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Iwakiri2000-Microstructure-W-He-Irradiation.pdf
Journal of Nuclear Materials 283±287 (2000) 1134±1138
www.elsevier.nl/locate/jnucmat
Microstructure evolution in tungsten during low-energy helium
ion irradiation
H. Iwakiri a,*, K. Yasunaga a, K. Morishita b, N. Yoshida c
a
Research Institute for Applied Mechanics, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University,
Kasugakoen 6-1, Fukuoka-ken 816-8580, Japan
b
Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan
c
Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka 816-8580, Japan
Abstract
In situ transmission electron microscopy (TEM) study was performed to investigate the microstructural changes in
tungsten during low-energy He‡ ion irradiations in an electron microscope linked with an ion accelerator. The irradiations were carried out with 8 and 0.25 keV He‡ ions at 293, 873 and 1073 K. In the case of the 8 keV irradiation,
irradiation-induced vacancies act as nucleation sites for dislocation loops and helium (He) bubbles. Accordingly, such
defects were formed even at the higher temperatures. With increasing irradiation temperature, the growth rates of
dislocation loops and He bubbles rise remarkably. Although no vacancies are produced during 0.25 keV irradiation, He
platelets, interstitial loops and He bubbles were formed. Impurity atoms may act as trapping centers for He atoms,
which form bubbles by ejecting W atoms from their lattice site. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction
Plasma facing materials su€er strong bombardment
from the plasma by particles such as hydrogen (H)
isotopes and helium (He) with the energies ranging
from 10 eV to several keV in addition to D±T
neutrons. These energetic particles along with neutrons,
induce irradiation damage in the plasma facing materials.
It was shown that in situ transmission electron
microscope (TEM) observation under ion irradiation
is a very useful technique to study mechanisms of
damage accumulation. Microstructural evolution in
Mo and W under H ion irradiation with energies
comparable to the boundary plasma has been investigated by the authors [1,2]. It was shown that the
irradiation e€ects of He are much stronger than H
[3,4], but the mechanism of defect accumulation under
*
Corresponding author. Tel.: +81-92 583 7719; fax: +81-92
583 7690.
E-mail address: [email protected] (H. Iwakiri).
He ion irradiation has not been seriously studied. In
the present work, in situ TEM observation of W under irradiation of He ions with fusion relevant energies
was carried out to study defect accumulation processes.
2. Experimental procedures
Material used in the present work was high purity
(99.95%) powder metallurgy W containing 40 wppm
Mo, 20 wppm Fe, 15 wppm C and 5 wppm O and N.
After rolling to 0.1 mm thickness, and cutting into disks
of 3 mm in diameter, they were annealed at 2273 K for
600 s in a vacuum of about 5 10ÿ4 Pa. Pre-thinned
samples for TEM observation were obtained by twin-jet
electro-polishing the disks.
The in situ observation under He ion irradiation was
conducted using a 200 kV transmission electron microscope equipped with a low energy ion accelerator. Details of the facility are described elsewhere [5]. Helium
ions at 8 or 0.25 keV were irradiated with the specimens
at 293, 873 and 1073 K. Microstructural evolution was
recorded either on ®lms or video.
0022-3115/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 3 1 1 5 ( 0 0 ) 0 0 2 8 9 - 0
H. Iwakiri et al. / Journal of Nuclear Materials 283±287 (2000) 1134±1138
3. Results
3.1. Microstructural evolution under 8 keV He ion
irradiation
Typical microstructural evolution in W at 293 K
under irradiation with 8 keV He ions is shown in Fig. 1.
Interstitial type dislocation loops appeared at ®rst and
increased in density with increasing dose. The density
saturated at a dose level of around 1:3 1019 ions=m2 ,
while the size of each loop continued to increase. At
4:3 1019 ions=m2 , the average size of the loops becomes about 5 nm, and the loops piled up as tangled
dislocations. The saturation density is about six times
higher than that for H ion irradiation at a comparable
energy [2]. Fig. 2 shows the temperature dependence of
loop formation. With increasing irradiation temperature
the density of the loops dropped drastically, while the
loop size increased. At 873 and 1073 K they grew rapidly
and tangled with each other.
Fig. 3 shows micrographs taken at rather high dose
with large s di€raction condition. Helium bubbles are
observed at all temperatures in addition to dislocation
loops. In the case of the 873 K irradiation, the bubbles
are aligned along the traces of {1 1 0} matrix planes, this
has been observed in bcc metals following high-dose
irradiation with He at around 0.2Tm [6], where Tm is the
melting temperature. In the case of irradiation at 1073 K,
Fig. 1. Microstructural evolution of W at room temperature
during irradiation with 8 keV He‡ ions.
1135
large bubbles above 20 nm in diameter are formed together with rather small bubbles of about 5 nm.
3.2. Microstructural evolution under 0.25 keV He ion
irradiation
Fig. 4 shows microstructural evolution at 293 K
under irradiation with 0.25 keV He ions. Despite insucient energy for knock-on damage, dense defects
appeared suddenly around 1:4 1019 He‡=m2 . According to stereoscopic observation, the defects distributed in a region about 20 nm from the beam
incident surface. The contrast of the defects is very
weak at low dose in comparison with a dislocation
loop, but it becomes stronger gradually above
3:0 1019 He‡ =m2 . Defects at low dose could be
clearly observed only under the Bragg condition. This
fact indicates that they are not dislocation loops as
observed in 8 keV He irradiation, but some other defect whose strain ®eld is weak at the beginning but
becomes stronger with dose. As discussed later, they
are probably plane agglomerates of injected He (called
Fig. 2. Temperature dependence of dislocation loop formation
during irradiation with 8 keV He‡ ions.
1136
H. Iwakiri et al. / Journal of Nuclear Materials 283±287 (2000) 1134±1138
Fig. 4. Microstructural evolution of W at room temperature
during irradiation with 0.25 keV He‡ ions.
Fig. 3. Temperature dependence of bubble formation during
irradiation with 8 keV He‡ ions.
here He platelets). As shown in Fig. 4(d) and demonstrated in serial video images in Fig. 5, a new defect
cluster with strong contrast appears suddenly beside
the He platelet above 3:0 1019 ions=m2 , and one or
two more defect clusters are formed for each He
platelet by prolonged irradiation. Image contrast of the
He platelet becomes weaker after formation of a new
cluster. The size of the new cluster is comparable to
the original platelet. This phenomenon suggests the
punching out of an interstitial loop from a platelet
precipitation [7]. The punched out dislocation loops
subsequently grow under irradiation. Hence, dislocation loops, which are usually formed as agglomerates
of interstitials formed by knock-on damage, are
formed even under non-displacement damage conditions. The formation of He platelets and dislocation
loops occurs even at temperatures as high as 1073 K
(see Fig. 6).
At higher dose, He bubbles were also formed at all
examined temperatures as shown in Fig. 7. Formation of
bubbles at the 0.25 keV irradiation is not much di€erent
from that at 8 keV, in spite of the large di€erence in ion
energy. This indicates that the major factor controlling
Fig. 5. Sequential micrographs of loop punching from He
platelet taken from a video camera.
He bubble formation is not knock-on damage but injection of He.
4. Discussion
4.1. Formation of interstitial loops under He irradiation
It is known that formation of interstitial loops under
He ion irradiation is enhanced by the trapping of interstitials around He±vacancy complexes [3,8,9]. Details
of the loop nucleation mechanism under He ion irradiation are discussed here within the content of this basic
idea. Thermal He desorption spectrometry (TDS)
H. Iwakiri et al. / Journal of Nuclear Materials 283±287 (2000) 1134±1138
1137
Fig. 6. Temperature dependence of platelet and dislocation
loop formation during irradiation with 0.25 keV He‡ ions.
Fig. 7. Temperature dependence of bubble formation during
irradiation with 0.25 keV He‡ ions.
experiments showed that injected He atoms with keVrange energies are trapped in radiation-induced vacancies and form He±vacancy complexes of various size,
i.e., Hei V1 …i 5 6† due to very strong He±vacancy
binding energy [10]. Here, the suxes denote the number
of He atoms or vacancies (V). Hei V1 complexes with
small i, however, may disappear by absorbing a mobile
interstitial [11,12]. Some fraction of the Hei V1 complexes, which reach a critical size (i ˆ 5 or 6) mutate into
a complex with two vacancies (Hei V2 ) by ejecting an
interstitial into the matrix [13]. By absorbing He atoms
and further ejecting interstitials, large complexes
Hei Vj …i > 6; j = 2† are formed. If the number of He (i)
is large enough, the complex cannot absorb interstitials
but may trap them around it. This trapping e€ect for
interstitials and the formation of excess interstitials by
the ejection may result in the enhancement of interstitial
loop formation. Because of the strong stability of large
Hei Vj complexes [11,14], interstitial loops are formed
even at 1073 K. Formation of interstitial loops at such
high temperature is a peculiar feature of He ion irradiation; in the case of H ion irradiation, for example, no
loops are formed above 873 K [2].
4.2. He bubble formation under 8 keV He ion irradiation
In the case of the 8 keV He ion irradiation, where
vacancies and interstitials are formed by knock-on
damage, He bubbles are formed at high dose depending
on irradiation temperature. At low temperatures, where
vacancies have no thermal mobility, He bubbles can
grow by ejecting interstitials due to a high He gas
pressure in the bubbles (gas-driven growth).
At elevated temperatures, where vacancies are thermally mobile, radiation-induced vacancies enhance
bubble formation. For example, the critical dose for
visible bubble formation at 1073 K is about 1/50 of that
at 293 K. By absorbing mobile vacancies, nucleation and
growth of bubbles occurs (see Fig. 3).
4.3. Defect formation under 0.25 keV He irradiation
In situ TEM observation under 0.25 keV He ion irradiation showed that the defects that appear at low
dose must not be dislocation loops because of their weak
contrast. This feature of the image indicates that the
strain ®eld around the defects is weak. A possible
1138
H. Iwakiri et al. / Journal of Nuclear Materials 283±287 (2000) 1134±1138
structure of the defects is platelets of He atoms formed
between W lattice planes, which were observed in Mo
irradiated with low energy 0.15 keV He ions [15]. Image
contrast of the defects becomes stronger when their size
exceeds about 5 nm. This indicates that platelets become
thicker and thicker and result in a stronger strain ®eld
around them. Finally, an interstitial type dislocation
loop is punched out to reduce the increased strain ®eld
as shown in Fig. 5. This loop punching process is repeated two or three times, but stops at high dose where
bubbles are formed. This indicates that absorption of He
by bubbles is more favorable than forming He platelets.
Since no vacancies are produced by knock-on process
under 0.25 keV He ion irradiation, an alternative mechanism for bubble formation is required. One possible
mechanism is trapping of He by impurities. TDS experiments showed that substitutional impurities such as Ag
and Cr strongly trap He atoms as well as vacancies [16].
If the number of trapped He atoms exceeds a critical
value, the impurity atom or the nearby W atom is pushed
out in a interstitial site and a Hei V1 complex is formed.
With increasing irradiation temperature, the probability
of trapping He around impurity atoms decreases and this
results in the reduction of bubble density at elevated
temperatures as shown in Fig. 7. Impurities may also act
as nucleation sites of He platelets.
Once Hei V1 complexes are formed they can easily
grow as He bubbles by continous absorption of He and
ejection of interstitials as discussed before. According to
a molecular dynamics calculation, interstitials ejected
from a high pressure Hei Vj complex, are bonded to the
complex at room temperature [17]. At elevated temperature, however, they are released thermally from the
complex and contribute to the growth of punched out
loops (see Fig. 7).
5. Conclusions
Microstructural evolution in W under irradiation of
0.25 keV or 8 keV He ion at 293, 873 and 1073 K was
observed in situ by TEM. In the case of the 8 keV He ion
irradiation, complexes of injected He and radiationinduced vacancies act as nuclei for He bubbles and they
enhance nucleation of interstitial loops. These phenomena were widely observed at all temperatures examined, though the density and size of the defects
strongly depend on temperature.
In the case of the 0.25 keV He ion irradiation, where
knock-on damage does not occur, He platelets, interstitial loops and bubbles were formed. Impurity atoms
may act as trapping centers for He atoms, which form
bubbles by ejecting W atoms from their lattice sites.
Formation of He platelets leads to nucleation of interstitial loops by punching-out process and the loops grow
by absorbing interstitials ejected from bubbles.
The present results indicate that the plasma facing
materials in D±T burning devices may su€er serious
radiation damage by the bombardment of He ions from
the plasma even at elevated temperatures and also even
when the particle energy is below the threshold for
knock-on damage.
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