Mechanism of oxygen adsorption on surfaces of γ-TiAl

Surface Science 606 (2012) 852–857

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Mechanism of oxygen adsorption on surfaces of γ-TiAl Y. Song a,⁎, J.H. Dai a, R. Yang b a b

School of Materials Science and Engineering, Harbin Institute of Technology at Weihai, 2 West Wenhua Road, Weihai 264209, China Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China

a r t i c l e

i n f o

Article history: Received 21 September 2011 Accepted 31 January 2012 Available online 9 February 2012 Keywords: Density functional theory γ-TiAl Oxygen adsorption

a b s t r a c t We studied the adsorption behavior of oxygen on low index surfaces of γ-TiAl via first principles to investigate the mechanism that drives the adsorption behavior. The (100) surface is the most stable surface energetically followed by the (111), (110) and (001) surfaces. A study of the adsorption of a single oxygen atom on surfaces of TiAl showed that the O atom prefers the Ti-rich environment that has a high potential of generating TiO2. Competition between O-Al bonding and O-Ti bonding was observed in the O adsorbed surface regions. However, the O-Ti interaction dominates the adsorption behavior in all considered systems except when O is adsorbed on an Al-terminated (001) surface as the O–Al bond is stronger than O–Ti bond. A linear relationship between adsorption energy and integration of orbital overlaps between the O atom and the metals is obtained, which indicates that the electronic structure controls the adsorption behavior of an O atom on a γ-TiAl surface — an opportunity to improve the oxidation resistance of γ-TiAl based alloys. © 2012 Elsevier B.V. All rights reserved.

1. Introduction γ-TiAl intermetallic alloys have recently received significant attention as they have the potential to replace heavy Ni-based alloys in certain applications due to their low density, high specific strength, high specific stiffness, and good creep resistance at elevated temperatures [1–3]. While the maximum temperature for an application of this type of material is up to 900 °C, practical use of γ-TiAl alloys remains limited to about 750–800 °C due to the rapid growth of a nonprotective intermixed Al2O3 + TiO2 scale formed by competitive oxidation of the Ti and Al at high temperatures [4–8]. This mixed oxide layer prevents the formation of a continuous and dense α-alumina that would provide a more effective oxidation barrier in high temperature applications. Improving the oxidation resistance of γ-TiAl alloys is critical for increasing both their use and their reliability in high temperature applications. Most studies of the environmental degradation of these alloys investigate the influence of alloying elements including Nb, Cr and Mo. Although confusing and contradictory results are presented for several alloying elements they appear to improve the oxidation resistance of TiAl-based intermetallic alloys at high temperatures [9–15]. There are several explanations for these results: the reduction of oxygen diffusion and solubility in the metallic substrate [16], the alloying element forming a protective oxide barrier, and changes in the mechanical properties improving oxide-scale adherence [17].

⁎ Corresponding author. E-mail address: [email protected] (Y. Song). 0039-6028/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2012.01.024

Several studies of the initial stages of oxidation of γ-TiAl have been carried out on polycrystalline surfaces [18–26]. At room temperature, oxidation is preceded by the dissociative adsorption of oxygen, which occurs preferentially on the Ti atom [23]. Titanium oxidation was then observed (producing a Ti(II) oxide [21]), followed by aluminum oxidation [23]. Simultaneous growth of Ti(II), Ti(III), Ti(IV) and Al(III) oxides was reported between temperatures 100 °C and 600 °C [19,21]. The amount of Ti(II) decreased and Ti(IV) increased as the temperature increased. At 850 °C, the oxidation of aluminum preceded the oxidation of titanium. Only Ti(IV) and Al(III) were observed in the simultaneous oxidation of the two alloying elements that followed [20]. More recently, Maurice et al. [22,24,25] studied the initial stages of oxidation of the γ-TiAl (111) surface at 650 °C under low oxygen pressure using X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and Auger-electron spectroscopy (AES). They found that the oxidation in the first stage produced an ultra-thin γ-like Al2O3 (111) film on the γ-TiAl (111) surface [24] and the lattice mismatch between the two surfaces is only 1.8%, which may be one of the reasons that works were mainly focused on the TiAl (111) surface in study of the oxidation of TiAl. They also found that the first phase of oxidation was characterized by the growth of a pure alumina layer resulting from the selective oxidation of aluminum, and that both metal elements were oxidized simultaneously in the second phase [22]. First principles computations provide a suitable means to gain insight in understanding the physical and chemical properties of surface. The methodology and reliability of such computations have been well studied [27–31]. For TiAl system, Li et al. performed firstprinciples total-energy calculations for oxygen adsorption on a γTiAl (111) surface [26]. They found that regardless of coverage the

Y. Song et al. / Surface Science 606 (2012) 852–857

most favorable adsorption site for oxygen is the one with more Ti atoms than its nearest neighbors on the surface. However, their results could not explain the experiments above [22,24,25]. More recently, Liu et al. studied the effect of surface-segregation on the adsorption of oxygen on a γ-TiAl (111) surface [32]. They concluded that under Al-rich conditions an Al antisite defect could occur on the surface, affecting adsorption behavior. Segregation of Si on a γ-TiAl (111) surface can enhance the interactions between O and Al atoms and weaken those between O and Ti atoms [33]. In addition, investigations into the selective oxidation behavior of a γ-TiAl (111) surface using ab initio DFT and thermodynamic calculations revealed competition between Al and Ti elements on the oxidation of γ-TiAl [34]. There are fewer studies of other surfaces of γ-TiAl. Gong et al. studied the properties of Pd/TiAl membranes and calculated the surface energy of the four lowest index surfaces of γ-TiAl and found that the (100) and Alterminated (110) surfaces have lower surface energies than the other two [35,36]. Most recently, Wang et al. performed ab initio calculations of the surface energy and cleavage energy of four lowest index surfaces of γ-TiAl and found that the (100) surface is the most stable surface with lowest surface energy and the Ti-terminated (001) surface is the least. The cleavage and surface energies are greatly affected by the covalent bonds between the Ti and Al atoms; the stronger the bonds, the more stable the surface [37]. However, there is little detailed analysis of how oxygen interacts with surfaces beyond the (111) surface of γ-TiAl. In this work, we study the adsorption properties of O atoms on the (100), (001), (110) and (111) surfaces of γ-TiAl using first principles calculations. Our analysis reveals an interesting relationship between each system's electronic structure and its adsorption energy. 2. Calculation method The (2 × 2) slab models that were used consisted of six atomic layers (each of 48 atoms for the (100) and (001) surfaces and 24 atoms for the (110) and (111) surfaces) separated by an approximately 1.1 nm vacuum in the c direction, as shown in Fig. 1. Calculations were performed under the framework of density functional theory using the Vienna Ab initio Simulation Package (VASP) [38,39]. For the exchange-correction functional, the generalized gradient approximation (GGA) in PW91 was employed with core radii of rc = 0.820 Å, rc = 1.402 Å, and rc = 1.476 Å for O, Al and Ti respectively. The Project Augmented Wave (PAW) potentials were used to span out the valence electron density [40,41] and the electron states were

853

expanded using a plane wave basis set with a cutoff energy of 450 eV. The total energy convergence was less than 0.1 meV and the forces on the atoms were each less than 0.01 eV/Å. As the total energy depends on the k-mesh, Monckhorst-Pack k-point grids of 5 × 6 × 1 for (110) surface, 5 × 5 × 1 for (001) and (111) surfaces and 6 × 6 × 1 for (100) surface were employed to guarantee that the convergence of the total energy was less than 0.01 eV. The dependence of surface energy on the layers of slab was checked by performing calculations on slabs that contained nine atomic layers for the (100) and (111) surfaces. Difference of the surface energies between the six and nine atomic layer surfaces is less than 0.2%. Therefore the six atomic layer slabs were used. 3. Results and discussion 3.1. Surface energy Relaxations were carried out for all atoms in the slab and the topmost three layers of the surface. The energy difference between the two processes was less than 0.08 eV for Al and Ti stoichiometric (100) and (111) surfaces. The total energies of the Al or Ti single atom terminated surfaces in the (001) and (110) systems were the same after the full relaxations as during the full relaxation both the topmost and bottommost layers are at the surface and each slab contains both the Al and Ti terminated surfaces. This paper studies the adsorption of oxygen during the relaxation of the topmost three layers. The thermodynamic stability of a given surface is determined by its surface energy, which is defined as [37]: Esurf ¼

1 ðE −NEbulk Þ: 2A slab

ð1Þ

Here the total energy of the slab model is made up of Esurf, Ebulk is the total energy of the bulk TiAl (estimated as −24.50 eV using a unit cell with four atoms after full relaxation), N is the number of TiAl unit cells in the slab model and A is the surface area. The estimated surface energies are listed in Table 1. The (100) surface is the most stable, with the lowest surface energy of 1.70 J/m 2. The Al and Ti stoichiometric surfaces, (100) and (111), are more stable than the Al or Ti terminated surfaces. Table 1 also shows that an atom's termination has very little influence over the surface stability, implying that the Al or Ti termination could appear randomly in both the (001) and (110) surfaces. There is lack of experimental values of γ-TiAl's surface

Fig. 1. Top and side views of the slab models used to simulate the adsorption sites of oxygen on (a) the (001), (b) the (110), (c) the (100) and (d) the (111) surfaces.

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Y. Song et al. / Surface Science 606 (2012) 852–857

Table 1 Surface area A and energy Esurf of considered surfaces. Surface

Al-(001) Ti-(001) Al-(110) Ti-(110) (100) (111)

A (nm2)

0.6416 0.6416 0.4610 0.4610 0.6520 0.2808

Table 2 Adsorption energy of oxygen on the considered surfaces.

Esurf (J/m2)

Site

Present

Others

2.17 2.18 2.08 2.06 1.70 1.75

2.033[37] 2.175[37] 1.48[36], 1.777[37] 2.55[36], 2.167[37] 1.41–2.47[36], 1.618[37] 1.65[26], 2.05[36], 1.691[37]

Al-(001)

Ti-(001)

Al-(110)

energy. Our calculation of (111) surface's energy reasonably agrees with the value of 1.65 J/m 2 reported by Li et al. [26], but differs from the value estimated by Gong [36]. Apart from a small discrepancy for the Al-terminated (110) surface, all estimated surface energies are consistent with the theoretical values obtained by Wang et al. [37]. 3.2. Adsorption of oxygen

(100)

(111)

We investigated the adsorption of oxygen atom on the (001), (100), (110) and (111) surfaces of γ-TiAl. Oxygen is first placed on one side of the slab, and a dipole correction is applied to counteract the induced dipole moment, which is parallel to the c direction [32,42,43]. Due to the calculation limit of an infinitely large supercell, the dipole effect should be considered in aperiodic systems (such as surface) [44]. It has been shown that dipole corrections could have remarkable influence in vinyl phosphonic acid-α-Al2O3 surface reaction [45] and water-MgH2 surface adsorption [46]. However, in the present work total energy difference between the corrected and noncorrected surface systems is about 10 meV. The oxygen atom and the topmost three layers of the slab are then relaxed, while the bottom three metal layers remain fixed. There are three non-equivalent sites on the (001) surface for oxygen to adsorb: the bridge site between the atoms in the topmost layer (labeled B in Fig. 1(a)) and the two top sites of the atoms in the topmost two layers (labeled T1 and T2). Fig. 1(b) shows the four adsorption sites chosen on the (110) surface; T1 on a top layer atom, T2 above a second layer atom, and two center sites (C1 and C2). The oxygen atoms are on top of the Al and Ti atoms in the topmost (A1 and T1 in Fig. 1(c)) and second (A2 and T2) layer of the (100) surface. Previous investigations of the adsorption of oxygen on the (111) surface of γ-TiAl have identified nine possible adsorption sites: fcc-Al, hcp-Al, fcc-Ti, hcp-Ti, bri-Al, bri-Ti, bri-TiAl, top-Al and top-Ti [26,32]. Relaxation moves oxygen atoms at some sites to others (bri-Al to hcp-Ti, bri-Ti to fcc-Ti, top-Al to hcp-Al, top-Ti to fcc-Ti, and bri-TiAl to fcc-Al), so only the four final sites (fcc-Al, hcp-Al, fcc-Ti and hcp-Ti) were studied and are shown in Fig. 1(d). The adsorption energy of the binding interaction between the oxygen and the surface is defined as follows, where E(x) is the total energy of system X [47–49]:   1 slab Eads ¼ EðO−TiAlÞ − EðTiAlÞ þ EðO2 Þ : 2

Ti-(110)

ð2Þ

Liu et al.'s estimate of the total energy of the oxygen molecule is −9.78 eV [32]. Table 2 lists the evaluated adsorption energies of an oxygen atom for the considered surfaces. The estimated adsorption energies for the (111) surface agree with the values estimated by Li et al. [26], but differ from those of Liu et al. [32]. As predicted theoretically, the fcc-Al site is the most favorable, followed by the hcp-Al, fccTi and hcp-Ti sites [26,32]. The A2 site (where the oxygen atom is on top of an Al atom in the surface's second layer) is the most favorable adsorption site of the (100) surface, followed by the T2 site (As the A2 site, but with a Ti atom in place of the Al one). The remaining sites (A1 and T1) have relatively high adsorption energies that give unstable

E(O-TiAl) (eV)

B T1 T2 B T1 T2 T1 T2 C1 C2 T1 T2 C1 C2 A1 A2 T1 T2 fcc-Al hcp-Al fcc-Ti hcp-Ti

− 285.454 − 283.557 − 285.133 − 286.506 − 287.196 − 284.938 − 140.811 − 142.212 − 144.114 − 143.654 − 143.911 − 145.203 − 145.015 − 145.401 − 287.049 − 289.560 − 288.023 − 289.548 − 150.912 − 150.815 − 149.918 − 149.871

Eads (eV) Present

Others

− 3.94 − 2.04 − 3.62 − 5.08 − 5.78 − 3.51 − 0.92 − 2.32 − 4.22 − 3.76 − 3.86 − 5.15 − 4.97 − 5.36 − 2.00 − 4.51 − 2.97 − 4.39 − 5.18 − 5.08 − 4.18 − 4.14

− 5.12[26] − 5.02[26] − 4.13[26] − 4.08[26]

configurations, and so are not discussed further. In the Al or Ti terminated surface systems, the large adsorption energies observed in Titerminated surfaces (− 5.78 eV and − 5.36 eV for the Ti-(001) and Ti-(110) surfaces respectively) imply stronger interactions between O and Ti atoms than between O and Al atoms. It is worth noting that the small differences in adsorption energy for C1, C2, and T2 sites of the Ti-(110) surface imply that O atoms will be adsorbed randomly on these sites. The following section analyses the electronic structure of the B site in Al-(001), the T1 site in Ti-(001), the C1 site in Al-(110), the C2 site in Ti-(110), the A2 site in (100), and all four sites in (111) surfaces to identify the mechanisms used in their interactions with the oxygen atoms. 3.3. Electronic structure Fig. 2 shows the partial densities of states (PDOS) of an O atom at its preferred site and of Al and Ti atoms for each system in Table 2. Inserts show the geometric structures of the oxygen and surface atoms. Equal strength O–Al bonds were created between each O atom and its two nearest Al atoms during adsorption on the Al terminated (001) surface (O + Al-(001) system) as the overlaps between the O p and Al p orbitals occurred both at −1.0 eV and between −4.0 and −3.0 eV (top panel in Fig. 2(a)). The charge difference distribution (CDD) on the (110) plane of this system also shows this bonding characteristic (left penal in Fig. 3(a)). The PDOS of the O p electrons in the O + Ti-(001) system shows a relatively isolated distribution; the main peak at − 5.8 eV below the Fermi energy causes the differences in the distribution of the Ti (mainly) d electrons between this system and the clean Ti terminated (001) surface. The amplitude of the Ti d electrons' PDOS is reduced near the Fermi energy and the two peaks induced between − 6.0 and − 5.0 eV overlap with the O p electrons and so create Ti–O bonds. Geometric structures consisting of an O atom and four Ti atoms held together by 0.205 nm O–Ti bonds are formed in the surface layer (insert in the second panel of Fig. 2(a)). The O atom equally bonds to four Ti atoms forming covalent bonds as shown in right penal of Fig. 3(a). In the O + Al-(110) system, the O atom has moved into surface and bonded with Al atoms from the topmost layer and Ti atoms from the second as shown in insert in the third panel of Fig. 2(a), and in its CDD of left penal in Fig. 3(b). A certain amount of CDDs distributed between the O and Al atoms and between the O and Ti atoms was

Y. Song et al. / Surface Science 606 (2012) 852–857

(a)

(b) EF

Op Al p Ti d

Al-(001)

6 O

4 2

Al

0 8

Ti-(001)

O

Density of states (state/eV)

8

Density of states (state/eV)

855

6 Ti

4 2 0 8

Al-(110)

O

6

Al

4

Ti

2 0 8

Ti-(110)

O

6 4

Ti Al

2 0

-10 -8 -6 -4 -2 0 2 Energy relative to the Fermi energy (eV)

EF

8 (100) O 6 4 Al 2 0 8 (111) fcc-Al 6 4 2 0 8 (111) hcp-Al 6 4 2 0 8 (111) fcc-Ti 6 4 2 0 8 (111) hcp-Ti 6 4 Op Al p 2 Ti d 0 -10 -8 -6

Ti

O Al

Ti O Al

Ti

O

Al

Ti

O Al Ti

-4

-2

0

2

Energy relative to the Fermi energy (eV)

Fig. 2. Partial densities of states of oxygen on (a) the Al or Ti single atom terminated surfaces and (b) the Al and Ti stoichiometric surfaces. Inserts are the geometric structures of the O atom and the surface atomic layers, where red, brown and yellow balls denote O, Al and Ti atoms, respectively.

Al-(001)

observed. The oxygen atom's influence over the PDOSs of the Al and Ti atoms is confined to the lower energy regions. O p electrons with energies between − 8.6 and 7.8 eV and between − 6.5 and −4.8 eV overlap with Al p and Ti d peaks resulting in competing O–Al and O–

Ti-(001) 0.0015 0.0015

0.0015

(a)

[001]

O Al

Al

[100]

0.0015 0.0015

(001) plane

0.0015

(100) plane

0.0015 0.0050

Ti

0.0015

-0.012

-0.012

Al -0.016

O

0.0015 0.0015

Ti -0.012

[100]

-0.012 Ti

0.0015 0.0015

-0.012

-0.016

O

Ti

Ti

[001]

(a)

0.030

Ti

0.0050

O

Al Ti

Al [010]

[110]

0.0050

(b) Al-(110)

(b)

0040

-0.0040

0.0083 -0.0040

Al

fcc-Al Ti

-0.028 [100]

-0.0040

Ti

O

Ti

O Ti

Ti

hcp-Al

Al

0.050

Al

Al Ti O Ti Al

Ti O

-0.0040

Al

hcp-Ti

O -0.0040 0.0040 0083

[010]

[010]

[010]

Ti-(110)

[010]

Fig. 3. Charge difference distributions of O adsorbed on the single atom terminated (a) (001) and (b) (110) surfaces.

b

Ti

Ti

a

fcc-Ti

Al

O

Al

Fig. 4. Charge difference distributions of O adsorbed on the Al and Ti stoichiometric (a) (100) and (b) (111) surfaces.

Y. Song et al. / Surface Science 606 (2012) 852–857

Ti bonds, which in turn result in TiO2 and Al2O3 compounds as observed experimentally [4-8]. The partial DOSs of the O + Ti-(110) system are shown in the bottom panel of Fig. 2(a). Unlike the cases above the O p peak shifts upwards to about − 4.0 eV below the Fermi energy and forms the O–Ti bonds due to fine overlap with the Ti d orbital between −5.0 and −3.5 eV. Geometric analysis shows that each O atom forms two 0.1859 nm bonds with Ti atoms on the surface, which dominate the adsorption of the oxygen atom. This is also confirmed by the CDD on the (001) plane of this system (right penal in Fig. 3(b)). Bonds originate from overlapping electronic orbitals. To estimate bond strength, PDOSs of O p and Ti d or Al p electrons in the overlapping areas were calculated using the following definition: E2   S ¼ ∫ DOSOp þ DOSTid=Alp dE

ð3Þ

E1

This gives S values of 4.25 between energies of − 6.83 (E1) and −5.81 eV (E2) (1.88 of which comes from a small number of O p–Al s and p overlaps) for a T2 site adsorption, 5.82 between −4.92 (E1) and −3.82 eV (E2) for a C1 site adsorption, and 6.51 between −5.03 (E1) and −3.63 eV (E2) for a C2 site adsorption. These values are linearly related to the adsorption energies listed in Table 2. This means the O–Ti interaction dominates the adsorption of the O atom in the O + Ti-(110) system, so we expect TiO2 formation. Oxygen atoms tend to adsorb at A2 site on the Al and Ti stoichiometric (100) surface with an adsorption energy of −4.39 eV. The top panel of Fig. 2(b) shows the O p electrons' two main bonding peaks at −7.0 eV and − 5.8 eV. Competition between O–Al and O–Ti bonding is expected as the peak at − 7.0 eV overlaps with Ti d orbital and the peak at − 5.8 eV overlaps with Al p and Ti d orbitals. The CDD on the (001) plane of this system shows that the O–Ti interactions are stronger than the O–Al interactions as the CDD along the O–Ti direction is larger than that along the O–Al direction (Fig. 4(a)). Integrating the PDOSs of the O p electrons and valence from surface atoms over the bonding regions (Eq. (3)) gives S = 3.20 for O–Ti and 2.09 for O–Al bonding, which implies that the O–Ti bond is the stronger of the two. The surface structure supports this conclusion; the length of O–Ti bond (0.207 nm) is slightly shorter than that of the O–Al bond (0.213 nm). For the O + (111) systems, the CDDs are illustrated in Fig. 4(b). One common feature for the four systems is that the O atom tends to interact with Ti atoms rather than Al atoms. The PDOSs of the system where oxygen was adsorbed at fcc-Al site on the (111) surface showed that O p electrons are concentrated in the energy range from −6.83 to −4.49 eV with four bonding peaks at −6.61, -6.02, −5.31 and −4.91 eV (second panel in Fig. 2(b)). The two lower-energy peaks mostly overlap with Al p orbital and so contribute to O–Al interactions, while the higher-energy peaks overlap with Ti d electrons to form O–Ti bonds. These overlaps integrate to S = 1.73 for the O–Al and 4.44 for the O–Ti bonding, implying that the O–Ti bonds are much stronger than the O–Al bonds and so O atoms are much more likely to interact with Ti atoms. The PDOSs for oxygen adsorbed at the hcp-Al site on the (111) surface showed a combination of O–Al and O–Ti bonding as the PDOSs of Al p and Ti d electrons have similar distributions between −7.62 and −4.52 eV (third panel in Fig. 2(b)). As with adsorption at the fcc-Al site, states with lower energies (−7.60 to −6.10 eV) contributed to O–Al bonds while states with higher energies (− 5.68 to − 4.52 eV) contributed to O–Ti bonds. Integrations of these overlaps give S = 2.17 and 3.83 respectively, so there is greater competition between O–Al and O–Ti interactions in this system than at the fcc-Al site. The tallest peak of the O p orbitals at the fcc-Ti site was at −6.5 eV and aligns closely with peaks of the Al p and Ti d orbitals, while the

other peak at − 5.1 eV formed a pd hybridization with Ti d orbitals in the region between − 5.8 and −4.4 eV (fourth panel in Fig. 2(b)). Integration gives 1.93 for the O p and Al p overlap and 3.00 for the O p and Ti d overlap, so this system shows competition between O– Al and O–Ti bonds too. The hcp-Ti site on the (111) surface is similar to the three cases above; low energy (−7.30 to −6.22 eV) O p electrons interact with Al p electrons to form O–Al bonds, while high energy (− 6.20 to −3.97 eV) O p electrons form O–Ti bonds with Ti d electrons (bottom panel in Fig. 2(b)). The PDOS integrations (S) give 1.67 for the O p and Al p orbitals and 3.19 for O p and Ti d orbitals, so the O–Ti bonds are stronger than the O-Al bonds. Integrations for the overlaps between O p and metal valence orbitals reveal a linear relationship between the adsorption energy Eads and the overlap integration S (with the exception of the Ti-(001) surface, as shown in Fig. 5). Fig. 5 also includes the cases where the O atoms occupy their second-most favorable sites: the T2 and C1 sites on the Ti-(110) surface and the T2 site on the (100) surface. This means the adsorption behavior of O atoms on the surface of γ-TiAl is controlled by the surface's electronic structures; we can use this property to improve the compound's oxidation resistance. The above analysis shows that O-Ti interaction dominates the adsorption of oxygen on the (111) surface. It drives the adsorption of O atoms in a Ti-rich environment (e.g. the fcc-Al and hcp-Al sites) with relatively large adsorption energies, a result consistent with other theoretical investigations [25,26]. However, competition between the O–Al and O–Ti bonds results in a competitive formation of Al2O3 and TiO2 (particularly noticeable at the hcp-Al and fcc-Ti sites). As a dense Al2O3 scale improves oxidation resistance, enhancing O–Al interactions through the surface modification, such as inducing Al antisite defects or generating Ti vacancies, may make Al2O3 formation more likely and so improve the oxidation resistance of γ-TiAl [32]. 4. Conclusions Adsorption of oxygen on low index surfaces of γ-TiAl was studied using first principles computations. Evaluations of surface energy showed that the (100) surface is the most stable surface energetically, followed by the (111), (110) and (001) surfaces. An examination of oxygen's adsorption behavior on these surfaces demonstrated that O atoms prefer to occupy a bridge site if they adsorb on the Al terminated (001) surface, but remain on top of the surface atoms of the Ti terminated (001) surface. O atoms can adsorb at three different sites on the Ti-terminated (110) surface with similar adsorption energies,

-3.5

-4.0 Adsorption energy Eads (eV)

856

-4.5

-5.0

-5.5

Al-(001) Ti-(001) Al-(110) Ti-(110) (100) (111)

-6.0 4.0 4.5 5.0 5.5 6.0 6.5 7.0 Integration of orbital overlaps between O and metals (state) Fig. 5. Relationship between adsorption energy and integrals of O, Ti and Al orbital overlaps.

Y. Song et al. / Surface Science 606 (2012) 852–857

but only one site on the Al-terminated (110) surface. The top sites on both Al and Ti atoms in the second layer of the (100) surface are preferred due to their lower adsorption energies, while four sites, fcc-Al, hcp-Al, fcc-Ti and hcp-Ti, are preferred on the (111) surface. Further analysis reveals that the adsorption behavior is determined by electronic structure and that there is a linear relationship between the adsorption energy and integrals of overlaps between the PDOSs of O, Ti and Al. O–Al and O–Ti bonding interactions compete when oxygen is adsorbed. O–Ti bonding is stronger than O–Al bonding in the considered systems except when oxygen is adsorbed on the Al-terminated (001) surface, where O–Al bonding dominates the adsorption of oxygen. This means oxygen is usually found in a Ti-rich environment where it is likely to generate TiO2, which negatively affects the oxidation resistance of γ-TiAl at high temperatures. However, enhancing O–Al bonding through surface modification such as inducing Al antisite defects or Ti vacancies could improve the oxidation resistance of γ-TiAl [32]. Acknowledgments This work was supported by the National Basic Research Programme of China, Grant No. 2011CB606400-G, the Natural Science Foundation of Shandong, China, Grant No. ZR2010BM034 and the Fundamental Research Funds for the Central Universities, Grant No. HIT.NSRIF.2009144. References [1] X. Wu, Intermetallics 14 (2006) 1114. [2] M. Yoshihara, Y.-W. Kim, Intermetallics 13 (2005) 952. [3] F. Appel, U. Brossmann, U. Christoph, S. Eggert, P. Janschek, U. Lorenz, J. Müllauer, M. Oehring, J.D.H. Paul, Adv. Eng. Mater. 2 (2000) 699. [4] A. Rahmel, W.J. Quadakkers, M. Schütze, Mater. Corros. 46 (1995) 217. [5] S. Becker, A. Rahmel, W. Quadakkers, M. Schütze, Oxid. Met. 38 (1992) 425. [6] C. Lang, M. Schütze, Oxid. Met. 46 (1996) 255. [7] C. Lang, M. Schütze, Mater. Corros. 48 (1997) 13. [8] M. Schmitz-Niederau, M. Schütze, Oxid. Met. 52 (1999) 225. [9] M. Mitoraj, E. Godlewska, O. Heintz, N. Geoffroy, S. Fontana, S. Chevalier, Intermetallics 19 (2011) 39. [10] T.T. Cheng, M.R. Willis, I.P. Jones, Intermetallics 7 (1999) 89.

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