J. Plasma Fusion Res. SERIES, Vol. 11 (2015)

Correlation of Microstructure Evolution and Hardening in Ion-irradiatedPure Tungsten
Zhexian ZHANGa,*, Wentuo HANb, Akihiko KIMURAbaGraduate School of Energy Science, Kyoto University, Uji, JAPANb

Institute of Advanced Energy, Kyoto University, Uji, Japan(Received: 17 September 2014 / Accepted: 19 January 2015)

Pure tungsten specimens in cold rolled condition and recrystallized condition were irradiated with 6.4 MeV Fe3+
at 300 oC, 700 oC, 1000 oC respectively. The damage profile was calculated by SRIM. The averaged dpa was 2 dpa
at 600nm. The irradiation hardness was tested by nano-indentation method with CSM mode and the bulk-equivalenthardness was evaluated by following Nix-Gao model. All the samples showed irradiation hardening irrespective ofirradiation temperature and specimen condition. The recrystallized ones showed larger irradiation hardening than theas-received ones. TEM observations revealed that voids were generated at 1000 oC. At the same time, dislocations
and loops were found at all the temperatures. No precipitate was found in the irradiated layer. Orowan equation wasapplied to estimate the strengthening contributed by dislocations, loops and voids. Although it is considered that thecontribution of voids is rather small, the estimated hardening based on the TEM observation results are far from themeasured one.
Keywords: tungsten, irradiation hardening, recrystallization, void formation
hardness was measured by continuous stiffness
measurement (CSM) by nano-indentation method andmicrostructures were observed by TEM at 200keV.
Irradiation hardening was correlated with microstructure
evolution.

1. Introduction

ITER has selected full-W divertor system in stead of
W/CFC[1-3]. The tungsten will endure 0.7 dpa after aneutron loading of 0.15 MWa/m2 at operation temperatures[4]. During neutron irradiation, radiation damage
structures such as dislocation, loops and voids, andprecipitates including transmutation helium atoms will begenerated and they may cause the deterioration ofmechanical properties. These structures will block
dislocation motion and cause irradiation hardening. Since
irradiation hardening is often accompanied by loss ofductility, understanding hardening mechanism is necessary.
Microstructure evolution induced by neutron
irradiation in tungsten and its alloys at different
temperatures were summarized by several researchers[5,6]. Loops are generated at low displacement per atom(dpa) and voids were found in many irradiation conditions.
As irradiation temperature increases or dpa increases,
loops are more likely replaced by voids. In the case of hightemperature irradiation over 1000oC, voids were generated
and considered to behave as pinning obstacles for
dislocations that account for hardening effect [7].
Since neutron irradiation consumes more time andfinancial cost, ion-irradiation has often been used to obtainhigh damage level within shorter time and with lower
expense [8-11]. The ion-accelerator, DuET, in Kyoto
University offers 6.4MeV Fe3+ irradiation beam and theirradiation temperatures are controlled by an infrared
thermal temperature control system. In this paper, puretungsten before and after recrystallization condition wasirradiated by DuET at 300oC, 700oC and 1000oC. The

2. Experimental procedure

The material used was a commercial pure tungsten(99.95%, Nilaco Corporation) and the texture was shown inFig. 1 which was investigated by EBSD analysis performed
on ZEISS Ultra55 with EBSP detector. A specimen wasannealed at 1400 oC for 1 h and called as recrystallized
specimen hereafter. All the specimens were polished by#500, #800, #1200, #2400, and #4000 SiC paper, followedby diamond polishing with the diameter of 6Ɋm, 3Ɋm, 1Ɋand 0.25Ɋm powders. The final electrical polishing wasdone in 2% NaOH solution under 15 V for 2 min. Vickers
hardness measurement was carried out for specimens before

Fig.1 EBSD analysis of as-received tungsten plate.
The grains of NR and NT surface are elongated andgrains on TR are isotropic.

author’s e-mail: zx-zhang@iae.kyoto-u.ac.jp

94©2015 by The Japan Society of Plasma
Science and Nuclear Fusion Research

Z. Zhang et al., Correlation of Microstructure Evolution and Hardening in Ion-Irradiated Pure Tungstenmethod. The indentation depth was 2000 nm and oscillation
was selected as 1 nm. Nix-Gao model (Fig.3) was used tocalculate the bulk-equivalent hardness [15]. Microstructures
were observed by JEOL-2010 TEM at 200keV. The samples
were prepared by FIB (JFIB-2100) with Ga3+ and then ionmilled with Model 1040 NanoMill with 900eV Ar+.

irradiation using ASAHI HM-103-HH with a load of 0.1 kgf
holding for 10s.
The ion-irradiation was carried out on as-received TR
surface, NR surface and recrystallized TR surface oftungsten sheets, respectively. The damage profile wascalculated by SRIM-2013[12] with different methods by thefollowing equations: (1) full damage cascade, FC, (2) quick
calculation of damage, vacancy model, (3) quick calculationof damage, NRT model, QD [13]. In equation (3), Tdam
means the energy loss to ions and phonons, ED [14]. Herethe author uses the average of 2 dpa at 600nm depth fromthe result of ED method to present for the irradiation
displacement damage.
dpa

dpa
dpa

vacancies  replacemen tu fluence
atom density
vacancies  recoils
u fluence
atom density
0.8Tdamu fluence
2 Ed

3. Result and discussion

The temperature dependence of irradiation hardeningof pure tungsten before and after recrystallization wasshown in Fig.4. All of the samples showed hardening at all
the temperatures. The recrystallized tungsten showed ahigher irradiation hardening than as-received ones. This isreasonable that the recrystallization eliminated most of thegrain boundaries which acts as absorption sink site ofirradiation induced interstitials and vacancies. As fortemperature dependence, as the temperature increases, thehardening increases in the case of before recrystallization,
while the recrystallized ones remain stable. Comparing thespecimens with different surface direction of as-received,
no difference was found between TR and RN, indicatingthat the grain shape change is not effective to change theamount of hardening.
Nano-hardness test is very sensitive to the surface
condition and testing environment. Any disturbance could
cause a big error. The total of absolute deviations makes it
even more difficult to present the accurate value. In general,
however, the trend of the curve is good enough, and theirradiation hardening was calculated by 'HV=HV(irr.) HV(unirr.). The error was calculated by ݁‫ݎݎ‬ሺ'‫ܸܪ‬ሻ  ൌඥ݁‫ݎݎ‬ሺ‫ݎݎܸ݅ܪ‬Ǥ ሻଶ ൅  ݁‫ݎݎ‬ሺ‫ݎݎ݅݊ݑܸܪ‬Ǥ ሻଶ .
The microstructures after ion-irradiation are shown inFig.5, which indicates high number densities and large
diameters of voids in the specimen irradiated at 1000 oC.
No void was observed in the specimens irradiated at 300o
C and 700 oC. The high temperature may excite themobility of vacancy clusters in stage IV [17] which

(1)
(2)
(3)

Nano-indentation hardness was measured by CSM

Fig.2 Displacement damage profile calculated by SRIM.
Nominal 2 dpa at 600 nm was selected as the damage level.

Fig.4 Temperature dependence of irradiation hardeningof pure tungsten before and after recrystallization.
Fig.3 The bulk-equivalent hardness, H0, based on Nix-Gaomodel. The irradiated curve has a shoulder which isconsidered as soft substrate effect.[16]95Z. Zhang et al., Correlation of Microstructure Evolution and Hardening in Ion-Irradiated Pure Tungsten

This could be the reason why voids were found in otherpapers[5-6,8] but not in this experiment.

corresponds to the dissociation of Vn൒5 formed in stage III[18]. At the same time, impurities will suppress thetendency of resolvable voids formation because impurities
could serve as individual trapping sites for vacancies[19].

Fig.5 Microstructures observed by TEM. Voids were found in all the specimens irradiated at 1000oC, while in the specimens irradiated at
300oC and 700oC, only the dislocations and loops were formed. The photos, a), b), d), e) g), and h) were observed in just focus
condition. c), f), and i) were taken in the under focused condition.
Table.1 Mean diameter and number density of dislocation/loops, voids, and the hardness estimated and measured.

3+Loopsdislocation

Fe into W

NumberSize

1/2(Nd)

1/2

ο

nm

X10 /m

1.28

4.72

7.77

0.54

1.47

1.38

5.21

8.48

0.57

4.64

1.33

1

6.51

8.07

0.55

o

6.40

1.56

1.02

7.99

9.03

o

8.36

1.78

8.7

9.5

9.09

142dpa10 /mo

222

10 /m

4.23

ο

1.27

TR,700 C

o

5.70

o

TR,300 C

TR,1000 C
Re,300 C
Re,700 C

3696

ο ሺ ሻ

Voids (Nd)GPa

X10 /m6

οEstimated Measured
1.81

1.09

2.04

1.41

3.15

1.62

0.58

2.14

2.14

0.59

2.37

2.1220.6

1.27

Z. Zhang et al., Correlation of Microstructure Evolution and Hardening in Ion-Irradiated Pure Tungsten

The relation between Vickers hardness and nanoindentation could be expressed in the following equation
based on the relationship between HV and nano-hardness
obtained from experiments in our lab:

Loops and dislocations were found in the 300oC
700oC and 1000oC. Dislocation number density wascalculated by line intercept method and computed by ߩ ൌܰȀ‫ܮ‬௥ ‫ݐ‬. N is the number of intercept points. Lr is the total
length of random lines and t is the thickness of TEM foil,
which was measured by CBED technique.
For bcc metals, after irradiation, the Burgers vectors ofdislocation loops are mainly 1/2<111> and <100>. Assume
the density of ½<111> loops was a, and <100> loops was b.
Images were taken at different reflection conditions: [200],[110], [211] and the densities in each condition wererecorded as A, B, C. According to invisibility criteriong൉b=0 (ignoring edge component), we have the visible
fraction at each reflection condition:ͳͳȀ͵‫ܣ‬ܽ൭ͳȀʹ ʹȀ͵൱ ቀ ቁ ൌ ൭‫ܤ‬൱ܾ͵ȀͶͳ‫ܥ‬H 0 [GPa] 0.001268 u H v [MPa]The estimated values are shown in Table.1. The
hardening values caused by loops are almost the sameamong the specimens irrespective of irradiation temperatureand recrystallization treatment. A remarkable difference isobserved for the case of irradiation at 1000 oC because ofwhere the contribution of voids to the hardening was taken
into account. However, the large difference of the calculated
value from the measured one, indicates that the voids are noteffective obstructs to dislocation motion. In this case,strengthening factor, D, of voids could be very small.
Impurities may also contribute to the hardening effect,
which will act as obstacles to dislocation motion. However
we didn’t find any visible precipitate by TEM observation.
Another possibility is that very fine invisible voids by TEM
existed in the specimens irradiated at 300 oC and 700 oC,
while some of the dislocations and loops were induced by
FIB and involved in the estimation. There may be residual
dislocation and loops that shouldn’t account for thehardening. Further investigation is necessary for thedetermination of the hardening factor ȽሺͻሻǤ(4)Solution of this equation is:ܽͶȀ͵ቀ ቁൌቀܾെͳOrെʹȀ͵ ‫ܣ‬ቁቀ ቁ‫ܤ‬ʹͶȀ͵ െͶȀͻ ‫ܣ‬ܽ൰ቀ ቁቀ ቁൌ൬ܾെͳͶȀ͵‫ܥ‬(5)(6)

4. Conclusion

Pure tungsten in as-received and recrystallized
conditions were irradiated with Fe3+ at 300 °C, 700 oC and1000 oC up to a nominal displacement damage of 2 dpa. The
following results were obtained:
1) Irradiation hardening was observed at all the irradiation
temperatures, 300°C, 700°C, 1000°C.
2) Dislocation loops were observed at all the temperatures,
while cavities were only observed at 1000°C. Thehardening appears to be due to the formation of
dislocation loops and possibly due to voids as well at1000°C.
3) Recrystallized W showed a higher irradiation hardening
than as-received W.
4) Large voids were found in 1000oC irradiated samples.
The density in recrystallized W is about two times higher
than as-received W.
5) Orowan analysis showed similar contribution ofdislocation loops to hardening, except the cases whenlarge voids were formed. It is considered that large voids
are ineffective in irradiation hardening. Fine voids may
contribute to the difference. The Orowan analysis needs
further investigation focusing on irradiation temperaturedependence.

So that total dislocation/loop density is:ͳͶ(7)ܽ൅ܾ ൌ ‫ܣ‬൅ ‫ܤ‬
͵
͵
orͳ
ͺ(8)
ܽ൅ܾ ൌ ‫ܣ‬൅ ‫ܥ‬͵ͻ
Grain boundaries were considered to be able to absorb
the defects generated from irradiation, these defects include
interstitial atoms and vacancies. Recrystallization will
eliminate such grain boundaries. As the migration energy ofinterstitials is much smaller than that of vacancies, eventhough there were no voids found in 300oC and 700oC
irradiated samples, the accumulated interstitial atoms could
form into and be absorbed by loops and line dislocations.
Thus, in recrystallized samples, the elimination of grainboundaries will lead to higher number of interstitials
comparing to as-received ones, and therefore a higher
number density of dislocations as well.
Orowan equation (equation 9) was used to estimate thehardening related to the microstructures observed by TEM.
οɐy stands for yield stress increase caused by obstacles.
Taylor factor, M, is selected as 6. The factor Ƚ is 0.2 for all
the microstructures. Burgers vector b for tungsten is 0.274
nm. The shear modulus Ɋ is 151 GPa.

'V yMDPb Nd'HV (MPa)

3'V y (MPa)(10)

Acknowledgements

The first author is sponsored by China ScholarshipCouncil. Mr. Hayashi and Dr. Kondo are gratefully
acknowledged for the technical support in operation of FIB,TEM, Nanomill. Mr. Okinobu Hashitomi is thanked for theion-irradiation experiments.

(9)

(10)

97

Z. Zhang et al., Correlation of Microstructure Evolution and Hardening in Ion-Irradiated Pure Tungsten

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