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AUGER ELECTRON SPECTROSCOPY STUDY OF GRAIN BOUNDARY AND SURFACE SULFUR
SEGREGATION ON SINTERED STEELS
C. Carbonnaux, R. Dessieux, G. Cizeron, A. Larere, T. Nguyen, G.
Saindrenan, D. Roptin
To cite this version:
C. Carbonnaux, R. Dessieux, G. Cizeron, A. Larere, T. Nguyen, et al.. AUGER ELECTRON SPECTROSCOPY STUDY OF GRAIN BOUNDARY AND SURFACE SULFUR SEGREGA- TION ON SINTERED STEELS. Journal de Physique Colloques, 1990, 51 (C1), pp.C1-789-C1-793.
�10.1051/jphyscol:19901123�. �jpa-00230032�
COLLOQUE DE PHYSIQUE
Colloque Cl, suppl6ment au n o l , Tome 51, janvier 1990
AUGER ELECTRON SPECTROSCOPY STUDY OF GRAIN BOUNDARY AND SURFACE SULFUR SEGREGATION ON SINTERED STEELS
C. CARBONNAUX, R. DESSIEUX, G. CIZERON, A. LARERE*, T.T. NGUYEN", G. SAINDRENAN*** and D. ROPTIN***
Structure des Materiaux Metalliques, 1.9~4, Bdt. 465, Universite Paris
$1, F-91405 Orsay Cedex, France
Laboratoire d e Mdtallurgie Structurale, ISMA, URA 1107, ~ d t . 413, ypiversite Paris XI, F-91405 Orsay Cedex, France
Centre des Materiaux, ENSMP, BP. 87, F-91003 Evry Cedex, France
* * *
Science des Materiaux de la Mecanique, ENSM, 1 Rue d e la Noe, F-40072 Nantes Cedex 03, France
Rksumk : Nous avons Btudik la fragilisation par du soufre skgrkgk d'aciers frittks B 1120°C pendant 30 minutes (Fe-4% Ni - 2% Cu
-
0,5 % MO - 0,3 % S-
0,5 % C). Dans ces aciers, le soufre amkliore I'usinabilitk et accklbre les processus de frittage par formation d'une phase 1iquide.La fragilitk intergranulaire a Ctk ktudi6e "in situ"par spectroscopie d'klectrons Auger (A.E.S.). La skgrkgation du soufre fut rkv86e par bombardement d'ions argon sur la surface de rupture.. En utilisant le modble de MC LEAN, nous avons dktermink l'enthalpie libre de skgrkgation : AG: B = - 88400J.m0l-~. Cette valeur est en accord avec celle obtenue par Seah et Hondros (en 1973) pour le Fe,: AGZ B = -84000J.m0l-~
Les Bchantillons pour l'etude de la sCgrCgation du soufre en surface ont CtC laminBs et recuits a 925°C durant 15 minutes avant de prockder aux analyses Auger.Les mesures de cinktique de skgregation du soufre en surface nous ont permis de dkterminer le coefficient de diffusion "apparent" de S comme ktant Bgal B 3.10-13et3, 8 . 1 0 - ' ~ c r n ~ . s - ~ B 400 et 500°C respectivement. Ces valeurs sont plus grandes que celles trouvkes pour D V mais sont du meme ordre de grandeur que D J . La taille de grain moyenne ktant kgale B lOpm, la skgrBgation du soufre en surface pourrait donc avoir comrne principale source la diffusion intergranulaire.
Abstract : The purpose of this paper is to study the embrittlement of sintered steels (Fe - 4% Ni
-
2% Cu - 0,5% MO - 0,3% S-
0,5% C) at 1120°C during 30 minutes. In these steels, the sulfur improves usually the machinability and decreases the temperature when the shrinkage occurs. The intergranular brittleness was studied "in situ" by A.E S. The S segregation was revealed by ion sputtering of the intergranular surface. MC LFANS's approach was used to determine the free energy of segregation, A G $ . ~ . = -88400Jmol-l. This value is close t o the value obtained by SEAH and HONDROS (1973) in Fe(&) : AG$ B = -84000Jmol-l.The samples for the free surface study were cold rolled and annealed at 925°C (15 mn) before analyses by A.E.S. The kinetic measurements allowed us to determine the "apparent" S diffusion coefficient as being equal to 3 ~ 1 0 - ' ~ and 3,8 X 10-'~crn~s-'at 400°C and 500°C respectively. These values are greater than the values found for S bulk diffusion coefficient but they are close t o intergranular diffusion coefficient. The average grain size is equal to 10pm ; this could explain that the surface sulfur segregation comes rather from the grain boundaries than from the bulk.
l - Introduction
Formally, in conventional metallurgy, additions of sulfur have sometime been achieved for long time during steelmaking practice to improve the machinability. In the case of sintered low alloy steels (Distaloy), we observed that a weak sulfur addition could produce a beneficial effect on the sintering processes, because there is an exothermic eutectic reaction between sulfur and iron on heating. Rapid sintering results from the formation of a liquid phase between these elements. With liquid formation there is rapid initial densification due to the capillary force exerted by the wetting liquid on the solid particles. The liquid phase enhances the rate of interparticle bonding during sintering. Accompanying interparticle bonding, there are significant changes in the pore structure and compact properties, including strength and ductility (1).
But such powder-metallurgy steels tend to fracture in a brittle manner along the grain boundaries rather than in a transgranular mode.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19901123
cl-790 COLLOQUE DE PHYSIQUE
This paper summarizes a part of a experimental study that determined the segregation of sulfur to grain boundaries and to free surfaces using Auger Electron Spectroscopy (A.E.S.).
2
-
Experimental ProcedureMaterials : Distaloy "A.E." partially prealloyed iron powder, containing nominally 4% nickel, 2% copper and 0,576 molybdenum, GUIMARD graphite, which has a residual ash contents of about 3% and sulfur powder or iron sulfide (FeS) were used to prepare samples. The concentrations of the principal impurities determined by spark mass spectroscopy are given in Table 1. The base of the prealloyed powder is HOGANAS ASC 100-29 atomized iron powder. Groups of compacts with graphite contents of "0%
"
(30 wt ppm) and 0,5%(wt) and with sulfur contents of "0%
"
(15 wt ppm), 0,15% (wt) and 0,30% (wt) were prepared. After sintering produced steels contain approximately 0,01 % and 0,40% carbon.Preparation and heat treatment of compacts : the iron powder-graphite mixtures with or no sulfur addition were mixed in a small ball mill for 30 minutes before compression to ensure uniform graphite and sulfur distribution. The "green compacts" were compressed under 600 MPa. Compacts were sintered singly at 1120°C for 30 minutes in cracked amnonia atmosphere, in a continuous industrial furnace.
Table 1 . Impurities contained by Distaloy A.E.
Mechanical Tesiing : The tensile tests were carried out in a DARTEC machine at strain rate of i = 1, l10-3sec-1, at room temperature.
Auger electron spectroscopy : Chemical was accomplished on freslliy prepared fractures in sintered material by A.E.S. Notched cylindrical specimens 4 mm in diameter by 25 mm in length were installed in the ultrahigh vacuum system and fractured by impact, thus generating a clean (i.e. uncontaminated) surface for analysis.
For free surfaces analysis, specimens were cold rolled from 1,l mm thickness t o about 0,6 mm, i.e. a deformation of 0,8, then polished and rinsedin pure acetone. These flat samples were annealed at 925OC for 30 minutes in sealed quartz capsules under a vacuum of 10-3Pa. For A.E.S experiments, a 2 X 3 mm sample is spot welded to a strip of resistively heated tantalum inside the Auger vacuum system. The temperature is measured with thin wire (50pm) thermocouples (chromel-alumel) spot welded to the sample. Such a device allows the temperature to rise 550°C within a minute.
The intergranular and surface segregations of the sulfur were studied by A.E.S. in a C.M.A. RIBER ISA Model A.S.C. 2000 analyses. The main characteristics of the analysis are : 3 keV*primary beam energy ; 0,l pA primary beam current ; 1 pm beam diameter ; 5.10-~ Pa residual pressure. All spectra were recorded in the differential mode dN(E)/dE. Quantification of the resulting spectra followed standard procedures (2) and basically assumes that the peak-to-peak height of a given characteristic Auger peak is proportional to the atomic concentration of the originating species on the surface being analysed. Concentration profiles were obtained away from exposed fracture surface by alternate use of A.E.S. and argon ion bombardment (600 V, 2 pA) under an argon pressure of 6.10-3Pa.
The superficial sulfur concentrations are calculated assuming :
. . r j
~ ~= k y r r j . H s ( m e v ) f
HP.(TOJ~V)
If we assume that sulfur has segregated in the first atomic layer, the calibration constant can be calculated using the following equation (3) :
~ y f
=ky
= 0,358w ~ t h a = 0,25 nm ; X = 0,50 nm and cos 0 = 0,74 3
-
Grain boundarv searenation - Experimental results.TensiIe t e s t s results : The results of the tensile tests a t room temperature are given in table 2. We observed that iron sulfide (FeS) addition enhances the tensile strength and decreases the elongation.
The Scanning Electron Microscopy (S.E.M.) observations of the fracture of the tensile test specimens show that the fracture is nearly 100 pct intergranular in the case of the iron sulfide addition (Fig 1) and at least 50 pct grain boundary character in the case of the sulfur ("fleur") addition.
Thus, the fresh fracture surfaces analysed by A.E.S. are primarily of grain boundary character.
Composition of the grain boundaries : Concentration profiles are outlined in Fig 2 . Both segregated and precipitated sulfur are seen. In order to confirm this interpretation of the sputtering profiles and to determine the nature of the sulfur-containing phase at the grain boundaries, the X-ray microanalysis by microprobe were achieved from the fracture surfaces. The only structure identified was that of sulfide MS. The &I element was essentially Fe, Ni(5 pct) and Cu (3 pct) (3).
Table 2 : Tensile test results
The profiles showing a rapid decrease followed by a long plateau above nominal level were interpreted as indicating that the boundary contained segregated S together with sulfides, and the segregated S coverage was calculated as (3) :
Dista1oyA.E.
Dista1oyA.E.
+
0,3%S(" fleur") Dista1oyA.E.+
0, 3%("FeS1')plateau
c y g =
ty [(z)'"" - (S) ]
the average contribution from the integranular sulfides and the matrix being
plateau
c $ l k =
(E)
with : Kgorn = 0,176The average value of the surface concentration detected on the fractured grain boundary surface is :
% S wt 0,0015
0,3 0,3
Usually,
' : C
represent the half of the intergranular sulfur concentration C$.B before fracture which is equal to :According t o MC Lean model (4), the relationship between the equilibrium intergranular concentration and
c ; o l i d a o l is :
H V 130 135 145
where
~ 2 . ~ .
is the ratio between the number of solute atoms present at equilibrium in the grain boundaries and the number of the sites available for these atoms in the grain boundary.o m ( M P a ) 460 430 490
AGg.B. is the grain boundary segregation energy. This value was determined at 1393 K.
This estimated value is close to the value determinated by Seah and Hondros (5).
Cl-792 COLLOQUE DE PHYSIQUE
Comment : Although both carbon and oxygen are detected, it is believed that contamination from the resid- ual atmosphere contributed to the quantity measured. Note that for several of the analyses, the quantities of oxygen and carbon increase as layers of metal are sputtered away ( 6 ) . It is also noteworthy that P, MO, Ca, Ni and Cu were detected on the grain boundaries.
4
-
Surface segregation-
Ex~erimental results.The kinetic experiments were carried out at 200, 300, 400, 500 and 550°C.
The "apparent" diffusivity constant of sulfur in iron was determined according to the following equation :
with a = 2,5.10-"cmand~?'~ = k y g . Hs(13a'V' H F ~ ( . I O J ~ V )
The main results are reported in table 3 together with the sulfur grain boundary and bulk heterodiffusivity constants in iron, calculated from equations proposed by (7) and ( 8 ) .
Table 3
Between 400 and 550°C, the values of D* are 2 to 3 orders of magnitude higher than the bulk heterodiffusion constants. However, they are close t o the grain boundary heterodiffusion constants.
Apparent diffusion constants and heterodiffusion constants of sulfur in iron
As soon as 500°C, the amount of surface segregated sulfur nearly reaches the saturated surface concentration value, i.e. 0,47. According t o Me Lean model and with
AGT;~~)
= -165000J.rnol-~ (g), we calculated Y:"'~ and thus : yiUrf N 1.Conclusions
~ v ( c n ~ . s - ' ) 6,2.10-'~
3,7.10-'g 2,7.10-l6 2,9.10-l4 2 , 0.10-l3 T ( O C )
200 300 400 500 550
The sulfur usually embrittles the sintered steels whatever the sulfur content.Thus 15 wt ppmS are suficient t o induce an intergranular Fracture in these steels.
The sulfur intergranular segregations are controlled by equilibrium me'chanism (MC Lean) and
The A.E.S. measurements on surfaces show that the sulfur atoms would supply the surface by grain bound- aries D ~ a r e n i ' '
"
D J ( s )D *
(crn2.s-l) 1 , 8.10-l4 4,0.10-l*3,0.10-'~
3, 8.10-l2 9,4.10-l2
Acknowledements
~ , ( c r n ~ . s - ' ) 3,5.10-l7 7 , 7.10-l5 3,4.10-l3 5,5.10-l2 1, 7.10-l1
The authors gratefully acknowledge the technicial assistance of J.P. Roche for surface Auger electron analysis and the help of J . Jodet (INSTN, Saclay), for the S.E.M.
References
1 -
R.M.
German, "Liquide phase sintering", New York, Plenum Press (1985).2 - L.E. Davis, N.C. Mac Donald, P.W. Palmberg, G.E. Riach and R.E. Weber : Handbook of Auger Electron Spectroscopy, 2 nd ed., Physical Electronics Industries, Inc., Eden Prairie, 19'76.
3 - P. Dumoulin, M. Guttmann, M. Foucault, M. Palmier, M. Wayman and M. Biscondi, Metal Sci. 14, 1 (1980).
4
-
D. Mac Lean, Grain Boundaries in Metals, Clarendon Press, Oxford (1957).5 - M.P. Seah and E.D. Hondros, Proc. R. Soc, 355, 191, (1973).
6
-
S.M. Tuomiven and S.P. Clough, Met. Trans, Vol 10A, 127, (1979).7 - G. Seibel, Mem.Sc. Rev. Met, 6l, 413, (1964).
8 - P.L. Gruzin, J.F. Kononiuk, M.M. Pavliuchenko and J.A. Polikarpov, Inzh. Fiz. Zh, 1, 64, (1958) 9
