Scripta METALLURGICA Vol. 26, pp. 1291-1296, 1992 Pergamon Press Ltd. et MATERIALIA Printed in the U.S.A. All rights reserved

NON-EQUILIBRIDM SURFACE SEGREGATION OF SULFUR DURING RECRYSTALLIZATION OF NICKEL

R. LE GALL', G. SAINDRENAN", D. ROPTIN 't',

' I.U.T, Laboratoire de thermique des mat~riaux, I0, Rue Jean Zay, F56100 LORIENT.

I.S.I.T.E.M, D~partement mat~riaux, *' La Chantrerie, C.P 3023, F44087 NANTES C~dex 03.

'" Ecole Centrale de Nantes, Laboratoire mat~riaux, l, Rue de la No~, F44072 NANTES Cedex 03.

(Received January 30, 1992) (Revised February 20, 1992)

Introduction

Impurity segregation can occur at surface or internal interfaces (e.g grain boundaries) of a metal. Saindrenan [i] showed that superficial sulfur segregation increases during the recrystallization of cold worked nickel: the apparent diffusion constants are then many orders of magnitude larger than the heterodiffusion constant of sulfur in nickel [2]. Consequently, we can observe in a few minutes the effects of segregation in nickel with a sulfur content as low as 1 atomic ppm. In a previous paper [3] we showed that surface segregation is directly associated with the recrystallization process.

One can wonder if the deformation ratio is a significant parameter of non- equilibrium segregation. In this paper we describe the influence of the amount of cold work on the surface segregation kinetics, in view of determining the mechanism of such a segregation.

M~terial. Exnerimental Techniaues.

The tests were carried out on polycrystalline nickel (270 nickel of Wiggins alloys). Several chemical analyses determined that the nickel content is higher than 99.98%, and that the sulfur content is about 0.5 weight ppm. The complete chemical analyses of this metal was previously published [4].

Prior to the initial deformation the samples were annealed at 1123 K during 5 h in sealed quartz capsules under a residual atmosphere of pure argon. The samples were then cold worked at room temperature. The surface segregation was studied by Auger Electron Spectroscopy (A.E.S) in a C.M.A RIBER ISA model ASC 2000 analyzer. The sample (5 x 1 x 0.5 mm) is spot welded to a strip of resistively heated tantalum inside the A.E.S vacuum system. The temperature is measured with a thin wire (50 ~m) thermocouple spot welded to the sample. Such a device allows the temperature to reach 728 K within 20 seconds. The residual pressure is maintained below 5.10-" Pa. The main characteristics of the analysis are : 3 keV primary energy, 0.i ~A primary beam current, 1 ~m beam diameter. We recorded in the differential mode dN(E)/d(E). The sulfur peak (152 eV) was normalized with respect to the nickel peak (848 eV). Whereas the atomic density of a polycristalline surface of nickel is 10.54 I0 *" at.m -2 [5], the superficial concentration of sulfur is:

Cs = ~i 10.54 1018 at.m -2 (I)

1291 0036-9748/92 $5.00 + .00

Copyright (c) 1992 Pergamon Press L t d .

1292 RECRYSTALLIZATION OF Ni Vol. 26, No. 8

in this expression C, is the sulfur concentration of sulfur (in at.m-2), H, and H,~ are the peak to peak heights of elements S (152 eV)and Ni (848 eV) and u=0.48 is a constant of the Auger analyzer.

$~rfac~ Segregation

A previous study showed that the recrystallization occurs above a strain ratio of 0.2 at a temperature of 728 K (threshold strain for recrystallization). At this temperature the segregation then occurs in a recrystallized material or during the recrystallization process. We checked the progress of the recrystallization using the secondary electron scanning image of the specimen which is formed in the Auger apparatus. The kinetics of sulfur segregation on specimens cold-rolled between 0.2 and 1.2 are recorded during annealing at 728 K; two representative kinetics are reported in figure I. The curves present a linear part with respect to Vt, from which we deduce an apparent diffusion constant (D*) for each curve according to the simplified McLean law [6]:

c , -

I n t h i s e q u a t i o n C, i s t h e s u p e r f i c i a l c o n c e n t r a t i o n o f s u l f u r ( i n a t .m-=] , Co i s t he b u l k c o n c e n t r a t i o n i n e q u i l i b r i u m w i t h t h e s u p e r f i c i a l c o n c e n t r a t i o n ( i n a t .m -3) and D* t h e a p p a r e n t h e t e r o d i f f u s i o n c o n s t a n t . These v a l u e s and t he heterodiffusion constant of sulfur in nickel extrapolated from an equation proposed by Wang and Grabke [7] are reported in table 1 for each deformation ratio.

TABLE 1 Apparent Diffusion Constant of Sulfur in Nickel During Recrystallization.

0.2 0.25

D*(m=.s -z) 4.96 10 -4 3.6 10 -4

D (m=.s -I )

D*/D

3.1 I0 -~

16 106

0.3 0.5 0.8 1.2

2.71 10 -4 1.80 10 -4 2.94 10 -4 2.41 10 -4

3.1 i0 -z~ 3.1 i0 -~

12 10 6 8 10 6 3.1 10 -11

6 10 6

3.1 i0 -z~

9 106

3.1 I0 -zz

8 i0"

One can see that sulfur diffusion is increased about i0" times with respect to the volume heterodiffusion constant of sulfur in nickel.

Hs/Hni848 D* (cm 2 . s !10 8)

I ,_, ,sKI

I

0s

0.6 ~0/

04

02

, d l

FIG. 1. Kinetics of superficial sulfur segregation during recrystallization of cold-worked nickel.

0.2 0.4 0.6

3

2 I I I i I

°o 20 40 eo so 0.0 0.8 1,0 ~,2 Vt (Vs) e

FIG. 2. Apparent diffusion constant of sulfur in nickel versus deformation strain.

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In figure 2 the evolution of D" versus the initial deformation is reported. The curve decreases rapidly then tends toward a constant value. This evolution can be explained using the impurity drag theory proposed by Cahn [8] and further developed by Lucke and St~we [9]. When a grain boundary moves in a material this theory predicts two cases: - the moving grain boundary can drag an atmosphere of impurity atoms under certain conditions of temperature, driving force and impurity concentration, - in the other case the grain boundary breaks away from this atmosphere and moves with a higher velocity.

Because of the high attractive interaction between the nickel interfaces and sulfur, we can apply qualitatively this model to our results. If we attribute the decrease of D* with the deformation ratio to a breakaway of the sulfur atmosphere, one can see that the transition ratio is situated between 0.2 and 0.25. So we must now consider two parts in the curve :

- between 0.2 (which is the lower strain which provokes recrystallization at 728 K) and the transition strain, the sulfur atmosphere is dragged by the moving grain boundaries until the recrystallization is finished: at the end of recrystallization a superconcentration of sulfur must exist in the vicinity of grain boundaries and the surface. Because of this high sulfur content in equilibrium with the interfaces, the diffusion flux will be higher than the nominal one. - if the ratio is higher than 0.25, the driving force for grain boundary migration (resulting from the dislocation density value) is too high and the sulfur atmosphere breaks away from the boundary. At the end of recrystallization, the distance between sulfur atoms and the surface is higher, and the segregation will be slower than in the first case.

The presence of sulfur atmosphere is difficult to probe because of the values of local concentrations. Nevertheless, it is possible to verify experimentally the existence of the sulfur superconcentration in the vicinity of the surface of the metal after recrystallization : after a first annealing of recrystallisation, the internal and external interfaces of the metal are stabilized and the defects superconcentration is eliminated. After removing the segregation layer on the surface by ion sputtering, we then let the annealing proceed. Figure 3 presents three successive kinetics obtained in this way.

H s 'Hni 848 1

0.8 . . . . . . . . . . . . . . y.-.-:.-.:.'. . . . . . . . . .

0 . 6 , [ .......... I T = 7 2 8 K / " . ...... No1 _ _

0 . 4 , , " ....""" N'2 . . . . . . .

0 t-" I 1 I i

0 20 40 60 80 100 Vt (Vs)

FIG. 3. Successive superficial segregration kinetics of sulfur in nickel after a deformation ratio of 0.2.

We noticed that the second and the third kinetics were slower than the first one, but these kinetics are still several orders of magnitude larger than equilibrium ones in an annealed structure. Assuming that the kinetics are represented by a McLean-like law, the slope of the linear part is given by :

1294 RECRYSTALLIZATION OF Ni Vol. 26, No. 8

c~ _ 2 %~ (3)

Thus the fast segregation can be explained either by an increase of the concentration near the surface, Co, or by an increase of the diffusion constant, D. Let us now examine the two hypotheses which are schematically represented in fig 4a and fig 4b.

Fig. 4a

...... . • 00 00 0. o oli

Fig. 4b

"'.'':%:" "'."~'&":%""" "~':'." "'I

: . "

Fig. 4c

Fig 4. Schematic representation of sulfur repartition after recrystallization.

- If we assume that the diffusion constant is equal to the heterodiffusion constant extrapolated from [7], the fast kinetics must result from a sulfur atmosphere in the vicinity of the surface (fig 4.a). In addition the form of the curve changes between the first and the second annealing : the straight lines passing through the origin for the second and the third kinetics reveal a supersaturation of sulfur (or sulfur sources) in the vicinity of the surface. Indeed this physical situation is necessary to maintain at a constant value the concentration in equilibrium with the surface, a necessary condition to observe linear kinetics versus ~t for surface segregation [I0]. We can then calculate the sulfur concentration needed to obtain the slope of the experimental curve. These values obtained at 728 K for a deformation ratio of 0.2 are reported in table 2.

Table 2. Volume Concentrations of Sulfur Needed to Explain The Kinetics During Successive Annealings (E=0.2, T=728 K).

third kinetics ~=0.2 - T=728 K first kinetics 2nd kinetics

Slope 4.55 lO -2 1.89 10 -2 1.16 10 -2 i

Co needed(at ppm) 4000 1600 i000

Such concentrations of sulfur cannot be dissolved in solid nickel whatever the temperature. The concentration of dissolved sulfur in nickel at 728 K is less than i0 atomic ppm [Ii]. So sulfur must exist as Ni,S2 precipitates or as an amorphous NiS layer. Studies of multilayer materials have pointed out amorphization of solids due to sharp concentration gradients (see for instance [12]). In our case, due to the impurity drag effect, sulfur supersaturation must exist after the end of the recrystallization process although we have no

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direct evidence for it.

- Because the recrystallization is ended after ~t=50, e.g. before the end of the first kinetics, for the following kinetics the increase of the diffusion constant cannot beexplainedbysuperconcentration of lattice defects resulting in an enhancement of the heterodiffusion constant. We must then consider the diffusion along grain boundaries. During recrystallization the sulfur segregates at grain boundaries as well as on the free surface (fig 4b). This intergranular sulfur can diffuse to the surface because surface segregation free enthalpy is higher than intergranular segregation enthalpy (absolute value). This diffusion is fast because of the high value of the intergranular diffusion constant of sulfur in nickel. Pierantoni [13] found at 973 K :

sSD~ = 1.2 10 -17 m3.s -I (4)

where s is the segregation factor, 6 the grain boundary width and D~ the diffusion constant along a grain boundary. So if we make the hypothesis that the ratio of heterodiffusion constants between 973 K and 728 K is equal to the ratio of grain boundary diffusion constants, it is possible to extrapolate :

SSDb = 4 10 -21 m3.s -I at 728 K (5)

With s=21000 (corresponding to a coverage of 0.9 monolayer) and 6=5 i0 -~° m, the diffusion constant D~=3.8 10 -~4 m2.s -2. So the mean depth of grain boundary affected by diffusion during 1600 s (the required time to obtain the surface saturation) is about 8 ~m. The sulfur content of the grain boundary area corresponding to this depth is about 20 % of the quantity needed to saturate the surface (with a mean grain diameter of 80 ~m and saturation of grain boundaries by sulfur). This second mechanism therefore cannot explain the kinetics eventhough its existence has been established [14].

The situation must be more complex, with both these mechanisms contributing simultaneously to the superficial segregation (fig 4c). Assuming a homogeneous nucleation of the recrystallized phase in the cold worked bulk, the surface is made of moving grain boundaries having emerged on the free surface, while grain boundaries consist of the meeting of two moving grain boundaries. From this point of view, there is no difference between the free surface and the grain boundaries, especially concerning the impurity drag. In other words, impurity atmospheres exist near grain boundaries as well as near the free surface. In these conditions the value of the superconcentration of table 2 can be reduced because of the contribution of intergranular diffusion and because sulfur superconcentration, which probably exists near the grain boundaries, has the same effect as on the surface.

Conclusion

Our results show clearly that the kinetics of superficial segregation of sulfur in nickel is highly increased by the recrystallization process. The amount of initial deformation does not influence the kinetics of segregation provided it is higher than the threshold strain for recrystallization ; at this value the kinetics are at a maximum. This evolution is explained qualitatively by the impurity drag theory.

After the end of recrystallization, the segregation remains abnormally fast. This is due to sulfur superconcentration near the free surface and near the grain boundaries after recrystallization.

Acknowledament

The authors acknowledge J.P. Roche for the A.E.S analysis.

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References

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(1989). 3. R. Le Gall, G. Saindrenan, D. Roptin, Mem. Et. Scient. Rev. Met., 87, 99

(1990). 4. J. Barbier-Vitart, G. Saindrenan, A. Lar~re, C. Roques-Carmes, Mat. Science, 17, 387 (1982). 5. M. Perdereau, J. Oudar, Surf. Sci., 20, 80 (1970). 6. C. Lea, M.P. Seah, Phil. Mag., 35, 213 (1977). 7. S.J. Wang, H.J. Grabke, Z. Metallkde., 61, 597 (1970). 8. J.W. Cahn, Acta Met., 10, 789 (1962). 9. K. Lucke, H.P. St~we, in "Recovery and recrystallization of metals", ed. L.

Himmel, Interscience New York, 171 (1963). 10. A. Rolland, J. Bernardini, Scripta Met., 19, 839 (1985). 11. N. Barbouth, J. Oudar, C.R. Acad. Sc., Serie C, 269 (1969), 1619. 12. P.J. Desr~, Acta Met., 39 (1991), 2309. 13. M. Pierantoni, B. Aufray, F. Cabane, J. Phys. C4, 46 (1985), 517. 14. F. Ferhat ; Th~se Universit~ de Nantes (1990).