TEM Observations of Radiation Damage in Tungsten Irradiated by 20 Mev W Ions

8/12/2019 TEM Observations of Radiation Damage in Tungsten Irradiated by 20 Mev W Ions

1/6

TEM observations of radiation damage in tungsten irradiatedby 20 MeV W ions

. Ciupinski a,, O.V. Ogorodnikova b, T. Pocinski a, M. Andrzejczuk a, M. Rasinski a, M. Mayer b,K.J. Kurzydowski a

a Warsaw University of Technology, ul. Woloska 141, PL-02507 Warsaw, Polandb Max-Planck-Institut fr Plasmaphysik, EURATOM Association, Boltzmannstr. 2, D-85748 Garching, Germany

a r t i c l e i n f o

Article history:

Received 24 September 2012

Received in revised form 21 February 2013

Accepted 4 March 2013

Available online 29 March 2013

Keywords:

Tungsten

Radiation damage

TEM observation

a b s t r a c t

Polycrystalline, recrystallized W targets were subjected to implantation with 20 MeV W6+ ions in order to

simulate radiation damage caused by fusion neutrons. Three samples with cumulative damage of 0.01,

0.1 and 0.89 dpa were produced. The near-surface zone of each sample has been analyzed by transmis-

sion electron microscopy (TEM). To this end, lamellae oriented perpendicularly to the targets implanted

surface were milled out using focused ion beam (FIB). A reference lamella from non-irradiated, recrystal-

lized W target was also prepared to estimate the damage introduced during FIB processing. TEM studies

revealed a complex microstructure of the damaged zones as well as its evolution with cumulative dam-

age level. The experimentally observed damage depth agrees very well with the one calculated using the

Stopping and Range of Ions in Matter (SRIM) software.

2013 Elsevier B.V. All rights reserved.

1. Introduction

Tungsten (W) will be used in the high-flux region of the divertor

in ITER and is a candidate material for plasma facing components

in future fusion devices [1]. This is mainly due to its favorable

physical properties under high heat and particle fluxes. Its low

sputtering yield can minimize impurity generation. Its good ther-

mal properties, i.e. one of the highest melting point of all elements

(3695 K) and high thermal conductivity (175 W m1 K1) [2] are

also required for efficient plasma facing components. Since tritium

is radioactive, safety requirements limit its in-vessel inventory to

the total of 700 g[3]. If this level would be overcome, a clean-up

of the vessel is necessary in order to reduce the radioactivity.

According to the present knowledge, the hydrogen isotope reten-

tion in pure W is not a concern [39]. However, a production of

neutron-like defects can result in a significant increase of the deu-terium retention in W as it was simulated by pre-irradiation of W

by fast heavy ions[10].

Neutron (n) irradiation of materials leads to a significant mod-

ification of their crystal structure due to the introduction and accu-

mulation of radiation induced structural defects[11].

Sources of fusion neutrons for materials irradiation are still not

available. Also, irradiation of materials in a fast nuclear reactor is

not very practical due to the long time required for the accumula-

tion of relevant damage levels and due to the different neutron en-

ergy spectra, as compared to fusion neutrons. Therefore, in the

present work, tungsten ions with a kinetic energy of 20 MeV were

used to simulate displacement damage created by fast neutrons.

This approach of mimicking neutron damage by fast ions has been

recently adopted by many authors (e.g. [10,1216]). Despite some

differences in primary knock-on atom energy spectra (>MeV from

high energy ion-bombardment and


2/6

the defects contained in this thin layer at a given depth from the

target surface. Moreover, often the exact location of this layer with

regard to the target surface is difficult to define due to specimen

preparation procedure limitations. Our cross section images pro-

vide information on defects right from the target surface down to

about 10 lm and allow evaluation of changes in defects distribu-

tion versus distance from the target surface.

2. Materials and methods

Polycrystalline European ITER reference tungsten (WI) pro-

duced by Plansee AG[21]was used in our study. The WI was cut

into 10 10 mm2 plates of the thickness of c.a. 0.5 mm and

mechanically polished to a mirror like finish. Prior to ion implanta-

tion the plates were recrystallized at 2470 K for 10 min. The radia-

tion damage was introduced by implantation with 20 MeV W6+

ions up to 0.01, 0.1 and 0.89 dpa in the damage maximum. This

corresponds to the fluences of 1.6 1016 W/m2, 1.6 1017 W/m2

and 1.4 1018 W/m2, respectively. The implantation was carried

out at IPP Garching in a chamber connected to the 3 MV tandem

accelerator [10]. The specimen holder was actively cooled during

implantation. The background pressure in the chamber was betterthan 105 Pa. The ion current during the experiment was 10

30 nA/cm2. The temperature of the sample was measured by a

thermocouple attached directly to the target and was kept around

290 K during the W ion bombardment. The damage profile was cal-

culated using the program SRIM 2008.03 [22], full cascade option,

with a threshold displacement energy ofEd= 90 eV. The damage

profile in W irradiated by 20 MeV W6+ is shown in Fig. 1. The

20 MeV W ions create an inhomogeneous profile up to about

2.35 lm with a maximum at a depth of about 1.35 lm.

The transmission electron microscopy (TEM) observations have

been performed on lamellae cut as cross-sections perpendicular to

the implanted sample surface. A focused ion beam system (FIB)

Hitachi FB2100 with Ga+ ions beam and 40 kV accelerating voltage

has been used for milling and extraction of TEM specimens. Extrac-

tion of the TEM lamella milled-out from the bulk of the material is

realized using a microprobe inside the FIB chamber. This operation

is called micro sampling [23]. Before ion milling the implanted sur-

face of the tungsten plates has been covered with a protective

tungsten layer using FIB-assisted chemical vapor deposition tech-

nique and tungsten hexacarbonyl W(CO)6 as processing gas.

During FIB processing a sidewall damage of the milled lamella

occurs[24]. In order to reduce this unwanted, additional damage

that would ad-up to the damage induced by self-implantation in

the TEM micrographs, low energy ion polishing has been em-

ployed. A LINDA Gentle Mill device with Ar ion beam operatedwith

an accelerating voltage in the range of 0.31 kV has been used for

FIB damage attenuation. The same procedure was used for extrac-

tion of a lamella from an undamaged polycrystalline tungsten

plate. This lamella was used as a reference sample for the evalua-

tion of damage introduced during samples preparation.

Finally, the TEM observations have been carried out on a scan-

ning-transmission electron microscope Hitachi STEM HD2700 with

an accelerating voltage of 200 kV. A scheme of cross-section micro

sampling showing the relative orientation of the beams used dur-

ing investigations is shown inFig. 2.

3. Results

A typical lamella is presented in Fig. 3. It is of uniform thickness,

approximately 70 nm thick and the area transparent to the elec-

trons is about 20 10lm. Roughly one half of this observable la-

mella width is presented inFig. 3. This lamella has been extracted

from the sample with the damage level of 0.1 dpa. The top of the

image corresponds to the top of the target surface. Several layers

with varying contrast and morphology can be observed. Theseare, listed from the top: (a) two amorphous layers of tungsten

deposited as protective coatings before ion milling, (b) a damaged

layer due to 20 MeV W ions implantation, (c) an undamaged dee-

0 1 2 310

-3

10-2

10-1

100

Eth

=90 eV

20 MeV W6+

-> W

dpa

Tungsten depth, m

Fig. 1. Damage profile calculated with SRIM for the 0.89 dpa damaged sample andEd= 90eV.

Fig. 2. TEM lamella sample preparation and observations scheme showing the

relative orientation of beams and samples.

Fig. 3. Typical lamella extracted from ion-implanted tungsten sample damaged to

0.1 dpa with characteristic features: (a) two amorphous protective layers deposited

prior FIB milling, (b) a damaged zone due to 20 MeV W ions implantation, (c) anundamaged deeper target region, and (d) material re-deposited during FIB milling.

160 . Ciupinski et al. / Nuclear Instruments and Methods in Physics Research B 317 (2013) 159164


3/6

per layer of the target and (d) a bright layer with a columnar inter-

face which is composed of material re-deposited during FIB mill-

ing. The top and bottom layers are inherent to the lamella

extraction method and as such are not taken into account in fur-

ther analyses.

The lamella extracted from the reference, non-implanted target

and the three damaged targets are juxtaposed in Fig. 4 in ascending

damage level order (0, 0.01, 0.1 and 0.89 dpa) looking from the left.

As the targets were recrystallized before implantation, coarse grain

microstructures developed. Therefore, only one or two grains are

usually visible in the cross sections as lamellae were smaller than

the grains of the polycrystalline targets.

Uniform contrast is observed for almost the whole depth of theundamaged sample except of the near surface region. The dotty

contrast of the sample is due to the damage induced by the Ga

ion beam of the FIB apparatus. This damage could not be removed

even by the low-energy ion polishing employed after FIB milling,

thus this unwanted feature is contributing to the damage images

of all investigated specimens.

Already at 0.01 dpa an additional damaged structure due to

20 MeV W6+ ions implantation is observable. This damaged struc-

ture extends from the surface down to roughly 2 lm in depth. Its

morphology resembles that of FIB induced damage, however it

can be noticed that it has seemingly a higher density of defects

than the deeper, unaltered layer or the reference sample. At

0.1 dpa, the damaged structure is also easily observable and some

differentiation of this region can be argued. Somewhere, in themiddle of the altered region a coarsening of the defects is apparent.

The deepest part of that damaged layer still preserves the dotty

morphology observed earlier. Finally, the sample with the highest

damage level (0.89 dpa) contains the most complex microstructure

of all investigated ones. Further coarsening of defects is easily

noticeable, especially in the middle of the damaged zone.

The three damaged samples with higher magnification com-

pared toFig. 4are shown inFig. 5. The images are presenting the

material volumes located at the depth range of approximately

0.5 1.5lm from the surface. The coalescence and redistribution

of defects are observed with increasing damage level. At 0.01 dpa

the image is dominated by defects, less than 20 nm in size, uni-

formly distributed in the observed volume. At 0.1 dpa these defects

begin to form chains and a web-like structure of defects emerges.At 0.89 dpa this web-like structure is already clearly visible. More-

over, we would argue that the material volumes inside that mesh

have a much lower defect density, as we believe that the small de-

fects visible inside those regions are due to FIB processing.

The microstructure of the whole cross-section of the damaged

zone of 0.89 dpa sample is shown inFig. 6. The changes in the con-

trast of the defects with the distance from the sample surface are

evident. The near surface area is characterized by a high density

of tangled dislocations. Deeper, the tangled dislocations network

grows, leaving larger areas with low defect density. Finally, at a

distance of c.a. 2 lm from the surface, the damaged zone is com-

posed of small dislocation loops densely distributed. Below this fi-

nal sub-layer small dislocation loops with lower areal density are

observed that we attribute to FIB milling. The depth of implanta-tion-induced damage in the sample with the highest level of dam-

Fig. 4. Overview of all analyzed materials (STEM bright field images). Looking from the left: (a) undamaged target, (b) 0.01 dpa damage, (c) 0.1 dpa damage and (d) 0.89 dpa

damage.

Fig. 5. Comparison of defect distributions in damaged zones (TEM bright fieldimages). Looking from the left: (a) 0.01 dpa, (b) 0.1 dpa and (c) 0.89 dpa.

. Ciupinski et al. / Nuclear Instruments and Methods in Physics Research B 317 (2013) 159164 161


4/6

age directly measured in TEM is equal to 2.3 lm (Fig. 6).Thisis ina

very good agreement with the damage profile calculated with

SRIM (Fig. 1)for the implantation of tungsten with 20 MeV W ions

using the displacement energy of 90 eV[10].

The defects that are visible in the low damage level sample and

form the dotty pattern mentioned above are dislocation loops.

These are shown inFig. 7 at high magnification for the 0.01 dpa

specimen. Their size is c.a. 5 nm and it can be noticed that already

at that damage level some agglomeration of the defects occurs.

Formation of the dislocation loops of similar contrast in bright field

TEM images and characteristic size around 5 nm, has been ob-

served by many authors [13,25,26] irrespective of the ions used

for implantation. Also in our investigations the images of defects

due to W ion implantation and FIB milling (that can be regardedas implantation of Ga ions) are very similar (compare Fig. 8). The

only noticeable difference is the size of the dislocation loops cre-

ated, that of FIB origin being smaller.

4. Discussion

Although the implantation depths calculated with SRIM and di-

rectly measured from TEM image are in a good agreement, the

damage intensities from calculations do not match the images.

SRIM predicts the damage density to peak at around 1.3lm,

whereas judging from the TEM image, obtained at 0.89 dpa, the

areal density of defects is the lowest at this depth. This can be ex-

plained in terms of recombination and rearrangement of defects,which is observed with the increasing dose as mentioned earlier.

There are two major driving forces influencing the mobility of

the structural defects, i.e., stress field and temperature. As every

dislocation loop is associated with a characteristic strain and stress

field, an increase of defect density will lead to the increase of the

overall stress field in the damaged material volume. That would

explain the observed disagreement between calculated damage

profiles and observed in STEM as the defects in the highly damaged

zone would have a higher tendency for recombination. However,

the rate of defects migration in tungsten at room temperature, as

discussed in [28], is extremely low. Therefore, for the defects to

rearrange within the time-scale of our experiment, an increase of

temperature seems indispensable.

There are two potential sources of heat in our sample prepara-tion procedure. The first one is the W6+ ion irradiation. Although

the whole implanted target is kept at room temperature, the local

rise of temperature in a small volume directly under the ion beam

cannot be excluded. The second one is FIB milling, which is basi-

cally also ion implantation. Park et al.[29]have shown that during

a typical ion milling operation, the sample temperature may rise

by 350for materials with low thermal conductivity such as glass.

They have also measured the temperature rise in a stainless steel

sample to be about 150. Taking into account that tungsten has

an approximately four times higher value of thermal conductivity

than stainless steel, in our experiment the temperature rise due

to ion-milling should not exceed several dozen degrees.

According to [30], the temperatures between 370 and 720 K

correspond to stage III, which is attributed to the migration ofself-interstitial atoms and a temperature of 720 K, are the end of

Fig. 6. STEM bright field damaged zone image of the 0.89 dpa damaged sample.

Fig. 7. STEM bright field image of dislocation loops in 0.01 dpa damaged sample.

Fig. 8. STEM bright field image of dislocation loops created by FIB milling.

162 . Ciupinski et al. / Nuclear Instruments and Methods in Physics Research B 317 (2013) 159164


5/6

stage III where vacancies became mobile. In [31]it was reported

that from 570 to 770 K monovacancies migrate through the crystal

lattice and either agglomerate with other vacancy-like defects to

form larger defects or annihilate at defect sinks such as grain

boundaries. Final recovery takes place only at temperatures higher

than 1273 K. Consequently, several dozen degrees should not sig-

nificantly modify the ion-induced defects. Therefore, FIB prepara-

tion of TEM lamella cannot be responsible for the observed

dislocation loop growth and coalescence.

Thus, the rearrangement of defects should be attributed solely

to the W ion implantation process. Kaoumi et al. [32]have studied

the grain growth in nano-structured metals and alloys under irra-

diation. They have shown that the grain boundaries become mobile

and resulting grain growth always occurs in the studied materials

irrespective of the irradiation temperature. In this extensive study

they have identified three temperature regimes: (1) a purely ther-

mal regime where thermal effects dominate the grain boundary

motion process, (2) a thermally assisted regime where thermal

and irradiation effects combine to increase the rate of grain bound-

ary mobility caused by either of these mechanisms and (3) a low-

temperature or nonthermal regime in which irradiation effects

dominate the grain boundary mobility, and the kinetics do not de-

pend on the irradiation temperature. Kaoumi et al. have also sta-

ted, that the transition temperature between the nonthermal and

thermally assisted regimes is material dependent but can be re-

lated to their melting temperature (Tm) and occurs at a homolo-

gous temperature between 0.15 and 0.20 Tm. Further, they have

developed a model for grain boundary migration in the nonthermal

regime. According to this model the migration occurs by atomic

jumps within the thermal spikes, due to incident high energy ions,

hitting the grain boundary. The kinetics of the process is also

biased by another driving force i.e. the local grain-boundary curva-

ture. The migration of dislocation loops that we observe, leading to

their growth and coalescence, may be triggered by the same mech-

anism since our samples have been irradiated at room tempera-

ture. This is certainly below 0.15 Tm(i.e., 550 K) for tungsten.

The rearrangement of defects can be attributed to the strong lo-cal overheating of the region of dense collisions during W ions

implantation. The kinetic energy of the atoms in the region of

dense collisions can be recalculated with temperature using the

basic equationE= 3/2NkBT. Therefore, the temperature is initially

of the order of 10,000 K for MeV ions and this region is called a

thermal spike[33,34]. The thermal spikes cool down to the ambi-

ent temperature in 1100 ps, so the temperature here does not

correspond to thermodynamic equilibrium temperature. A curious

feature of collision cascades is that the final amount of damage

produced after thermal spike may be much less than the number

of atoms initially displaced in the thermal spike[35]. Typically, a

heat spike is characterized by the formation of a transient under-

dense region in the center of the cascade, and an overdense region

around it[35,36]. After the cascade, the overdense region becomesa region of interstitial defects, and the underdense region typically

becomes a region of vacancies.

It is also worth mentioning that in fusion devices tungsten ele-

ments will work at elevated temperatures above the 0.150.2 Tmlimit, so the thermal regimes would prevail. Therefore, our future

research will concern the samples irradiated at elevated

temperatures.

5. Summary

TEM investigations of radiation damage in self-irradiated tung-

sten have been performed. Samples with cumulative damage levels

of 0.01, 0.1 and 0.89 dpa have been studied. The damaged zones

have been analyzed in cross sections perpendicular to the im-planted surface. This approach allowed the examination of changes

in the defect structure not only in relation to the radiation dose but

also as function of the distance from the sample surface.

It has been observed that at low damage level (0.01) the de-

fected structure is composed of uniformly distributed dislocation

loops of some 5 nm size. This structure further develops both with

the radiation dose and distance from the implanted surface. At

moderate damage (0.1 dpa) coarsening and entangling of defects

occurs. This effect is more pronounced in the middle of the ob-

served damaged zone depth. The defect structure in the sample

damaged up to 0.89 dpa can be subdivided into three segments:

(1) the near surface area down to about 0.4 lm characterized

by a high density of tangled dislocations, (2) the intermediate area

from about 0.4lm down to about 1.9lm with the tangled dis-

locations network growing/becoming coarser and leaving larger

areas with low defect density and (3) the deep area at the depth

of c.a. 2 lm, which is composed of small dislocation loops with a

high density and a uniform distribution. The driving force of this

observed defects rearrangement is related to the 20 MeV W6+ ion

implantation and could be described by atomic jumps within ther-

mal spikes.

Acknowledgements

The technical assistance of J. Dorner and M. Fueder during

tungsten irradiation is gratefully acknowledged. This work, sup-

ported by the European Community, was carried out within the

framework of the European Fusion Development Agreement. The

views and opinions expressed herein do not necessarily reflect

those of the European Commission. The support by the Polish Min-

istry of Science and Education through Grant No. 2077/7.PR-EUR-

ATOM/2011/2 is also acknowledged. This work was partly

supported by the Impuls- and Vernetzungs fund of the Helmholtz

Society.

References

[1] V. Philipps, J. Nucl. Mater. 415 (1) (2011) S2.

[2] E. Lassner, W.-D. Schubert, Tungsten: Properties, Chemistry, Technology of the

Element, Alloys, and Chemical Compounds, Kluwer Academic/Plenum

Publishers, New York, 1999. p. 30.

[3] J. Roth et al., J. Nucl. Mater. 390391 (2009) 1.

[4] O.V. Ogorodnikova, J. Roth, M. Mayer, J. Nucl. Mater. 373 (2008) 254.

[5] O.V. Ogorodnikova, T. Schwarz-Selinger, K. Sugiyama, T. Drbeck, W. Jacob,

Phys. Scr. T 138 (2009) 014053.

[6] Z. Tian, J.W. Davis, A.A. Haasz, J. Nucl. Mater. 399 (2010) 101.

[7] V.Kh. Alimov, W.M. Shu, J. Roth, K. Sugiyama, S. Lindig, M. Balden, K. Isobe, T.

Yamanishi, Phys. Scr. T 138 (2009) 014048.

[8] R.A. Causey, C.L. Kunz, D.F. Cowgill, J. Nucl. Mater. 337339 (2005) 600.

[9] M. Poon, A.A. Haasz, J.W. Davis, J. Nucl. Mater. 374 (2008) 390.

[10] O.V. Ogorodnikova, B. Tyburska, V.Kh. Alimov, K. Ertl, J. Nucl. Mater. 415(2011)

S661.

[11] P. Vladimirov, S. Bouffard, in: C.R. Physique 9 (2008) 303.

[12] M.H.J. t Hoen, B. Tyburska-Pschel, K. Ertl, M. Mayer, J. Raap, A.W. Kleyn, P.A.

Zeijlmans van Emmichoven, in: Nucl. Fusion 52 (2012) 023008.

[13] D.E.J. Armstrong, X. Yi, E.A. Marquis, S.G. Roberts, J. Nucl. Mater. 432 (2013)428.

[14] M. Fukumoto, H. Kashiwagi, Y. Ohtsuka, Y. Ueda, M. Taniguchi, T. Inoue, K.

Sakamoto, J. Yagyu, T. Arai, I. Takagi, T. Kawamura, J. Nucl. Mater. 390391

(2009) 572.

[15] W.R. Wampler, R. Doerner, Nucl. Fusion 49 (2009) 115023.

[16] Y. Oya, M. Shimada, M. Kobayashi, T. Oda, M. Hara, H. Watanabe, Y. Hatano, P.

Calderoni, K. Okuno, Phys. Scr. T 145 (2011) 014050.

[17] J.M. Steichen, J. Nucl. Mater. 60 (1) (1976) 13.

[18] R. Sakamoto, T. Muroga, N. Yoshida, J. Nucl. Mater. 220222 (1995)

819.

[19] H. Iwakiri, K. Yasunaga, K. Morishita, N. Yoshida, J. Nucl. Mater.

283287 (2000) 1134.

[20] T. Matsuia, S. Mutob, T. Tanabe, J. Nucl. Mater. 283287 (2000) 1139.

[21] ITER material assessment report 2001 G 74 MA 10 0107-11 W 0.2.

[22] , SRIM.

[23] .

[24] J.P. McCaffrey, M.W. Phaneuf, L.D. Madsen, Ultramicroscopy 87 (2001) 97.

[25] W. Jger, M. Wilkens, Phys. Status Solidi A 1 (1975) 89.

[26] Y. Watanabe, H. Iwakiri, N. Yoshida, K. Morishita, A. Kohyama, Nucl. Instrum.Methods Phys. Res. B 255 (2007) 32.

. Ciupinski et al. / Nuclear Instruments and Methods in Physics Research B 317 (2013) 159164 163
http://www.srim.org/http://www.hht-eu.com/hht-eu/nte/FB-2100Brochure.pdfhttp://www.hht-eu.com/hht-eu/nte/FB-2100Brochure.pdfhttp://www.srim.org/


6/6

Documents

TEM Observations of Radiation Damage in Tungsten Irradiated by 20 Mev W Ions