22D12
Impact Property of Low-activation Vanadium Alloy and its weld joint after
heavy neutron irradiation
中性子重照射した低放射化バナジウム合金及びその溶接材の衝撃特性
Takuya Nagasaka1, 2, Takeo Muroga1, 2, Hideo Watanabe3, Takeshi Miyazawa2,
Masanori Yamazaki4, Kenji Shinozaki5
長坂琢也 1, 2, 室賀健夫 1, 2, 渡辺英雄 3, 宮澤健 2, 山崎正徳 4, 篠崎賢二 5
1

National Institute for Fusion Science, Toki, Gifu 509-5292, Japan
核融合科学研究所 〒509-5292 岐阜県土岐市下石町 322-6
2
The Graduate University for Advanced Studies, Toki, Gifu 509-5292, Japan2
総合研究大学院大学 〒509-5292 岐阜県土岐市下石町 322-6
3
Research Institute for Applied Mechanics, Kyushu University, Kasuga 816-8580, Japan3
九州大学応用力学研究所 〒816-8580 福岡県春日市春日公園 6-1
4International Research Center for Nuclear Materials Science, Institute for Materials Research, TohokuUniversity, Oarai, Ibaraki 311-1313, Japan
4
東北大学 金属材料研究所附属量子エネルギー材料科学国際研究センター 〒311-1313 茨城県東茨1

城郡大洗町成田町
5

Department of Machanical System Engineering, Graduate School of Engineering, Hiroshima University,
Higashi Hiroshima 739-8527, Japan5
広島大学大学院工学研究科機械物理工学専攻 〒739-8527 東広島市鏡山 1-4-1
The reference low-activation alloy, NIFS-HEAT-2, with a composition of V-4Cr-4Ti and itsweld joints were neutron-irradiated in JOYO reactor up to 0.98 dpa and 8.5 dpa at 670 K and 720 K,
respectively. Impact properties were evaluated by Chrapy V-notch impact test with miniature
specimens. The base metal of NIFS-HEAT-2 exhibited superior resistance to neutron-irradiation
embrittlement for the present irradiation condition, while the weld metal indicated enhanced irradiation
embrittlement. It was found that the embrittlement for the weld metal was caused by not only
irradiation hardening but also additional hardening due to dissolution of Ti-CON precipitates during
the welding before irradiation. A post-irradiation annealing at 873 K for 1 hr was effective to recover
the irradiation hardening in the weld metal.

1. Introduction
Welding is a key technology for lowactivation vanadium alloys to fabrication of blanket
components for fusion reactors. The Japanesereference V-4Cr-4Ti alloy, NIFS-HEAT-2, haveexhibited superior impact properties at annealedcondition and also at as-welded condition. Heavy
neutron irradiation is necessary to evaluatemechanical performance of the alloy at the blanket
irradiation condition. The purpose is to evaluate
irradiation embrittlement of the alloy and its weld
joints by heavy neutron irradiation and to clarify themechanism for the embrittlement.
2. Experimental
Material used was NIFS-HEAT-2 plateswith a thickness of 4 mm. The plates were annealed
at 1273 K for 2 hr. Weld samples were made bybead-on-plate welding with 1.6 kW YAG laser in ahigh purity Ar. They were machined into Charpy Vnotch (CVN) specimens with a size of 3.3 x 3.3 x
25.4 mm (1/3 size CVN) and 1.5 x 1.5 x 20 mm(1.5 size CVN). The V-notch was positioned at thebase metal (BM) and the center of the weld metal(WM). The CVN specimens were irradiated in
JOYO reactor in Japan. The irradiation conditions
were 1.5 x 1025 neutron m-2 (E > 0.1 MeV) at 670 K
and 1.3 x 1026 neutron m-2 at 720 K, which areequivalent to 0.98 dpa and 8.5 dpa, respectively.
After irradiation, Charpy impact tests, Vickers
micro-hardness tests with a load of 100 g for 30 sec
andmicrostructuralobservations
withatransmission electron microscope (TEM) wereconducted.
3. Results and discussion
Figures 1 and 2 show absorbed energy for
the Charpy impact tests for the base metal and theweld metal. In the present paper, ductile-to-brittle
transition temperature (DBTT) is defined as thetemperature where the absorbed energy is half theupper-shelf energy (EU). DBTT is an importantparameter to evaluate the toughness and ductility ofthe materials. RT (~300 K) is recognized as a limit

22D12for DBTT of the structural materials, assuming theyare cooled down to RT with some pressure aftershutdown of the fusion reactor. DBTT for the basemetal and the weld metal before irradiation was 122
and 103 K, respectively, with 1/3 size CVNspecimens (Fig. 1), while it was below 77 K for
both the base metal and the weld metal with 1.5 size
CVNs (Fig. 2). DBTT was shifted to 189 K and 188
K after 8.5 dpa irradiation at 720 K with 1/3 size
(Fig. 1) and 1.5 size (Fig. 2) CVNs, respectively.
Since the DBTTs were still much lower than RT,
the base metal maintained the superior resistance toneutron irradiation embrittlement up to 8.5 dpa at
720 K. The weld metal maintained 188 K in DBTTwith 1.5 size CVNs after 0.98 dpa irradiation at 670
K (Fig. 2). Absorbed energy for weld metal after
8.5 dpa irradiation with 1/3 size CVNs wasscattered, however DBTT was tentatively estimatedas 332 K to estimated the worst one (Fig. 1), while
it was above 423 K with 1.5 size CVNs (Fig. 2).
These results revealed that irradiation embrittlement
was enhanced in the weld metal. Recovery of theabsorbed energy and DBTT was examined for theweld metal by a post-irradiation annealing at 873 K.
DBTT was successfully recovered to 243 K by thepost-irradiation annealing (PIA) as indicated by anarrow in Fig. 1.
Figure 3 presents the relationship between
DBTT with 1.5 size CVNs and hardness of the basemetal and the weld metal before and after theneutron irradiations. DBTT is increased withincreasing hardness. The recovery of DBTT for theweld metal by the post-irradiation annealing is
attributed to the recovery of the hardness indicatedby an arrow in Fig. 3.
According to TEM observations before
irradiation, the base metal contained Ti-CON
precipitates (size ~ 100-1000 nm), while no
precipitates were observed in the weld metal. It wasindicated that the decomposition of the precipitates
released the interstitial C, O and N into alloy matrix,
and increased hardness of the weld metal by solid
solution hardening. After irradiation, dislocation
loops and network were produced as irradiation
defects. In addition, irradiation-induced precipitates(size ~ 10 nm) were observed. No significant
difference was found in dislocation structure
between the base metal and the weld metal, while
precipitates in the weld metal seemed slightly morethan the base metal. The dislocations were partly
recovered by the post-irradiation annealing. The
larger hardness of the weld metal after irradiation isexplained as the combination of the hardening by
the decomposition of the precipitates before
irradiation, and the irradiation hardening by theFig. 1 Absorbed energy for Charpy impact tests with
1/3 size specimens.

Fig. 2 Absorbed energy for Charpy impact tests with
1/3 size specimens.

Fig. 3 Relation ship between DBTT and hardness ofthe base metal and the weld metal before andafter the neutron irradiations.

irradiation-induced dislocations and precipitates. It
is considered that the recovery of the dislocations
resulted in the decrease in the hardness andrecovery of DBTT of the weld metal.
4. Conclusion
The base metal of NIFS-HEAT-2 exhibitedsuperior
resistancetoneutron-irradiation
embrittlement, while the weld metal indicatedenhanced irradiation embrittlement. A postirradiation annealing at 873 K for 1 hr was effective
to recover the irradiation hardening in the weld
metal, mainly due to the recovery of the irradiationinduced dislocations.