Physica B 407 (2012) 2321–2328

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Physica B

0921-45

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/physb

Comparison of O adsorption on Ni3Al (0 0 1), (0 1 1), and (1 1 1) surfacesthrough first-principles calculations

Qiong Wu, Shusuo Li, Yue Ma, Shengkai Gong n

School of Materials Science and Engineering, Beihang University, Beijing 100191, China

a r t i c l e i n f o

Article history:

Received 9 November 2011

Received in revised form

23 February 2012

Accepted 6 March 2012Available online 13 March 2012

Keywords:

Ni3Al

First-principles calculations

Oxidation

Adsorption

Ni-based superalloys

26/$ - see front matter & 2012 Elsevier B.V. A

016/j.physb.2012.03.021

esponding author. Tel.: þ86 10 82339003; fa

ail addresses: queong@126.com, gongsk@bua

a b s t r a c t

First-principles calculations were performed to study the properties of O adsorption on Ni3Al (0 0 1),

(0 1 1), and (1 1 1) surfaces using the Cambridge serial total package (CASTEP) code. Stable adsorption

sites are identified. The atomic and electronic structures and adsorption energies are predicted. The

adsorption sites for O on the Ni3Al (0 0 1) surface are at the 2Ni–2Al fourfold hollow site, whereas O

prefers to adsorb at the Ni–Al bridge site on (0 1 1) surface and 2Ni–Al threefold hollow site on (1 1 1)

surface. It is found that O shows the strongest affinity for Al and the state of O is the most stabilized

when O adsorbs on (0 0 1) surface, while the affinity of O for Al on (0 1 1) surface is weaker than (0 0 1)

surface, and (1 1 1) surface is the weakest. The stronger O and Al affinity indicates more stable Al2O3

when oxidized. The experiment has shown that the oxidation resistance of single crystal superalloy in

different orientations improves in the order of (1 1 1), (0 1 1), and (0 0 1) surface, suggesting that the

oxidation in different crystallographic orientations may be related to the affinity of O for Al in the

surface.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Single crystal Ni(Ni3Al)-based superalloys, with remarkablemechanical and good anti-oxidation properties, are widely usedfor modern turbine blades and vanes in the modern aviationindustry. g0-Ni3Al’s volume fraction is usually as high as �70%and is the key microstructure constituent as the precipitationstrengthener and anti-oxidation phase in superalloys [1]. Theabsence of grain boundaries in single crystal superalloys leads tomaterial anisotropy including the oxidation behavior [2,3]. Yuanet al. [3] showed the dependence of oxidation on the crystal-lographic orientation in directionally cast single crystal alloys (DSCM 247LC). The specimen orientated 451 to the (0 0 1) directionoxidized at the fastest rate, and the (0 1 1) orientated specimenoxidized at medium rate, while the (0 0 1) orientated specimengave the smallest weight gains.

The engine components made from Ni-based superalloys areprotected by a multilayer thermal barrier coating (TBC) thatincludes a metallic bond coat. For the new kinds of bond coatingsdeveloped recently, like, Ni(Pt)Al coating [4,5], g0 based EQ coat-ings [6,7], and PtþHf modified gþg0 based coating [8], NiAl orNi3Al phase are used as the major anti oxidation phase. Both NiAland Ni3Al have its own certain advantages. The ab initio studies ofthe initial oxidation of NiAl (1 1 0) have been presented in many

ll rights reserved.

x: þ86 10 82338200.

a.edu.cn (S. Gong).

references [9–12]. However, the oxidation of Ni3Al is differentfrom NiAl. Therefore, the investigation of atomic and electronicproperties of O adsorption on Ni3Al surfaces is important to makea correct interpretation of the obtained experimental data and tounderstand the microscopic process which governs the initialstages of oxidation [13].

Lots of researchers have studied the Ni3Al surfaces [13–23],and the adsorption behavior of CO [24–27], Pb [27], methanol andmethoxy [28,29] on Ni3Al surfaces for the chemical reaction andnanostructure. Few researchers have reported the adsorptionbehavior of O on the Ni3Al surfaces, which govern the initialstages of oxidation [9]. In this paper, the adsorption behavior of Oon Ni3Al (0 0 1), (0 1 1), and (1 1 1) surfaces was studied throughfirst-principles calculations. The purpose of the present study wasto predict the stable adsorption sites of O on Ni3Al surfaces, andhopefully to characterize the initial oxidation behavior of Ni3Al indifferent orientations.

2. Methodologies

The low-energy electron diffraction (LEED) [14–16] and Auger-electron spectroscopy (AES) analysis [30] showed that theclean (0 0 1), (0 1 1), and (1 1 1) surfaces of Ni3Al are all bulk-terminated, with no evidence of surface reconstruction or surfacesegregation. For the Ni3Al (0 0 1) and (0 1 1) surfaces, there aretwo possible types of atomic planes: an equal number of Ni and Alatoms (mixed-layer termination) and solely Ni atoms (Ni-layer

Q. Wu et al. / Physica B 407 (2012) 2321–23282322

termination). LEED [14–16] analysis showed that only the mixed-layer (equal number of Ni and Al atoms) occurs in reality, andtherefore, only this termination was considered in our work. Inthe case of the Ni3Al (1 1 1) surface, only one type of surfacetermination with 75% Ni and 25% Al occurs, and all the atomicplanes are identical to each other [15].

Fig. 1 shows the ideal, unrelaxed slab structures for the threesurfaces. The three low-index surfaces were modeled by repeatedslabs with a 2�2 surface unit cell with four atoms in each layer.The slabs for the (0 0 1) and (1 1 1) surfaces consisted of fivelayers, while the slab for (0 1 1) surface consisted of seven layers.The slabs were separated with 12 A spacing of vacuum. The atomsat the two bottom layers of the slab were fixed at their bulkposition (the calculated equilibrium lattice constant of bulk Ni3Alis 3.58 A), while the top three (for (0 0 1), (1 1 1) surfaces) or five(for (0 1 1) surface) layers of the slab were allowed to relax. Thesurface areas of the 2�2 surface unit cell are 25.64 A2 for (0 0 1)surface, 36.26 A2 for (0 1 1) surface, and 22.20 A2 for (0 0 1)surface. This shows that the (1 1 1) surface is the closest packed,and (0 0 1) surface is second close packed, while (0 1 1) surface isthe least close packed.

Fig. 2 shows the possible high-symmetry adsorption sites forO on Ni3Al (0 0 1), (0 1 1), and (1 1 1) surfaces. Fig. 2a shows thefour possible adsorption sites on the Ni3Al (0 0 1) surface, with two

Fig. 1. Slab models for the clean low-index (a) (0 0 1), (b) (0 1 1), and (c) (1 1 1) su

Fig. 2. Schematic representation top view of the possible adsorption sites for O on t

represent Ni and Al atoms in the surface layer, respectively. Gray black and gray white

on-top sites such as Al on top (as labeled A) and Ni on top(as labeled B), one Ni–Al bridge site (as labeled C), and one fourfold2Ni–2Al hollow site (labeled as D). The six possible adsorption siteson the Ni3Al (0 1 1) surface are shown in Fig. 2b. There are two on-top sites: Al on top (as labeled A) and Ni on top (as labeled B), oneNi–Al short-bridge site (C), two long-bridge site (Al–Al bridge site,labeled as D and Ni–Ni bridge, labeled as E), and one fourfoldcoordination hollow site (2Ni–2Al, labeled F). Nine possible adsorp-tion sites exist on the Ni3Al (1 1 1) surfaces, as shown in Fig. 2c:two on top sites (Al on top as labeled A, Ni on top as labeled B),three bridge sites (Ni–Al bridge site, as labeled C, two kinds ofNi–Ni sites, as labeled D, E), four threefold coordination hollowsites (two kinds of 3Ni hollow sites (F, G), and two kinds of 2Ni–Alhollow sites (H, I)).

We define 1 ML of adsorbed O atoms corresponding to thesame atoms as the atomic sites in the surface layer. One O atomadsorbing on the Ni3Al (0 0 1), (0 1 1), and (1 1 1) (2�2) surfacescorresponds to an adsorption coverage of 0.25 ML. The adsorptionenergy (Eb) is calculated from the following equation [27]:

Eb ¼ Eadsorb�Esub�EO ð1Þ

where Eadsorb is the energy of an adsorbed slab, Esub is the energyof a clean surface, and EO is the energy of a free O atom.

rfaces of Ni3Al. Dark and light spheres represent Ni and Al atoms, respectively.

he Ni3Al (a) (0 0 1), (b) (0 1 1), and (c) (1 1 1) surfaces. Black and white spheres

spheres represent Ni and Al atoms in the second (subsurface) layer, respectively.

Table 1Atomic distances between atomic planes of the relaxed Ni3Al system, and the relative changes (%) with respect to an ideal structure, also the corresponding values given by

LEED measurements [14–16] and other calculation results [13,23].

(0 0 1) (0 1 1) (1 1 1)

This work Ref. [23] LEED [14] This study Ref. [13] LEED [16] This study Ref. [13] LEED [15]

A % A % A % A % A % A % A % A % A %

Bulk value 1.790 1.784 1.78 1.266 1.232 1.259 2.067 2.011 2.055

dAlNi11

0.011 0.007 0.02 0.012 0.037 0.015 0.072 0.098 0.06

dNiNi12

1.733 �3.169 1.733 �2.859 1.73 �2.809 1.132 �10.595 1.080 �12.338 1.11 �11.835 2.016 �2.480 1.957 �2.685 2.045 �0.487

dAlNi12

1.745 �2.539 1.740 �2.466 1.8 1.124 1.144 �9.618 1.137 �7.711 1.125 �10.643 2.088 1.014 2.055 2.188 2.105 2.433

dNiNi23

1.785 �0.275 1.781 �0.168 1.78 0 1.318 4.119 1.279 3.815 1.3 3.257 2.058 �0.426 2.014 0.149 2.055 0

dNiAl23

1.776 �0.787 1.791 0.392 1.299 2.637 1.287 4.464 2.054 �0.642 1.990 �1.044

dAlNi33

0.009 0.019 �0.008 0.004 0.011

dNiNi34

1.785 �0.291 1.793 0.504 1.247 �1.472 1.213 �1.542 2.052 �0.705 2.004 �0.348

dAlNi34

1.794 0.221 1.783 �0.056 1.266 0.011 1.205 �2.192 2.057 �0.489 2.015 0.199

Fig. 3. (a) Total DOS distribution with all atoms in the surface layer and DOS contributions connected with Ni and Al atoms of the (0 0 1) surface layer. (b) DOS contribution

connected with Al atoms forming the Ni3Al (0 0 1) surface layer and its components built up by s, p states of the surface Al atoms. (c) The same as in (b), but for surface Ni

atoms in (0 0 1) surface. (d) The same as in (a), but for (0 1 1) surface. (e) The same as in (b), but for Al atoms in (0 1 1) surface. (f) The same as in (c), but for Ni atoms in

(0 1 1) surface. (g)–(i) are the same as in (d)–(f) but for (1 1 1) surface.

Q. Wu et al. / Physica B 407 (2012) 2321–2328 2323

Q. Wu et al. / Physica B 407 (2012) 2321–23282324

The DFT [31–34] calculations were performed through theCambridge serial total package (CASTEP) code. The effects ofexchange correlation interaction are treated with the generalizedgradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE)[35]. The ultra-soft pseudo-potentials describe this electron–ioninteraction system to high accuracy with a plane wave cutoff of380 eV. Self-consistent solutions were obtained by employing the(4�4�1) Monkhorst–Pack [36] grid of k-points for the integra-tion over the Brillouin zone for the (2�2) surface unit cell.

3. Results and discussion

3.1. Structures of clean Ni3Al surfaces

The Ni3Al surface geometries were obtained from the fullyrelaxed clean surface slabs. The calculated atomic layer distancesare shown in Table 1 and compared with the results from otherwork (both experimental and calculation results [13–16,23]).dxy

ij is the distance along the surface normal direction betweenthe x atom at the i atomic layer and the y atom at the j atomiclayer. Our calculation results agreed with the values from LEEDmeasurements [14–16] and Jurczyszyn [13] and Kurnosikov et al.’s

Table 2Calculation results of O adsorption on Ni3Al (0 0 1) surface, including the adsorp-

tion energy, the local surface rippling upon O adsorption, the distance to nearest

(NN) and next-nearest (NNN) neighbors to the O adsorb.

(0 0 1) Site Eb (eV) dAlNi11 (A) rNN (A) rNNN (A)

Bulk 0.016

Al on top A �5.924 �0.200 1.681(Al) 2.933(Ni)

Ni on top B �5.206 �0.187 1.705(Ni) 3.160(Al)

Ni–Al C �6.926 0.020 1.745(Al) 1.860(Ni)

2Ni–2Al D �7.728 0.144 1.833(Al) 2.318(Ni)

Fig. 4. DOS for the surface layers when O adsorbs on 2Ni–2Al site minima at Ni3Al (0 0

(b) DOS of surface Al, and its components built up by s, p states. (c) The same as (b), b

work [23] very well. The rippling in the first atomic layer(Al atoms move outwards with respect to Ni atoms) was calculatedto be 0.016 A for Ni3Al (0 0 1) surfaces, 0.012 A for Ni3Al (0 1 1)surfaces, and 0.072 A for Ni3Al (1 1 1) surfaces. The LEED results[14–16] showed the surface Al atom located 0.02 A above Ni atom in(0 0 1) surface, 0.015 A in (0 1 1) surface, and 0.06 A in (1 1 1)surface. The Al atom always moves outwards with Ni atom at bothB2-NiAl [9,10,37–39] and L12-Ni3Al [13–16,23] surfaces, which maybe due to the larger atom size of Al (atomic radii 1.82 A) than Ni(atomic radii 1.62 A) [40] and the smaller surface energy of Althan Ni.

Fig. 3 presents the total and partial density of states (DOS)distributions at the top atomic layer of the clean Ni3Al (0 0 1),(0 1 1), and (1 1 1) surfaces. The DOS contributions connectedwith s, p and d states of the surface Al and surface Ni atoms of(0 0 1) surface are shown in Fig. 3b and c, respectively. For theenergy range from �3 to 0 eV, the electronic structure of Ni3Al(0 0 1) surface is dominated by d states of the surface Ni atoms.For (0 1 1) and (1 1 1) surfaces, the energy ranges that dominatedby d states of the surface Ni atoms are wider, which arerespectively from �3 to 0.5 eV and from �3.5 to 0.5 eV. Withinthe rest of the considered energy range, the DOS contributionsconnected with surface Al and Ni atoms are comparable.

3.2. Adsorption behavior of O on Ni3Al surfaces

The three surfaces are very different in rippling, atomic layerdistance, atomic arrangement at the surface and so on. The atomicradius of O is 0.65 A, much smaller than both Ni (1.62 A) and Al(1.82 A) [40]. Adsorption site preferences correlate with the sizeand electronegativity of the adatoms, which is how strongly anatom can attract electrons from the surrounding atoms [10,37].

3.2.1. Adsorption on Ni3Al (0 0 1) surface

Table 2 displays the predicted adsorption energy of O on Ni3Al(0 0 1) surface, the local surface rippling upon adsorption of O, the

1) surface. (a) Total DOS at surface and the contributions of surface Al, Ni and O.

ut for Ni. (d) DOS of adsorbed O atom.

Q. Wu et al. / Physica B 407 (2012) 2321–2328 2325

distance of O to the nearest (NN) and next-nearest (NNN)neighbors. The 2Ni–2Al fourfold is the most stable minima. Thus,the Ni–Al bridge site can be a transition state for diffusionbetween the neighboring fourfold sites, and the diffusion barrierenergy is 0.802 eV. For the fourfold 2Ni–2Al hollow site minima,the rippling is much induced, with a deeper Ni trough of 0.144 Athan surface Al, while the clean surface is only 0.016 A rippling.When O diffused to the Ni–Al bridge site, the rippling is 0.020 A.So as O diffuses from the fourfold minima to the twofold state,many local structural changes respond to the diffusion, with Almoving a normal distance of 0.124 A. Also if O moves from the Alon top site to the 2Ni–2Al fourfold site, the surface Al would movea normal distance of 0.344 A. The NN and NNN distance to theadsorbed O atom is also shown in Table 2. The O–Al distances areshorter than O–Ni distances, except for O at Ni on top site,suggesting a somewhat stronger interaction between O and Althan between O and Ni. This is consistent with the relaxedstructure when O adsorbs on the 2Ni–2Al fourfold hollow site,where Al moves upward from the surface, and surface Ni movesdownwards below the surface Al, with a normal distance of0.144 A.

Fig. 4 shows the total and partial density of states (DOS) forsurface layers when O adsorbed at the 2Ni–2Al site minima. TheDOS is characterized by two regions separated by a gap. The regionranging from �19.5 to �18 eV is dominated by O-s orbital. The

Table 3Calculation results of O adsorption on Ni3Al (0 1 1) surface.

(0 1 1) Site Eb (eV) dAlNi11 (A) rNN (A) rNNN (A)

Bulk 0.012

Al on top A �6.037 �0.170 1.669(Al) 2.942(Ni)

Ni on top B �5.695 �0.204 1.666(Ni) 3.147(Al)

Ni–Al C �7.719 �0.037 1.740(Al) 1.788(Ni)

Ni–Ni E �7.053 �0.073 1.908(Ni) 1.988(Al)

Al–Al D �7.358 0.279 1.794(Al) 2.737(Ni)

2Ni-2Al F �7.379 0.200 1.839(Al) 2.191(Ni)

Fig. 5. DOS for the surface layers when O adsorbs on Ni–Al bridge site minima at Ni3

valence band DOS below the Fermi energy (from �10 to 0 eV) ismainly due to O-2p and Ni-d orbitals. Some electrons with energyregion ranging from �20 to �18 eV appear in Ni and Al afterO adsorption, which means Ni and Al have some hybridizationwith O atom. The peak of Al (ranging from �19.5 to �18 eV) isstronger than Ni, which suggests the interaction of O with Al isstronger than Ni.

3.2.2. Adsorption on Ni3Al (0 1 1)

Table 3 shows the adsorption energies of O on the Ni3Al (0 1 1)surface, the local surface rippling induced by the O adsorption,and the distance between the O atom and its nearest and nextnearest neighbors. The comparison of the adsorption energiesreveals the preferred position of the oxygen on the Ni3Al (0 1 1)surface is at the Ni–Al bridge site. Adsorption of O at the 2Ni–2Alhollow site and Al–Al bridge site is close in energy. For the Ni–Albridge site minima, Ni is located 0.037 A above Al site. Thedistance between the oxygen and the nearest Al is shorter thanthe distance between the oxygen and the nearest Ni, which showsthe stronger affinity of O for Al than Ni. When O adsorbs at the2Ni–2Al hollow site, the O atom moves toward the two Al atoms,and pushes two Ni neighbors away. As a result, Al locatesabove Ni.

When O moves from the Ni–Al bridge site minima to the nextNi–Al bridge site, it will cross Al–Al bridge site rather than Ni–Nibridge site, and the diffusion barrier is 0.361 eV. Al moves aheight distance of about 0.315 A. If O moves from the Ni–Al bridgesite to the next-nearest Ni–Al bridge site, it will cross 2Ni–2Alhollow site. The diffusion barrier is 0.340 eV, and Al moves anormal distance of 0.237 A.

Fig. 5 shows the DOS for the Ni3Al (0 1 1) surface when Oadsorbs at the Ni–Al bridge site minima. The band ranging from�19 to �17.5 eV is contributed by the O, Al and Ni. The peak of Al(ranging from �19 to �17 eV) is only a little stronger than Ni,which means the interactions of O with Al and Ni are comparable.

Al (0 1 1) surface. (a)–(d) are the same as (a)–(d) in Fig. 4 but for (0 1 1) surface.

Q. Wu et al. / Physica B 407 (2012) 2321–23282326

The overlapped region of O and Ni between �10 and 0 eV in(0 1 1) surface is much larger than (0 0 1) surface.

3.2.3. Adsorption on Ni3Al (1 1 1)

The calculation results of O adsorption on Ni3Al (1 1 1) surfaceare shown in Table 4, which includes the adsorption energy, thelocal surface rippling induced by the O adsorption, and thedistance between the O atom and its nearest and next nearestneighbors. O atom adsorbs on the stoichiometric Ni3Al (1 1 1)surface preferably at the two 2Ni–Al threefold sites, which are theminima sites. The Ni on top site, Ni–Al bridge site and two Ni–Nibridge sites are unstable, O atom deposited there moves towardsthe 3Ni threefold sites, or 2Ni–Al threefold sites.

For the two 2Ni–Al hollow site, the adsorption energy of O on2Ni–Al(H) site (EH) is a little larger than 2Ni–Al(I) site (EI), whichmay be due to the atomic arrangements in the subsurface layer.There is no atom under 2Ni–Al(H) site, but a Ni atom under2Ni–Al(I) site in the subsurface layer, so the EH is larger than EI.The same reason caused the adsorption energy difference in thetwo 3Ni threefold hollow sites. When there is an Al atom under

Table 4Calculation results of O adsorption on Ni3Al (1 1 1) surface.

(1 1 1) Site Eb (eV) dAlNi11 (A) rNN (A) rNNN (A)

Bulk 0.072

Al on top A �5.624 �0.156 1.687(Al) 1.987(Ni)

Ni on top B-F

Ni–Al C-H

Ni–Ni D-F

Ni–Ni E-I

3Ni F �7.348 �0.202 1.360(Ni) 1.798(Al)

3Ni G �7.086 �0.193 1.317(Ni) 1.942(Al)

2Ni–Al H �7.598 0.069 1.277(Al) 1.348(Ni)

2Ni–Al I �7.603 0.053 1.266(Al) 1.328(Ni)

Fig. 6. DOS for the surface layers when O adsorbs on 2Ni–Al(I) threefold site minima at

the 3Ni(F) hollow site, and no atom is under 3Ni(G) hollow site inthe subsurface layer. So the adsorption energy of O at 3Ni(F) siteis lower than 3Ni(G) site.

Fig. 6 shows the DOS for the Ni3Al (1 1 1) surface when Oadsorbs at the 2Ni–2Al(I) hollow site minima. The peak of Nibetween �20 and �18.5 eV is stronger than Al, which meansmore Ni electrons interact with O electrons than Al. Also thatmore Ni electrons overlaps with O between �7 and �3 eV, whichindicates that the interaction of O with Ni is stronger than Al.

3.3. Comparison of O adsorption behaviors on the three kinds of

surfaces

The total and partial densities of states of the O adsorption onNi3Al (0 0 1), (0 1 1) and (1 1 1) surfaces (energy minima) areshown in Fig. 7. Fig. 7a shows the total energies of the threesurfaces when O adsorbs in minima. The region ranging from�20 eV to �17.5 eV is mainly dominated by O. The spectra of(0 1 1) surface shift to the range of higher energy than (0 0 1)surface, and (1 1 1) surface shift to lower energy range than(0 0 1) surface. By comparing the DOS of Al and Ni ranging from�20 eV to �17.5 eV (Fig. 7b, c), the peak of Al at the (0 0 1)surface is the strongest of the three surfaces and the peak of Ni in(0 0 1) surface in this range is the weakest (Fig. 7c), whichindicates the interactions of O with Al is the strongest in (0 0 1)surface. Fig. 7d displays the DOS of O when O adsorbs at theminima sites on the three surfaces. The spectra of O on (0 0 1)surface are the widest in the region between �9 and �2 eV,which means O has the most stable state, and shows a strongaffinity with Al. The former analysis has shown that the peak ofAl (ranging from �19.5 to �18 eV) is stronger than Ni at (0 0 1)surface (Fig. 4), the peak of Al (ranging from �19 to �17 eV) is alittle stronger than Ni at (0 1 1) surface (Fig. 5), and the peak ofNi (ranging from �20 to �18.5 eV) is stronger than Al at (1 1 1)surface (Fig. 6). So the affinity of O for Al in the three surfacesare rated as (from strong to weak) (0 0 1), (0 1 1), (1 1 1) surface.

Ni3Al (1 1 1) surface. (a)–(d) are the same as (a)–(d) in Fig. 4 but for (1 1 1) surface.

Fig. 7. The comparison of DOS of O adsorption on Ni3Al (0 0 1) (0 1 1) (1 1 1) surfaces (energy minima): (a) total DOS, (b) DOS for surface Al, (c) DOS for surface Ni, and

(d) DOS of O.

Q. Wu et al. / Physica B 407 (2012) 2321–2328 2327

The affinity of O for Al may relate to the surface atoms. Whenboth (0 0 1) and (0 1 1) surface have 50% Al and 50% Ni, (1 1 1)surface has 25% Al and 75% Ni. So the affinity of O for Al at the(1 1 1) surface is the weakest. If the affinity of O for Al is stronger,it is likely that the Al2O3 is more stable if there is an Al2O3 layerformed after oxidation. The experimental study by Yuan et al. [3]showed the oxidation resistance of single crystal superalloy indifferent crystallographic orientations increased as (1 1 1), (0 1 1),and (0 0 1) surface. Since g0-Ni3Al is the major oxidation resis-tance phase in superalloys, this indicates that maybe the strongeraffinity of O for Al results in better oxidation resistance. However,the oxidation of superalloy is complicated and likely due tomultiple cases, but this work provides evidence that the affinityof O for Al may be responsible for the oxidation resistance ofsingle crystal alloys in different orientations.

4. Summary

In this work, we have investigated the adsorption behavior ofO on Ni3Al (0 0 1), (0 1 1), and (1 1 1) surfaces, in order tocharacterize the initial oxidation behavior in different orienta-tions of Ni3Al. We find that O prefers to adsorb at the 2Ni–2Alfourfold hollow site on the Ni3Al (0 0 1) surface, whereas O favorNi–Al bridge site on (0 1 1) surface and 2Ni–Al threefold hollowsite on (1 1 1) surface. The DOS of the surface were present whenO adsorbs at the energy minima site. O has the strongest affinitywith Al at (0 0 1) surface, and the (0 1 1) surface is weaker, whilethe (1 1 1) surface is the weakest. The stronger affinity of O withAl demonstrates stable Al2O3 phase. The oxidation resistance ofsuperalloy in different crystallographic orientations increased as(1 1 1), (0 1 1), and (0 0 1) surface, so the affinity of O for Al maybe one of the reasons that caused difference in the oxidationresistance of single crystal alloy in different orientations.

Acknowledgments

Computations were carried out at the Supercomputing Centerof Chinese Academy of Science.

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