The effect of He bubbles on the swelling and hardening of UNS N10003 alloy

Accepted Manuscript The effect of He bubbles on the swelling and hardening of UNS N10003 alloy Guanhong Lei, Ruobing Xie, Hefei Huang, Renduo Liu, Qing Huang, Jianjian Li, Yuying Zhou, Cheng Li, Qiantao Lei, Qi Deng, Yongqi Wang, Chengbin Wang, Wei Zhang, Long. Yan, Ming Tang PII:

S0925-8388(18)30786-2

DOI:

10.1016/j.jallcom.2018.02.291

Reference:

JALCOM 45167

To appear in:

Journal of Alloys and Compounds

Received Date: 3 November 2017 Revised Date:

6 February 2018

Accepted Date: 24 February 2018

Please cite this article as: G. Lei, R. Xie, H. Huang, R. Liu, Q. Huang, J. Li, Y. Zhou, C. Li, Q. Lei, Q. Deng, Y. Wang, C. Wang, W. Zhang, L. Yan, M. Tang, The effect of He bubbles on the swelling and hardening of UNS N10003 alloy, Journal of Alloys and Compounds (2018), doi: 10.1016/ j.jallcom.2018.02.291. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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The effect of He bubbles on the swelling and hardening of UNS N10003 alloy Guanhong Lei

a,b

, Ruobing Xie a, Hefei Huang a, Renduo, Liu a, Qing Huang a, Jianjian Li a,

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Yuying Zhou a,b, Cheng Li a, Qiantao Lei a, Qi Deng a, Yongqi Wang a, Chengbin Wang a, Wei Zhang a, Long. Yan a,*, Ming Tang c

b c

Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China

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a

Los Alamos National Laboratory, Los Alamos, NM 87545, USA

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ABSTRACT

In order to investigate the effect of He bubbles on the swelling and hardening of nickel based alloys, 1.2 MeV He ion irradiation was performed on UNS N10003 alloy at 650 oC. The number densities and sizes of He bubbles increase with increasing irradiation dose. Neither “black spots” nor dislocation loops were observed in TEM

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images of the as-irradiated samples. Irradiation dose up to 6.18 displacements per atom (dpa) causes a swelling of 2.67%, which is mainly ascribed to He bubbles. He bubbles induced hardening increases with increasing irradiation dose. The irradiation hardening

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behavior was compared with heavy ion irradiation. The result reveals that the obstacle strength of He bubbles is less than that of the saturated heavy ion induced irradiation

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defects.

Key words: Nickel based alloy, ion irradiation, He bubbles, swelling, irradiation hardening.

1. Introduction Nickel based alloys have reached much attention since the Gen IV nuclear reactors concept was proposed [1-2]. They are considered as promising candidate structural materials for advanced reactors due to their high temperature strength, excellent corrosion resistance and good creep rupture properties [3-4]. In recent years, a variety 1

ACCEPTED MANUSCRIPT of commercial nickel based alloys were evaluated in neutron reactors for potential applications in Gen IV reactor systems above 650 oC [5].The available information indicates that He bubbles should be one of the most concerned issues in nickel based alloys, since they are capable of substantially deteriorating mechanical properties of

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structural materials especially under high flux and high temperature neutron irradiation environment [6-11]. However, He bubbles are usually concurrent with other irradiation defects, such as point defects, dislocation loops, voids and so on. Thus, it is difficult to investigate single-type irradiation defect by neutron irradiation.

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Ion beam irradiation is a useful tool to gain fundamental insights into the damage processes [12-13]. Many sequential dual-ion irradiations and heavy ion irradiations

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with He pre-injection experiments were carried out to simulate neutron damage in alloys [14-17]. On the other hand, specific irradiation damage can be acquired by adjusting the irradiation rate, temperature, energy and ion species to evaluate the irradiation induced property changes. In pure nickel and nickel based alloys, void swelling has been widely studied by heavy ion irradiation at different irradiation

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temperatures and dose rates [18-22]. Irradiation hardening induced by dislocation loops has also been reported [23-26]. Recently, the separate contribution of different irradiation induced defects, including dislocation loops, vacancy clusters and

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precipitates, to the hardening behavior was investigated in iron based alloys by different characterization methods [27]. Nevertheless, the effect of He bubbles on mechanical property changes is still not fully understood.

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In this study, we introduced He bubbles in the UNS N10003 alloy, which is a

candidate nickel based structural material in Gen IV Molten Salt Reactors (MSR), by He ion irradiation at high temperature. Neither “black spots” nor dislocation loops were observed. The effect of He bubbles on the swelling and hardening of UNS N10003 alloy was investigated. Furthermore, He bubbles induced swelling and hardening were compared with those induced by heavy ion irradiation defects, including voids and dislocation loops.

2. Experiment procedure 2

ACCEPTED MANUSCRIPT 2.1 Material preparation The material used in this study is nickel based UNS N10003 alloy. The nominal chemical composition of this alloy is shown in Table 1. The alloy samples were cut from a 4 mm thick hot rolled plate, which was quenched after annealing at 1177 oC for

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40 min. The samples for irradiation were prepared in geometry size of 10 × 10 × 1 mm3 by wire-electrode cutting. All the samples were mechanically polished with silicon carbide paper from grade 80 to 2000, followed by diamond polishing paste and alumina suspensions to get a mirror-like surface. Then vibration polishing was applied

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to ensure that the surface stress induced from mechanical polishing was removed. Finally the samples were cleaned for 10 min using an ultrasonic bath with alcohol and

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deionized water respectivly.

Table 1 The chemical composition of the UNS N10003 alloy (wt. %). Elements Ni Mo Cr Fe Mn Si C, W, Ti, Al, Co, S, P UNS N10003 alloy

base

17.20 6.96 4.06 0.70 0.43

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2.2 Irradiation

< 0.1

Ion irradiation was carried out with a 4 MV pelletron accelerator at Shanghai Institute of Applied Physics, Chinese Academy of Science (SINAP-CAS). The samples were irradiated with 1.2 MeV He+ at the fluence of 1 × 1017 and 3 × 1017

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ions/cm2 (corresponding to the peak doses of 2.06 and 6.18 dpa) at 650 oC. The beam current was 0.5 µA/cm2, giving a displacement rate of 6.4× 10-5 dpa/s. In order to

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investigate the swelling after irradiation, the irradiation surface of all the samples were half covered with metal foils. The ion irradiation damage profile was simulated by SRIM 2006 program using the “Kinchin-Pease quick calculation” mode. The threshold displacement energy Ed was set to 40 eV [28]. The dpa profile and He concentration were calculated and shown in Fig.1 for 1.2 MeV He+ at the fluence of 3 × 1017 ions/cm2 (6.18 dpa). He ion penetration depth is about 2.25 µm, and the damage peak position is 1.95 µm. As for He concentration, the peak value is up to 12.6%, while the He accumulation peak depth is slightly deeper than irradiation damage peak depth.

3

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15

5 10 4

3 5

2

1

0 0

500

1000

1500

Depth (nm)

2000

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Displacement per atom (dpa)

6

He concentration (at.%)

dpa concentration

0 2500

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Fig. 1 Depths profile of dpa (black line) and He concentration distribution (red line) in the 1.2 MeV He+ irradiation corresponding to the dose of 6.18 dpa calculated by SRIM-2006.

2.3 Characterization

The swelling behavior of irradiated samples was characterized with Multimode-8 AFM by measuring step height of the interface between irradiated and unirradiated region. Nano-indentation tests were performed by using G200

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Nano Indenter. A Berkovich diamond indenter tip was used under a continuous stiffness measurement (CSM) mode. Over 20 indentations were made for each sample to acquire an averaged hardness information. The indentation depth was

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selected as 2.5 µm, which covered the whole irradiation damage region. The distance between adjacent indentations was larger than 20 µm. The ion irradiation-induced microstructural evolution was characterized using a Tecnai

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G2 F20 TEM operated at 200 kV. The cross-sectional TEM samples were prepared using a focused ion beam (FIB), where the thickness of the samples was approximately 60 nm [29].

3. Results 3.1 Swelling characterization Fig. 2 (a) is the AFM image of UNS N10003 alloy irradiated with He ions at the dose of 6.18 dpa at 650 oC. Both the irradiated and unirradiated regions show a flat surface, while a clear step can be discerned at the interface, indicating a 4

ACCEPTED MANUSCRIPT severe swelling in the irradiated alloy. Three tests of step height at different areas were collected. Fig. 2(b) shows the step height profiles of three white lines in Fig. 2 (a). The average step height is ~ 60 nm. The step height measurements can be used to estimate the total integrated swelling along the entire range modified by

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the ion irradiation [18]. The swelling was calculated based on the step height divided by the nominal damage range [19]. Here, the damage depth of 1.2 MeV He ion is 2250 nm, which is based on both simulations using the SRIM code (Fig. 1) and TEM observation. Thus, the swelling of the irradiated sample is 2.67%.

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As for the sample irradiated at the dose of 2.06 dpa, no obvious step height was observed.

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Our result is also compared with the swelling effects of Ni ion and neutron irradiated pure Ni [20-21], as shown in Fig.3. It can be seen that the He ion irradiated swelling of UNS N10003 alloy at 6.18 dpa is higher than Ni ion irradiated swelling of pure Ni at 13dpa. Details of the swelling will be further discussed in Section 4.

120

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(b)

100

60

unirradiated region

40

irradiated region

20 0 -20

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Height (nm)

80

0

5

10

15

20

25

30

Section (µm)

Fig. 2 Swelling behavior of UNS N10003 alloy irradiated with He ions at the dose of 6.18 dpa at 650 oC (a) AFM image; (b) profile of step height.

5

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void swelling in pure Ni by neutron irradiation-ref [20] void swelling in pure Ni by Ni ion irradiation -ref [21] He bubbles induced swelling in this work

4

6.4×10-5dpa/s ( 6.18dpa) 7×10-4dpa/s (13dpa )

2

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Swelling(%)

3

1

1×10-7dpa/s (0.8dpa )

300

400

500

600

Irradiation temperature (

700

800

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0 200

)

3.2 Microstructure observation

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Fig. 3 Temperature dependence of swelling in UNS N10003 alloy and pure Ni, the line represents the void swelling, blue line (1× 10-7 dpa/s, neutron irradiated pure Ni at 0.8dpa); black line (7× 10-4 dpa/s, Ni ion irradiated pure Ni at 13dpa). The red circle represents the bubbles swelling in this work (6.4× 10-5 dpa/s, He ion irradiated UNS N10003 alloy at 6.18 dpa).

Fig. 4 (a) is the cross-sectional TEM observation of UNS N10003 alloy

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irradiated with He ions at 6.18 dpa. SRIM simulation results are overlapped on TEM micrograph. He bubbles are observed in the depth range of 1000 nm - 2250 nm, which is consistent with the calculated He concentration using SRIM. Moreover, the distribution of He bubbles is not homogeneous. Four typical

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regions, “Zone A” (1000- 1250 nm), “Zone B” (1250- 1650 nm) “Zone C” (1650- 1900 nm & 2200- 2250 nm) and “Zone D” (1900- 2200 nm) can be

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distinguished according to the size and the density distribution of He bubbles. High resolution TEM (HRTEM) images of the four regions are shown in Fig. 4(b) - (e). It should be noted that no obvious “black spot” defects or dislocation loops were observed in all the irradiation zones. The swelling induced by bubbles can be calculated with equation (1) [19]. S% =

  ∑





× ∑  

× 100%

(1)

The parameter A and δ are the area and the thickness of the irradiated damaged region; N and di represent the total number and the diameter of He bubbles respectively. 6

ACCEPTED MANUSCRIPT Fig. 5 (a) shows the number density distribution of He bubbles at 6.18 dpa. The number density at the peak damage region (Zone D) is up to 7.13 × 1023/m3. The number densities at Zone A, Zone B and Zone C are 5.5 × 1022/m3, 1.98 × 1023/m3, 2.89 × 1023/m3, respectively. Fig. 5 (b) shows the average bubble sizes

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and the swelling values for the four regions. The average sizes of He bubbles in the four regions are 1.94, 2.67, 3.96 and 5.66 nm, respectively. And the weighted average of He bubbles number density and size are 3.15 ×1023/m3 and 3.55 nm for the entire irradiated region. The swelling calculated for the four typical regions

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are 0.02%, 0.21%, 1.1% and 7.7%, respectively. Since most observable He bubbles exist within the four typical regions (from 1000 to 2250 nm), we use the

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weighted average of swelling in these regions to estimate the swelling induced by He bubbles. The estimated swelling is ~ 2.18%, which is close to the swelling value, 2.67%, acquired by AFM. It suggests that He irradiation induced swelling in UNS N10003 alloy is mainly caused by He bubbles formation. The sample irradiated at 2.06 dpa is also characterized by TEM, as shown in

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Fig. 6. He bubbles can be seen in the depth range of 1850 nm - 2250 nm. According to the calculated He concentration of SRIM, very small bubbles are not detected under TEM observation. At the peak damage region, the average size of He bubbles is 3.5 nm, while the number density of He bubbles is 4.08 ×

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1022/m3, much less than that at 6.18 dpa. Thus, we estimate that the swelling induced by He bubbles is less than 0.12%. That’s the reason why no obvious

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swelling was observed by AFM.

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Zone C

Zone B

Zone A

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2

8 6 4

500

1000

1500

2000

2500

9 8 7

5.0

6

4.5 5 4.0 4 3.5 3

3.0

2

2.5

0

2.0

0

0

He bubble size swelling

5.5

He bubble size

3 23

4

10

10

(b)

6.0

14 12

6

6.5

16

Zone D

number density He concentrition

8

Number density (10 /m )

7.0

18

(a)

He concentrition (%)

10

2 1 0

1.5

3000

Zone A

Depth (nm)

Swelling (%)

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Fig. 4 (a) Cross-sectional TEM image of UNS N10003 alloy irradiated with He ions at the dose of 6.18 dpa. The HRTEM images of the Zone A, Zone B, Zone C and Zone D denoted in (a) are displayed in (b), (c), (d) and (e), respectively.

Zone B

Zone C

Zone D

Fig.5 Distribution of He bubbles in UNS N10003 alloy irradiated with He ions at the dose of 6.18 dpa in four typical regions. (a) He bubble number densities and He concentration distribution; (b) He bubble sizes and swelling values.

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Fig. 6 (a) Cross-sectional TEM image of UNS N10003 alloy irradiated with He ions at 2.06 dpa in the peak damage region (1900 nm-2200 nm). (a) low magnification image; (b) HRTEM image.

3.3 Hardening analysis

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The hardness (H) of irradiated region can be calculated using Nix-Gao model [30-31] as follows:

∗

H =  1    .

(2)

where H0 is the hardness at infinite depth, h* is a characteristic length that depends on the materials and the shape of the indenter tip and h is the indentation depth.

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Fig. 7 (a) shows the indentation depth profiles of average nano-hardness of the irradiated and unirradiated UNS N10003 alloys characterized by Nano-indentation. Within top 50 nm indented range, the hardness is increasing, which is known as

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reverse indentation size effect (RISE) [32-33]. The hardness data at indentation depth where h < 125 nm is ignored due to RISE which is usually attributed to the testing

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artifacts. It can be seen that the hardness (H) decreases with indentation depth where h > 50nm. This behavior has been noticed as an indentation size effect (ISE). Note that there is an inflection at a critical indentation depth (hc) of about 500 nm for the sample irradiated at the dose of 6.18 dpa. It is due to the delayed onset of plasticity in the substrate (unirradiated region) under the irradiated layer, i.e. softer substrate effect. Figure 7(a) also shows that the hardness is increasing with increasing irradiation dose, although the hardness varies with indention depths. As Fig. 7 (b) shows, the hardness data were plotted as H2 versus 1/h to compare with the Nix-Gao model. For the irradiated samples, two clearly different slopes can 9

ACCEPTED MANUSCRIPT be seen at the critical depth hc. This is caused by the softer substrate effect. H0 can be derived from the least square fitting for 125 nm < h < hc, which reflects the irradiation hardening in the materials [33]. The H0 of the original sample is 3.00 GPa, which is consistent with previous studies [23, 31, 34]. As for the irradiated samples, the

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hardness values are 4.08 GPa and 5 GPa corresponding to the doses of 2.06 dpa and 6.18 dpa, respectively. That is, the hardness of UNS N10003 alloys is increasing with increasing irradiation dose. 50

8

unirradiated 2.06 dpa 6.18 dpa

hc

hc

40

6

H2(GPa2)

35

5

4

(b)

30 25 20

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Nanohardness(GPa)

7

45

Unirradiated Fitting Curve 2.06 dpa Fitting Curve 6.18 dpa Fitting Curve

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(a)

15

3

10

2 0

500

1000

1500

Indentation depth(nm)

2000

5 0.000

2500

0.002

0.004

0.006

0.008

-1

1/h(nm )

4. Discussion

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Fig.7 Hardening behavior of irradiated and unirradiated UNS N10003 alloys (a) indentation depth (h) profiles of nano-hardness (H); (b) plots of H2 vs. 1/h for the average nano-hardness.

Voids and bubbles are the two main factors inducing swelling. Void swelling

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induced by heavy ions is mainly due to large number of vacancy and interstitials (Frenkel pairs) produced by injected exotic ions. Nevertheless, He bubbles induced swelling is caused by He atoms produced by (n, α) reactions or directly implanted. He

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atoms can gather and form bubbles in materials, especially at high temperatures [35]. Fig.3 shows the temperature dependence of swelling in UNS N10003 alloy and

pure Ni [20-21]. In Ni ion irradiated pure Ni, void swelling is dominant in the temperature range of 425 ~ 625 oC. The lines represent the void swelling, while the red circle represents the swelling in our work. The comparison shows that the swelling in UNS N10003 alloy irradiated with 650 oC He ions at 6.18 dpa is much higher than the voids induced swelling in pure Ni at the peak swelling temperature (550 oC), although the pure Ni was irradiated at a higher dose up to 13 dpa. We know that void swelling is sensitive to the irradiation dose rate, temperature and composition of alloys 10

ACCEPTED MANUSCRIPT [20-21, 36-37]. The voids are difficult to survive above 0.5 Tm (Tm is the absolute melting temperature) due to the recovery of excess vacancies over the thermal equilibrium [38]. Therefore, when temperature is over 625 oC (above 0.5 Tm of Ni), the void swelling of pure Ni can be negligible at the range of 7× 10-4 - 1× 10-7 dpa/s. For

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UNS N10003 alloy, the irradiation temperature in this work is above 0.5 Tm, and the irradiation dose rate is 6.4× 10-5 dpa/s, between 7× 10-4 and 1× 10-7 dpa/s. On the other side, a regime of zero void swelling was observed at 625 oC over a region of compositional space embracing 7-20 wt% Cr and 45-75 wt% Ni in the early work of

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Johnston et al. [39], and the composition of UNS N10003 alloy is well within the range. Furthermore, at high temperatures (T > 0.5 Tm), He bubbles are easy to form [6, 30],

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which has also been confirmed by TEM observation. Therefore, the main reason for the swelling of UNS N10003 alloy at 650 oC can be ascribed to He bubbles, while void induced swelling can be neglected.

To understand the contribution of He bubbles to the hardening behavior of UNS N10003 alloy, the nano-hardness of the alloy irradiated by He ions is compared with

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  and the that of Xe ion irradiation, as listed in Table1. The average nano-hardness ( irradiation induced increment of nano-hardness (∆H) are calculated based on previously published data and this work [23-24, 31, 34]. The hardness of Xe ion

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irradiated UNS N10003 alloy increases rapidly at low doses (0- 1dpa). At higher doses (1- 10 dpa), it tends to saturate at 5.44 GPa, which is comparable with that of He ion

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irradiated UNS N10003 alloy at 6.18 dpa (5.00 GPa). Table 1. The average nano-hardness and irradiation induced increment of nano-hardness of UNS N10003 alloy under different conditions [23-24, 31, 34]

Material

Irradiation

Irradiation

Peak dose

 (GPa) 

∆H (%)

--

3.05±0.09

0

25

1

5.44±0.18

78.4

Xe

25

10

5.42±0.15

77.7

He

650

2.06

4.08±0.33

33.3

He

650

6.18

5.00±0.38

63.4

o

Ion species

Temperature ( C)

(dpa)

--

--

UNS

Xe

N10003 alloy

11

ACCEPTED MANUSCRIPT It is well known that irradiation induced defects, such as black dots, dislocation loops and He bubbles, will act as barriers to gliding dislocations in the slip plane, impede their movements, and result in irradiation hardening [27, 40]. An equation established by Orowan et al. can be applied to estimate the hardening induced by

∆ =

∙ " ∙ # ∙ $ ∙ √& ∙ '

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irradiation defects [27]: ,

(3)

where α is the obstacle strength, varied from 0 to 1, which strongly depends on the type of defects. M is the Taylor factor, µ is the shear modulus, b is the module of the Burgers

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vector, N and D are the defects number densities and their average sizes, respectively. For a given material, the parameters of M, µ and b are all constants. Hence, the type of

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defects is inversely proportional to the square root of N and D at a given hardness. The obstacle strength of He bubbles is compared with that of Xe ion irradiation defects. It should be noted that the size of He bubbles is small at 2.06 dpa, due to the low concerntration of He atoms, and many small He bubbles are not detectable by TEM. However, the undetectable small defects have profounding effect on hardening

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bahaviors of alloys [27]. The sample irradiated at 6.18 dpa was selected for comparation, since He bubbles can be observed in the whole irradiation damage region (1000 - 2250 nm from the surface). What is more, the consistent results between AFM

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and TEM observation indicate that He bubbles in the sample irradiated at 6.18 dpa have the maximum value statistical significance in the TEM observation.

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For the Xe ion irradiated UNS N10003 alloy, the square root of N and D of room-temperature irradiation defects tend to saturate above 1 dpa, which consists well with the irradiation hardening behavior of the alloy. At 10 dpa, most of the irradiation defects are dislocation loops. And the average N and D of the dislocation loops are 4.4 ×1022/m3 and 11 nm at saturated hardness (5.42 GPa), respectively [24, 34]. For the He ion irradiated alloy at 650 oC, the average N and D of He bubbles are 3.15 ×1023/m3 and 3.55 nm at 6.18 dpa (5.00 GPa), respectively. It can be calculated from equation (3) that the square root of N and D for the Xe ion induced irradiation defects is 1.5 times less than that of He bubbles. While the hardening induced by Xe ion irrdiation is only 1.08 12

ACCEPTED MANUSCRIPT times higher than that of He bubbles. Thus, the obstacle strength of He bubbles is about 0.72 times that of the saturated Xe ion induced irradiation defects in UNS N10003 alloy. The comparison leads to a better understanding of the contribution of He bubbles to the irradiation hardening behavior. It indicates that He bubble is also a strong barrier to

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impede dislocation glide, although slightly weaker than the saturated heavy ion irradiation induced defects.

5. Conclusion

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He bubbles were introduced in UNS N10003 alloy by 1.2 MeV He ion irradiation at 650 oC. He bubbles induced swelling and hardening were compared with those

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induced by heavy ion irradiation defects. The main conclusions are as follows: 1. Major He ion irradiation defects in UNS N10003 alloy at 650 oC are He bubbles. No obvious “black spots” or small dislocation loops were observed by TEM. Both the number densities and sizes of He bubbles increases with increasing irradiation dose.

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2. At 6.18 dpa, the irradiation swelling of UNS N10003 alloy is 2.67%, which is mainly ascribed to He bubbles. Void swelling is negligible at 650 oC in the alloy.

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3. The nano-hardness of He irradiated UNS N10003 alloy increases with increasing irradiation dose. The obstacle strength of He bubbles is less than

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that of the saturated Xe ion induced irradiation defects in UNS N10003 alloy.

Acknowledgements This work is supported by the strategic priority research program of

Thorium-based Molten-Salt Reactor (TMSR) (Grant No. XDA02040100), the National Science Foundation of China (Grant Nos. 11675246, 11505266), and the Natural Science Foundation of Shanghai, China (Grant No. 14ZR1448300).

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Highlights


He bubbles are the major irradiation defects in He ion irradiated UNS N10003 alloy at 650 oC. No “black spots” or small dislocation loops are observed




Irradiation dose up to 6.18 dpa causes a swelling of 2.67%, which is mainly

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ascribed to He bubbles.


The obstacle strength of He bubbles is less than that of heavy ion induced

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irradiation defects.