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Effects of neutron irradiation on microstructures and hardness of stainless steel weld-overlay cladding of nuclear reactor pressure vessels
Journal of Nuclear Materials 449 (2014) 273–276
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Effects of neutron irradiation on microstructures and hardness of stainless steel weld-overlay cladding of nuclear reactor pressure vessels T. Takeuchi a,⇑, Y. Kakubo b, Y. Matsukawa b, Y. Nozawa b, T. Toyama b, Y. Nagai b, Y. Nishiyama c, J. Katsuyama c, Y. Yamaguchi c, K. Onizawa c a b c
Oarai Research and Development Center, Japan Atomic Energy Agency, Oarai, Ibaraki 311-1393, Japan The Oarai Center, Institute for Materials Research, Tohoku University, Oarai, Ibaraki 311-1313, Japan Nuclear Safety Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
a r t i c l e
i n f o
Article history: Available online 3 February 2014
a b s t r a c t The microstructures and the hardness of stainless steel weld overlay cladding of reactor pressure vessels subjected to neutron irradiation at a dose of 7.2 1019 n cm2 (E > 1 MeV) and a flux of 1.1 1013 n cm2 s1 at 290 °C were investigated by atom probe tomography and by a nanoindentation technique. To isolate the effects of the neutron irradiation, we compared the results of the measurements of the neutron-irradiated samples with those from a sample aged at 300 °C for a duration equivalent to that of the irradiation. The Cr concentration fluctuation was enhanced in the d-ferrite phase of the irradiated sample. In addition, enhancement of the concentration fluctuation of Si, which was not observed in the aged sample, was observed. The hardening in the d-ferrite phase occurred due to both irradiation and aging; however, the hardening of the irradiated sample was more than that expected from the Cr concentration fluctuation, which suggested that the Si concentration fluctuation and irradiation-induced defects were possible origins of the additional hardening. Ó 2014 Published by Elsevier B.V.
1. Introduction Stainless steel weld overlay cladding on the inner surfaces of commercial water-cooled reactor pressure vessels (RPVs) constitutes a protective barrier that protects RPVs from corrosion by cooling water. The cladding material is a duplex phase structure composed of approximately 90% austenite phase and 10% d-ferrite phase with net-like structures [1]. The mixed d-ferrite and austenitic phases bring about enhanced corrosion resistance of the cladding [2]. It is known that the d-ferrite phase is significantly hardened during thermal aging and neutron irradiation, giving rise to an increase in the ductile-to-brittle transition temperature (DBTT) and a decrease in upper-shelf energy [3–8]. However, the relationship between microstructures and the corresponding mechanical properties have not been revealed for neutron-irradiated cladding. Recently, we investigated the microstructural changes and the hardness in claddings aged at 400 °C for 100–10,000 h by threedimensional atom probe tomography (APT) [9–11]. Fluctuation in the Cr concentration due to spinodal decomposition was observed, in addition to NiSiMn clusters in the d-ferrite phase. The hardening ⇑ Corresponding author. Tel.: +81 29 266 7032; fax: +81 29 266 7071. E-mail address: [email protected] (T. Takeuchi). http://dx.doi.org/10.1016/j.jnucmat.2014.01.004 0022-3115/Ó 2014 Published by Elsevier B.V.
of the d-ferrite phase with aging showed a good linear relationship with the degree of fluctuation in the Cr concentration, independent of the change in the number and density of the NiSiMn clusters. We also investigated the microstructural changes in cladding that was neutron-irradiated with a dose of 7.2 1019 n cm2 (E > 1 MeV) and a flux of 1.1 1013 n cm2 s1 at 290 °C by APT [12]. Neutron irradiation resulted in a slight progression in Cr spinodal decomposition and an increase in the fluctuation of the Si, Ni, and Mn concentrations in the d-ferrite phase. However, the hardness of neutron-irradiated cladding has not been measured. The objective of this research was to clarify using nanoindentation the contribution of neutron irradiation to the hardness of the cladding and to reveal its origins based on the differences in effects of neutron irradiation and thermal aging on microstructures observed by APT. 2. Experimental The cladding used was fabricated by an electroslag weldoverlay method, which is same used for actual reactor cladding. The chemical composition is listed in Table 1. The cladding was subjected to post-weld heat treatment (PWHT) at 615 °C for 7 h with subsequent furnace cooling. The sample contained approximately 8% d-ferrite phase with net-like structures in the matrix
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T. Takeuchi et al. / Journal of Nuclear Materials 449 (2014) 273–276
Table 1 Chemical composition (wt.%) of the sample. C
Si
Mn
P
S
Ni
Cr
Mo
0.013
0.62
1.28
0.02
0.002
10.7
19.5
0.02
austenite phase. The width of the d-ferrite phase was a few microns. We prepared three samples for experiments by subjecting then to the following treatments: (1) PWHT followed by furnace cooling (as received); (2) neutron irradiation at a dose of 7.2 1019 n cm2 (E > 1 MeV) and a flux of 1.1 1013 n cm2 s1 at 290 °C (with irradiation); and (3) thermal aging at 300 °C for 2000 h (without irradiation), which was equivalent to the temperature (at 290 °C) and time (for 1800 h) during neutron irradiation. The samples for APT analysis were first lifted using a lift-out method with a dual-beam-focused ion beam scanning electron microscope (FIB/SEM) ion milling system [13], and then they were sharpened to select the austenite or d-ferrite phase at the tips of the needle-like specimens. To analyze the gallium-free area, the damaged layer caused by gallium implantation during FIB fabrication was minimized by applying low-energy ions during the final process. A UV laser-pulse APT (CAMECA Instruments LEAP 4000X-HR), equipped with an energy-compensating reflection lens [14–16], was used for the microstructure analysis. The measurements were performed at a laser pulse power of 60 pJ and a repetition rate of 200 kHz at approximately 60 K. The high DC bias was typically in the range of 3–10 kV. A few tens of millions of atoms were typically collected for each measurement. The samples for nanoindentation were treated by the following procedure as a surface treatment. The samples were mounted in epoxy resin, followed by mechanical polishing with sandpaper and an electrochemical polish in an ethanol solution with a mixture of 5% perchloric acid at 8 °C and finally by a chemical polish in nitro-hydrochloric acid. Hardness testing was performed using a nanoindenter (ENT1100a by Elionix Inc.). Because the width of the net-like d-ferrite phase was only a few microns, as mentioned above, the indentation size needed to be less than a few hundreds nanometers to avoid the influence of the surrounding austenite phase. However, the effect of the surface was likely to appear if the indentation load was overly light. We tried loads of 1, 2, 3, 5, 7, and 9 mN and observed the sizes of the indentations. The SEM image of the indentation at 2 mN is shown in Fig. 1 as an example. With increasing the load, the lowest and mean hardness values tended to shift toward lower values, which likely included the hardness of the
indentations
10 µm Fig. 1. SEM image of indentations made with loads of 2 mN at the d-ferrite phase.
austenite phase surrounding the targeted d-ferrite phase. Therefore, the lower load was as desirable as possible. However, the indentation depth with 1 mN, with which the SEM image of the indentation was no longer clear, was estimated at less than 100 nm where surface effects could exist. As a result, 2 mN was applied in this study as the optimal load, which was the same as that used in Ref. [11]. The average values of hardness were obtained from approximately 10–20 indentations for each sample.
3. Results and discussion The 3D elemental maps of Fe, Si, Mn, Ni, and Cr in the d-ferrite phase of the as-received samples, both with and without irradiation, are shown in Fig. 2(a)–(c), respectively. The atom maps are constructed from a slice of 20 nm in thickness to illustrate the detailed chemical composition fluctuations clearly. In Fig. 2, the Cr, Ni, Mn, and Si concentration fluctuations seem to be enhanced after irradiation. However, these fluctuations were too small to be analyzed by a commonly used maximum separation algorithm based on a friend-of-friends concept. Thus, to estimate the concentration fluctuations of Cr, Ni, Si, and Mn, we introduced a parameter V, which is defined as
V¼
100 X jOðiÞ BðiÞj;
ð1Þ
i¼0
where i is the number of atoms of the element per 100 atoms, and O(i) and B(i) are the observed and binominal distributions of the mean concentration of each element, respectively [17,18]. The distributions should have been similar to a binominal distribution in shape, had the atoms been randomly distributed, resulting in V = 0. Fig. 3 shows the correlation between the degrees of the Cr concentration fluctuations (VCr) and VNi, VSi, and VMn for the three samples. VCr increased with aging without irradiation and increased more with irradiation. In addition, VNi and VMn increased while VSi slightly decreased in the sample without irradiation; however, they all increased in the sample with irradiation, compared to the asreceived sample. Generally, thermal aging enhances the diffusion of mainly oversized atoms because of the vacancy mechanism [19,20]. In contrast, self-interstitial atoms and vacancies are induced by neutron irradiation, resulting in enhancement of the diffusion of undersized and oversized atoms via mixed-dumbbell and vacancy mechanisms, respectively. For the studied samples, the order of the atomic radii was Cr > Mn > (Fe) > Ni > Si [21]. Therefore, the inverse behavior of the VSi value between the irradiated and aged samples was quite reasonable from the viewpoint of the difference in atom sizes [12]. Next, the relationship between the microstructural changes and the hardening at the d-ferrite phase was studied. The relationship between the VCr and the nanoindentation hardness in the d-ferrite phase in the three samples is shown in Fig. 4. A line denoting the results for previously studied thermal-aged samples [11,22,23] was also drawn. The plotted points for the as-received sample and the sample without irradiation were consistent with the correlation line within the margin of error. After irradiation, the point deviated approximately 1 GPa beyond the line outside its margin of error, which implies a cause of hardening other than the Cr concentration fluctuation in the d-ferrite phase of the sample with irradiation. The fluctuations in Si, Mn, and Ni concentrations and the irradiation-induced defects invisible to APT were possible causes. However, the former is unlikely because the change in V was less than 0.1. These trivial concentration fluctuations were considered to hardly affect the motion of dislocations. Therefore, the irradiation-induced defects, such as vacancies and/or their
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T. Takeuchi et al. / Journal of Nuclear Materials 449 (2014) 273–276
VNi, VSi, and VMn
0.15
Ni Si Mn
With irradiation
0.1
As-received
0.05
0 0.4
Without irradiation
0.6
0.8
VCr Fig. 3. VCr vs. VNi, VSi, and VMn for the samples that are as-received, without irradiation, and with irradiation.
complexes, are believed to be the dominant cause of excess hardening over thermal aging. In the austenite phase, no microstructural changes were found in the samples that were as-received and without irradiation, although the elemental maps are not shown. While no concentration fluctuations were detected, clusters containing Si were observed. The 1D concentration profile penetrating the clusters revealed that Ni also aggregated at the clusters, and the Ni:Si ratio was estimated to be approximately 3:1 [12]. Fig. 5 shows the nanoindentation hardness at the austenite phase at 400 °C with aging time. Thermal aging did not affect the microstructure [11] or the hardness of the austenite phase, even for a long duration. Fig. 6 shows the nanoindentation hardness at the austenite phase of the as-received and irradiated samples. After irradiation, the hardness increased beyond the
Nanoindentation Hardness [GPa]
Fig. 2. Solute atom maps of Fe, Cr, Ni, Mn, and Si in the d-ferrite phase of the samples that are (a) as-received, (b) without irradiation, and (c) with irradiation.
7.5
With irradiation
6.0 Without irradiation As-received
4.5
3.0
0
0.5
1.0
VCr Fig. 4. Relationship between the VCr and the nanoindentation hardness at the dferrite phase in the samples that are as-received, without irradiation, and with irradiation.
margin of error. The above-mentioned NiSi clusters or/and defects caused by irradiation are possible causes of the hardening at the austenite phase with irradiation. It has been reported that yield stress is increased by neutron irradiation, but it is hardly affected by thermal aging, although DBTT is increased by both neutron irradiation and thermal aging [8]. For the tensile property, because the deformation of materials was controlled by a phase with a large volume fraction, the irradiation-induced increases in yield stress and tensile strength were almost caused by the changes in austenite phase. Regarding the Charpy impact property, the DBTT was related to the d-ferrite phase because the austenite phase did not exhibit the ductile– brittle transition behavior in general. Therefore, it is expected that
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T. Takeuchi et al. / Journal of Nuclear Materials 449 (2014) 273–276
4.0
samples. The causes of the hardening in the d-ferrite phase of the neutron-irradiated cladding were likely irradiation-induced defects. The concentration fluctuations of Cr, Si, Mn, and Ni enhanced by irradiation might also have contributed to the hardening in particular.
3.0
Acknowledgements
2.0
0
2000
4000
6000
Nanoindentation Hardness [GPa]
Fig. 5. The nanoindentation hardness at the austenite phase at 400 °C with aging time.
4.0 Austenite phase
This study was performed under the Cooperative Research Program of the International Research Center for Nuclear Materials Science, the Institute for Materials Research (IMR), Tohoku University. A part of this study was the result of ‘‘Study of degradation mechanism of stainless steel weld overlay cladding of nuclear reactor pressure vessels’’, undertaken under the Strategic Promotion Program for Basic Nuclear Research by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. This study was partially supported by a Grant-in-Aid for Scientific Research (A) (21246142) from MEXT and by the Budget for Nuclear Research of MEXT, based on screening and counseling by the Atomic Energy Commission. The authors would like to thank M. Narui and M. Yamazaki at IMR for their support in the hot laboratory experiments. References
3.0
2.0
As-received With irradiation
Fig. 6. The nanoindentation hardness at the austenite phase.
the increase in the DBTT was due to the hardening at the d-ferrite phase by both irradiation and aging, as shown in this study. Recently, Fujii et al. reported the effects of irradiation on Cr spinodal decomposition in delta-phase of duplex stainless steel [24]. In their study, CF8M cast duplex stainless steel, which had been previously aged at 400 °C for 10,000 h, was irradiated with Fe ions at a dose of 1.0 dpa and a dose rate of 1.0 104 dpa s1 at approximately 300 °C. The results showed Cr concentration fluctuation, that is, the spinodal decomposition and hardness at the d-ferrite phase decreased due to the irradiation. A very high dose rate of ion irradiation, unlike neutron irradiation, could be related to the different results with neutron-irradiated materials, in which spinodal decomposition has progressed. 4. Summary Hardening by neutron irradiation and concomitant aging were measured using a nanoindentation technique. The results were compared with those of previously observed thermally aged
[1] J.A. Brooks, A.W. Thompson, Int. Mater. Rev. 36 (1) (1991) 16. [2] I. Alvarez-Armas, S. Degallaix-Moreuil, Duplex Stainless Steels, Wiley-ISTE, New York, 2009. [3] F.M. Haggag, S.K. Iskander, Nuclear Enginieering and Design 118 (1990) 297. [4] F.M. Haggag, W.R. Corwin, R.K. Nanstad, Nuclear Enginieering and Design 124 (1990) 129. [5] F.M. Haggag, R.K. Nanstad, NUREG/CR-6363 ORNL/TM-13047, 1997. [6] W.R. Corwin, R.G. Berggren, R.K. Nanstad, R.J. Gray, Nucl. Eng. Des. 89 (1985) 199. [7] W.R. Corwin, Nucl. Eng. Des. 98 (1987) 171. [8] Y. Nishiyama, K. Onizawa, Y. Idei, M. Suzuki, JAERI-Research 2000-47 (2000) (in Japanese). [9] T. Takeuchi, J. Kameda, Y. Nagai, T. Toyama, Y. Nishiyama, K. Onizawa, J. Nucl. Mater. 415 (2011) 198. [10] T. Takeuchi, J. Kameda, Y. Nagai, T. Toyama, Y. Matsukawa, Y. Nishiyama, K. Onizawa, J. Nucl. Mater. J. Nucl. Mater. 425 (2012) 60. [11] T. Takeuchi, Y. Kakubo, Y. Matsukawa, Y. Nozawa, T. Toyama, Y. Nagai, Y. Nishiyama, J. Katsuyama, Y. Yamaguchi, K. Onizawa and M. Suzuki, J. Nucl. Mater., submitted for publication. [12] T. Takeuchi, Y. Kakubo, Y. Matsukawa, Y. Nozawa, Y. Nagai, Y. Nishiyama, J. Katsuyama, K. Onizawa, M. Suzuki, J. Nucl. Mater. 443 (2013) 266. [13] D.J. Larson, D.T. Foord, A.K. Petford-Long, H. Liew, M.G. Blamire, A. Cerezo, G.D.W. Smith, Ultramicroscopy 79 (1999) 287. [14] M.K. Miller, Atom Probe Tomography, Kulwar Academic/Plenum Publishing, New York, NY, 2000. [15] S.J. Sijbrandij, A. Cerezo, T.J. Godfrey, G.D.W. Smith, App. Surf. Sci. 94 (95) (1996) 428. [16] N.M. Chung, O.K. Chopra, International symposium on environmental degradation of materials in nuclear power systems, Traverse City, MI, USA, 30 August 1987. [17] D. Blavette, G. Grancher, A. Bostel, J. Phys. 49-C6 (1988) 433. [18] P. Auger, A. Menand, D. Blavette, J. Phys. 49-C6 (1988) 439. [19] P.R. Okamoto, H. Wiedersich, J. Nucl. Mater. 53 (1974) 242. [20] R.A. Johnson, N.Q. Lam, Phys. Rev. B 13 (1976) 4364. [21] D.C. Ghosh, R. Biswas, Int. J. Mol. Sci. 3 (2002) 87. [22] F. Danoix, P. Auger, D. Blavette, Microsc. Microanal. 10 (2004) 349. [23] C. Pareige, S. Novy, S. Saillet, P. Pareige, J. Nucl. Mater. 411 (2011) 90. [24] K. Fujii, K. Fukuya, J. Nucl. Mater. 440 (2013) 612.
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Effects of neutron irradiation on microstructures and hardness of stainless steel weld-overlay cladding of nuclear reactor pressure vessels T. Takeuchi a,⇑, Y. Kakubo b, Y. Matsukawa b, Y. Nozawa b, T. Toyama b, Y. Nagai b, Y. Nishiyama c, J. Katsuyama c, Y. Yamaguchi c, K. Onizawa c a b c
Oarai Research and Development Center, Japan Atomic Energy Agency, Oarai, Ibaraki 311-1393, Japan The Oarai Center, Institute for Materials Research, Tohoku University, Oarai, Ibaraki 311-1313, Japan Nuclear Safety Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
a r t i c l e
i n f o
Article history: Available online 3 February 2014
a b s t r a c t The microstructures and the hardness of stainless steel weld overlay cladding of reactor pressure vessels subjected to neutron irradiation at a dose of 7.2 1019 n cm2 (E > 1 MeV) and a flux of 1.1 1013 n cm2 s1 at 290 °C were investigated by atom probe tomography and by a nanoindentation technique. To isolate the effects of the neutron irradiation, we compared the results of the measurements of the neutron-irradiated samples with those from a sample aged at 300 °C for a duration equivalent to that of the irradiation. The Cr concentration fluctuation was enhanced in the d-ferrite phase of the irradiated sample. In addition, enhancement of the concentration fluctuation of Si, which was not observed in the aged sample, was observed. The hardening in the d-ferrite phase occurred due to both irradiation and aging; however, the hardening of the irradiated sample was more than that expected from the Cr concentration fluctuation, which suggested that the Si concentration fluctuation and irradiation-induced defects were possible origins of the additional hardening. Ó 2014 Published by Elsevier B.V.
1. Introduction Stainless steel weld overlay cladding on the inner surfaces of commercial water-cooled reactor pressure vessels (RPVs) constitutes a protective barrier that protects RPVs from corrosion by cooling water. The cladding material is a duplex phase structure composed of approximately 90% austenite phase and 10% d-ferrite phase with net-like structures [1]. The mixed d-ferrite and austenitic phases bring about enhanced corrosion resistance of the cladding [2]. It is known that the d-ferrite phase is significantly hardened during thermal aging and neutron irradiation, giving rise to an increase in the ductile-to-brittle transition temperature (DBTT) and a decrease in upper-shelf energy [3–8]. However, the relationship between microstructures and the corresponding mechanical properties have not been revealed for neutron-irradiated cladding. Recently, we investigated the microstructural changes and the hardness in claddings aged at 400 °C for 100–10,000 h by threedimensional atom probe tomography (APT) [9–11]. Fluctuation in the Cr concentration due to spinodal decomposition was observed, in addition to NiSiMn clusters in the d-ferrite phase. The hardening ⇑ Corresponding author. Tel.: +81 29 266 7032; fax: +81 29 266 7071. E-mail address: [email protected] (T. Takeuchi). http://dx.doi.org/10.1016/j.jnucmat.2014.01.004 0022-3115/Ó 2014 Published by Elsevier B.V.
of the d-ferrite phase with aging showed a good linear relationship with the degree of fluctuation in the Cr concentration, independent of the change in the number and density of the NiSiMn clusters. We also investigated the microstructural changes in cladding that was neutron-irradiated with a dose of 7.2 1019 n cm2 (E > 1 MeV) and a flux of 1.1 1013 n cm2 s1 at 290 °C by APT [12]. Neutron irradiation resulted in a slight progression in Cr spinodal decomposition and an increase in the fluctuation of the Si, Ni, and Mn concentrations in the d-ferrite phase. However, the hardness of neutron-irradiated cladding has not been measured. The objective of this research was to clarify using nanoindentation the contribution of neutron irradiation to the hardness of the cladding and to reveal its origins based on the differences in effects of neutron irradiation and thermal aging on microstructures observed by APT. 2. Experimental The cladding used was fabricated by an electroslag weldoverlay method, which is same used for actual reactor cladding. The chemical composition is listed in Table 1. The cladding was subjected to post-weld heat treatment (PWHT) at 615 °C for 7 h with subsequent furnace cooling. The sample contained approximately 8% d-ferrite phase with net-like structures in the matrix
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T. Takeuchi et al. / Journal of Nuclear Materials 449 (2014) 273–276
Table 1 Chemical composition (wt.%) of the sample. C
Si
Mn
P
S
Ni
Cr
Mo
0.013
0.62
1.28
0.02
0.002
10.7
19.5
0.02
austenite phase. The width of the d-ferrite phase was a few microns. We prepared three samples for experiments by subjecting then to the following treatments: (1) PWHT followed by furnace cooling (as received); (2) neutron irradiation at a dose of 7.2 1019 n cm2 (E > 1 MeV) and a flux of 1.1 1013 n cm2 s1 at 290 °C (with irradiation); and (3) thermal aging at 300 °C for 2000 h (without irradiation), which was equivalent to the temperature (at 290 °C) and time (for 1800 h) during neutron irradiation. The samples for APT analysis were first lifted using a lift-out method with a dual-beam-focused ion beam scanning electron microscope (FIB/SEM) ion milling system [13], and then they were sharpened to select the austenite or d-ferrite phase at the tips of the needle-like specimens. To analyze the gallium-free area, the damaged layer caused by gallium implantation during FIB fabrication was minimized by applying low-energy ions during the final process. A UV laser-pulse APT (CAMECA Instruments LEAP 4000X-HR), equipped with an energy-compensating reflection lens [14–16], was used for the microstructure analysis. The measurements were performed at a laser pulse power of 60 pJ and a repetition rate of 200 kHz at approximately 60 K. The high DC bias was typically in the range of 3–10 kV. A few tens of millions of atoms were typically collected for each measurement. The samples for nanoindentation were treated by the following procedure as a surface treatment. The samples were mounted in epoxy resin, followed by mechanical polishing with sandpaper and an electrochemical polish in an ethanol solution with a mixture of 5% perchloric acid at 8 °C and finally by a chemical polish in nitro-hydrochloric acid. Hardness testing was performed using a nanoindenter (ENT1100a by Elionix Inc.). Because the width of the net-like d-ferrite phase was only a few microns, as mentioned above, the indentation size needed to be less than a few hundreds nanometers to avoid the influence of the surrounding austenite phase. However, the effect of the surface was likely to appear if the indentation load was overly light. We tried loads of 1, 2, 3, 5, 7, and 9 mN and observed the sizes of the indentations. The SEM image of the indentation at 2 mN is shown in Fig. 1 as an example. With increasing the load, the lowest and mean hardness values tended to shift toward lower values, which likely included the hardness of the
indentations
10 µm Fig. 1. SEM image of indentations made with loads of 2 mN at the d-ferrite phase.
austenite phase surrounding the targeted d-ferrite phase. Therefore, the lower load was as desirable as possible. However, the indentation depth with 1 mN, with which the SEM image of the indentation was no longer clear, was estimated at less than 100 nm where surface effects could exist. As a result, 2 mN was applied in this study as the optimal load, which was the same as that used in Ref. [11]. The average values of hardness were obtained from approximately 10–20 indentations for each sample.
3. Results and discussion The 3D elemental maps of Fe, Si, Mn, Ni, and Cr in the d-ferrite phase of the as-received samples, both with and without irradiation, are shown in Fig. 2(a)–(c), respectively. The atom maps are constructed from a slice of 20 nm in thickness to illustrate the detailed chemical composition fluctuations clearly. In Fig. 2, the Cr, Ni, Mn, and Si concentration fluctuations seem to be enhanced after irradiation. However, these fluctuations were too small to be analyzed by a commonly used maximum separation algorithm based on a friend-of-friends concept. Thus, to estimate the concentration fluctuations of Cr, Ni, Si, and Mn, we introduced a parameter V, which is defined as
V¼
100 X jOðiÞ BðiÞj;
ð1Þ
i¼0
where i is the number of atoms of the element per 100 atoms, and O(i) and B(i) are the observed and binominal distributions of the mean concentration of each element, respectively [17,18]. The distributions should have been similar to a binominal distribution in shape, had the atoms been randomly distributed, resulting in V = 0. Fig. 3 shows the correlation between the degrees of the Cr concentration fluctuations (VCr) and VNi, VSi, and VMn for the three samples. VCr increased with aging without irradiation and increased more with irradiation. In addition, VNi and VMn increased while VSi slightly decreased in the sample without irradiation; however, they all increased in the sample with irradiation, compared to the asreceived sample. Generally, thermal aging enhances the diffusion of mainly oversized atoms because of the vacancy mechanism [19,20]. In contrast, self-interstitial atoms and vacancies are induced by neutron irradiation, resulting in enhancement of the diffusion of undersized and oversized atoms via mixed-dumbbell and vacancy mechanisms, respectively. For the studied samples, the order of the atomic radii was Cr > Mn > (Fe) > Ni > Si [21]. Therefore, the inverse behavior of the VSi value between the irradiated and aged samples was quite reasonable from the viewpoint of the difference in atom sizes [12]. Next, the relationship between the microstructural changes and the hardening at the d-ferrite phase was studied. The relationship between the VCr and the nanoindentation hardness in the d-ferrite phase in the three samples is shown in Fig. 4. A line denoting the results for previously studied thermal-aged samples [11,22,23] was also drawn. The plotted points for the as-received sample and the sample without irradiation were consistent with the correlation line within the margin of error. After irradiation, the point deviated approximately 1 GPa beyond the line outside its margin of error, which implies a cause of hardening other than the Cr concentration fluctuation in the d-ferrite phase of the sample with irradiation. The fluctuations in Si, Mn, and Ni concentrations and the irradiation-induced defects invisible to APT were possible causes. However, the former is unlikely because the change in V was less than 0.1. These trivial concentration fluctuations were considered to hardly affect the motion of dislocations. Therefore, the irradiation-induced defects, such as vacancies and/or their
275
T. Takeuchi et al. / Journal of Nuclear Materials 449 (2014) 273–276
VNi, VSi, and VMn
0.15
Ni Si Mn
With irradiation
0.1
As-received
0.05
0 0.4
Without irradiation
0.6
0.8
VCr Fig. 3. VCr vs. VNi, VSi, and VMn for the samples that are as-received, without irradiation, and with irradiation.
complexes, are believed to be the dominant cause of excess hardening over thermal aging. In the austenite phase, no microstructural changes were found in the samples that were as-received and without irradiation, although the elemental maps are not shown. While no concentration fluctuations were detected, clusters containing Si were observed. The 1D concentration profile penetrating the clusters revealed that Ni also aggregated at the clusters, and the Ni:Si ratio was estimated to be approximately 3:1 [12]. Fig. 5 shows the nanoindentation hardness at the austenite phase at 400 °C with aging time. Thermal aging did not affect the microstructure [11] or the hardness of the austenite phase, even for a long duration. Fig. 6 shows the nanoindentation hardness at the austenite phase of the as-received and irradiated samples. After irradiation, the hardness increased beyond the
Nanoindentation Hardness [GPa]
Fig. 2. Solute atom maps of Fe, Cr, Ni, Mn, and Si in the d-ferrite phase of the samples that are (a) as-received, (b) without irradiation, and (c) with irradiation.
7.5
With irradiation
6.0 Without irradiation As-received
4.5
3.0
0
0.5
1.0
VCr Fig. 4. Relationship between the VCr and the nanoindentation hardness at the dferrite phase in the samples that are as-received, without irradiation, and with irradiation.
margin of error. The above-mentioned NiSi clusters or/and defects caused by irradiation are possible causes of the hardening at the austenite phase with irradiation. It has been reported that yield stress is increased by neutron irradiation, but it is hardly affected by thermal aging, although DBTT is increased by both neutron irradiation and thermal aging [8]. For the tensile property, because the deformation of materials was controlled by a phase with a large volume fraction, the irradiation-induced increases in yield stress and tensile strength were almost caused by the changes in austenite phase. Regarding the Charpy impact property, the DBTT was related to the d-ferrite phase because the austenite phase did not exhibit the ductile– brittle transition behavior in general. Therefore, it is expected that
276
T. Takeuchi et al. / Journal of Nuclear Materials 449 (2014) 273–276
4.0
samples. The causes of the hardening in the d-ferrite phase of the neutron-irradiated cladding were likely irradiation-induced defects. The concentration fluctuations of Cr, Si, Mn, and Ni enhanced by irradiation might also have contributed to the hardening in particular.
3.0
Acknowledgements
2.0
0
2000
4000
6000
Nanoindentation Hardness [GPa]
Fig. 5. The nanoindentation hardness at the austenite phase at 400 °C with aging time.
4.0 Austenite phase
This study was performed under the Cooperative Research Program of the International Research Center for Nuclear Materials Science, the Institute for Materials Research (IMR), Tohoku University. A part of this study was the result of ‘‘Study of degradation mechanism of stainless steel weld overlay cladding of nuclear reactor pressure vessels’’, undertaken under the Strategic Promotion Program for Basic Nuclear Research by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. This study was partially supported by a Grant-in-Aid for Scientific Research (A) (21246142) from MEXT and by the Budget for Nuclear Research of MEXT, based on screening and counseling by the Atomic Energy Commission. The authors would like to thank M. Narui and M. Yamazaki at IMR for their support in the hot laboratory experiments. References
3.0
2.0
As-received With irradiation
Fig. 6. The nanoindentation hardness at the austenite phase.
the increase in the DBTT was due to the hardening at the d-ferrite phase by both irradiation and aging, as shown in this study. Recently, Fujii et al. reported the effects of irradiation on Cr spinodal decomposition in delta-phase of duplex stainless steel [24]. In their study, CF8M cast duplex stainless steel, which had been previously aged at 400 °C for 10,000 h, was irradiated with Fe ions at a dose of 1.0 dpa and a dose rate of 1.0 104 dpa s1 at approximately 300 °C. The results showed Cr concentration fluctuation, that is, the spinodal decomposition and hardness at the d-ferrite phase decreased due to the irradiation. A very high dose rate of ion irradiation, unlike neutron irradiation, could be related to the different results with neutron-irradiated materials, in which spinodal decomposition has progressed. 4. Summary Hardening by neutron irradiation and concomitant aging were measured using a nanoindentation technique. The results were compared with those of previously observed thermally aged
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