Nam-Jin Heo4, Kenji Shinozaki5 and Minoru Narui6. 1National ..... 15) H. Watanabe, M. Nagamine, K. Yamasaki, T. Muroga, T. Nagasaka,. N. J. Heo and K.
Materials Transactions, Vol. 46, No. 3 (2005) pp. 498 to 502 Special Issue on Fusion Blanket Structural Materials R&D in Japan #2005 The Japan Institute of Metals
Recovery of Hardness, Impact Properties and Microstructure of Neutron-Irradiated Weld Joint of a Fusion Candidate Vanadium Alloy Takuya Nagasaka1 , Takeo Muroga1 , Hideo Watanabe2 , Kazuhiro Yamasaki3 , Nam-Jin Heo4 , Kenji Shinozaki5 and Minoru Narui6 1
National Institute for Fusion Science, Toki 509-5292, Japan Research Institute for Applied Mechanics, Kyushu University, Kasuga 816-8580, Japan 3 Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga 816-8580, Japan 4 Center for Advanced Research of Energy Conversion Materials, Hokkaido University, Sapporo 060-8628, Japan 5 Department of Mechanical System Engineering, Graduate School of Engineering, Hiroshima University, Higashi Hiroshima 739-8527, Japan 6 International Research Center for Nuclear Materials Science, Institute for Materials Research, Tohoku University, Oarai 311-1313, Japan 2
Weld samples of fusion reactor vanadium alloy (NIFS-HEAT-2) were neutron-irradiated at 563 K up to 4:5 1023 neutrons/m2 (0.08 dpa). The recovery of irradiation hardening, degradation of impact properties, and damage structures in the weld metal were investigated after post-irradiation isothermal annealing at temperatures between 673 and 1073 K. Irradiation hardening and the decrease in impact absorbed energy at 77 K were larger for the weld metal than for the base metal. Recovery of the hardening by post-irradiation annealing was coincident with recovery of the impact absorbed energy in both the weld metal and the base metal. Recovery of the mechanical properties required postirradiation annealing at 1073 K for 1 h for the weld metal, which was 100 to 200 K higher than that for the base metal. The dislocation loops introduced by the neutron irradiation are likely to account for the hardening. The dislocation loops were observed even after annealing at 973 K in the weld metal, whereas they disappeared in the base metal. The characteristics of the radiation defects in the weld metal and the mechanisms for irradiation hardening and embrittlement are discussed. (Received October 21, 2004; Accepted January 18, 2005) Keywords: low activation materials, laser welding, radiation defect
1.
Introduction
Energy, E/(Bb) 3/2 / J mm
-3
High-purity low-activation vanadium alloy for fusion reactors has good mechanical properties after welding.1,2) Figure 1 shows that the ductile-brittle transition temperature (DBTT) of the weld is sufficiently low after neutron irradiation to 0.08 displacement per atom (dpa), although irradiation hardening and embrittlement are enhanced in the weld metal compared with the base metal.3) Further irradiation hardening and embrittlement in the welds are critical issues for fusion blanket components, which are subjected to much heavier neutron irradiation. From a practical view-
0.5 NIFS-HEAT-2
EU
0.4 0.3 0.2 0.1 0
EU / 2 DBTT = 113 K
BM, un-irrad. WM, un-irrad. BM, irrad. WM, irrad.
point, the recovery temperature of the absorbed energy of the irradiated weld metal is especially important. In the present study, recovery of hardness, impact properties, and microstructure of vanadium weld metal were investigated by post-irradiation annealing and testing. The characteristics of the radiation defects contributing to the changes in the mechanical properties are discussed to understand the mechanisms of irradiation hardening and embrittlement of the weld metal. 2.
Experimental Procedure
A reference low-activation V-4Cr-4Ti alloy, NIFS-HEAT2,4) was used in the present study. The alloy plates of 4 mm thickness were annealed at 1273 K for 2 h prior to welding. The weld samples were made by bead-on-plate welding with 1.6 kW YAG laser in high-purity Ar. Welding speed was 0.33 m/min. Input power was 290 J/m. The laser focal point was at the plate surface. Table 1 lists the impurity levels in the base metal and the weld metal. Miniature V-notch impact specimens with a size of 1:5 mm 1:5 mm 20 mm were sampled from the center
Recovery was investigated
100 200 300 Test Temperature, T/ K
Fig. 1 Temperature dependence of impact absorbed energy for the base metal (BM) and the weld metal (WM) before and after the neutron irradiation up to 4:5 1023 neutrons/m2 at 563 K. The figure is cited in Ref. 3). The present study investigated recovery of the absorbed energy at 77 K, hardness and microstructures by post-irradiation annealing.
Table 1 Impurity levels in the base metal and the weld metal of a NIFSHEAT-2 weld sample (mass ppm). Cr*
Ti*
H
C
N
O
Base metal
4.00
4.02
29
51
123
139
Weld metal
NA
NA
35
49
129
158
*mass%, NA: Not analyzed
Recovery of Mechanical Properties of Irradiated Weld Joint of Vanadium Alloy
NIFS-HEAT-2
0.5
BM, un-irrad. WM, un-irrad. BM, irrad. WM, irrad.
Energy, E/(Bb)3/2 / J mm-3
Hardness, H / Hv
250
As-irrad.
200
150 As-welded 600
700 800 900 1000 1100 1200 Annealing temperature, T / K
Fig. 2 Hardness of the base metal and the weld metal just after laser welding (As-welded) and neutron irradiation (As-irrad.). Temperature dependences of the hardness after post-irradiation annealing (irrad.) and after post-weld annealing (un-irrad.) are also shown. The data for un-irrad. are cited in Ref. 6).
position in the plate thickness. The notch was 0.3 mm in depth and was placed on either the base metal or the center of the weld metal. Coupons for the hardness test with a size of 1 mm 4 mm 20 mm and disks for Transmission electron microscope (TEM) with a size of 3 mm 0:25 mm were also prepared from both the base metal and the weld metal. The specimens were annealed at 673 K for 1 h prior to neutron irradiation for degassing hydrogen picked up during the machining and grinding. It was shown that the impact properties were not influenced by the degassing.5) The samples were irradiated in the Japan Materials Testing Reactor (JMTR) at 563 K. The neutron fluence was 4:5 1023 neutrons/m2 (0.08 dpa). After the irradiation, the samples were isothermally annealed at 673–1073 K for 1 h in a vacuum. Charpy impact tests, Vickers hardness tests with a load of 1 N for 30 s, and microstructural analyses by TEM operated at 200 keV were conducted. 3.
Results
Figure 2 shows the Vickers hardness (Hv) of the base metal and the weld metal before and after neutron irradiation, and their recovery behavior during post-irradiation annealing for 1 h. The hardness of the weld metal was defined as the average of the hardnesses measured in the area within 1 mm from the center of the weld bead, where the hardness was almost homogeneous.3) The hardness of the base metal and the weld metal after the irradiation was 190 Hv and 239 Hv. The irradiation hardening of the weld metal was 64 Hv, which is 40% greater than that of the base metal (46 Hv). Irradiation hardening of the base metal decreased with increasing annealing temperature. The hardness of the base metal recovered to 152 Hv at 873 K, which is the same hardness level as that of the un-irradiated base metal. In contrast, the weld metal kept a high hardness level above 213 Hv up to 973 K and then showed a recovery to 164 Hv at 1073 K. The hardness recovery temperatures of the base metal and weld metal were estimated to be 873 K and 1073 K. The recovery behavior of the impact absorbed energies at 77 K during annealing is shown in Fig. 3, as well as the energy levels before irradiation (dotted lines). In the as-
Before irrad. 0.4 WM BM
NIFS-HEAT-2 77 K
499
BM, irrad. WM, irrad.
0.3 0.2 As-irrad.
0.1 0
600
700 800 900 1000 1100 1200 Annealing temperature, T / K
Fig. 3 Absorbed energy in the impact tests at 77 K after neutron irradiation and post-irradiation annealing. The absorbed energies are normalized by specimen width (B) and ligament size (b), which is given by B – d, where d is the notch depth.
irradiated condition, the absorbed energy of the base metal (weld metal) was 0.28 J/m3 (0.033 J/m3 ), which is 83% (8.7%) of that before irradiation. The absorbed energy of the base metal was fully recovered by annealing at 973 K, while that of the weld metal required annealing at 1073 K for recovery. Figure 4 shows the microstructures of the base metal and the weld metal at as-irradiated condition and after postirradiation annealing. Black dots (5 nm or less in diameter) were observed (except in the base metal after post irradiation annealing at 973 K). Figure 5 presents high magnification images of Fig. 4(b) with different reflections. From the visible and invisible combination, the black dots were characterized as dislocation loops with a Burgers vector of a=2h111i and ah100i or a=2h110i. The ah100i and a=2h110i types could not be separated from each other in the present analysis. Precipitates aligned to the h100i direction have been reported in the weld metal after post-weld annealing at 873 K and higher.5,6) However, no precipitate was identified in the present study. Figure 6 presents the number density of the dislocation loops. The number density of the dislocation loops in the base metal started to decrease at 873 K and disappeared at 973 K, while that in the weld metal decreased only after annealing at 973 K. 4.
Discussion
Recovery of the impact absorbed energy shown in Fig. 3 is associated with the decrease in the hardening by postirradiation annealing as indicated in Fig. 2. Figure 7 illustrates the correlation between the hardness and the absorbed energy. Irradiation embrittlement is promoted by irradiation hardening in both the weld and the base metals. The dislocation loops observed in Fig. 4 are considered to be obstacles that interrupt the dislocation movement during deformation and induce irradiation hardening. The contribution of the dislocation loops can be estimated as follows.7) pffiffiffiffiffiffi ¼ b Nd ð1Þ : Increase in shear stress by hardening : Barrier strength for the dislocation loops
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Fig. 4 Radiation defect structures in the base metal and the weld metal as-irradiated and after the post-irradiation isothermal annealing at temperature between 673 K and 973 K.
: shear modulus = 46.7 GPa b: Burgers vector = 0.26 nm N: Number density of the dislocation loops d: Average diameter of the dislocation loops. This can be converted to the hardness increase, H as shown below.
H ¼ Cy ¼ CM
ð2Þ
C: constant y : Yield stress M: Taylor factor Previous studies have reported that ranges 0.34 to 0.56 for the base metal (BM ).8–10) Rice et al. reported 0.4 to 0.5 as
Recovery of Mechanical Properties of Irradiated Weld Joint of Vanadium Alloy
Fig. 5
High magnification images of Fig. 4(b) with different reflection conditions.
22
Number density, N / m
-3
10
BM, irrad. WM, irrad.
Semi-log plot NIFS-HEAT-2
21
10
As-irrad. 20
10
Disappeared
19
10
600
700 800 900 1000 1100 1200 Annealing temperature, T / K
Fig. 6 Number density of the dislocation loops shown in Fig. 4.
Energy, E/(Bb)3/2 / J mm-3
0.5 NIFS-HEAT-2 77 K
BM, as-welded 0.4
WM, as-welded
0.3
BM, as-irrad. WM, Irrad. + annealing
0.2 0.1 0
BM, Irrad. + annealing 150
501
200 Hardness, H / Hv
WM, as-irrad. 250
Fig. 7 Correlation between hardness and absorbed energy in the impact tests at 77 K.
mainly for a=2h110i type loops,10) which have high stacking fault energy; therefore they are considered to have the largest in the observed dislocation loop types. It has also been suggested that for the weld metal (WM ) is 1.25 times larger than BM ,3) which is interpreted as follows. The base metal contains Ti-(C, N, O) precipitates before the welding.11) They were decomposed during the welding, and the interstitial impurities were released into the matrix of the weld metal. The impurities are considered to decorate and stabilize the dislocation loops during irradiation. Such interaction has been reported as radiation anneal hardening at 473 to 773 K in pure vanadium,12–14) where the residual interstitial impurities are as free from Ti as is the weld metal. The decorating impurities could enhance thermal stability of the dislocation loops and delay their recovery. In order to estimate the maximum contribution of the dislocation loops to the hardening, let BM be the maximum value reported, 0.56, and WM be 1:25 BM ¼ 0:70. Assuming d ¼ 5 nm, C ¼ 3 and M ¼ 310) for rough estimation, the hardening estimated from the number density of the dislocation loops is given in Fig. 8. The tendency of the estimated hardening to decrease with the annealing temperature is likely to be consistent with that in the experimental hardening calculated from Fig. 2. The dislocation loops and the decrease in number density could account for the irradiation hardening and its recovery. However, the estimated values are less than 50% of the experimental ones. Other obstacles smaller than the limit of resolution of the 200 kV TEM should be tape into account. Possible obstacles are invisible radiation defect clusters, Ti(C, N, O) clusters, and precipitates. In the base metal, the decrease in the difference between the estimated and
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Hardening, ∆ H / Hv
70 60 50
Estimated Experimental
WM
BM
40 30 20 10 0 As-irrad. 773 K 973 K 673 K 873 K 673 K 873 K As-irrad. 773 K 973 K
Fig. 8 Comparison of the hardening estimated from the number density of the dislocation loops and that experimentally obtained at as-irradiated and post-irradiation annealed conditions.
experimental value can be understood by annihilation of unstable, small, and invisible radiation defect clusters. In contrast, the weld metal posses a larger gap between the estimated and experimental hardening than does the base metal. Irradiation-induced precipitates of 1 to 2 nm in size have been identified in the weld metal of NIFS-HEAT-2 by high-resolution TEM after Cu ion irradiation up to 7.5 dpa at 573 K.15) In another work, irradiation-induced precipitates have been identified at doses of 0.013 dpa and higher at 613 K in V-4Cr-4Ti-Si alloy. At 873 K and higher temperature, thermal precipitates have been identified in NIFSHEATs.5,6,11) The apparent start of the thermal precipitation varied from 873 to 1073 K. Assuming the obstacles are 1 nm in size and 3 1022 m3 in number density, they may produce 43 Hv of hardening, which is comparable to the observed difference between the estimated and experimental values in the weld metal. Appropriate post-weld heat treatment is required in order to avoid the additional hardening and the delay in the recovery of the mechanical and microstructural changes of the weld metal. It has been reported that post-weld annealing at 873 to 1223 K effectively reduced neutron irradiation hardening in the weld metal to the level of the base metal, where the impurities were re-trapped by Ti and produced precipitates before irradiation.3) Thus, the post-weld heat treatment at 1073 K has been recommended6) to suppress precipitation hardening and embrittlement. Neutron irradiation programs for weld samples after the post-weld heat treatment at 1073 K have been initiated by using JMTR and JOYO reactors. 5.
Conclusion
Neutron-irradiation hardening and irradiation embrittlement in the weld metal of NIFS-HEAT-2 (V-4Cr-4Ti alloy)
were larger than in the base metal, although the same type radiation defects and similar number density of the radiation defects were found in both metals. The recovery of the hardening by post-irradiation annealing was accompanied by the recovery of the impact properties of the weld metal as well as of the base metal. The recovery of the mechanical properties for the weld metal required post-irradiation annealing at 1073 K for 1 h, which was 100 to 200 K higher than that required for the base metal. Dislocation loops were observed until 973 K in the weld metal, but disappeared in the base metal at 973 K. The observed dislocation loops cannot account for all the hardening. Additional hardening and delay in the recovery of the mechanical and the microstructural properties are considered to be due to visible dislocation loops as well as invisible radiation defect clusters, such as small loops, Ti-(C, N, O) clusters, and precipitates, in the 200 keV TEM observation. Acknowledgements This work was partly performed under the inter-university cooperative Research program of the Institute for Materials Research, Tohoku University. The authors are grateful to the staff of the International Research Center for Nuclear Materials Science, Institute for Materials Research, Tohoku University, for the post-irradiation experiments. REFERENCES 1) T. Nagasaka, M. L. Grossbeck, T. Muroga and J. F. King: Fusion Technol. 39 (2001) 664–668. 2) N. J. Heo, T. Nagasaka, T. Muroga, A. Nishimura, K. Shinozaki and N. Takeshita: Fusion Eng. Design 61–62 (2002) 749–755. 3) T. Nagasaka, N. J. Heo, T. Muroga, A. Nishimura, H. Watanabe, M. Narui and K. Shinozaki: J. Nucl. Mater. 329–333 (2004) 1539–1543. 4) T. Nagasaka, N. J. Heo, T. Muroga and M. Imamura: Fusion Eng. Design 61–62 (2002) 757–762. 5) T. Nagasaka, T. Muroga, M. L. Grossbeck and T. Yamamoto: J. Nucl. Mater. 307–311 (2002) 1595–1599. 6) N. J. Heo, T. Nagasaka, T. Muroga, H. Watanabe, A. Nishimura and K. Shinozaki: To be published in J. Nucl. Mater. 7) A. L. Bement, Jr.: Am. Soc. Met. 2 (1970) 693. 8) T. Chuto, Msatou and K. Abe: J. Nucl. Mater. 283–287 (2000) 503– 507. 9) K.-i. Fukumoto, H. Matsui, Y Candra, K. Takahashi, H. Sasanuma, S. Nagata and K. Takahiro: J. Nucl. Mater. 283–287 (2000) 535–539. 10) P. M. Rice and S. J. Zinkle: J. Nucl. Mater. 258–263 (1998) 1414–1419. 11) N. J. Heo, T. Nagasaka, T. Muroga and H. Matsui: J. Nucl. Mater. 307– 311 (2002) 620–624. 12) K. Shiraishi, K. Fukuya and Y. Katano: J. Nucl. Mater. 44 (1972) 228– 238. 13) J. T. Stanley, J. M. Williams, W. E. brundage and M. S. Wechsler: Acta Metall. 20 (1972) 191–198. 14) S. Morozumi, M. Goto, Y. Tukaue and K. Kayano: J. Japan Inst. Metals 39 (1975) 801–808. 15) H. Watanabe, M. Nagamine, K. Yamasaki, T. Muroga, T. Nagasaka, N. J. Heo and K. Shinozaki: J. Plasma Fusion Res. 80 (2004) 889–894.
