~ journal of magnetism

, ~ and magnetic , ~ malerials

ELSEVIER Journal of Magnetism and Magnetic Materials 133 (1994) 229-232

AES studies of antimony surface segregation in non-oriented silicon steel

M. Jenko a,*, F. Vodopivec a, B. Pra~ek b M. Godec a, D. Steiner a a Institute o f Metals and Technologies, Lepi pot 1l, 61000 Ljubljana, Slovenia

6 Institute for Electronics and Vacuum Techniques, Teslova 30, 61111 Ljubljana, Slovenia

Abstract

Antimony surface segregation in decarburized non-oriented silicon steel doped with 0.05 wt% Sb and 0.1 wt% Sb was investigated by a previously developed experimental method, based on Auger Electron Spectroscopy, its kinetics determined in the temperature range from 500 to 850°C. It was found that antimony segregation proceeded with perceivable velocity at 600°C and increased with increasing temperature. The thickness of the equilibrium antimony surface segregation layer reached after 30 minutes of annealing at 700°C was estimated to correspond to a monolayer. From the segregation kinetics and its temperature dependence in the temperature range from 650 to 750°C the diffusion coefficient and the activation energy for antimony diffusion in bulk were also estimated. The influence of antimony on the growth of recrystallized grains is discussed from obtained results.

1. Introduction

The segregation of antimony on the surface and interfaces of iron-based alloys is interesting from dif- ferent points of view and has been discussed in several papers [1,2]. A beneficial effect of a small amount of antimony 0.03-0.1 wt% Sb in silicon electrical steels on the recrystallization behaviour and on energy losses was also found [1,2]. It has been recognized that the small addition of Sb results in a substantial texture improvement in non-oriented silicon steel [1,2]. The possible explanation for this effect is that antimony, being a surface active element, segregates on the sur- face and grain boundaries and thus, affects the recrys- tallization behaviour of grains emerging to the surface of the sheet producing an increase of the number of ferrite grains with soft magnetic lattice space orienta- tion in the sheet plane. It is suggested that the nucle- ation of grains with (111) orientation occurs in the vicinity of the hot rolled band grain boundaries [1] and

* Corresponding author.

antimony might be responsible for retarding the nucle- ation rate of the ( l I D orientation [1,2].

The aim of the present investigation was to study the kinetics of surface antimony segregation in non-ori- ented silicon steel doped with 0.05% Sb and 0.1% Sb, using an AES surface analytical method and to under- stand its influence on the recrystallization.

2. Experimental

The composition of steels used for this investigation is given in Table 1. Both steels were produced in laboratory vacuum furnace from the same base materi- als and doped with 0.05 wt% Sb and 0.1 wt% Sb respectively. Ingots were hot rolled to a 2.5 mm thick strip, decarburized and cold rolled with intermediate recrystallization annealing to the final thickness of 0.1 m m .

For in situ investigation of surface segregation, a new sensitive experimental method, based on AES was developed and described in our earlier reports [3].

The AES analyses were performed in the addition- ally equipped Physical Electronics Scanning Auger Mi- croprobe, SAM 545A, using a static electron beam of

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230 M. Jenko et al. /Journal of Magnetism and Magnetic Materials 133 (1994) 229-232

Table ] Composition in wt% of the experimental steels

Code C Mn Si S AI Sb

A 0.005 0.18 1.85 0.001 0.19 0.05 B 0.004 0.20 1.94 0.001 0.11 0.1 a 0.004 0.22 2.12 0.001 0.19 -

a Comparative steel without antimony.

3 keV, 1 IxA, 45 l~m in diameter at an incidence angle of 30 ° .

The kinetics of the segregated antimony on the surface of non-oriented silicon was investigated by di- rect AlES measurements following a time dependence ratio (PHR, for peak-height ratio) of amplitudes be- tween the dominant Sb(MsN4,sN4, 5) and Fe(LM2,3V) Auger transitions at kinetics energies of 454 and 651 eV respectively.

3. Results and d i scuss ion

In Fig. 1 the kinetics of antimony surface segrega- tion measured with AES method at the constant tem- peratures of 650, 700, 750, 800 and 850°C, for both steels doped with antimony are shown.

The antimony bulk concentration in the investigated steels A and B were approximately 0.02 at% and 0.04 at% respectively and were lower than the sensitivity of ALES method. Only after heating at constant tempera- ture of 600°C and higher the antimony enrichment was detected by AES measurements. It can be thus con- cluded, that the surface segregation of antimony in the investigated steels proceeds with perceivable velocity at 600°C for steel A doped with 0.05 wt% Sb and at the

(23

E

- 2 4

-28 - -

-32

- 3 6 I 1 I 9 10 11 12

lo T K

Fig. 2. Arrhenuis plots for steels doped with 0.05 wt% Sb (A) and with 0.1 wt% Sb (B).

temperature of 700°C, the equilibrium antimony segre- gation with a concentration about 15 at% was found after 60 minutes of annealing. For this steel, at the temperature of 750°C a smaller segregation of anti- mony was found. On contrary, for steel B, doped with 0.1 wt% Sb the maximal amount of segregated anti- mony was reached at 700°C and only at 850°C a signifi- cantly lower segregation was found.

The saturated layer of segregated antimony of 0.3 nm for both steels was estimated with AES depth profile analysis. The calculated value for one mono- layer of antimony d = (M /pNA ) ]/3, where d is thick- ness of monolayer, M the molecular or atomic mass, p density and N A the Avogadro constant, is 0.3 nm [4]. It can thus be concluded that the thickness of antimony equilibrium segregation layer correspond to one mono- layer.

In the temperature range from 600 to 700°C no segregation of solute elements, beside that of antimony was detected. From segregation kinetics and its tem- perature dependence and from Arrhenius plots, for

0.4

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o - . i l I Z 0 10 20 30 40 50 60

Time , rain

(Q)

0.4

0 .3

0,2

° i o 0

_ / ! /

i : i t I _ _ L _

i l /75o °c

J l i

i

10 20 30 40

T i m e , mtn

(b)

50 60

Fig. 1. Kinetics of antimony surface segregation at temperatures of 650, 700, 750, 800 and 850°C in non-oriented silicon steels: (a) doped with 0.05 wt% Sb; (b) doped with 0.1 wt% Sb.

hi. Jenko et al. ~Journal of Magnetism and Magnetic Materials 133 (1994) 229-232 231

steels doped with 0.05 wt% Sb (A) and with 0.1 wt% Sb (B) shown in Fig. 2, the activation energy of anti- mony diffusion in the bulk was calculated for both steels.

Cranck's equation [5] cs= 2cb(Dt/~r) ~/2, with c s the surface concentration of the segregant, at anneal- ing time t, c b and D the solute bulk concentration and diffusion coefficient respectively, was applied to the measured kinetics.

The calculated value for activation energy of diffu- sion of antimony in steel doped with 0.05 wt% Sb is 284 kJ /mol and 274 kJ /mol for steel doped with 0.1 wt% Sb. The difference is small and both values are in

(a) ~z

4~

//" I ', rT - - ' , I l)m~

/

;7 u,~--Z ~ 30 3~ I

/_ 031 ~LI

reasonable agreement with the data given by Brugge- man [6] and Nishida [7].

After AES measurements, the microtexture on the specimens of both steels as well as on specimens of steel free of antimony was determined by the etch pit method described in Refs. [8,9]. The space grain orien- tation was determined from scanning electron micro- graphs. In Fig. 3 number of grains with the space orientation within a space angle + 5 ° is given in the standard stereographic triangle for both antimony doped steels and a comparative steel without antimony submitted an identical deformation-thermal history. The density of grains with soft magnetic space orienta-

(b)

_,-4 C, /_~26 T o I 3 loL

O01 OT1

(c) 1 ~t

.IoO o-011

/2 Y~-[ o ~ ~ o ~ 0 [0022_ J o, o ooi 0~1

(d) IT1

001 011

Fig. 3. Positions of ND projections of etch figures in standard triangle (8) (a); number of grains with space orientation within a space angle _+5 ° for non-oriented silicon steel doped with 0.05 wt% Sb (b); with 0.l wt% Sb (c); and for a comparative steel without antimony (d).

232 M. Jenko et al. / Journal of Magnetism and Magnetic Materials 133 (1994) 229-232

tion near the plane (001) is significantly higher in steel doped with 0.05 wt% antimony than in steel without and in steel with 0.I wt% of antimony.

The influence of antimony on the growth of recrys- tallized grains was studied on steel A and on a steel manufactured from the same base materials and with- out antimony [10,11]. The kinetics of recrystallized grain growth and the grain size were determined in the temperature range from 700 to 800°C. From obtained results and previously reported findings, for instance after recrystallization and significant growth the grains were coarser in the antimony steel [10,11], it can be concluded that antimony segregation hindered the for- mation of recrystallized nuclei in the temperature range from 500 to 700°C. The coarser recrystallized grains can be explained as a product of longer growth of nuclei in a deformed matrix [10].

4. Conclusions

Kinetics of surface antimony segregation for both non-oriented silicon steels doped with Sb was mea- sured at constant temperatures of 650, 700, 750, 800 and 850°C using an earlier developed experimental method based on AES [3].

Antimony segregation on the surface of non-ori- ented silicon steel doped with 0.05 wt% Sb increased with the increasing temperature and reached the satu- ration layer corresponding to an approximate thickness of 1 monolayer after 30 min of annealing at 700°C. At a temperature above 750°C a lower amount of the segregated antimony was measured. For the steel doped with 0.1 wt% Sb the diminution of segregated layer was measured only at 850°C.

The diffusion coefficient and the activation energy of antimony diffusion in bulk of 284 kJ /mol and 274 kJ /mol were calculated from surface segregation ki- netics and its temperature dependence for both steels. Both values are in good agreement with data of Bruggeman and Nishida [6,7].

It was found that the density of grains with soft magnetic orientation near the plane (001) is signifi- cantly higher in steel doped with 0.05 wt% Sb than in the other two investigated steels.

References

[1] G. Lyudkovski and P.K. Rastogi, Metall. Trans. A 15 (1984) 257.

[2] F. Vodopivec, F. Marin~ek, D. Gnidovec, B. Pra~:ek and M. Jenko, J. Magn. Magn. Mater. 97 (1991) 281.

[3] M. Jenko, F. Vodopivec and B. Prafiek, Appl. Surf. Sci. 70/71 (1993) 118.

[4] M. Wutz, H. Adam and W. Walcher, Theory and Prac- tice of Vacuum Technology (Vieweg, Braunschweig/ Wiesbaden, 1989).

[5] J. Cranck, The Mathematics of Diffusion (Clarendon, Oxford 1967).

[6] G. Bruggeman and J. Roberts, J. Met. 20 (1968) 54. [7] K. Nishida H. Murohashi and T. Yamamoto, Trans. Jpn.

Inst. Met. 20 (1979) 269. [8] M. Godec, M. Jenko, M. Lovre~i~, F. Vodopivec and L.

Kosec, Kovine, Zlitine, Tehnologije 28 (1994) in press. [9] A. Bottcher, T. Gerber and K. Lucke, Mater. Sci. and

Technol. 8 (1992) 16. [10] F. Vodopivec, M. Jenko, A. Rodin: and B. Breskvar, J.

Magn. Magn. Mater.(1993) (in print). [11] D.Steiner, M. Jenko, F. Vodopivec, L. Kosec and M.

Godec, Kovine, Zlitine, Tehnologije 28 (1994) in press.