GRAIN BOUNDARY EMBRITTLEMENT STUDY ON NICKEL 270 BY EQUILIBRIUM AND DYNAMIC INTERFACE SULFUR SEGREGATION

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GRAIN BOUNDARY EMBRITTLEMENT STUDY ON NICKEL 270 BY EQUILIBRIUM AND DYNAMIC

INTERFACE SULFUR SEGREGATION

A. Larere, G. Saindrenan

To cite this version:

A. Larere, G. Saindrenan. GRAIN BOUNDARY EMBRITTLEMENT STUDY ON NICKEL 270 BY

EQUILIBRIUM AND DYNAMIC INTERFACE SULFUR SEGREGATION. Journal de Physique

Colloques, 1990, 51 (C1), pp.C1-563-C1-568. �10.1051/jphyscol:1990188�. �jpa-00230356�

COLLOQUE DE PHYSIQUE

Colloque Cl, supplement au nO1, Tome 51, janvier 1990

GRAIN BOUNDARY EMBRITTLEMENT STUDY ON NICKEL 270 BY EQUILIBRIUM AND DYNAMIC INTERFACE SULFUR SEGREGATION

A. LARERE and G. SAINDRENAN*

Laboratoire de Mdtallurgie Structurale ISMA, URA CNRS 1107, Universit6 raris-Sud, Bdt. 413, F-91405 Orsay Cedex, France

Laboratoire de Sciences des Materiaux de la Mtlcanique, ENSM, 1 Rue de la No.!+, F-44072 Nantes Cedex 03, France

Resume : Le but de cet article est de pr6senter quelques resultats relatifs aux s6gregations superficielles de soufre, obtenus par spectrom6trie des electrons AUGER pour interpreter les rtsultats des essais m6caniques rdalises sur le nickel 270 (99,98% ; 1 ppm pds (S).Les mesures cinktiques de s6gregations par S.E.A. ont Bt6 effectuees sur des 6chantillons recuits 4 heures 1150°C. Les analyses ont 6td r6alis6e "in situ" 950, 900, et 800°C. La valeur du coefficient de diffusion en volume du soufre a 6t6 obtenue en utilisant le modale de McLEAN. Le formalisme de FOWLER (solution r6guliG1-e) a 6th utilis6 pour determiner l'enthalpie libre de segregation : bGs =

-

¹⁴⁰

-

3 Ys kJ.mol-l (Y : taux de s6gr6gation) .Les dkfauts en sursaturation tels que les lacunes de trempe et ceux dus B la deformation plastique (metal lamin6) augmentent considerablement les cinetiques de s6gr6gation superficielle du soufre (par rapport aux cinhtiques de McLEAN). Les cinetiques de segregation superficielle de soufre sur des structures laminees presentent deux mecanismes : klimination des lacunes en sursaturation et drainage des atomes de soufre par les joints de grain durant la recristallisation. Les r6sultats des essais mkcaniques apras des recuits spkcifiques peuvent &re expliquts par des mkcanismes de segregation dynamique.

Abstract : The purpose of this paper is to present some information obtained about sulfur segregation at surfaces using A.E.S. to explain the results of mechanical testing on nickel 270 (99,98%, 1 wt ppmS). The kinetic segregation measurements by A.E.S. were carried out on samples quenched after annealing 4 hours at 1150°C. The analyses were performed in situ at 950, 900 and 800°C. The bulk S diffusion coefficients were obtained using McLean's model.

FOWLER'S formalism (regular solution) was used to determine surface segregation energy

&Gs : - 140

-

3Ys k~.mol-' (Y : rate of segregation).The defects in supersaturation, such as quenched-in vacancies and those produced by plastic strain (cold worked metals) increase considerably the surface kinetics of sulfur segregation (in comparison with McLEAN's kinetics). The sulfur surface segregation kinetics, on cold worked structures presented two mechanisms : elimination of over saturated vacancies and sulfur atoms dragging by grain boundaries during recrystallization.

The results of mechanical testing after specific annealings can be explained by dynamic segregation mechanisms.

I

-

Introduction

The ability of sulfur to segregate at interfaces (free surfaces and grain boundaries) of nickel is a well known phenomenon (1). The intergranular sulfur segregation is responsible for two detrimental effects on the properties of nickel : intergranular corrosion and grain boundary embrittlement.

The purpose of this paper is to present some information obtained about sulfur segregation surfaces, using Auger Electron Spectroscopy (A.E.S.) to explain the results of mechanical testing on nickel INCO 270 (99,98%, 1 wt ppmS).

This work will be discussed in terms of thermodynamical and kinetical aspects, The thermodynamical approach will allow us to specify the notion of equilibrium segregation as well as non-equilibrium segregation that we will call "dynamic" segregation according to a short discussion in a previous paper (2).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990188

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Equilibrium segregation

The driving force of such a segregation arises from the difference between the equilibrium and actual concentrations in the boundary at time t :

[*"Thermodynamic" 1 ^a [C: (Equili.)

-

^C: (t)]

and cancels out when they become equal. (3).

It is calculated by minimizing the total free energy of the system : matrix

+

boundary :

This is ideal solution : McLEAN's model (4)

with Y, = C, / CS(nau) (segregation rate) (cf (1) and ( 6 1 , ch I).

The regular model for interfaces takes into account lateral interactions between solute segregated atoms with interaction coefficient (5).

with AG: = AGZ*

+

^{2 a:-S :Y (a:-s < 0 =} attractive ~nteraction) and a:-, = jy Z E

with S( AVOGADRO1s number

2: : coordinence of S in interface

e : regular solution parameter (interaction between the nearest neighbours).

Kinetic approach , : The solute heterodiffusion constant can he easily obtained from segregation kinetics using the following relation ( 4 ) :

. .. . ..

c

0 = a x LA1

with a = 2 (free surface) a = 4 (grain boundary)

d = segregated thickness = 0.25 nm (assumed approximately equal to 1 interatomic distance : cf (1).

We have proposed in (6) a further discussion about the assumptions for using the above relation:

it is a satisfactory approximation for usual conditions of segregations.

Non-equilibrium or Dynaaic Segregation

Mechanism and driving force. In non equilibrim segregation, the thermodynamic driving force does not originate in a difference between intergranular concentrations but between bulk concentrations or other bulk quantities (3). Thus, this driving force results from the difference of chemical potential between the metal containing reticular defects -with a concentration higher

than that of thermodynamic equilibrium

-

and the metal in the thermodynamic equilibrium. This mechanism is over when the bulk equilibrium is restored. However the grain boundary concentration is allowed to reach the value dictated by the law of equilibrium concentration, if this concentration was not reached before by non-equilibrium mechanisms.

These non-equilibrium segregations could be associated with several 'types of defects :

-

the vacancies in supersaturation resulting from quenching, sintering, irradiation, plastic deformation,

...

-

the dislocations during recrystallisation of a previously cold worked metal. The segregation is associated with dragging of impurities by moving grain boundaries. When the metal is recrystallised, the grain boundary segregations could be important.

We observed (2) these two mechanism relative to intergranular sulfur segregation in nickel 270.

2

-

Materials and experimental procedure

Investigated samples are 270 nickel quality (Wigging Alloys) obtained by sintering from excarbonyl powder. Purity is higher than 99,98%. This metal contains 1 ppm (wtppm) sulfur and the compldte analysis of this metal was published earlier(7).

Specimens were cold rolled (from initial material) and then annealed 4 hours at 1150°C in sealed quartz capsule (under a vacuum of 1 0 - ~ ~ a ) and finally air cooled.

Specimens were tempered either directly in the Auger analysis chambers ("in situ") by resistive heating or in sealed quartz capsules for electrical resistivity measurements.

The heating system and the sample dimensions (A.E.S.) were designed to allow the temperature to stabilize with in a few seconds.

Electrical resistivity measurements were carried out on sheets (0.3 X 2 X 50 mm) at 77 K and at room temperature.

The tensile tests were carried out in an INSTRON machine at strain of

;

= 1.1x10-~ S-' at 77 K and 293 K.

The A.E.S. system consists of a RIBER I.S.A. model ASC 2000 analyser with a cylindrical mirror analyser (C.M.A.) and coaxial gun.

The primary beam energy was Ep = 2.5 KeV and the modulation voltage 3V peak to peak. Sulfur concentration is obtained from the ratio hS/hNi ,(,, e,, of the peak to peak heights in the derivative AUGER spectra :

hs (152 e Y )

C, = K with K, = 0.398 (cf (1))

S hNi (848 eV) 3

-

Results and discussion Equilibrium segregation

Thermodynamical approach : the experimental results obtained at 95O0C, 900°C and.8OOoC show that sulfur segregates to surface of nickel 270 (Fig 1).

From the levels of equilibrium segregation

caUPf

at these temperatures, be calculated

S(&¶)

using FOWLER formalism :

.iurf = ^csurf / ^Csurf

surf

S ( E q ) ,(Flax) S ( M a w = 0 , 4 4 ) (Cf (l))

We obtained (6) : A =

-

¹⁴⁰

-

Zx(1.5) X kJ mol-l (FOWLER'S model).

Cl-566 COLLOQUE DE PHYSIQUE

asurf = ,

S-S 1.5 kJ mol-l would suggest that the attractive interactions between sulfur atoms in the surface are quite weak. However the regular solution model (FOWLER) gives only some information about the "balance sheet" of actual interactions (6). Moreover, on heterogeneous surfaces, &G can remain fairly constant, while OH and AS decrease simultaneously (8). MYAHARA (9) obtained OG~"'* =

-

135 kJ mol-l from sulfur surface segregation on polycrystalline nickel measured by AES

S

between 1073 K and 1223K (200 at ppmS).

Kinetic approach : According to equation [A], the values of are calculated from experimental curves of fig 1. These values are reported in table I with-other ones available from literature (cf ref [6], ch IV).

Table I

et al.

-

GRABKE 1.7 10-g

800 6.2 10"

The values of (Ni 270) are two orders of magnitude higher than the literature values. This abnormaly fast sulfur segregation is associated with a close to surface high density of defects (dislocations, vacancies). These defects would be generated by mechanical polishing or air cooling. However the value of D:"~~ at 720°C calculated from other A.E.S. measurements (12) on dilute alloy Ni-S (30 wtppms) is similar to the value given in the literature.

Dynamic segregation resulting from a flow of vacancies :

The quenching from 1150°C of a sample (1 mm thick) produces a supersaturation of point defects (vacancies or polyvacancies). The elimination of vacancies in quenched nickel occurs during the subsequent annealings (Fig 2). The electrical resistivity is restored at about 300°C. At temperatures upper than 300°C, sulfur surface segregation is observed. However kinetics are 102 or lo3 higher than those calculated by the equilibrium mechanism (13).

We observed intergranular segregation as if the segregations result from an equilibrium mechanism (6). However these segregations does not generate brittle failure. Thus, the ability of deformation is not significantly affected by the subsequent annealing after quenching (Fig 3a) while the failure stress decreases (Fig 3b).

SEM images of failure surfaces present a complex morphology : a brittle peripheral zone surrounding a ductile core. This feature explains the mechanical properties observed. In the core, the cooling rate is not high enough to generate the vacancy trapping and then the dynamic segregation does not occur. The time of annealing (1 or 2 hours) is too brief for an important sulfur intergranular segregation by equilibrium mechanism. This interpretation is confirmed by the results relative to a thin sample (0.5 mm) : the brittle zone reaches nearly the core.

Other experimental results are now available showing that the sulfur diffusion coefficients are equilavent on surfaces and grain boundaries. Probably, the same mechanism generates these two types of interfacial segregation.

Dynamic segregation generated by boundary motion :

On a nickel specimen, cold rolled from 0.7 mm to about 0.5 mm (i.e. a deformation of 30 X ) , we observed a superficial sulfur segregation in isothermal condition at 450°C (curve 1

-

^{figure 4)}

.

Such a temperature induces the primary recrystallisation of the metal. The apparent diffusion constant of sulfur extracted from the kinetic curve is 6 orders of magnitude higher than the heterodiffusion constant extrapolated from literature data.

If the same sample is heated at 500, 550, 600°C, each temperature step is responsible for a new segregation (see curves 2, 3, 4 on figure 4). These last segregations are not due to the recrystallisation of the nickel which was achieved by the previous annealing at 450°C, but they are concomitant with the grain growth as it can be seen on micrographs 1, 2, and 3.

The apparent diffusion constants of sulfur extracted from the kinetic curves are almost independant of the temperature. Concerning the grain boundaries, it is clear that sulfur is also dragged by their movement because of the lost of ductility observed on the metal during a tensile test. Presently, our results are too scattered to precise data about kinetics or mechanisms.

4

-

Conclusions

The sulfur surface segregation on Nickel 270 is controled by the equilibrium mechanism (FOWLER'S formalism) :

Using McLEAN's kinetic model, the apparent diffusion constant in bulk has been calculated from A.E.S. measurements.

Dynamic segregations are distinguished by faster kinetics (2 or 6 orders of magnitude) than equilibrium segregations.

At -low temperatures, when the equilibrium segregations are not observed in Nickel 270, ( Chulk

S = I w t p p m )

,

the dynamic segregations take place in the grain boundaries as well as on the surfaces. However, the reticular defects (vacancies, dislocations,...), which are the source of the driving force, need to be in the bulk in supersaturation.

Their effects on intergranular properties of nickel 270 are similar to those of equilibrium segregation (embrittlement, corrosion).

REFERENCES

1

-

A. LARERE, M. GUTTMANN, P. DUMOULIN, C. ROQUES-CARMES, Acta Met., 30, 685, (1982).

2'- G. SAINDRENAN, A. LARERE, Scripta Met., l8, 969, (1984).

3

-

M. GUTTMANN, "Advances in Mechanics and Physics of Surfaces", ed by R.M. LATANISION and A.J.

COURTEL, v01 1, (1981).

4

-

MC LEAN, "Grain Boundaries in metals", ch5, Oxford, (1957).

5

-

R. H. FOWLER, E, A, GUG~ENHEIM, "Statictical Thermodynamics", Cambridge University Press, (1965).

6

-

A. LARERE, Thbse Doctorat d'Etat Thesis, Universit6 Paris XI, (1983).

7

-

J. BARBIER-VITART, G. SAINDRENAN, A. LARERE, C. ROQUES-CARMES, J. Mat. Sci., 17, 387, (1982).

8

-

S. OUDAR, "La chimie des Surfaces", PUF (1973).

9

-

T. MIYAHARA, K. STOLT, D.A. REED, H.K. BIRNBAUM, Scripta Met,

a,

117, (1985).

10

-

A.B. VLADIMIROV, V.N. KAIGORODOV, S.M. KLOTZMAN, I.S. IRAMTENBERG, Fiz. Met. Metall.

a,

319, (1975).

11

-

S.J. WANG, H.J. GRABKE, Z. Metall., 61, 597, (1970).

12

-

S. BOUQUET, G. LORANG, J.P. LANGERON, P. MARCUS, J. Spec. Mic. Elec.,

1,

447, (1982).

13

-

F. FERHAT, D. ROPTIN, G. SAINDRENAN, Scripta Met.,

a,

223, (1988).

Fig 1. Nickel 270

-

Surface segregation Fig 2. isochronoal recovery curves electrical of sulfur (950°C, 900'C, 800°C)

-

Equili- resistivity ratio p 7 , / p s o 0 (tempering time :

brium Mechanism. 30 min).

Cl-568 COLLOQUE DE PHYSIQUE

Fig 3 : Variations of strain for failure ( a ) and failure stress i b i versus anneaiing temperature. (deformation temperature iik).

Fig 4 : Surface segregation of sulfur on cold-worked nickel. lsothermal kinetics

during recrystallisation and grain growth stages.

Photo n" 1 : Annealed 450°C.

photo N o 2 : Annealed 5 5 6 ° C Photo N" 3 : Annealed 600°C

Photos n" 1 , 2 , 3 MICROSTRUCTtiKE OF N l c h ~ ~ 270 ( s . E . M . ) .