O3+α2-Ti3Al, Z-Ti5Al3O2+AlNb2+Laves-(Ti,Nb)Cr2, and H-Ti2AlN/α2-Ti3Al lamellae from the outside to inside after high-temperature oxidation. The γ-TiAl/α2-Ti3Al lamellae near the scale/substrate interface were first transformed into H-Ti2AlN/α2-Ti3Al lamellae, with orientation relationships identified as (0001)α2//(0001)Ti2AlN, (101¯0)α2//(101¯0)Ti2AlN and [12¯10]α2//[12¯10]Ti2AlN. The H-Ti2AlN/α2-Ti3Al lamellae were then transformed into a metastable Z-Ti5Al3O2 phase at the scale/substrate interface. The Z-phase was decomposed to Ti3Al and Al2O3 as the scale/substrate interface moved inwardly. Ti3Al reacted further with oxygen and nitrogen to form Ti2AlN, which was finally oxidized to form TiO2 and α-Al2O3. A Nb-rich layer was present beneath the scale along with the formation of AlNb2 and Laves phase, and the doping effect of Nb to suppress the diffusion of oxygen occurred mainly in the TiO2+Al2O3 compound layer. The results obtained in this study would pave the way for the development of advanced oxidation-resistant TiAl-based materials for high-temperature applications.