Ab-initio study of surface segregation in aluminum alloys

Applied Surface Science 399 (2017) 351–358

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Ab-initio study of surface segregation in aluminum alloys Yifa Qin ∗ , Shaoqing Wang Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 3 August 2016 Received in revised form 10 November 2016 Accepted 8 December 2016 Available online 9 December 2016 Keywords: Aluminum Surface segregation Ab-initio calculation

a b s t r a c t We have calculated surface segregation energies of 41 impurities by means of density functional theory calculations. An interesting periodical variation tendency was found for surface segregation energies derived. For the majority of main group elements, segregation energies are negative which means solute elements enrichment at Al surface is energetically more favorable than uniformly dissolution. Half of transition elements possess positive segregation energies and the energies are sensitive to surface crystallographic orientations. A strong correlation is found between the segregation energies at the Al surface and the surface energ of solute elements. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Surface segregation, i.e. the alloying ingredients segregates at the material surfaces, is common in aluminum alloys which may introduce a distinct change of properties relative to homogenized alloys, e.g. corrosion potential shifts towards negative directions and anode current increases significantly [1–3]. It is widely reported that some elements, especially those from Group IIIA to VA (Group 13–15) are prone to enrich at aluminum surface after heat-treatment [4–6]. This article is devoted to the surface segregation/anti-segregation of common additional or impurity elements in aluminum alloys. Surface segregation has motivated intensive theoretical researches for decades due to the applications in catalysis domain, thus the research mainly focus on segregation of transition elements [7–9]. The theoretical models can be roughly categorized into two types, i.e. Ising-type and Non-Ising-type. By Ising-type thermodynamic models [10,11], surface segregation energy could be decomposed into three parts, i.e. cohesive effect, alloying effect and size-mismatch effect. These calculations were primarily performed on a rigid lattice on which empirical pair interaction was employed, recently first-principle Ising-type model was used to calculate Pd/Au segregation [11]. Non-Ising-type models are usually based on electronic structure calculations [12] or Monte-Carlo approaches [8] by which segregation energy or equilibrium concentration profile can be derived respectively. However, the contribution of each effect to segregation energy cannot be clarified.

∗ Corresponding author. E-mail address: [email protected] (Y. Qin). http://dx.doi.org/10.1016/j.apsusc.2016.12.055 0169-4332/© 2016 Elsevier B.V. All rights reserved.

Experimental research about surface segregation of Al alloys are widely concerned since trace amount (ppm level) of certain elements can considerably change the alloy electrochemical properties [3]. Obviously, a remarkable property change induced by trace amount elements means huge economic benefits or losses. A comprehensive understanding of solute elements segregation at Al alloys surfaces is of vital importance. It provides a low-cost approach to modify Al surface property to obtain corresponding Al materials, like corrosion-resistant Al alloys or Al sacrificial anodes [13] which corrode uniformly in service. Various mechanisms were proposed to reveal how segregated elements activate aluminum alloys in aggressive environments [13–17], but some basic questions are neglected and remaining unsolved, i.e., which elements are prone to surface-segregation in Al and how large the respective tendencies are. The past theoretical researches of impurities segregation in Al mainly focused on Al grain boundaries (GB) [18–28]. In some researches, surface segregation energies are also calculated in comparison with GB counterpart, but only a few elements are concerned. For instance, Uesugi et al. studied segregation of alkali and (113) [110] grain boundary and the alkaline earth metals at Al (113) free surface, Razumovskiy investigated the segregation of B,  Si, P, Cr, Ni, Zr and Mg impurities in Al (210) [100] and the (210) surface. As far as we know, an over-all quantitative comparison of surface segregation tendency for solute elements in Al alloys is still unavailable. In fact, quantitatively reliable segregation energies can be derived from photoemission spectroscopy of surface core-level shifts (SCLS) [29,30]. The shift in the core-level binding energy of surface impurity atoms relative to its bulk counterpart is approximately equal to the impurity surface segregation energy [31]. However, the methodology is only suitable for Z ± 1/Z combination (the atomic numbers of impurity/host elements are Z ± 1/Z

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respectively). Namely, by SCLS it is infeasible to derive the surface segregation energies of a series of impurities in Al. By technique like glow discharge optical emission spectrometer (GD-OES) or other approaches, the concentration-depth profiles of solute elements in Al alloys can be obtained [1,2,32,33]. The contrast between the surface and interior concentration qualitatively reveals the segregating tendency of solute elements. Nevertheless, the contrast cannot be used to uniformly and quantitatively compare the tendency due to the diversity of the Al alloy types and experiment conditions in published literature. To gain a more general understanding of the segregation on Al surface, we calculated the segregation energies of 41 elements by density functional theory methods. Most of common alloying elements and impurity or trace elements in Al alloys were included in the calculation. Since the model adopted here did not contain grain boundaries, intermetallic particles (IMP) and other defects, the comparison of segregation tendency can be performed on equal footing without disturbing factors. Segregation energy variations with the surface orientation, the atomic numbers and the depth of impurities were analyzed. 2. Computational methods and modeling Surface segregation energy Eseg is used to characterize the tendency of a solute element enriching at the alloy surface and defined as the difference between the surface/bulk permutation enthalpy [11] of solute element in the alloy. Surface/bulk permutation enthalpy [11] is the change of system enthalpy when an isolated impurity atom is exchanged with a surface/bulk solvent atom, obtained as follows: perm

Hsurf = (Edoped,surface + Eisolated,Al ) − (Epure,surface + Eisolated,X ) perm

Hbulk = (Edoped,bulk + Eisolated,Al ) − (Epure,bulk + Eisolated,X ) Edoped/pure,surface/bulk is the total energy of doped/pure surface/bulk model respectively. A negative value of Hperm means the permutation is energetically favorable. Segregation energy is then given by [11]: perm

perm

E seg = Hsurf − Hbulk

= (Edoped,surface + Epure,bulk ) − (Epure,surface + Edoped,bulk ) Corresponding models are shown in Fig. 1. Negative segregation energy indicates that the solute element tends to segregate at Al surface in comparison with in the bulk, positive means the opposite. All calculations were performed with Vienna Ab Initio Simulation Package (VASP)[34] which implements density functional theory[35] with periodic boundary condition. The electron-ion interaction was described by the projector augmented plane wave method [36] with generalized gradient approximation (GGA). A plane-wave basis with an energy cutoff of 400 eV was used to expand electronic wave functions. The electron exchange and correlation were treated by the Perdew-Burke-Ernzerh functional [37]. A maximum Hellmann Feynman force tolerance of 0.03 eV/Å was set as the convergence criterion for the structure optimization. To calculate the (111), (110) and (100) surface segregation energies of an Al-X system, slab models containing 81, 135, 135 atoms (9 atoms per atomic layer) were exploited respectively. The top 6, 10, 10 atomic layers were relaxed for (111), (110), (100) slabs respectively and the remained layers were fixed. The concentration of impurity atoms in the surface layer is 1/9 ML (1 impurity atom and 8 Al atoms). The bulk models (108 atoms) were set 3 × 3 × 3 times as an FCC unit cell in which the supercell volume and all of the atoms were free to relax. All the models were set big enough to ensure that the concentration of X element in the models is as close

as possible to dilute limit. For those elements of which solubility is particular low in Al alloys, like Sn [38], In [38], etc, the concentration in the models is high enough to form precipitate phases or intermetallic particles. Yet due to the computing power we could not enlarge the model size arbitrarily to lower the concentration and we considered the model size was still a reasonable compromise. 3. Results and discussion 3.1. Surface segregation energy of alloying elements in Al 3.1.1. Crystallographic orientation We calculated the surface segregation energy in aluminum of alkaline and alkaline-earth metals from Period 2–5, all the Period 4 and 5 transition elements, and 11 elements from Group IIIA to VA. For every element, Eseg for (110), (111), (100) were computed to ensure the calculation comprehensiveness. Results are listed in Table 1 and Fig. 2. Experimental and theoretical segregation energies from literatures are also listed in Table 1. Uesugi et al. [21] derived segregation energies of Na (−1.94), Mg(−0.20), K(−3.42), Ca(−1.16), Sr(−2.38 eV/atom) on the Al (1–13) surface using the first-principles calculation, Razumovskiy et al. [18] calculated the Al (210) segregation energies of Mg(-0.28), Ni(0.02), Zr(1.40), Si(-0.45), Cr(1.43 eV/atom) etc, our data are consistent with both of the two works. For Zr, the deviation between this work (0.30–0.90 eV/atom) and the literature (1.40 eV/atom) may be attributed to that segregation energy of Zr is sensitive to the Al surface orientation and the impurity concentration. The experimental data of impurities segregation energies at Al surfaces are quite rare, we only found Mg segregation energy (about −0.21 eV/atom), which was obtained by optical second harmonic generation (SHG) on the (111) surface of single crystal Al-1.45at%Mg. Again, they are in good agreement. As shown in Table 1 and Fig. 2, surface segregation energy in Al has a huge range, the minimum (most negative) is Eseg 110 of Cs (-5.73 eV/atom) and the maximum (most positive) is Eseg 100 of Mo (1.62 eV/atom). Two interesting features are revealed by Fig. 2. The first one is the asymmetry of positive/negative segregation energy range. As we can see, for elements with negative Eseg , the values are distributed in a range which is 5.73/1.62–3.5 times as that of the positive Eseg elements. The other feature is that, for an impurity, the (111) segregation energy is usually smaller (in absolute value) than those of (110) and (100). In Fig. 2, when the Eseg 111 < 0 eV/atom, most of dots are below the solid diagonal line (Eseg 110/100 < Eseg 111), and vice versa. It is in consistence with a previous study [40] that the driving forces of segregation/depletion at close-packed surfaces are weaker than those at non-close-packed ones. The (111) surface is close-packed, i.e., the number of missing bonds per surface atom is smaller than that of open surface. In other words, for alloying elements, the (111) surface is more close to the bulk environment compared with the (110) and (100) surfaces. 3.1.2. Depth For the (111) surface, we investigated the variation of segregation energy induced by the depth of solute atoms. The segregation energy when a solute atom is located at the n-th layer (n = 1 being the outmost layer) were computed and listed (n = 2, 3, 4) in Table 2. Since the segregation energies of the three surfaces share similar features, here only the (111) surface is concerned. In Table 2, the segregation energies of 2–4th layers are much smaller (in absolute value) than those of the 1st ones (Table 1). The largest |Eseg | is that of cesium (0.86 eV/atom), all the rest |Eseg | are below 0.3 eV/atom and more than half Eseg are within ±0.1 eV/atom. For elements with high negative Eseg at the 1st layer, e.g. Na (−1.14), K (−3.30), Rb (−4.30), Ba (−3.21), the Eseg at the 2nd layer are only

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Fig. 1. Surface and bulk models used in the computation of Eseg.

Table 1 Surface segregation energies (eV/atom) of impurities in aluminum (111), (110) and (100). The solute atom is located at the outmost layer of the slab (1/9 ML). Segregation energies from other theoretical literature are listed in comparison with the values in this work. Eseg 111 (eV/atom)

Eseg 110

Eseg 100

Li Na K Rb Cs Be Mg

−0.18 −1.14 −3.30 −4.30 −5.20 0.12 −0.28

−0.43 −1.56 −3.81 −4.85 −5.73 −0.10 −0.42

−0.21 −1.36 −3.61 −4.65 −5.53 0.04 −0.31

Ca Sr Ba Sc Y Ti Zr V Nb Cr Mo Mn Tc Ru

−1.10 −2.07 −3.21 0.02 −0.47 1.35 0.30 0.82 0.77 0.84 0.93 0.72 0.79 0.56

−1.39 −2.50 −3.73 0.35 −0.38 0.58 0.90 1.30 1.43 1.21 1.51 0.93 1.17 0.72

−1.32 −2.42 −3.60 0.30 −0.47 1.13 0.81 1.48 1.37 1.32 1.62 0.44 1.23 0.53

Ref. Eseg

−1.94,a −3.42,a

−0.20,a −0.22,b −0.21, c −1.16,a −2.38,a

1.40,b

1.43,b

Eseg 111 (eV/atom)

Eseg 110

Eseg 100

Fe Co Rh Ni Pd Cu Ag

0.54 0.38 0.31 0.22 0.10 0.07 −0.18

0.59 0.27 0.27 0.01 −0.04 −0.12 −0.34

0.00 −0.18 0.21 −0.13 0.03 0.00 −0.21

Zn Cd Al Ga In Tl Si Ge Sn Pb P As Sb Bi

−0.11 −0.60 0.00 −0.25 −0.85 −1.62 −0.03 −0.29 −0.83 −1.52 −0.17 −0.35 −0.94 −1.54

−0.27 −0.78 0.00 −0.37 −1.08 −1.93 −0.22 −0.53 −1.12 −1.84 −0.65 −0.77 −1.30 −1.88

−0.16 −0.72 0.00 −0.36 −1.09 −1.85 −0.15 −0.45 −1.08 −1.75 −0.45 −0.61 −1.20 −1.76

Ref. Eseg

0.02,b

−0.45, b

Notes: a.Cited from Uesugi [21] et al. The surface concentration of impurities is 1/4ML. The surface is free (1–13). b. Cited from Razumovskiy [18] et al. 1/4 ML. (210). c. Cited from Bloch [39] et al. (111) surface of Al-1.45at%Mg.

small negative values, i.e. −0.01, −0.03, −0.12, −0.04 eV/atom, the contrast means the tendency of subsurface segregation is much weaker than that of the outmost layer. The sharply reduced |Eseg | reveals that the subsurface area significantly differs to the surface, instead it approximates the bulk environment. As the distance of solute atoms increases, the segregation energies decay to zero since the locations of solute atoms are getting more and more similar to the bulk environment. The decay of Eseg is manifested by two ways, i.e., monotonous and oscillating. Alloying effect determines which way the decay of segregation energy will take [10]. Na, K, etc are typical examples of the oscillating category. For Na, the segregation energy shifts towards positive direction from −1.14 (n = 1) to −0.03 (n = 2), and then towards negative direction from a small positive values 0.06

(n = 3) to 0.02 eV/atom (n = 4). For Na, K, Sr, Cs etc, the small positive Eseg at 3rd and 4th layer indicate that a weak depletion may occur here. Namely, for these impurities, the preference of sites is as follows: outmost surface (n = 1) ≥ bulk ≥≈ subsurface (n = 3,4,etc). With regard to impurities with positive Eseg at 1st ∼ 2nd and negative ones at 3rd ∼4th layers, e.g. V, Mo, etc, the preference is reversed. Some impurities like Ba, have relatively high Eseg at 2nd ∼4th layers, thus the oscillation may take more than 4 layers to decay the segregation energy to close to zero.

3.1.3. Periodicity By combined with Tables 1 and 2 data and atomic numbers, it is found that explicit periodicity exists in surface segregation energy

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Fig. 2. Correlation between Eseg 111 and Eseg 110or Eseg 100. The x axis is Eseg 111 resorted in ascending order, the y axis is Eseg 110or Eseg 100 of corresponding elements.

Table 2 (111) Segregation energy (eV/atom) of n-th layer solute elements (n = 2, 3, 4) in Al. Bolding is applied for the elements with|Eseg |> 0.1 eV/atom. Element

2nd

3rd

4th

Element

2nd

3rd

4th

Li Na K Rb Cs Be Mg Ca Sr Ba Sc Y Ti Zr V Nb Cr Mo Mn Tc Ru

0.01 −0.01 −0.03 −0.12 −0.86 −0.02 0.00 0.04 0.03 −0.04 0.13 0.11 0.18 0.19 0.22 0.23 0.22 0.24 0.19 0.20 0.14

0.01 0.06 0.12 0.19 0.27 −0.01 0.00 0.08 0.16 0.26 0.05 0.11 −0.01 0.06 −0.05 −0.02 −0.07 −0.07 −0.06 −0.07 −0.04

0.01 0.02 0.09 0.16 0.24 −0.01 −0.02 0.08 0.15 0.27 0.04 0.12 0.04 0.06 −0.01 0.01 −0.01 −0.01 0.01 0.01 0.05

Fe Co Rh Ni Pd Cu Ag Zn Cd Al Ga In Tl Si Ge Sn Pb P As Sb Bi

0.14 0.07 0.07 0.01 0.01 −0.03 −0.02 −0.04 −0.03 0.00 −0.02 −0.02 0.18 0.03 0.01 0.00 0.00 0.03 0.02 0.03 0.00

−0.03 −0.01 0.00 −0.01 0.02 −0.02 0.02 −0.02 0.03 0.00 0.01 0.06 −0.01 0.06 0.07 0.10 0.18 0.08 0.10 0.15 0.24

0.05 0.06 0.08 0.04 0.07 0.00 0.02 −0.01 0.01 0.00 0.02 0.03 0.00 0.04 0.06 0.08 0.15 0.02 0.06 0.12 0.21

in Al. If not specified, the following Eseg refers to that of the solute element which locates at the outmost layer (n = 1). As shown in Table 1, Eseg 111 of IA Group are −0.18, −1.14, −3.30, −4.30, −5.20 eV/atom for Li, Na, K, Rb, Cs respectively. Obviously, there is an enhanced segregation tendency with atomic number for alkaline metal elements. The tendency remains mainly the same for (100) and (110) surfaces. The extraordinary high values of Eseg signify that alkaline metal elements have a strong trend of enriching at aluminum surface except for Li, which has relatively weak Eseg from about −0.2 to −0.4 eV/atom. Alkaline-earth metal elements share similar variation trend with their IA group neighbors. For Be, Mg, Ca, Sr, Ba, respective Eseg 111 are 0.12, −0.28, −1.10, −2.07, −3.21 eV/atom. Beryllium has small positive Eseg 111 and Eseg 100 which means that it is more inclined to migration into Al matrix instead of remaining at (111)/(100) surface zone, though the tendency is weak. Mg has a less negative Eseg 111 (0.28 eV/atom) compared to that of Na (-1.10 eV/atom), as Ca to K, and Sr to Rb, etc. Overall, the surface segregation trend of IIA

elements is weaker than IA elements of the same period. The gradient of Eseg 111 for Group IIA is also smaller than Group IA. The periodicity of main group elements Eseg is clearly shown in Fig. 3. Segregation energies of elements alloyed in Al are negative for all calculated members from Group IIIA to VA except for Al itself (Eseg of Al is 0 according to the definition). Surface crystallographic orientations have no effect on the sign of segregation energy. The change of Eseg from IIIA to VA is much small relative to that from IA to IIA as depicted in Fig. 3. For elements in same group, Eseg reduces as atomic number increases. Energy curves of Period 3, 4, 5, 6 share a similar shape which is composed of a steep slope and a flat hilltop. In case of transition elements, the periodicity still exists and manifests as three significant features as shown in Fig. 4. Primarily, the transition zone concentrates the majority of elements with positive Eseg at Al surface. It means that for half transition elements in Period 4 and 5, enrichment at Al surface is energetically unfavorable compared to uniformly dissolution. Y, Ag, Zn and Cd possess negative Eseg for each surface, corresponding Eseg 111 are

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As n increases to 3 or 4, the periodicity of segregation energy of transition elements vanishes due to the decay of energy and probably the calculation precision. Based on Table 1, the calculated elements can be divided into anti-segregation, segregation, bidirectional groups. Beryllium, cobalt, nickel, copper and palladium are categorized as bidirectional group since the sign of Eseg differs in the three surfaces. Anti-segregation elements are those with positive Eseg for (111), (110) and (100), i.e. Sc, Ti, V, Cr, Mn, Fe, Zr, Nb, Mo, Tc, Ru, Rh. Those elements with negative segregation energies fall in segregation group. Bidirectional Be, Co, Ni, Cu and Pd can be regarded as intermediates between segregation and anti-segregation elements. Interestingly, the same type elements gather together in Periodic Table.

Fig. 3. Eseg 111 of Group IA to VA elements. Eseg 100 and Eseg 110 are not shown due to similarity.

Fig. 4. Eseg 111/110/100 of Period 4 and 5. For (111) surface, the segregation energy of outmost layer (n = 1) and the sublayer (n = 2) are both plot, the latter are magnified 2 times for clarity.

−0.47,−0.18,−0.11,−0.60 eV/atom respectively. The segregation or depletion of Cu and Pd are weak since all the absolute values of segregation energies are within or close to 0.1 eV/atom, which means they have negligible preference between the Al bulk and the surface. Secondly, segregation energy of transition elements alloyed in Al is sensitive to surface crystallographic orientations. The remarkable dispersion about Eseg for various surfaces is in contrast with Group IA to VA elements. A typical example is Nb, which has 0.77, 1.43, 1.37 eV/atom segregation energy for (111), (110), (100) respectively, the gap between the maximum and minimum is noticeable 0.66 eV/atom. For magnetic metals like Fe, Co, Ni, Eseg is positive for (110) and (111) surfaces while negative or zero for (100). In addition, the dispersion of Eseg shrinks for late transition elements especially for Period 5 as depicted in Fig. 4. Finally, Eseg increases (namely shifting towards positive direction) and then reduces with atomic number for elements in the same period. Peak value locates at V and Mo for Period 4 and 5 respectively. Even for the subsurface layer (n = 2), the segregation energy of (111) still shares the same variation trend with its outmost layer counterpart.

3.1.4. Effect factors of surface segregation energy Three driving forces of surface segregation were identified according to the Tight-Binding Ising model [41,42]. The first is named as site effect or cohesive effect, namely, the element with a lower surface energy is driven to enrich at surface area by the difference in the surface energy or cohesive energy between the solute and the solvent elements. Alloying effect is another import driving force which promotes the segregation of the solvent/solute element if they tend to form heteroatomic/homoatomic bonding (bonding between different/same elements), respectively [10]. Size effect favors the surface segregation of atoms with larger radius. To qualitatively estimate the contribution of each effect to Eseg , the correlation between Eseg and surface energy of pure solute metals was depicted as Fig. 5. It is clearly seen that the majority of calculated metal elements fall in two areas: surface energy larger than Al and Eseg > 0 (upper right corner), or surface energy smaller than Al and Eseg <0 (lower left corner). The data of surface energies in Fig. 5 are from the study of F.R. de Boer, et al(35), the atom radius are cited from Kelly, etc. [43]. For most of the metal elements, size effect is synergic with site effect, i.e. if the element has Esurf exp larger than Al, the atom size is also bigger than Al, vice versa. Zr, Ti and Nb are exceptions, since the radius of them(0.160, 0.147, 0.146 nm) are bigger than that of aluminum(0.143 nm), yet they are anti-segregation group elements with high positive Eseg . It seems that the size effect is too weak to overcome the sum of site effect and alloying effect. For zinc (0.134 nm), the situation is also similar. As shown in Fig. 5, alkaline and alkaline earth metals have surface energy (atom radius) much lower (larger) than Al (1.143J/m2 , 0.143 nm) respectively, hence the fierce synergic effect results in extraordinarily negative segregation energy. However, some elements (Cu, Pd, Ni, Be, etc) have Eseg close to zero, though the radius are smaller and the surface energies are larger than Al. It is probably due to a strong alloying effect which counteracts the site and size effects. 3.2. Segregation energy and experiment concentration depth profile Negative Eseg of X element indicates that X enrichment at alloy surface brings down system free energy relative to dissolution in the alloy bulk. For elements with negative Eseg , the surface concentration is supposed to be higher than that in bulk provided that appropriate heat treatment is applied to prompt solute atoms to overcome migration energy barrier. Conversely, positive Eseg leads to surface depletion of solute elements. Thus, the validity of our calculation and classification can be qualitatively evaluated by the ratios between the surface and total concentration of solute elements (R = cs /ct ). The ratios and corresponding alloys and heat treatment are collected from literatures and listed in Table 3. Mn and Cr are anti-segregation group elements with positive minimum segregation energy, 0.44 and 0.84 eV/atom respectively.

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Fig. 5. Correlation between (111)/(100) segregation energy in Al and experimental surface energy of solute elements. The radius of every dot in the picture is proportional to the atomic radius. Blue/red dots correspond to elements with a radius smaller/larger than that of aluminum (1.43 nm). Eseg 110 was not shown due to the similarity with Eseg 100. Notes: Sn and In almost coincide in Fig. 5.

Table 3 The comparison between the surface and the total concentration of solute elements in aluminum alloys. Cs and Ct

R = Cs /Ct

Eseg (eV/atom)

Alloy

Methodology

Ref

Cr

Not provided

<1

0.84–1.32

AA3012 heat-treated at 600 ◦ C for 1h

[1]

Mn

∼0,0.25 wt% Not provided, 0.288at% 100 ppm,100 ppm ∼1.5 wt%,0.5 wt% 2.0at%,0.9at% 40at%,12.7at% 17at%,6.5at%

<1 <1 ≈1 3 2.2 3.1 2.6

−0.12–0.07 0.00–+ 0.59 −0.27–−0.11

6 wt%, < <0.05 wt% 2.5 wt%,222 ppm Not provided

> >120 250 >1

−0.42 to −0.28

AA3012 heat-treated at 600 ◦ C for 1h AlMg220Pb30 heat-treated in air at 450 ◦ C for 1h AA8006

In

Not provided, 20 ppm

>1

−1.09 to−0.85

AlIn20 heat-treated for 1 h at 300 ◦ C

Sn

Not provided, 100 ppm

>1

−1.12 to−0.83

AlSn100 heat-treated at 300 ◦ C followed by water quenching

GD-OES depth profiling GD-OES DSIMS, XPS RBS GD-OES Theory AES, LEED AES, LEED DSIMS, etc GD-OES TEM, GD-OES TEM, GD-OES GD-OES, FEG-SEM SEM, GD-OES DSIMS, XPS GD-OES, EDS, TEM GD-OES TEM, GD-OES TEM, GD-OES

Cu Fe Zn Li

Mg

Na

Not provided, 0.012 wt%

>1

Pb

0.9 wt%,50 ppm

180

0.5 ∼ 1 wt%, < <0.05 wt% 0.4 wt%,20 ppm

> >10 200

0.9 wt%,20 ppm

450

0.44–0.93

−0.43 to−0.18

−1.56 to−1.14

AA3012 heat-treated at 600 ◦ C for 1h LM6 (Al–12%wt Si) heat-treated at 440 ◦ Cfor 360 min AlCu100Pb1.5 heat-treated at 600 ◦ C for 1h AA3012 heat-treated at 600 ◦ C for 1h Al-Zn liquid alloy (110)Al-12.7at%Li single crystal alloy heat-treated at 418 K for 10 min (111)Al-6.5at%Li single crystal alloy annealed at about 500 K

◦

LM6 (Al–12%wt Si) heat-treated at 440 C for 360 min AlPb50

−1.84 to−1.52

AA3102 polished and heat-treated for 60 min at 600 ◦ C AlPb20 heat-treated in air at 450 ◦ C for 1h AlMg220Pb20 heat-treated in air at 450 ◦ C for 1h

It means surface depletion is energetically more favorable for the two elements. Surface depletion of manganese [44] is found in LM6 (Al–12%Si) alloy since enhancement of Mn signal strength from the surface to the interior is clearly depicted in the dynamic secondary ion mass spectrometry (DSIMS) depth profile of LM6. Glow discharge optical emission spectroscopy (GD-OES) profiles for aluminum alloy 3102 (heat-treated for 60 min at 600 ◦ C) also reveals that Mn concentration increases from oxide-metal interface metal side to the bulk [1]. The author also mentions that segregation

[1] [44] [45] [1] [48] [47] [46] [1] [32] [6] [3] [2] [44] [4] [1] [32] [32]

of chromium is confined to the outer part of oxide which means enrichment at metal side is not observed [1]. In other words, the R values of Mn (in LM6, AA3102) and Cr (AA3102) are below 1. Specific values of the ratios are not available in the articles but it can be deduced that R is less than 1. The classification based on Eseg is also in agreement with experimental literature for anti-segregation and segregation group elements. Significant enrichment of lead and weak enrichment of copper at AlCu50Pb1.5 foil (50 ppm Cu and

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1.5 ppm Pb) surface was revealed by Rutherford backscattering spectroscopy (RBS) [45] after heat-treatment in vacuum. This experiment result is consistent with our data of Cu (0.12 ∼ + 0.07 eV/atom) and Pb (-1.84 ∼ −1.52 eV/atom), which means a nearly negligible and violent inclination of surface enrichment respectively. The surface peak concentrations of lead are approximately 0.9 wt%, 0.4 wt%, 0.85 ∼ 0.9 wt% and corresponding R values are respectively about 180, 200, 450 for AlPb50 [4], AlPb20 [32], AlMg200Pb20 [32] after respective heat treatment. Small R values usually correlates with weakly negative Eseg , e.g. 17at%/6.5at% = 2.6 [46], 40at%/12.7at% = 3.1 [47] for Li (-0.43 ∼ −0.18 eV/atom), ∼1.5 wt%/0.5wt% = 3 [1] for Fe(0.00 ∼ 0.59 eV/atom), ∼2.0at%/0.9at% = 2.2 [48] for Zn(0.27 ∼ −0.11 eV/atom). Since total concentration of Li is too high in the cited literature, higher R values for Al alloy with trace Li are possible but not available in literature as far as we know. Significant surface enrichment induced by heat-treatment is also reported for Al alloys containing In [3], Sn [2], Na [44], which are highly negative segregation-group elements. As shown in Table 3, for the majority of the listed elements with negative/positive Eseg , the corresponding ratios are greater/less than 1 respectively. It is clear that our calculation distinguishes solute elements of segregation group from anti-segregation and bidirectional counterparts. It is worth noting that Pb has a much more negative Eseg (-1.84 ∼ −1.52 eV/atom) than Mg (−0.42 to −0.28 eV/atom) in Al alloys, whereas Mg can reach a high surface concentration (6 wt%) [1] relative to Pb (0.9 wt%) [32] and they share R with the same magnitude (100 ∼ 300). There are several possible reasons for this phenomenon. The first reason may be attributed to solute-solute interaction. According to the definition, surface segregation energy is the driving force in the dilute limit for the exchange of an interior solute atom with a surface solvent atom. As the solute atoms assembling at surface, the local solute concentration increases and then the driving force may be influenced by solute-solute interaction. This interaction is not confined to the same type solute atoms. Secondly, oxide layer, grain boundaries and defects and IMPs may also be involved in the scramble of solute atoms. From another perspective, Eseg can be regarded as an indicator of evaluating the competence of bulk/surface to attract solute elements, positive Eseg means Al bulk preceding Al surface, vice versa. Our model is actually designed for ideal metal without oxide layer, grain boundaries, IMPs and defects. These competitors may attract or reject solute atoms and then narrow or broaden the gap between surface and interior concentration. Finally, heat treatment also significantly alters surface concentration of solute elements. Heat treatment at appropriate temperature gives solute atoms (elements with negative Eseg ) enough energy to climb over energy barrier and then diffuse to surface area. Higher temperature and longer time grant more solute atoms the capability of migration, on the other hand the diffusion ability of surface solute atoms is also enhanced which means an increasing homogenization inclination [3,5]. The compromise between segregation and homogenization leads to an optimum surface segregation heat treatment temperature for a binary Al-X alloy (X with negative Eseg ). Consequently, the comparison between the segregation inclinations of solute elements may be confused by heat-treatment temperature and time.

4. Conclusion By analysis of surface segregation energy of 41 elements alloyed in aluminum, interesting tendencies are found and several conclusions are drawn as follows:

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(1) All the main group elements from Group IA to VA have negative surface segregation energy in aluminum except for Be (−0.10 to + 0.12 eV/atom). Generally, surface segregation tendency increases from Period 3–6. (2) Half of the transition elements in Period 4 and 5 have positive segregation energies at the Al surfaces. The energy shifts toward positive direction and then negative direction from early transition elements to late ones. (3) Surface crystallographic orientations significantly influence segregation energy of transition elements but do not change the signs of energy except for Co, Ni. The influence diminishes for early and late transition elements. Close-packed plane (111) has a weaker segregation/anti-segregation tendency for elements with negative/positive segregation energy in relative to (100) and (110). (4) Calculated elements are classified as segregation, antisegregation, and bidirectional groups based on surface segregation energy. The calculation result is in good agreement with experimental literature. (5) Solute elements with a lower/higher surface energy than that of Al are inclined to enrichment/depletion at the Al surface.

Acknowledgment The project was supported by the National Major Research Program of China (No.2016YFB0701302) and the National Natural Science Foundation of China (No.51471164). The computational support from the Informalization Construction Project of Chinese Academy of Sciences during the 11th Five-Year Plan Period (No.INFO-115-B01) and Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) are also highly acknowledged.

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