SURFACE AND INTERFACE ANALYSIS, VOL. 10, 92-96 (1987)

Surface Segregation and Oxidation Studies of Al-Be Alloysf

M. AhmadS and J. M. Blakely Materials Science and Engineering, Cornell University, Ithaca, N.Y. 14853, USA

Surface segregation and oxidation of AI-Be alloys have been studied using the LEED-AES technique. Results obtained from one polycrystalline sample and three single crystals with (Ool), (011) and (111) orientations show no segregation of Be to the clean A1 surface. The annealing temperatures ranged from 300 "C to 600 OC for a few minutes to a few hours. Results from the polycrystalline sample suggest that in the presence of oxygen or A1203 on the surface, Be diffuses to the surface during annealing (at 400 OC or above) to form Be0 and reduces A1203. The difference in the free energies of formation of the bulk B e 0 and A1203 favors this reaction. A one-to-one correspondence has been found between the surface coverage of Be0 formed to that of A1,03 present prior to annealing the sample. The three single crystals had much less Be in the bulk and this affects the reduction reaction by limiting the rate of Be diffusion to the surface. LEED investigation for the (111) oriented crystal showed that B e 0 grows as the (OOO1) basal plane on the Al(111) surface. The magnitude of the misfit between unrelaxed Al(111) and BeO(OOO1) as measured from the relative positions of superlattice beams indicate some compression or expansion in the interface region.

INTRODUCTION

The effect of trace impurity addition on the corrosion and mechanical properties of alloys has been widely reported in the literature. It has been found that a small amount of beryllium in various aluminum alloys effectively reduces oxidation, improves adhesion of aluminum films and prevents mold reaction in casting alloys. The effects are attributed to Be diffusion to the surface and formation of a protective layer.' From exten- sive thermogravimetric measurements Whitaker and Heath found that the addition of 100 ppm Be to Al-Mg alloys was effective in reducing the rate of Al-Mg alloy oxidation.' Electron microscopy studies by Field et al., revealed that Be at these concentrations inhibits MgO formation in favor of Be0.3 Degreve et al., have shown by ion microprobe analysis of the thermal oxide layer on an Al-Zn-Mg alloy with Be addition that Be becomes enriched at the interface between amorphous oxide and metal after heat treatment.4

In this paper we report preliminary results from a LEED-AES investigation of the structure, composition and kinetics of oxide formations on Al-Be alloy surfaces. The aim of our work was to study the surface segregation of Be and its effect on the oxidation of aluminum.

Chemisorption and initial oxidation of aluminum have been the subjects of numerous studies using a variety of UHV surface sensitive tools. Much of this work has been reviewed re~ent ly .~ Comparatively few investigations on Be have been reported.6 In the case of an alloy surface, any preferential oxidation of one of the species will depend on the rate of chemisorption, the mobilities of Al, Be and 0 and the relative values of the free energies of formation of the stable oxides of Al and Be.

t This work was supported by the Aluminum Company of America

$ To whom enquiries regarding the work should be directed. and by the Materials Science Center at Cornell University.

0 142-242 1 /87/02OO92-05 $05.00 @ 1987 by John Wiley & Sons Ltd

EXPERIMENTAL

LEED and AES were performed in a UHV chamber equipped with a four-grid retarding field electron energy analyzer. The working base pressure was S 2 x lo-'' Torr and gas partial pressure was monitored by a Varian VGA-100 quadrupole mass analyser. Ion sputtering with 500 eV argon ions at 300 "C was effective in cleaning the surface. Oxidations were done at room temperature at Oz pressures in the 4 x Torr to 8 x lo-' Torr range. During the oxidations, AES signals were monitored using a 1.5 kV glancing incidence electron gun at a beam current of 5 PA. After a specified amount of oxygen was chemisorbed, or surface aluminum oxide was formed, the sample was annealed in vacuum usually at 400 "C for 5 minutes or longer. The resulting Auger spectrum was monitored for changes in surface composition. In some instances the sample was annealed during a con- tinuous flow of oxygen. The LEED pattern was monitored at various stages in the oxidation and anneal- ing sequence.

Al-Be alloy ingots containing approximately 0.13 at% Be were used to grow single crystals from the melt. Sample discs oriented by real time x-ray diffraction were spark-cut to about 1 cm in diameter and 3 mm in thick- ness. The samples were cleaned and polished extensively resulting in a mirror-like finish. The samples were moun- ted on Ta ribbons spot welded to Ta heating rods.

RESULTS AND DISCUSSION

One polycrystalline sample and three single crystal samples with the three principle orientations were investigated in this study.

Received 18 July 1986 Accepted 25 August 1986

STUDIES OF AI-Be ALLOYS 93

Clean surface composition

The clean surface was first checked for segregation by annealing in vacuum at va.rious temperatures starting at 300 "C and going up to 600 "C in steps of 100 "C. The annealing periods varied from a few minutes to several hours. After each anneal the sample was quenched to room temperature and the Auger spectrum was obtained. No detectable segregation of Be was found in any of the samples. From the phenomenological bond breaking model: segregation of Be is not expected since there is very little difference between the heats of sublimation of the alloying constituents; for Al and Be these are respectively 78.7 and 7'1.5 kcal mol-'.8 However, the difference in size between A1 and Be is significant, the effective atomic radii being 1.43 and 1.11 A respectively.8 According to the model proposed by Abraham and Brundle Be should segregate to the Al surface.' Within the detection capability of our instrument (i.e., approxi- mately 0.02 monolayers of Be ) we found no evidence to support this prediction.

Oxidation-polycrystal

The polycrystalline sample was annealed at 600°C for 30 min to grow large grains and then quenched to room temperature. The individual grain sizes were of the order of 2mm and LEED patterns showed them to be pre- dominantly of (001) normal orientation. At room tem- perature, a controlled amount of oxygen exposure resul- ted in the chemisorption of oxygen followed by the oxidation of A1 as evidenced by the appearance of the 55 eV Auger peak corresponding to A1203. The Auger peaks at 68 eV (metallic Al), 55 eV (A1203) and 510 eV (oxygen) were monitoled during oxidation. Oxidation at higher temperatures, inostly at 400 "C or above showed that Be diffused to the surface and was preferentially oxidized to form BeO.

For the annealing case where the surface was pre- covered with oxygen or A1203 (grown at room tem- perature), Be0 was formed by reacting with the chemisorbed oxygen OI by complete reduction of A1203. This is shown in Fig. 1 for three cases with different initial Al-oxide thicknesses and with annealing at 400 "C for 5minutes. In each of these cases the surface was sputter-cleaned and annealed before the next oxygen exposure. The thickness of A1203 and of Be0 on the surface can be estimated from the attenuation of the 68 eV aluminum peak-to-peak height and the inelastic mean free path ( A ) values of 68 eV Auger electrons in A1203 and in BeO. The reported value of A for 68 eV electrons in A1203 is approximately 6A. '" For 100eV electrons the A value in Be0 is approximately 13 A." Assuming this same value for A in Be0 for 68 eV elec- trons we found that the ratios of the initial A1203 coverage to the Be0 coverage following the 400°C anneal in the cases of 48, 144 and 1800 Langmuirs (1 Langmuir = 1.0 x ~ C I - ~ Torr for 1 sec) of O2 exposure were 1.2, 1.3 and 1 respectively. These ratios indicate to within experimental error a complete reduction of A1203 on the surface with apparently no oxygen dissolution into the bulk at the annealing temperature. With the limited amount of data now available we cannot deter-

ROOM TEMP. 40O0C, 5 MIN 36 L

510

68

X I ( & 1 I x2.5n x5 I\ I40 L

(b)

Figure 1. Oxidation of aluminum at room temperature (left column) and the subsequent reduction of AI,O, by Be0 formation (right column) after annealing in vacuum at 400 "Cfor 5 min. The exposures are (a) 48 L. (b) 144 L and (c) 1800 L at room temperature. Surface was sputter-cleaned prior to each exposure.

mine the individual oxide thickness accurately; more careful calibration and measurement are underway.

The reduction reaction at the surface can be explained in terms of the free energy of formation, AGf, for the stable bulk oxides of Al and Be. The AGf values (obtained from Ref. 12) of Al,03 and of Be0 along with the free energy of mixing of the A1-0.13 at% Be alloy, AGmi,, plotted as a function of temperature are shown in Fig. 2(a) AGmi,, the partial molar free energy of mixing of Be in the alloy, was calculated from the solid solubility curve of the Al-Be phase diagram.I3 Figure 2(a) shows that AGf for bulk Be0 is slightly greater than that for A1203 (by approximately 10 kcal mol)-'), sug- gesting that Be would reduce A20, with the formation of Be0 This is shown in the energy level diagram of Fig. 2(b). It can also be seen that the positive free energy of mixing of Be in Al might enhance the reduction reaction if the Be formed a supersaturated solid solution. However, at true thermodynamic equilibrium, the posi- tive AGmix would lead to precipitation of Be in the alloy and thus reduce the concentration of Be atoms available to diffuse to the surface. Some care should also be taken in using the difference in the AGf values of the bulk oxides to characterize the surface reaction involving only a few monolayers; contributions to the reaction from interfacial free energies are undoubtedly very important.

The effect of a very thin layer of Be0 on the further oxidation of A1 is shown in Fig. 3. The Be0 coverage (approximately one monolayer) shown in Fig. 3(a) was formed by annealing the crystal at 400°C in vacuum after an initial oxygen exposure of 48 Langmuirs. This surface was then exposed to 2600 L of O2 at room temperature and the resulting spectrum is shown in Fig. 3(b). The Be0 peak showed very little or no change while the attenuation of the A1 68 eV peak was significant

94 M. AHMAD AND J. M. BLAKELY

Temperature ( K ) (a)

Figure 2. (a) Free energy of formation (in kcal mol-' of $0,) of A1,03 and Be0 and the Free energy of mixing (AGmi,) of Be in AI-0.13 at% Be alloy plotted as a function of temperature. AG,,, of Be calculated from the solid solubility limit in the phase diagram (Ref. 13). (b) Energy level diagram showing that a positive AG,,, for Be in Al may enhance A1203 reduction.

and the 5 5 eV peak indicated the growth of A1203. The rate of this Al-oxidation was however much slower than in the case of room temperature oxidation of A1 starting from a clean surface. It was not possible to determine whether the initial Be0 on the surface grew as a complete oxide monolayer or as islands of Be0 as good LEED patterns were difficult to obtain for this polycrystalline sample. Nevertheless, it is clear that the oxidation of A1 is inhibited by the presence of Be0 on the surface. It will be of interest to study the details of the primary oxidation of A1 (grown at the Al-Be0 interface) for various Be0 coverages. It can be noted that, when the sample with both Be0 and A1203 was annealed at 400 "C for 5 min in vacuum, the A1203 was completely reduced and the Be0 peak grew significantly larger as shown in Fig. 3(c).

Oxidation-single crystals

The three single crystals showed considerably different thermal oxidation behavior. The reduction of the surface AI20, was limited by kinetic and perhaps thermo- dynamic factors. These samples were found to have much lower bulk Be content due to unavoidable zone refining during crystal growth from the melt. A spark emission analysis (done at the Alcoa Laboratories) indi- cated that the Be concentrations were in the range of 100ppm or less, Furthermore, the very slow cooling process during crystal growth might also have led to near equilibrium Be precipitation; slow dissolution of these precipitates could limit the amount of Be in solid solution.

The (001) and the (011) oriented samples showed very limited Be0 formation after prolonged annealing in oxygen. The (01 1) sample was vacuum annealed for 65 h at 480 "C to equilibrate the Be distribution and also to increase the Be content in the solid solution by partially dissolving Be precipitates. This sample showed slightly more Be segregation to form Be0 but not enough to completely reduce the A1203 as was the case for the polycrystalline sample. The [ 11 1) crystal was obtained

from a different part of the single crystal ingot and had a slightly higher Be concentration. This sample showed more Be0 growth but it was still limited to a few monolayers. This limited coverage of Be0 was formed by annealing the sample at 400 "C for 20 min in 2.0 x lop7 Torr 02. Further annealing in O2 up to 12 h did not increase the Be0 coverage. At -100ppm the bulk is undersaturated at this temperature and this will result in a negative AGmi, for Be in Al; this in turn may offset the difference in the AG,-values for the oxide and prevent continued formation of BeO.

LEED observations

A LEED pattern of the overlayer Be0 on A1 was observed only for the (111) surface on which sufficient Be0 was formed to allow such observations. The clean surface was exposed to oxygen at room temperature in exposure steps of 10 L and the sample was annealed in

t 51OeV

f 68 eV t

510 eV

68 eV i

54 eV

x 2

f 51OeV

68 eV

p8L @, 400'C onneal , I 2600L O,, 2 5 O C , ,400'C anneal, 5 minutes , (a) ( b) (Cl

Figure 3. Oxidation of Al with precovered Be0 on the surface: (a) Be0 formed (approx. one monolayer) after 48 L 0, exposure and vacuum-annealing at 400 "C for 5 min; (b) spectrum resulting from further O2 exposure up to 2600 L at room temperature. Be0 (95 eV) p-p height almost unchanged; (c) reduction of A120, at Al-Be0 interface after annealing in vacuum at 400 "C for 5 min.

STUDIES OF AI-Be ALLOYS 95

Figure 4. (a) Room temperature LEED pattern (67 eV) obtained after 30 L 0, exposure at room temperature and subsequent annealing at 400 "C for 20 min. The pattern is essentially that from the Al(111) clean surface; (b) 49 eV pattern after 70 L 0, and 20 minutes annealing showing a p(1 x 1) Be0 overlayer structure; (c) after long term annealing in 0, this 64 eV pattern shows superlattice spots representing either double diffraction from a Be0 overlayer and Al substrate (or diffraction from the coincidence mesh)..(d) Reciprocal lattice spot spacing for substrate. overlayer and superlattice corresponding to an unrelaxed Be0 layer on Al.

vacuum at 400 "C for 20 min after each exposure step. LEED pattern was monitored at the end of each O2 exposure and annealing step. We did not observe any ordered oxygen structure at the end of the first exposure. Very little change was observed in the (111) LEED pattern after the first few exposure and annealing steps as shown in Fig. 4(a). This pattern corresponded to the stage where a small shoulder appeared at 95 eV in the Auger spectrum after ;I total of approximately 30 L O2 exposure and subsequent 400 "C annealing. An ordered Be0 overlayer p(1 x I) pattern was observed after approximately 70 L O2 exposure and vacuum-annealing. This is shown in Fig. 4(b) in which the inside first order beams correspond to the substrate Al(111) and the

outside ones correspond to the BeO(0001) overlayer. The Be0 overlayer grew undistorted with its sixfold symmetry of the (0001) basal plane. Further long term annealing in oxygen resulted in the limiting Be0 cover- age shown in Fig. 4(c). In this case hexagons of superlat- tice beams were visible around both sets of primary beams corresponding to the threefold A1 substrate and the sixfold Be0 overlayer. Such patterns have been reported for other overlayer structures, e.g., ZnO on Zn.14 The extra reflections correspond to double scatter- ing (where one scattering event occurs in the A1 substrate and the other in the Be0 overlayer) or to diffraction from an AI-Be0 coincidence lattice. The magnitudes of the real two dimensional lattices of Al(111) and

96 M. AHMAD AND J. M. BLAKELY

BeO(0001) are respectively 2.86A and 2.698w. The reciprocal lattice spacings for the primary beams and for the superlattice beams for an unrelaxed Be0 layer on Al are shown in Fig. 4(d). For such a case the misfit ratio would be approximately 6%. Estimates of this ratio from the relative positions of superlattice beams in the LEED pattern varied from 7 to 10% as the Be0 growth (to the limited thickness) was repeated a few times. The deviation of the measured ratio from the predicted one appeared to be slightly outside the range of experimental error possibly indicating some Be0 compression or Al expansion in the interface region; further work would be required to confirm this observation. Due to the thermodynamic or kinetic limitations mentioned earlier it was not possible to grow Be0 sufficiently thick to make all the superlattice and substrate beam spots dis- appear.

SUMMARY

A preliminary study of surface segregation and oxidation in Al-Be alloys was camed out. No segregation of Be to the clean surface was observed. In the presence of oxygen or A1203 on the surface Be diffused to the surface and formed Be0 by reducing A1203. The reduction of A1203 can be understood from the difference in the free energies of formation of the bulk oxides of A1 and Be.

The amount of Be0 growth and the reduction of Al- oxide are strongly affected by the bulk Be content and the level of Be precipitation in the bulk. It has also been found that a small amount of Be0 on Al surfaces can affect the rate of oxidation of Al. A more careful study of the effect of Be0 on Al-oxidation should shed some light on the mechanism by which Be limits the oxidation of Al-alloys.

LEED investigation revealed that on the (1 11) surface of Al, Be0 formed as an epitaxial layer with the sixfold symmetry of its basal plane, (0001). The magnitude of the misfit between the substrate and the overlayer, as measured from the relative positions of the superlattice beams, was not in perfect agreement with the value for unrelaxed Al(111) and BeO(0001). Further work is needed to determine the exact nature of any possible relaxation of the substrate and the overlayer. It will be of interest to investigate whether the hexagonal face of Be0 forms on other Al surface orientations; such orien- ted layers may be responsible for the strong influence that Be has on Al oxidation.

Acknowledgment

Thanks are due to Dr Douglas Granger of Alcoa Laboratories for providing the alloy ingots and to Mr B. F. Addis of the Materials Preparations Laboratory at Cornell for growing the single crystal and preparing the samples.

REFERENCES

1. Aluminum. Properties and Physical Metallurgy, p. 224 ed. by

2. M. Whitaker and A. R. Heath, J. lnst. Metals, 82, 107 (1953). 3. D. J. Field, E. P. Butler and G. M. Scamans, Proc. 1st Int. Conf

4. F. Degreve, R. Figaret and P. Laty, Int. J. Mass. Spect. and Ion

5. 1. P. Batra and L. Kleinman, J. Elect. Spect. Re/. Phenom. 33,

6. D. E. Fowler and J. M. Blakely, Surf. Sci. 148, 265 (1984). 7. F. L. Williams and D. Nason, Surf. Sci. 45, 377 (1974). 8. 0. Kubaschewski and C. B. Alcock in Metallurgical Ther-

J. E. Hatch, ASM (1984).

on A/-U Alloys, p. 325 AIME, (1980).

Physics 29. 351 (1979).

175 (1 984).

chemistry, p. 204, Pergamon Press (1979).

9. F. F. Abraham and C. R. Brundle, J. Vac. Sci. Technol. 18, 506 (1 981 ).

10. D. Norman and D. P. Woodruff, Surf. Sci. 75, 179 (1978). 11. D. E. Fowler, J. Gyulai and C. Palmstrom. J. Vac. Sci. Technol.

Al, 1021 (1983). 12. JANAF Thermodynamic Tables, 2nd Ed., D. R. Stull and

H. Prophet, National Bureau of Standards, Washington, D.C. (1971).

13. R. P. Elliot and F. A. Shunk, Bull. of Alloy Phase Diagrams 2.

14. W. N. Unertl and J. M. Blakely, Surf. Sci. 69, 23 (1977). 145 (1980).