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Microstructure change and swelling of helium irradiated beryllium
Fusion Engineering and Design 140 (2019) 62–66
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Microstructure change and swelling of helium irradiated beryllium a,⁎
a
b
b
a
a,⁎
P.P. Liu , L.W. Xue , L.P. Yu , J.L. Liu , W. Hu , Q. Zhan , F.R. Wan a b
a,⁎
T
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Beijing Institute of Aerospace Control Devices, Beijing 100854, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Beryllium Irradiation damage Dislocation loop type Helium bubble Swelling
Beryllium, as an outstanding nuclear metal, will be exposed to high-dose irradiation of high-energy neutrons during services in reactor, which will produce a large number of helium and significant irradiation damage resulting in extreme performance degradation. In this paper, microstructure change and swelling of helium irradiated beryllium have been investigated using transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). Inside the grain, some impurity particles were observed and be revealed to be Fe/Al/Mn/Cr-rich beryllide phase with fcc structure. Beryllium metal was irradiated with 30 keV helium ion to a dose of 1 × 1017 He+/cm2 at room temperature. A high density of dislocation loop was induced in the sample after the irradiation. The types of loops in beryllium were summarized. Meanwhile, helium bubbles with average size of 2.7 nm were observed. Helium irradiation induced swelling and temperature effect were discussed.
1. Introduction
as the reaction shows [7–9]. Neutron radiation will produce severe irradiation damage, such as vacancies, dislocation loops, bubbles, etc. Characterization of dislocation loops and/or bubbles allows theoretically predict the dynamics of microstructure development, calculate materials swelling and predict performance degradation [10]. Transmission electron microscopy (TEM) has proved to be a powerful method for investigating such defects. The combination of conventional imaging with energy dispersive X-ray spectrum (EDS) on the nanoscale allows a detailed material characterization. Klimenkov et al. have studied gas bubble morphology and evolution by TEM in beryllium with helium content up to 3000 appm [11]. Chakin et al. reported the evolution of beryllium microstructure including grain size and damage structure under highdose neutron irradiation by using an optical microscope and electron microscope [12]. However, it will produce an extremely high helium content in beryllium, which may be ten times larger than that in structural steel and will greatly affect the performance properties and macrostructure of beryllium [12]. Precipitates and irradiation induced dislocation loops and a large number of bubbles will affect the properties and irradiation behavior of beryllium, that need to be analyzed in detail. Nanoscale microstructure analysis before and after irradiation plays a key role in understanding the changes of mechanical and physical properties [13]. In the present work, pure beryllium metal was chosen to study helium and irradiation damage effect. It was irradiated by 30 keV helium
Beryllium is a newcomer to the ever-increasing list of industrially important metals. Because of the combination of high flexural rigidity, thermal stability, thermal conductivity and low density, beryllium metal services in aircraft components, missiles, spacecraft, and satellites gyroscope, scan mirrors, sports equipment, and electronics industries as a structural and functional material [1–4]. Meanwhile, with many outstanding nuclear properties, like low atomic number, low neutron-capture cross section and a high neutron scattering cross section, beryllium is also used as a neutron moderator in fission reactor [5,6]. After used successfully in several tokamak as plasma facing material (FM), beryllium has been selected as the first wall of ITER nuclear fusion reactor [7]. The beryllium will undergo a (n, 2n) neutron reaction with neutron energy over 1.9 MeV, to produce two alpha particles. Thus, for high-energy fusion neutron, beryllium is a neutron multiplier, releasing more neutrons than it absorbs. This nuclear reaction is: 9 4 Be
+ n→ 224 He + 2n (E > 1.9 MeV)
Therefore, beryllium will be used as a neutron multiplier material (NM) in the helium-cooled ceramic breeder (HCCB) test blanket module (TBM). During service as FM and NM in reactor, the beryllium metal will be exposed to high-dose irradiation of high-energy neutrons, resulting in significant irradiation damage and a large number of helium
⁎
Corresponding authors. E-mail addresses: [email protected] (P.P. Liu), [email protected] (Q. Zhan), [email protected] (F.R. Wan).
https://doi.org/10.1016/j.fusengdes.2019.01.126 Received 14 December 2018; Received in revised form 25 January 2019; Accepted 26 January 2019 0920-3796/ © 2019 Published by Elsevier B.V.
Fusion Engineering and Design 140 (2019) 62–66
P.P. Liu et al.
Table 1 Chemical composition of beryllium sample (wt.%). BeO
C
Ti
Fe
Al
Mg
Si
Mn
Be
˜0.2
0.08
< 0.01
0.33
0.038
< 0.005
0.12
0.021
Balance
at room temperature. TEM and scanning transmission electron microscopy (STEM) investigations of the microstructure in beryllium metal before and after the high dose helium ion irradiation were presented. The composition, morphology and structure of precipitates were investigated in detail, which might play an important role in controlling mechanical properties. After irradiation, dislocation loops and bubbles were observed and analyzed. The swelling of the beryllium metal after helium ion irradiation was discussed.
2. Material and methods The material analyzed in this study was commercial beryllium metal. The chemical compositions are listed in Table 1. Specimens with 3 mm in diameter and 0.1 mm in thickness made by manual thinning. Then the sample was electrochemically polished using a twin-jet electro-polisher with a polishing solution of perchloric acid and ethyl alcohol compound. The TEM samples were mounted on a pure-Cu sample holder to ensure good thermal and electrical conductivity, followed by irradiation with helium ions in an ion accelerator. Helium ions with an energy of 30 keV were implanted perpendicularly into the surface of the sample at room temperature. Ion fluence was 1 × 1017 He+/cm2. The helium ions content in the sample was estimated by the Monte Carlo program SRIM 2013. The peak helium content was about 7.82 at.% based on the calculation as shown in Fig. 1. Microstructure investigations were carried out in a Tecnai F20 TEM. The foil thickness of the TEM image was estimated primarily with convergent beam electron diff ;raction (CBED) and with the thickness fringes to maintain consistent thickness of the observation area. The scanning unit for performing scanning TEM (STEM) was equipped with a high-angle annular dark field (HAADF) detector and an energy dispersive X-ray spectrum (EDS) analysis system. The accelerator voltage of the microscope was 200 kV.
Fig. 2. Morphology of the beryllium metal before helium ion irradiation.
3. Results and discussion 3.1. Microstructure of unirradiated beryllium TEM observation of the beryllium sample before irradiation was conducted. According to the observation, the grain size of the metal fall in the range from several microns to several hundred microns. Some long dislocation lines were observed in the sample, which could be induced during the manufacturing processes, as reported in previously ref. [13]. Another atypical morphology of the beryllium is that some impurity particles are inside the grain. The low magnification morphology of the beryllium sample is shown in Fig. 2. To understand these impurity particles more clearly, a HAADF image and a composition mapping by STEM-EDS were given. Fig. 3a shows a typical low-magnification Z-contrast imaging of the beryllium specimens before ion irradiation. Z-contrast imaging helps eliminate the contrast contributions from coherent strain effects and highlight the mass-thickness differences. The impurity containing heavy atoms may exhibit a bright contrast. Electron diffraction pattern (EDP) of the impurity particle marked in the Fig. 3a are shown in Fig. 3b. Corresponding TEM bright filed image of the particle is also shown in Fig. 3c. The STEM-EDS mapping results of the area marked by a line segment (from position 0.0 to 0.7 um) in Fig. 3a is shown in Fig. 3d. Klimenkov et al. have demonstrated these impurity particles to contain of complex Fe/Cr/Al/Mn beryllide phases by using EELS [11]. In this study, The EDP results reveal that this particle should be a single phase. This precipitate with ellipsoidal shape shows a complex composition of 34.14 wt.% Al, 25.77 wt.% Mn, 14.45 wt. % Fe, and 12.22 wt.% Cr. In combination with the EDP analysis, it should be Be4Al (FeCrMn) phase with a face-centered cubic structure as shown in embedded figure in Fig. 3c. The enrichment of heavy atoms of Fe, Al, Mn or Cr, makes the precipitate exhibit brighter contrasts than those of the beryllium matrix. Those precipitates could disrupt continuity of the matrix, reduce the strength of the grain boundary and accelerate the forming and expanding of the crack, resulting in degradation of mechanical properties [11].
3.2. Microstructure of helium ion irradiated beryllium After helium ion irradiation, a large number of defects and defect clusters appear in the specimens, such as supersaturated vacancies and interstitial atoms, clusters of vacancy and interstitial atom, dislocations, dislocation cluster, and so on. Fig. 4 shows the image of the helium ion irradiated beryllium specimens. These defects were not uniform in size
Fig. 1. Helium content profile in sample calculated by SRIM. 63
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P.P. Liu et al.
Fig. 3. STEM and EDS Analysis of precipitate in unirradiated beryllium sample. (a) The lowmagnification Z-contrast image of the beryllium specimen; (b) Electron diffraction pattern of the impurity particle marked in the (a); (c) Corresponding TEM bright filed image of the particle; (d) The composition analysis of the precipitate.
phenomenon was similar. In addition, a long dislocation with Burgers vector b = 1/3 < 11–20 > (a-type) was observed in this grain. Renterghem et al. have reported that a-type dislocations were observed in irradiated and un-irradiated beryllium sample [16]. The results here agreed well with their report. In the beryllium sample after helium irradiation with fluence of 1 × 1017 He+/cm2, the irradiation-induced bubbles were imaged with under-focus image conditions with Δf = −6 um as shown in Fig. 6. One partial enlargement image was also shown in Fig. 6b. Bubbles were identified with under- and over-focused conditions as shown in inserted figures of Fig. 6b. Most of observed bubbles were homogeneously distributed throughout the investigated materials volume. The statistically measurement shows that the bubble’s size is about 2–5 nm and the density is about 6.8 × 1021/m3. The statistical results are shown in Fig. 7. 3.3. Ion irradiation swelling in beryllium A large number of helium bubbles are produced after helium ion irradiation, which can change the volume of material resulting in irradiation swelling. The effective swelling rate S of materials can be calculated by Formula (1) based on the parameters such as the average size and the number density of helium bubbles.
Fig. 4. Microstructure image of beryllium after helium ions irradiation.
and some of them were tangled. Slightly defocused bright field images were taken with g = 10–10 under two beam condition (as shown in the inserted diffraction pattern of Fig. 4) in the current study to enhance the contrast of dislocation loops. There are no extra diffraction spots except for the matrix ones in the correspond EDP. This finding confirms that the tiny black spots are not precipitates but irradiation defects like dislocation loops induced by helium irradiation. Some dislocation loops are entangled with each other and hard to be distinguished experimentally. For easy analysis, the dislocation loops in beryllium were simply divided into three categories here: a-type with b = 1/3 < 11–20 > , ctype including b = 1/2 < 0001 > and b = < 0001 > , and c-complex type including b = 1/6 < 20–23 > and b = 1/3 < 11–23 > . For a-type dislocation loops, it is visible under g = 10-10 but invisible under g = 0002. On the contrary, c-type dislocation loops are invisible under g = 10–10 but visible under g = 0002. While c-complex type loops are both visible under g = 10–10 and g = 0002. Morphology of the same grain of beryllium under different g vectors are shown in Fig. 5. It is found that a-type, c-type, c-complex type dislocation loops are generated under the helium ions irradiation. According to the observation, a-type dislocation loop should be dominant. Average size of the dislocation loop was about 36.8 nm, and number density was 7.6 × 1019/m3. The density and size of dislocation loop were smaller and slightly larger than that of Chakin et al. and Renteghem, respectively [14–16]. The irradiation conditions are different, but the main
s=
Δv × 100% v
(1)
where ΔV is the increased volume of material which is equal to the sum of all helium bubbles volume in the corresponding volume V. In order to simplify the calculation, the helium bubble was treated as a sphere. The Eq. (1) can be given as:
s=
∑ 4πr Δv × 100 %= v v
3
3
× 100%
(2)
where r is radius of sphere [17]. Irradiation swelling, for example, can be calculated according to the formula (2) in beryllium after high doses helium ion irradiation. Size of helium bubbles ranged from 0.2 nm to 5.8 nm approximately in diameter for the helium ion irradiated beryllium sample. Area with the thickness of about 200 nm has been selected to carry out the bubbles investigation after irradiation by using CBED technology. Average size was measured to be 2.7 nm with standard deviation of 0.3 nm. Density of the helium bubbles was counted to be 6.8 × 1021/m3. According to the formula (2), the swelling was calculated to be about 0.1%. At an early stage of bubbles growth, density of bubble may be insusceptible by bubble growth as size of bubble is so small that the bubble will not contact with each other and merge. Thus, swelling affected by the growth of bubble. The growth rate of a gas64
Fusion Engineering and Design 140 (2019) 62–66
P.P. Liu et al.
Fig. 5. Dislocation loops generated in beryllium after helium ions irradiation. (a) bright filed image of dislocation loop with g = 0002; (b) bright filed image of dislocation loop with g = 10–10.
filled bubble can be described as [18,19]:
drc 1 = (zcv Dv C v − zci Di Ci − zcv Dv Ccv ) dt rc
(3)
the point defect fluxes captured by a where bubble of is the rate of vacancy emission from the bubble. If the vacancy concentration, Ccv , is in local equilibrium with the bubble, it gives: and zci Di Ci are radius rc , and zcv Dv Ccv
zcv Dv C v
Ccv = Cev exp[
Ω ( σ− P)] kT
(4)
where Cev is the thermal equilibrium vacancy concentration, Ω is the atomic volume and kT has its usual meaning. P is the bubble’s internal gas pressure. σ is the surface tension of the cavity, given by:
σ=
2γ rc
(5) Fig. 7. Bubble size distribution in the beryllium after helium ions irradiation.
where γ is the free surface energy. According to the formula (4), the Ccv is depending on temperature, surface energy and gas pressure. A schematic diagram was shown in Fig. 8 according formula (4) (Fig. 8a) and formula (3) (Fig. 8b). Two sets of data were selected from Table 1 of reference [19] and used for example here. When the bubble is very small and stabilized by their internal gas pressure, i.e. P ˜ σ, temperature will not affect the concentration of vacancy. That may be an ideal condition. In practically, σ is always larger than P and the concentration of vacancy increase with the increase of value of σ-P as shown in Fig. 8a. As the temperature increase, the concentration always decreased and the growth rate of bubbles increased as shown in Fig. 8b. The growth also increased with
increase of σ-P value. It was shown in other neutron irradiation work that the values of swelling of irradiated beryllium can range from almost zero to about 10%. Klimenkov et al. have reported that swelling of the beryllium pebbles can range from 0.6% up to 6.5% with the temperature increase from 686 K to 968 K after neutron irradiation with 3000 appm helium content [20]. In this study, however, the swelling of beryllium is small as 0.1% after high fluence helium ion irradiation. One reason of this big difference may be the irradiation temperature as discussed above.
Fig. 6. Bubbles induced by helium ion irradiation in beryllium sample. 65
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Fig. 8. A schematic diagram of the relationship between concentration of vacancy and growth rate of bubble with temperature.
4. Conclusions
Int. Technical Conf. (2004). [6] A. Goldberg, Atomic, Crystal, Elastic, Thermal, Nuclear, and Other Properties of Beryllium, (2006). [7] F. Elio, K. Ioki, P. Barabaschi, et al., Engineering design of the ITER blanket and relevant research and development results, Fusion Eng. Des. 46 (2–4) (1999) 159–175. [8] A. Khomutov, V. Barabash, V. Chakin, et al., Beryllium for fusion application—recent results, J. Nucl. Mater. 307 (1) (2002) 630–637. [9] K.M. Feng, C.H. Pan, G.S. Zhang, et al., Progress on design and R&D for heliumcooled ceramic breeder TBM in China, Fusion Eng. Des. 87 (7–8) (2012) 1138–1145. [10] V.A. Borodin, P.V. Vladimirov, Damage production in atomic displacement cascades in beryllium, Nucl. Mater. Energy 9(C (2016). [11] M. Klimenkov, V. Chakin, A. Moeslang, et al., TEM study of impurity segregations in beryllium pebbles, J. Nucl. Mater. 455 (1–3) (2014) 660–664. [12] V.P. Chakin, S.V. Belozerov, A.O. Posevin, Accumulation and diffusion of radiogenic helium in beryllium, Phys. Met. Metallogr. 104 (3) (2007) 257–261. [13] P. Liu, Q. Zhan, W. Hu, et al., Microstructure evolution of beryllium with argon ion irradiation, Nucl. Mater. Energy 13(C (2017). [14] V.P. Chakin, Z. Ye Ostrovsky, Evolution of beryllium microstructure under highdose neutron irradiation, J. Nucl. Mater. 307–311 (2002) 657–663. [15] A. Leenaers, G. Verpoucke, A. Pellettieri, et al., Microstructure of long-term annealed highly irradiated beryllium, J. Nucl. Mater. 372 (2008) 256–262. [16] W. Van Renterghem, A. Leenaers, S. Van den Berghe, Investigation of long-term annealed highly irradiated beryllium, J. Nucl. Mater. 374 (2008) 54–60. [17] P.P. Liu, Q. Zhan, Z.Y. Fu, et al., Surface and internal microstructure damage of Heion-irradiated CLAM steel studied by cross-sectional transmission electron microscopy, J. Alloys Compd. 649 (2015) 859–864. [18] R.E. Stoller, G.R. Odette, Analytical solutions for helium bubble and critical radius parameters using a hard sphere equation of state, J. Nucl. Mater. 131 (2) (1985) 118–125. [19] N. Oono, S. Ukai, S. Kondo, et al., Irradiation effects in oxide dispersion strengthened (ODS) Ni-base alloys for Gen. IV nuclear reactors, J. Nucl. Mater. 465 (2015) 835–839. [20] M. Klimenkov, V. Chakin, A. Moeslang, et al., TEM study of beryllium pebbles after neutron irradiation up to 3000 appm helium production, J. Nucl. Mater. 443 (1–3) (2013) 409–416.
In summary, microstructure analysis (TEM and STEM) were performed in beryllium before and after helium ion irradiation at room temperature. Some impurity particles were observed inside the grain and investigated by STEM-EDX. These precipitates could be Fe/Al/Mn/ Cr-rich beryllide phases. After the high-dose irradiation, a-type, c-type and c-complex type dislocation loops were induced in the beryllium sample. A large number of helium bubbles with average size of 2.7 nm were observed and resulted in small swelling (about 0.1%) of the specimen as room temperature irradiation. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant Nos. 51601012, U1637210, 51571021, 11775018) and sponsored by Fundamental Research Funds for the Central Universities with Grant Nos. FRF-BR-17-025A. The authors acknowledge the help from Haibao Special Metal Materials Co., Ltd and Beijing Institute of Aerospace Control Devices, China. References [1] G.E. Darwin, J.H. Buddery, Beryllium, Academic Press, 1960. [2] G. Tuer, A. Kaufmann, The Metal Beryllium, ASM, Cleveland, 1955, p. 383. [3] K. Ashley, Beryllium: Sampling and Analysis, ASTM International, West Conshohocken, 2006. [4] W.J. Haws, New trends in powder processing beryllium-containing alloys, JOM 52 (5) (2000) 35. [5] T.A. Tomberlin, Beryllium—a unique material in nuclear applications, Proc. Sample
66
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Microstructure change and swelling of helium irradiated beryllium a,⁎
a
b
b
a
a,⁎
P.P. Liu , L.W. Xue , L.P. Yu , J.L. Liu , W. Hu , Q. Zhan , F.R. Wan a b
a,⁎
T
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Beijing Institute of Aerospace Control Devices, Beijing 100854, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Beryllium Irradiation damage Dislocation loop type Helium bubble Swelling
Beryllium, as an outstanding nuclear metal, will be exposed to high-dose irradiation of high-energy neutrons during services in reactor, which will produce a large number of helium and significant irradiation damage resulting in extreme performance degradation. In this paper, microstructure change and swelling of helium irradiated beryllium have been investigated using transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). Inside the grain, some impurity particles were observed and be revealed to be Fe/Al/Mn/Cr-rich beryllide phase with fcc structure. Beryllium metal was irradiated with 30 keV helium ion to a dose of 1 × 1017 He+/cm2 at room temperature. A high density of dislocation loop was induced in the sample after the irradiation. The types of loops in beryllium were summarized. Meanwhile, helium bubbles with average size of 2.7 nm were observed. Helium irradiation induced swelling and temperature effect were discussed.
1. Introduction
as the reaction shows [7–9]. Neutron radiation will produce severe irradiation damage, such as vacancies, dislocation loops, bubbles, etc. Characterization of dislocation loops and/or bubbles allows theoretically predict the dynamics of microstructure development, calculate materials swelling and predict performance degradation [10]. Transmission electron microscopy (TEM) has proved to be a powerful method for investigating such defects. The combination of conventional imaging with energy dispersive X-ray spectrum (EDS) on the nanoscale allows a detailed material characterization. Klimenkov et al. have studied gas bubble morphology and evolution by TEM in beryllium with helium content up to 3000 appm [11]. Chakin et al. reported the evolution of beryllium microstructure including grain size and damage structure under highdose neutron irradiation by using an optical microscope and electron microscope [12]. However, it will produce an extremely high helium content in beryllium, which may be ten times larger than that in structural steel and will greatly affect the performance properties and macrostructure of beryllium [12]. Precipitates and irradiation induced dislocation loops and a large number of bubbles will affect the properties and irradiation behavior of beryllium, that need to be analyzed in detail. Nanoscale microstructure analysis before and after irradiation plays a key role in understanding the changes of mechanical and physical properties [13]. In the present work, pure beryllium metal was chosen to study helium and irradiation damage effect. It was irradiated by 30 keV helium
Beryllium is a newcomer to the ever-increasing list of industrially important metals. Because of the combination of high flexural rigidity, thermal stability, thermal conductivity and low density, beryllium metal services in aircraft components, missiles, spacecraft, and satellites gyroscope, scan mirrors, sports equipment, and electronics industries as a structural and functional material [1–4]. Meanwhile, with many outstanding nuclear properties, like low atomic number, low neutron-capture cross section and a high neutron scattering cross section, beryllium is also used as a neutron moderator in fission reactor [5,6]. After used successfully in several tokamak as plasma facing material (FM), beryllium has been selected as the first wall of ITER nuclear fusion reactor [7]. The beryllium will undergo a (n, 2n) neutron reaction with neutron energy over 1.9 MeV, to produce two alpha particles. Thus, for high-energy fusion neutron, beryllium is a neutron multiplier, releasing more neutrons than it absorbs. This nuclear reaction is: 9 4 Be
+ n→ 224 He + 2n (E > 1.9 MeV)
Therefore, beryllium will be used as a neutron multiplier material (NM) in the helium-cooled ceramic breeder (HCCB) test blanket module (TBM). During service as FM and NM in reactor, the beryllium metal will be exposed to high-dose irradiation of high-energy neutrons, resulting in significant irradiation damage and a large number of helium
⁎
Corresponding authors. E-mail addresses: [email protected] (P.P. Liu), [email protected] (Q. Zhan), [email protected] (F.R. Wan).
https://doi.org/10.1016/j.fusengdes.2019.01.126 Received 14 December 2018; Received in revised form 25 January 2019; Accepted 26 January 2019 0920-3796/ © 2019 Published by Elsevier B.V.
Fusion Engineering and Design 140 (2019) 62–66
P.P. Liu et al.
Table 1 Chemical composition of beryllium sample (wt.%). BeO
C
Ti
Fe
Al
Mg
Si
Mn
Be
˜0.2
0.08
< 0.01
0.33
0.038
< 0.005
0.12
0.021
Balance
at room temperature. TEM and scanning transmission electron microscopy (STEM) investigations of the microstructure in beryllium metal before and after the high dose helium ion irradiation were presented. The composition, morphology and structure of precipitates were investigated in detail, which might play an important role in controlling mechanical properties. After irradiation, dislocation loops and bubbles were observed and analyzed. The swelling of the beryllium metal after helium ion irradiation was discussed.
2. Material and methods The material analyzed in this study was commercial beryllium metal. The chemical compositions are listed in Table 1. Specimens with 3 mm in diameter and 0.1 mm in thickness made by manual thinning. Then the sample was electrochemically polished using a twin-jet electro-polisher with a polishing solution of perchloric acid and ethyl alcohol compound. The TEM samples were mounted on a pure-Cu sample holder to ensure good thermal and electrical conductivity, followed by irradiation with helium ions in an ion accelerator. Helium ions with an energy of 30 keV were implanted perpendicularly into the surface of the sample at room temperature. Ion fluence was 1 × 1017 He+/cm2. The helium ions content in the sample was estimated by the Monte Carlo program SRIM 2013. The peak helium content was about 7.82 at.% based on the calculation as shown in Fig. 1. Microstructure investigations were carried out in a Tecnai F20 TEM. The foil thickness of the TEM image was estimated primarily with convergent beam electron diff ;raction (CBED) and with the thickness fringes to maintain consistent thickness of the observation area. The scanning unit for performing scanning TEM (STEM) was equipped with a high-angle annular dark field (HAADF) detector and an energy dispersive X-ray spectrum (EDS) analysis system. The accelerator voltage of the microscope was 200 kV.
Fig. 2. Morphology of the beryllium metal before helium ion irradiation.
3. Results and discussion 3.1. Microstructure of unirradiated beryllium TEM observation of the beryllium sample before irradiation was conducted. According to the observation, the grain size of the metal fall in the range from several microns to several hundred microns. Some long dislocation lines were observed in the sample, which could be induced during the manufacturing processes, as reported in previously ref. [13]. Another atypical morphology of the beryllium is that some impurity particles are inside the grain. The low magnification morphology of the beryllium sample is shown in Fig. 2. To understand these impurity particles more clearly, a HAADF image and a composition mapping by STEM-EDS were given. Fig. 3a shows a typical low-magnification Z-contrast imaging of the beryllium specimens before ion irradiation. Z-contrast imaging helps eliminate the contrast contributions from coherent strain effects and highlight the mass-thickness differences. The impurity containing heavy atoms may exhibit a bright contrast. Electron diffraction pattern (EDP) of the impurity particle marked in the Fig. 3a are shown in Fig. 3b. Corresponding TEM bright filed image of the particle is also shown in Fig. 3c. The STEM-EDS mapping results of the area marked by a line segment (from position 0.0 to 0.7 um) in Fig. 3a is shown in Fig. 3d. Klimenkov et al. have demonstrated these impurity particles to contain of complex Fe/Cr/Al/Mn beryllide phases by using EELS [11]. In this study, The EDP results reveal that this particle should be a single phase. This precipitate with ellipsoidal shape shows a complex composition of 34.14 wt.% Al, 25.77 wt.% Mn, 14.45 wt. % Fe, and 12.22 wt.% Cr. In combination with the EDP analysis, it should be Be4Al (FeCrMn) phase with a face-centered cubic structure as shown in embedded figure in Fig. 3c. The enrichment of heavy atoms of Fe, Al, Mn or Cr, makes the precipitate exhibit brighter contrasts than those of the beryllium matrix. Those precipitates could disrupt continuity of the matrix, reduce the strength of the grain boundary and accelerate the forming and expanding of the crack, resulting in degradation of mechanical properties [11].
3.2. Microstructure of helium ion irradiated beryllium After helium ion irradiation, a large number of defects and defect clusters appear in the specimens, such as supersaturated vacancies and interstitial atoms, clusters of vacancy and interstitial atom, dislocations, dislocation cluster, and so on. Fig. 4 shows the image of the helium ion irradiated beryllium specimens. These defects were not uniform in size
Fig. 1. Helium content profile in sample calculated by SRIM. 63
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P.P. Liu et al.
Fig. 3. STEM and EDS Analysis of precipitate in unirradiated beryllium sample. (a) The lowmagnification Z-contrast image of the beryllium specimen; (b) Electron diffraction pattern of the impurity particle marked in the (a); (c) Corresponding TEM bright filed image of the particle; (d) The composition analysis of the precipitate.
phenomenon was similar. In addition, a long dislocation with Burgers vector b = 1/3 < 11–20 > (a-type) was observed in this grain. Renterghem et al. have reported that a-type dislocations were observed in irradiated and un-irradiated beryllium sample [16]. The results here agreed well with their report. In the beryllium sample after helium irradiation with fluence of 1 × 1017 He+/cm2, the irradiation-induced bubbles were imaged with under-focus image conditions with Δf = −6 um as shown in Fig. 6. One partial enlargement image was also shown in Fig. 6b. Bubbles were identified with under- and over-focused conditions as shown in inserted figures of Fig. 6b. Most of observed bubbles were homogeneously distributed throughout the investigated materials volume. The statistically measurement shows that the bubble’s size is about 2–5 nm and the density is about 6.8 × 1021/m3. The statistical results are shown in Fig. 7. 3.3. Ion irradiation swelling in beryllium A large number of helium bubbles are produced after helium ion irradiation, which can change the volume of material resulting in irradiation swelling. The effective swelling rate S of materials can be calculated by Formula (1) based on the parameters such as the average size and the number density of helium bubbles.
Fig. 4. Microstructure image of beryllium after helium ions irradiation.
and some of them were tangled. Slightly defocused bright field images were taken with g = 10–10 under two beam condition (as shown in the inserted diffraction pattern of Fig. 4) in the current study to enhance the contrast of dislocation loops. There are no extra diffraction spots except for the matrix ones in the correspond EDP. This finding confirms that the tiny black spots are not precipitates but irradiation defects like dislocation loops induced by helium irradiation. Some dislocation loops are entangled with each other and hard to be distinguished experimentally. For easy analysis, the dislocation loops in beryllium were simply divided into three categories here: a-type with b = 1/3 < 11–20 > , ctype including b = 1/2 < 0001 > and b = < 0001 > , and c-complex type including b = 1/6 < 20–23 > and b = 1/3 < 11–23 > . For a-type dislocation loops, it is visible under g = 10-10 but invisible under g = 0002. On the contrary, c-type dislocation loops are invisible under g = 10–10 but visible under g = 0002. While c-complex type loops are both visible under g = 10–10 and g = 0002. Morphology of the same grain of beryllium under different g vectors are shown in Fig. 5. It is found that a-type, c-type, c-complex type dislocation loops are generated under the helium ions irradiation. According to the observation, a-type dislocation loop should be dominant. Average size of the dislocation loop was about 36.8 nm, and number density was 7.6 × 1019/m3. The density and size of dislocation loop were smaller and slightly larger than that of Chakin et al. and Renteghem, respectively [14–16]. The irradiation conditions are different, but the main
s=
Δv × 100% v
(1)
where ΔV is the increased volume of material which is equal to the sum of all helium bubbles volume in the corresponding volume V. In order to simplify the calculation, the helium bubble was treated as a sphere. The Eq. (1) can be given as:
s=
∑ 4πr Δv × 100 %= v v
3
3
× 100%
(2)
where r is radius of sphere [17]. Irradiation swelling, for example, can be calculated according to the formula (2) in beryllium after high doses helium ion irradiation. Size of helium bubbles ranged from 0.2 nm to 5.8 nm approximately in diameter for the helium ion irradiated beryllium sample. Area with the thickness of about 200 nm has been selected to carry out the bubbles investigation after irradiation by using CBED technology. Average size was measured to be 2.7 nm with standard deviation of 0.3 nm. Density of the helium bubbles was counted to be 6.8 × 1021/m3. According to the formula (2), the swelling was calculated to be about 0.1%. At an early stage of bubbles growth, density of bubble may be insusceptible by bubble growth as size of bubble is so small that the bubble will not contact with each other and merge. Thus, swelling affected by the growth of bubble. The growth rate of a gas64
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Fig. 5. Dislocation loops generated in beryllium after helium ions irradiation. (a) bright filed image of dislocation loop with g = 0002; (b) bright filed image of dislocation loop with g = 10–10.
filled bubble can be described as [18,19]:
drc 1 = (zcv Dv C v − zci Di Ci − zcv Dv Ccv ) dt rc
(3)
the point defect fluxes captured by a where bubble of is the rate of vacancy emission from the bubble. If the vacancy concentration, Ccv , is in local equilibrium with the bubble, it gives: and zci Di Ci are radius rc , and zcv Dv Ccv
zcv Dv C v
Ccv = Cev exp[
Ω ( σ− P)] kT
(4)
where Cev is the thermal equilibrium vacancy concentration, Ω is the atomic volume and kT has its usual meaning. P is the bubble’s internal gas pressure. σ is the surface tension of the cavity, given by:
σ=
2γ rc
(5) Fig. 7. Bubble size distribution in the beryllium after helium ions irradiation.
where γ is the free surface energy. According to the formula (4), the Ccv is depending on temperature, surface energy and gas pressure. A schematic diagram was shown in Fig. 8 according formula (4) (Fig. 8a) and formula (3) (Fig. 8b). Two sets of data were selected from Table 1 of reference [19] and used for example here. When the bubble is very small and stabilized by their internal gas pressure, i.e. P ˜ σ, temperature will not affect the concentration of vacancy. That may be an ideal condition. In practically, σ is always larger than P and the concentration of vacancy increase with the increase of value of σ-P as shown in Fig. 8a. As the temperature increase, the concentration always decreased and the growth rate of bubbles increased as shown in Fig. 8b. The growth also increased with
increase of σ-P value. It was shown in other neutron irradiation work that the values of swelling of irradiated beryllium can range from almost zero to about 10%. Klimenkov et al. have reported that swelling of the beryllium pebbles can range from 0.6% up to 6.5% with the temperature increase from 686 K to 968 K after neutron irradiation with 3000 appm helium content [20]. In this study, however, the swelling of beryllium is small as 0.1% after high fluence helium ion irradiation. One reason of this big difference may be the irradiation temperature as discussed above.
Fig. 6. Bubbles induced by helium ion irradiation in beryllium sample. 65
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Fig. 8. A schematic diagram of the relationship between concentration of vacancy and growth rate of bubble with temperature.
4. Conclusions
Int. Technical Conf. (2004). [6] A. Goldberg, Atomic, Crystal, Elastic, Thermal, Nuclear, and Other Properties of Beryllium, (2006). [7] F. Elio, K. Ioki, P. Barabaschi, et al., Engineering design of the ITER blanket and relevant research and development results, Fusion Eng. Des. 46 (2–4) (1999) 159–175. [8] A. Khomutov, V. Barabash, V. Chakin, et al., Beryllium for fusion application—recent results, J. Nucl. Mater. 307 (1) (2002) 630–637. [9] K.M. Feng, C.H. Pan, G.S. Zhang, et al., Progress on design and R&D for heliumcooled ceramic breeder TBM in China, Fusion Eng. Des. 87 (7–8) (2012) 1138–1145. [10] V.A. Borodin, P.V. Vladimirov, Damage production in atomic displacement cascades in beryllium, Nucl. Mater. Energy 9(C (2016). [11] M. Klimenkov, V. Chakin, A. Moeslang, et al., TEM study of impurity segregations in beryllium pebbles, J. Nucl. Mater. 455 (1–3) (2014) 660–664. [12] V.P. Chakin, S.V. Belozerov, A.O. Posevin, Accumulation and diffusion of radiogenic helium in beryllium, Phys. Met. Metallogr. 104 (3) (2007) 257–261. [13] P. Liu, Q. Zhan, W. Hu, et al., Microstructure evolution of beryllium with argon ion irradiation, Nucl. Mater. Energy 13(C (2017). [14] V.P. Chakin, Z. Ye Ostrovsky, Evolution of beryllium microstructure under highdose neutron irradiation, J. Nucl. Mater. 307–311 (2002) 657–663. [15] A. Leenaers, G. Verpoucke, A. Pellettieri, et al., Microstructure of long-term annealed highly irradiated beryllium, J. Nucl. Mater. 372 (2008) 256–262. [16] W. Van Renterghem, A. Leenaers, S. Van den Berghe, Investigation of long-term annealed highly irradiated beryllium, J. Nucl. Mater. 374 (2008) 54–60. [17] P.P. Liu, Q. Zhan, Z.Y. Fu, et al., Surface and internal microstructure damage of Heion-irradiated CLAM steel studied by cross-sectional transmission electron microscopy, J. Alloys Compd. 649 (2015) 859–864. [18] R.E. Stoller, G.R. Odette, Analytical solutions for helium bubble and critical radius parameters using a hard sphere equation of state, J. Nucl. Mater. 131 (2) (1985) 118–125. [19] N. Oono, S. Ukai, S. Kondo, et al., Irradiation effects in oxide dispersion strengthened (ODS) Ni-base alloys for Gen. IV nuclear reactors, J. Nucl. Mater. 465 (2015) 835–839. [20] M. Klimenkov, V. Chakin, A. Moeslang, et al., TEM study of beryllium pebbles after neutron irradiation up to 3000 appm helium production, J. Nucl. Mater. 443 (1–3) (2013) 409–416.
In summary, microstructure analysis (TEM and STEM) were performed in beryllium before and after helium ion irradiation at room temperature. Some impurity particles were observed inside the grain and investigated by STEM-EDX. These precipitates could be Fe/Al/Mn/ Cr-rich beryllide phases. After the high-dose irradiation, a-type, c-type and c-complex type dislocation loops were induced in the beryllium sample. A large number of helium bubbles with average size of 2.7 nm were observed and resulted in small swelling (about 0.1%) of the specimen as room temperature irradiation. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant Nos. 51601012, U1637210, 51571021, 11775018) and sponsored by Fundamental Research Funds for the Central Universities with Grant Nos. FRF-BR-17-025A. The authors acknowledge the help from Haibao Special Metal Materials Co., Ltd and Beijing Institute of Aerospace Control Devices, China. References [1] G.E. Darwin, J.H. Buddery, Beryllium, Academic Press, 1960. [2] G. Tuer, A. Kaufmann, The Metal Beryllium, ASM, Cleveland, 1955, p. 383. [3] K. Ashley, Beryllium: Sampling and Analysis, ASTM International, West Conshohocken, 2006. [4] W.J. Haws, New trends in powder processing beryllium-containing alloys, JOM 52 (5) (2000) 35. [5] T.A. Tomberlin, Beryllium—a unique material in nuclear applications, Proc. Sample
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