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Characterization of neutron-induced copper-enriched clusters in pressure vessel steel weld: an APFIM study
applied surface science ELSEVIER
Applied Surface Science 94/95 (1996) 370-377
Characterization of neutron-induced copper-enriched clusters in pressure vessel steel weld: an APFIM study P. Pareige, M.K. Miller * Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, MS 6376, Oak Ridge, TN 37831-6376, USA
Received 3 August 1995; accepted 8 September 1995
Abstract Embrittlement of pressure vessel steels is controlled by complex interactions between a large number of variables making purely empirical predictions unreliable. It is necessary to have an accurate description of the changes that occur in the microstructure during neutron irradiation. A detailed atom probe field ion microscope examination of the microstructure of a neutron-irradiated commercial A533B-type weld (3.5 x 1019 n cm -2 (E > 1 MeV) at 283°C) is reported in this study. The plane-by-plane type analysis provides an accurate description of the neutron-induced copper-enriched clusters. The size, the morphology, the spatial distribution of solutes and the chemical composition of these features are determined. The matrix chemistry of this irradiated material was determined and compared with the as-fabricated weld and weld material that had been thermally aged for 93 000 h at 280°C. This comparison isolated the effect of neutron irradiation from any thermally induced microstructural evolution.
1. Introduction Research on radiation effects in materials is performed to increase our understanding of materials, to support the broad field of materials processing with particle beams, and to underpin fission and fusion reactor technologies. In this latter field, irradiationinduced or -enhanced embrittlement of reactor pressure vessels may limit the operation of a number of nuclear plants worldwide. Embrittlement is controlled by complex interactions between a large number of variables, making purely empirical predictions unreliable. It is also necessary to have an accurate
* Corresponding author. Tel.: + 1 423 574 4719; fax: + 1 423 574 0641; e-mail: [email protected].
description of the changes that occur in the mlcrostructure under long-term aging conditions. Copper-enriched " p r e c i p i t a t e s " or " c l u s t e r s " are one of the dominant features in industrial pressure vessel steels containing significant trace quantities of this element ( ~ > 0.1 at% Cu). Progress in understanding this component of embrittlement has been largely the consequence of the development and application of advanced characterization tools such as the atom probe field ion microscope (APFIM). This technique has already answered a number of questions in this field of material science. However, information about the chemistry and the morphology of these neutron-induced clusters is often controversial and further experimental investigation is required to resolve some of the outstanding questions. The extremely small size ( ~ 3 nm) and the dark
0169-4332/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 01 69-4332(95)00399- 1
P. Pareige, M.K. Miller~Applied Surface Science 94/95 (1996) 370-377
contrast in field ion images (using FIM technique) of these neutron-induced phases are additional effects that increase the difficulties in answering these questions. Since the atom probe can accurately determine the chemical composition on a single plane of the crystal, a set of experiments have been undertaken that included plane-by-plane type analysis in an attempt to resolve some of these outstanding issues. Thus, a detailed examination of the microstructure of a neutron-irradiated commercial pressure vessel weld (3.5× 1019 n cm -2 ( E > 1 MeV)) from the B & W Master Integrated Reactor Vessel Surveillance Program has been performed in the ORNL energy-compensated instrument [1]. This type of analysis permits an accurate description of the size, the chemistry, the solute distribution, the morphology and the behavior of such clusters under field ionization and evaporation to be determined.
2. Material The bulk chemistry of the pressure vessel weld used in this study is given in Table 1. The weld, typical of M n - M o - N i - w e l d wire/Linde 80 flux submerged arc weld, is representative of the materials used to fabricate the beltline shell course regions of the Oconee Unit 3 and Arkansas Unit 1 reactors. The weld was subjected to a stress relief heat treatment for 29 h at 593-621°C and furnace cooled at a rate of ~ 8°C/h to ~ 310°C prior to neutron irradiation. The weld was irradiated at 283°C and at a flux of 7.4X 10 ~° n cm -2 s -~ to a neutron dose of 3 . 5 x 1019 n cm -2 ( E > 1 MeV). The increase in the Charpy transition temperature during irradiation was determined to be 128°C [2,3]. In order to isolate the effect of neutron irradiation, as-received (i.e., stress-relieved) and long-term thermally aged (i.e., stress-relieved + 93 000 h at 280°C) specimens have also been characterized.
371
3. Experimental technique In order to have a reliable description of the microstructure of the material, both random area and plane-by-plane analyses were performed [4]. The probe aperture was positioned to collect atoms from the (020) plane of the body-centered cubic (bcc) ferritic matrix (Fig. 1). Both field ion microscopy and atom probe experiments (with different effective probe apertures) were performed in order to determine the full set of parameters (i.e. FIM local magnification, detector efficiency, and lateral resolution) required to perform the interpretation of the "linear" data chain obtained from the atom probe, which is a one-dimensional representation from the deconstruction of the three-dimensional material. The experimental conditions required to obtain accurate APFIM data have been established for these ferritic steels [5,611 In particular, it is necessary to cool the specimen to a temperature of 50 K to avoid a systematic error in the copper level measurement. Field-ion needles were electropolished using standard procedures [4] from blanks that were cut from Charpy V-notch specimens. All compositions reported in this paper are quoted in atomic percent.
4. Results and discussion The measured matrix compositions obtained from the as-received, the long-term thermally aged and the neutron-irradiated materials are reported in Table 2. A copper depletion from 0.26 at% (bulk chemistry) to 0.15 at% can be noted after the stress relief heat treatment. This lower value is in agreement with the predicted copper solubility limit in the FeCu binary system for the temperature of the stress relief (and slow fumace cooling) treatment [7]. The absence of any copper particles in the matrix suggests there was thermal diffusion and precipitation of copper at grain
Table 1 Chemical composition (bulk chemistry) of the commercial weld (typical Mn-Mo-Ni-weld wire/Linde 80 flux submerged-arc weld) pressure vessel steel (balance is iron) % Cu Ni Mn Si P C S Mo Cr Weld A wt% 0.28 0.59 1.49 0.51 0.016 0.09 0.016 0.39 0.06 at% 0.24 0.56 1.5 1.01 0.03 0.42 0.03 0.23 0.06
372
P. Pareige, M.K. Miller~Applied Surface Science 94 / 95 (1996) 370-377
Fig. 1. Field ion micrograph of the ferrite matrix of the neutronirradiated weld. The probe aperture is positioned to collect atoms from the (020) crystallographic plane of the body-centered cubic matrix.
boundaries or carbide/matrix interfaces during the heat treatment at 610°C as previously observed in these types of welds [8]. The similar matrix compositions measured in the as-received and the long-term thermally aged materials show that there is no effect of a long-term thermal aging heat treatment on the ferrite matrix. The results of the neutron-irradiated matrix composition measurements show that the copper content was considerably further reduced from the nominal bulk level. This copper depletion of the matrix is also associated with the phosphorus depletion. Both random area and plane-by-plane atom probe analyses of the neutron-irradiated material revealed the presence of intragranular neutron-induced features with a number density of ~ 3 × 1023 m -3. A
short portion of the data from a plane-by-plane experiment is shown in the form of an evaporation diagram in Fig. 2. The number of collected ions is plotted as a function of the number of evaporation pulses (the pulse frequency was 50 Hz). It clearly exhibits the evaporation sequence of each (020) crystallographic plane. The effective probe aperture diameter of 2.5 nm and the 60% detector efficiency resulted in the collection of a mean value of 37 ions per plane. The loss of the regular "steps" in the ladder diagram is due to the presence of a second phase (i.e., a copper cluster) which increases the difficulty in the interpretation of the data. However, the unique behavior (evaporation field) of each chemical species under the electric field allows the exact ion evaporation sequences to be determined through the new phase. The diagram of the number of ions for each chemical species as a function of the total number of collected ions (Fig. 3) illustrates this effect. The preferential evaporation of the copper ions can be distinguished in Fig. 3. Due to their lower evaporation field a " h i g h " number of copper ions are detected at the beginning of the evaporation of each atomic layer. Their discontinuous evaporation is an indication of the plane-by-plane evaporation sequence of the cluster. It is thus possible to accurately identify each plane and to determine their true chemistry. This plane-by-plane detection provides several pieces of information about the neutron-induced copper-rich clusters. As mentioned above, the cumulative profile, Fig. 3, reveals the plane-by-plane evaporation sequence through the cluster and permits the composition of each plane to be determined. Thus a spatial planeby-plane distribution of the different elements analyzed in the small volume of material can be reconstructed with three-dimensional visualization software. The reconstruction, with an accurate depth
Table 2 APFIM measured compositions (at%) of the ferrite matrix obtained from the as-received, the long-term thermally aged and the neutron-irradiated weld material (the compositions are an average of several experiments (5:2 o-))
As-received Thermally aged Neutron-irradiated
Cu
Ni
Mn
Si
P
C
Mo
Cr
0.14+_0.03 0.15 ± 0.05 0.05 4- 0.01
0.45±0.06 0.45 ± 0.09 0.57_+ 0.05
1.20±0.10 0.90 + 0.12 1.08 + 0.07
1.05±0.10 0.80 ± 0.12 1.22_+ 0.07
0.03±0.03 0.02 ± 0.02 0.013 ± 0.007
0.005±0.005 0.005 ± 0.005
0.18±0.04 0.14 ± 0.05 0.23 4_-_0.03
0.03±0.03 0.03 ± 0.01 0.08 +_ 0.02
P. Pareige, M.K. Miller/Applied Surface Science 94/95 (1996) 370-377
373
9 104
u)
E O
6 10'
J
O o.
~6
J
f 3 10'
E Z
f 0 100
I 200
I 300
I 400
I 500
I 600
l 700
I 800
I 900
1000
Total number of ions collected Fig. 2. Portion of an evaporation diagram of the data from a (020) plane-by-plane experiment. A mean number of 37 ions are collected per plane. The reduced slope between about 300 and 700 ions is due to the presence of the copper-enriched cluster.
resolution of 0.145 nm is shown in Fig. 4. It must be noted that on each plane the X and Y spatial positions of the detected ions have been randomly distributed within the extent of the 2.5 nm diameter cylinder of analysis.
In this representation the copper atoms were mainly distributed in the core of the cluster. However, one or two copper atoms were detected at 2 or 3 interatomic distances from the central core of the cluster. This indicates the diffuse or ramified nature
70 b~
Q)
"5 (D 60 O. m
~ 50 E Q cO ¢-
L
40
A
0
~ 3o tO
.,- 20 0
.Q
E 10 z
0
200
400
600
800
Totalnumberof
1000
1200
1400
1600
ions collected
Fig. 3. Diagram of the number of ions for each element as a function of the total number of ions collected within the copper-enriched cluster. The different evaporation fields of each chemical species allow the exact plane-by-plane evaporation sequence within the cluster to be determined.
374
P. Pareige, M.K. Miller~Applied Surface Science 94 / 95 (1996) 370-377 8 at.% o
6
'~
4
0.
2
g
g m
,ili ii
o
.I..
2
m
~ 4
IIIllllll i:11l!i!!!1 fflllllllll
•i
!lii'ii" |
•
m
g~ 8 Fig. 5. Copper concentration measured on each plane of copper-enriched cluster. The increase and the decrease of copper level is representative of a spherical morphology. The planes marked FIM have been recorded (see text and Fig. 1) manually field evaporated.
@
the the two and
0 0
0
o~ 0
G Jlb~ O. 145
0 0
0I
••
O
D
FLItl
of the local copper enrichment. Moreover, Mn, Si and to a lesser extent Ni exhibit a wider spatial distribution around the center of the cluster than the copper atoms. This representation also provides an accurate estimate of the size of the cluster. The range of the copper atoms is a reasonable parameter to use in describing the size of the core of the cluster, 20 planes in this case (see below), i.e. 2.9 nm. The number of copper atoms detected per each plane is represented in Fig. 5. This schematic representation shows an increase and then a decrease in the copper content on the evaporated planes as the analysis proceeds through the cluster. Such a distribution is representative of a spherical spatial distribution of copper atoms, and other solutes as well. In Fig. 5, two planes are not represented and are marked " F I M " . The x abscissa of these two missing planes indicates the exact position of their FIM observation and their manual field evaporation. The field ion image in Fig. 1 was taken after the evaporation of the second plane. The calculation of the FIM magnification (6 x 106) and the local radius of curvature
• 0
2.5rim
Fig. 4. Reconstruction of the spatial distribution of the different elements analyzed in a small volume of material. On each reconstructed (020) plane, the x and y spatial positions of the ions are randomly distributed. The figure clearly shows the (020) planes. Iron atoms are not represented for the clarity of the image.
P. Pareige, M.K. Miller/Applied Surface Science 94/95 (1996) 370-377
375
Table 3 Apparent composition (at%) of the clusters detected from random area analysis or plane-by-plane (in the core) atom probe analyses
Cu P Ni Mn Si Cr Mo Fe
Plane-by-plane (in the core)
Random area analysis (at% 5:2 tr)
8.5 5:2.3 0.3+0.3 4.4 5:1.7 5.6+ 1.9 4.1 5:0.4 0.5 5:0.5 76.6 5:3.5
4.0 + 1.6 0.7+0.7 7.2 5:2.2 7.45:2.2 4.1 + 1.7 0.2 5:0.2 0.2 +_ 0.2 76.2 5:3.6
2.0 + 1.4 0.5+0.5 3.3 _ 1.8 4.3 5:2.0 1.8 5:1.3 -
0.8 5:0.8 " 87.4 5:3.3
1.3 + 0.9 0.2-t-0.2 1.8 5:0.3 2.75:1.3 1.2 +_ 0.9 0.2 5:0.2 0.3 5:0.3 92.3 5:2.2
5.8 _+ 2.8 6.9 5:2.9 6.9+2.9 4.0 + 2.2 0.4 5:0.3 0.7 +_0.6 75.4 5:5.0
13.8 4,6 4.1 2,0
+ 5.0 5:3.0 5:2.8 + 2.0
75.4 + 6.1
5.2 1.1 7.2 5.5 3.2 77.8
5:1.5 +0.7 5:1.8 _+ 1.6 ± 1.2
5:2.3
Table 4 Average enrichment factors (ratio of composition in the clusters to composition in the matrix) for each chemical species detected in the neutron-induced clusters Cu
P
Ni
Mn
Si
Cr
Mo
116
31
9
5
2
1.3
1.4
Enrichment factor
on the (020) pole (35 nm) permitted the diameter of the first atomic terrace to be determined. The calculated value, on the FIM image, of ~ 3 nm is of the same order of the size of the cluster. The darkest region in the center of the pole has a diameter of 1.5 nm. As it is shown, even if the contrast is dark, there is no evidence of the presence of the cluster. No distortions of the patterns are noticeable; the rings of
the (020) crystallographic pole are concentric. It indicates the absence of interfacial dislocations with the Burger vector component along the (020) crystallographic direction. This, in addition to the planeby-plane evaporation sequence of the cluster, supports the idea of the coherency (or semi-coherency) of the crystal structure of the cluster with the bodycentered cubic ferrite matrix.
70.
_m
D.
6o
w
E o 5O
"6 .D
E 4O Z
30 0
I 1
I 2
I 3
I 4
I 5
I 6
7
Distance (nm)
Fig. 6. Representation of the numbers of atoms collected per plane within the copper-enriched cluster. The local magnification effect is characterized by the addition of ~ 12 ions per plane.
376
P. Pareige, M.K. Miller~Applied Surface Science 9 4 / 9 5 (1996) 370-377
From these FIM observations and AP analyses an accurate description of the morphology, the solute distribution and the size of the cluster was possible. The composition of this cluster as well as the average composition of the six other clusters encountered in random area analyses are reported in Table 3. The comparison of the solute compositions, between random and plane-by-plane (in the core) analyses, show a significant variation in the copper content. The composition of the other solutes are of the same order. This variation in the copper composition is in agreement with the fact that copper is more concentrated in the center of the cluster. Random analyses through the edge of the cluster underestimate the copper content. It should also be noted that similar concentrations are often detected for Ni and Mn solutes ( ~ 5 at%). The different enrichment factors (ratio of local composition in the cluster to the matrix composition) for each chemical species detected in these neutron-induced clusters are summarized in Table 4. Copper and phosphorus show the highest level of enrichment in accordance with their well-known behavior to segregate under neutron irradiation [7-10]. However, the measured concentrations in the clusters could be an underestimation of their true values because of the local magnification [4]. Indeed, the plot shown in Fig. 6, of the analyzed distance as a function of the number of ions collected per plane shows this local magnification effect. There were, on average, 12 additional ions systematically detected on each plane within the cluster. Assuming (with no evident reasons) that this excess of ions comes from the surrounding iron matrix (and very few other substitutional solutes), the estimate of the copper concentration would increase from 8.5 to 14 at%. Even with this (hypothetical) correction, the idea of a pure copper precipitate does not appear to be valid in these materials. This detailed description of the neutron-induced copper-enriched clusters is in agreement with the different clusters described in the literature from atom probe analyses of pressure vessel steel materials. In high copper materials, such as the submergedarc weld in Westem pressure vessel steels [7,11] or low copper level used in the C core shell metal of the French CHOOZ A reactor [12,13], high copper enrichment factors are found in the cluster. Copper is
always associated with P, Ni, Mn and Si and their spatial distribution follows the one described in this work. The nonhomogeneous spatial distribution of the different solutes detected in the clusters suggests that they behave differently under neutron irradiation. Copper seems to behave as observed under higher temperature thermal aging conditions where it precipitates from the metastable solid solution. However, the precipitation of Si, Ni and Mn is surprising since their concentrations are well below the solubility limits as reported in the Si-, N i - and M n - F e binary systems. Although they are all detected in the same clusters, different processes of precipitation of these chemical species may be involved, as discussed in the companion paper [14].
5. Conclusion A detailed atom probe field ion microscope examination of the microstmcture of a commercial A533B-type weld neutron-irradiated (3.5 X 1019 n cm -2 ( E > 1 MeV)) at 283°C was performed. A high number density ( ~ 3 X 1017 cm -3) of ultrafine (2-3 nm) intragranular Cu-, P-, Ni-, Mn-, and Si-enriched clusters was observed and analyzed. The fact that these small clusters were not observed in the as-received and the long-term (100000 h) thermally aged (300°C) materials confirms that their formation was either radiation-induced or radiation-enhanced. The atom probe plane-by-plane type analysis of these clusters provides an accurate description of their spherical morphologies. From the three-dimensional reconstruction of these copper-enriched clusters, it is observed that copper atoms are mainly distributed in the core of the cluster, with a concentration of ~ 8.5 at%. Mn, Si and Ni exhibit a wider spatial distribution around the center of the cluster. Since copper precipitation is observed in copper-containing steel aged at higher ( ~ 500°C) temperatures, the formation of copper-rich clusters in material irradiated at ~ 300°C could be attributed to simple radiation-enhanced diffusion. However, the effect of thermal aging or irradiation on the other solutes (P, Ni, Mn and Si) that also segregate to these clusters is not well understood. Although they are all detected in the same clusters, different processes of precipitation
P. Pareige, M.K. Miller~Applied Surface Science 94/95 (1996) 370-377
of these chemical species, such as radiation-enhanced for copper and radiation-induced for the other solutes, may be involved.
Acknowledgements The materials for this study were provided by the B&W Owners Group. The authors would like to thank A.L. Lowe, M.J. De Van and W.A. Pavinich of B &W Nuclear Technologies for their assistance. This research was sponsored by the Division of Materials Sciences, US Department of Energy, under contract DE-AC05-96OR22464 with Lockheed Martin Energy Research, by the Office of Nuclear Regulatory Research, US Nuclear Regulatory Commission under inter-agency agreement DOE 1886-8109-8L with the US Department of Energy. This research was also supported in part by appointment to the Oak Ridge National Laboratory Postdoctoral Research Associates Program administrated jointly by Oak Ridge National Laboratory and the Oak Ridge Institute for Science and Education. This research was conducted utilizing the Shared Research Equipment (SHARE) User Program facilities at Oak Ridge National Laboratory. We would like to thank K,F. Russell, Dr. J.M. Hyde and Dr. R.E. Stoller for fruitful discussions and their technical assistance.
377
References [1] M.K. Miller, J. Phys. (Paris) 47 (1986) C2-493. [2] A.L. Lowe et al., Analysis of Capsule RSl-D, Sacramento Municipal Utility District, Rancho Seco Unit 1, BAW 1792, Babcock and Wilcox Co., Lynchburg, VA, October 1983. [3] A.L. Lowe et al., Analysis of Capsule S, Wisconsin Electric Power Co., Point Beach Unit 2 Reactor Material Surveillance Program, BAW 2140, B&W Nuclear Technologies, Lynchburg, VA, August 1991. [4] M.K. Miller and G.D.W. Smith, Atom Probe Microanalysis: Principles and Applications to Materials Science (Materials Research Society, Pittsburgh, PA, 1989). [5] G.M. Worrall and G.D.W. Smith, J. Phys. (Paris) 47 (1986) C2-245. [6] P. Pareige, J.C. Van Duysen and P. Auger, Appl. Surf. Sci. 67 (1993) 342. [7] M.K. Miller and M.G. Burke, J. Nucl. Mater. 195 (1992) 68. [8] M.K. Miller, M.G. Hetherington and M.G. Burke, Met. Trans. 20A (1989) 2651. [9] U. Potapovs and J.R. Hawthorn, Nucl. Appl. 1 (1969) 27. [10] R.B. Jones, in: Proc. 3rd Int. Symp. on Environmental Degradation of Materials in Nuclear Power Systems - Water Reactor, Traverse City, USA, 1988, p. 111. [11] M.K. Miller and M.G. Burke, J. Phys. (Paris) 48 (1987) 429. [12] P. Auger, P. Pareige, M. Akamatsu and D. Blavette, J. Nucl. Mater. 225 (1995) 225. [13] P. Pareige, PhD Thesis, Rouen University (1994). [14] P. Pareige, K.F. Russell and M.K. Miller, Appl. Surf. Sci. 94/95 (1996) 362.
Applied Surface Science 94/95 (1996) 370-377
Characterization of neutron-induced copper-enriched clusters in pressure vessel steel weld: an APFIM study P. Pareige, M.K. Miller * Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, MS 6376, Oak Ridge, TN 37831-6376, USA
Received 3 August 1995; accepted 8 September 1995
Abstract Embrittlement of pressure vessel steels is controlled by complex interactions between a large number of variables making purely empirical predictions unreliable. It is necessary to have an accurate description of the changes that occur in the microstructure during neutron irradiation. A detailed atom probe field ion microscope examination of the microstructure of a neutron-irradiated commercial A533B-type weld (3.5 x 1019 n cm -2 (E > 1 MeV) at 283°C) is reported in this study. The plane-by-plane type analysis provides an accurate description of the neutron-induced copper-enriched clusters. The size, the morphology, the spatial distribution of solutes and the chemical composition of these features are determined. The matrix chemistry of this irradiated material was determined and compared with the as-fabricated weld and weld material that had been thermally aged for 93 000 h at 280°C. This comparison isolated the effect of neutron irradiation from any thermally induced microstructural evolution.
1. Introduction Research on radiation effects in materials is performed to increase our understanding of materials, to support the broad field of materials processing with particle beams, and to underpin fission and fusion reactor technologies. In this latter field, irradiationinduced or -enhanced embrittlement of reactor pressure vessels may limit the operation of a number of nuclear plants worldwide. Embrittlement is controlled by complex interactions between a large number of variables, making purely empirical predictions unreliable. It is also necessary to have an accurate
* Corresponding author. Tel.: + 1 423 574 4719; fax: + 1 423 574 0641; e-mail: [email protected].
description of the changes that occur in the mlcrostructure under long-term aging conditions. Copper-enriched " p r e c i p i t a t e s " or " c l u s t e r s " are one of the dominant features in industrial pressure vessel steels containing significant trace quantities of this element ( ~ > 0.1 at% Cu). Progress in understanding this component of embrittlement has been largely the consequence of the development and application of advanced characterization tools such as the atom probe field ion microscope (APFIM). This technique has already answered a number of questions in this field of material science. However, information about the chemistry and the morphology of these neutron-induced clusters is often controversial and further experimental investigation is required to resolve some of the outstanding questions. The extremely small size ( ~ 3 nm) and the dark
0169-4332/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 01 69-4332(95)00399- 1
P. Pareige, M.K. Miller~Applied Surface Science 94/95 (1996) 370-377
contrast in field ion images (using FIM technique) of these neutron-induced phases are additional effects that increase the difficulties in answering these questions. Since the atom probe can accurately determine the chemical composition on a single plane of the crystal, a set of experiments have been undertaken that included plane-by-plane type analysis in an attempt to resolve some of these outstanding issues. Thus, a detailed examination of the microstructure of a neutron-irradiated commercial pressure vessel weld (3.5× 1019 n cm -2 ( E > 1 MeV)) from the B & W Master Integrated Reactor Vessel Surveillance Program has been performed in the ORNL energy-compensated instrument [1]. This type of analysis permits an accurate description of the size, the chemistry, the solute distribution, the morphology and the behavior of such clusters under field ionization and evaporation to be determined.
2. Material The bulk chemistry of the pressure vessel weld used in this study is given in Table 1. The weld, typical of M n - M o - N i - w e l d wire/Linde 80 flux submerged arc weld, is representative of the materials used to fabricate the beltline shell course regions of the Oconee Unit 3 and Arkansas Unit 1 reactors. The weld was subjected to a stress relief heat treatment for 29 h at 593-621°C and furnace cooled at a rate of ~ 8°C/h to ~ 310°C prior to neutron irradiation. The weld was irradiated at 283°C and at a flux of 7.4X 10 ~° n cm -2 s -~ to a neutron dose of 3 . 5 x 1019 n cm -2 ( E > 1 MeV). The increase in the Charpy transition temperature during irradiation was determined to be 128°C [2,3]. In order to isolate the effect of neutron irradiation, as-received (i.e., stress-relieved) and long-term thermally aged (i.e., stress-relieved + 93 000 h at 280°C) specimens have also been characterized.
371
3. Experimental technique In order to have a reliable description of the microstructure of the material, both random area and plane-by-plane analyses were performed [4]. The probe aperture was positioned to collect atoms from the (020) plane of the body-centered cubic (bcc) ferritic matrix (Fig. 1). Both field ion microscopy and atom probe experiments (with different effective probe apertures) were performed in order to determine the full set of parameters (i.e. FIM local magnification, detector efficiency, and lateral resolution) required to perform the interpretation of the "linear" data chain obtained from the atom probe, which is a one-dimensional representation from the deconstruction of the three-dimensional material. The experimental conditions required to obtain accurate APFIM data have been established for these ferritic steels [5,611 In particular, it is necessary to cool the specimen to a temperature of 50 K to avoid a systematic error in the copper level measurement. Field-ion needles were electropolished using standard procedures [4] from blanks that were cut from Charpy V-notch specimens. All compositions reported in this paper are quoted in atomic percent.
4. Results and discussion The measured matrix compositions obtained from the as-received, the long-term thermally aged and the neutron-irradiated materials are reported in Table 2. A copper depletion from 0.26 at% (bulk chemistry) to 0.15 at% can be noted after the stress relief heat treatment. This lower value is in agreement with the predicted copper solubility limit in the FeCu binary system for the temperature of the stress relief (and slow fumace cooling) treatment [7]. The absence of any copper particles in the matrix suggests there was thermal diffusion and precipitation of copper at grain
Table 1 Chemical composition (bulk chemistry) of the commercial weld (typical Mn-Mo-Ni-weld wire/Linde 80 flux submerged-arc weld) pressure vessel steel (balance is iron) % Cu Ni Mn Si P C S Mo Cr Weld A wt% 0.28 0.59 1.49 0.51 0.016 0.09 0.016 0.39 0.06 at% 0.24 0.56 1.5 1.01 0.03 0.42 0.03 0.23 0.06
372
P. Pareige, M.K. Miller~Applied Surface Science 94 / 95 (1996) 370-377
Fig. 1. Field ion micrograph of the ferrite matrix of the neutronirradiated weld. The probe aperture is positioned to collect atoms from the (020) crystallographic plane of the body-centered cubic matrix.
boundaries or carbide/matrix interfaces during the heat treatment at 610°C as previously observed in these types of welds [8]. The similar matrix compositions measured in the as-received and the long-term thermally aged materials show that there is no effect of a long-term thermal aging heat treatment on the ferrite matrix. The results of the neutron-irradiated matrix composition measurements show that the copper content was considerably further reduced from the nominal bulk level. This copper depletion of the matrix is also associated with the phosphorus depletion. Both random area and plane-by-plane atom probe analyses of the neutron-irradiated material revealed the presence of intragranular neutron-induced features with a number density of ~ 3 × 1023 m -3. A
short portion of the data from a plane-by-plane experiment is shown in the form of an evaporation diagram in Fig. 2. The number of collected ions is plotted as a function of the number of evaporation pulses (the pulse frequency was 50 Hz). It clearly exhibits the evaporation sequence of each (020) crystallographic plane. The effective probe aperture diameter of 2.5 nm and the 60% detector efficiency resulted in the collection of a mean value of 37 ions per plane. The loss of the regular "steps" in the ladder diagram is due to the presence of a second phase (i.e., a copper cluster) which increases the difficulty in the interpretation of the data. However, the unique behavior (evaporation field) of each chemical species under the electric field allows the exact ion evaporation sequences to be determined through the new phase. The diagram of the number of ions for each chemical species as a function of the total number of collected ions (Fig. 3) illustrates this effect. The preferential evaporation of the copper ions can be distinguished in Fig. 3. Due to their lower evaporation field a " h i g h " number of copper ions are detected at the beginning of the evaporation of each atomic layer. Their discontinuous evaporation is an indication of the plane-by-plane evaporation sequence of the cluster. It is thus possible to accurately identify each plane and to determine their true chemistry. This plane-by-plane detection provides several pieces of information about the neutron-induced copper-rich clusters. As mentioned above, the cumulative profile, Fig. 3, reveals the plane-by-plane evaporation sequence through the cluster and permits the composition of each plane to be determined. Thus a spatial planeby-plane distribution of the different elements analyzed in the small volume of material can be reconstructed with three-dimensional visualization software. The reconstruction, with an accurate depth
Table 2 APFIM measured compositions (at%) of the ferrite matrix obtained from the as-received, the long-term thermally aged and the neutron-irradiated weld material (the compositions are an average of several experiments (5:2 o-))
As-received Thermally aged Neutron-irradiated
Cu
Ni
Mn
Si
P
C
Mo
Cr
0.14+_0.03 0.15 ± 0.05 0.05 4- 0.01
0.45±0.06 0.45 ± 0.09 0.57_+ 0.05
1.20±0.10 0.90 + 0.12 1.08 + 0.07
1.05±0.10 0.80 ± 0.12 1.22_+ 0.07
0.03±0.03 0.02 ± 0.02 0.013 ± 0.007
0.005±0.005 0.005 ± 0.005
0.18±0.04 0.14 ± 0.05 0.23 4_-_0.03
0.03±0.03 0.03 ± 0.01 0.08 +_ 0.02
P. Pareige, M.K. Miller/Applied Surface Science 94/95 (1996) 370-377
373
9 104
u)
E O
6 10'
J
O o.
~6
J
f 3 10'
E Z
f 0 100
I 200
I 300
I 400
I 500
I 600
l 700
I 800
I 900
1000
Total number of ions collected Fig. 2. Portion of an evaporation diagram of the data from a (020) plane-by-plane experiment. A mean number of 37 ions are collected per plane. The reduced slope between about 300 and 700 ions is due to the presence of the copper-enriched cluster.
resolution of 0.145 nm is shown in Fig. 4. It must be noted that on each plane the X and Y spatial positions of the detected ions have been randomly distributed within the extent of the 2.5 nm diameter cylinder of analysis.
In this representation the copper atoms were mainly distributed in the core of the cluster. However, one or two copper atoms were detected at 2 or 3 interatomic distances from the central core of the cluster. This indicates the diffuse or ramified nature
70 b~
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~ 50 E Q cO ¢-
L
40
A
0
~ 3o tO
.,- 20 0
.Q
E 10 z
0
200
400
600
800
Totalnumberof
1000
1200
1400
1600
ions collected
Fig. 3. Diagram of the number of ions for each element as a function of the total number of ions collected within the copper-enriched cluster. The different evaporation fields of each chemical species allow the exact plane-by-plane evaporation sequence within the cluster to be determined.
374
P. Pareige, M.K. Miller~Applied Surface Science 94 / 95 (1996) 370-377 8 at.% o
6
'~
4
0.
2
g
g m
,ili ii
o
.I..
2
m
~ 4
IIIllllll i:11l!i!!!1 fflllllllll
•i
!lii'ii" |
•
m
g~ 8 Fig. 5. Copper concentration measured on each plane of copper-enriched cluster. The increase and the decrease of copper level is representative of a spherical morphology. The planes marked FIM have been recorded (see text and Fig. 1) manually field evaporated.
@
the the two and
0 0
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of the local copper enrichment. Moreover, Mn, Si and to a lesser extent Ni exhibit a wider spatial distribution around the center of the cluster than the copper atoms. This representation also provides an accurate estimate of the size of the cluster. The range of the copper atoms is a reasonable parameter to use in describing the size of the core of the cluster, 20 planes in this case (see below), i.e. 2.9 nm. The number of copper atoms detected per each plane is represented in Fig. 5. This schematic representation shows an increase and then a decrease in the copper content on the evaporated planes as the analysis proceeds through the cluster. Such a distribution is representative of a spherical spatial distribution of copper atoms, and other solutes as well. In Fig. 5, two planes are not represented and are marked " F I M " . The x abscissa of these two missing planes indicates the exact position of their FIM observation and their manual field evaporation. The field ion image in Fig. 1 was taken after the evaporation of the second plane. The calculation of the FIM magnification (6 x 106) and the local radius of curvature
• 0
2.5rim
Fig. 4. Reconstruction of the spatial distribution of the different elements analyzed in a small volume of material. On each reconstructed (020) plane, the x and y spatial positions of the ions are randomly distributed. The figure clearly shows the (020) planes. Iron atoms are not represented for the clarity of the image.
P. Pareige, M.K. Miller/Applied Surface Science 94/95 (1996) 370-377
375
Table 3 Apparent composition (at%) of the clusters detected from random area analysis or plane-by-plane (in the core) atom probe analyses
Cu P Ni Mn Si Cr Mo Fe
Plane-by-plane (in the core)
Random area analysis (at% 5:2 tr)
8.5 5:2.3 0.3+0.3 4.4 5:1.7 5.6+ 1.9 4.1 5:0.4 0.5 5:0.5 76.6 5:3.5
4.0 + 1.6 0.7+0.7 7.2 5:2.2 7.45:2.2 4.1 + 1.7 0.2 5:0.2 0.2 +_ 0.2 76.2 5:3.6
2.0 + 1.4 0.5+0.5 3.3 _ 1.8 4.3 5:2.0 1.8 5:1.3 -
0.8 5:0.8 " 87.4 5:3.3
1.3 + 0.9 0.2-t-0.2 1.8 5:0.3 2.75:1.3 1.2 +_ 0.9 0.2 5:0.2 0.3 5:0.3 92.3 5:2.2
5.8 _+ 2.8 6.9 5:2.9 6.9+2.9 4.0 + 2.2 0.4 5:0.3 0.7 +_0.6 75.4 5:5.0
13.8 4,6 4.1 2,0
+ 5.0 5:3.0 5:2.8 + 2.0
75.4 + 6.1
5.2 1.1 7.2 5.5 3.2 77.8
5:1.5 +0.7 5:1.8 _+ 1.6 ± 1.2
5:2.3
Table 4 Average enrichment factors (ratio of composition in the clusters to composition in the matrix) for each chemical species detected in the neutron-induced clusters Cu
P
Ni
Mn
Si
Cr
Mo
116
31
9
5
2
1.3
1.4
Enrichment factor
on the (020) pole (35 nm) permitted the diameter of the first atomic terrace to be determined. The calculated value, on the FIM image, of ~ 3 nm is of the same order of the size of the cluster. The darkest region in the center of the pole has a diameter of 1.5 nm. As it is shown, even if the contrast is dark, there is no evidence of the presence of the cluster. No distortions of the patterns are noticeable; the rings of
the (020) crystallographic pole are concentric. It indicates the absence of interfacial dislocations with the Burger vector component along the (020) crystallographic direction. This, in addition to the planeby-plane evaporation sequence of the cluster, supports the idea of the coherency (or semi-coherency) of the crystal structure of the cluster with the bodycentered cubic ferrite matrix.
70.
_m
D.
6o
w
E o 5O
"6 .D
E 4O Z
30 0
I 1
I 2
I 3
I 4
I 5
I 6
7
Distance (nm)
Fig. 6. Representation of the numbers of atoms collected per plane within the copper-enriched cluster. The local magnification effect is characterized by the addition of ~ 12 ions per plane.
376
P. Pareige, M.K. Miller~Applied Surface Science 9 4 / 9 5 (1996) 370-377
From these FIM observations and AP analyses an accurate description of the morphology, the solute distribution and the size of the cluster was possible. The composition of this cluster as well as the average composition of the six other clusters encountered in random area analyses are reported in Table 3. The comparison of the solute compositions, between random and plane-by-plane (in the core) analyses, show a significant variation in the copper content. The composition of the other solutes are of the same order. This variation in the copper composition is in agreement with the fact that copper is more concentrated in the center of the cluster. Random analyses through the edge of the cluster underestimate the copper content. It should also be noted that similar concentrations are often detected for Ni and Mn solutes ( ~ 5 at%). The different enrichment factors (ratio of local composition in the cluster to the matrix composition) for each chemical species detected in these neutron-induced clusters are summarized in Table 4. Copper and phosphorus show the highest level of enrichment in accordance with their well-known behavior to segregate under neutron irradiation [7-10]. However, the measured concentrations in the clusters could be an underestimation of their true values because of the local magnification [4]. Indeed, the plot shown in Fig. 6, of the analyzed distance as a function of the number of ions collected per plane shows this local magnification effect. There were, on average, 12 additional ions systematically detected on each plane within the cluster. Assuming (with no evident reasons) that this excess of ions comes from the surrounding iron matrix (and very few other substitutional solutes), the estimate of the copper concentration would increase from 8.5 to 14 at%. Even with this (hypothetical) correction, the idea of a pure copper precipitate does not appear to be valid in these materials. This detailed description of the neutron-induced copper-enriched clusters is in agreement with the different clusters described in the literature from atom probe analyses of pressure vessel steel materials. In high copper materials, such as the submergedarc weld in Westem pressure vessel steels [7,11] or low copper level used in the C core shell metal of the French CHOOZ A reactor [12,13], high copper enrichment factors are found in the cluster. Copper is
always associated with P, Ni, Mn and Si and their spatial distribution follows the one described in this work. The nonhomogeneous spatial distribution of the different solutes detected in the clusters suggests that they behave differently under neutron irradiation. Copper seems to behave as observed under higher temperature thermal aging conditions where it precipitates from the metastable solid solution. However, the precipitation of Si, Ni and Mn is surprising since their concentrations are well below the solubility limits as reported in the Si-, N i - and M n - F e binary systems. Although they are all detected in the same clusters, different processes of precipitation of these chemical species may be involved, as discussed in the companion paper [14].
5. Conclusion A detailed atom probe field ion microscope examination of the microstmcture of a commercial A533B-type weld neutron-irradiated (3.5 X 1019 n cm -2 ( E > 1 MeV)) at 283°C was performed. A high number density ( ~ 3 X 1017 cm -3) of ultrafine (2-3 nm) intragranular Cu-, P-, Ni-, Mn-, and Si-enriched clusters was observed and analyzed. The fact that these small clusters were not observed in the as-received and the long-term (100000 h) thermally aged (300°C) materials confirms that their formation was either radiation-induced or radiation-enhanced. The atom probe plane-by-plane type analysis of these clusters provides an accurate description of their spherical morphologies. From the three-dimensional reconstruction of these copper-enriched clusters, it is observed that copper atoms are mainly distributed in the core of the cluster, with a concentration of ~ 8.5 at%. Mn, Si and Ni exhibit a wider spatial distribution around the center of the cluster. Since copper precipitation is observed in copper-containing steel aged at higher ( ~ 500°C) temperatures, the formation of copper-rich clusters in material irradiated at ~ 300°C could be attributed to simple radiation-enhanced diffusion. However, the effect of thermal aging or irradiation on the other solutes (P, Ni, Mn and Si) that also segregate to these clusters is not well understood. Although they are all detected in the same clusters, different processes of precipitation
P. Pareige, M.K. Miller~Applied Surface Science 94/95 (1996) 370-377
of these chemical species, such as radiation-enhanced for copper and radiation-induced for the other solutes, may be involved.
Acknowledgements The materials for this study were provided by the B&W Owners Group. The authors would like to thank A.L. Lowe, M.J. De Van and W.A. Pavinich of B &W Nuclear Technologies for their assistance. This research was sponsored by the Division of Materials Sciences, US Department of Energy, under contract DE-AC05-96OR22464 with Lockheed Martin Energy Research, by the Office of Nuclear Regulatory Research, US Nuclear Regulatory Commission under inter-agency agreement DOE 1886-8109-8L with the US Department of Energy. This research was also supported in part by appointment to the Oak Ridge National Laboratory Postdoctoral Research Associates Program administrated jointly by Oak Ridge National Laboratory and the Oak Ridge Institute for Science and Education. This research was conducted utilizing the Shared Research Equipment (SHARE) User Program facilities at Oak Ridge National Laboratory. We would like to thank K,F. Russell, Dr. J.M. Hyde and Dr. R.E. Stoller for fruitful discussions and their technical assistance.
377
References [1] M.K. Miller, J. Phys. (Paris) 47 (1986) C2-493. [2] A.L. Lowe et al., Analysis of Capsule RSl-D, Sacramento Municipal Utility District, Rancho Seco Unit 1, BAW 1792, Babcock and Wilcox Co., Lynchburg, VA, October 1983. [3] A.L. Lowe et al., Analysis of Capsule S, Wisconsin Electric Power Co., Point Beach Unit 2 Reactor Material Surveillance Program, BAW 2140, B&W Nuclear Technologies, Lynchburg, VA, August 1991. [4] M.K. Miller and G.D.W. Smith, Atom Probe Microanalysis: Principles and Applications to Materials Science (Materials Research Society, Pittsburgh, PA, 1989). [5] G.M. Worrall and G.D.W. Smith, J. Phys. (Paris) 47 (1986) C2-245. [6] P. Pareige, J.C. Van Duysen and P. Auger, Appl. Surf. Sci. 67 (1993) 342. [7] M.K. Miller and M.G. Burke, J. Nucl. Mater. 195 (1992) 68. [8] M.K. Miller, M.G. Hetherington and M.G. Burke, Met. Trans. 20A (1989) 2651. [9] U. Potapovs and J.R. Hawthorn, Nucl. Appl. 1 (1969) 27. [10] R.B. Jones, in: Proc. 3rd Int. Symp. on Environmental Degradation of Materials in Nuclear Power Systems - Water Reactor, Traverse City, USA, 1988, p. 111. [11] M.K. Miller and M.G. Burke, J. Phys. (Paris) 48 (1987) 429. [12] P. Auger, P. Pareige, M. Akamatsu and D. Blavette, J. Nucl. Mater. 225 (1995) 225. [13] P. Pareige, PhD Thesis, Rouen University (1994). [14] P. Pareige, K.F. Russell and M.K. Miller, Appl. Surf. Sci. 94/95 (1996) 362.