applied surface science

Applied Surface Science 70/71 (1993) 118-122

North-Holland

Surface segregation of antimony in Fe-Si steel for grain oriented sheets

M. Jenko, F. Vodopivec

Institute of Metals and Technologies, Lepi pot II, Ljubljana, Slovenia

and

B. PraEek

Institute for Electronics and Vacuum Techniques, Teslova 30, Ljubljana, Slovenia

Received 1 September 1992; accepted for publication 18 November 1992

Surface segregation of antimony in a polycrystalline Fe-Si alloy with 0.049 wt% Sb was investigated by Auger electron

spectroscopy in the temperature range from 4.50 to 800°C and the segregation kinetics of antimony were described. From the

surface segregation kinetics and its temperature dependence the bulk diffusion coefficient of antimony and the activation energy

were determined in the temperature range from 500 to 600°C.

1. Introduction

The segregation of antimony on the surface and interfaces of iron-base alloys is interesting from different points of view and has been dis- cussed in several papers [l-13]. It is well known that several elements act as severely embrittling impurities in steel, among them antimony, and that they strongly segregate to grain boundaries of body centered cubic iron-base alloys.

A beneficial effect of a small amount of anti- mony 0.03-0.1 wt% Sb in electrical silicon steels on the recrystallization behaviour and on energy losses was also found [l-lo]. It has been recog- nized that the small addition of Sb results in substantial texture improvement in non-oriented and oriented silicon steels. The possible explana- tion of this effect is that antimony being a surface active element, segregates on the surface and grain boundaries and affects the recrystallization behaviour producing an increase of the number

of ferrite grains with soft magnetic lattice space orientation in the sheet plane. It is suggested that the nucleation of grains with (111) orientation occurs in the vicinity of the original hot band grain boundary [13] and antimony might be re- sponsible for retarding the nucleation rate of the (111) orientation.

The kinetics is mainly determined by the bulk concentration and the diffusion rate of the impu- rity. Grabke’s group investigated binary systems like Fe-C, Fe-Si, Fe-Al, Fe-P, Fe-S, Fe-Sn, Fe-Sb [12-221 in order to estimate the equilibria of segregation and its interaction with various elements.

Commercial steels are very complex systems and in order to estimate the segregation in tech- nical steels, the steel concerned has to be investi- gated. The aim of this work was to determine the kinetics of surface antimony segregation in Fe-Si alloys doped with antimony using an AES surface analytical method 1271. Such Fe-Si alloys with

0169-4332/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved

M. Jenko et al. / Surface segregation of Sb in Fe-Si steel for grain oriented sheets 119

approximately 3% Si are widely used as oriented silicon electrical sheets, where grain orientation improves the magnetic properties [l-lo].

2. Experimental

A laboratory cast steel of composition Fe, 2.85% Si, 0.15% Mn, 0.002% C, 0.049% Sb with a minimal content of uncontrolled elements and impurities was prepared. Ingots were hot rolled to a 2.5 mm thick strip, de-scaled and then cold rolled with intermediate recrystallization anneal- ing to a final thickness of 0.1 mm.

For “in situ” investigations of surface segrega- tion, a new sensitive experimental method based on Auger electron spectroscopy was developed. The specimen of dimensions 3 X 30 X 0.1 mm was mounted in an ultrahigh vacuum system operat- ing at a base pressure of 1 x lo-’ Pa. The speci- men was resistively heated up to 850°C. A 0.1 mm Fe-CuNi thermocouple was spot-welded to the rear side of the specimen to control the tempera- ture. A constant temperature was attained 3-5 min after establishing a fixed current. At the beginning of each experiment, a constant temper- ature was established and impurities were re- moved by Ar+ sputtering just before the AES measurements. By alternative argon ion sputter- ing and annealing almost all impurities were re- moved.

The AES analyses were performed in an addi- tionally equipped Physical Electronics scanning Auger microprobe, SAM 545A, using a static electron beam of 3 keV/l PA, 45 pm in diame- ter at an incidence angle of 30”.

Argon ion sputtering was performed with two ion guns at an Ar pressure of 5 X 10M3 Pa, at energies of 1 keV and 3 keV, and ion current of l-10 mA. The sputtering rate was estimated by sputtering a Sb thin film with known thickness.

3. Results and discussion

The kinetics of the segregated antimony on the surface of an electrical sheet was investigated by direct AES measurements following the time de-

(min)

Fig. 1. The kinetics of antimony surface segregation in Fe-Si steel for a grain oriented sheet at temperatures of 500, 600,

700,750 and 800°C.

pendence ratio (PHR, for peak-height-ratio) of peak-to-peak amplitudes between the dominant Sb(M,N, gN4 J and Fe(LM,,V) Auger transi- tions at kinetic energies of 454 and 651 eV [261, respectively. The kinetics of the Sb surface segre- gation at the constant temperatures 500,600,700, 750 and 800°C is shown in fig. 1. The antimony bulk concentration in the investigated steel was approximately 0.02 at% and was lower than sensi- tivity of the AES method. Only after heating at a constant temperature of 500°C and higher the antimony enrichment was detected by AES mea- surements. We can conclude, that the surface segregation of antimony in the investigated steel proceeds with perceivable velocity at 500°C. At a temperature of 600°C the maximal amount of segregated antimony with a concentration of ap- proximately 17 at% was found after 20 min of annealing. At a temperature of 700°C a decrease of the amount of segregated antimony was found and with increasing temperature the antimony amount decreased.

Fig. 2 shows Auger electron spectra of Fe-Si steel with Auger peaks of Fe (598, 651, and 703 eV), Sb (454 eV), C (272 eV1, 0 (510 eV1, P (120 eV), S (152 eV) and N (379 eV) in a segregated layer at the sheet surface after 20 min of anneal- ing at 400, 600, 700, 750 and 800°C respectively.

In fig. 3 the thickness of segregated antimony layer after annealing for 30 min at 6OO”C, was estimated to be 0.3 nm by AES depth profile analysis. The calculated value for one monolayer of antimony is the same [lo]. It can thus be

120 M. Jenko et al. / Surface segregation of Sb in Fe-52 steel for grain oriented sheets

dN(E

dE

6OO'C

700°C

aoocc

85ooc

Sputter time (set)

Fig. 3. AES depth profile of the saturated Sb segregation

layer after 30 min of annealing at 600°C.

tion would have been possible. Carbon segregates in the temperature range up to 500°C nitrogen desorption from the surface becomes important at T > 500'72, while segregation of sulfur and phosphor was detected at temperatures T 2 700°C.

From the surface segregation kinetics and its temperature dependence in the temperature range from 500 to 600°C the diffusion coefficient and from Arrhenius plots, shown in fig. 4, the activation energy of antimony diffusion in the bulk was calculated. Cranck’s equation [18]

c, = 2c,( D+r)“*,

with c, as the surface concentration of the segre- gant, at annealing time t, cb and D solute bulk concentration and diffusion coefficient respec-

tively, was applied to the measured kinetics. The calculated value for the activation energy

of 260 kJ/mol using the data for D, = 2 X lop4

Electron energy (eV)

Fig. 2. Auger electron spectra of Fe-Si steel with Auger

peaks of Fe (598, 651, and 703 eV), Sb (454 eV), C (272 eV),

0 (510 eV), P (120 eV), S (152 eV) and N (379 eV) in a

segregated layer at the sheet surface after 20 min of annealing

at 400, 600, 700,750 and 8Oo”C, respectively.

concluded that the Sb saturation segregation layer is 1 ML thick.

In the temperature range from 500 to 700°C only antimony segregation was found, and no segregation of other elements, otherwise no measurements of equilibrium antimony segrega-

$.103 K

Fig. 4. Determination of activation energy and coefficient of diffusion of Sb in o-iron from surface kinetics on Fe-Si steel

for an oriented electrical sheet.

M. Jenko et al. / Surface segregation of Sb in Fe-Si steel for grain oriented sheets 121

cm2/s is in a good agreement with the value of 262 kJ/mol for the activation energy evaluated for the trace diffusion of Sb in iron given by Nishida et al. [32]. This value is also in reasonable agreement with the value of 270 kJ/mol reported by Bruggeman et al. [30]. The frequency term D, was calculated from the equation InD, = 9.3 X

10P5Q - 25.9, given by Nishida et al. [32]. The very similar values for the activation energy of antimony bulk diffusion confirm the hypothesis suggested by Riisenberg and Viefhaus [13] that the bulk diffusion is the rate-determining process in segregation. A possible reason for the lack of a uniform antimony distribution in the bulk sub- strate might also be due to surface evaporation of antimony, because of the very high vapour pres- sure.

4. Conclusions

The kinetics of surface antimony in a Fe-Si alloy in the temperature range from 450 to 800°C was measured using a new developed experimen- tal method based on Auger electron spec- troscopy.

The amount of segregated antimony on the surface of transformer steel increased in the tem- perature range from 500 to 650°C. The saturated layer thickness of 0.3 nm was estimated with AES depth profile analysis, which corresponds to the calculated value of one Sb monolayer. At temper- atures T 2 700°C a diminution of the amount of segregated antimony was measured, probably provoked by the surface evaporation process.

In the temperature range from 500 to 700°C only Sb segregation was measured, carbon segre- gated in the temperature range up to 500°C nitrogen desorption from the surface became im- portant at T.2 500°C while a segregation of sulfur and phosphor was detected at temperatures T a 700°C.

From the surface segregation kinetics and its temperature dependence in the temperature range from 500 to 600°C the diffusion coefficient and activation energy of Sb diffusion in bulk of 260 kJ mol- ’ were determined in good agree- ment with data of Nishida et al. [32].

On the basis of the obtained results we assume that Sb segregation could decrease the surface energy of grains emerging to the surface and through it also the kinetics of grain growth.

The investigation of recrystallized grain growth in silicon electrical sheet shows that the antimony grain boundary segregation affects the retarda- tion of formation of recrystallized nuclei in the temperature range from 500 to 700°C [331 and we suppose that the surface segregation affects the recrystallization by a decrease of surface energy of the low index orientation grains, their growth of coarse grains to the determined texture.

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