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hydrogen irradiations in reduced-activation martensitic steel
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TEM observation of dislocation loops induced by single and sequential helium/hydrogen irradiations in reduced-activation martensitic steel Zheng Yang a , Fengfeng Luo b , Zhongcheng Zheng a , Weiping Zhang a , Yanxia Yu c , Zhenyu Shen a , Liping Guo a,∗ , Jinping Suo d a Hubei Nuclear Solid Physics Key Laboratory, Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education and School of Physics and Technology, Wuhan University, Wuhan, Hubei, 430072, China b Institute of Applied Physics, Jiangxi Academy of Science, Nanchang, Jiangxi, 330029, China c Department of Physics, Guangdong University of Petrochemical Technology, Maoming, Guangdong, 525000, China d State Key Laboratory of Mould Technology, Institute of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei, 430074, China
a r t i c l e
i n f o
Article history: Received 30 October 2016 Received in revised form 11 May 2017 Accepted 11 May 2017 Available online xxx Keywords: Dislocation loops Synergistic effect Reduced-activation martensitic steel Transmission electron microscopy
a b s t r a c t Dislocation loops induced by single (He or H) ion beam and sequential (H/He or He/H) ion beam irradiation in reduced activation martensitic steels were investigated with transmission electron microscopy (TEM). Specimens were irradiated with 30 keV He+ and 18 keV H+ respectively under three different doses at 450 ◦ C. Dislocation loops with mean size of 41.3 nm were observed under single He ion beam irradiated to 0.1 dpa. Sequential ion irradiation could induce larger dislocation loops than the case of single ion irradiation. It was interesting that mean size of dislocation loops induced by He/H sequential ion irradiation was larger than that induced by H/He sequential ion irradiation at the same dose. Possible explanations were presented and synergistic effects of He and H on dislocation loops were discussed. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Reduced-activation ferritic/martensitic (RAFM) steels are the prime candidate structural materials for fusion reactors because of their low activation, better mechanical properties, microstructural stability and swelling resistance compared with austenitic steels [1–3]. In fusion conditions, high energy neutron irradiation will generate a continuous production of He and H in structural materials by (n, ␣) and (n, p) nuclear transmutation reactions. Both He and H are known to have detrimental effects on the structural materials, such as increasing in ductile-to-brittle transition temperature (DBTT), hardening and embrittlement [4–8]. Moreover, computer simulations and experiments evidences have shown that synergistic effects should occur when both He and H are present, though either He or H alone may be detrimental [9–12]. However, most studies concerning the synergistic effect on microstructure in RAFMs were focused on swelling [11,12], whilst few were reported to investigate the dislocation structures induced by energetic He/H particles. In our previous work, the synergistic effect on dislocation
∗ Corresponding author. E-mail address: [email protected] (L. Guo).
loops under very high concentration (∼5000 appm) of He irradiation had been investigated [13]. However, the concentration of He ion was much lower than 5000 appm in the actual reactor. Thus the synergistic effect on dislocation loops under low dose is more important and is our primary concern. The aim of the present work is to study the synergistic effect on dislocation loops in super-clean reduced-activation martensitic steels under single (He or H) ion and sequential (He/H or H/He) ion beam irradiation. He and H were compared with one another to get the roles of He and H atoms on the formation of dislocation loops. It is interesting to find that dislocation loops induced by sequential ion irradiation were larger than that induced by single ion irradiation, and the mean size of dislocation loops induced by He/H sequential ion irradiation were much larger than the case of H/He sequential ion irradiation under the same dose. The physical mechanism of synergistic effect on dislocation loops is discussed.
2. Experimental The used specimens are super-clean reduced-activation martensitic (SCRAM) steel supplied by Huazhong University of Science and Technology which has a composition of Fe-9.24%Cr, 2.29%W, 0.49%Mn, 0.25%V, 0.25%Si, 0.088%C and 0.0059%P in wt.%
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Please cite this article in press as: Z. Yang, et al., TEM observation of dislocation loops induced by single and sequential helium/hydrogen irradiations in reduced-activation martensitic steel, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.05.057
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Table 1 A summary of dislocation loops in the specimens under different irradiation conditions. Fluence (ions/cm2 )
Low-dose
Mid-dose
High-dose
Loop density (1020 /m3 )
Average size of loops (nm)
He 8.0 × 1013 H 2.8 × 1014 He 8.0 × 1014 H 2.8 × 1015 He 1.7 × 1015 H 6.0 × 1015
Single H
Single He
Sequential H/He
Sequential He/H
Single H
Single He
Sequential H/He
Sequential He/H
–
–
–
4.7 ± 0.3
–
–
–
28.1
–
–
21.5 ± 0.9
34.6 ± 1.3
–
–
45.2
20.0
–
41.3 ± 2.7
45.8 ± 0.8
47.6 ± 0.7
–
9.3
13.2
24.4
“-” refers to no visible dislocation loops.
[14]. The SCRAM steel was first quenched at 980 ◦ C for 0.5 h and tempering at 760 ◦ C for 2 h; then quenching at 960 ◦ C for 0.5 h and tempered at 740 ◦ C for 2 h. Specimens had a final fully tempered martensite lath structure after these treatments. Plates were first cut into 0.5 mm thick sheets and then mechanically polished down to ∼0.1 mm. 3 mm standard TEM disk specimens were punched and milled to a thickness of 40 ∼ 50 m with silicon carbide papers with grades 800–2500. Finally, TEM specimens were polished with solution of 5% perchloric acid and 95% ethanol in vol.% using a MTPA-5 twin-jet electro-polishing machine (produced by Shanghai Jiaotong University, China). The specimens were irradiated with 30 keV He+ and 18 keV H+ at 450 ◦ C on an ion implanter in the Accelerator Laboratory of Wuhan University. Detailed irradiation conditions were listed in Table 1. Three different doses had been signed as Low-dose, Middose and High-dose in the paper for easy identification. In the case of sequential irradiation H/He or He/H, the interval between two irradiations was about 15 min. The damage depths of 30 keV He+ and 18 keV H+ in the SCRAM steel were both around 100 nm from the calculation of SRIM2008 using a displacement energy of 40 eV as recommended in ASTME521-89 [15]. For the specimens under He+ irradiation with the fluence of 8.0 × 1013 He+ /cm2 , 8.0 × 1014 He+ /cm2 and 1.7 × 1015 He+ /cm2 , the corresponding damage dose was about 0.005 dpa, 0.05 dpa and 0.1 dpa, the irradiation time was about 90 s, 900 s and 1800 s, respectively. For the specimens
under H+ irradiation with 2.8 × 1014 H+ /cm2 , 2.8 × 1015 H+ /cm2 and 6.0 × 1015 H+ /cm2 , the corresponding damage dose was about 0.0014 dpa, 0.014 dpa and 0.03 dpa, the irradiation time was about 180 s, 1800 s and 3600 s, respectively. During ion implantation, the specimens were silver-glued to a thin SCRAM steel plate of the same composition, which was then mounted onto the heating holder. Temperature of the ion-facing surface of the SCRAM steel plate was tested with a thermocouple and was maintained at 450 ◦ C throughout the implantation. The investigation of the microstructure of the specimens was performed with a JEM-2010HT TEM operated at 200 kV, below the threshold energy for knock-on damage in Fe. Foil thickness at the observed area was fixed at ∼100 nm, which was estimated by counting the number of thickness fringes from the edge of the thin foil.
3. Results and discussion 3.1. Single (He or H) ion irradiation The microstructure of unirradiated steel was shown in Fig. 1. The steel exhibited a typical martensitic lath structure with a variety of M23 C6 precipitates, which distributed mainly along martensite lath boundaries. Line dislocation pileups were observed with the BF imaging taken along [111] direction as shown in Fig. 1(b). The den-
Fig. 1. Typical BF micrographs of unirradiated specimen of SCRAM steel (a) martensite lath and (b) line dislocation.
Please cite this article in press as: Z. Yang, et al., TEM observation of dislocation loops induced by single and sequential helium/hydrogen irradiations in reduced-activation martensitic steel, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.05.057
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Fig. 2. BF micrographs of specimens under single He ion irradiation with a fluence of (a) 8.0 × 1013 He+ /cm2 , (b) 8.0 × 1014 He+ /cm2 and (c) 1.7 × 1015 He+ /cm2 using g = 110 near pole [001].
Fig. 3. BF micrographs of specimens under single H ion irradiation with a fluence of (a) 2.8 × 1014 H+ /cm2 , (b) 2.8 × 1015 H+ /cm2 and (c) 6.0 × 1015 H+ /cm2 using g = 110 near pole [001].
Fig. 4. BF micrographs of specimens under H/He sequential ion irradiation with (a) Low-dose, (b) Mid-dose and (c) High-dose using g = 110 near pole [001].
sity of dislocations was measured to be ∼1021 /m3 . No dislocation loop could be found in unirradiated specimens. Fig. 2(a–c) showed the BF micrograph of specimens under single He ion irradiation with a fluence up to 1.7 × 1015 He+ /cm2 , almost no dislocation loops could be found after single He ion irradiation to a fluence below 8.0 × 1014 He+ /cm2 , possibly due to TEM resolution limit. However, when the He irradiation fluence increased to 1.7 × 1015 He+ /cm2 , dislocation loops were observed using g = 110 in a foil orientation near [001]. The mean size and number density of dislocation loops under different irradiation conditions were summarized in Table 1. For single He ion irradiation to 1.7 × 1015 He+ /cm2 , the mean size and number density of these loops were 41.3 nm and 9.3 × 1020 /m3 , respectively.
Microstructure of specimens under single H ion irradiation using g = 110 were shown in Fig. 3, no dislocation loop could be observed up to the fluence of 6.0 × 1015 H+ /cm2 . 3.2. H/He sequential ion irradiation Fig. 4 showed the microstructure of specimens under H/He sequential ion irradiation. No dislocation loops could be observed when irradiated at low-dose (e.g. 2.8 × 1014 H+ /cm2 and 8.0 × 1013 He+ /cm2 ). Small dislocation loops were observed when irradiated to the fluence of 2.8 × 1015 H+ /cm2 and 8.0 × 1014 He+ /cm2 as shown in Fig. 4(b). The mean size and number density of these loops were
Please cite this article in press as: Z. Yang, et al., TEM observation of dislocation loops induced by single and sequential helium/hydrogen irradiations in reduced-activation martensitic steel, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.05.057
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Fig. 5. BF micrograph of specimens under He/H sequential ion irradiation with (a) Low-dose, (b) Mid-dose and (c) High-dose using g = 110 near pole [001].
21.5 nm and 4.5 × 1021 /m3 , respectively. Compared to the result that no dislocation loop could be observed in the single (H or He) ion irradiated specimen for mid-dose, the appearance of dislocation loops (i.e. 21.5 nm) in the H/He sequential ion irradiation revealed that synergistic effect did exist for the dislocation loops. When the irradiation dose increased to high-dose (e.g. 6.0 × 1015 H+ /cm2 and 1.7 × 1015 He+ /cm2 ), larger dislocation loops could be found as shown in Fig. 4(c). The mean size of dislocation loops was 45.8 nm, which was larger than the lower dose in the H/He sequential irradiated specimens; and the number density of these loops was 1.3 × 1021 /m3 , which was smaller than the lower dose in the H/He sequential irradiated specimens. Compared with the case of single (He or H) ion irradiation at the same dose, the mean size and number density of dislocation loops was larger. 3.3. He/H sequential ion irradiation Microstructure of specimens under He/H sequential ion irradiation were shown in Fig. 5. When irradiated at low-dose (i.e. 8.0 × 1013 He+ /cm2 and 2.8 × 1014 H+ /cm2 ), some black dots were found as shown in Fig. 5(a). The mean size and number density of these black dots were 4.7 nm and 2.8 × 1021 /m3 , respectively. With the increasing fluence to 8.0 × 1014 He+ /cm2 and 2.8 × 1015 H+ /cm2 , dislocation loops were observed as shown in Fig. 5(b). The mean size and number density of these loops were 34.6 nm and 2.0 × 1021 /m3 , respectively. The mean size was significantly larger and the density was a little smaller than the case of H/He sequential on irradiation at the same dose. When the irradiation dose increased to high-dose (i.e. 1.7 × 1015 He+ /cm2 and 6.0 × 1015 H+ /cm2 ), large dislocation loops were found as shown in Fig. 5(c). The mean size and number density of these loops increased to 47.6 nm and 2.4 × 1021 /m3 , respectively. Compared with the single (He or H) ion and H/He sequential irradiation at the same dose, the size of these loops was much larger and the density was much smaller. No obvious voids or cavities could be observed in our specimens even at the High-dose condition. 3.4. Discussion Fig. 6(a) summarizes the mean size of dislocation loops in the steels under different irradiations conditions. Large dislocation loops were observed under single He ion irradiation to high-dose (0.1 dpa). Compared with single (He or H) ion irradiation, the mean size of dislocation loops induced by sequential ion irradiation were visibly larger. The differences were most profound among the middose specimens. And the mean size of dislocation loops induced by He/H sequential ion irradiation were the largest at the same dose. The size distribution of dislocation loops in the specimens under
Fig. 6. (a) The mean size of dislocation loops under different irradiation conditions; (b) The size distribution of dislocation loops in the specimens under single and sequential ion irradiations at high dose.
single and sequential ion irradiations at high dose was shown in Fig. 6(b). The overall loop density of the He/H irradiated specimen was larger than that in H/He specimen, which was again larger than in the single He irradiated specimen. The size of dislocation loops induced by sequential H/He or He/H irradiation was larger than the simple sum of the size of those loops induced by single He and H, indicating that the synergistic effect on dislocation loops was obvious. Large dislocation loops with mean size of 41.3 nm were observed under single He ion irradiation to 0.1 dpa at 450 ◦ C. In our previous work, large interstitial (I) loops induced by He ion irradiation were found and these loops were larger than that induced by other types of irradiations (e.g. proton, heavy ions and neutron irradiation) [16]. He atoms could knock away Fe atoms from the equilibrium positions to form self-interstitial atoms (SIAs) and then were trapped
Please cite this article in press as: Z. Yang, et al., TEM observation of dislocation loops induced by single and sequential helium/hydrogen irradiations in reduced-activation martensitic steel, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.05.057
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by resulting vacancies to form He-vacancy (He-V) clusters. More He atoms and vacancies would join the He-V clusters, more freely migrating SIAs could be created since the SIAs could be easily emitted away from the positions surrounding the He-V cluster [17], leading to the formation of interstitial type loops (I-loops), which is called as the loop punching mechanism [18]. Furthermore, it had been reported that I-loop formation could be enhanced by He atoms in bcc steels, since He-V clusters could act as nucleation sites for Iloops [19–21]. Birmbal found that the density of dislocation loops increased when helium presented and concluded that helium could enhance the nucleation of dislocation loops [22]. The reason might be that the binding of migrating vacancies at He-V clusters not only decreases the chance of re-combination with SIAs but also enhances the chance of agglomerate of SIAs to nucleation of I-loops. In contrast, for other types of irradiations, the emitted SIAs would easily recombine with the vacancies thus getting annihilated. In the H/He sequential ion irradiated specimens, the mean sizes and number densities of dislocation loops were larger than the case of single (He or H) ion irradiated specimens at both mid- and high-dose, showing that strong evidence of synergistic effect. Hvacancy (H-V) clusters could be formed by the pre-implanted H, however, H-V clusters were not stable and decomposed at above 350 ◦ C [23]. Subsequent implanted He ions could be trapped by the vacancies decomposed from H-V clusters and then form He-V clusters. Compared with single ion irradiation, more He-V and H-V clusters existed in the specimens that could act as nucleation sites for I-loops [20], leading to more and larger SIAs or dislocation loops. It was worth nothing that mean size of dislocation loops in the case of He/H sequential ion irradiation were larger than the case of H/He sequential ion irradiation at the same dose. A possible mechanism may be as follows: He-V clusters and SIAs were formed in advance by the pre-implanted He. It was reported that H was more stably trapped in the He-associated trapped sites [24] though the binding energy between H and defects was small. Therefore, more H atoms could be trapped by He-V clusters to form He-Hvacancy (He-H-V) complexes during sequential H ion irradiation, compared to when H was implanted before He, which was predicted by positron-annihilation spectroscopy [25]. Large He-H-V complexes could trap SIAs around them and act as nucleation sites for I-loops through loop punching mechanism. Thus, SIAs or I-loops became larger by absorbing the excess ejected interstitials. 4. Conclusion Dislocation loops induced by single (He or H) ion and sequential H/He or He/H ion irradiation at 450 ◦ C were investigated via TEM. Large dislocation loops were observed under single He ion
5
irradiation to 1.7 × 1015 He+ /cm2 (0.1 dpa). Compared with single He or H ion irradiation, the mean size of dislocation loops induced by sequential ion irradiation were much larger. And the dislocation loops induced by He/H sequential ion irradiation were the largest at the same dose. Acknowledgements The financial supports from the International Science & Technology Cooperation Program of China (with Grant No. 2015DFR60370) and the National Natural Science Foundation of China (with Grant Nos. 11275140 and U1532134) are gratefully acknowledged. The authors would also like to express our gratitude to the help from the staff of the Center for Electron Microscopy in Wuhan University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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Please cite this article in press as: Z. Yang, et al., TEM observation of dislocation loops induced by single and sequential helium/hydrogen irradiations in reduced-activation martensitic steel, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.05.057
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FUSION-9541; No. of Pages 5
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Contents lists available at ScienceDirect
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TEM observation of dislocation loops induced by single and sequential helium/hydrogen irradiations in reduced-activation martensitic steel Zheng Yang a , Fengfeng Luo b , Zhongcheng Zheng a , Weiping Zhang a , Yanxia Yu c , Zhenyu Shen a , Liping Guo a,∗ , Jinping Suo d a Hubei Nuclear Solid Physics Key Laboratory, Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education and School of Physics and Technology, Wuhan University, Wuhan, Hubei, 430072, China b Institute of Applied Physics, Jiangxi Academy of Science, Nanchang, Jiangxi, 330029, China c Department of Physics, Guangdong University of Petrochemical Technology, Maoming, Guangdong, 525000, China d State Key Laboratory of Mould Technology, Institute of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei, 430074, China
a r t i c l e
i n f o
Article history: Received 30 October 2016 Received in revised form 11 May 2017 Accepted 11 May 2017 Available online xxx Keywords: Dislocation loops Synergistic effect Reduced-activation martensitic steel Transmission electron microscopy
a b s t r a c t Dislocation loops induced by single (He or H) ion beam and sequential (H/He or He/H) ion beam irradiation in reduced activation martensitic steels were investigated with transmission electron microscopy (TEM). Specimens were irradiated with 30 keV He+ and 18 keV H+ respectively under three different doses at 450 ◦ C. Dislocation loops with mean size of 41.3 nm were observed under single He ion beam irradiated to 0.1 dpa. Sequential ion irradiation could induce larger dislocation loops than the case of single ion irradiation. It was interesting that mean size of dislocation loops induced by He/H sequential ion irradiation was larger than that induced by H/He sequential ion irradiation at the same dose. Possible explanations were presented and synergistic effects of He and H on dislocation loops were discussed. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Reduced-activation ferritic/martensitic (RAFM) steels are the prime candidate structural materials for fusion reactors because of their low activation, better mechanical properties, microstructural stability and swelling resistance compared with austenitic steels [1–3]. In fusion conditions, high energy neutron irradiation will generate a continuous production of He and H in structural materials by (n, ␣) and (n, p) nuclear transmutation reactions. Both He and H are known to have detrimental effects on the structural materials, such as increasing in ductile-to-brittle transition temperature (DBTT), hardening and embrittlement [4–8]. Moreover, computer simulations and experiments evidences have shown that synergistic effects should occur when both He and H are present, though either He or H alone may be detrimental [9–12]. However, most studies concerning the synergistic effect on microstructure in RAFMs were focused on swelling [11,12], whilst few were reported to investigate the dislocation structures induced by energetic He/H particles. In our previous work, the synergistic effect on dislocation
∗ Corresponding author. E-mail address: [email protected] (L. Guo).
loops under very high concentration (∼5000 appm) of He irradiation had been investigated [13]. However, the concentration of He ion was much lower than 5000 appm in the actual reactor. Thus the synergistic effect on dislocation loops under low dose is more important and is our primary concern. The aim of the present work is to study the synergistic effect on dislocation loops in super-clean reduced-activation martensitic steels under single (He or H) ion and sequential (He/H or H/He) ion beam irradiation. He and H were compared with one another to get the roles of He and H atoms on the formation of dislocation loops. It is interesting to find that dislocation loops induced by sequential ion irradiation were larger than that induced by single ion irradiation, and the mean size of dislocation loops induced by He/H sequential ion irradiation were much larger than the case of H/He sequential ion irradiation under the same dose. The physical mechanism of synergistic effect on dislocation loops is discussed.
2. Experimental The used specimens are super-clean reduced-activation martensitic (SCRAM) steel supplied by Huazhong University of Science and Technology which has a composition of Fe-9.24%Cr, 2.29%W, 0.49%Mn, 0.25%V, 0.25%Si, 0.088%C and 0.0059%P in wt.%
http://dx.doi.org/10.1016/j.fusengdes.2017.05.057 0920-3796/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: Z. Yang, et al., TEM observation of dislocation loops induced by single and sequential helium/hydrogen irradiations in reduced-activation martensitic steel, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.05.057
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Table 1 A summary of dislocation loops in the specimens under different irradiation conditions. Fluence (ions/cm2 )
Low-dose
Mid-dose
High-dose
Loop density (1020 /m3 )
Average size of loops (nm)
He 8.0 × 1013 H 2.8 × 1014 He 8.0 × 1014 H 2.8 × 1015 He 1.7 × 1015 H 6.0 × 1015
Single H
Single He
Sequential H/He
Sequential He/H
Single H
Single He
Sequential H/He
Sequential He/H
–
–
–
4.7 ± 0.3
–
–
–
28.1
–
–
21.5 ± 0.9
34.6 ± 1.3
–
–
45.2
20.0
–
41.3 ± 2.7
45.8 ± 0.8
47.6 ± 0.7
–
9.3
13.2
24.4
“-” refers to no visible dislocation loops.
[14]. The SCRAM steel was first quenched at 980 ◦ C for 0.5 h and tempering at 760 ◦ C for 2 h; then quenching at 960 ◦ C for 0.5 h and tempered at 740 ◦ C for 2 h. Specimens had a final fully tempered martensite lath structure after these treatments. Plates were first cut into 0.5 mm thick sheets and then mechanically polished down to ∼0.1 mm. 3 mm standard TEM disk specimens were punched and milled to a thickness of 40 ∼ 50 m with silicon carbide papers with grades 800–2500. Finally, TEM specimens were polished with solution of 5% perchloric acid and 95% ethanol in vol.% using a MTPA-5 twin-jet electro-polishing machine (produced by Shanghai Jiaotong University, China). The specimens were irradiated with 30 keV He+ and 18 keV H+ at 450 ◦ C on an ion implanter in the Accelerator Laboratory of Wuhan University. Detailed irradiation conditions were listed in Table 1. Three different doses had been signed as Low-dose, Middose and High-dose in the paper for easy identification. In the case of sequential irradiation H/He or He/H, the interval between two irradiations was about 15 min. The damage depths of 30 keV He+ and 18 keV H+ in the SCRAM steel were both around 100 nm from the calculation of SRIM2008 using a displacement energy of 40 eV as recommended in ASTME521-89 [15]. For the specimens under He+ irradiation with the fluence of 8.0 × 1013 He+ /cm2 , 8.0 × 1014 He+ /cm2 and 1.7 × 1015 He+ /cm2 , the corresponding damage dose was about 0.005 dpa, 0.05 dpa and 0.1 dpa, the irradiation time was about 90 s, 900 s and 1800 s, respectively. For the specimens
under H+ irradiation with 2.8 × 1014 H+ /cm2 , 2.8 × 1015 H+ /cm2 and 6.0 × 1015 H+ /cm2 , the corresponding damage dose was about 0.0014 dpa, 0.014 dpa and 0.03 dpa, the irradiation time was about 180 s, 1800 s and 3600 s, respectively. During ion implantation, the specimens were silver-glued to a thin SCRAM steel plate of the same composition, which was then mounted onto the heating holder. Temperature of the ion-facing surface of the SCRAM steel plate was tested with a thermocouple and was maintained at 450 ◦ C throughout the implantation. The investigation of the microstructure of the specimens was performed with a JEM-2010HT TEM operated at 200 kV, below the threshold energy for knock-on damage in Fe. Foil thickness at the observed area was fixed at ∼100 nm, which was estimated by counting the number of thickness fringes from the edge of the thin foil.
3. Results and discussion 3.1. Single (He or H) ion irradiation The microstructure of unirradiated steel was shown in Fig. 1. The steel exhibited a typical martensitic lath structure with a variety of M23 C6 precipitates, which distributed mainly along martensite lath boundaries. Line dislocation pileups were observed with the BF imaging taken along [111] direction as shown in Fig. 1(b). The den-
Fig. 1. Typical BF micrographs of unirradiated specimen of SCRAM steel (a) martensite lath and (b) line dislocation.
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Fig. 2. BF micrographs of specimens under single He ion irradiation with a fluence of (a) 8.0 × 1013 He+ /cm2 , (b) 8.0 × 1014 He+ /cm2 and (c) 1.7 × 1015 He+ /cm2 using g = 110 near pole [001].
Fig. 3. BF micrographs of specimens under single H ion irradiation with a fluence of (a) 2.8 × 1014 H+ /cm2 , (b) 2.8 × 1015 H+ /cm2 and (c) 6.0 × 1015 H+ /cm2 using g = 110 near pole [001].
Fig. 4. BF micrographs of specimens under H/He sequential ion irradiation with (a) Low-dose, (b) Mid-dose and (c) High-dose using g = 110 near pole [001].
sity of dislocations was measured to be ∼1021 /m3 . No dislocation loop could be found in unirradiated specimens. Fig. 2(a–c) showed the BF micrograph of specimens under single He ion irradiation with a fluence up to 1.7 × 1015 He+ /cm2 , almost no dislocation loops could be found after single He ion irradiation to a fluence below 8.0 × 1014 He+ /cm2 , possibly due to TEM resolution limit. However, when the He irradiation fluence increased to 1.7 × 1015 He+ /cm2 , dislocation loops were observed using g = 110 in a foil orientation near [001]. The mean size and number density of dislocation loops under different irradiation conditions were summarized in Table 1. For single He ion irradiation to 1.7 × 1015 He+ /cm2 , the mean size and number density of these loops were 41.3 nm and 9.3 × 1020 /m3 , respectively.
Microstructure of specimens under single H ion irradiation using g = 110 were shown in Fig. 3, no dislocation loop could be observed up to the fluence of 6.0 × 1015 H+ /cm2 . 3.2. H/He sequential ion irradiation Fig. 4 showed the microstructure of specimens under H/He sequential ion irradiation. No dislocation loops could be observed when irradiated at low-dose (e.g. 2.8 × 1014 H+ /cm2 and 8.0 × 1013 He+ /cm2 ). Small dislocation loops were observed when irradiated to the fluence of 2.8 × 1015 H+ /cm2 and 8.0 × 1014 He+ /cm2 as shown in Fig. 4(b). The mean size and number density of these loops were
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Fig. 5. BF micrograph of specimens under He/H sequential ion irradiation with (a) Low-dose, (b) Mid-dose and (c) High-dose using g = 110 near pole [001].
21.5 nm and 4.5 × 1021 /m3 , respectively. Compared to the result that no dislocation loop could be observed in the single (H or He) ion irradiated specimen for mid-dose, the appearance of dislocation loops (i.e. 21.5 nm) in the H/He sequential ion irradiation revealed that synergistic effect did exist for the dislocation loops. When the irradiation dose increased to high-dose (e.g. 6.0 × 1015 H+ /cm2 and 1.7 × 1015 He+ /cm2 ), larger dislocation loops could be found as shown in Fig. 4(c). The mean size of dislocation loops was 45.8 nm, which was larger than the lower dose in the H/He sequential irradiated specimens; and the number density of these loops was 1.3 × 1021 /m3 , which was smaller than the lower dose in the H/He sequential irradiated specimens. Compared with the case of single (He or H) ion irradiation at the same dose, the mean size and number density of dislocation loops was larger. 3.3. He/H sequential ion irradiation Microstructure of specimens under He/H sequential ion irradiation were shown in Fig. 5. When irradiated at low-dose (i.e. 8.0 × 1013 He+ /cm2 and 2.8 × 1014 H+ /cm2 ), some black dots were found as shown in Fig. 5(a). The mean size and number density of these black dots were 4.7 nm and 2.8 × 1021 /m3 , respectively. With the increasing fluence to 8.0 × 1014 He+ /cm2 and 2.8 × 1015 H+ /cm2 , dislocation loops were observed as shown in Fig. 5(b). The mean size and number density of these loops were 34.6 nm and 2.0 × 1021 /m3 , respectively. The mean size was significantly larger and the density was a little smaller than the case of H/He sequential on irradiation at the same dose. When the irradiation dose increased to high-dose (i.e. 1.7 × 1015 He+ /cm2 and 6.0 × 1015 H+ /cm2 ), large dislocation loops were found as shown in Fig. 5(c). The mean size and number density of these loops increased to 47.6 nm and 2.4 × 1021 /m3 , respectively. Compared with the single (He or H) ion and H/He sequential irradiation at the same dose, the size of these loops was much larger and the density was much smaller. No obvious voids or cavities could be observed in our specimens even at the High-dose condition. 3.4. Discussion Fig. 6(a) summarizes the mean size of dislocation loops in the steels under different irradiations conditions. Large dislocation loops were observed under single He ion irradiation to high-dose (0.1 dpa). Compared with single (He or H) ion irradiation, the mean size of dislocation loops induced by sequential ion irradiation were visibly larger. The differences were most profound among the middose specimens. And the mean size of dislocation loops induced by He/H sequential ion irradiation were the largest at the same dose. The size distribution of dislocation loops in the specimens under
Fig. 6. (a) The mean size of dislocation loops under different irradiation conditions; (b) The size distribution of dislocation loops in the specimens under single and sequential ion irradiations at high dose.
single and sequential ion irradiations at high dose was shown in Fig. 6(b). The overall loop density of the He/H irradiated specimen was larger than that in H/He specimen, which was again larger than in the single He irradiated specimen. The size of dislocation loops induced by sequential H/He or He/H irradiation was larger than the simple sum of the size of those loops induced by single He and H, indicating that the synergistic effect on dislocation loops was obvious. Large dislocation loops with mean size of 41.3 nm were observed under single He ion irradiation to 0.1 dpa at 450 ◦ C. In our previous work, large interstitial (I) loops induced by He ion irradiation were found and these loops were larger than that induced by other types of irradiations (e.g. proton, heavy ions and neutron irradiation) [16]. He atoms could knock away Fe atoms from the equilibrium positions to form self-interstitial atoms (SIAs) and then were trapped
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by resulting vacancies to form He-vacancy (He-V) clusters. More He atoms and vacancies would join the He-V clusters, more freely migrating SIAs could be created since the SIAs could be easily emitted away from the positions surrounding the He-V cluster [17], leading to the formation of interstitial type loops (I-loops), which is called as the loop punching mechanism [18]. Furthermore, it had been reported that I-loop formation could be enhanced by He atoms in bcc steels, since He-V clusters could act as nucleation sites for Iloops [19–21]. Birmbal found that the density of dislocation loops increased when helium presented and concluded that helium could enhance the nucleation of dislocation loops [22]. The reason might be that the binding of migrating vacancies at He-V clusters not only decreases the chance of re-combination with SIAs but also enhances the chance of agglomerate of SIAs to nucleation of I-loops. In contrast, for other types of irradiations, the emitted SIAs would easily recombine with the vacancies thus getting annihilated. In the H/He sequential ion irradiated specimens, the mean sizes and number densities of dislocation loops were larger than the case of single (He or H) ion irradiated specimens at both mid- and high-dose, showing that strong evidence of synergistic effect. Hvacancy (H-V) clusters could be formed by the pre-implanted H, however, H-V clusters were not stable and decomposed at above 350 ◦ C [23]. Subsequent implanted He ions could be trapped by the vacancies decomposed from H-V clusters and then form He-V clusters. Compared with single ion irradiation, more He-V and H-V clusters existed in the specimens that could act as nucleation sites for I-loops [20], leading to more and larger SIAs or dislocation loops. It was worth nothing that mean size of dislocation loops in the case of He/H sequential ion irradiation were larger than the case of H/He sequential ion irradiation at the same dose. A possible mechanism may be as follows: He-V clusters and SIAs were formed in advance by the pre-implanted He. It was reported that H was more stably trapped in the He-associated trapped sites [24] though the binding energy between H and defects was small. Therefore, more H atoms could be trapped by He-V clusters to form He-Hvacancy (He-H-V) complexes during sequential H ion irradiation, compared to when H was implanted before He, which was predicted by positron-annihilation spectroscopy [25]. Large He-H-V complexes could trap SIAs around them and act as nucleation sites for I-loops through loop punching mechanism. Thus, SIAs or I-loops became larger by absorbing the excess ejected interstitials. 4. Conclusion Dislocation loops induced by single (He or H) ion and sequential H/He or He/H ion irradiation at 450 ◦ C were investigated via TEM. Large dislocation loops were observed under single He ion
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irradiation to 1.7 × 1015 He+ /cm2 (0.1 dpa). Compared with single He or H ion irradiation, the mean size of dislocation loops induced by sequential ion irradiation were much larger. And the dislocation loops induced by He/H sequential ion irradiation were the largest at the same dose. Acknowledgements The financial supports from the International Science & Technology Cooperation Program of China (with Grant No. 2015DFR60370) and the National Natural Science Foundation of China (with Grant Nos. 11275140 and U1532134) are gratefully acknowledged. The authors would also like to express our gratitude to the help from the staff of the Center for Electron Microscopy in Wuhan University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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Please cite this article in press as: Z. Yang, et al., TEM observation of dislocation loops induced by single and sequential helium/hydrogen irradiations in reduced-activation martensitic steel, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.05.057