SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 30, 350–353 (2000)

Effects of selenium surface segregation on thetexture of a selenium-doped FeSi alloy

M. Jenko,1* J. Fine2 and D. Mandrino1

1 Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia2 National Institute of Standards and Technology, Gaithersburg, MD, USA

High-resolution Auger electron spectroscopy has been used to study selenium surface segregation on an Se-doped FeSi alloy. The surface segregation of Se and impurities such as C, P and S was measuredin situin the analysis chamber of an Auger spectrometer under ultrahigh vacuum conditions in the temperaturerange 200–850°C. At elevated temperatures (T ≥ 850°C), co-segregation of Se and S was observed to occur bymeans of x-ray photoelectron spectroscopy. It was also found that the surface segregation of selenium criticallyaffected reconstruction of the (110) surface microstructure of the FeSi alloy, resulting in the formation of(100) planes at 850°C. Copyright 2000 John Wiley & Sons, Ltd.

KEYWORDS: Fe–Si alloy; selenium; segregation; co-segregation; SEM; HRAES; XPS

INTRODUCTION

The segregation of surface-active elements such as anti-mony and tin on the surface and interfaces of iron andiron-based alloys has been discussed in several papersand is known to affect the material properties of suchalloys (surface energy, surface diffusion, adhesion, corro-sion, catalytic reactions, recrystallization).1 – 3 In iron andiron-based alloys, impurities such as Ni, Cu, P, S, Pb,As, Sb, Sn, Se, etc. are generally deleterious.5 However,positive effects of doping with Sb, Sn and Se have beenobserved and studied at the Institute of Metals and Tech-nology, Ljubljana, and at Max-Planck-Institute for IronResearch (MPIE), Dusseldorf.2 – 6

Impurities Sb, Sn and Se have a strong tendency tosurface segregation on iron and FeSi alloys; such seg-regation can affect the texture of these alloys and canimprove the electrical properties of silicon steels.9,10 Inprevious studies, we found that the addition of surface-active elements, such as Sb and Sn, into silicon steel hada beneficial effect on the texture and electrical propertiesof these steels.4 – 6,9,10 Selenium also is a surface-activeelement but there are virtually no data available as to itssegregation behaviour.10

EXPERIMENTAL

The high-resolution Auger electron spectroscopy (HRAES)instrument used in this study was a VG-Scientific Micro-lab 310-F spectrometer, equipped with a spherical-sectoranalyzer and a field-emission electron gun that could pro-duce a beam 10 nm in diameter at a current of 1 nA.

* Correspondence to: M. Jenko, Institute of Metals and Technology,Lepi pot 11, 1000 Ljubljana, PO Box 431, Slovenia.E-mail: monika.jenko@imt.si

Contract/grant sponsor: Ministry of Science and Technology, Slove-nia; Contract/grant number: J2-0718.

This small beam size makes it possible to do scanningauger microscopy (SAM) with very high lateral resolution,as well scanning electron microscopy (SEM). For surfacechemical analysis, the instrument was equipped with anx-ray photoelectron spectrometer. In order to study freshlyprepared fractured surfaces of metallic samples and tostudy themin statu nascendi, this instrument was equippedwith an ultrahigh vacuum (UHV) fracture stage. Forin situsurface segregation studies, the analysis chamber of theinstrument was equipped with a heating device capable ofannealing a sample to 1000°C.

The samples used to investigate Se surface segregationwere prepared from a laboratory-cast FeSi alloy (2.0% Si,0.95% Al, 0.002% C and 0.05% Se) with minimal impu-rities (<0.001% S and<0.0001% P). The ingots werehot-rolled into a 2.5 mm thick strip, descaled and thencold-rolled with intermediate recrystallization annealingto a final thickness of 0.15 mm. Microsections of the sam-ples were obtained by standard metallographic proceduresand introduced immediately into the preparation chamberof the instrument. The surface of the specimen was thensubjected to a cleaning procedure that included a num-ber of cycles of argon ion sputtering to remove adsorbedimpurities; sample cleanliness was assessed by HRAES.In the analysis chamber, the sample was mounted on asample holder that could be heated to 1000°C. Temper-ature was monitored by a thermocouple and a precisioninfrared pyrometer. At the beginning of each experiment,the sample was heated to the selected measurement tem-perature (500–850°C) and impurities were removed byargon ion sputtering before the HRAES measurementswere made. Regions of analysis were determined by SEMand then analyzed with HRAES or SAM. Chemical anal-ysis of surface-segregated ultrathin films was performedby XPS with Mg K 1456 eV radiation.

Cylindrical fracture samples used for the study of grainboundary segregation were prepared from a strip thatwas both hot- and cold-rolled, and then machined to afinal diameter of 3 mm. The samples were evacuated to¾10�6 mbar and encapsulated in quartz tubes, annealed at

Copyright 2000 John Wiley & Sons, Ltd. Received 15 July 1999Revised 1 November 1999; Accepted 1 November 1999

Se SURFACE SEGREGATION IN Se-DOPED FeSi ALLOY 351

1050°C, quenched in water and aged at 550°C for 200 and500 h before being introduced into the UHV system of theinstrument. After cooling to approximately�120°C, thesamples were impact fracturedin situ at a base vacuumof 5ð 10�10 mbar, and HRAES analyses were performedon as many intergranular fracture sites as possible.

Surface texture also was observed using an improvedpit etching technique.11

RESULTS AND DISCUSSION

We have already indicated that surface-active elementscan affect the surface reconstruction of FeSi alloys.7 – 10

Figure 1 shows a typical Auger spectrum of a polycrys-talline FeSi sample at maximum Se surface concentrationafter segregation had reached equilibrium at 800°C. Herewe observed both selenium and sulphur segregation. Afterseveral cycles of heating from 500 to 850°C, the value ofthe selenium (as well as the sulphur) surface coveragebecame constant, indicating that equilibrium segregationhad been achieved. The AES depth profile shown in Fig. 2

Figure 1. Auger spectrum of an FeSi alloy at equilibrium Se Ssurface segregation at T ½ 800 °C.

Figure 2. The AES depth profile of the Se S segregatednear-surface composition of an FeSi alloy that was annealedat T ½ 800 °C. This depth profile was obtained at 20 °C.

Figure 3. X-ray photoelectron spectra, excited by Mg K˛ radia-tion, of an FeSi alloy. The Se 3s and S 2s photoelectron lines weremeasured at the equilibrium of surface segregation (T ½ 800 °C).The measured binding energies for both lines (Se at 230.6 eV andS at 227.9 eV) are characteristic of those of the pure elements.

(takenat 20°C) showsthat the equilibrium seleniumsur-faceconcentrationwas¾10 at.%,whereasthatof sulphurwas ¾20 at.%, and that there was very little seleniumat ¾1 nm below the surface.It was also clear that theobservedphenomenonwasnot a competitionbetweenSeand S for the free siteson the surface,becausethe Se/Sratio wasconstantfor T ½ 800°C. Onepossibleexplana-tion could involve either the co-segregationof Se and Sor the formation of surfacecompounds.The segregatedsamplealso was analyzedby XPS; core-level electronintensitiesweremeasuredasa functionof bindingenergyrelativeto theFermilevel.Figure3 showstheSe3sandS2sphotoelectronlinesobtainedat theequilibriumconcen-trationstate(at 800°C). Themeasuredbindingenergiesofboth Se(230.6eV) and S(227.9eV) are characteristicofthoseof thepureelements,indicatingthatno seleniumorsulphursurfacecompoundshadbeenformed.

The XPS lines of Fe areshownin Fig. 4 for the samesegregatedsample.Again, the measuredbinding energiesof both the Fe 2p1/2 and Fe 2p3/2 lines at 707.0 eV and720.1 eV, respectively,are thoseof pure Fe, providingfurther evidencethat no surfacecompoundscontainingFe,Seor S hadbeenformed.From all of theseXPS datait seemsclear that surfacecompoundformationdoesnot

Figure 4. The XPS lines Fe 2p1/2 and Fe 2p3/2 for the same sampleas in Fig. 3. These line energies are characteristic of pure iron.

Copyright 2000JohnWiley & Sons,Ltd. Surf. InterfaceAnal. 30, 350–353 (2000)

352 M. JENKOET AL.

occur, and that the process taking place is indeed that ofthe co-segregation of Se and S.

Grain growth and micromorphology of the sample sur-face were monitored by SEM. After several cycles ofheating from 500 to 850°C, the microstructure changeddrastically and on some grains facetting occurred. It isknown that facetting occurs if new planes are energeticallyfavoured that minimize the total surface energy. Facettingalso is known to occur as a result of surface adsorption,even on single crystals. Grant and Hass have observedfacetting of the (110) surface of Cr and its reconstructionto (100) upon oxygen and carbon monoxide absorption;8

Russeberg and Viefhaus9 also found that facetting of (110)surfaces occurred during Sb segregation.

During the selenium–sulphur co-segregation, the (110)grains at the surface of the sample were observed to facet,as shown in Fig. 5 for the P1 grain. Figure 6(a) shows aSEM image of the first stage of surface reconstructionand confirms our concept that the process of surfacegrain reconstruction from (110) to (100) occurs duringSe–S segregation, as can be seen by the square facettingin the region of P1 and P2. The AES spectra taken atpositions P1–P4 are shown in Fig. 6(b); they stronglysuggest that surface reconstruction is promoted by thepresence of a fractional near-surface layer of segregatedSe and S. We plan to investigate this type of surfacereconstruction process on single-crystal surfaces in thenear future.

CONCLUSION

Selenium surface segregation on polycrystalline surfacesof an FeSi alloy (with¾0.05 mass.% Se) was studied byHRAES, SEM and XPS. In the temperature range inves-tigated (500–850°C), we have observed the segregationof C at 500°C, P at 600°C and co-segregation of Se andS atT ½ 800°C. For T ½ 800°C, the equilibrium con-centrations of Se and S at the co-segregated surface werefound to be constant at levels of about 10 and 20 at.%,respectively.

On the (110) faceted grains of this FeSi alloy, evidencewas found for surface reconstruction promoted by the

Figure 5. The SEM image of the surface microstructure of thesample annealed several times to 850 °C, which shows thefacetting of (110) grains during Se S co-segregation.

Figure 6. (a) The SEM image of the first stage of surfacereconstruction of (110) grains to (100) planes that occurredduring Se S co-segregation. (b) The AES spectra measuredat positions P1 P4; it is evident that the surface consists of afractional near-surface layer of Se and S that can induce grainreconstruction.

segregation of Se and S. After several hours of annealingat 850°C, we have observed reconstruction of (110)-oriented grains into (100) planes. In conjunction withprevious work,10,11 we have shown that the decreasedsurface energy of the (100) grains due to selective surfacesegregation of Sn and Sb can induced grain growth andthat this cubic growth can be increased by the additionof small concentrations of such surface-active elements.We conclude that small concentrations of surface-activeelements (e.g. Sb, Sn and Se) can effectively alter thesurface texture of silicon steels, leading to lower corelosses and to a consequent improvement in their electricalproperties.

Acknowledgements

Financial support of this work (project J2-0718) was provided by theMinistry of Science and Technology, Republic of Slovenia, and isgratefully acknowledged. This work was also part of a joint USA/NIST-97 Slovenia project.

Surf. Interface Anal. 30, 350 353 (2000) Copyright 2000 John Wiley & Sons, Ltd.

Se SURFACE SEGREGATION IN Se-DOPED FeSi ALLOY 353

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Copyright 2000 John Wiley & Sons, Ltd. Surf. Interface Anal. 30, 350–353 (2000)