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SOLUTE CLUSTERING AND PRECIPITATION IN PRESSURE VESSEL STEELS UNDER LOW
FLUENCE IRRADIATION CONDITIONS
M. Burke, M. Miller
To cite this version:
M. Burke, M. Miller. SOLUTE CLUSTERING AND PRECIPITATION IN PRESSURE VESSEL
STEELS UNDER LOW FLUENCE IRRADIATION CONDITIONS. Journal de Physique Colloques,
1988, 49 (C6), pp.C6-283-C6-288. �10.1051/jphyscol:1988649�. �jpa-00228146�
SOLUTE CLUSTERING AND PRECIPITATION IN PRESSURE VESSEL STEELS UNDER LOW FLUENCE IRRADIATION CONDITIONS
M.G. BURKE and M . K . MILLER*
Westinghouse R and D Center, Pittsburgh, PA 15235. U.S.A.
" ~ e t a l s and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A.
Abstract - The microstructural features present in reactor pressure vessel steels after low fluence neutron irradiation have been characterized by atom probe field-ion microscopy. Unlike the clusters and precipitates which have been reported in pressure vessel steels irradiated to intermediate and higher fluences, very diffuse solute-enriched atmospheres have been observed. Although copper is associated with a high proportion of these atmospheres, phosphorus appears to play a more dominant role in the formation of these features.
I. INTRODUCTION
Irradiatibn embrittlement of pressure vessel steels in light water reactors is manifested by a reduction in the Charpy V-Notch test upper shelf energy and a reduction in the ductile-to-brittle transition temperature.['l The radiation-induced microstructural changes which are responsible for this degradation in mechanical behavior have been the subject of considerable research and speculation. Recent SANS investigations have revealed the existence of ultra-fine features in a variety of model and ex-service pressure vessel steels and APFIM has provided direct evidence of irradiation-induced copper precipitates, copper-enriched solute clusters containing manganese and nickel, and a variety of ultra-fine carbides, nitrides, and p h o s ~ h i d e s . [ ~ - ~ ~ l This microchemical information has a significant impact on the interpretation of complementary SANS data, and in the development of irradiation embrittlement models.
Previous atom probe field-ion microscopy studies of radiation embrittlement have featured pressure vessel steels that had been irradiated to fluences greater than 1019 neutrons/cm2 (E > IMeV). Pressure vessel steels irradiated to lower fluence levels also exhibit observable shifts in the ductile-to-brittle transition temperature.
However, these reactor materials have not been the subject of extensive microstructural investigations due to the experimental difficulties in characterizing the ultrafine features which are presumed to be responsible for the degradation in mechanical properties. In this paper, the results of an APFIM investigation of pressure vessel steels exposed to lower fluences are reported.
2. EXPERIMENTAL
The pressure vessel steels used in this investigation were A533B type weld and plate surveillance specimens, plate trepanned from the decommissioned Gundremmigen KRB-A reactor, and Gundremmigen archive material which was' irradiated in the University of Buffalo test reactor. The nominal compositions of these materials are summarized in Table 1. The composition of the A533B weld was substantially higher in copper than the plate. The archive and trepan KRB-A materials had almost identical compositions. The most significant compositional difference between these two steels was the inclusion of vanadium and the higher chromium and molybdenum contents in the KRB-A material. These steels were irradiated to fluences between 2 X 1017 and 8.5 X 1018 neutrons/cm2 (E > IMeV) at a temperature of approximately 288OC as shown in Table 2.
TABLE 1. Nominal composition of the alloys in atomic percent
Alloy Type Cu Ni Mn MO Cr Si P C V Fe
A533B plate 0.13 0.57 1.31 0.28 0.04 0.49 0.018 0.92 0 Balance A533B weld 0.27 0.68 1.52 0.29 0.04 0.12 0.027 0.35 0 Balance KRB-A plate 0.14 0.71 0.71 0.36 0.41 0.43 0.023 1.01 0.03 Balance
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988649
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All atom probe field-ion experiments were conducted in the ORNL energy-compensated atom probe with a specimen temperature between 50 , ~ n d 6 0 ~ . [ ' ~ ] The errors quoted in this paper are one standard deviation. It should be noted that a high mass resolution energy-compensated type of atom probe is essential for the characterization of these multi-element commercial materials due to the large number of peaks in the mass spectra.
For example, no less than 25 isotope peaks occur in the mass range from 25 to 35 amu. All field-ion micrographs were recorded with neon as the imaging gas. The atom probe ion by ion data were examined for clustering with the use of both the Johnson and Klotz Markov chain method[14] and the mean separation method developed by Hetherington and h ill er.['^] The presence of vanadium complicated the statistical analysis for the early stages of copper clustering, since the stable VN2+ complex ion species occurs at exactly the same mass-to-charge ratio as the '%u2+ species. Therefore, a context-sensitive approach was adopted where the ions at a mass-to-charge ratio of 32.5 amu were assigned to copper only when no vanadium ions were encountered in the region and to VN when vanadium ions were observed. This approach will therefore underestimate the segregation of copper to the vanadium carbonitrides.
TABLE 2. Irradiation conditions
Alloy Type Fluence Temperature
neutrons ("C)
(E > IMeV)
A533B plate 2 loi7 288
A533B weld 2 l0l7 288
KRB-A trepan 2.7 X 10l8 288
KRB-A test-reactor 8.5 X 1018 288 3. RESULTS AND DISCUSSION
Several coarse microstructural features were observed in both the irradiated and unirradiated A533B and KRB-A steels. A darkly-imaging MSC cementite precipitate in the A533B weld material is shown in Fig. 1. The compositio'n of this precipitate was determined as 24.620.4% C, 10.5f0.3% Mn, 1.420.11 MO, 0.420.06Yo Cr, balance Fe. No interface decoration was observed in contrast to previous cementite-ferrite interfaces in A302B pressure vessel steels.[4] The composition of a cementite precipitate in unirradiated KRB-A material was 26.520.8% C, 7.020.5% Cr, 4.6+0.4% Mn, 2520.3% MO, 0.420.196 V, balance Fe. The variation in solute content of these alloy cementites reflects the difference in the nominal composition of the two alloys and is an important parameter in understanding the contribution of the various alloying elements and hence matrix chemistry.
Brightly-imaging MozC precipitates were observed on grain boundaries in A533B welds. A typical example is shown in Fig. 2. The composition of this precipitate was determined to be 60.8+1.1% MO, 32.921.1% C, 3.4*0.4% Mn, 2.1+0.3% Fe, and 0.8+0.2% Cr. The three sections of grain boundaries shown in Fig. 2 revealed various levels of decoration. Some boundaries showed almost n o visible decoration in the field-ion micrographs, whereas others had decoration ranging from isolated bright spots to almost continuous films. These bright regions were fine precipitates of molybdenum carbide and nitride. Some phosphorus segregation to grain boundaries was also observed.
In addition, several small darkly-imaging oxide particles were observed in the irradiated A533B-type weld and plate. These coarse precipitates were associated with the processing, the thermomechanical treatments which are commonly employed in the fabrication of pressure vessel steel plate, and with the heat treatments used to stress- relieve the welds.
A variety of ultra-fine intragranular precipitates were detected in these materials. These included rod and disc-shaped carbides and nitrides. Very fine Mo2C and MoN precipitates were observed in the A533B-type weld and plate, in agreement with previous APFIM results on other A533B materials.
Brightly-imaging, -10-nm-diameter, roughly spherical V(C,N) precipitates were observed throughout the KRB-A steel, as shown in Fig. 3. These features were expected since the Gundremmigen KRB-A pressure vessel was
phosphorus, these precipitates contained virtually no molybdenum or other elements.
In addition to the relatively large carbonitrides, some ultra-fine carbonitride precipitates were detected in the KRB-A steel. Both disc-shaped and spherical morphologies were observed for these ultra-fine carbonitrides, as shown in Fig. 4. The thicknesses of the discs approached atomic dimensions and their diameters were less than 10 nm. The ultra-fine spherical examples featured diameters of less than approximately 2 nm. The precise classification of these features was difficult to ascertain from APFIM since their composition varied widely. For example, a disc-shaped cluster-precipitate shown in Fig. 4(a) contained 12 molybdenum, 9 vanadium, 3 chromium, 11 carbon and 4 nitrogen atoms, whereas 11 molybdenum, 2 vanadium, 4 chromium, 3 carbon, 2 nitrogen and 1 phosphorus atoms were collected from the spherical cluster-precipitate shown in Fig. 4(b). Since similar fine V(C,N) precipitates and clusters have also been observed in both unirradiated KRB-A control material and in other unirradiated vanadium-bearing forging steels, it is most likely that these features formed during thermomechanical processing prior to irradiation. However, the possibility that the number density and size distribution were altered during irradiation cannot be excluded. These ultra-fine cluster-precipitates represent yet another complicating microstructural component to be accounted for in the mechanical properties-structure correlations.
No darkly-imaging features corresponding to copper precipitates or copper clusters were observed in the field- ion images in any of these alloys. Such darkly-imaging features had previously been observed in A533B welds irradiated to higher f l u e n ~ e s . [ ~ ~ ~ ~ ~ ~ ~ ]
The matrix compositions of these materials are summarized in Table. 3. The A533B weld revealed a substantial depletion in copper from the nominal composition of 0.27% Cu. This copper depletion was most probably due to previously reported precipitation of copper on grain boundaries and other defects during the stress relief treatment.[8~11] The KRB-A and the A533B plate materials exhibited relatively small reductions in copper content over the nominal bulk levels.
TABLE 3. Matrix composition of the alloys in atomic percent
Alloy Type Cu Ni Mn MO Cr Si P C V Fe
A533B plate errors A533B weld
errors KRB-A trepan
errors KRB-A test
errors
0.04
-
Balance 0.020.06
-
Balance 0.020.10 0.016 Balance 0.01 0.005
0.04 0.002 Balance 0.01 0.002
The statistical significance of the distribution of copper and phosphorus in the matrix was estimated from the mean separation technique of Hetherington and ille er.['^] The preliminary results are summarized in Table 4. A statistically significant deviation from randomness is detected when the significance is greater than 2. These results are underlined. The Johnson and Klotz order parameter B was 0 for all materials examined indicating that no nearest neighbor Cu or P atoms were detected. This is consistent with the low solute content of these elements.
Unlike the clustering reported in A533B steels irradiated to higher fluences, no copper containing precipitates or clusters were revealed either in the field-ion micrographs or from preliminary statistical analysis of the atom probe data for any of the materials examined in this investigation.
Phosphorus-enriched regions were detected from the statistical analysis of the atom probe data for both the actual KRB-A pressure vessel and test reactor-irradiated materials. Examination of the ion-by-ion evaporation
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sequences indicated that these features were diffuse and best described as "pre-clusters" or atmospheres. Some copper was associated with these phosphorus atmospheres.
TABLE 4. Statistical parameters for copper and phosphorus distribution in the matrix
Alloy Type Element Mean Separation Method
variance variance error significance experimental expected
A533B plate Cu 3.9 X 105 1.0 X 106 7.9 X 105 -0.83 P 8.5 X 106 2.2 X 107 3.7 X 107 -0.38 A533B weld Cu 9.1 X 105 5.2 X 106 3.7 X 105 1.04
P 2.1 X 106 1.1 X 106 8.4 X 105 1.13 KRB-A trepan Cu 6.4 X 106 3.4 X 106 1.6 X 106 I .85
P 3.6 X 106 1.7 X 106 6.5 X 105
m
KRB-A test Cu 5.4 X 106 3.7 X 106 4.8 X 106 0.30
P 4.1 X 107 5.3 X 106 7.5 X 106
a
4. CONCLUSIONS
Detailed chemical and structural analyses have revealed the presence in the ferrite of phosphorus-enriched atmospheres and a variety of ultra-fine carbides and nitrides. The results of the microstructural analyses indicated that a variety of mechanisms should be invoked to explain the changes in mechanical properties of these materials.
The applicability of the mechanisms is both material dependent and also is influenced by the neutron fluence. One important experimental parameter is the matrix chemistry before irradiation, since this determines the quantity of solute that is available for clustering or precipitation. It should be noted that the thermal history of the steel is an important parameter in understanding the chemistry of the material and its subsequenr susceptibility to irradiation embrittlement.
5. Acknowledaments
This research was sponsored by the Division of Materials Sciences, U. S. Department of Energy, under contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc., through the SHaRE program under contract DE-AC05-760R00033 with Oak Ridge Associated Universities, and by Carolina Power and Light Company (contract monitor: Mr. S. P. Grant). The authors would like to thank Dr. M. G. Hetherington of M.
I.
T. and Dr. J. R. Hawthorne of Materials Engineering Associates for helpful discussions and K. F. Russell for her technical assistance.5.
REFERENCES1. G.R. Odette and G.E. Lucas, ASTM-STP 909, American Society for Testing and Materials, (1986) 206-241.
2. J.T. Buswell, C.A. English, M.G. Hetherington, W.J. Phythian, G.D.W.'Smith and G.M. Worrall, Proc. 1 4 ' ~ Int.
Sym. on the Effects of Radiation on Materials, ASTM, Andover, Mass. June 1988, in press.
3. F. Frisius, R. Kampmann, R. Wagner, P.A. Beaven and J.R. Hawthorne, Proc. 1 4 ~ ~ Int. Sym. on the Effects of Radiation on Materials, ASTM, Andover, Mass., June 1988, in press.
4. M.K. Miller and S.S. Brenner, Res Mechanica,
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(1984) 161-168.5. M.K. Miller, J.A. Spitznagel, S.S. Brenner and M.G. Burke, Proc. 2nd Int. Sym. on Environmental Degradation of Materials in Nuclear Power Systems
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Water Reactors, Monterey, 1985, eds. J.T.A. Robert, J.R. Weeks and G.Theus, American Nuclear Society, pp. 523-528.
6. S.P. Grant, S.L. Earp, S.S. Brenner and M.G. Burke, Proc. 2nd Int. Sym. on Environmental Degradation of Materials in Nuclear Power Systems
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Water Reactors, Monterey, 1985, eds. J.T.A. Robert, J.R. Weeks and G.Theus, American Nuclear Society, pp. 385-392.
7. M.G. Burke and S.S. Brenner, J. de Physique, -, (1986) 239-244,
10. M.K. Miller, D.T. Hoelzer, F. Ebrahimi, J.R. Hawthorne and M.G. Burke, Proc. 3rd Int. Sym. on Environmental Degradation of Materials in Nuclear Power Systems
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Water Reactors, Traverse City, 1987, eds. J.T.A. Robert, J.R. Weeks and G. Theus, pub. TMS-AIME, 1988, pp. 133-139.11. M.K. Miller and M.G. Burke, Proc. 3rd Int. Sym. on Environmental Degradation of Materials in Nuclear Power Systems - Water Reactors, Traverse City, 1987, eds. J.T.A. Robert, J.R. Weeks and G. Theus, pub. TMS-AIME, 1988, PP. 141-149.
12. M.K. Miller and M.G. Burke, Proc. 1 4 ~ ~ Int. Sym. on the Effects of Radiation on Materials, ASTM, Andover, Mass. June 1988, in press.
13. M.K. Miller, J. de Physique, -, (1986) 493-498.
14. C.A. Johnson and J.H. Klotz, Technomefrics,
E,
(1974) 495 15. M.G. Hetherington and M.K. Miller, J. de Physique, this volume.Fig. I. Darkly-imaging cementite and brightly-imaging ferrite in irradiated A533B weld material. No decoration was evident at the interphase interface.
Fig. 2. Coarse brightly-imaging MozC carbide and decorated grain boundaries in A533B weld materials.
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"r. ' *4 , $,,'
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; C , - 9*.&***$.+y':*,
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Fig. 3. Small brightly-imaging -10 nm diameter vanadium carbide precipitates in irradiated KRB-A trepan material.
Fig. 4. (a) disc-shaped and (b) spherical vanadium carbonitride precipitates in irradiated KRB-A material.
