Oxidation behavior of Ti45Al8.5Nb alloy anodized in NH4F containing solution

Journal Pre-proof Oxidation behavior of Ti45Al8.5Nb alloy anodized in NH4 F containing solution Lian-Kui Wu (Funding acquisition) (Writing - review and editing), Jun-Jie Xia, Mei-Yan Jiang, Qi Wang, Hai-Xin Wu, Dong-Bai Sun, Hong-Ying Yu, Fa-He Cao (Conceptualization) (Writing - review and editing)

PII:

S0010-938X(19)31247-8

DOI:

https://doi.org/10.1016/j.corsci.2020.108447

Reference:

CS 108447

To appear in:

Corrosion Science

Received Date:

22 July 2019

Revised Date:

21 November 2019

Accepted Date:

6 January 2020

Please cite this article as: Wu L-Kui, Xia J-Jie, Jiang M-Yan, Wang Q, Wu H-Xin, Sun D-Bai, Yu H-Ying, Cao F-He, Oxidation behavior of Ti45Al8.5Nb alloy anodized in NH4 F containing solution, Corrosion Science (2020), doi: https://doi.org/10.1016/j.corsci.2020.108447

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Oxidation behavior of Ti45Al8.5Nb alloy anodized in NH4F containing solution

Lian-Kui Wu a, b, Jun-Jie Xia b, Mei-Yan Jiang b, Qi Wang c, Hai-Xin Wu c, Dong-Bai Sun a, c, Hong-Ying Yu a, Fa-He Cao a, *

School of Materials, Sun Yat-sen University, Guangzhou 510275, China

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a

College of Materials Science and Engineering, Zhejiang University of Technology,

Hangzhou 310014, China

South China Sea Institution, Sun Yat-sen University, Guangzhou 510275, China

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c

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E-mail: [email protected]

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Ti45Al8.5Nb was anodized in NH4F containing solution. Nb can reduce the critical Al content to form a continuous alumina layer. The anodized Ti45Al8.5Nb exhibited excellent oxidation resistance at 1000 °C. The improved oxidation resistance can be ascribed to halogen effect.

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Highlights

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Abstract: Ti45Al8.5Nb was anodized in ethylene glycol and NH4F containing solution to improve the oxidation resistance. Isothermal oxidation test shows that the anodized specimens can provide excellent oxidation resistance and spallation resistance under 1000 °C. During the high temperature oxidation process, a compact and continuous alumina layer would generate on the anodized Ti45Al8.5Nb alloy. This protective

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alumina layer can efficiently decrease the inward diffusion of oxygen and outward diffusion of the substrate element. The improved oxidation resistance is assigned to the halogen effect caused by the fluorine component generated during the anodization process.

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Keywords: Titanium alloy; Anodization; Halogen effect; High temperature oxidation

1. Introduction

The high specific strength, low density, and good creep resistance at elevate

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temperature make TiAl alloy become promising candidate structural materials for

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application in aerospace, automotive and so on [1-6]. However, the inferior oxidation resistance at service temperature above 800 °C restricts the potential utilization of TiAl

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alloy due to the formation of non-protective oxide scale consisting of TiO2 and Al2O3

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[7-10].

Over the past several decades, various strategies have been developed to enhance the

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high temperature oxidation resistance of TiAl alloy [11-15]. Among them, increase the Al content or addition of a foreign element, such as Nb, Si, Zr, and rare element, into

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the alloy has received great attention [16-20]. It is regarded that the addition of suitable amount of niobium (>7% at.) affords the best oxidation protection and the derived alloy has already found its application for turbine wheels in turbocharger [21]. While the excessive high Nb content would deteriorate the oxidation resistance due to the generation of TiNb2O7, AlNbO4 or Nb2O5 phases in the scale [22]. It has been also

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reported that the addition of Nb is beneficial to enhance the activity of Al and reduce the critical Al content to form an external alumina layer [16, 23], therefore promoting the formation of continuous TiN layer at the oxide scale/substrate interface [22, 24, 25], lowering the dissolution of oxygen in the TiAl and suppressing the internal oxidation [26]. In spite of this, serious oxidation will occur on Nb containing TiAl alloy at

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temperature over 900 °C. The oxidation resistance of TiAlNb alloy can be further improved by surface treatment, such as introduction of halogen elements into the alloy, the so-called halogen effect [5, 27-30]. The mechanism of halogen effect to improve

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the oxidation resistance of TiAl alloy can be concluded as two aspects. Firstly, the

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volatilization of titanium halide at high temperature results in the enrichment of Al at the oxide scale/substrate interface and promotes the formation of protective Al2O3 [31,

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32]. Secondly, the outward diffusion of aluminum halide can reach to the interface of

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oxide scale/substrate via the cracks or pores and then oxidized to form Al2O3 [33-35]. Therefore, compact Al2O3 scale can generate and dramatically reduce the oxidation rate

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at high temperature [4]. Schütze M. has devoted a great deal of efforts to improve the oxidation resistance of TiAl alloy based on halogen effect [31, 33, 36-42]. Additionally,

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it has been reported that among all halogens fluorine can provide the best corrosion resistance for TiAl alloy under cyclic conditions [3]. Till now, several techniques have been developed to introduce fluorine into the TiAl surface, including beam-line ion implantation, plasma immersion ion implantation, spraying, painting or immersing of the TiAl with fluorine-contained compounds and

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dipping in fluorine containing acids [5, 28]. Recently, we proposed to prepare fluorine containing layer on TiAl alloy via anodization, a universal and ease operation strategy. In detail, TiAl alloy was acted as anode and anodized in a fluorine containing solution, including

1-butyl-3-methylimidazolium hexafluorophosphate

(BmimPF6)

[43],

ethylene glycol with BmimPF6 [44] or NH4F [45], and methanol with NaF [46]. Results manifested that the fluorine enriched anodic film could promote the formation of

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continuous Al2O3 scales under high temperature oxidation process.

The present work aims to enhance the high temperature oxidation resistance of Nb

containing TiAl alloy (Ti45Al8.5Nb) via anodization in a low-cost electrolyte system

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consisting of ethylene glycol (EG) and NH4F under moderate operation condition. The

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oxidation behavior of the anodized Ti45Al8.5Nb alloy was investigated at 1000 °C and

2. Experimental

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the protection mechanism of the anodized Ti45Al8.5Nb alloy was carefully discussed.

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Ti45Al8.5Nb alloy with chemical composition of Ti-45Al-8.5Nb-0.2W-0.02B-0.02Y was used as the substrate. The homogenized ingots were cut into 15 × 15 × 1.2 mm. All

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specimens were ground with emery paper with grit of # 60, then cleaned ultrasonically in acetone and ethanol for 5 min sequentially, rinsed with deionized water, finally blew-

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dried with warm air.

The ground and cleaned Ti45Al8.5Nb specimens were anodized in EG containing

0.05 M NH4F. Two graphite plates (100 × 25 mm2) were acted as the counter electrodes and placed face to face with a distance of 5 cm. Ti45Al8.5Nb alloy was hung in the middle of the two graphite plates, acted as the working electrode. Anodization was

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carried out at voltages varied from 1 to 5 V on an electrochemical workstation at 25 °C. After anodization, the specimens were taken out and ultrasonically cleaned in deionized water, acetone, and ethanol, respectively, finally dried with warm air. The oxidation behavior of the anodized Ti45Al8.5Nb alloy was investigated at 1000 °C in a box-type sintering furnace (KSL-1200X, Hefei Kejing Materials Technology Co., Ltd, China). Prior to the oxidation test, the crucible was heated at

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1200 °C until no mass change was occurred. The oxidation were performed at 1000 °C and lasted for 100 h. Each specimen was put in the crucible after treatment and naturally lean on the inner wall. The specimens were removed from the furnace at select time

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intervals and cooled down to room temperature for at least 60 min in air. After the

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measurement of total mass (including corundum crucible, substrate and spalled oxide scale), the specimens were put back into the furnace again. The weight gain per exposed

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area (mg cm-2) was used to evaluate the oxidation resistance, which was achieved by

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an electronic balance with a sensitivity of 0.1 mg. Microstructure and element analysis were characterized by scanning electron

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microscope (SEM, Carl Zeiss, Supra 55 ) and transmission electron microscopy (TEM, FEI, Talos F200). X-ray diffraction (XRD) patterns of the anodized Ti45Al8.5Nb alloy

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before and after oxidation were recorded on a Panalytical X'Pert PRO equipped with Cu Kα radiation (λ=0.154056 nm) at 40 kV and 40 mA. After oxidation, the phase composition of the Ti45Al8.5Nb was analyzed by Grazing Incidence XRD (GIXRD) with a grazing incidence angle of 0.8°. X-ray photo electron spectroscopy (XPS, Kratos Axis ultra DLD, Al Kα X-ray source, hυ = 1486.6 eV) was used to analyze the

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composition of the anodized Ti45Al8.5Nb alloy before and after oxidation. 3. Results and discussion 3.1 Current-time response during anodization Fig. 1 shows the evolution of anodic current recorded at different voltages. For all cases, the initial currents are high and dependent on applied voltage. Then the current sharply drops at the first seconds and the current-time curves exhibit various shapes

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dependent strongly on the applied voltage. Specifically, the current falls rapidly to negligible value when the applied voltage is low (1 V, curve 1), suggesting the

generation of compact anodic film [44]. While increase the voltage to 3 V and 5 V

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results in the current-time curves exhibiting different shapes (curve 2 and 3). The

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current-time response recorded at 5 V is enlarged in Fig. 1b. The curve can be divided into three stages, i.e., (I) formation of initiation anodic film, (II) formation of nanoscale

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pore, (III) progressive growth of the nanoscale [47]. This manifests that anodization

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under high voltage, such as, 3 V or 5 V, may result in the generation of porous anodic film.

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3.2 Morphology and composition of the anodic film The surface morphologies of Ti45Al8.5Nb (non-anodized) and anodized

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Ti45Al8.5Nb alloy are shown in Fig. 2. As for Ti45Al8.5Nb alloy, scratches derived from ground process are still observed (Fig. 2a). When the specimen was anodized at 1 V, no significant difference can be found on the surface, even if prolonging the anodization time from 10 min (Fig. 2b) to 1 h (Fig. 2c). While high magnified image indicates that a lot of nano-pores can be found on the specimen anodized at 1 V for 1 h

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(the inset in Fig. 2c). Further increase the anodization voltage to 3 V, different phenomenon emerges (Fig. 2d). It is revealed that some part of the anodic film is quite compact, while the other part is full of nano-pores whose sizes are larger than those on the specimen anodized at 1 V. Additionally, a great deal of micro-cracks are observed on the surface. As shown in Table 1, the O and F contents in the anodic film increase with the anodization voltage or duration time. While Al, Ti, and Nb contents perform

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opposite direction.

TEM bright field image directly shows that the specimen anodized at 1 V for 1 h exhibits porous structure (Fig. 3a). And the SAED pattern reveals that Al2O3 and TiO2

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with poor crystallization are contained in the anodic film (Fig. 3b). Fig. 3c shows the

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cross-sectional high-angle annular dark field (HAADF) image and corresponding elemental distribution maps of the anodic film. It can be seen that Al, Ti, O, and F are

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uniformly distributed in the anodic film.

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The surface composition and chemical state of the anodized Ti45Al8.5Nb alloy were analyzed by XPS (Fig. 4). The survey spectra reveals that the top-surface of this

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specimen consists of Ti, Al, Nb, F, and O (Fig. 4a). It should be noted that before XPS analysis the specimen was rinsed thoroughly to remove the possible residual NH4F

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contained in the pores. Therefore, the extremely high content of F indicates that the anodic film is fluorine enriched. The refined spectra of Ti 2p is displayed in Fig. 4b. The peaks located at binding energies of 459.1 eV and 464.6 eV correspond to the 2p 3/2 and 2p 1/2 spin-orbit doublet for TiO2. Besides, a small amount of titanium fluoride can also be detected. While the Al 2p spectra can be fitted with three independent peaks

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(Fig. 4c). Specifically, the peaks at 75.6 eV and 74.7 eV are attributed to Al-F and AlO, respectively. And the peak at 72.3 eV is assigned to metallic Al derived from the substrate. Notably, no metallic Ti is detected from the anodic film. The existence of metallic Al in the anodic film is beneficial for the formation of protective Al2O3 during the high temperature oxidation process then provide good high temperature oxidation resistance. The spectra of Nb 3d can be fitted well with a single peak and is assigned to

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Nb2O5 (Fig. 4d). The F 1s spectrum can be fitted with two peaks corresponding to Ti-F and Al-F, respectively (Fig. 4e). As shown in Fig. 4f, the O 1s spectra can be fitted by four independent peaks located at 533.2 eV, 532.4 eV, 531.4 eV, and 530.9 eV,

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indicating that there are four kinds of oxygen species in the anodic film. Specifically,

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the peaks at 532.4 eV, 531.4 eV, and 530.9 eV are ascribed to the lattice oxygen, i.e., Ti-O-Ti, Nb-O-Nb, and Al-O-Al bonds, respectively. While the peak at 533.2 eV is

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3.3 Oxidation test

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assigned to the adsorbed oxygen species.

Fig. 5 depicts the isothermal oxidation kinetics of Ti45Al8.5Nb and anodized

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Ti45Al8.5Nb alloys at 1000 °C in air. After oxidation for 100 h, the total mass gain (including the spallation) of Ti45Al8.5Nb alloy is 7.03 mg cm-2 (curve 1 in Fig. 5a).

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Whereas, anodization in NH4F can provide efficient oxidation resistance for Ti45Al8.5Nb, therefore significantly reduce the total mass gain (curve 2-5, Fig. 5). In detail, the mass gains are 1.67, 0.84, 0.85, and 1.0 mg cm-2 for Ti45Al8.5Nb alloys anodized at 1 V for 10 min, 1 V for 1 h, 3 V for 1 h, and 5 V for 1 h, respectively. Notably, the mass gain of the anodized specimens increases with applied voltage. This

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is consistent with the result observed from the micro-morphology which indicates cracks will generate in the anodic film prepared at high anodization voltage and the pores sizes in the anodic film also increase with the anodization voltage (Fig. 2). Interestingly, it is found that the optimal anodization voltage for Ti45Al8.5Nb is much lower than that for Ti50Al alloy [43-45]. This may be related to the synergetic effect of anodization and Nb element. And the existence of Nb makes the substrate more

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sensitive to exhibit halogen effect.

The spalling behavior was analyzed by only recording the mass change of the

substrate, i.e. the oxide scale spalled from the substrate was not weighted (Fig. 5b). It

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is clearly revealed that for Ti45Al8.5Nb alloy (curve 1), a rapid increase in mass change

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is observed until 20 h. Then, the mass change switches to dramatic loss process due to the spallation of the scale. After oxidation for 100 h, the net mass gain for Ti45Al8.5Nb

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alloy is -9.18 mg cm-2. Whereas, anodization in NH4F containing solution significantly

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improves the spallation resistance. This can be manifested from the kinetic curves, which depict the continuous and slight increase in the mass gain for anodized specimens

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(curves 2-5, Fig. 5b). Optical images directly reveal the enhanced spallation resistance (insets in Fig. 5b). It is shown that only small part of Ti45Al8.5Nb is covered by the

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newly generated oxide scale. As for the anodized specimens, however, no spallation is observed.

As shown in Fig. 6a, after oxidation at 1000 °C for 100 h, the remained oxide scale on Ti45Al8.5Nb alloy is composed of TiAl, Ti3Al, TiO2, Al2O3, and AlNb2. Moreover, the diffraction intensity of the substrate, i.e., TiAl is much stronger than other

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components. This is because that nearly all outmost oxide scale spalls off from the substrate (as shown in Fig. 5b), so that the substrate is easily to be detected by X-ray. The diffraction peaks of TiO2 and Al2O3 are ascribed to the residual oxide scale. In order to directly confirm the phase composition of the residual oxide scale formed on Ti45Al8.5Nb alloy, GIXRD characterization was conducted. As shown in Fig. 6b, all diffraction peaks are ascribed to TiO2, Al2O3, and NbO. This demonstrates that the

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diffraction peaks of TiAl, Ti3Al, and AlNb2 in Fig. 6a are derived from the inner oxide scale and substrate.

As for the specimen anodized at 1 V for 10 min (Fig. 7a), besides the diffraction

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peaks (TiAl and Ti3Al) derived from the Ti45Al8.5Nb substrate, TiO2 with strong

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intensity is also detected. This indicates that the oxide scale generated on anodized Ti45Al8.5Nb has good adhesion to the substrate. Prolonging the oxidation time to 1 h

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(Fig. 7b), the diffraction intensity of TiO2 decreases remarkably, and γ-TiAl emerges as

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the dominated phase again. This manifests that the anodic film can provide good oxidation resistance and the derived oxide scale is quite thin. While further increase the

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anodization voltage to 3 V (Fig. 7c) and 5 V (Fig. 7d) strengthens the diffraction intensity of TiO2. This is consistent with the kinetic oxidation results, which

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demonstrates that anodization at high voltage deteriorates the oxidation resistance. 3.4 Surface morphology and composition After 100 h oxidation at 1000 °C, the top-surface morphologies of Ti45Al8.5Nb and anodized Ti45Al8.5Nb alloy were characterized by SEM. Due to the poor spallation resistance, the oxide scale generated on Ti45Al8.5Nb has spalled for several times,

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leading to the newly exposed substrate oxidized again and formation of multi-layer structure. As shown in Fig. 8a, most of the oxide scale spalls off from the substrate, which is consistent with the optical images (the inset in Fig. 5b). EDS results show that the residual oxide scale and newly generate oxide scale have similar composition (Table 2). Anodization in NH4F containing electrolyte has great influence on the morphology

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of the oxide scale. In detail, for Ti45Al8.5Nb alloy anodized at 1 V for 10 min, two typical morphologies are observed (Fig. 8b-8d). This may be because that the anodic

film generated under such a short anodization time is quite inhomogeneous. Fig. 8c and

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8d are magnified images from the rectangles marked in Fig. 8b. The rough region with

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large quantities of rod-like oxide grains (Fig. 8d) are related to rutile TiO2 based on the EDS measurement (Table 2) and XRD analysis (Fig. 7a). While the other flat region

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with small oxide grains mainly consists of Al2O3 (Fig. 8c, and Table 2). The newly

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emerged Al2O3 region indicates that the high temperature oxidation resistance of this specimen is improved to some extent.

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Lengthening the anodization time to 1 h, the rod-like TiO2 grains nearly disappear and compact oxide scale composed of refined Al2O3 particles emerges (Fig. 8e-8f and

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Table 2). Further increase the anodization voltage to 3 V has no obvious influence on the morphology except for the increased surface rumpling (Fig. 8g and 8h), implying the deterioration in the oxidation resistance [48]. Despite this, no spallation or cracks is found on this specimen, indicating the good spallation resistance. 3.5 Cross-sectional microstructures

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After oxidation at 1000 °C for 100 h, the cross-sectional microstructures, corresponding elements depth profiles and distribution maps of Ti45Al8.5Nb alloy and anodized Ti45Al8.5Nb alloy were characterized. As displayed in Fig. 9, a multilayered oxide scale is observed on Ti45Al8.5Nb alloy. One should keep in mind that most of the oxide scale has spalled off from the substrate during the cooling down process and the observed structure is the residual oxide scale. Element depth profiles (Fig. 9b) and

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distribution maps (Fig. 9c) results indicate that the outmost layer of the oxide scale is

composed of mixed TiO2 and Al2O3. EDS analysis reveal that beneath this non-

protective mixed layer is a continuous Nb enriched layer (Table 3). It is known that the

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generation of TiO2 layer is controlled by oxygen diffusion via a vacancy mechanism.

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While Nb substitutes Ti4+ in TiO2 as a cation with valence 5, thus the growth of TiO2 is suppressed leading to the generation of dense and chemically uniform Nb enriched layer.

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After 100 h oxidation, the cross-sectional morphology of the Ti45Al8.5Nb alloy

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anodized at 1 V for 10 min is shown in Fig. 10. The corresponding elements distribution maps are also given. As mentioned above, the top surface morphology of this specimen

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is non-uniform (Fig. 8b). So, two typical cross-sectional morphologies are taken to give a comprehensive investigation. As shown in Fig. 10a and 10b, a thin and compact

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Al2O3-enriched outmost layer with thickness of 4 μm can be observed. Beneath this layer, there is a thick Al-depleted inner layer dispersed with island-like precipitates. EDS analysis indicates that this precipitate is composed of Al2O3 (Table 3). The generation of Al2O3 islands in the inner layer is directly related to the formation of compact Al2O3 outmost layer, which can efficiently retard the inward diffusion of

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oxygen, and decrease the oxygen partial pressure at the Al2O3 outmost layer/alloy interface, therefore resulting in the selective oxidation of Al in the Ti45Al8.5Nb alloy. Undoubtedly, oxide scale with similar structure can provide excellent oxidation resistance and is responsible to the improved oxidation resistance as shown in the isothermal oxidation test (Fig. 5). While, as shown in Fig. 10c and 10d, a thick oxide scale with mixed structure implies the severe internal oxidation and poor oxidation

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resistance. Based on XRD analysis (Fig. 7a) and EDS measurement (Table 3), the oxide scale at this region can be defined as TiO2 layer/Al2O3-rich layer/(Ti, Nb)O2-rich layer/Nb-rich layer from outmost layer to inner layer. This structure is quite similar to

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that formed on Ti45Al8Nb (Fig. 9). The generation and development of this non-

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protective oxide scale would deteriorate the long-term oxidation resistance of the specimen as observed in the isothermal oxidation test (Fig. 5).

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As shown in Fig. 11a, a dense and compact thin oxide scale is observed on the

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Ti45Al8.5Nb alloy anodized at 1 V for 1 h. Element depth profiles (Fig. 11b) and distribution maps (Fig. 11c) together with EDS measurement (Table 3) indicate that the

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continuous and compact outmost layer of the oxide scale is composed of Al2O3. Besides, some Al2O3 islands are dispersed in the Al-depleted inner layer. Notably, the size of the

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dispersed Al2O3 islands in the specimen anodized for 1 h is much smaller than that anodized for 10 min. This is attributed to the improved oxygen barrier property because the outmost Al2O3 layer becomes more and more compact when the anodization process was lasted for 1 h. For the Ti45Al8.5Nb alloy anodized at 3 V for 1 h (Fig. 12), the morphology and

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structure of the oxide scale are quite similar to those of the specimen anodized at 1 V for 1 h (Fig. 11). Despite this, it is shown that with the increase of anodization voltage the Al2O3 outmost layer becomes rougher and rougher. Moreover, some ridges and pegs are emerged in the oxide scale. These are derived from the different structure of the anodic film, which changes from compact to porous with the increase of anodization voltage (Fig. 2).

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After 100 h oxidation, the chemical composition of the Ti45Al8.5Nb anodized at 1 V for 1 h was characterized by XPS (Fig. 13). As shown in the survey spectra (Fig. 13a), signals of Al, Ti, Nb, O, and F are detected and semi-quantitative analysis reveal that

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the top surface of the oxide scale is Al-enriched. This is consistent with the SEM results,

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which demonstrate that the outmost oxide scale mainly consists of Al2O3 (Fig. 11). Moreover, it is found that Al is enriched on the top surface of the oxide scale . This can

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be ascribed to the halogen effect. In detail, during the high temperature oxidation

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process, titanium fluorine with boiling point of 284 °C will sublimate, while the aluminum fluorine will transfer from the anodic film to the surface to form Al2O3,

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resulting in an Al-enriched oxide scale and an Al-depleted inner layer (Fig. 11). The refined spectra of Al, Ti, and Nb can be fitted well with a single peak, corresponding to

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Al2O3, TiO2, and Nb2O5, respectively (Fig. 13b-d). As revealed in Fig. 13e, the O 1s spectra can be fitted with four peaks, which can be ascribed to Al2O3, TiO2, Nb2O5, and adsorbed oxygen species, respectively. Interestingly, 0.43% F is detected on this oxidized specimen (Fig. 13f). Whereas, our previous investigations shown that no matter the Ti50Al alloy was anodized in fluorine-containing ionic liquid system [43] or

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methanol containing NaF [46], no fluorine could be detected from the oxide scale. This difference may be related to the Nb element contained in the substrate, which makes the anodization is more sensitive to the applied potential. Specifically, the optimized anodization voltage dramatically reduces from 30 V to 1 V when Ti50Al was replaced by Ti45Al8.5Nb. During the high temperature oxidation process, Nb is beneficial for holding and storing fluorine in the oxide scale and providing long-term protectiveness.

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It has been demonstrated that the addition of Nb can reduce the critical Al content to

form a continuous alumina layer [1]. Therefore, much less aluminum-fluorine is needed to transfer to the surface to form compact and protective Al2O3 layer, which will

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efficiently suppress the further consumption of aluminum-fluorine. Schütze M. et al.

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also detected fluorine from the fluorine treated TiAl/Nickel-based alloys underwent for 1000 h oxidation [49].

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4. Conclusions

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Ti45Al8.5Nb was anodized in ethylene glycol and NH4F system to improve the high temperature oxidation resistance. Anodization in this electrolyte system results in the

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generation of Al- and F-enriched anodic film with porous structure. After oxidation at 1000 °C for 100 h, no cracks or spallation is observed on the anodized specimen, and

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the weight gain is only 0.84 mg cm-2 which is nearly one magnitude lower than that of the un-anodized specimen. The improved oxidation resistance is attributed to halogen effect caused by the contained fluorine element. When exposed to air at high temperature, the sublimation of titanium-fluorine compound and the selective transport of aluminum-fluorine can induce the formation of an Al-enriched outmost layer, which

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is beneficial for the generation of a dense and continuous alumina oxide scale. Besides, Nb can reduce the critical Al content to form a continuous alumina layer and therefore promote the formation of protective alumina oxide scale.

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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Author Contribution

Lian-Kui Wu: Data curation and analysis, Funding acquisition, Writing-Reviewing

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and Editing. Jun-Jie Xia: Oxidation test, Writing-Original draft preparation. Mei-Yan Jiang: TEM characterization. Qi Wang: XPS characterization. Hai-Xin Wu: SEM

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characterization. Dong-Bai Sun: Results discussion. Hong-Ying Yu: Results

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discussion. Fa-He Cao: Conceptualization, Writing- Reviewing and Editing.

Acknowledgement

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This work was financially supported by the National Natural Science Foundation of

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China (No. 51971205), the Fundamental Research Funds for the Central Universities (No. 19lgpy20), and Guangdong MEPP Fund (No. GDOE[2019]A16).

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temperature oxidation mechanisms of a titanium aluminide based alloy, Acta Mater, 148 (2018) 300-310. [9] S.W. Zeng, A.M. Zhao, H.T. Jiang, L. Luo, F. He, X. Li, Y.S. Ren, Evolution of Surface Morphology of Oxide Formed During Initial-Stage Oxidation of gamma-TiAl Alloy Using In Situ Environmental

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SEM, Oxid Met, 90 (2018) 649-655.

[10] Y. Huang, X. Peng, Z. Dong, Y. Cui, Thermal growth of exclusive alumina scale on a TiAl based alloy: Shot peening effect, Corros Sci, 143 (2018) 76-83. [11] L.-K. Wu, J.-J. Wu, W.-Y. Wu, G.-Y. Hou, H.-Z. Cao, Y.-P. Tang, H.-B. Zhang, G.-Q. Zheng, High

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temperature oxidation resistance of γ-TiAl alloy with pack aluminizing and electrodeposited SiO2 composite coating, Corros Sci, 146 (2019) 18-27. [12] L.-K. Wu, W.-Y. Wu, J.-L. Song, G.-Y. Hou, H.-Z. Cao, Y.-P. Tang, G.-Q. Zheng, Enhanced high

ur

temperature oxidation resistance for γ-TiAl alloy with electrodeposited SiO2 film, Corros Sci, 140 (2018) 388-401.

[13] W.Y. Gui, J.P. Lin, M.D. Liu, Y.H. Qu, Y.C. Wang, Y.F. Liang, Effects of nano-NiO addition on the

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microstructure and corrosion properties of high Nb-TiAl alloy, J Alloy Compd, 782 (2019) 973-980. [14] S.Q. Wang, F.Q. Xie, X.Q. Wu, L.Y. Chen, CeO2 doped Al2O3 composite ceramic coatings fabricated on gamma-TiAl alloys via cathodic plasma electrolytic deposition, J Alloy Compd, 788 (2019) 632-638. [15] K. Bobzin, T. Brögelmann, C. Kalscheuer, T. Liang, Thermal cyclic oxidation behavior of γ-TiAl with in situ post-annealed Al-Si-Y coating, Journal of Vacuum Science & Technology A, 37 (2019) 041401. [16] J.P. Lin, L.L. Zhao, G.Y. Li, L.Q. Zhang, X.P. Song, F. Ye, G.L. Chen, Effect of Nb on oxidation behavior of high Nb containing TiAl alloys, Intermetallics, 19 (2011) 131-136. [17] X. Gong, R.R. Chen, H.Z. Fang, H.S. Ding, J.J. Guo, Y.Q. Su, H.Z. Fu, Synergistic effect of B and Y on the isothermal oxidation behavior of TiAl-Nb-Cr-V alloy, Corros Sci, 131 (2018) 376-385.

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[18] Y. Wu, K. Hagihara, Y. Umakoshi, Influence of Y-addition on the oxidation behavior of Al-rich γTiAl alloys, Intermetallics, 12 (2004) 519-532. [19] X. Wu, Review of alloy and process development of TiAl alloys, Intermetallics, 14 (2006) 11141122. [20] Y. Garip, O. Ozdemir, Comparative study of the oxidation and hot corrosion behaviors of TiAl-Cr intermetallic alloy produced by electric current activated sintering, J Alloy Compd, 780 (2019) 364-377. [21] T. Tetsui, S. Ono, Endurance and composition and microstructure effects on endurance of TiAl used in turbochargers, Intermetallics, 7 (1999) 689-697. [22] H. Jiang, M. Hirohasi, Y. Lu, H. Imanari, Effect of Nb on the high temperature oxidation of Ti–(0– 50 at.%)Al, Scripta Mater, 46 (2002) 639-643. [23] Y. Shida, H. Anada, The effect of various ternary additives on the oxidation behavior of TiAl in high-temperature air, Oxid Met, 45 (1996) 197-219. [24] V.A.C. Haanappel, J.D. Sunderkötter, M.F. Stroosnijder, The isothermal and cyclic high temperature

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oxidation behaviour of Ti–48Al–2Mn–2Nb compared with Ti–48Al–2Cr–2Nb and Ti–48Al–2Cr, Intermetallics, 7 (1999) 529-541.

[25] S.K. Varma, A. Chan, B.N. Mahapatra, Static and Cyclic Oxidation of Ti–44Al and Ti–44Al–xNb Alloys, Oxid Met, 55 (2001) 423-435.

[26] Y. Shida, H. Anada, Role of W, Mo, Nb and Si on Oxidation of TiAl in Air at High Temperatures,

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Materials Transactions, JIM, 35 (1994) 623-631.

[27] M. Schütze, G. Schumacher, F. Dettenwanger, U. Hornauer, E. Richter, E. Wieser, W. Möller, The halogen effect in the oxidation of intermetallic titanium aluminides, Corros Sci, 44 (2002) 303-318.

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[28] S. Friedle, R. Pflumm, A. Seyeux, P. Marcus, M. Schütze, ToF-SIMS Study on the Initial Stages of the Halogen Effect in the Oxidation of TiAl Alloys, Oxid Met, 89 (2018) 123-139. [29] N. Laska, S. Friedle, R. Braun, M. Schütze, Lifetime of 7YSZ thermal barrier coatings deposited on

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fluorine-treated γ-TiAl-based TNM-B1 alloy, Mater Corros, 67 (2016) 1185-1194. [30] M. Kumagai, K. Shibue, M.-S. Kim, M. Yonemitsu, Influence of chlorine on the oxidation behavior of TiAl-Mn intermetallic compound, Intermetallics, 4 (1996) 557-566. [31] H.E. Zschau, V. Gauthier, G. Schumacher, F. Dettenwanger, M. Schütze, H. Baumann, K. Bethge,

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M. Graham, Investigation of the Fluorine Microalloying Effect in the Oxidation of TiAl at 900°C in Air, Oxid Met, 59 (2003) 183-200.

[32] G. Schumacher, F. Dettenwanger, M. Schütze, Investigations of the microalloying effect of chlorine in the oxidation of TiAl alloys, Mater High Temp, 17 (2000) 53-58.

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[33] A. Donchev, M. Schütze, Improving the oxidation resistance of γ-titanium aluminides by halogen treatment, Mater Corros, 59 (2008) 489-493.

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[34] S.A. Tsipas, E. Gordo, A. Jiménez-Morales, Oxidation and corrosion protection by halide treatment of powder metallurgy Ti and Ti6Al4V alloy, Corros Sci, 88 (2014) 263-274. [35] S. Friedle, N. Laska, R. Braun, H.-E. Zschau, M.C. Galetz, M. Schütze, Oxidation behaviour of a fluorinated beta-stabilized γ-TiAl alloy with thermal barrier coatings in H2O-and SO2-containing atmospheres, Corros Sci, 92 (2015) 280-286. [36] G. Schumacher, C. Lang, M. Schütze, U. Hornauer, E. Richter, E. Wieser, Improvement of the oxidation resistance of gamma titanium aluminides by microalloying with chlorine using ion implantation, Mater Corros, 50 (1999) 162-165. [37] A. Donchev, B. Gleeson, M. Schütze, Thermodynamic considerations of the beneficial effect of halogens on the oxidation resistance of TiAl-based alloys, Intermetallics, 11 (2003) 387-398.

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[38] H.E. Zschau, M. Schütze, H. Baumann, K. Bethge, Application of ion beam analysis for the control of the improvement of the oxidation resistance of TiAl at 900 °C in air by fluorine ion implantation and HF-treatment, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 240 (2005) 137-141. [39] A. Donchev, E. Richter, M. Schütze, R. Yankov, Improvement of the oxidation behaviour of TiAlalloys by treatment with halogens, Intermetallics, 14 (2006) 1168-1174. [40] H.-E. Zschau, M. Schütze, H. Baumann, K. Bethge, The time behaviour of surface applied fluorine inducing the formation of an alumina scale on gamma-TiAl during oxidation at 900°C in air, Intermetallics, 14 (2006) 1136-1142. [41] A. Donchev, E. Richter, M. Schütze, R. Yankov, Improving the oxidation resistance of TiAl-alloys with fluorine, J Alloy Compd, 452 (2008) 7-10. [42] P.J. Masset, M. Schütze, Thermodynamic Assessment of the Alloy Concentration Limits for the Halogen Effect of TiAl Alloys, Adv Eng Mater, 10 (2008) 666-674.

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[43] M.-H. Mo, L.-K. Wu, H.-Z. Cao, J.-P. Lin, D.-H. Lu, G.-Q. Zheng, High temperature oxidation behavior and anti-oxidation mechanism of Ti-50Al anodized in ionic liquid, Surf Coat Technol, 307 (2016) 190-199.

[44] M.-H. Mo, L.-K. Wu, H.-Z. Cao, J.-P. Lin, G.-Q. Zheng, Improvement of the high temperature oxidation resistance of Ti–50Al at 1000 °C by anodizing in ethylene glycol/BmimPF6 solution, Surf Coat

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Technol, 286 (2016) 215-222.

[45] M.-H. Mo, L.-K. Wu, H.-Z. Cao, J.-P. Lin, G.-Q. Zheng, Halogen effect for improving high temperature oxidation resistance of Ti-50Al by anodization, Appl Surf Sci, 407 (2017) 246-254.

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[46] L.-K. Wu, J.-J. Xia, H.-Z. Cao, W.-J. Liu, G.-Y. Hou, Y.-P. Tang, G.-Q. Zheng, Improving the HighTemperature Oxidation Resistance of TiAl Alloy by Anodizing in Methanol/NaF Solution, Oxid Met, 90 (2018) 617-631.

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[47] D. Kowalski, D. Kim, P. Schmuki, TiO2 nanotubes, nanochannels and mesosponge: Self-organized formation and applications, Nano Today, 8 (2013) 235-264. [48] N.P. Padture, M. Gell, E.H. Jordan, Thermal barrier coatings for gas-turbine engine applications, Science, 296 (2002) 280-284.

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[49] A. Donchev, H.E. Zschau, M. Schütze, The halogen effect for improving the oxidation resistance of

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TiAl-alloys, Mater High Temp, 22 (2005) 309-314.

19

List of figure captions Fig. 1 (a, b) Current-time response recorded on Ti45Al8.5Nb alloy at different anodic voltages for 1 h: 1 V (1), 3 V (2), and 5 V (3). (b) is the enlarged from curve 3 in (a).

Fig. 2 Top-surface SEM images of Ti45Al8.5Nb alloy (a) and Ti45Al8.5Nb alloy

The insets in a-d show the high-magnification images.

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anodized at various conditions: 1 V for 10 min (b), 1 V for 1 h (c), and 3 V for 1 h (d).

Fig. 3 TEM bright filed image (a), selected area electron diffraction (SAED) (b), high-

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angle annular dark field (HAADF) image and corresponding elemental distribution

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maps of Al, Ti, Nb, O, and F for Ti45Al8.5Nb alloy anodized at 1 V for 1 h. Fig. 4 The survey (a) and refined XPS spectra of Ti 2p (b), Al 2p (c), Nb 3d (d), F 1s

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(e), and O 1s (f) spectra for Ti45Al8.5Nb alloy anodized at 1 V for 1 h.

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Fig. 5 Oxidation kinetics of Ti45Al8.5Nb alloy (1) and anodized Ti45Al8.5Nb alloy (25) at 1000 °C for 100 h: with spallation (a), without spallation (b). Anodization was

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conducted at: 1 V for 10 min (2), 1 V for 1 h (3), 3 V for 1 h (4), and 5 V for 1 h (5). The insets in (b) are the optical images of the specimens after oxidation for 100 h.

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Fig. 6 XRD (a) and GIXRD (b) patterns of Ti45Al8.5Nb alloy oxidized at 1000 °C for 100 h.

Fig. 7 XRD patterns of anodized Ti45Al8.5Nb alloys after oxidation at 1000 °C for 100 h. Anodization was conducted at: 1 V for 10 min (a), 1 V for 1 h (b), 3 V for 1 h (c), and 5 V for 1 h (d).

20

Fig. 8 Top-surface SEM images of Ti45Al8.5Nb alloy (a) and anodized Ti45Al8.5Nb alloy (b-h) after oxidation at 1000 °C for 100 h. Anodization was conducted at: 1 V for 10 min (b-d), 1 V for 1 h (e, f ), and 3 V for 1 h (g, h). Fig. 9 After oxidation at 1000 °C for 100 h, the cross-sectional SEM image (a), corresponding elements depth profiles (b) and distribution maps (c) of Al, Ti, Nb, and O for Ti45Al8.5Nb alloy. The arrow and rectangle marked in (a) represent the direction

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and position where elements line scan and mapping analysis were carried out.

Fig. 10 After oxidation at 1000 °C for 100 h, the cross-sectional SEM image (a, c) and

corresponding element distribution maps (b, d) of Al, Ti, Nb and O for Ti45Al8.5Nb

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alloy anodized at 1 V for 10 min. (a) and (c) were taken from typical regions at the

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interface. The rectangles marked in (a, c) show the position where elements mapping analysis were carried out.

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Fig. 11 After oxidation at 1000 °C for 100 h, the cross-sectional SEM image (a),

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corresponding elements depth profiles (b) and distribution maps (c) of Al, Ti, Nb and O for Ti45Al8.5Nb alloy anodized at 1 V for 1 h. The arrow and rectangle marked in

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(a) show the direction and position where elements line scan and mapping analysis were carried out.

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Fig. 12 After oxidation at 1000 °C for 100 h, the cross-sectional SEM image (a), corresponding elements depth profiles (b) and distribution maps (c) of Al, Ti, Nb and O for Ti45Al8.5Nb alloy anodized at 3 V for 1 h. The arrow and rectangle marked in (a) show the direction and position where elements line scan and mapping analysis were carried out.

21

Fig. 13 After oxidized at 1000 °C for 100 h, the survey (a) and refined XPS spectra of Ti 2p (b), Al 2p (c), Nb 3d (d), O 1s (e), and F 1s (f) spectra for Ti45Al8.5Nb alloy anodized at 1 V for 1 h.

b) 2.0

1: 1 V, 1 h 2: 3 V, 1 h 3: 5 V, 1 h

I

4

2

3 2 1

0

900

1800

2700

III

1.8

1.6

1.4

1.2

-p

0

II

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Current / mA

Current / mA

a) 6

3600

0

100

200

300

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Time / s Time / s Fig. 1 (a, b) Current-time response recorded on Ti45Al8.5Nb alloy at different anodic

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voltages for 1 h: 1 V (1), 3 V (2), and 5 V (3). (b) is the enlarged from curve 3 in (a).

22

a)

b) 1

500 nm

500 nm

5 μm

5 μm

c)

d)

500 nm

4

Crack

500 nm

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2

3

5 μm

5 μm

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Fig. 2 Top-surface SEM images of Ti45Al8.5Nb alloy (a) and Ti45Al8.5Nb alloy

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anodized at various conditions: 1 V for 10 min (b), 1 V for 1 h (c), and 3 V for 1 h (d).

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The insets in a-d show the high-magnification images.

23

a)

b) Al2O3 (113) Al2O3 (104)

TiO2 (200)

1 μm

Ti

O

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Al

-p

c)

5 1/nm

5 μm

F

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Nb

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Fig. 3 TEM bright filed image (a), selected area electron diffraction (SAED) (b), highangle annular dark field (HAADF) image and corresponding elemental distribution

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maps of Al, Ti, Nb, O, and F for Ti45Al8.5Nb alloy anodized at 1 V for 1 h.

24

b)

Ti-F

0

468

466

Almet

464

462

458

78

456

76

e)

74

70

f)

F-Al

Nb-O

F-Ti

689 688 687 686 685 684 683 682

Binding Energy / eV

72

Binding Energy / eV

Binding Energy / eV

Ti-O

Al-O Oab

ro of

206

460

Binding Energy / eV

Intensity / cps

Intensity / cps

Nb-O

208

Al-F

Intensity / cps

Nb-O

210

Al-O

Al 2s Al 2p

Nb 3d

Ti-O

Binding Energy / eV

212

Intensity / cps

Intensity / cps

O 1s Ti 2p

F 1s

Ti 2s

1200 1000 800 600 400 200

d)

c)

Ti-O

C 1s

at / % 10.64 10.38 3.25 57.31 18.42 O KLL

C KLL

Ti LMMb

Intensity / cps

Element Ti Al Nb O F

F KLL

a)

534

532

530

528

Binding Energy / eV

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Fig. 4 The survey (a) and refined XPS spectra of Ti 2p (b), Al 2p (c), Nb 3d (d), F 1s

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(e), and O 1s (f) spectra for Ti45Al8.5Nb alloy anodized at 1 V for 1 h.

25

6

1

4

2

2 5 4 3

-2

b) 2

1: bare 2: 1 V, 10 min 3: 1 V, 1 h 4: 3 V, 1 h 5: 5 V, 1 h

Mass change / mg cm

Mass gain / mg cm

-2

a) 8

2 5 4 3

0 -2 1: bare 2: 1 V, 10min

-4 3: 1 V, 1 h 4: 3 V, 1 h 5: 5 V, 1 h

-6 -8

Oxide scale 1

0

-10 0

25

50

75

0

100

25

50

75

100

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Time / h Time / h Fig. 5 Oxidation kinetics of Ti45Al8.5Nb alloy (1) and anodized Ti45Al8.5Nb alloy (25) at 1000 °C for 100 h: with spallation (a), without spallation (b). Anodization was conducted at: 1 V for 10 min (2), 1 V for 1 h (3), 3 V for 1 h (4), and 5 V for 1 h (5).

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The insets in (b) are the optical images of the specimens after oxidation for 100 h.

26

a)

b)

1: TiAl

1

3: TiO2 3

2: Ti3Al

6

Intensity /.a.u

Intensity /.a.u

5: AlNb2

1 55 5

5 1 5 5 1 5 3 5 5 43 4 5

10

20

30

40

5 1 1 4 23 455

50

1 5 45

60

70

4 4 4 4 3

4

6 3 4

4

5

1

1 4

6: NbO

3

4: Al2O3

5

4: Al2O3

6

3: TiO2

4 3

1

80

10

20

30

40

50

60

4 4 3

6 3 3

70

6

80

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2 / degree 2 / degree Fig. 6 XRD (a) and GIXRD (b) patterns of Ti45Al8.5Nb alloy oxidized at 1000 °C for

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100 h.

27

a)

b)

1: TiAl

2: Ti3Al

3: TiO2

3: TiO2

4: Al2O3

1

5: AlNb2

4

4 3

2

4 5 31

10

20

30

40

50

5: AlNb2

3

2 3 4 4 3

4 3

3 4 211 4

60

70

80

10

20

30

12

Intensity /.a.u

20

30

3 531

40

4 2

50

3

4

5 211 44

60

70

4

4

3 4

2

21

10

80

4: Al2O3

5: AlNb2

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10

55

2: Ti3Al

-p

Intensity /.a.u

5: AlNb2

2

80

3: TiO2

4: Al2O3

34

70 1: TiAl

1

3: TiO2

3

2 3

60

2

2: Ti3Al

5 4

50

21

21 4 14

43 4

2 / degree d)

1: TiAl

4

40

3

2

4 43 4 3 31 5

42

21

2 / degree c)

4

4: Al2O3

2

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Intensity /.a.u

2

1: TiAl

1

2: Ti3Al

Intensity /.a.u

3

20

30

2

5 34 5 3 5 5 31

2

40

50

43

3 4

2 4 5 1 5

60

70

21

80

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2 / degree 2 / degree Fig. 7 XRD patterns of anodized Ti45Al8.5Nb alloys after oxidation at 1000 °C for 100

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h. Anodization was conducted at: 1 V for 10 min (a), 1 V for 1 h (b), 3 V for 1 h (c),

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and 5 V for 1 h (d).

28

a)

b)

Spallation 2

c) 3

Residual oxide scale

2 μm

d)

1

100 μm

e)

4

25 μm

f)

5 μm

h)

g)

6

5 10 μm

2 μm

10 μm

2 μm

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Fig. 8 Top-surface SEM images of Ti45Al8.5Nb alloy (a) and anodized Ti45Al8.5Nb

alloy (b-h) after oxidation at 1000 °C for 100 h. Anodization was conducted at: 1 V for

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10 min (b-d), 1 V for 1 h (e, f ), and 3 V for 1 h (g, h).

29

b)

a)

Ⅰ

Ⅱ

Ⅲ

1 2 3

Counts / a.u.

Al

Ni coating

Ti

Nb O

3 μm

c)

0

Ti

2

3

Distance / m Nb

4

5

O

ro of

Al

1

Fig. 9 After oxidation at 1000 °C for 100 h, the cross-sectional SEM image (a),

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corresponding elements depth profiles (b) and distribution maps (c) of Al, Ti, Nb, and

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O for Ti45Al8.5Nb alloy. The arrow and rectangle marked in (a) represent the direction

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and position where elements line scan and mapping analysis were carried out.

30

a)

b)

Ni coating

Al

Ti

Nb

O

Al

Ti

4 5 6 7 8

5 μm

c)

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d) 9 10

Nb

10 μm

O

-p

11 12

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Fig. 10 After oxidation at 1000 °C for 100 h, the cross-sectional SEM image (a, c) and corresponding element distribution maps (b, d) of Al, Ti, Nb and O for Ti45Al8.5Nb

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alloy anodized at 1 V for 10 min. (a) and (c) were taken from typical regions at the

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interface. The rectangles marked in (a, c) show the position where elements mapping

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analysis were carried out.

31

b)

a)

Ⅰ

Ⅱ

Ⅲ

Counts / a.u.

Ni coating 13 14 15 16

Al Ti Nb O

17

c)

5 μm

0

Ti

10

15

O

ro of

Al

5

Distance / m Nb

Fig. 11 After oxidation at 1000 °C for 100 h, the cross-sectional SEM image (a),

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corresponding elements depth profiles (b) and distribution maps (c) of Al, Ti, Nb and

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O for Ti45Al8.5Nb alloy anodized at 1 V for 1 h. The arrow and rectangle marked in (a) show the direction and position where elements line scan and mapping analysis were

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carried out.

32

b)

a)

Counts / a.u.

Ni coating 18 19 20 21 22

Al Ti Nb O

5 μm

Al

Ti

5

10

Distance / m Nb

15

O

ro of

c)

0

Fig. 12 After oxidation at 1000 °C for 100 h, the cross-sectional SEM image (a),

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corresponding elements depth profiles (b) and distribution maps (c) of Al, Ti, Nb and

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O for Ti45Al8.5Nb alloy anodized at 3 V for 1 h. The arrow and rectangle marked in (a) show the direction and position where elements line scan and mapping analysis were

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carried out.

33

b)

Ti-O

468

0

Intensity / cps

77

76

75

74

73

72

Binding Energy / eV f)

Al-O

Intensity / cps 208

456

206

Ti-O Nb-O Oab

536

534

532

530

528

Binding Energy / eV

Binding Energy / eV

ro of

210

459

e)

Nb-O

Nb-O

212

462

Intensity / cps

d)

465

Binding Energy / eV

Binding Energy / eV

Al-O

Intensity / cps

C 1s Nb 3d Al 2s Al 2p

1200 1000 800 600 400 200

c)

Ti-O

Intensity / cps

O 1s Ti 2s

at / % 10.94 25.68 1.66 61.29 0.43

Ti 2p

C KLL

Ti LMM

Intensity / cps

Element Ti Al Nb O F

O KLL

a)

688

686

684

682

680

Binding Energy / eV

-p

Fig. 13 After oxidized at 1000 °C for 100 h, the survey (a) and refined XPS spectra of

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Ti 2p (b), Al 2p (c), Nb 3d (d), O 1s (e), and F 1s (f) spectra for Ti45Al8.5Nb alloy

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anodized at 1 V for 1 h.

34

List of table captions Table 1 EDS results derived from the spots marked in Fig. 2. Composition / at.% Position Al

Nb

O

F

1

42.78

41.32

7.04

7.45

1.41

2

38.53

40.37

7.29

10.63

3.18

3

30.84

29.53

5.42

21.53

12.68

4

30.23

29.46

5.20

20.98

14.13

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ro of

Ti

35

Table 2 EDS results derived from the spots marked in Fig. 8. Composition / at.% Position Al

Nb

O

1

7.91

24.36

1.52

66.21

2

13.02

17.80

0.60

68.58

3

7.36

28.65

0.36

63.63

4

32.33

0.49

-

67.18

5

8.26

31.79

0.37

59.58

6

6.73

31.75

0.37

61.15

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Ti

36

Table 3 EDS results derived from the spots marked in Fig. 9-12. Composition / at.% Position Al

Nb

O

1

16.43

19.17

-

64.40

2

11.99

25.41

19.23

43.37

3

33.03

45.46

9.26

12.25

4

5.68

23.82

0.16

70.34

5

43.32

26.86

7.55

22.27

6

5.55

29.08

1.13

64.24

7

47.34

28.88

7.86

8

39.62

38.65

9.89

9

4.26

31.82

-

10

22.76

3.34

2.59

11

23.48

2.28

12

21.37

13

0.95

11.84

63.92

71.94

re

-p

71.31

4.70

4.65

69.28

32.12

-

66.93

39.67

26.16

5.28

28.89

12.66

23.72

1.90

61.72

47.97

30.01

6.71

15.31

17

41.21

43.06

8.16

7.57

18

37.07

0.26

-

62.67

19

23.74

31.89

4.44

39.93

20

29.01

47.14

7.16

16.69

21

28.88

45.84

6.95

18.33

22

32.00

45.05

8.70

14.25

15

ur

na

16

Jo

15.92

2.30

lP

14

ro of

Ti

37