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Grain boundary cosegregation and diffusion in Cu(Fe,S) solid solution
oool-6l60~3
~cro merull. Vol. 31. No. 7, pp. 1047-1052. 1983 Printed in Great Britain. All rights ~CSWWSI
so3.00+
.oo
Copyright 0.1983 Pergamon Press Ltd
GRAIN BOUNDARY COSEGREGATION AND DIFFUSION IN Cu(Fe,S) SOLID SOLUTION A. PINEAU, Laboratoire de Metallurgic-ERA
B. AUFRAY, F. CABANE-BROUTY and J. CABANE 552. Facultt des Sciences et Techniques St Jerome, rue H. Poincare. 13397, Marseille, C&dex 13, France (Received I6 Drcemhcr
1982)
Abstract-We have experimentally studied the effect of a very slightly soluble metallic impurity (Fe) on the grain boundary (G.B.) segregation and diffusion of sulphur (S) in copper. Auger Electron Spectroscopy and radiochemical (‘3) techniques have been used. A large increase in the segregation is observed and associated with a large decrease in the G.B. diffusion (D&. These results are explained by Fe-S cosegregation which can be linked to Fe-S interactions which are stronger than Cu-S, as in the sulfides. In the low energy G.B., the segregated atoms can be randomly distributed, but in the higher energy G.B.,
they rather form a phase similar to a 2D compound; the number of boundaries concerned by this last situation increases when the iron level increases. All the results cannot be correctly explained without taking into account structural changes in the G.B. due to segregation or cosegregation. R&sat&-Nous avons Ctudii exp&rirnentalementl’influence dune impurete mballique t&s peu soluble (Fe) sur la segregation et la diffusion intergranulaire du soufre dans le cuivre. La spectrometrie Auger et des techniques radiochimiques (I’S) ont izti utili&es. Nous observons une augmentation importante de la segregation et une diminution speetaculaire de la diffusion intergranulaire (0,). Ces risultats s’expliquent par une cosC&ation Fe-S qui peut itre rel& Bdes interactions Fe-S plus forte que les interactions Cu-S, comme dans les sulfures. Daus les joints de faibles energies, les atomes s&g&g&s peuvent &n distribues au hasard mais dans les joints de plus fortes energies, ils ferment plutot une phase semblable Bun compose 2D; le nombre de joints concern&spar cette demi&e situation augrnente quand la concentration volumique en fer augmente. L’ensemble des resultats ne peut pas itre explique de fa9on satisfaisante sans envisager des changements de structure dans les joints lies a la segregation ou a la costgregation.
EinfluB der sehr wenig l&lichen metallischen Verunreinigung Fe auf die Komgrenxensegregation und die Diffusion von Schwefel wurde experimentell in Kupfer untersucht. Augerelektronen-Spektroskopie und radiochemische Q’S) Methoden wurden angewandt. Die Segregation ist sehr verstirkt und die Komgrenzdiffusion (0,) ist stark vennindert. Diese Beobachtungen k&men mit Zusammenfassung-Der
einer Kosegregation von F*S erklirt werden, die von e-iner stgrkeren Wechselwirkung Fe-S gegeniiber der von Cu-S wie bei den Sulfiden herriihren kann. In Komgrenxen niedriger Energie k&men die segregierten Atome zufallig verteilt sein. In Komgrenzen mit hoher Energie bilden sie jedoch eher eine
Phase iihnlich einer ZD-Zusammensetxung. Die Anzahl der auf diese Weise dekorierten Komgrenxen nimmt mit ansteigender Volumkonxentration des Eisens zu. All diese Ergebnisse k&men nur zufriedenstellend erkhirt werden, wenn Strukturiinderungen der Komgrenze wlhrend der Segregation oder der Kosegregation
beriicksichtigt werden.
1. INTRODUCTION
When several alloying elements are present in a material, it is often observed that the interfacial segregation of each element is changed by the presence of the others; two types of interactions may be considered: -in the case of repulsive for the segregation of each -in the case of attractive for the segregation of each
interactions, element to interactions, element to
it is possible decrease, it is possible increase.
This last situation is called cosegregation and interests us in particular [l]. After a substantial study of the intergranular diffusion and segregation of sulphur in high purity copper and silver [2,3], we present the results of the elfect of a slightly soluble metallic impurity on the
intergranular diffusion and segregation of sulphur in copper; the experimental data are discussed within the frame of cosegregation phenomena. 2. EXPERIMENTAL PROCEDURES AND RESULTS The techniques are similar to those previously described for grain boundary studies in copper and silver [2,3]; therefore we give an outline and only specify the particular points in the presence of a second solute. 2.1. Preparation
of the alloys Cu(Fe)
Firstly, iron is electrolytically deposited on small copper bars (ASARCO: 99.999%); these bars are then annealed at 95O’C. in purified hydrogen, for 3 days; finally the homogeneity of the alloys is improved by I047
PINEAt: (‘I (I/.: GRAIN
1048
BOUNDARY
COSEGREGATION segregation
AND DIFFUSION
coefficient
us. (e.g. Fig. I)
4..
I
IO
30
20
40
x, pm
Fig. I. Typical curve of the lJS activity as a function of depth, in a segregated sample.
melting at about 12Oo”C, in a graphite mould, in purified argon. The homogeneity, composition and state of dispersion of Fe are checked by resistivity measurements, performed at a low temperature; thus, we are sure we actually have a Cu(Fe) solid solution, at 6OO”C, for an iron concentration of 0.2 at.%. 2.2. Sulphur deposit Elementary sulphur or its radioisotope 35S, is brought from the vapour phase into the sample, by a diffusion process. The same apparatus is used for all the thermal treatments; a (Hz-H,S) or (HIH2 35S) mixture, the composition of which is known and remains constant, circulates above the sample, for the whole time of treatment. In this apparatus the sulphur chemical potential can be arranged in the vapour so that no 3D sulphide can form on the sampIps; the range is determin@ from the stability of the various binary and ternary sulphide [4], taking into account the activity of the metallic solute. Therefore, this technique appears to be a useful means to study the ternary systems. 2.3. Segregation equilibrium: sulphur grain boundary segregation The segregated amounts have been measured with the technique previously used by Moya and known as the “flat grain boundary technique”. It is based on the use of 35 sulphur which is a soft b em&or. A set of flat grain boundaries, approximately parallel to the free surface, is formed following the procedure described in Refs [2,3]. An analysis is performed by alternatively cutting parallely to the surface and counting the superficial activity; it allows us to obtain the 35 sulphur distribution near the plane of grain boundaries. The curve of the superficial “S activity A, as a function of the depth (x) allows us to obtain the
with 11 the absorption coefficient of the 3’S /I rays by copper (51, S the apparent width of the grain boundary, taken to be 50mm (5.10-‘0m), (X,,,)” the molar fraction of sulphur in the grain boundary and (X,,)’ the molar fraction of sulphur in the bulk. Note that the segregated amount, determined by this technique is a mean value relative to several families of grain boundaries. The results obtained at 600°C are represented in Table 1 as a function of the iron bulk concentration. These experiments have been performed with the ratio pHIS/pHI= 1.2. lo-’ which corresponds to a sulphur bulk concentration of 0.5 +lo-‘at.%. The percentage of a sulphur monolayer in the grain boundary (0”) has been calculated with respect to the surface monolayer [6]. In pure copper we obtained the value foreseen by Moya; in the other samples we have observed a large increase of as with the iron bulk concentration; nevertheless the segregated amount is always less than a monolayer.
2.4. Sulphur surface segregation We performed a similar work on the surface in order to confirm the influence of iron on sulphur segregation, at the interfaces. For this purpose, we followed the kinetics of sulphur and iron segregation at 5OO”C, with Auger Electron Spectroscopy; the standardization of the sulphur segregated amounts was done by a radio chemical analysis of the samples. A single crystal containing 0.1 at.% of iron and a single crystal of high purity copper were saturated with radioactive sulphur in such a way that the superficial activity of the solid solution corresponded to the sulphur solubility limit at 500°C [7]; the A.E.S., analysis was carried out after an argon sputtering in order to obtain a perfectly clean initial surface. Figure 2 shows the variations of the sulphur Auger peak (h’) when annealing the alloys at 500°C. We ascertain a very large iron effect; although the corresponding signal at 47eV is not observed, the presence of iron very close to the surface is shown by high energy peaks of relatively low intensivity. It is clearly seen that the maximum segregated amount is larger for Cu(Fe,S) alloys than Cu(S) alloys
h&~+S) -
1.88.
%s,
By alternatively cutting slices and counting the radio-
Table 1. Grain boundary segregation results (X,)S. W(at.%) 0.5
(W”
(at.%) 0 0.1 0.2
a = (~#*)s/(~,)s
11,500 51,ooo 72.000
---
0s
3.3”‘, 14.5% 20.5%
PINEAU
CI
GRAIN
(I/.:
Cu
BOUNDARY
COSEGREGATION
AND
approximatively
constant
(FeS)
1049
DIFFUSION composition
upto
about
I nun depth, -- annealing with radioactive sulphur which is the actual diffusion treatment we will study, --cooling. 0.
T=500 *C
3-
t
(h)
Fig. 2. Relative intensity (/I~) of the Auger peak of sulphur (I 50 eV) during an annealing of Cu(S) and Cu(Fe,S) alloys
at 500°C.
This process allows us to realize a self diffusion experiment, 35 sulphur diffusion only being an isotopic exchange. The concentration-depth curve X =.f’(x) is deduced from the activity measurement A =.f(x) using the Gruzin’s method [9]. With Whipple’s solution [IO] for the integration of Fick’s law and Leclaire’s approximation [I 11, the diffusion parameter P is calculated from either A =/(x) or X =f (x)
activity of the samples, we found the bulk concentration near the surface and the data [(X2) es] characteristic of the equilibrium [8]: bulk+surface.
(a~,F~,,=
6.4. IO-’
for 2 < rj$-‘.‘2 < 10
O&c.sj = 14%
(X&,s, = 19.2 * 10-9
with
8&s, = 9% P=
A:Wti, T&-= *
1 63
aSD*h 2D,(D#
Assuming that the segregation coefficient does not depend on the composition in this concentration range, these results lead to 7a:“(Fc.SJ= 4 . 7. a c~(s) We add that an identical result has been obtained for a polycristalline sample. Hence the introduction of iron moves the sulphur segregation isotherm towards low chemical potentials both on the surface and in the grain boundaries. 2.5. Grain boundary d@sion 2.5.1. Determination of the grain boundary djksion parameter. The same techniques as those described in
Ref. [3], have been used. Let us recall the main stages: -recrystallisation annealing, -electrolytical polishing, -annealing with inactive sulphur, in such a way to obtain Cu(S) and Cu(Fe,S) solid solutions with an
experimentally from the tist part of the penetration curves, t the diffusion time and S the apparent width of the boundary (50 mm). The errors in Dx,, were estimated to within 15%. 2.5.2. Results. In the Table 2, we have gathered, the experimental data concerning sulphur grain boundary diffusion at 6OO”C, it is clear that the diffusion parameter Dti for sulphur increases with the sulphur bulk concentration and decreases with the iron bulk concentration. 3. DISCUSSION
3.1. Effect of iron on the equilibrium sulphur segregation The values in Table 1, show that the addition of iron leads to a very great increase of the equilibrium sulphur segregation; it means that a Fe-S cosegregation occurs. In order to obtain the iron concentration in the boundaries and some thermodynamical quantities relative to cosegregation, we have used the results outlined by the cosegregation model proposed by Guttmann [12]. The bulk is taken as an ideal solid solution and the grain boundaries as a regular one limited to a monolayer. This model is defective [13],
Table 2. Grain boundary diffusion results iO’(at.%) 0.5
1.14
x
D, is the bulk diffusion coefficient which is found
which is in good agreement with the precedent value for
(XJs.
--
’ - (D&“2
W,)”
(at.%) 0 0.1 0.2 0 0.1 0.2
cfs(DJs.
IO4(cm2 s-l) ~4.4 3.3 1.3 8.3 4.7 2.7
PINEAU er ul.: GRAIN BOUNDARY COSEGREGATION AND DIFFUSlON
1050
but it allows to get an analytical solution to the and to show up the effect of interactions between segregated atoms. Indeed, for a bulk composition (X,)’ (i =sulphur or iron) is given by
interactions.
problem
CXJ I_oi
Cu(Fe)
The relationship between the segregation free enthalpy (AG)s or (AG)* in a ternary Cu(Fe,S) solid solution and the free enthalpy (AG”)S or (AGO)” in the binary solution is written in the form (AG)s = (AC”)’ + ,W” (AG)F’ = (AG”)Fe + /If?? Taking into account the values given in Table 1, this leads to a system of equations which enables us to calculate:
Cu(Fe) alloys Cu(Fe,S) alloys -the
0.1%
50% 60%
in
(X,)” = 0.2% 67% 78%
segregation free enthalpy:
for binary system (AC”)’ = -81.5 kJ mol-’ and (AG”)Fe = - 50.6 kJ mol-‘, for ternary system: - (X,)re = 0.1% (AG)s = -92.9 kJ mol-’ and (AG)Fe = - 53.6 kJ mol-’ - (X,,)re = 0.2% (AG)s = -96.0 kJ mol-’ and -the
interaction
~~~~e~t-s4’3
kJ m”‘-’
a&
/J = -18.8kJmol-‘. This last parameter is usually written as a function of the binary interactions f&&Fc, fLWs, &_, similar to those defined in the regular solution model; which reduces to &_s - GWs to the extent that the interactions between metallic atoms (Cu-Fe) are weaker than those between sulphur and metallic atoms. Under these conditions, the sign of/J accounts for relatively stronger Fe-S attractions than Cu-S ones; its value is nearly the same as the difference between the enthalpies for formation of the copper and iron sulphides (Table 3); this result shows that the interactions in the segregated phase are probably nearer those in an ordered compound (2D or 3D sulphide) than those in a solid solution. This set of results is apparently consistent and agrees with the data relative to the Cu-S and Fr+S
in
the iron coverages in the fair
(AH), (kJ mob ‘)
with
the
< 2.10’ and &(Fes) = 5. lo3
whereas in a polycristal a&, = lo4 and a&s) = 5. to” The effect of iron is the same in the two cases but the sulphur coverage is smaller in the low energy grain boundary. Besides, similar observations have been made on the surface: a :u(Fe.S Ja&,
E
4.7.
These results lead to the following conclusion: -each grain boundary can be characterized by an isotherm, the shape and position in the sulphur chemical potential scale depending on the grain boundary energy, -when iron is added, every isotherm is shifted by roughly the same amount, towards a lower sulphur chemical potential.
Table 3. Enthalpy formation for 2D and 3D sulphides [14]
Sulphide
agreement
[ 151related to the iron solubility
We are led to the following conclusion: cosegregation of iron and sulphur in the grain boundaries is proved by strong interactions between atoms in the boundaries; it is connected to large coverages of both iron and sulphur. This conclusion does not easily agree with the assumption of randomly distributed segregated atoms; the above pattern implying strong Fe-S interactions associated with high grain boundary concentrations would be rather favourable to ordered clusters of segregated atoms, setting up a new phase [16]. It is difficult to specify the structure and the distribution of this 2D phase with the results obtained in polycrystals. Actually the grain boundary behaviour, as well as the surface one, is quite likely dependant on the crystallographical characteristics; thus each grain boundary must be considered in its own right 1171. Nevertheless, we can roughly distinguish two types of grain boundary: on the one hand, those in which the segregation equilibrium corresponds to sulphur and iron concentrations sufficiently low to be consistent with a random distribution; on the other hand, those in which the equilibrium segregation corresponds to the formation of an ordered concentrated phase. The first family corresponds to quite low intergranular energies, the second one to rather high intergranular energies. A comparison between the results obtained with a low energy boundary and those in grain boundaries of a polycrystal, confirms this hypothesis. In a twin boundary it has been found [I81 for (X,,)” = 0.1%
(AG)’ = 1 -:(,yJ exp - RT’
GvFe=
arc
Hondros’s prevision limit in copper.
W*)
-the iron coverage (0” in % of a monolayer) each alloy:
In particular,
alloys
FeM2D)
FeS (3D)
Cu,S (2D)
Cu,S (3D)
-195
-IS0
-175
-130
PINEAU
ri ul.:
GRAIN
BOUNDARY
COSEGREGATlON
AND
All these comments enable us to understand cosegregation in a Cu(Fe.S) alloy, in the following
1051
DIFFUSION
ir- 600
*C
manner: -in the low energy grain boundaries, such as the twin boundary, some not very favourahle sites for sulphur segregation in pure copper, would fix sulphur atoms when copper atoms are substituted by iron atoms in neighbouring sites, -in higher energy grain boundaries, the presence of iron would permit the formation of a 2D compound. The number of these grain boundaries would increase with the iron buik concentration, at a given sulphur chemical potential. However, there is a possibility that both these types of behaviours occur in the different grain boundaries of a polycrystal. 3.2. Ins~rgranu~ar dl~~io~ An evolution of the intergranular diffusion parameter P, is generally difficult to explain precisely [19]; it is mainly due to the expression of P which simultaneously involves ~uilibri~ and kinetic terms; an additional faculty consists in the lack of knowl~ge about the structure of the “bad crystal” zone which is responsible for the rapid diffusion; finally when measured in a polycrystalline sample, P is a mean value, relative to various families of grain boundaries. Nevertheless, since the ~gre~atian ~uilib~um was studied at the same time, we have been able to eliminate the first difficulty by separating the contribution of segregation from the total experimentai measurements; thus when a and DR,,were both determined under the same thermodynamical conditions, the diffusion coefficient Bgb was dire&y deduced; in the other cases, it was assumed that the segregation coefficient did not depend on the sulphur concentration. This hypothesis is justified by the following remarks: -in Cu(S) alloys, Moya has not found significant variation with the bulk sulphur ~n~ntration; we have performed some experiments at different bulk concentrations confirming the earlier results, -even when the segregation coefficient is very large, the greatest amount of segregated sulphur remains less than one monolayer, which is assumed to be the maximum amount (e.g. Bs = 45% monolayer when as = 72,000 and (X,)s = I. 14 1 IO-* at.%)_ Moreover, the only variation of as would probably be a decrease as in Giver [3] when the sutphur bulk concentration increases; the shape of the curves would not be changed. Assuming that as does not depend on the sulphur chemical potential, enables us to obtain a set of dif&ion coefficients and to discuss their variation with the sulphur grain boundary concentration. 3.2. I. SuIphur intergranutur d#ksion in Cu(S) al/o_vs. We observe (Fig. 3) a large increase in the diffusion coefficient with the sulphur jntergranuIar
20
Cu Fe (0.1 *A)
_+.--
_*__---I
_...__
-e-l 40
_-60
r-CuFe(O.2%) 80
Fig. 3. Sulphur diffusion coefficients (Q.) as a function of the grain boundary con~ntration (X,).
concentration. This variation is of the same form as in the Ag(S) solution [3] but it is more pronounced in the Cu(S) one. This resuit could be connected to a decrease in the enthalpy for vacancy formation in grain boundaries. This assumption is based on the fact that a rapid sulphur diffusion (~*)s>>~~~)~ is correctly explained by an increase of the vacancy concentration associated with the solute atoms [20]. However, a recent study in Ag(Sn) solid solution [21] shows that the effect of a rapid element is much smaller in the grain boundary than in the bulk; it is easily understood, for grain boundaries are disordered stabs in a solid, where the number of vacancies added with the solute probably remains small with respect to the total number of vacancies. Moreover, both in copper and in silver, sulphur is a slow element in the grain boundary ~~~~)s<<(~~~Au.Therefore, we conclude that it seems difficult to explain the intergranular transport phenomenon from values relative to bulk diffusion. The very special behaviour of suIp~ur-low diffusion coefficients associated with an increase in the grain ~undary con~ntration~an only be interpreted by taking into account a segregation effect on the grain boundary structure. However, our results taken as a whole do not allow to specify the structural changes due to the presence of sulphur. From this point of view, only an accurate study with high resolution electron microscopy could give some further information. 3.2.3. rntergratiufar dl~usia~ in Cu(Fe,S) alloys: a comparison with the Cu(S) alloy. The introduction of iron gives rise to a drastic decrease in sulphur intergranular diffusion; this result proves suiphur and iron cosegregation; it is in good agreement with Fe-S bonds being stronger than Cu-S in grain boundaries, just as in the sulphides; indeed as far as a sulphur atom is more bonded, it has more difficulties to jump from one vacancy to another; whence it seems likely to observe a decrease of the diffusion. However, taking into account the large coverages, it is difficult to confine the discussion to energy aspects of the bonds: the assumption of grain boundary structures, linked to segregation must also be considered when cosegregation occurs. It could explain the large effect that we observe.
I052
PINEAU e/ c/l.: GRAIN BOUNDARY COSEGREGATION AND 4. CONCLUSION
We studied the inlluence of dissolved iron on the sulphur intergranular behaviour in copper, with techniques that enable us to separately vary the amounts of bulk iron and sulp;hur. Thus we carried out measurements of grain boundary segregation (a) and diffusion (D,,,) and observations of the surface. All the results are consistent, on the one hand, with the previous works on sulphur behaviour in copper and silver, in the case where pure copper is concerned and on the other hand with an enhancement of the effects due to sulphur-grain boundary interactions in the case where iron is added to copper. In Cu(Fe) alloys (e.g. G = 0.2 at.%), we observed: -an
increase in the segregation coefficients - 7) with segregated amounts less than a monolayer, -a decrease in the sulphur diffusion coefficients, c lo-‘] though they were rather r(o,*)~“~F.s,/(Dr*)s”~s~ small in pure copper, compared to the intergranular diffusion of metallic elements (D,,)s/(D,,)Ay z lo-‘. (a$Fcf~a$sI
We conclude that Fe-S cosegregation is very pronounced in the interfaces; this is expressed by a shift of the sulphur segregation isotherm towards low bulk sulphur concentrations; the amount of this shift does not seem to depend on the grain boundary energy. Presently we are undertaking a study of bicrystals with various misorientations; this would enable us to specify the r6le of grain boundary energy and structure, if any, on the shifts of isotherms, and to obtain further informations on the existence of 2D com-
DIFFUSION
pounds in some type of grain boundaries suggested by the results in polycrystals.
as it. is
REFERENCES 2. 3. 4. 5. 6.
A. Pineau, These 3ime cycle, Marseille (1980). F. Moya and G. E. Moya-Gontier, J. Phys. 36C4, 157 (1975). B. Aufray, F. Cabane-Brouty and J. Cabane, Acfu mercdl. 27, 1849 (1979). J. Barin and 0. Knacke, ThermodynumiculProperlies of Inorganic Substance, Vol. 1. Springer, Berlin (I 973). J. Oudar and N. Barbouth. Bull. Sot. Chim. 3. 834 (1970). J. Benard, 1. Oudar and F. Cabane-Brouty, Sur$ Sci.
3, 4 (1965).
F. Moya, G. E. Moya-Gontier, F. Cabane-Brouty and J. Oudar, Acru metoll. 19, 1189 (1971). 8. P. Petrino, F. Moya and F. Cabane-Brouty, J. Solid St. Chem. 2, 439 (1974). 9. G. Seibel, Int. J. appl. Rod. Isotop. 15, 679 (1964). IO. R. T. P. Whipple, Phil. Mug. 45, 1225 (1954). Il. A. D. Le Claire, Br. J. Appf. Phys. 14, 351 (1963). 12. M. Guttmann, Surf. Sci. 53, 213 (1975). 13. R. Defay and I., Prigogine, Surface Tension and Adsorprion, p. 177. Longmans, London (1966). 14. J. Benard, J. Oudar, N. Barbouth, E. Margot and Y. Berthier, Surf. Sci. 88, L3S (1979). 15. E. D. Hondros and M. P. Seah. Metall. Truns. 8A. 1363 (1977). 16. M. Guttmann, Metall. Trans. 8A, 1383 (1977). 17. J. W. Cahn. Proc. NSF-CNRS WorkshoDon Structures and Properties of Intergranular Bohdaries. Caen (I 982). To be published. 18. B. Aufray and J. Cabane, Scripta metall. 15, 1339 (1981). 19. N. L. Peterson, Int. metall. Rev. To be published. 20. F. Moya and F. Cabane-Brouty, C.r. Acad. Sci. paris I.
264c, 1543 (1967).
21. P. Gas and J. Bemardini, Sur/. Sci. 72, 365 (1978).
~cro merull. Vol. 31. No. 7, pp. 1047-1052. 1983 Printed in Great Britain. All rights ~CSWWSI
so3.00+
.oo
Copyright 0.1983 Pergamon Press Ltd
GRAIN BOUNDARY COSEGREGATION AND DIFFUSION IN Cu(Fe,S) SOLID SOLUTION A. PINEAU, Laboratoire de Metallurgic-ERA
B. AUFRAY, F. CABANE-BROUTY and J. CABANE 552. Facultt des Sciences et Techniques St Jerome, rue H. Poincare. 13397, Marseille, C&dex 13, France (Received I6 Drcemhcr
1982)
Abstract-We have experimentally studied the effect of a very slightly soluble metallic impurity (Fe) on the grain boundary (G.B.) segregation and diffusion of sulphur (S) in copper. Auger Electron Spectroscopy and radiochemical (‘3) techniques have been used. A large increase in the segregation is observed and associated with a large decrease in the G.B. diffusion (D&. These results are explained by Fe-S cosegregation which can be linked to Fe-S interactions which are stronger than Cu-S, as in the sulfides. In the low energy G.B., the segregated atoms can be randomly distributed, but in the higher energy G.B.,
they rather form a phase similar to a 2D compound; the number of boundaries concerned by this last situation increases when the iron level increases. All the results cannot be correctly explained without taking into account structural changes in the G.B. due to segregation or cosegregation. R&sat&-Nous avons Ctudii exp&rirnentalementl’influence dune impurete mballique t&s peu soluble (Fe) sur la segregation et la diffusion intergranulaire du soufre dans le cuivre. La spectrometrie Auger et des techniques radiochimiques (I’S) ont izti utili&es. Nous observons une augmentation importante de la segregation et une diminution speetaculaire de la diffusion intergranulaire (0,). Ces risultats s’expliquent par une cosC&ation Fe-S qui peut itre rel& Bdes interactions Fe-S plus forte que les interactions Cu-S, comme dans les sulfures. Daus les joints de faibles energies, les atomes s&g&g&s peuvent &n distribues au hasard mais dans les joints de plus fortes energies, ils ferment plutot une phase semblable Bun compose 2D; le nombre de joints concern&spar cette demi&e situation augrnente quand la concentration volumique en fer augmente. L’ensemble des resultats ne peut pas itre explique de fa9on satisfaisante sans envisager des changements de structure dans les joints lies a la segregation ou a la costgregation.
EinfluB der sehr wenig l&lichen metallischen Verunreinigung Fe auf die Komgrenxensegregation und die Diffusion von Schwefel wurde experimentell in Kupfer untersucht. Augerelektronen-Spektroskopie und radiochemische Q’S) Methoden wurden angewandt. Die Segregation ist sehr verstirkt und die Komgrenzdiffusion (0,) ist stark vennindert. Diese Beobachtungen k&men mit Zusammenfassung-Der
einer Kosegregation von F*S erklirt werden, die von e-iner stgrkeren Wechselwirkung Fe-S gegeniiber der von Cu-S wie bei den Sulfiden herriihren kann. In Komgrenxen niedriger Energie k&men die segregierten Atome zufallig verteilt sein. In Komgrenzen mit hoher Energie bilden sie jedoch eher eine
Phase iihnlich einer ZD-Zusammensetxung. Die Anzahl der auf diese Weise dekorierten Komgrenxen nimmt mit ansteigender Volumkonxentration des Eisens zu. All diese Ergebnisse k&men nur zufriedenstellend erkhirt werden, wenn Strukturiinderungen der Komgrenze wlhrend der Segregation oder der Kosegregation
beriicksichtigt werden.
1. INTRODUCTION
When several alloying elements are present in a material, it is often observed that the interfacial segregation of each element is changed by the presence of the others; two types of interactions may be considered: -in the case of repulsive for the segregation of each -in the case of attractive for the segregation of each
interactions, element to interactions, element to
it is possible decrease, it is possible increase.
This last situation is called cosegregation and interests us in particular [l]. After a substantial study of the intergranular diffusion and segregation of sulphur in high purity copper and silver [2,3], we present the results of the elfect of a slightly soluble metallic impurity on the
intergranular diffusion and segregation of sulphur in copper; the experimental data are discussed within the frame of cosegregation phenomena. 2. EXPERIMENTAL PROCEDURES AND RESULTS The techniques are similar to those previously described for grain boundary studies in copper and silver [2,3]; therefore we give an outline and only specify the particular points in the presence of a second solute. 2.1. Preparation
of the alloys Cu(Fe)
Firstly, iron is electrolytically deposited on small copper bars (ASARCO: 99.999%); these bars are then annealed at 95O’C. in purified hydrogen, for 3 days; finally the homogeneity of the alloys is improved by I047
PINEAt: (‘I (I/.: GRAIN
1048
BOUNDARY
COSEGREGATION segregation
AND DIFFUSION
coefficient
us. (e.g. Fig. I)
4..
I
IO
30
20
40
x, pm
Fig. I. Typical curve of the lJS activity as a function of depth, in a segregated sample.
melting at about 12Oo”C, in a graphite mould, in purified argon. The homogeneity, composition and state of dispersion of Fe are checked by resistivity measurements, performed at a low temperature; thus, we are sure we actually have a Cu(Fe) solid solution, at 6OO”C, for an iron concentration of 0.2 at.%. 2.2. Sulphur deposit Elementary sulphur or its radioisotope 35S, is brought from the vapour phase into the sample, by a diffusion process. The same apparatus is used for all the thermal treatments; a (Hz-H,S) or (HIH2 35S) mixture, the composition of which is known and remains constant, circulates above the sample, for the whole time of treatment. In this apparatus the sulphur chemical potential can be arranged in the vapour so that no 3D sulphide can form on the sampIps; the range is determin@ from the stability of the various binary and ternary sulphide [4], taking into account the activity of the metallic solute. Therefore, this technique appears to be a useful means to study the ternary systems. 2.3. Segregation equilibrium: sulphur grain boundary segregation The segregated amounts have been measured with the technique previously used by Moya and known as the “flat grain boundary technique”. It is based on the use of 35 sulphur which is a soft b em&or. A set of flat grain boundaries, approximately parallel to the free surface, is formed following the procedure described in Refs [2,3]. An analysis is performed by alternatively cutting parallely to the surface and counting the superficial activity; it allows us to obtain the 35 sulphur distribution near the plane of grain boundaries. The curve of the superficial “S activity A, as a function of the depth (x) allows us to obtain the
with 11 the absorption coefficient of the 3’S /I rays by copper (51, S the apparent width of the grain boundary, taken to be 50mm (5.10-‘0m), (X,,,)” the molar fraction of sulphur in the grain boundary and (X,,)’ the molar fraction of sulphur in the bulk. Note that the segregated amount, determined by this technique is a mean value relative to several families of grain boundaries. The results obtained at 600°C are represented in Table 1 as a function of the iron bulk concentration. These experiments have been performed with the ratio pHIS/pHI= 1.2. lo-’ which corresponds to a sulphur bulk concentration of 0.5 +lo-‘at.%. The percentage of a sulphur monolayer in the grain boundary (0”) has been calculated with respect to the surface monolayer [6]. In pure copper we obtained the value foreseen by Moya; in the other samples we have observed a large increase of as with the iron bulk concentration; nevertheless the segregated amount is always less than a monolayer.
2.4. Sulphur surface segregation We performed a similar work on the surface in order to confirm the influence of iron on sulphur segregation, at the interfaces. For this purpose, we followed the kinetics of sulphur and iron segregation at 5OO”C, with Auger Electron Spectroscopy; the standardization of the sulphur segregated amounts was done by a radio chemical analysis of the samples. A single crystal containing 0.1 at.% of iron and a single crystal of high purity copper were saturated with radioactive sulphur in such a way that the superficial activity of the solid solution corresponded to the sulphur solubility limit at 500°C [7]; the A.E.S., analysis was carried out after an argon sputtering in order to obtain a perfectly clean initial surface. Figure 2 shows the variations of the sulphur Auger peak (h’) when annealing the alloys at 500°C. We ascertain a very large iron effect; although the corresponding signal at 47eV is not observed, the presence of iron very close to the surface is shown by high energy peaks of relatively low intensivity. It is clearly seen that the maximum segregated amount is larger for Cu(Fe,S) alloys than Cu(S) alloys
h&~+S) -
1.88.
%s,
By alternatively cutting slices and counting the radio-
Table 1. Grain boundary segregation results (X,)S. W(at.%) 0.5
(W”
(at.%) 0 0.1 0.2
a = (~#*)s/(~,)s
11,500 51,ooo 72.000
---
0s
3.3”‘, 14.5% 20.5%
PINEAU
CI
GRAIN
(I/.:
Cu
BOUNDARY
COSEGREGATION
AND
approximatively
constant
(FeS)
1049
DIFFUSION composition
upto
about
I nun depth, -- annealing with radioactive sulphur which is the actual diffusion treatment we will study, --cooling. 0.
T=500 *C
3-
t
(h)
Fig. 2. Relative intensity (/I~) of the Auger peak of sulphur (I 50 eV) during an annealing of Cu(S) and Cu(Fe,S) alloys
at 500°C.
This process allows us to realize a self diffusion experiment, 35 sulphur diffusion only being an isotopic exchange. The concentration-depth curve X =.f’(x) is deduced from the activity measurement A =.f(x) using the Gruzin’s method [9]. With Whipple’s solution [IO] for the integration of Fick’s law and Leclaire’s approximation [I 11, the diffusion parameter P is calculated from either A =/(x) or X =f (x)
activity of the samples, we found the bulk concentration near the surface and the data [(X2) es] characteristic of the equilibrium [8]: bulk+surface.
(a~,F~,,=
6.4. IO-’
for 2 < rj$-‘.‘2 < 10
O&c.sj = 14%
(X&,s, = 19.2 * 10-9
with
8&s, = 9% P=
A:Wti, T&-= *
1 63
aSD*h 2D,(D#
Assuming that the segregation coefficient does not depend on the composition in this concentration range, these results lead to 7a:“(Fc.SJ= 4 . 7. a c~(s) We add that an identical result has been obtained for a polycristalline sample. Hence the introduction of iron moves the sulphur segregation isotherm towards low chemical potentials both on the surface and in the grain boundaries. 2.5. Grain boundary d@sion 2.5.1. Determination of the grain boundary djksion parameter. The same techniques as those described in
Ref. [3], have been used. Let us recall the main stages: -recrystallisation annealing, -electrolytical polishing, -annealing with inactive sulphur, in such a way to obtain Cu(S) and Cu(Fe,S) solid solutions with an
experimentally from the tist part of the penetration curves, t the diffusion time and S the apparent width of the boundary (50 mm). The errors in Dx,, were estimated to within 15%. 2.5.2. Results. In the Table 2, we have gathered, the experimental data concerning sulphur grain boundary diffusion at 6OO”C, it is clear that the diffusion parameter Dti for sulphur increases with the sulphur bulk concentration and decreases with the iron bulk concentration. 3. DISCUSSION
3.1. Effect of iron on the equilibrium sulphur segregation The values in Table 1, show that the addition of iron leads to a very great increase of the equilibrium sulphur segregation; it means that a Fe-S cosegregation occurs. In order to obtain the iron concentration in the boundaries and some thermodynamical quantities relative to cosegregation, we have used the results outlined by the cosegregation model proposed by Guttmann [12]. The bulk is taken as an ideal solid solution and the grain boundaries as a regular one limited to a monolayer. This model is defective [13],
Table 2. Grain boundary diffusion results iO’(at.%) 0.5
1.14
x
D, is the bulk diffusion coefficient which is found
which is in good agreement with the precedent value for
(XJs.
--
’ - (D&“2
W,)”
(at.%) 0 0.1 0.2 0 0.1 0.2
cfs(DJs.
IO4(cm2 s-l) ~4.4 3.3 1.3 8.3 4.7 2.7
PINEAU er ul.: GRAIN BOUNDARY COSEGREGATION AND DIFFUSlON
1050
but it allows to get an analytical solution to the and to show up the effect of interactions between segregated atoms. Indeed, for a bulk composition (X,)’ (i =sulphur or iron) is given by
interactions.
problem
CXJ I_oi
Cu(Fe)
The relationship between the segregation free enthalpy (AG)s or (AG)* in a ternary Cu(Fe,S) solid solution and the free enthalpy (AG”)S or (AGO)” in the binary solution is written in the form (AG)s = (AC”)’ + ,W” (AG)F’ = (AG”)Fe + /If?? Taking into account the values given in Table 1, this leads to a system of equations which enables us to calculate:
Cu(Fe) alloys Cu(Fe,S) alloys -the
0.1%
50% 60%
in
(X,)” = 0.2% 67% 78%
segregation free enthalpy:
for binary system (AC”)’ = -81.5 kJ mol-’ and (AG”)Fe = - 50.6 kJ mol-‘, for ternary system: - (X,)re = 0.1% (AG)s = -92.9 kJ mol-’ and (AG)Fe = - 53.6 kJ mol-’ - (X,,)re = 0.2% (AG)s = -96.0 kJ mol-’ and -the
interaction
~~~~e~t-s4’3
kJ m”‘-’
a&
/J = -18.8kJmol-‘. This last parameter is usually written as a function of the binary interactions f&&Fc, fLWs, &_, similar to those defined in the regular solution model; which reduces to &_s - GWs to the extent that the interactions between metallic atoms (Cu-Fe) are weaker than those between sulphur and metallic atoms. Under these conditions, the sign of/J accounts for relatively stronger Fe-S attractions than Cu-S ones; its value is nearly the same as the difference between the enthalpies for formation of the copper and iron sulphides (Table 3); this result shows that the interactions in the segregated phase are probably nearer those in an ordered compound (2D or 3D sulphide) than those in a solid solution. This set of results is apparently consistent and agrees with the data relative to the Cu-S and Fr+S
in
the iron coverages in the fair
(AH), (kJ mob ‘)
with
the
< 2.10’ and &(Fes) = 5. lo3
whereas in a polycristal a&, = lo4 and a&s) = 5. to” The effect of iron is the same in the two cases but the sulphur coverage is smaller in the low energy grain boundary. Besides, similar observations have been made on the surface: a :u(Fe.S Ja&,
E
4.7.
These results lead to the following conclusion: -each grain boundary can be characterized by an isotherm, the shape and position in the sulphur chemical potential scale depending on the grain boundary energy, -when iron is added, every isotherm is shifted by roughly the same amount, towards a lower sulphur chemical potential.
Table 3. Enthalpy formation for 2D and 3D sulphides [14]
Sulphide
agreement
[ 151related to the iron solubility
We are led to the following conclusion: cosegregation of iron and sulphur in the grain boundaries is proved by strong interactions between atoms in the boundaries; it is connected to large coverages of both iron and sulphur. This conclusion does not easily agree with the assumption of randomly distributed segregated atoms; the above pattern implying strong Fe-S interactions associated with high grain boundary concentrations would be rather favourable to ordered clusters of segregated atoms, setting up a new phase [16]. It is difficult to specify the structure and the distribution of this 2D phase with the results obtained in polycrystals. Actually the grain boundary behaviour, as well as the surface one, is quite likely dependant on the crystallographical characteristics; thus each grain boundary must be considered in its own right 1171. Nevertheless, we can roughly distinguish two types of grain boundary: on the one hand, those in which the segregation equilibrium corresponds to sulphur and iron concentrations sufficiently low to be consistent with a random distribution; on the other hand, those in which the equilibrium segregation corresponds to the formation of an ordered concentrated phase. The first family corresponds to quite low intergranular energies, the second one to rather high intergranular energies. A comparison between the results obtained with a low energy boundary and those in grain boundaries of a polycrystal, confirms this hypothesis. In a twin boundary it has been found [I81 for (X,,)” = 0.1%
(AG)’ = 1 -:(,yJ exp - RT’
GvFe=
arc
Hondros’s prevision limit in copper.
W*)
-the iron coverage (0” in % of a monolayer) each alloy:
In particular,
alloys
FeM2D)
FeS (3D)
Cu,S (2D)
Cu,S (3D)
-195
-IS0
-175
-130
PINEAU
ri ul.:
GRAIN
BOUNDARY
COSEGREGATlON
AND
All these comments enable us to understand cosegregation in a Cu(Fe.S) alloy, in the following
1051
DIFFUSION
ir- 600
*C
manner: -in the low energy grain boundaries, such as the twin boundary, some not very favourahle sites for sulphur segregation in pure copper, would fix sulphur atoms when copper atoms are substituted by iron atoms in neighbouring sites, -in higher energy grain boundaries, the presence of iron would permit the formation of a 2D compound. The number of these grain boundaries would increase with the iron buik concentration, at a given sulphur chemical potential. However, there is a possibility that both these types of behaviours occur in the different grain boundaries of a polycrystal. 3.2. Ins~rgranu~ar dl~~io~ An evolution of the intergranular diffusion parameter P, is generally difficult to explain precisely [19]; it is mainly due to the expression of P which simultaneously involves ~uilibri~ and kinetic terms; an additional faculty consists in the lack of knowl~ge about the structure of the “bad crystal” zone which is responsible for the rapid diffusion; finally when measured in a polycrystalline sample, P is a mean value, relative to various families of grain boundaries. Nevertheless, since the ~gre~atian ~uilib~um was studied at the same time, we have been able to eliminate the first difficulty by separating the contribution of segregation from the total experimentai measurements; thus when a and DR,,were both determined under the same thermodynamical conditions, the diffusion coefficient Bgb was dire&y deduced; in the other cases, it was assumed that the segregation coefficient did not depend on the sulphur concentration. This hypothesis is justified by the following remarks: -in Cu(S) alloys, Moya has not found significant variation with the bulk sulphur ~n~ntration; we have performed some experiments at different bulk concentrations confirming the earlier results, -even when the segregation coefficient is very large, the greatest amount of segregated sulphur remains less than one monolayer, which is assumed to be the maximum amount (e.g. Bs = 45% monolayer when as = 72,000 and (X,)s = I. 14 1 IO-* at.%)_ Moreover, the only variation of as would probably be a decrease as in Giver [3] when the sutphur bulk concentration increases; the shape of the curves would not be changed. Assuming that as does not depend on the sulphur chemical potential, enables us to obtain a set of dif&ion coefficients and to discuss their variation with the sulphur grain boundary concentration. 3.2. I. SuIphur intergranutur d#ksion in Cu(S) al/o_vs. We observe (Fig. 3) a large increase in the diffusion coefficient with the sulphur jntergranuIar
20
Cu Fe (0.1 *A)
_+.--
_*__---I
_...__
-e-l 40
_-60
r-CuFe(O.2%) 80
Fig. 3. Sulphur diffusion coefficients (Q.) as a function of the grain boundary con~ntration (X,).
concentration. This variation is of the same form as in the Ag(S) solution [3] but it is more pronounced in the Cu(S) one. This resuit could be connected to a decrease in the enthalpy for vacancy formation in grain boundaries. This assumption is based on the fact that a rapid sulphur diffusion (~*)s>>~~~)~ is correctly explained by an increase of the vacancy concentration associated with the solute atoms [20]. However, a recent study in Ag(Sn) solid solution [21] shows that the effect of a rapid element is much smaller in the grain boundary than in the bulk; it is easily understood, for grain boundaries are disordered stabs in a solid, where the number of vacancies added with the solute probably remains small with respect to the total number of vacancies. Moreover, both in copper and in silver, sulphur is a slow element in the grain boundary ~~~~)s<<(~~~Au.Therefore, we conclude that it seems difficult to explain the intergranular transport phenomenon from values relative to bulk diffusion. The very special behaviour of suIp~ur-low diffusion coefficients associated with an increase in the grain ~undary con~ntration~an only be interpreted by taking into account a segregation effect on the grain boundary structure. However, our results taken as a whole do not allow to specify the structural changes due to the presence of sulphur. From this point of view, only an accurate study with high resolution electron microscopy could give some further information. 3.2.3. rntergratiufar dl~usia~ in Cu(Fe,S) alloys: a comparison with the Cu(S) alloy. The introduction of iron gives rise to a drastic decrease in sulphur intergranular diffusion; this result proves suiphur and iron cosegregation; it is in good agreement with Fe-S bonds being stronger than Cu-S in grain boundaries, just as in the sulphides; indeed as far as a sulphur atom is more bonded, it has more difficulties to jump from one vacancy to another; whence it seems likely to observe a decrease of the diffusion. However, taking into account the large coverages, it is difficult to confine the discussion to energy aspects of the bonds: the assumption of grain boundary structures, linked to segregation must also be considered when cosegregation occurs. It could explain the large effect that we observe.
I052
PINEAU e/ c/l.: GRAIN BOUNDARY COSEGREGATION AND 4. CONCLUSION
We studied the inlluence of dissolved iron on the sulphur intergranular behaviour in copper, with techniques that enable us to separately vary the amounts of bulk iron and sulp;hur. Thus we carried out measurements of grain boundary segregation (a) and diffusion (D,,,) and observations of the surface. All the results are consistent, on the one hand, with the previous works on sulphur behaviour in copper and silver, in the case where pure copper is concerned and on the other hand with an enhancement of the effects due to sulphur-grain boundary interactions in the case where iron is added to copper. In Cu(Fe) alloys (e.g. G = 0.2 at.%), we observed: -an
increase in the segregation coefficients - 7) with segregated amounts less than a monolayer, -a decrease in the sulphur diffusion coefficients, c lo-‘] though they were rather r(o,*)~“~F.s,/(Dr*)s”~s~ small in pure copper, compared to the intergranular diffusion of metallic elements (D,,)s/(D,,)Ay z lo-‘. (a$Fcf~a$sI
We conclude that Fe-S cosegregation is very pronounced in the interfaces; this is expressed by a shift of the sulphur segregation isotherm towards low bulk sulphur concentrations; the amount of this shift does not seem to depend on the grain boundary energy. Presently we are undertaking a study of bicrystals with various misorientations; this would enable us to specify the r6le of grain boundary energy and structure, if any, on the shifts of isotherms, and to obtain further informations on the existence of 2D com-
DIFFUSION
pounds in some type of grain boundaries suggested by the results in polycrystals.
as it. is
REFERENCES 2. 3. 4. 5. 6.
A. Pineau, These 3ime cycle, Marseille (1980). F. Moya and G. E. Moya-Gontier, J. Phys. 36C4, 157 (1975). B. Aufray, F. Cabane-Brouty and J. Cabane, Acfu mercdl. 27, 1849 (1979). J. Barin and 0. Knacke, ThermodynumiculProperlies of Inorganic Substance, Vol. 1. Springer, Berlin (I 973). J. Oudar and N. Barbouth. Bull. Sot. Chim. 3. 834 (1970). J. Benard, 1. Oudar and F. Cabane-Brouty, Sur$ Sci.
3, 4 (1965).
F. Moya, G. E. Moya-Gontier, F. Cabane-Brouty and J. Oudar, Acru metoll. 19, 1189 (1971). 8. P. Petrino, F. Moya and F. Cabane-Brouty, J. Solid St. Chem. 2, 439 (1974). 9. G. Seibel, Int. J. appl. Rod. Isotop. 15, 679 (1964). IO. R. T. P. Whipple, Phil. Mug. 45, 1225 (1954). Il. A. D. Le Claire, Br. J. Appf. Phys. 14, 351 (1963). 12. M. Guttmann, Surf. Sci. 53, 213 (1975). 13. R. Defay and I., Prigogine, Surface Tension and Adsorprion, p. 177. Longmans, London (1966). 14. J. Benard, J. Oudar, N. Barbouth, E. Margot and Y. Berthier, Surf. Sci. 88, L3S (1979). 15. E. D. Hondros and M. P. Seah. Metall. Truns. 8A. 1363 (1977). 16. M. Guttmann, Metall. Trans. 8A, 1383 (1977). 17. J. W. Cahn. Proc. NSF-CNRS WorkshoDon Structures and Properties of Intergranular Bohdaries. Caen (I 982). To be published. 18. B. Aufray and J. Cabane, Scripta metall. 15, 1339 (1981). 19. N. L. Peterson, Int. metall. Rev. To be published. 20. F. Moya and F. Cabane-Brouty, C.r. Acad. Sci. paris I.
264c, 1543 (1967).
21. P. Gas and J. Bemardini, Sur/. Sci. 72, 365 (1978).