Vacancy like defects and hardening of tungsten under irradiation with He ions at 800 °C

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Vacancy like defects and hardening of tungsten under irradiation with He ions at 800 ◦ C M.H. Cui a,∗ , T.L. Shen a , H.P. Zhu c , J. Wang d , X.Z. Cao b , P. Zhang b , L.L. Pang a , C.F. Yao a , K.F. Wei a , Y.B. Zhu a , B.S. Li a , J.R. Sun a , N. Gao a , X. Gao a , H.P. Zhang a , Y.B. Sheng a , H.L. Chang a , W.H. He a , Z.G. Wang a,∗ a

Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou, 710000, China Key Laboratory of Nuclear Analysis Techniques, Institute of High Energy Physics, CAS, Beijing 100049, China c North China Electric Power University, Beijing 102206, China d Ningbo Institute of Industrial Technology, Chinese Academy of Sciences, Ningbo, 315200, China b

h i g h l i g h t s • • • •

The bulk-equivalent hardness values and their increments of tungsten irradiated with 200 keV He+ at 800 ◦ C were investigated. Vacancy like defects in tungsten induced by 200 keV He+ at 800 ◦ C were investigated. The irradiation induced hardening behavior in tungsten could be explained by the dispersed barrier hardening model. The irradiation induced hardening in tungsten was consistent with the vacancy like defects.

a r t i c l e

i n f o

Article history: Received 27 September 2016 Received in revised form 10 March 2017 Accepted 9 May 2017 Available online xxx Keywords: Vacancy like defect Hardening Tungsten He irradiation Hen Vm complex

a b s t r a c t The irradiation induced hardening and vacancy like defects are investigated as a function of irradiation fluences with 200 keV He ion implantation at 800 ◦ C in this work. Nano-indentation tests show the irradiation induces hardening and the hardness values increase with increasing irradiation fluences. Doppler broadening spectroscopy analyses of slow positron annihilation find that a large amount of vacancy like defects are produced under irradiation. When the He/dpa ratio is less than about 2.2% He/dpa, the vacancy like defects include mainly empty vacancy clusters and loops, and Hen Vm complexes. When the He/dpa ratio is more than about 2.2% He/dpa, the vacancy like defects become mainly Hen Vm complexes with increasing irradiation fluences. Meanwhile, these complexes increase in size and He/V ratio while the irradiation fluences increase. Based on the Orowan hardening mechanism, we discuss the relationship between irradiation induced hardening and defects under indentation, especially the contribution of Hen Vm complexes to the hardening. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Nuclear fusion energy is one important method to solve the future energy of the world. One most important issue in the development of future reactors is the material problem, such as the selection and estimation of divertor and structural components. Tungsten is a potential candidate divertor material, due to its high melting point, high thermal conductivity, low activation, low sputtering and low H isotope retention etc. The divertor component,

∗ Corresponding authors. E-mail addresses: [email protected] (M.H. Cui), [email protected] (Z.G. Wang).

i.e. tungsten, serves under severe environments such as the bombardment of hydrogen isotopes (D and T) and helium (He) plasma, the irradiation of neutron and high temperatures etc. Irradiation induced damages such as the displacement damage (up to 15 dpa within 5 years in DEMO-like reactor for the armor materials of divertor) and He atom depositions often influence the microstructure and mechanical properties of materials and possibly relates to the safe operation of reactors. He ion irradiation has been testified to be an effective means to simulate the He effects in materials under the irradiation in reactors. There were lots of He irradiation experiments on many kinds of materials such as low activation ferrite/martensite steels [1–4], tungsten [5–11] and Si [12]. The researches showed that He irradiation produced dislocations, and vacancy like defects such as vacancies (Vs), complexes of He and

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6.0

1.0

16

(a) 200 keV He in tungsten

2

1.0X 10 ions/cm 16 2 5.0X 10 ions/cm 17 2 1.0X 10 ions/cm

5.5 5.0 4.5

0.8

(b) 200 keV He in tungsten 16

2

1.0 X10 ions/cm 16 2 5.0 X10 ions/cm 17 2 1.0 X10 ions/cm

4.0 3.5

dpa

He at.%

0.6

0.4

3.0 2.5 2.0 1.5

0.2

1.0 0.5

0.0 0

200

400

600

0.0 0

100

Depth(nm)

200

300 400 Depth(nm)

500

600

700

Fig. 1. depth profiles of (a) dpa and (b) He concentration (CHe ) for irradiated tungsten with 200 keV He to three fluences calculated with SRIM.

vacancies (Hen Vm , in which n and m are the number of He atoms and vacancies, respectively), and He bubbles. Vacancy like defects, especially the Hen Vm complexes, play a very important role in the degradation of materials’ mechanical properties [5,8,13] and surface morphology [14], because the vacancies are very attractive to He atoms [15–17] and the complexes are the nuclei of He bubbles. The fundamental understanding of He irradiation induced vacancy like defects and their effects on hardening in tungsten is very important in understanding the mechanical behavior under irradiation. The investigation about vacancy like defects and their effects on hardening of tungsten induced by He irradiation under different temperatures has been investigated by several papers. Monovacancies were introduced by 800 keV He irradiation in the track region of tungsten [10]. They became mobile from about 200 ◦ C, which gave rise to the formation of vacancy clusters [18]. The complexes of Hen Vm formed by 350 keV He ion irradiation in the near surface of tungsten. The ratios of He/V in complexes at 600 ◦ C were lower than the ratios at room temperature (RT). The hardness at 600 ◦ C was also lower than the hardness at RT [5]. However, the studies at higher temperatures are rare but important because the operation temperature is in a wide region of about 400–1400 ◦ C [6] and the nature of vacancy like defects are different at different temperatures. Recently developed nano-indentation technology (NIT) has been widely used to characterize the hardness in the thin damage layer induced by ion irradiation [3,5,7,19–22]. The positron annihilation technology was an effective method in detecting the vacancy like defects because of its sensitivity on point defects with atomic scales [10]. Many works related the S parameters derived from the Doppler Broadening Spectrum (DBS) of positron annihilation with the nano-hardness from the NIT to investigate the effects of vacancy like defects to nano-hardness under irradiation [3,5,7,19,20]. In this work, we choose pure tungsten as the study objects. To investigate the fundamental process and effects of Hen Vm complexes on hardening at higher temperatures, 200 keV He ion irradiation in tungsten were completed at 800 ◦ C which is in the working temperature range of divertor. Three different irradiation fluence of 1.0 × 1016 , 5.0 × 1016 , 1.0 × 1017 ions/cm2 were completed to get a relatively low displacement damage and He centration in shallower depth, which are favorable in the investigations of DBS and NIT. The changes of hardness and vacancy like defects after irradiation are investigated by the NIT tests and the DBS measurements of slow positron annihilation, respectively. The influences of irradiation induced defects on the hardness is discussed.

2. Experiments The investigated tungsten samples were cut from the high pure tungsten plate provided by Advanced Technology & Materials Co., Ltd. The plate was produced by the powder metallurgy method followed by warm rolling at 1200 ◦ C to 75% reduction in thickness (final thickness ∼3 mm). The elongated grains varied in size from several to a few hundred ␮m. The tungsten’s purity was higher than above 99.99% and contained impurities such as ∼23 appm Mo, ∼13 appm Fe, ∼9 appm Cr and ∼3 appm Ni. The samples were mechanically polished until the diamond paste is 0.25 ␮m before irradiation and then were irradiated with 200 keV He ions provided by the 320 kV Multi-Discipline Research Platform for Highly Charged Ions in Institute of Modern Physics, Chinese Academy of sciences (IMP, CAS), Lanzhou, China. The ion beam was swept in two perpendicular directions to a uniform distribution. The irradiation temperature is 800 ◦ C and the irradiation fluences are the lowest 1.0 × 1016 ions/cm2 , the modest 5.0 × 1016 ions/cm2 and the highest 1.0 × 1017 ions/cm2 . When the incident He ions irradiate into the samples, the majority damages are the displacement damage and the He atom deposition. The theoretical results of displacement damage levels (i.e. dpa, displacement per atom) and He concentrations (CHe ) for irradiated tungsten with three fluences are calculated with SRIM (The stopping and Range of Ions in Matter) and shown in Fig. 1(a) and (b). The displacement energy of tungsten was set to be 90 eV [10]. As recommended in Ref. [23], the surface and binding energies were set to be 0 eV, meanwhile we selected the “Ion Distribution and Quick Calculation of Damage” option to gain the data related to dpa and CHe . CHe /dpa ratios are equal to the CHe divided by dpa. It should be noted that the dpa values are possible maxima because the calculation does not consider the recombination of point defects and the diffusion of atoms. We can see that the dpa and CHe peak are the depths of 330 and 400 nm, respectively from Fig. 1(a) and (b). The integrated average dpa and CHe in two regions of first 160 nm and 160–300 nm from surface are given in Table. 1. In addition, CHe /dpa ratio is another important factor in the discussion of hardness and defect evolution. Its profile has been shown in Ref. [7] which shows its value increases with increasing depths and equals to about 2.2 at.%/dpa at 160 nm from surface. After irradiation, NIT tests were carried out using an Agilent Nano Indenter G200 with a Berkovich tip in the continuous stiffness mode (CSM). The indenter was normal to the samples’ surface. Six indentations were carried out on each sample and hence six hardness continuous distributions versus depths were given. Each

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200

15 (a)

(0) un-implanted

2

16

2

17

2

16

2

17

2

150

(3)1.0x 10 ions/cm

(3)1.0 x 10 ions/cm

(2)

16

(2)5.0x 10 ions/cm

(2)5.0 x 10 ions/cm

(1)

(3) (2)

2

5

(1) 50

(0)

o

+

200keV He -->W@800 C 0

200keV He -->W@800 C

100

2

(0)

0

0

200

400 Dpeth(nm)

600

67 nm o

+

(1)1.0x 10 ions/cm

H (GPa )

Hardness(GPa)

(b) (0) un-implanted

2

16

10

125

500

(1)1.0 x 10 ions/cm

(3)

3

0.002

0.004

0.006

0.008

0.010

0.012

0.014

-1

1/h (nm )

Fig. 2. (a) depth profiles of hardness with NIT tests and (b) H2 vs. 1/h curves in the unirradiated and irradiated samples.

Table 1 the integrated average dpa and CHe in two regions of first 160 nm and 160–300 nm from surface. Fluence

0–160 nm 2

160–300 nm

environment around the vacancy like defects. The type and density of vacancy like defects can be identified from the S vs. W plot. 3. Results and discussion

ions/cm

Average dpa

Average CHe

Average dpa

Average CHe

1.0 × 1016 5.0 × 1016 1.0 × 1017

0.04 0.19 0.37

0.06 at.% 0.28 at.% 0.55 at.%

0.08 0.38 0.76

0.23 at.% 1.15 at.% 2.30 at.%

indentation was set approximately 30 ␮m apart in order to avoid any overlap of the deformation region caused by other indentations. In addition, the DBS of Positron Annihilation measurements were carried out in Beijing 22 Na slow positron beam line to infer the vacancy like defects in tungsten induced by He irradiation. The principle of this measurement was described in many other papers [6]. In our measurements, the technology is implemented with the use of a slow positron beam which enables to implant positrons to specimens with variable energy from 0.18 to 22.18 keV. The relation between the energy of incident positron E and the mean incident depth Z¯ is given as follows: Z¯ = 40 E1.6 /␳, where Z¯ is in nm, E is in keV, ␳ is the density of the material in g/cm3 , (for tungsten, it is about 19.37 g/cm3 ). According to this formula, the maximum incident positron energy corresponds to a mean incident depth of about 300 nm without considering the broadening of the implanted positron profile. We draw a line at this depth in Figs. 1 and 2. As seen from Figs. 1 and 2, this depth is below the depth of irradiation induced damage peak. Each spectrum collected at each energy is characterized by two integral parameters S and W, which are defined as the ratio of the area calculated around the central low momentum part of the spectrum (511 ± 0.76 keV) over the total number of annihilation, and the ratio of the area calculated in a high-momentum region far from the center of the spectrum (from 511 ± 2.6 keV to 511 ± 6.8 keV) over the total number of annihilation respectively as given elsewhere [7]. The S and W parameters are the annihilation fractions with the low and high momentum electrons, respectively, and therefore they mainly related to positron annihilation with the valance and core electrons, respectively. When the positrons are trapped and annihilated at open volume defects or vacancy like defects, S increases and W decreases. It should be noticed that S parameters are related to the total open volume, which is effected by both the type and density of vacancy like defects, and W parameters are related to the chemical

3.1. NIT test Hardness profiles obtained by NIT tests of unirradiated and irradiated samples are shown in Fig. 2(a). Each curve is the average of the data in the six indentations of each sample. The error bars in each curve are also the statistics on six indentations every depth region of 10 nm–40 nm. For the unirradiated sample, nano-indentation hardness shows a decrease trend with increasing depths, which should be the indentation size effects (ISE) [2,21,24,25]. The hardness values of irradiated samples are higher than those of the unirradiated sample, which indicates that the ion irradiation induces hardening of tungsten as previously reported by us [7], Armstrong [8], Ou [5] and Zhang [22]. The hardness values at the same depth increase with increasing irradiation fluences. In addition, for the irradiated samples, the hardness values increase form surface to about 75 nm and then decrease with increasing depths. The increase of hardness is explained as the reverse indentation size effect (ISE), which usually attributes to testing artifacts and is neglected [21]. The decrease of hardness should be analyzed from many effects, such as the ISE, softer substrate effect (SSE), damage degrade effect (DGE) [2,21,24,26]. To explain the ISE, Nix and Gao have developed a model based on the concept of geometrically necessary dislocation [25]. The measured hardness (H) in this model is given by the following function:



H = H0

1+

h∗ h

in which, H0 is the hardness at infinite depths (i.e. macroscopic hardness), h* is a characteristic length which depends on the material and the shape of indenter tip, h is the indent depth. After the assumption of an irradiation-induced harder layer than bulk for simplicity, Kasada et al. [21] developed an approach to evaluate nominal bulk-equivalent hardness values of low energy ion irradiated samples from the experimentally obtained hardness profile data. This approach has been widely used to explain the irradiation induced hardness change [1,2,19,21,22], in which they have plotted the curve of H2 versus 1/h and found a bilinearity behavior. The obtained average hardness data are analyzed in the same way and the H2 vs. 1/h plots are given in Fig. 2(b). The error bars

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Table 2 H0 , Hexp.aver. , H0 and Hexp.aver. in the unirradiated and irradiated samples with different fluences. The four values have been defined in the text. The data in brackets are corresponding standard deviations. Fluence ions/cm2

Hexpaver. GPa

H expaver. GPa

H0 GPa

H0 GPa

h* nm

0 1.0 × 1016 5.0 × 1016 1.0 × 1017

7.3(0.49) 8.1(0.29) 10.8(0.82) 12.0(0.73)

– 0.8 3.5 4.7

7.0(0.2) – 9.5(0.49) 11.6(0.59)

– 2.5 4.6

7.8 – 25.1 6.3

3.2. DBS measurement 12 Hexp.aver.

Hardness(GPa)

10

H0

8

4 ΔHexp.aver.

2

0

ΔH0

0.0

16

2.0x10

16

4.0x10

16

6.0x10

16

8.0x10

17

1.0x10

2

Fluence (ions/cm ) Fig. 3. the change of H0 , Hexp.aver. , H0 and Hexp.aver. with irradiation fluences.

in Fig. 2(b) are also the statistics on six indentations every depth region of 10 nm to 40 nm. A multiple linearity relation between H2 and 1/h can be seen in Fig. 2(b). It suggests that an obvious damage degrade effect in He irradiation tungsten other than a simple flat damage as seen in other works. We should analyze the data in different zones separately. The indenter produces a plastic deformation zone with a radius r under the surface. Although the r/h ratio maybe smaller in the harder zone, the ratio is often higher than 2 can reaches 5 [24]. For example, when the h is 120 nm, the obtained hardness may be affected by the defects in 300–550 nm or even deeper under the surface. For the sample irradiated with the lowest fluence, the hardness changes little from 75 to 150 nm from surface and is about 8.1 ± 0.29 GPa. While for the unirradiated and the irradiated samples with modest and highest fluences, we fit the hardness data from 70 nm to 120 nm by using the least square method as shown by the dotted lines in Fig. 2(b), in order to correlate the hardness values with vacancy like defects obtained by DBS. For the unirradiated sample, the curve does not show a good linearity because of a probably work hardening due to the surface mechanical polishing. Therefore, we should compare the equivalent hardness values of irradiated samples with the values of the unirradiated sample in the same depth range to eliminate the hardening effects of surface polishing induced dislocations before the irradiation. The solved bulk-equivalent hardness values H0 and characteristic h* are given in Table 2. The average hardness Hexp.aver. vales are obtained directly from the experimental values in this region. Hexp.aver. , H0 and Hexp.aver. are also given in Table 2. Hardness changes of H0 and Hexp.aver. equal to the hardness values of irradiated samples minus that of unirradiated samples with two methods, respectively. H0 , Hexp.aver. , H0 and Hexp.aver. of samples are plotted with fluences and given in Fig. 3. The calculated hardness H0 is a little lower than the average hardness Hexp.aver. obtained directly from the experimental values in this region. With increasing fluences, H0 , Hexp.aver. , H0 and Hexp.aver. increase.

DBS measurement results of unirradiated and irradiated samples are shown in Fig. 4(a)–(c). Based on the S and W parameters under different positron energies, S and W parameters are shown in Fig. 4(a) and (b) as functions of the positron energies and the corresponding depths, and the S versus W plots are given in Fig. 4(c). The S parameters of irradiated samples are higher than those of unirradiated sample and the W parameters of irradiated samples are lower than those of unirradiated sample, which indicates that vacancy like defects are introduced in tungsten by He ion irradiation. The results of irradiated samples are discussed from the two regions I (before ∼ 160 nm) and II (∼ 160–300 nm). In the region I, (S, W) points of three irradiated samples are on one line as show in Fig. 4(c) which suggests that the type of vacancy like defects in region I of different irradiated samples are the same. With increasing irradiation fluences, the S parameters increase and the W parameters decrease. It indicates the density of vacancy like defects increases. In the region II, (S, W) points of three irradiated samples are on three lines whose slopes are different from the line of the region I. Therefore, the types of vacancy like defects in region II are different from the type in region I. The slope of two samples with lowest and medium fluences are similar, and obviously different from the slope of the sample with highest fluence. It suggests that the type of vacancy like defects in two samples with lower fluences are the same but different from the sample with the highest fluence. With increasing fluences, the S parameters do not change and the W parameters decrease, which indicates that the total open volumes do not change but the chemical environment around the vacancy like defects change. When the energetic He ions interact with tungsten atoms, elastic interaction induces atom displacements and the formation of initial defects such as Frenkel pairs, vacancy loops [27]. Besides, incident He atoms lose their energies and deposit near their range as seen in Fig. 1(b). At high temperature of 800 ◦ C, mono-vacancies [18], He atoms and self-interstitials are highly mobile. Meanwhile, both the experiments and theoretical simulation show that vacancies are strong traps for He atoms because of high binding energies between them [9,10,15–17,28]. Therefore, some processes may happen in materials, such as the recovery of dislocations from work hardening, the migration of mono-vacancies and then the formation of vacancy clusters; the binding of He atoms with vacancies and then the formation of Hen Vm complexes; the formation of interstitial loops as a result of the accumulation of self-interstitials; Hen Vm complexes with large inner pressures eject lattice atoms to form larger bubbles and interstitial loos; loops attract He atoms, and the above mentioned defects increase in density and size. According to the in situ TEM observation of 8 keV He ions in tungsten, TEM invisible He bubbles occurs under a fluence of 5.0 × 1015 ions/cm2 [11] which corresponds to about 1.4 He at.%. The He concentrations and He/dpa ratios in the region I are below 1.4 He at.% and 2.2% He/dpa, respectively, as seen in the Table 1. Therefore, we suggest not all the vacancies are occupied with He atoms and the main vacancy like defects in He irradiated samples are empty vacancy clusters and vacancy loops, as

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9

0.48

27

52

Depth(nm) 82 117

249

un-implanted 1.0x1016ions/cm2 5.0x1016ions/cm2 1.0x1017ions/cm2

(a)

0.46

9

0.09

0.44

+

27

52

Depth(nm) 82 117

(b)

201

157 +

un-implanted 1.0 x1016ions/cm2 5.0 x1016ions/cm2 1.0 x1017ions/cm2

0.08

W parameter

S parameter

201

157

5

249 o

200keV He -->W@800 C

0.07

o

200keV He -->W@800 C 0.42

0

5

10 15 Energy of positrons (keV)

0.080

0.06

20

0

5

10 15 Energy of positrons (keV)

20

(c)

W parameter

0.075 depth increasing 0.070 +

o

200keV He -->W@800 C un-implanted 0.065 1.0x 1016ions/cm2 5.0x 1016ions/cm2 1.0x 1017ions/cm2 0.060 0.42

0.43

0.44 0.45 S parameter

0.46

0.47

Fig. 4. (a) S and (b) W parameter profiles with energy of positrons and corresponding depth, (c) S vs. W parameters in unirradiated and irradiated samples. The dotted vertical lines in (a) and (b) are drawn to divide region I and II. The dotted arrows in (c) indicate the direction of depth increasing.

well as Hen Vm complexes etc. The densities of theses vacancy like defects increase with increasing irradiation fluences. This is similar to the results which show that the irradiation induced monovacancies [10] and vacancy loops [27,29,30] increase in density with increasing irradiation fluences in irradiated tungsten at lower temperatures. The He/dpa ratios in the region II are above 2.2% which is higher than that in region I. Therefore, there are more possibilities of the He atom occupation of empty vacancies and the absorption of vacancies by formed Hen Vm complexes. The main vacancy like defects in the region II become Hen Vm complexes, especially under the highest fluence. This may be a reason for the change of vacancy like defect’s type. TEM visible He bubbles may arise in the region II of the sample with the highest fluence due to the high average He concentration of 2.3 He at.%. Comparing with empty vacancies, He atoms in Hen Vm complexes reduce effective open volumes and hence the measured S parameters [12,31]. This is also in accordance with the lower S values in the region II than in the region I of a sample. The Hen Vm complexes should be more in number or larger in size with increasing irradiation fluences as results of the higher dpa levels and He concentrations. The unchanging S values in the region II with increasing irradiation fluences could be explained by the higher He/V ratios in Hen Vm complexes with increasing irradiation fluences, which could induce the similar effective open volume to annihilate with positions.

Irradiation induced defects are the obstacles of dislocation movements during indentations. The obstacle strength factors are different for different defects. Hen Vm complexes are strong obstacles based on the previous reports [5,8,13,32]. Hardness increase relates with both the density and size of Hen Vm complexes, based on the dispersed barrier hardening model of the Orowan hardening mechanism [1]. The increase of complex’s density in the region I and the increase of complex’s size in the region II has a positive contribution to the hardness increase with increasing irradiation fluences. In addition,recent calculations indicate that higher He/V ratios in Hen Vm complexes induce larger obstacle strength factors than empty voids [13,32]. The He induced additional hardening than only the displacement damage is also observed. Therefore, the hardness increase with increasing irradiation fluences also relate to the increasing He/V ratios. Of course, there are some other sources of hardening in He irradiated tungsten, besides Hen Vm complexes, for example dislocation loops [8,22]. In our irradiated tungsten samples, dislocation loops also exist and attribute to the irradiation hardening. The dislocation loops may originate from work hardening and the irradiation. The dislocations from work hardening may partly recover at the irradiation temperature of 800 ◦ C, which has a negative effect on the hardness increments. While the irradiation induced dislocations have a positive effect on the hardness increments. The increase of density and size with increasing fluence as seen in

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[11] may cause the increase of hardness increments. However, the contributions ratios of Hen Vm complexes and dislocations to the hardness increase cannot be weighed from our results and should be researched in future. 4. Summary The irradiation induced vacancy like defects and hardening in tungsten at 800 ◦ C is investigated as a function of irradiation fluences with the 200 keV He ion implantation in this work. Irradiation induced hardening is found with the NIT tests. The average and bulk-equivalent hardness values of irradiated samples increases with increasing irradiation fluences. The DBS measurements shows that a large amount of vacancy like defects are produced in the irradiated samples, especially Hen Vm complexes, which are strong hardening sources. When He irradiation induces less than about 2.2% He/dpa, irradiation induced vacancy like defects are mainly empty vacancy cluster and loops, and Hen Vm complexes. These defects increase in density when the irradiation fluences increase, which has a positive contribution to the hardness increase based on the dispersed barrier hardening model of the Orowan hardening mechanism. When He irradiation induces more than about 2.2% He/dpa, the irradiation induced vacancy like defects become mainly Hen Vm complexes with increasing irradiation fluences. These complexes increase in size and He/V ratio in the complexes with increasing irradiation fluences. This also attributes to the higher hardening under the higher irradiation fluence. Acknowledgements The authors would like to express our sincere gratitude to all the staffs of 320 kV multi-discipline research platform. This research is funded by the National Natural Science Foundation of China under Grant Nos. 11605256, 91426301, 11505246, 11405231, 11575258, 11375242, and the Strategic Priority Research Program of the Chinese Academy of Sciences, Grant No. XDA03010301. References [1] Z.Y. Fu, P.P. Liu, F.R. Wan, Q. Zhan, Helium and hydrogen irradiation induced hardening in CLAM steel, Fusion Eng. Des. 91 (2015) 73–78. [2] Y. Takayama, R. Kasada, Y. Sakamoto, K. Yabuuchi, A. Kimura, M. Ando, D. Hamaguchi, H. Tanigawa, Nanoindentation hardness and its extrapolation to bulk-equivalent hardness of F82H steels after single- and dual-ion beam irradiation, J. Nucl. Mater. 442 (2013) S23–S27. [3] Y. Xin, X. Ju, J. Qiu, L. Guo, J. Chen, Z. Yang, P. Zhang, X. Cao, B. Wang, Vacancy-type defects and hardness of helium implanted CLAM steel studied by positron-annihilation spectroscopy and nano-indentation technique, Fusion Eng. Des. 87 (2012) 432–436. [4] M. Ando, E. Wakai, T. Sawai, H. Tanigawa, K. Furuya, S. Jitsukawa, H. Takeuchi, K. Oka, S. Ohnuki, A. Kohyama, Synergistic effect of displacement damage and helium atoms on radiation hardening in F82H at TIARA facility, J. Nucl. Mater. 329–333 (2004) 1137–1141. [5] X. Ou, W. Anwand, R. Kogler, H.-B. Zhou, A. Richter, The role of helium implantation induced vacancy defect on hardening of tungsten, J. Appl. Phys. 115 (2014) 123521. [6] P.E. Lhuillier, T. Belhabib, P. Desgardin, B. Courtois, T. Sauvage, M.F. Barthe, A.L. Thomann, P. Brault, Y. Tessier, Helium retention and early stages of helium-vacancy complexes formation in low energy helium-implanted tungsten, J. Nucl. Mater. 433 (2013) 305–313. [7] M.H. Cui, Z.G. Wang, L.L. Pang, T.L. Shen, C.F. Yao, B.S. Li, J.Y. Li, X.Z. Cao, P. Zhang, J.R. Sun, Y.B. Zhu, Y.F. Li, Y.B. Sheng, Temperature dependent defects evolution and hardening of tungsten induced by 200 keV He-ions, Nucl. Instrum. Methods Phys. Res. Sect. B 307 (2013) 507–511.

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Please cite this article in press as: M.H. Cui, et al., Vacancy like defects and hardening of tungsten under irradiation with He ions at 800 ◦ C, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.05.043