Thermodynamic and microstructural study of Ti2AlNb oxides at 800 °C

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.

Scientific RepoRTS | (2018) 8:12761 | DOI: 10.1038/s41598-018-31196-w represented by the Nb enrichment in TiO 2 grains of the mixture layer. Vojtěch et al. 36 studied the role of the addition of 2 at.% Nb to the eutectic TiAl-Ti 5 Si 3 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 Gedanitz 12 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 TiO 2 and a small amount of AlNbO 4 with needle-like TiO 2 crystals present all over the surface. Ralison et al. 38 along with an oxygen-affected zone in a Ti-27Al-15Nb alloy at 800 °C in air, and Al 2 O 3 /(TiO 2 + AlNbO 4 )/Ta-rich Al 2 O 3 /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 TiO 2 , AlNbO 4 , and Nb 2 O 5 .
These studies revealed a complex scale structure containing oxidation products of Al 2 O 3 , TiO 2 , Nb 2 O 5 , AlNbO 4 , etc., with an outer layer consisting mainly of TiO 2 . When the content of the element Nb is high enough, Nb 2 O 5 or AlNbO 4 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, Małecka 41 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: TiO 2 -rich layer/AlNbO 4 -rich layer/ TiO 2 -rich layer/AlNbO 4 -rich layer/oxygen and a nitride-enriched zone. Leyens 14 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 Al 2 O 3 can be co-existent in the oxidation of Ti 2 AlNb-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 Ti 2 AlNb 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 Ti 2 AlNb-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 Ti 2 AlNb alloy. Ti 2 AlNb-based alloy contains different volumes of the ordered phases β 0 (Strukturbericht: B2, space group: − Pm3 m, Person symbol: cP2), α 2 -Ti 3 Al (Strukturbericht: DO 19 , space group: P6 3 /mmc, Pearson symbol: hP8), and the ordered orthorhombic O-Ti 2 AlNb phase (Strukturbericht: A 2 BC, space group: CmCm, Person symbol: oC16) 43 . There exist crystallographic orientations of these phases 1-3 :  . 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 Ti 2 AlNb alloy are shown in Fig. 1(a-g). XRD results reveal the presence of O-Ti 2 AlNb phase, B2 phase, and α 2 phase in Fig. 1(a). The stereoscopic image indicates coarse grains in the Ti 2 AlNb-cast alloy ( Fig. 1(b)). The SEM image in Fig. 1(c) shows dark α 2 phase, gray O-Ti 2 AlNb 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 α 2 phase is mainly located at grain boundaries in the Ti 2 AlNb-based Isothermal oxidation kinetics. Figure 2(a) shows a curve of isothermal oxidation kinetics of Ti 2 AlNb-cast alloy at 800 °C. The obtained weight gain of this alloy after 100 h at 800 °C was about 9.3 mg/cm 2 . The relationship between oxidation and mass gain could be obtained by fitting the experimental data using the following equation, where ΔM represents the weight gain per unit area (mg/cm 2 ), n is an oxidation exponent (n = 1, liner relationship; n = 2, parabolic relationship), k n is a rate constant (mg n /cm 2n ·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 Ti 2 AlNb-cast alloy at 800 °C obeyed basically a linear relationship and the oxide layer is not protective at this temperature.
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).  Fig. 2(c). It is seen that a large amount of AlNbO 4 and TiO 2 is present in the scale after oxidation at 800 °C. According to our previous studies 30 , 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 Al 2 O 3 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, AlNbO 4 and TiO 2 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 TiO 2 . The consumption of oxygen leads to a reduction in the oxygen partial pressure, but there is still sufficient oxygen (compared to the interior of  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 alloy 28 .
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 AlNbO 4 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-Al 2 O 3 , δ-Al 2 O 3 and α-Al 2 O 3 . This suggests the co-existence of different types of Al 2 O 3 oxides. It should be noted that, to the best of the authors' knowledge, such a co-existent phenomenon of different forms of Al 2 O 3 oxides observed via HRTEM has not been reported in the literature, where α-Al 2 O 3 is a thermodynamically stable form (corundum form) while δ-Al 2 O 3 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 TiO 2 . Figure 4 shows the microstructures of interface between Ti 2 AlNb 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-TiO 2 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 AlNb 2 (Fig. 4(e)), which is also one of the common oxidation products of the Ti 2 AlNb-based alloy. Point 3 shows a TiO polycrystalline ring (Fig. 4(f)), which is further oxidized to become TiO 2 . In Fig. 4(g,h) for point 4, Nb 2 O 5 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-Ti 2 AlNb phase where brookite TiO 2 and O-Ti 2 AlNb phases are present, which are shown in Fig. 4(i). In Fig. 4

Discussion
Evolution of B2 phase and O-Ti 2 AlNb phase. As mentioned above, the Ti 2 AlNb-based alloy consists of 3 phases, i.e., α 2 -Ti 3 Al, B2, and O-Ti 2 AlNb. In the present Ti 2 AlNb-cast alloy there is fewer α 2 -Ti 3 Al phase existing in the vicinity of grain boundaries and the O-Ti 2 AlNb 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 phases 44 . 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 TiO 2 . The brightest regions on the HAADF image in Fig. 4(b) are Nb-rich areas and have been identified to be AlNb 2 . It is sandwiched between the complete and residual O-Ti 2 AlNb laths, which corresponds to the former position of the B2 phase. AlNb 2 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 elements 45 . There are a large number of Nb element in the B2 phase 43 . During oxidation, the Ti element in the B2 phase reacts with the O element to form TiO/TiO 2 . The consumption of Ti causes the reaction of Al and Nb. As a result, AlNb 2 is surrounded by gray TiO/TiO 2 in Fig. 4(b). AlNb 2 is stably present at the interface, indicating that its oxidation resistance is relatively strong 45 , however, the discontinuous nature of AlNb 2 at the interface makes the effect limited. After this the O-Ti 2 AlNb phase is oxidized. Combined with Fig. 4(f-j), there are brookite-TiO 2 , rutile-TiO 2 , γ-Al 2 O 3 , and Nb 2 O 5 in the O-Ti 2 AlNb phase. It can thus be considered that the O-Ti 2 AlNb phase has been oxidized and decomposed into oxides of TiO 2 , Al 2 O 3 , and Nb 2 O 5 . Al 2 O 3 blocks the transport channel, hinders further growth of TiO 2 , and improves oxidation resistance. As one of the oxide products, Al 2 O 3 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 TiO 2 and amorphous Al 2 O 3 coexist in the scale. This is similar to the structure in Fig. 4(c). It should be noted that TiO 2 and Al 2 O 3 occur almost simultaneously because of their similar  Fig. 2(d).
Scientific Formation of AlNbO 4 . As described above, there are TiO 2 , Al 2 O 3 , and AlNbO 4 in the outer oxide layer, as presented in Fig. 3. Zheng et al. 40 assumed that the fast-growing Nb 2 O 5 could react with Al 2 O 3 that developed at the early stage of oxidation to form AlNbO 4 . In the present work, Dmol3 module in Materials Studio 6.0 is used to calculate the Gibbs free energy change of reaction: Nb 2 O 5 + Al 2 O 3 → 2AlNbO 4 , 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, where H 0 , a, b, c, and I are fitting parameters. The results show an excellent matching close to 100%, as shown in Fig. 5 and     14 . Furthermore, Nb 2 O 5 is not observed in the XRD results ( Fig. 2(c)), which can be mainly attributed to its reaction with Al 2 O 3 as discussed above, in conjunction with the lower diffusion coefficient of Nb, the rapid growth of TiO 2 , 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 Ti 2 AlNb-based alloy can be summarized below and schematically shown in Fig. 6. Stage 1: Oxygen absorbs on the surface of the Ti 2 AlNb-based alloy, which later penetrates into it. The B2 phase is oxidized to produce TiO, and oxygen dissolves in O-Ti 2 AlNb phase. Stage 2: TiO is transformed into rutile-TiO 2 and AlNb 2 is formed in the areas of the B2 phase. Oxidation occurs, i.e., brookite-TiO 2 is generated inside the O-Ti 2 AlNb phase and Nb 2 O 5 outside the O phase. Stage 3: O-Ti 2 AlNb phase breaks down to TiO 2 , Nb 2 O 5 , and Al 2 O 3 . In the scale there is a reaction: Al 2 O 3 + Nb 2 O 5 → 2AlNbO 4 , 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 Ti 2 AlNb-based alloy are summarized in Table 3.

Conclusions
1. After being exposed at 800 °C in static air for 100 h, the Ti 2 AlNb-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-Al 2 O 3 and δ-Al 2 O 3 . Al 2 O 3 reacted with Nb 2 O 5 to form AlNbO 4 , however, Nb 2 O 5 and AlNbO 4 were not able to hinder the diffusion of oxygen. 3. The B2 phase was oxidized to form TiO 2 , where Nb and Al were transformed into AlNb 2 at the interface during oxidation. AlNb 2 could hinder the diffusion of oxygen and improve the oxidation resistance of the Ti 2 AlNb-based alloy, but its discontinuous nature allowed only a limited effect. 4. After long-term oxidation at 800 °C, O-Ti 2 AlNb was oxidized to form TiO 2 , Al 2 O 3 and Nb 2 O 5 . Al 2 O 3 could hinder the growth of TiO 2 in the O-Ti 2 AlNb laths and form a compact scale. Increasing the amount of the O-Ti 2 AlNb phase in the alloy contributes to the improvement in its high-temperature oxidation resistance.
Data Availability. The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.  Table 3. Crystallographic structure of the related oxides.