Thermodynamic and microstructural study of Ti₂AlNb oxides at 800 °C

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.

Introduction

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 °C^10,11, leading to severe oxidation of the alloy surface^12,13,14. There are three potential approaches to improve high-temperature oxidation resistance: alloying^15,16,17, pre-oxidation¹⁸, and coating^{19,20,21,22,23,24,25,26}. For example, Wang et al.²⁷ reported that a two-step voltage-controlled microarc oxidation (MAO) method can be used to produce ceramic coatings on a Ti₂AlNb-based alloy. However, after a prolonged exposure to air at elevated temperatures, intermetallics exhibit oxygen-induced embrittlement characteristics such as low ductility and brittle fracture^16,28,29. Thus, an understanding of high-temperature oxidation mechanisms is essential for improving the oxidation resistance of materials. In our previous study³⁰, a Ti₂AlNb-based alloy was observed to exhibit fairly good oxidation resistance below 750 °C. After reaching 800 °C, the oxidation resistance decreased dramatically. Thus, the oxidation behavior and mechanisms are investigated at a higher temperature of 800 °C in this study.

Mass transfer is known to be the essence of oxidation reaction. During the high-temperature oxidation of a Ti₂AlNb-based alloy, O and N elements diffuse inward, whereas Al, Ti and Nb elements diffuse outward. Among the many elements that can improve the oxidation resistance, such as Al, Nb, Mo, Si, Zr, etc., Al and Nb are the most important elements³¹. While Al atoms and O atoms are able to generate a continuous and dense Al₂O₃ protective layer on the alloy surface and thus improve the oxidation resistance of alloys, this is not the case in Ti₂AlNb-based alloys. The Gibbs free energy of Al₂O₃ and TiO₂ is so similar that both oxides are produced almost simultaneously³². The addition of the element Nb can improve the oxidation resistance of the alloy: Nb substitutes for Ti in TiO₂ as a cation with a valence of 5, while no Nb is present in Al₂O₃^{33,34,35,36,37}. The doping of Nb in TiO₂ grains reduces oxygen vacancy and Ti cations, which impedes the mass transfer in TiO₂³⁴. Lu et al.³⁵ observed the substitution of Ti by Nb via high-resolution transmission electron microscopy (HRTEM) Z-contrast imaging, as represented by the Nb enrichment in TiO₂ grains of the mixture layer. Vojtěch et al.³⁶ studied the role of the addition of 2 at.% Nb to the eutectic TiAl-Ti₅Si₃ alloy, and reported that Nb markedly influences oxidation kinetics, with a six-fold decrease of oxidation rate.

Some fundamental aspects of oxidation behavior, such as weight gain, scale morphology, and structure, have been investigated^{12,21,38,39,40,41}. Leyens and Gedanitz¹² studied the mass gain and oxidation rate of a Ti-22Al-25Nb alloy in air between 650 °C and 800 °C, and reported a fairly good oxidation resistance at 650 °C up to 4000 hours and at 700 °C up to 500 hours, whereas at 800 °C “breakaway” oxidation occurred after about 100 hours. Wang et al.²¹ observed layers of TiO₂ and a small amount of AlNbO₄ with needle-like TiO₂ crystals present all over the surface. Ralison et al.^38,39 reported a multi-layered scale $({{\rm{TiO}}}_{2}+\frac{1}{2}{{\rm{AlNbO}}}_{4}/({\rm{eventually}}\,{\rm{Ta}},\,{\rm{Mo}})-{\rm{rich}}\,{{\rm{AlNbO}}}_{4})$ along with an oxygen-affected zone in a Ti-27Al-15Nb alloy at 800 °C in air, and Al₂O₃/(TiO₂ + AlNbO₄)/Ta-rich Al₂O₃/oxygen-affected zone in a Ti-27Al-10Nb alloy at the same temperature of 800 °C. Some cracks were present in the multi-layered scale. Zheng et al.⁴⁰ studied the oxidation behavior of a Ti-22Al-25Nb alloy at 800 °C for 300 hours and observed the formation of a mixed oxide scale on the alloy surface, which was predominantly composed of TiO₂, AlNbO₄, and Nb₂O₅.

These studies revealed a complex scale structure containing oxidation products of Al₂O₃, TiO₂, Nb₂O₅, AlNbO₄, etc., with an outer layer consisting mainly of TiO₂. When the content of the element Nb is high enough, Nb₂O₅ or AlNbO₄ would be present, however, they are prone to spall-off and are unfavorable to the oxidation resistance. As for the structure of inner oxide layer, Małecka⁴¹ observed that it consists of an Al-rich layer and Ti, Al (Nb, Mo, V)-rich zone. Li et al.⁴² reported that it has such a structure: TiO₂-rich layer/AlNbO₄-rich layer/TiO₂-rich layer/AlNbO₄-rich layer/oxygen and a nitride-enriched zone. Leyens¹⁴ reported that when the temperature is above 900 °C there exists a nitrogen-enriched layer underneath the oxide scale, i.e., a nitride-containing layer. However, the questions remain as to how the oxide scale containing various oxides is formed; in what form (crystalline or amorphous) the substance/scale interface region would be; and if different types of Al₂O₃ can be co-existent in the oxidation of Ti₂AlNb-based alloys. The objective of the present study is to address these questions via detailed microstructrual examinations using different advanced techniques along with thermodynamic calculations.

Materials and Methods

The selected material is as-cast Ti₂AlNb alloy with a nominal composition of Ti-22Al-20Nb-2V-1Mo-0.25Si (in at.%). The alloy ingot was cut into small plates with a size of 8 × 8 × 3 mm by electro-discharge machining. The surface of the samples was ground with sandpaper from grit #400 to #1200, ultrasonically cleaned in acetone for 15 mins. The dimensions were measured using a Vernier caliper and the samples were weighted using an analytical balance with an accuracy of 0.00001 g. During the isothermal oxidation in air at 800 °C, the samples were taken out of the furnace at intervals of 1, 3, 6, 12, 24, 36, 50, 62, 74, 86, and 100 h, cooled to room temperature and weighed. It should be noted that the Ti₂AlNb-based alloy was observed to exhibit a fairly good oxidation resistance below 750 °C, while its oxidation resistance decreased considerably above 800 °C. Thus, the oxidation behavior and mechanism are studied in detail at 800 °C in the present investigation. The time of oxidation experiments was selected according to the standard HB5258-2000 of aerospace sector in China, where a duration of 100 hours is suggested to be sufficient. Also, if the oxidation time was too long, the oxide layer would peel off, causing difficulties for the study of the oxide scale.

A stereoscope was used to observe the grain sizes. XRD (Rigaku D/Max-2550) with a Cu K_α radiation (λ = 1.5418 Å) was used to identify the phases in the oxide scale at 50 kV and 200 mA with a diffraction angle (2θ) from 10° to 100° at a step size of 0.02° and 1 s in each step. SEM (Nova Nano SEM 450) was used to observe the surface morphology and cross-sectional structure of the oxide layer. For the observation of the oxide cross-section, it was necessary to mount the sample with resin, use sandpapers from grit #400 to #1200 to grind, and then diamond paste to polish the sample to a smooth mirror surface. Since the oxide is an insulator, it is necessary to perform carbon deposition on the polished surface before SEM observations. Elemental distribution in different regions on the cross-section of the oxide scale was characterized by using an electron probe microanalyzer (EPMA, Shimadzu 1720) with a resolution of 1 μm and secondary-electron image resolution of 6 nm using a beam current of 10 nA. TEM (FEI TECNAI G2 S-TWIN F20) was used to examine the structures of the oxide scale as well as the scale/substrate interface. To locate the scale/substrate interface more precisely TEM samples were prepared via the cutting of a focused ion beam (FIB, FEI, Helios nanolab 600). The dimension of FIB-TEM samples is: 5 μm in length, 4 μm in width, ~35 nm in thickness for the scale, and ~65 nm in thickness for the scale/substrate interface.

Results

Microstructures of as-cast Ti₂AlNb alloy

Ti₂AlNb-based alloy contains different volumes of the ordered phases β₀ (Strukturbericht: B2, space group: ${\rm{Pm}}\mathop{3}\limits^{-}{\rm{m}}$, Person symbol: cP2), α₂-Ti₃Al (Strukturbericht: DO₁₉, space group: P6₃/mmc, Pearson symbol: hP8), and the ordered orthorhombic O-Ti₂AlNb phase (Strukturbericht: A₂BC, space group: CmCm, Person symbol: oC16)⁴³. There exist crystallographic orientations of these phases^1,2,3: ${[1\bar{1}1]}_{B2}//{[11\bar{2}0]}_{{\alpha }_{2}}$, ${(011)}_{B2}//{(0001)}_{{\alpha }_{2}}$, ${[0001]}_{{\alpha }_{2}}//{[001]}_{O}$, ${(10\bar{1}0)}_{{\alpha }_{2}}//{(110)}_{O}$, ${[\bar{1}11]}_{B2}//{[1\bar{1}0]}_{O}$, ${(110)}_{B2}//{(001)}_{O}$. X-ray diffraction pattern, stereoscopic image, back-scattered electron (BSE) SEM micrograph, and TEM bright field image along with the relevant selected area electron diffraction (SAED) patterns of as-cast Ti₂AlNb alloy are shown in Fig. 1(a–g). XRD results reveal the presence of O-Ti₂AlNb phase, B2 phase, and α₂ phase in Fig. 1(a). The stereoscopic image indicates coarse grains in the Ti₂AlNb-cast alloy (Fig. 1(b)). The SEM image in Fig. 1(c) shows dark α₂ phase, gray O-Ti₂AlNb phase and B2 phase, which can be better seen in a magnified TEM image in Fig. 1(d). In Fig. 1(c), a small amount of α₂ phase is mainly located at grain boundaries in the Ti₂AlNb-based alloy. Then there are only B2 phase (matrix) and O-Ti₂AlNb phase (lath) presented in Fig. 1(d–g), with crystallographic orientations between them: ${[\bar{1}11]}_{B2}//{[1\bar{1}0]}_{O}$, ${(110)}_{B2}//{(001)}_{O}$.

Figure 1

X-ray diffraction pattern (a) and microstructure (b–g) of as-cast Ti₂AlNb-based alloy. (b) Stereoscopic image showing coarse grains; (c) SEM back-scattered electron micrograph showing dark α₂-Ti₃Al, gray B + O-Ti₂AlNb phases; (d) TEM bright field image along with (e–g) the corresponding SAED patterns of points 1 and 2, and crystallographic orientations, where 1 stands for O-Ti₂AlNb phase, and 2 denotes B2 phase.

Isothermal oxidation kinetics

Figure 2(a) shows a curve of isothermal oxidation kinetics of Ti₂AlNb-cast alloy at 800 °C. The obtained weight gain of this alloy after 100 h at 800 °C was about 9.3 mg/cm². The relationship between oxidation and mass gain could be obtained by fitting the experimental data using the following equation,

$${\rm{\Delta }}{M}^{n}=k{}_{n}t,$$

(1)

where ΔM represents the weight gain per unit area (mg/cm²), n is an oxidation exponent (n = 1, liner relationship; n = 2, parabolic relationship), k_n is a rate constant (mgⁿ/cm²ⁿ·h), and t is oxidation time (h). The obtained oxidation exponent was ~0.83, being close to 1, thus suggesting that the oxidation kinetics of Ti₂AlNb-cast alloy at 800 °C obeyed basically a linear relationship and the oxide layer is not protective at this temperature.

Figure 2

Oxidation of Ti₂AlNb-cast alloy at 800 °C for 100 h. (a) Oxidation kinetics; (b) surface morphology of scale; (c) XRD pattern of the oxide scale on the surface and substrate; (d) SEM back-scattered electron micrograph of cross-structure scale; (e) EPMA mapping of a selected area in (d).

Surface morphology and structure of scale

Secondary electron micrograph, XRD pattern, back-scattered electron SEM micrograph, and EPMA mapping are shown in Fig. 2(b–e). Randomly-oriented and fairly-dense laminar-shaped oxides of about 2~3 μm long are observed to cover the alloy surface, as seen in Fig. 2(b).

X-ray diffraction patterns of oxide scale on Ti₂AlNb alloy are shown in Fig. 2(c). It is seen that a large amount of AlNbO₄ and TiO₂ is present in the scale after oxidation at 800 °C. According to our previous studies³⁰, the laminar-shaped oxide is AlNbO_4. The cross-section of the overall scale can be seen from an SEM back-scattered electron (BSE) image shown in Fig. 2(d). The scale appears dense with a thickness of ~60 μm, and it consists of three layers based on the EPMA mapping in Fig. 2(e). The structure can be deduced to be (Al,Nb)-rich mixed oxide layer (I)/mixed oxide layer (II)/oxygen-rich layer (III) from the outside to inside. However, the presence of some Al₂O₃ is also found in layer I through TEM investigations.

EPMA point microanalyses were used to reveal the chemical composition in various locations of the cross-sectional scales in Fig. 2(d). According to XRD results in Fig. 2(c) and the chemical composition of points 1–10 in Table 1, AlNbO₄ and TiO₂ can be confirmed to be the main oxides in the scale. However, Al and Nb are richer at points 1–2 (in the outermost scale of ~5 μm thick) than at points 3–6, i.e., the (Al,Nb)-rich mixed outer layer (I) and mixed mid-layer (II). This should be a result of the rapid growth of TiO₂. The consumption of oxygen leads to a reduction in the oxygen partial pressure, but there is still sufficient oxygen (compared to the interior of the alloy) in the O-rich zone (III), as represented by points 7–10 where the content of oxygen was determined to be ~15 at.% or higher, as shown in Table 1. Also, a low content of nitrogen indicates that there is no nitride present in the oxide scale (points 1–6). N and O penetrated into O-rich zone (III) will cause an environmental brittleness and reduce the mechanical properties of the alloy²⁸.

Table 1 Elemental composition (at.%) determined via EPMA at points 1–10 in Fig. 2(d).

TEM study of oxide layer

TEM samples were taken from the outer scale (right) and at the scale/substrate interface (left) via FIB, as shown in Fig. 3(a). Figure 3(b) shows a TEM bright-field image, with the SAED pattern of point 1 given in Fig. 3(c), indicating the presence of AlNbO₄ phase. Figure 3(d) is an HRTEM image of point 2 in Fig. 3(b), where Fourier transformations of 2A, 2B, and 2 C were performed to obtain diffraction patterns shown in Fig. 3(e) to (g), corresponding to α-Al₂O₃, δ-Al₂O₃ and α-Al₂O₃. This suggests the co-existence of different types of Al₂O₃ oxides. It should be noted that, to the best of the authors’ knowledge, such a co-existent phenomenon of different forms of Al₂O₃ oxides observed via HRTEM has not been reported in the literature, where α-Al₂O₃ is a thermodynamically stable form (corundum form) while δ-Al₂O₃ is one of metastable transition forms/polymorphs of alumina. Figure 3(h) is a SAED pattern of point 3 in Fig. 3(b), which corroborates the presence of TiO₂.

Figure 3

Analysis of oxide scale after oxidation at 800 °C for 100 h. (a) Position of TEM samples prepared via FIB; (b) TEM bright field image; (c) and (h) the corresponding SAED patterns of point 1 and 3 in (b), and (d–g) the corresponding HRTEM and FFT images of point 2 in (b), where 1 represents AlNbO₄, 2 A denotes α-Al₂O₃, 2B stands for δ-Al₂O₃, 2 C signifies α-Al₂O₃, and 3 indicates rutile-TiO₂.

Figure 4 shows the microstructures of interface between Ti₂AlNb substrate and oxide scale, from a TEM sample taken at the interface of oxide layer/substrate (i.e., the left TEM sample shown in Fig. 3(a)). Figure 4(a,b) present a TEM bright-field image and a high angle annular dark-field (HAADF) image, respectively, where the left side represents the substrate and the right side represents the oxide scale. It should be noted that the white spots in the oxide layer in Fig. 4(a) are pores. The rutile-TiO₂ is found at point 1 in Fig. 4(c,d), with an amorphous surrounding. Nb content at point 2 is very high, and it has been identified as AlNb₂ (Fig. 4(e)), which is also one of the common oxidation products of the Ti₂AlNb-based alloy. Point 3 shows a TiO polycrystalline ring (Fig. 4(f)), which is further oxidized to become TiO₂. In Fig. 4(g,h) for point 4, Nb₂O₅ is revealed to be present at the boundary of two phases. This is due to the fact that the diffusion of oxygen at the phase boundary is faster as a result of the presence of phase (or grain) boundary energy, along with the reduced reaction activity of Ti and Al. Point 5 shows a lamellar O-Ti₂AlNb phase where brookite TiO₂ and O-Ti₂AlNb phases are present, which are shown in Fig. 4(i). In Fig. 4(j), TiO₂, γ-Al₂O₃, and Nb₂O₅ are observed to co-exist in the O-Ti₂AlNb laths, with the orientation relationships of γ-Al₂O₃ and TiO₂: ${(211)}_{\gamma -A{l}_{2}{O}_{3}}//{(220)}_{rutile-Ti{O}_{2}}$, ${(003)}_{\gamma -A{l}_{2}{O}_{3}}//{(112)}_{rutile-Ti{O}_{2}}$, $\,{[1\bar{2}0]}_{\gamma -A{l}_{2}{O}_{3}}//{[1\bar{1}0]}_{rutile-Ti{O}_{2}}$.

Figure 4

Microstructures of interface between as-cast Ti₂AlNb-based alloy substrate and oxide scale. (a) TEM bright field image of interface, where white regions reflect pores, (b) high angle annular dark field (HAADF) image of (a); (c) HRTEM of point 1, and (d) FFT image of (c); (g) HRTEM of point 4, and (h) FFT image of (g); (e), (f), (i) and (j) the corresponding SAED patterns of points 2, 3, 5, 6, where point 1 represents rutile-TiO₂, point 2 represents AlNb₂, point 3 represents TiO, point 4 represents Nb₂O₅, point 5 represents brookite TiO₂ and O-Ti₂AlNb phase, point 6 represents rutile TiO₂, Nb₂O₅ and γ-Al₂O₃.

Discussion

Evolution of B2 phase and O-Ti₂AlNb phase

As mentioned above, the Ti₂AlNb-based alloy consists of 3 phases, i.e., α₂-Ti₃Al, B2, and O-Ti₂AlNb. In the present Ti₂AlNb-cast alloy there is fewer α₂-Ti₃Al phase existing in the vicinity of grain boundaries and the O-Ti₂AlNb laths are present in the original coarsened B2 grains. The diffusion rate of oxygen atoms in the B2 phase is larger than that in the other two phases⁴⁴. It follows that no B2 phase is present at the interface. This means that oxidation reaction occurs first in the B2 phase, producing metastable TiO, which is further oxidized to TiO₂. The brightest regions on the HAADF image in Fig. 4(b) are Nb-rich areas and have been identified to be AlNb₂. It is sandwiched between the complete and residual O-Ti₂AlNb laths, which corresponds to the former position of the B2 phase. AlNb₂ is considered as a by-product during the oxidation of the Nb-containing TiAl-based alloy, which is due to the reaction of enriched Nb and Al elements⁴⁵. There are a large number of Nb element in the B2 phase⁴³. During oxidation, the Ti element in the B2 phase reacts with the O element to form TiO/TiO₂. The consumption of Ti causes the reaction of Al and Nb. As a result, AlNb₂ is surrounded by gray TiO/TiO₂ in Fig. 4(b). AlNb₂ is stably present at the interface, indicating that its oxidation resistance is relatively strong⁴⁵, however, the discontinuous nature of AlNb₂ at the interface makes the effect limited.

After this the O-Ti₂AlNb phase is oxidized. Combined with Fig. 4(f–j), there are brookite-TiO₂, rutile-TiO₂, γ-Al₂O₃, and Nb₂O₅ in the O-Ti₂AlNb phase. It can thus be considered that the O-Ti₂AlNb phase has been oxidized and decomposed into oxides of TiO₂, Al₂O₃, and Nb₂O₅. Al₂O₃ blocks the transport channel, hinders further growth of TiO₂, and improves oxidation resistance. As one of the oxide products, Al₂O₃ has been reported extensively in the literature. During the early stage of oxidation, the oxides at the interface exist in an amorphous form. Lu et al.²⁹ reported that polycrystalline TiO₂ and amorphous Al₂O₃ coexist in the scale. This is similar to the structure in Fig. 4(c). It should be noted that TiO₂ and Al₂O₃ occur almost simultaneously because of their similar Gibbs free energy⁴³, however, the energy is not high enough at an oxidation temperature of 800 °C, resulting in the presence of metastable alumina. To the best of the authors’ knowledge, there is no report about the structure of Al₂O₃ in the oxidation of a Ti₂AlNb-based alloy. In the present work, the unique formation of alumina is observed via HRTEM in the alloy after oxidation at 800 °C for 100 h. That is, the presence of three adjoining oxide grains and oxides in O-Ti₂AlNb laths, as shown in Figs 3(d) and 4(j), suggests the occurrence of phase change among three types of Al₂O₃, i.e., α-Al₂O₃, δ-Al₂O_3, and γ-Al₂O₃. The γ-Al₂O₃, θ-Al₂O₃, and κ-Al₂O₃ are common metastable phases in TiAl alloys at 900 °C^29,46,47. According to Yang et al.⁴⁸, γ-Al₂O₃ formed with twins in the oxidation of NiAl, and γ-Al₂O₃ twins were observed to play an important role in the scale growth. Cowley et al.⁴⁹ reported that {111} γ-Al₂O₃ twin boundaries provide a fast diffusion path for Al cations. This would improve the oxidation resistance of an alloy. However, in the present study of the Ti₂AlNb-based alloy at 800 °C, there exists δ-Al₂O₃ (monoclinic, a = 11.74 Å, b = 5.72 Å, c = 11.24 Å, β = 103.34°⁵⁰), while no γ-Al₂O₃ twins are observed. As reported by Levin and Brandon⁵¹, there is a route for the formation of Al₂O₃: Amorphous (anodic film) → γ → δ → θ → α-Al₂O₃. This would be the phase change route of Al₂O₃ in the present Ti₂AlNb-based alloy. Further studies in this aspect are needed at different oxidation temperatures.

Formation of AlNbO₄

As described above, there are TiO₂, Al₂O₃, and AlNbO₄ in the outer oxide layer, as presented in Fig. 3. Zheng et al.⁴⁰ assumed that the fast-growing Nb₂O₅ could react with Al₂O₃ that developed at the early stage of oxidation to form AlNbO₄. In the present work, Dmol3 module in Materials Studio 6.0 is used to calculate the Gibbs free energy change of reaction: Nb₂O₅ + Al₂O₃ → 2AlNbO₄, however, the temperature in the software is up to 1000 K only, which is lower than the temperature of the present oxidation experiment, 1073.15 K. Thus, the following equation is used to fit the relationship between the Gibbs free energy and temperature,

$$G={H}_{0}+aT\,\mathrm{ln}\,T+b{T}^{2}+\frac{c}{T}+IT,$$

(2)

where H₀, a, b, c, and I are fitting parameters. The results show an excellent matching close to 100%, as shown in Fig. 5 and Table 2. The relationships between the Gibbs free energy and temperature for Al₂O₃, Nb₂O₅, and AlNbO₄ can thus be expressed as follows,

$${{\rm{G}}}_{{{\rm{Al}}}_{2}{{\rm{O}}}_{3}}=1.18577-0.00368T\,\mathrm{ln}\,T-1.4271\times {10}^{-6}{T}^{2}+\frac{3.88868}{T}+0.01932T,$$

(3)

$${{\rm{G}}}_{{{\rm{Nb}}}_{2}{{\rm{O}}}_{5}}=2.07990-0.01388T\,\mathrm{ln}\,T-2.54067\times {10}^{-6}{T}^{2}+\frac{10.8132}{T}+0.05475T,$$

(4)

$${{\rm{G}}}_{{{\rm{AlNbO}}}_{4}}=6.74954-0.01287T\,\mathrm{ln}\,T-8.12388\times {10}^{-6}{T}^{2}+\frac{16.08122}{T}+0.06692T.$$

(5)

Figure 5

Gibbs free energy curves of Al₂O₃, Nb₂O₅ and AlNbO₄ before and after fitting.

Table 2 Fitting results of free energy curve of Al₂O₃, Nb₂O₅ and AlNbO₄ in Fig. 5.

Substituting T = 1073.15 K into the above equations yields the Gibbs free energy values for the formation of Al₂O₃, Nb₂O₅, and AlNbO₄, respectively, in the present oxidation condition of a higher temperature. As a result, the Gibbs free energy change of the reaction Nb₂O₅ + Al₂O₃ → 2AlNbO₄ becomes −1.00873 kJ/mol. This means that the reaction is thermodynamically possible. Also, Ai et al.⁵² reported that Nb₂O₅ reacted completely with Al₂O₃ to form AlNbO₄ in Nb₂O₅-Al₂O₃ ceramics. While Nb₂O₅ is consumed as a reactant, the formed AlNbO₄ makes the scale unprotective as well¹⁴. Furthermore, Nb₂O₅ is not observed in the XRD results (Fig. 2(c)), which can be mainly attributed to its reaction with Al₂O₃ as discussed above, in conjunction with the lower diffusion coefficient of Nb, the rapid growth of TiO₂, and the hindering effect of alumina.

Oxidation process at the interface

Based on the above observations and analyses, the high-temperature oxidation process of the Ti₂AlNb-based alloy can be summarized below and schematically shown in Fig. 6. Stage 1: Oxygen absorbs on the surface of the Ti₂AlNb-based alloy, which later penetrates into it. The B2 phase is oxidized to produce TiO, and oxygen dissolves in O-Ti₂AlNb phase. Stage 2: TiO is transformed into rutile-TiO₂ and AlNb₂ is formed in the areas of the B2 phase. Oxidation occurs, i.e., brookite-TiO₂ is generated inside the O-Ti₂AlNb phase and Nb₂O₅ outside the O phase. Stage 3: O-Ti₂AlNb phase breaks down to TiO₂, Nb₂O₅, and Al₂O₃. In the scale there is a reaction: Al₂O₃ + Nb₂O₅ → 2AlNbO₄, and N atoms are dissolved in the alloy because of the consumption of O atoms. The corresponding crystallographic structures of the related oxides of the Ti₂AlNb-based alloy are summarized in Table 3.

Figure 6

Schematic diagram showing a summary of high-temperature oxidation process of Ti₂AlNb-based alloy. Stage 1, by inward diffusion of oxygen B2 phase is transformed into TiO, and Ο-Ti₂AlNb is abound with oxygen. Stage 2, TiO is transformed into rutile-TiO₂ and AlNb₂ is formed in the areas of B2 phase. Oxidation occurs in O-Ti₂AlNb phase. Stage 3, O-Ti₂AlNb phase breaks down into Al₂O₃, Nb₂O₅ and TiO₂. Al₂O₃ reacts with Nb₂O₅ to form (AlNbO₄ + TiO₂) mixed oxide layer. At last, nitrogen dissolves in the alloy.

Table 3 Crystallographic structure of the related oxides.

Conclusions

• 1. After being exposed at 800 °C in static air for 100 h, the Ti₂AlNb-based alloy followed an almost liner kinetic law of oxidation and exhibited a multi-layered structure consisting of an (Al,Nb)-rich mixed oxide layer (I), mixed oxide layer (II), and oxygen-rich layer (III) from the outside to inside.

• 2. In the mixed outer scale, there existed α-Al₂O₃ and δ-Al₂O₃. Al₂O₃ reacted with Nb₂O₅ to form AlNbO₄, however, Nb₂O₅ and AlNbO₄ were not able to hinder the diffusion of oxygen.

• 3. The B2 phase was oxidized to form TiO₂, where Nb and Al were transformed into AlNb₂ at the interface during oxidation. AlNb₂ could hinder the diffusion of oxygen and improve the oxidation resistance of the Ti₂AlNb-based alloy, but its discontinuous nature allowed only a limited effect.

• 4. After long-term oxidation at 800 °C, O-Ti₂AlNb was oxidized to form TiO₂, Al₂O₃ and Nb₂O₅. Al₂O₃ could hinder the growth of TiO₂ in the O-Ti₂AlNb laths and form a compact scale. Increasing the amount of the O-Ti₂AlNb phase in the alloy contributes to the improvement in its high-temperature oxidation resistance.