Abstract
The high-temperature structural applications of Ti2AlNb-based alloys, such as in jet engines and gas turbines, inevitably require oxidation resistance. The objective of this study is to seek fundamental insight into the oxidation behavior of a Ti2AlNb-based alloy via detailed microstructural characterization of oxide scale and scale/substrate interface after oxidation at 800 °C using X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM). The oxide scale exhibits a complex multi-layered structure consisting of (Al,Nb)-rich mixed oxide layer (I)/mixed oxide layer (II)/oxygen-rich layer (III)/substrate from the outside to inside, where the substrate is mainly composed of B2 and O-Ti2AlNb phases. High-resolution TEM examinations along with high-angle annular dark-field (HAADF) imaging reveal: (1) the co-existence of two types (α and δ) of Al2O3 oxides in the outer scale, (2) the presence of metastable oxide products of TiO and Nb2O5, (3) an amorphous region near the scale/substrate interface including the formation of AlNb2, and (4) O-Ti2AlNb phase oxidized to form Nb2O5, TiO2 and Al2O3.
Highlights
Ti2AlNb-based alloy, sometimes referred to as orthorhombic alloy[1,2], is a class of highly promising lightweight high-temperature materials
An understanding of high-temperature oxidation mechanisms is essential for improving the oxidation resistance of materials
The oxidation behavior and mechanisms are investigated at a higher temperature of 800 °C in this study
Summary
Ti2AlNb-based alloy, sometimes referred to as orthorhombic alloy[1,2], is a class of highly promising lightweight high-temperature materials This type of alloy is considered to partially substitute the high-density (ρ = 8~8.5 g/ cm3) Ni-based superalloys in the aerospace industry due to its low density, high strength, superior plasticity, high fracture toughness and excellent creep resistance at elevated temperatures[3,4,5,6,7,8,9]. In such applications, the operating temperatures could go beyond 600–650 °C10,11, leading to severe oxidation of the alloy surface[12,13,14]. Lu et al.[35] observed the substitution of Ti by Nb via high-resolution transmission electron microscopy (HRTEM) Z-contrast imaging, as www.nature.com/scientificreports/
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