22D14
Surface Modification on Tungsten Materials
by Repeated Pulse High Heat Loading
繰り返しパルス高熱負荷によるタングステン材の表面損傷
Kazutoshi Tokunaga1, Kuniaki Araki , Tadashi Fujiwara , Yoshio Miyamoto , Kazuo Nakamura ,1

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Tomohiro Hotta and Hiroaki Kurishita
徳永和俊 , 荒木邦明 ，藤原 正1, 宮本好雄1、中村一男1、堀田智宏2、栗下裕明3
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Research Institute for Applied Mechanics, Kyushu University
6-1 Kasuga kouen, Kasuga, Fukuoka 816-8580, Japan
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Interdisciplinary Graduate School of Engineering Sciences, Kyushu Uiniversity
6-1 Kasuga kouen, Kasuga, Fukuoka 816-8580, Japan3
International Research Center for Nuclear Materials Science, IMR, Tohoku UniversityOarai, Ibaraki 311-1313, Japan1

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
九州大学大学院総合理工学府 〒816-8580 福岡県春日市春日公園6-1
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東北大学金属材料研究所量子エネルギー材料科学国際研究センター〒311-1313 茨城県東茨城郡大洗町成田町2145-2

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Steady state (1700 oC, 180 s) and repeated (450 oC – 1250 oC, 380 times) heat loading experiments of
ITER grade W and Toughened, Fine Grained, Recrystallized W-1.1TiC (TFGR W-1.1TiC) have been
performed using an electron beam heat loading system. For ITER grade W, the irradiation around 1700oC
caused recrystallization and grain growth up to the average diameters of 50~100 µm. The subsequent
repeated irradiations caused significant surface roughening, cracking at the grain boundaries and surface
exfoliation. On the other hand, grains of the surface of TFGR W-1.1TiC are clearly observed after theirradiation around 1700 oC for 180s by steady state heat loading. In addition, TFGR W-1.1TiC did notexhibit any surface roughening and cracking by the repeated heat loading.

1. Introduction
Tungsten (W) is the primary candidate for use as
plasma facing materials/components (PFM/PFC).
PFM/PFC will be subjected to heavy thermal loads
in the steady state or transient mode (disruptions
and edge localized modes (ELMs)) combined withhigh energy neutron irradiation that will cause
serious material degradation and transmutation.
Material degradation in W mainly comes fromrecrystallization embrittlement which is themanifestation of brittle fracture at weak,high-energy grain boundaries (GBs) formed byrecrystallization: The recrystallization temperatureof commercially available pure W plates which aresupplied in the stress relieved state is 1200~1300oC,
much lower than the melting point of pure W(3410oC).
Recently one of the coauthors has developed
nanostructured W-1.1%TiC compacts that exhibit
appreciable room temperature ductility even in therecrystallized state (Toughened, Fine Grained,
Recrystallized W-1.1TiC: TFGR W-1.1TiC) [1].
Given the recent progress in materials research on
the W-TiC alloys, it is very appropriate andmeaningful to assess the alloys under closer

conditions to the anticipated ambience in fusion
reactors. The purpose of this study is to simulatethe heat load tolerance capability of TFGR
W-1.1TiC and pure W by steady state and repeatedheat loading of an electron beam heating device.
2. Experimental
TFGR W-1.1TiC compacts with equiaxed grains
were fabricated through the application ofsuperplasticity-based microstructural modification(SPMM) at 1650oC to ultra-fine grained W-1.1TiC.
As a precursor for the SPMM treatment, thecompacts were produced by powder metallurgicalmethods utilizing mechanical alloying (MA) andhot isostatic pressing (HIP) at 1350oC for 3 hours.
Two kind of TFGR W-1.1TiC (A: High oxygen
impurity: 850 ppm, B : Low oxygen impurity: 170ppm) which have different oxygen content havebeen prepared to investigate effect of concentrationof oxygen impurity. The W-1.1TiC compacts andITER grade W were machined to the dimensions of10mm x 10mm x 1mm, followed by mechanicaland electro polishing.
All of the polished specimens were placed on awater-cooled Cu block and subjected to high heatload experiments by an electron beam irradiation
test simulator of the Research Institute for AppliedMechanics, Kyushu University [2].
The
experiments were conducted at two irradiation
conditions; (1) surface temperature of around
1700oC, duration of 180 seconds, (2) repeatedirradiations of 2 second-irradiation and 7.5
second-rest with one cycle of 9.5 seconds for totally
1 hour. In the case of (2), surface temperature of thesamples changes from below 450oC to 1250oC by 2
second-irradiation. Gas emission from the heatedsample was measured using QMS. Before and after
the irradiation, the specimen surfaces wereexamined by SEM. Weight loss of the sample wasalso measured with an electronic balance.
3. Results
For ITER grade W, the irradiation around 1700oC
caused recrystallization and grain growth up to theaverage diameters of 50~100µm. The subsequent
repeated irradiations caused significant surface
roughening, cracking at the grain boundaries andsurface exfoliation as shown in Fig.1. The surface
roughening is due to plastic deformation caused bydifference in the thermal stresses between theadjacent grains with different orientations.
On the other hand, TFGR W-1.1TiC did notexhibit any surface roughening and cracking as
shown in Fig. 2. This can be attributed to the finegrains with dispersoids where the generated thermal
stresses are smaller and can also be relaxed by grainboundary sliding which is characteristic of a finegrained structure. These results indicate that TFGR
W-TiC has a superior thermal property under
repeated high heat load such as ELM heat loadconditions.

Fig.1. SEM micrograph observed on the surface ofITER grade W, is heated by steady state heat loading at
1700 oC for 180s, and 380 repeated heat loading shots.

Fig.2. SEM micrograph observed on the surface ofTFGR W-1.1TiC, is heated by steady state heat loadingat 1700 oC for 180s, and 380 repeated heat loading shots.

References[1] H.Kurishita, S.Matsuo, H.Arakawa, T.Sakamoto,S.Kobayashi, K.Nakai, T.Takida, M.Kato, M.Kawai,N.Yoshida, J. Nucl. Mater. 398 (2010) 87.
[2] K.Tokunaga, K.Matsumoto, Y.Miyamoto, T.Muroga,N.Yoshida, J. Nucl. Mater. 212-215 (1994) 1323.