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

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/α 2 -Ti 3 Al 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 TiO 2 , Al 2 O 3 -rich, Al 2 O 3 +TiO 2 , H-Ti 2 AlN+Al 2 O 3 +α 2 -Ti 3 Al, Z-Ti 5 Al 3 O 2 +AlNb 2 +Laves-(Ti,Nb)Cr 2 , and H-Ti 2 AlN/α 2 -Ti 3 Al lamellae from the outside to inside after high-temperature oxidation. The γ-TiAl/α 2 -Ti 3 Al lamellae near the scale/substrate interface were first transformed into H-Ti 2 AlN/α 2 -Ti 3 Al lamellae, with orientation relationships identified as ( 0001 ) α 2 / / ( 0001 ) T i 2 A l N , ( 10 1 ¯ 0 ) α 2 / / ( 10 1 ¯ 0 ) T i 2 A l N and [ 1 2 ¯ 10 ] α 2 / / [ 1 2 ¯ 10 ] T i 2 A l N . The H-Ti 2 AlN/α 2 -Ti 3 Al lamellae were then transformed into a metastable Z-Ti 5 Al 3 O 2 phase at the scale/substrate interface. The Z-phase was decomposed to Ti 3 Al and Al 2 O 3 as the scale/substrate interface moved inwardly. Ti 3 Al reacted further with oxygen and nitrogen to form Ti 2 AlN, which was finally oxidized to form TiO 2 and α-Al 2 O 3 . A Nb-rich layer was present beneath the scale along with the formation of AlNb 2 and Laves phase, and the doping effect of Nb to suppress the diffusion of oxygen occurred mainly in the TiO 2 +Al 2 O 3 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. [ABSTRACT FROM AUTHOR]