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Submitted on 1 Jan 1988
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PREFERENTIAL FIELD EVAPORATION DURING ATOM PROBE ANALYSIS OF ZIRCALOY-4
B. Wadman, H.-O. Andrén, U. Rolander
To cite this version:
B. Wadman, H.-O. Andrén, U. Rolander. PREFERENTIAL FIELD EVAPORATION DURING
ATOM PROBE ANALYSIS OF ZIRCALOY-4. Journal de Physique Colloques, 1988, 49 (C6), pp.C6-
323-C6-327. �10.1051/jphyscol:1988656�. �jpa-00228153�
PREFERENTIAL FIELD EVAPORATION DURING ATOM PROBE ANALYSIS OF ZIRCALOY-4
B. WADMAN, Ha-0. ANDREN and U. ROLANDER
Department of Physics, Chalmers University of Technology, S-412 96 Gbteborg, Sweden
Abstract
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Previous experiments using atom probe analysis to determine the matrix composition of Zircaloy-4, a zirconium alloy used for nuclear fuel tubes in pressurised water reactors, showed a higher Sn concentration in some analysis, compared to the chemical analysis of the materials studied. This effect had no evident metallurgical explanation, and was considered to depend on preferential field evaporation of zirconium. A study has been performed in order to reduce the loss of zirconium between pulses. By varying the conditions of analysis, i.e., temperature and pulse fraction, it was found that the correct values for the matrix composition were obtained using a temperature of 60 K and a pulse fraction of 15 percent of the holding field.1
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INTRODUCTIONIn order to understand the relation between heat treatment and corrosion resistance of nuclear fuel tubes of Zircaloy, a thorough knowledge of the microstructure in the alloy is needed. The formation of intermetallic precipitates such as Z~-(l?e,Cr)~ has been carefully studied recently [I-31. The depletion of the matrix due to this precipitation is, however, difficult to examine with other microanalytical methods than the atom probe. The small interprecipitate distances (down to 0.1 pm), and the low concentrations of Fe and Cr added to the alloys (0.2 and 0.1 weight percent respectively), demands a combination of spatial resolution and detection limit that is only fulfilled by atom probe analysis. Therefore, a method for analysing the matrix composition between precipitates in Zircaloy-4 (Zr-4) materials has been developed [4,5].
Analysis has been performed of two Zr-4 materials with very different heat treatment history and corrosion properties 151. The temperature of these analyses was 90 K, and the pulse fraction was 15 percent of the holding field. The results of these analysis showed an enhanced concentration of Sn in the matrix of one material, compared to the chemical analysis of the tube studied, Table 1. As the C and 0 concentrations were also higher than expected from the chemical analysis, the effect was considered to be caused by preferential field evaporation of zirconium. The data were therefore compensated for the loss of zirconium, by using the chemically determined Sn concentration as a reference. As the major reason to perform the analysis was to measure the Fe and Cr concentrations in the matrix, compensations had to be made in two ways; assuming that only Zr was lost between pulses, and assuming that also Fe and Cr were preferentially field evaporated.
To obtain a safer method of analysing Zr-4, a study has been performed by varying the conditions of analysis. The object has been to find a reliable method for analysing ZI-4 without losing zirconium between pulses.
2
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EXPERIMENTALThe study has been performed on a material with a composition given by Table 1.The method used to prepare and examine the specimens has been described elsewhere [4,5]. The atom probe used in this study has been described before [6,7].
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988656
C6-324 JOURNAL DE PHYSIQUE
After being introduced in the atom probe, the specimens were field evaporated, until the metal was freed from the oxide layer produced during specimen preparation. This was done at a temperature of 90 K while observing the specimen by field ion microscopy, using Ne as image gas. Then atom probe analysis was performed using a series of different specimen temperatures and pulse fractions.
The temperatures used were 3 5 , 4 5 6 0 and 90 K. The cooling of the specimen was performed with a cold helium gas cryostat. A pulse fraction (pulse amplitude divided by standing voltage) of 15% was used. At 90 K, analysis at a pulse fraction of 25% was also performed.
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RESULTSThe results of varied temperature of analysis are shown in Table 2. The analyses at 35 K and 45 K are composed of a small number of ions, because of the high occurrence of specimen fracture at low temperatures. When analysing at 60 K, it was easier to obtain a sufficient number of ions to achieve the statistical accuracy needed. The values for Sn, C and 0 were compared with the chemical analysis in Table 1. At this temperature, the Sn concentration in the matrix agreed within statistical limits with the chemical analysis.
When the pulse fraction is increased in order to decrease the field between the pulses, another effect, field adsorption of residual gases on the specimen, can be suspected to occur. This is shown by a higher 0 concentration in the analyses at a pulse fraction value of 25 percent, Table 3.
The two straightfomard methods to decrease the field evaporation of zirconium between pulses used in this study, decreasing the temperature and increasing the pulse fraction, both have disadvantages. The lower temperature demands a higher electric field in order to get the same rate of field evaporation. This increases the absolute difference between the holding field and the pulse fie1d;which is desired. However, the increased holding field also produces higher mechanical stress on the specimen during analysis. This results in an increased risk of specimen fracture, which is highly unwanted for a material with a high incidence of specimen fracture already at 90 K, as is the case for 2-4.
By increasing the pulse fraction, one can obtain a higher difference in field evaporation rate between holding field and pulses without increasing the mechanical stress. However, as the field between pulses becomes lower, the risk for field adsorption of residual gases on the specimen will increase.
The observed increase in 0 concentration during analysis with a pulse fraction of 25 percent is assumed to result from field adsorption of residual gases. This is generally an undesired effect in atom probe analysis, but has a special significance for the analysis of Zr-4 matrix depletion of Fe. Any risk for adsorption of CO, which may field evaporate and appear in atom probe spectra at mass-to-charge ratio 28, would highly affect the possibility of measuring small concentrations of Fe. At the evaporation field of 21-4, Fe normally field evaporates as Fe2+, with its most abundant isotope at m/n = 28. For this reason, we decided not to increase the pulse fraction during analysis.
So to obtain both a low mechanical stress on the specimen and negligible field adsorption of residual gases, we have decided to use the following analysing conditions for Zr-4: Removal of the oxide at T > 90 K, and atom probe analysis at 60 K, using a pulse fraction of 15 percent.
In order to facilitate the specimen cooling process, a closed-circuit helium refrigerator has been installed.
The refrigerator is connected to the vacuum vessel by a bellows assemblv. Firmre 1. ~ermittine the removal of the entire refrigerator during 250 O C bake-out of the vacuum system.
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CONCLUSIONSA study has been performed concerning the influence of specimen temperature and pulse fraction on the preferential evaporation of zirconium in Zircaloy-4.
By decreasing the specimen temperature, preferential field evaporation of zirconium was avoided.
Decreasing the temperature below 50 K resulted in an increased occurrence of specimen fracture.
Zirconium alloys were found to be very sensitive to the high mechanical stress produced during atom probe analysis.
Conditions of analysis giving accurate results without increased risk for tip fracture or field adsorption of residual gases are the following:
1) Removal of the anodic oxide at a specimen temperature of 90 K 2) Atom probe analysis using a pulse fraction of 15 percent at 60 K.
Acknowledgements
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This work was supported by ABB ATOM AB, AB Sandvik Steel and the Swedish State Power Board.REFERENCES
[I] Grange, J.M., Charquet, D., and Molin, L., in Zirconium in the Nuclear Industry, ASTM STP 754, American Society of Testing and Materials (1982) 96
[2] Charquet, D. and Alheritiere, E., in Proceedings, KTG Specialists Workshop on Second Phase Particles and Matrix Properties of Zircaloys, Erlangen, E. Baroch and H. Weidmger, Eds., Kerntechnische Gesellschaft, e.V., Bonn (1987) 9
[3] Andersson, T., Thomaldsson, T., Wilson, A., and Wardle, A.M., in Improvements in Water Reactor Fuel Technology and Utilization, IAEA-SM-288159, Vienna (1987) 435
[4] Andrkn, H.-O., Mattson, L. and Rolander, U., J. de Physique 47 (1986) C2-191 [S] Wadman, B., Rolander, U., and Andren, H.-0.. J. de Physique 48 (1987) C6-299 [6] Andrkn, H.-O., and NordCn, H., Scand. J. Metall. 8 (1979) 147
[7] Andrkn, H.-O., J. de Physique 47 (1986) C7-483
I I
Table 1. Chemical analysis ofstudied Zr-4 fuel tube
I I
Element Composition, wt.%
1 . 6 0 0 . 1 9 5
Z r b a l .
C6-326 JOURNAL DE PHYSIQUE
Table 2. Variation of température of analysis
Température of Analysis
Bernent
Sn ft Cr 0
c
2r Sl+N
Number of Ions
35K
0.93 ± 0.53 0.15 ± 0 . 1 5
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0.13 ± 0 . 0 7
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98.82 ± 0.88
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400
45 K
2.04 ± 0 . 4 7
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0.04 ± 0 . 0 4 0.14 ± 0 . 0 5 0.01 ± 0.01 97.74 ± 0 . 4 7
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1496
60 K
1.73 ± 0 . 2 2 0.04 ± 0 . 0 1 0.01 ± 0.01 0.14 ± 0 . 0 2 0.03 ± 0.01 98.05 ± 0.29
0.01 ± 0.01
8192
90 K
2.21±0.16 0.06 ± 0.02 0.03 ± 0 . 0 1 0.17 ± 0 . 0 2 0.05 ± 0.01 97.48 ± 0 . 1 7 0.01 ± 0.01
11274
Table 3. Variation of puise fraction
Puise fraction, T = 90 K
Bsrrent
S n Fa Cr O C 2s Sl+N
Numbei of Ions
15%
2.21±0.16 0.06 ± 0.02 0.03 ± 0.01 0.17 ± 0 . 0 2 0.05 ± 0.01 97.48 ± 0 . 1 7 0.01 ± 0.01
11 274
25%
1.28 ± 0 . 2 1 0.14 ± 0 . 0 5
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0.26 ± 0.03 0.03 ± 0.01 98.30 ± 0.29 0.02 ± 0.01
3 613
Refrigerator stage 1 77 K
11 I
Copper braid\\
Fig. 1 -Refrigerator connection. The retaining rings prevent bellows from deforming dwing removal of the refigerator. A good heat transfer between the base of the bellows arrangement and stage 2 of the refrigerator is achieved by spring loading and a droplet of dz@iaion pump oil between the two surfaces.
