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Hardness distribution and tensile properties in an electron beam weldment of F82H irradiated in HFIR
Journal of Nuclear Materials 455 (2014) 454–459
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Hardness distribution and tensile properties in an electron beam weldment of F82H irradiated in HFIR H. Oka a,⇑, N. Hashimoto a, T. Muroga b, A. Kimura c, M.A. Sokolov d, T. Yamamoto e, S. Ohnuki a a
Hokkaido University, Sapporo, Hokkaido, Japan National Institute for Fusion Science, Toki, Gifu, Japan c Kyoto University, Uji, Kyoto, Japan d Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA e University of California Santa Barbara, Santa Barbara, CA 93106, USA b
a r t i c l e
i n f o
Article history: Available online 8 August 2014
a b s t r a c t F82H-IEA and its EB-weld joint were irradiated at 573 and 773 K up to 9.6 dpa and the irradiation effect on its mechanical properties and microstructure were investigated. A hardness profile across the weld joint before irradiation showed the hardness in transformed region (TR) was high and especially that in the edge of TR was the highest (high hardness region: HHR) compared to base metal (BM). These hardness distribution was correspond to grain size distribution. After irradiation, hardening in HHR was small compared to other region in the sample. In tensile test, the amount of hardening in yield strength and ultimate tensile strength of F82H EB-weld joint was almost similar to that of F82H-IEA but the fracture position of EB-weld joint was at the boundary of TR and BM. Therefore, the TR/BM boundary is the structural weak point in F82H EB-weld joint after irradiation. As the plastic instability was observed, the dislocation channeling deformation can be expected though the dislocation channel was not observed in this study. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
In the fusion reactor, there will be a lot of joint parts of a reduced activation ferritic/martensitic steel F82H, which is a promising structural material for first wall and blanket components of fusion reactor [1–3]. An electron beam (EB) welding is one of candidates because the volume affected by heating is small compared with the other welding method such as a tungsten inert gas welding [4]. Microstructure of the joint part is different from the base metal (BM) due to the heating during welding, resulting in the mechanical property changes [5–9]. Further, the joint part is used under neutron-irradiation environment. However, the effect of neutron irradiation on microstructure evolution and mechanical property change in EB welded F82H has not been revealed yet. In this study, neutron-irradiated F82H-IEA and its EB-weld joint were compared to investigate the irradiation effect in F82H EB-weld joint through conducting the hardness distribution measurement across the joint, tensile test and transmission electron microscopy (TEM).
The chemical compositions of F82H-IEA and F82H EB-weld used in this study have been given in Ref. [10]. The F82H EB-weld was subject to post weld heat treatment at 993 K for 1 h. A small sheet-type tensile specimen with the gauge section of 5 mm long, 1.2 mm wide and 0.5 mm thick was machined from the joint center so as to coincide with the specimen center for F82H EB-weld joint. The irradiation experiments for F82H-IEA and F82H EB-weld joint were conducted under the US–Japan collaboration program TITAN. The specimens were irradiated up to 9.6 dpa at 573 and 773 K in the rabbit capsule of the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL) [11]. The cycle number of the rabbit irradiation was from 422 to 437. 1 cycle is almost corresponding to 1 month. Neutron fluence (E > 0.1 MeV) was 2.7– 22.3 1025 n/m2. Irradiation temperature was monitored with SiC thermometry. Environment in rabbits was inert gas. Thermal neutron shield was not used. Post-irradiation hardness test was performed along the gauge section to obtain the profile of irradiation hardening using Mitutoyo AAV-500 with the load of 9.8 N. Then, strain-controlled tensile test with the strain rate of 1 10 3 s 1 in vacuum condition was conducted by loading on the shoulders of the specimen in MTS test machines in ORNL hot cell. The fracture surface observation were performed on a
⇑ Corresponding author. Address: Faculty of Engineering, Hokkaido University, N-13, W-8, Kita-ku, Sapporo 060-8628, Japan. Tel./fax: +81 11 706 6772. E-mail address: [email protected] (H. Oka). http://dx.doi.org/10.1016/j.jnucmat.2014.07.076 0022-3115/Ó 2014 Elsevier B.V. All rights reserved.
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H. Oka et al. / Journal of Nuclear Materials 455 (2014) 454–459
Base BaseMetal Metal
Transformed Transformed region region (TR) (TR)
Hardness, Hv
600 600
Mitutoyo AAV-500, Mitutoyo AAV-500, F=9.8N F=9.8N
500 500 400 400 300 300
On the other hand, HHR contains very fine grains (less than 0.5 lm) and needle-like grains (width of 0.3–0.5 lm). This structure may include over-tempered martensite and freshly formed martensite produced due to be heated above Ac1 temperature during the EB welding. Obviously, the distribution of these microstructure is responsible for the distribution of hardness (Fig. 1). Irradiation at 573 K resulted in hardening of about 100 in Hv for F82H EB-weld joint, as shown in Fig. 3a. HHR showed much less irradiation hardening compared with BM. It must be pointed out that the profile of the irradiation hardening shown in Fig 3 was obtained based on the difference between before and after irradiation where the raw data was smoothed. The irradiation hardening was attributable to the dislocation loops induced by irradiation [12–14]. The less irradiation hardening in HHR can be interpreted
200 200 High High hardness hardness region region (HHR) (HHR)
scanning electron microscopy. The samples for microstructural observation were prepared by a focused ion beam machine, and then observed in TEM (Philips CM200) operated at 200 kV.
(a) 500 400 300
200
200
100 0
3. Results and discussions 3.1. Hardness distribution Fig. 1 shows the distribution of hardness in an unirradiated F82H EB-weld joint used in this study. Horizontal axis of Fig. 1 represents the distance from the edge of the tensile specimen. The center region of the specimen with the length of 5 mm was very hard compared to the shoulder of the tensile specimen. In the following, the 5 mm-length region is called transformed region (TR). Notably, both ends in the TR showed the highest hardness in the specimen, so called high hardness region (HHR). Fig. 2 shows the microstructure of BM, the center of the TR and the HHR in F82H EB-weld joint. In the center of TR, the grains with enormously high-aspect ratio (0.2–1.0 lm width and 5–10 lm length) were observed, which is in contrast with the microstructure observed in the BM; a typical ferritic/martensitic microstructure (Fig. 2a).
Hardness, Hv
-100
Change in hardness, ΔH
Fig. 1. Distribution of hardness in an unirradiated F82H EB-weld joint. The configuration of tensile specimen and the range of transformed region (TR) and high hardness region (HHR) are also shown.
600
400
(b) 300 200 0
100
-100 -200 -300 0
2
4
6
8
10
12
14
16
Distance from the specimen edge / mm
Change in hardness, ΔH
22 44 66 88 10 10 12 12 14 14 16 16 Distance Distancefrom fromedge edge/ mm / mm
Hardness, Hv
100 100 00
Fig. 3. Distribution of the hardness and the irradiation hardening in F82H EB-weld joint irradiated at (a) 573 K up to 9.6 dpa and (b) 773 K up to 5.5 dpa. Axis for the irradiation hardening is provided in right hand side.
Fig. 2. Microstructure of (a) base metal (BM), (b) the center of transformed region (TR) and (c) high hardness region (HHR) in unirradiated F82H EB-weld joint.
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H. Oka et al. / Journal of Nuclear Materials 455 (2014) 454–459
F82H-IEA
F82H/F82H EB-weld *1 N. Hashimoto et al., Mat. Res. Soc. Symp. Proc. Vol. 650 (2001) R1.10.
1000
Engineering stress / MPa
(a)
(b)
9.6 dpa
800
9.6 dpa
1.2 dpa 1.2 dpa
600 0 dpa*1
400 0 dpa
200
Irradiated at 573 K, Tested at RT
Irradiated at 573 K, Tested at 573 K
0
0
5 5 % 10
15
20
25
30
0
5 5 % 10
Engineering strain / %
15
20
25
30
Engineering strain / %
1000
Engineering stress / MPa
(c) 800
(d)
0 dpa 5.5 dpa
0 dpa 5.5 dpa
0 dpa*1 5.5 dpa
9.6 dpa
9.6 dpa
Irradiated at 773 K, Tested at 773 K
9.6 dpa
0 dpa 5.5 dpa 9.6 dpa
600
400
200
Irradiated at 773 K, Tested at RT 0
0
5 5 % 10
15
20
25
Engineering strain / %
30
0
5 5 % 10
15
20
25
30
Engineering strain / %
Fig. 4. Stress–strain curve of F82H-IEA and F82H EB-weld joint irradiated up to 9.6 dpa at 573 K tested at (a) room temperature and (b) 573 K, and irradiated at 773 K tested at (c) room temperature and at (d) 773 K. In each graph, the curves in the left and right hand side represent F82H-IEA and F82H EB-weld joint, respectively.
that the high density of grain boundaries attributed to the finegrained structure act as sink sites for irradiation-induced defects and suppress the formation of dislocation loops [13]. In contrast to the irradiation at 573 K, irradiation softening of about 200 in Hv was obtained in the TR after irradiation at 773 K (Fig. 3b). Note that the distance of 9–16 mm from the specimen edge was excluded from hardness measurement due to the inappropriate surface condition.
3.2. Tensile properties Stress–strain curve of F82H-IEA and F82H EB-weld joint before and after irradiation were shown in Fig. 4a–d. In each graph, the curves in the left hand side represent F82H-IEA, and that in right hand side represent F82H EB-weld joint. Materials, irradiation conditions and the results of the tensile test were summarized in Table 1. The results of unirradiated F82H-IEA tested at 573 and 773 K were quoted from Ref. [12]. Before irradiation (0 dpa), F82H EB-weld joint had slightly higher ultimate tensile strength and less total elongation compared with F82H-IEA at room temperature. This tendency were also exhibited in 573 or 773 K testing as well. The less total elongation of F82H-IEA might be due to the TR which has higher strength and lower ductility than the BM. Additionally, the work hardening capability of F82H EB-weld joint was small compared to that of F82H-IEA. The elongation after
achieving ultimate tensile strength, namely the elongation in inhomogeneous deformation, was small in F82H EB-weld joint, as well. The significant hardening resulting from the irradiation at 573 K was observed and it increased with increasing irradiation dose in both F82H-IEA and F82H EB-weld joint. The yield strength increased almost 80–100% after irradiation to 9.6 dpa. Moreover, the degradation of total elongation were exhibited after the irradiation. Notably in room temperature test, the reduction in total elongation of about 50% were observed in F82H-IEA. However, unlike the F82H-IEA, the reduction in total elongation in F82H EB-weld joint were small, about 30%. Although F82H-IEA and F82H EB-weld joint were different in the reduction of total elongation, plastic instability was exhibited for both material tested at room temperature and 573 K after irradiation, shown in Fig. 4a and b. The details of the plastic instability was discussed later in Section 3.4. In contrast to the irradiation at 573 K, changes in tensile properties for both materials after irradiation at 773 K are comparatively small, as shown in Fig. 4c and d.
3.3. Fracture surface and fracture position The fracture surface of F82H EB-weld joint after 573 K irradiation exhibited that dimples with the size of 20–30 lm and a very fine (1 lm) dimple distribution were observed, indicating that duc-
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H. Oka et al. / Journal of Nuclear Materials 455 (2014) 454–459 Table 1 Tensile test results for all materials in the unirradiated and irradiated condition. Material
Irradiation temperature/K
Capsule ID
Irradiation dose/dpa
Specimen ID
Test temperature/K
Yield strength/MPa
Ultimate tensile strength/MPa
Uniform elongation/%
Total elongation/%
F82H-IEA
573
T9G1
1.2
T9G2
9.6
T9C1
5.5
T9C2
9.6
1 4 5 OWAQ H02 G02 H03 G03 3 6 2 7 W6 8 W5 9
298 298 573 573 298 298 573 573 298 298 773 773 298 298 773 773
727 996 895 744 982 829 995 943 486 498 454 446 450 474 496 430
731 1000 911 744 988 836 997 946 594 608 473 487 546 587 519 453
5.5 5.1 5.4 3.8 5.8 5.9 5.4 5.2 9.4 10.1 5.4 5.8 8.8 10.3 4.9 5.7
13.7 13.1 11.0 6.9 11.7 12.1 11.1 10.7 21.2 23.3 14.2 15.6 20.0 25.6 14.5 15.8
T9G1
1.2
T9G2
9.6
T9C1
5.5
T9C2
9.6
HA01 HA03 HA31 HA33 HA18 HA16 HA17 HA46 HA47 HA48
298 573 298 573 298 773 773 298 773 773
760 789 973 930 526 451 426 479 412 451
766 789 973 930 623 465 441 574 459 467
4.3 4.0 4.8 4.4 5.9 5.0 4.0 6.7 4.2 4.4
8.4 7.7 7.6 6.8 10.5 9.8 9.3 12.3 9.3 11.0
– – – –
– – – –
F04 HA57 HA04 HA55
298 298 573 773
540 591 530 528
635 655 574 555
9.6 6.6 4.8 4.5
22.2 10.4 9.3 8.8
773
F82H/F82H EB-weld
573
773
F82H-IEA F82H/F82H EB-weld
– – – –
tile fracture was the major fracture mechanism, as seen in Fig. 5. Focusing on the broken point on the tensile specimen, it appeared to break at the boundary between TR and BM (Fig. 5c and g) though the size of TR and BM were too small to evaluate the exact point of a break. Therefore, technically, the boundary between TR and BM is a structural weak point of F82H EB-weld joint after irradiation. This results seems to be valid because the distribution of hardness in TR/BM boundary drastically change even after irradiation; the hardness of TR and BM is 550 and 300 for 573 K irradiation, respectively. Inhomogeneous distribution of hardness in a structural material produces the structural weakness in itself.
(a)
(b)
3.4. Microstructure Post irradiation TEM study was carried out for the tensile tested F82H EB-weld joint in order to investigate its deformation mechanism. Fig. 6a shows the configuration indicating lift-out position of FIB specimen. Position A corresponds to BM, meaning that microstructure of position A is as-irradiated condition. Position B is corresponding to the center of TR. Simultaneously, it is a uniformly-deformed region. Position C, which is the necked region during tensile test, corresponds to the HHR. The observed microstructure are shown in Fig. 6b–g, and the direction of tension are
(c)
1 mm 5 µm
100 µ
(d)
(g)
(e)
1 mm 100 µ m
5 µm
Fig. 5. Fracture surface and the overview of the broken tensile specimen of F82H EB-weld joint irradiated at 573 K up to (a–c) 1.2 dpa and (d–f) 9.6 dpa.
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H. Oka et al. / Journal of Nuclear Materials 455 (2014) 454–459
(a) A
B
C
4.1 7.2 10.6 Unit: mm
A (Base material)
(b)
B
C
(Center of transformed region)
(c)
(d)
Tension
(e)
(High hardness region)
Tension
(f)
(g)
N. D.
Tension
Fig. 6. Microstructure of F82H EB-weld joint irradiated at 573 K up to (b–d) 1.2 dpa and (e–g) 9.6 dpa. FIB/TEM sample lifted out from position A: (b) and (e), position B: (c) and position C: (d) and (g).
also shown by white arrow. Note that the ion beam tracks (diagonal in Fig. 6) were produced during FIB preparation. As-irradiated microstructure (1.2 dpa for Fig. 6b and 9.6 dpa for Fig. 6e) exhibited the irradiation-induced defect cluster (shown in Fig. 7) but the grain size and/or precipitation distribution seemed to be retained after irradiation. The microstructure in the center of TR including the grains with high-aspect ratio (0.2–1.0 lm width and 5–10 lm length) was also retained after irradiation (see Fig. 6c). In the same manner, the very fine grain structure in position C was also retained (Fig. 6d and g) after irradiation at 573 K up to 9.6 dpa. The irradiation hardening observed in this study might be attributed to the dislocation loops induced by irradiation. Hashimoto et al. [12–14] found the dislocation channeling deformation in the deformed region of F82H irradiated up to 5 dpa at 573 K in HFIR, which is due to the existence of the dislocation loops with high density, leading to the plastic instability. Although the dislocation channel was not observed microstructurally in this study, the dislocation channeling deformation can be expected for F82H EB-weld joint because the plastic instability was observed in its stress–strain curve. The further investigation to assess the density of dislocation loops need to be done because the dislocation channeling deformation is strongly related to the density of dislocation loops. Furthermore, the microstructure of the necked region in the shoulder side of tensile (the other half of tensile) sample also need
Fig. 7. Dislocation loops in the F82H EB-weld joint irradiated at 573 K up to 9.6 dpa.
to be investigated. Since that region is the deformed BM, the dislocation channel structure can be expected.
H. Oka et al. / Journal of Nuclear Materials 455 (2014) 454–459
4. Summary F82H-IEA and its EB-weld joint irradiated at 573 and 773 K up to 9.6 dpa in the HFIR were investigated. F82H EB-weld joint had a hardness distribution in accordance with microstructural distribution. Although HHR exhibited much less irradiation hardening in hardness compared with BM in F82H EB-weld joint, the boundary between TR and BM is a structural weak point after irradiation. As both F82H-IEA and its EB-weld joint exhibited a plastic instability in tensile test, it can be considered that the dislocation channeling deformation was dominant. Acknowledgements This work was supported by US–Japan collaboration program TITAN. Authors are grateful to Janet Robertson, Pat Bishop and the 3025E operators, Daniel Lewis, Keith Leonard, Kiran, Marie Williams, Patricia Tedder, Kiyohiro Yabuuchi, Takashi Nozawa and Dai Hamaguchi for helping this research. References [1] K. Shiba, M. Suzuki, A. Hishinuma, J. Nucl. Mater. 233–237 (1996) 309.
459
[2] M. Enoeda, M. Akiba, S. Tanaka, A. Shimizu, A. Hasegawa, S. Konishi, A. Kimura, A. Kohyama, A. Sagara, T. Muroga, Fusion Eng. Des. 81 (2006) 415. [3] H. Serizawa, S. Nakamura, H. Tanigawa, H. Ogiwara, J. Nucl. Mater. 442 (2013) S535. [4] H. Tanigawa, T. Hirose, K. Shiba, R. Kasada, E. Wakai, H. Serizawa, Y. Kawahito, S. Jitsukawa, A. Kimura, Y. Kohno, A. Kohyama, S. Katayama, H. Mori, K. Nishimoto, R.L. Klueh, M.A. Sokolov, R.E. Stoller, S.J. Zinkle, Fusion Eng. Des. 83 (2008) 1471. [5] A. Alamo, A. Castaing, A. Fontes, P. Wident, J. Nucl. Mater. 283–287 (2000) 1192. [6] J. Rensman, E.V. van Osch, M.G. Horsten, D.S. D’Hulst, J. Nucl. Mater. 283–287 (2000) 1201. [7] J. Rensman, J. van Hoepen, J.B.M. Bakker, R. den Boef, F.P. van den Broek, E.D.L. van Essen, J. Nucl. Mater. 307–311 (2002) 245. [8] T. Sawai, K. Shiba, A. Hishinuma, J. Nucl. Mater. 283–287 (2000) 657. [9] T. Hirose, M.A. Sokolov, M. Ando, H. Tanigawa, K. Shiba, R.E. Stoller, G.R. Odette, J. Nucl. Mater. 442 (2013) S557. [10] K. Shiba, R.L. Klueh, Y. Miwa, N. Igawa, J.P. Robertson, Fusion materials semiannual progress report, DOE/ER-0313/28 (2000) 131. [11] Y. Katoh, J.K. McDuffee, Fusion materials semiannual progress report, DOE/ER0313/53 (2012) 137. [12] N. Hashimoto, S.J. Zinkle, R.L. Klueh, A.F. Rowcliffe, K. Shiba, Mat. Res. Soc. Symp. Proc. 650 (2001) R1.10. [13] N. Hashimoto, R.L. Klueh, M. Ando, H. Tanigawa, T. Sawai, K. Shiba, Fusion Sci. Technol. 44 (2003) 490. [14] N. Hashimoto, H. Oka, T. Muroga, T. Nagasaka, A. Kimura, S. Ukai, T. Yamamoto, M.A. Sokolov, Mater. Trans. 54 (2013) 442.
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Hardness distribution and tensile properties in an electron beam weldment of F82H irradiated in HFIR H. Oka a,⇑, N. Hashimoto a, T. Muroga b, A. Kimura c, M.A. Sokolov d, T. Yamamoto e, S. Ohnuki a a
Hokkaido University, Sapporo, Hokkaido, Japan National Institute for Fusion Science, Toki, Gifu, Japan c Kyoto University, Uji, Kyoto, Japan d Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA e University of California Santa Barbara, Santa Barbara, CA 93106, USA b
a r t i c l e
i n f o
Article history: Available online 8 August 2014
a b s t r a c t F82H-IEA and its EB-weld joint were irradiated at 573 and 773 K up to 9.6 dpa and the irradiation effect on its mechanical properties and microstructure were investigated. A hardness profile across the weld joint before irradiation showed the hardness in transformed region (TR) was high and especially that in the edge of TR was the highest (high hardness region: HHR) compared to base metal (BM). These hardness distribution was correspond to grain size distribution. After irradiation, hardening in HHR was small compared to other region in the sample. In tensile test, the amount of hardening in yield strength and ultimate tensile strength of F82H EB-weld joint was almost similar to that of F82H-IEA but the fracture position of EB-weld joint was at the boundary of TR and BM. Therefore, the TR/BM boundary is the structural weak point in F82H EB-weld joint after irradiation. As the plastic instability was observed, the dislocation channeling deformation can be expected though the dislocation channel was not observed in this study. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
In the fusion reactor, there will be a lot of joint parts of a reduced activation ferritic/martensitic steel F82H, which is a promising structural material for first wall and blanket components of fusion reactor [1–3]. An electron beam (EB) welding is one of candidates because the volume affected by heating is small compared with the other welding method such as a tungsten inert gas welding [4]. Microstructure of the joint part is different from the base metal (BM) due to the heating during welding, resulting in the mechanical property changes [5–9]. Further, the joint part is used under neutron-irradiation environment. However, the effect of neutron irradiation on microstructure evolution and mechanical property change in EB welded F82H has not been revealed yet. In this study, neutron-irradiated F82H-IEA and its EB-weld joint were compared to investigate the irradiation effect in F82H EB-weld joint through conducting the hardness distribution measurement across the joint, tensile test and transmission electron microscopy (TEM).
The chemical compositions of F82H-IEA and F82H EB-weld used in this study have been given in Ref. [10]. The F82H EB-weld was subject to post weld heat treatment at 993 K for 1 h. A small sheet-type tensile specimen with the gauge section of 5 mm long, 1.2 mm wide and 0.5 mm thick was machined from the joint center so as to coincide with the specimen center for F82H EB-weld joint. The irradiation experiments for F82H-IEA and F82H EB-weld joint were conducted under the US–Japan collaboration program TITAN. The specimens were irradiated up to 9.6 dpa at 573 and 773 K in the rabbit capsule of the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL) [11]. The cycle number of the rabbit irradiation was from 422 to 437. 1 cycle is almost corresponding to 1 month. Neutron fluence (E > 0.1 MeV) was 2.7– 22.3 1025 n/m2. Irradiation temperature was monitored with SiC thermometry. Environment in rabbits was inert gas. Thermal neutron shield was not used. Post-irradiation hardness test was performed along the gauge section to obtain the profile of irradiation hardening using Mitutoyo AAV-500 with the load of 9.8 N. Then, strain-controlled tensile test with the strain rate of 1 10 3 s 1 in vacuum condition was conducted by loading on the shoulders of the specimen in MTS test machines in ORNL hot cell. The fracture surface observation were performed on a
⇑ Corresponding author. Address: Faculty of Engineering, Hokkaido University, N-13, W-8, Kita-ku, Sapporo 060-8628, Japan. Tel./fax: +81 11 706 6772. E-mail address: [email protected] (H. Oka). http://dx.doi.org/10.1016/j.jnucmat.2014.07.076 0022-3115/Ó 2014 Elsevier B.V. All rights reserved.
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H. Oka et al. / Journal of Nuclear Materials 455 (2014) 454–459
Base BaseMetal Metal
Transformed Transformed region region (TR) (TR)
Hardness, Hv
600 600
Mitutoyo AAV-500, Mitutoyo AAV-500, F=9.8N F=9.8N
500 500 400 400 300 300
On the other hand, HHR contains very fine grains (less than 0.5 lm) and needle-like grains (width of 0.3–0.5 lm). This structure may include over-tempered martensite and freshly formed martensite produced due to be heated above Ac1 temperature during the EB welding. Obviously, the distribution of these microstructure is responsible for the distribution of hardness (Fig. 1). Irradiation at 573 K resulted in hardening of about 100 in Hv for F82H EB-weld joint, as shown in Fig. 3a. HHR showed much less irradiation hardening compared with BM. It must be pointed out that the profile of the irradiation hardening shown in Fig 3 was obtained based on the difference between before and after irradiation where the raw data was smoothed. The irradiation hardening was attributable to the dislocation loops induced by irradiation [12–14]. The less irradiation hardening in HHR can be interpreted
200 200 High High hardness hardness region region (HHR) (HHR)
scanning electron microscopy. The samples for microstructural observation were prepared by a focused ion beam machine, and then observed in TEM (Philips CM200) operated at 200 kV.
(a) 500 400 300
200
200
100 0
3. Results and discussions 3.1. Hardness distribution Fig. 1 shows the distribution of hardness in an unirradiated F82H EB-weld joint used in this study. Horizontal axis of Fig. 1 represents the distance from the edge of the tensile specimen. The center region of the specimen with the length of 5 mm was very hard compared to the shoulder of the tensile specimen. In the following, the 5 mm-length region is called transformed region (TR). Notably, both ends in the TR showed the highest hardness in the specimen, so called high hardness region (HHR). Fig. 2 shows the microstructure of BM, the center of the TR and the HHR in F82H EB-weld joint. In the center of TR, the grains with enormously high-aspect ratio (0.2–1.0 lm width and 5–10 lm length) were observed, which is in contrast with the microstructure observed in the BM; a typical ferritic/martensitic microstructure (Fig. 2a).
Hardness, Hv
-100
Change in hardness, ΔH
Fig. 1. Distribution of hardness in an unirradiated F82H EB-weld joint. The configuration of tensile specimen and the range of transformed region (TR) and high hardness region (HHR) are also shown.
600
400
(b) 300 200 0
100
-100 -200 -300 0
2
4
6
8
10
12
14
16
Distance from the specimen edge / mm
Change in hardness, ΔH
22 44 66 88 10 10 12 12 14 14 16 16 Distance Distancefrom fromedge edge/ mm / mm
Hardness, Hv
100 100 00
Fig. 3. Distribution of the hardness and the irradiation hardening in F82H EB-weld joint irradiated at (a) 573 K up to 9.6 dpa and (b) 773 K up to 5.5 dpa. Axis for the irradiation hardening is provided in right hand side.
Fig. 2. Microstructure of (a) base metal (BM), (b) the center of transformed region (TR) and (c) high hardness region (HHR) in unirradiated F82H EB-weld joint.
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F82H-IEA
F82H/F82H EB-weld *1 N. Hashimoto et al., Mat. Res. Soc. Symp. Proc. Vol. 650 (2001) R1.10.
1000
Engineering stress / MPa
(a)
(b)
9.6 dpa
800
9.6 dpa
1.2 dpa 1.2 dpa
600 0 dpa*1
400 0 dpa
200
Irradiated at 573 K, Tested at RT
Irradiated at 573 K, Tested at 573 K
0
0
5 5 % 10
15
20
25
30
0
5 5 % 10
Engineering strain / %
15
20
25
30
Engineering strain / %
1000
Engineering stress / MPa
(c) 800
(d)
0 dpa 5.5 dpa
0 dpa 5.5 dpa
0 dpa*1 5.5 dpa
9.6 dpa
9.6 dpa
Irradiated at 773 K, Tested at 773 K
9.6 dpa
0 dpa 5.5 dpa 9.6 dpa
600
400
200
Irradiated at 773 K, Tested at RT 0
0
5 5 % 10
15
20
25
Engineering strain / %
30
0
5 5 % 10
15
20
25
30
Engineering strain / %
Fig. 4. Stress–strain curve of F82H-IEA and F82H EB-weld joint irradiated up to 9.6 dpa at 573 K tested at (a) room temperature and (b) 573 K, and irradiated at 773 K tested at (c) room temperature and at (d) 773 K. In each graph, the curves in the left and right hand side represent F82H-IEA and F82H EB-weld joint, respectively.
that the high density of grain boundaries attributed to the finegrained structure act as sink sites for irradiation-induced defects and suppress the formation of dislocation loops [13]. In contrast to the irradiation at 573 K, irradiation softening of about 200 in Hv was obtained in the TR after irradiation at 773 K (Fig. 3b). Note that the distance of 9–16 mm from the specimen edge was excluded from hardness measurement due to the inappropriate surface condition.
3.2. Tensile properties Stress–strain curve of F82H-IEA and F82H EB-weld joint before and after irradiation were shown in Fig. 4a–d. In each graph, the curves in the left hand side represent F82H-IEA, and that in right hand side represent F82H EB-weld joint. Materials, irradiation conditions and the results of the tensile test were summarized in Table 1. The results of unirradiated F82H-IEA tested at 573 and 773 K were quoted from Ref. [12]. Before irradiation (0 dpa), F82H EB-weld joint had slightly higher ultimate tensile strength and less total elongation compared with F82H-IEA at room temperature. This tendency were also exhibited in 573 or 773 K testing as well. The less total elongation of F82H-IEA might be due to the TR which has higher strength and lower ductility than the BM. Additionally, the work hardening capability of F82H EB-weld joint was small compared to that of F82H-IEA. The elongation after
achieving ultimate tensile strength, namely the elongation in inhomogeneous deformation, was small in F82H EB-weld joint, as well. The significant hardening resulting from the irradiation at 573 K was observed and it increased with increasing irradiation dose in both F82H-IEA and F82H EB-weld joint. The yield strength increased almost 80–100% after irradiation to 9.6 dpa. Moreover, the degradation of total elongation were exhibited after the irradiation. Notably in room temperature test, the reduction in total elongation of about 50% were observed in F82H-IEA. However, unlike the F82H-IEA, the reduction in total elongation in F82H EB-weld joint were small, about 30%. Although F82H-IEA and F82H EB-weld joint were different in the reduction of total elongation, plastic instability was exhibited for both material tested at room temperature and 573 K after irradiation, shown in Fig. 4a and b. The details of the plastic instability was discussed later in Section 3.4. In contrast to the irradiation at 573 K, changes in tensile properties for both materials after irradiation at 773 K are comparatively small, as shown in Fig. 4c and d.
3.3. Fracture surface and fracture position The fracture surface of F82H EB-weld joint after 573 K irradiation exhibited that dimples with the size of 20–30 lm and a very fine (1 lm) dimple distribution were observed, indicating that duc-
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H. Oka et al. / Journal of Nuclear Materials 455 (2014) 454–459 Table 1 Tensile test results for all materials in the unirradiated and irradiated condition. Material
Irradiation temperature/K
Capsule ID
Irradiation dose/dpa
Specimen ID
Test temperature/K
Yield strength/MPa
Ultimate tensile strength/MPa
Uniform elongation/%
Total elongation/%
F82H-IEA
573
T9G1
1.2
T9G2
9.6
T9C1
5.5
T9C2
9.6
1 4 5 OWAQ H02 G02 H03 G03 3 6 2 7 W6 8 W5 9
298 298 573 573 298 298 573 573 298 298 773 773 298 298 773 773
727 996 895 744 982 829 995 943 486 498 454 446 450 474 496 430
731 1000 911 744 988 836 997 946 594 608 473 487 546 587 519 453
5.5 5.1 5.4 3.8 5.8 5.9 5.4 5.2 9.4 10.1 5.4 5.8 8.8 10.3 4.9 5.7
13.7 13.1 11.0 6.9 11.7 12.1 11.1 10.7 21.2 23.3 14.2 15.6 20.0 25.6 14.5 15.8
T9G1
1.2
T9G2
9.6
T9C1
5.5
T9C2
9.6
HA01 HA03 HA31 HA33 HA18 HA16 HA17 HA46 HA47 HA48
298 573 298 573 298 773 773 298 773 773
760 789 973 930 526 451 426 479 412 451
766 789 973 930 623 465 441 574 459 467
4.3 4.0 4.8 4.4 5.9 5.0 4.0 6.7 4.2 4.4
8.4 7.7 7.6 6.8 10.5 9.8 9.3 12.3 9.3 11.0
– – – –
– – – –
F04 HA57 HA04 HA55
298 298 573 773
540 591 530 528
635 655 574 555
9.6 6.6 4.8 4.5
22.2 10.4 9.3 8.8
773
F82H/F82H EB-weld
573
773
F82H-IEA F82H/F82H EB-weld
– – – –
tile fracture was the major fracture mechanism, as seen in Fig. 5. Focusing on the broken point on the tensile specimen, it appeared to break at the boundary between TR and BM (Fig. 5c and g) though the size of TR and BM were too small to evaluate the exact point of a break. Therefore, technically, the boundary between TR and BM is a structural weak point of F82H EB-weld joint after irradiation. This results seems to be valid because the distribution of hardness in TR/BM boundary drastically change even after irradiation; the hardness of TR and BM is 550 and 300 for 573 K irradiation, respectively. Inhomogeneous distribution of hardness in a structural material produces the structural weakness in itself.
(a)
(b)
3.4. Microstructure Post irradiation TEM study was carried out for the tensile tested F82H EB-weld joint in order to investigate its deformation mechanism. Fig. 6a shows the configuration indicating lift-out position of FIB specimen. Position A corresponds to BM, meaning that microstructure of position A is as-irradiated condition. Position B is corresponding to the center of TR. Simultaneously, it is a uniformly-deformed region. Position C, which is the necked region during tensile test, corresponds to the HHR. The observed microstructure are shown in Fig. 6b–g, and the direction of tension are
(c)
1 mm 5 µm
100 µ
(d)
(g)
(e)
1 mm 100 µ m
5 µm
Fig. 5. Fracture surface and the overview of the broken tensile specimen of F82H EB-weld joint irradiated at 573 K up to (a–c) 1.2 dpa and (d–f) 9.6 dpa.
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H. Oka et al. / Journal of Nuclear Materials 455 (2014) 454–459
(a) A
B
C
4.1 7.2 10.6 Unit: mm
A (Base material)
(b)
B
C
(Center of transformed region)
(c)
(d)
Tension
(e)
(High hardness region)
Tension
(f)
(g)
N. D.
Tension
Fig. 6. Microstructure of F82H EB-weld joint irradiated at 573 K up to (b–d) 1.2 dpa and (e–g) 9.6 dpa. FIB/TEM sample lifted out from position A: (b) and (e), position B: (c) and position C: (d) and (g).
also shown by white arrow. Note that the ion beam tracks (diagonal in Fig. 6) were produced during FIB preparation. As-irradiated microstructure (1.2 dpa for Fig. 6b and 9.6 dpa for Fig. 6e) exhibited the irradiation-induced defect cluster (shown in Fig. 7) but the grain size and/or precipitation distribution seemed to be retained after irradiation. The microstructure in the center of TR including the grains with high-aspect ratio (0.2–1.0 lm width and 5–10 lm length) was also retained after irradiation (see Fig. 6c). In the same manner, the very fine grain structure in position C was also retained (Fig. 6d and g) after irradiation at 573 K up to 9.6 dpa. The irradiation hardening observed in this study might be attributed to the dislocation loops induced by irradiation. Hashimoto et al. [12–14] found the dislocation channeling deformation in the deformed region of F82H irradiated up to 5 dpa at 573 K in HFIR, which is due to the existence of the dislocation loops with high density, leading to the plastic instability. Although the dislocation channel was not observed microstructurally in this study, the dislocation channeling deformation can be expected for F82H EB-weld joint because the plastic instability was observed in its stress–strain curve. The further investigation to assess the density of dislocation loops need to be done because the dislocation channeling deformation is strongly related to the density of dislocation loops. Furthermore, the microstructure of the necked region in the shoulder side of tensile (the other half of tensile) sample also need
Fig. 7. Dislocation loops in the F82H EB-weld joint irradiated at 573 K up to 9.6 dpa.
to be investigated. Since that region is the deformed BM, the dislocation channel structure can be expected.
H. Oka et al. / Journal of Nuclear Materials 455 (2014) 454–459
4. Summary F82H-IEA and its EB-weld joint irradiated at 573 and 773 K up to 9.6 dpa in the HFIR were investigated. F82H EB-weld joint had a hardness distribution in accordance with microstructural distribution. Although HHR exhibited much less irradiation hardening in hardness compared with BM in F82H EB-weld joint, the boundary between TR and BM is a structural weak point after irradiation. As both F82H-IEA and its EB-weld joint exhibited a plastic instability in tensile test, it can be considered that the dislocation channeling deformation was dominant. Acknowledgements This work was supported by US–Japan collaboration program TITAN. Authors are grateful to Janet Robertson, Pat Bishop and the 3025E operators, Daniel Lewis, Keith Leonard, Kiran, Marie Williams, Patricia Tedder, Kiyohiro Yabuuchi, Takashi Nozawa and Dai Hamaguchi for helping this research. References [1] K. Shiba, M. Suzuki, A. Hishinuma, J. Nucl. Mater. 233–237 (1996) 309.
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