23. - 25. 5. 2012, Brno, Czech Republic, EU
AES ANALYSIS OF SULFUR GRAIN BOUNDARY EQUILIBRIUM SEGREGATION IN NICKEL
WITH SMALL BULK CONTENT IN SULFUR
Djamal Boutassouna(1), René Le Gall(2), Ibn Khaldoun. Lefkaier(3)
(1), (3) LSF, Université de Laghouat, Laghouat 03000, Algeria. email: [email protected]
(2) LGMPA, Ecole Polytechnique de l’Université de Nantes, France. [email protected]
Abstract
Auger electron spectroscopy has been the main method used to study equilibrium segregation of sulfur to
the grain boundaries of a metallic Nickel based system having composition of 0.8 mass ppm sulfur.
All samples were first annealed at adequate temperatures for sufficient equilibrium time then quenched in
water at room temperature. The influence of the annealing temperature on the grain boundary segregation of
sulfur is investigated.
For this system with 0.8 mass ppm bulk sulfur concentration, Auger quantitative analysis reveals that only
samples annealed at 650 °C show existence of sulfur in their grain boundaries. The average grain boundary
composition for this temperature is estimated to 15.08 % atomic of sulfur which is in agreement with other
studies on the critical sulfur concentration for the embrittlement of Nickel. The other samples annealed at
higher temperatures exhibit all ductile Fractures.
Keywords: sulfur segregation, grain boundary, Auger spectroscopy, in-situ fracture, brittle.
1. INTRODUCTION
For decades segregation phenomenon has been the subject of several theoretical and experimental studies.
The interest in this phenomenon is consistent with the rapid progress of the technology and the materials
science. Nowadays, the tendency to the conception of new materials is obviously eminent from one day to
another. In this way, surface and interface phenomena play a determinant role, and the understanding of the
segregation phenomenon acquires a great practical importance. Indeed, this phenomenon often occurs
during the preparation of materials for various applications in industry and technology. Grain boundary
segregation affects continuously and in a remarkable way the behavior of the elaborated materials when
faced to mechanical, chemical, electrical or even magnetic external stresses.
The experimental study of grain boundary segregation in the last decades has been carried out using a
variety of methods. Auger electron spectroscopy (AES) along is the most important technique that has been
often the tool to quantify grain boundary segregation. Although the spectroscopic measurements of the grain
boundaries composition, obtained through in-situ fracture along the boundaries, give fairly accurate results,
such experiments are not reliable when the material is not likely to be intergranularly fractured. Under these
conditions, an idea of segregation to grain boundaries can be obtained by direct analysis of free surface
segregation sample [1-3]. Nevertheless, Auger spectroscopy analysis proved to be a powerful tool in
materials science and especially in the field of grain boundary segregation analysis.
2. STUDIED NI(S) SYSTEM
The nickel-based alloys are model systems for studying the effects of impurity elements (few ppm) on the
general behavior of metals. Indeed, Nickel with its crystallographic and physico-chemical properties, is very
similar to austenitic stainless steels with the advantage of the not exhibition of phase transformations [4].
23. - 25. 5. 2012, Brno, Czech Republic, EU
Due to the segregation of sulfur to the grain boundaries of nickel, this later becomes more fragile and
particularly susceptible to in-situ fracturing [5,6,7].
In this work we were interested to the experimental study by AES of sulfur segregation to the grain
boundaries of a nickel based alloy carried out at an appropriate temperature range. The Ni(S) studied
system (noted SV220) is a synthetic Nickel based matrix, prepared in vacuum oven then hot-rolled in the
form of tape of 8 cm width and 3 mm thick. The chemical composition is presented in table 1.
(Tab. 1) Chemical composition of the SV220 Nickel based system
Element Ni
S
Al
O
N
C
Mg
Si
ppm
0,80
150
5
~0,4
~25
1,3
83
Cr
Ti
Mn
Fe
Co
Cu
2.0
0,19
0,87
45
4,3
2,0
(mass)
3. PREPARATION OF SAMPLES FOR ANALYSIS
3
The material (metallic alloy SV220) is cut in the form of bars with the dimensions 20x8x3 mm , then polished,
placed in vacuum sealed ampoules and annealed at different temperatures ranging from 600 to 850 °C [7].
The annealing time is determined using the kinetics heterodiffusion properties of sulfur in Nickel [8]. In this
way, it is necessary first to calculate the coefficient of diffusion for every temperature; then using the diffusion
kinetics model, the value of the annealing time is determined. The values of the obtained annealing time for
each temperature are gathered in table 2.
(Tab. 2) Annealing time for the different studied temperatures.
T (°C)
850
800
775
750
725
t (h)
44
140
288
144
720
700
650
1586
4968
After this stage, the annealed samples are subsequently quenched at room temperature in water.
4. AUGER SPECTROSCOPY ANALYSIS
In order to evaluate the sulfur concentration in the grain boundaries of our samples, test tubes long of 15 mm
and thick of about 0.4 mm are prepared (figure 1). Notches in the middle of the thin part of the tube are
achieved to facilitate the fracture in-situ.
0,4 mm
Notches
15 mm
10 mm
(Fig. 1) Schematic illustration of the test tube for in-situ AES analysis
23. - 25. 5. 2012, Brno, Czech Republic, EU
The sampled are in-situ cold fractured in vacuum. The achievement of temperatures values of about -180 °C
was insured by a liquid nitrogen flow. Upon this cold fracture, the sample is transferred into the analysis part
of the AES spectrophotometer.
5. GRAIN BOUNDARY ANALYSIS RESULTS
According to the chosen values of annealing temperatures that range from 650°C to 850°C, 7 Ni(S) samples
were submitted to the Auger grain boundary analysis. Figure 2 shows some images obtained upon cold
fracture of the samples which are subsequently analyzed by Auger spectroscopy.
(Fig. 2) SEM images obtained upon cold fracture of NiS samples annealed
at different temperatures
As it can be noted, the sample annealed at 850 °C shows complete ductile fracture wich is agreement with
other works for nickel-sulfur systems [9]. On the other hand, it can be observed as well that the samples
annealed at temperatures ranging from 700 to 800 °C show also absolutely ductile fracture. The fracture
surfaces for these samples do not present sulfur rich facets that may insure brittle fracture of the grain
boundaries. The only sample that presents a partial intergranular fracture is that one annealed at 650 °C
(figure 3).
(Fig. 3) Auger image obtained upon cold fracture of NiS (0.8 atom ppm) sample
annealed at 650 °C, “p” refers to “point” and indicates the analysis position
23. - 25. 5. 2012, Brno, Czech Republic, EU
d(E.N)/dE
d(E.N)/dE
We believe that the value of the amount of sulfur in the grain boundaries in the samples plays the main role
in the mode of fracture, that is, to obtain a brittle fracture, the amount of sulfur in the grain boundaries should
reach a certain threshold istimated in some works to 15.5 ± 3.4 at. %. [10,11]. Figure 4 presents the
derivative spectrum obtained for the sample annealed at 650 °C.
E
E (eV)
(eV)
(Fig. 4) Auger derivative spectrum for NiS (0.8 ppm) sample annealed
at 650 °C, and cold fractured in-situ
The amount of sulfur in the grain boundaries is calculated according the following formula [12]:
HS
XS 
S
HS
S

(1)
H Ni
 Ni
where : X is the atomic concentration in the grain boundary,  the sensibility parameter with : S=13 ,76
and Ni=1,53. We have calculated the average sulfur grain boundary concentration for more than 22 analysis
points for this sample and the value obtained upon calculation of the standard deviation is 15.08 % atomic.
Table 3 shows the results and the remarks brought about the analysis of our Ni(S) system.
(Tab. 3) Results of Auger grain boundary analysis of the Nickel based system with 0.8 mass ppm of sulfur.
Annealing
850
800
775
750
725
700
650
temperature (°C)
Result / feature
ductile
ductile
ductile
ductile
ductile
ductile
15.08
Fracture Fracture
Fracture
Fracture
Fracture
Fracture
%partial
The main feature with this system is the failure to achieve intergranular fracture of the sample annealed
above 650 °C. We underline here that for each annealing temperature we have prepared at least 3 samples
and in cases that show unusual behavior, we were obliged to redo the analysis, however the result was the
same. The real mechanism of this behaviour is still misunderstood because of the difficulty to obtain
intergranular fractures in vacuum, and also because in our case, the brittle feature of the grain boundaries is
only partial. Nevertheless, one can note the increase of the ductility with the augmentation of the annealing
temperature.
23. - 25. 5. 2012, Brno, Czech Republic, EU
In iron and nickel based alloys containing sulfur, cracking is believed to occur often by S segregation and
induced decohesion. However, this embrittling effect of sulfur can be affected by the interaction of the other
alloying elements and impurities such as manganese and chromium. In iron based alloys, the presence of
manganese considerably decreases the solubility of sulfur. However, it has been shown that in nickel based
alloys, manganese has no effect on sulfur segregation [13]. the only reason that only 650°C exhibits partial
brittle fracture while at higher temperatures only ductile feature are seen, is the very small bulk content in
sulfur in our system.
6. CONCLUSION
Equilibrium segregation of sulfur to the grain boundaries of metallic nickel depends on its content in the bulk.
In ou work, Auger electron spectroscopy has been the main method to study equilibrium segregation of
sulfur to the grain boundaries of a metallic Nickel based system with sulfur bulk content of 0.8 mass ppm.
The main feature is the failure to achieve intergranular fracture of the sample annealed at temperatures
higher than 650 °C where the samples exhibit all ductile Fractures. For the sample annealed at 650, Auger
quantitative analysis provides an estimation of an average grain boundary composition of about 15.08 %
atomic of sulfur which is in agreement with other studies on the critical sulfur concentration to the
embrittlement of Nickel.
ACKNOWLEDGMENT
The experimental work has been realized in the LGMPA laboratory (Nantes-France) and sustained by
the OTAN (Grant Reference PST.CLG.979608). We also express gratitude to Mr. Thierry BROUSSE for
his kind reception in the LGMPA.
LITERATURE
[1]
J. Woodward and G. T. Burstein, Metal Sci. 14, (1980) 529.
[2]
C. Yen, W. R. Graham and G. R. Belton, Metall. Trans. 9A, (1978) 1461.
[3]
J. J. Burton, B. J. Berkowitz and R. D. Kane, Metall. Trans. 10A, (1979) 677.
[4]
Saindrenan, R. Le Gall, F. Christien, Endommagement interfacial des métaux, coll. Technosup, Ellipse, 2002.
[5]
L. Ben Mostepha, G. Saindrenan, N. Barbouth, A.M. Brass, J. Chêne, Scripta Metall. Mater. 24 (1990) 773.
[6]
R. Le Gall, G. Dazelle, O. Danylova, S. Witzke, Mater. Sci. Forum 426–432 (2003) 1041.
[7]
Larère, A. thèse de doctorat d’état, University of Paris XI Orsay, 1982.
[8]
AB Vladimirov, VN Kaigorodov, SM Klotzman, IS Trakhtenberg” diffusion of Sulphur in Nickel” Fiz. Met. Met. 39:
319-323.
[9]
M Cornen "Ségrégations intergranulaires d'impuretés dans le système modèle Ni-S" - ED STIM 2006.
[10]
M Yamaguchi, M Shiga and H Kaburaki, "Grain Boundary Decohesion by Impurity Segregation in a NickelSulfur System", Science 21 Vol. 307 no. 5708 pp. 393-397, 2005
[11]
J. K. Heuer, P. R. Okamoto, N. Q. Lam, J. F. Stubbins, J. Nucl. Mater. 301, 129 (2002).
[12]
Lawrence E. David, et al, Handbook of Auger Electron Spectroscopy, second ed., Physical Electronics
Industries, Inc.
[13]
Lambret Eric, Saindrenan Guy « cent ans d’Invar » LGM 1996.