Effects of Helium Implantation and Subsequent Electron Irradiation on Microstructures of Fe-11\,wt.{\%} Cr Model Alloy

Bing-Sheng Li(李炳生)$^{1,2\hyperlink{s}{**}}$, Zhi-Guang Wang(王志光)$^{2}$, Tie-Long Shen(申铁龙)$^{2}$, Kong-Fang Wei(魏孔芳)$^{2}$, Yan-Bin Sheng(盛彦斌)$^{2}$, Tamakai Shibayama(柴山环树)$^{3}$, Xi-Rui Lu(卢喜瑞)$^{4}$, An-Li Xiong(熊安利)$^{4}$

doi:10.1088/0256-307X/36/4/046104

⇦Chinese Physics Letters, 2019, Vol. 36, No. 4, Article code 046104⇨ Effects of Helium Implantation and Subsequent Electron Irradiation on Microstructures of Fe-11 wt.% Cr Model Alloy * Bing-Sheng Li (李炳生)1,2**, Zhi-Guang Wang (王志光)2, Tie-Long Shen (申铁龙)2, Kong-Fang Wei (魏孔芳)2, Yan-Bin Sheng (盛彦斌)2, Tamakai Shibayama (柴山环树)3, Xi-Rui Lu (卢喜瑞)4, An-Li Xiong (熊安利)4 Affiliations 1State Key Laboratory for Environment-Friendly Energy Materials, Southwest University of Science and Technology, Mianyang 621010 2Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000 3Center for Advanced Research of Energy and Materials, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan 4School of National Defense Science and Technology, Southwest University of Science and Technology, Mianyang 621010 Received 1 January 2019, online 23 March 2019 ^*Supported by the National Natural Science Foundation of China under Grant Nos U1832133, 11475229 and 91426301.
^**Corresponding author. Email: libingshengmvp@163.com Citation Text: Li B S, Wang Z G, Shen T L, Wei K F and Sheng Y B et al 2019 Chin. Phys. Lett. 36 046104 Abstract Helium effects on dislocation and cavity formation of Fe-11 wt.% Cr model alloy are investigated. Single-beam (electron) and dual-beam (He$^{+}$/e$^{-}$) irradiations are performed at 350$^\circ\!$C and 400$^\circ\!$C using an ultra-high voltage electron microscope combined with ion accelerators. In-situ observation shows that the growth rate of dislocation loops is reduced in the helium pre-injected specimen. The mean size of cavities decreased in the helium pre-injected specimen. The possible mechanisms are discussed. DOI:10.1088/0256-307X/36/4/046104 PACS:61.80.Jh, 61.82.Bg, 68.37.Lp, 81.40.Wx © 2019 Chinese Physics Society Article Text After decades of research, reduced activation ferritic/martensitic steels (RAFMs)—which possess good mechanical and thermal properties, low irradiation-induced swelling, and reasonably fast radioactive decay—have been considered as the primary candidate materials for the first wall and/or blanket of fusion reactors.[1-3] Several kinds of RAFMs, with chromium contents ranging from 9 to 12 wt.%, have been produced and tested by neutron irradiation, ion irradiation and electron irradiation. These include Eurofer-97, F82H, JLF-1, CLAM, CLF-1 and Optimax-IX. The most concerning drawback of these steels is the irradiation embrittlement, which is manifested by a ductile-to-brittle transition temperature (DBTT) shift to higher temperatures after neutron irradiation.[4-7] Its amplitude depends on the irradiation conditions, including temperature, neutron flux, neutron fluence, and helium concentration. It is well-known that the increase in DBTT is due to irradiation-induced dislocation loops, bubbles and precipitates. Therefore, the studies of formation and evolution of these defects are essential to clarify mechanism of the DBTT shift and they play an important role in the future design of advanced embrittlement-resistant steels. Using an in-situ method via high voltage electron transmission microscopy can provide valuable information of irradiation-induced lattice defects. For example, the migration energies of interstitials and vacancies, lattice swelling in ferritic/martensitic steels can be obtained by electron irradiation and then in-situ observation.[8-10] The China Initiative Accelerator Driven System (CIADS) is being founded in China. To resist the corrosion of liquid lead-bismuth eutectic, neutron irradiation and high temperatures, a novel RAFM steel with high silicon steel (SIMP) has been cooperatively developed by Institute of Modern Physics and Institute of Metal Research, Chinese Academy of Sciences. To ascertain the irradiation effects on SIMP steel, ion irradiation experiments have been carried out recently. Irradiation-induced hardening and lattice swelling in SIMP steel have been investigated in He or Fe ion irradiation.[11-13] Consequently, it is worth studying whether only one kind of ion beam can simulate the neutron irradiation. To solve this problem, the influence of He atoms on irradiation-induced defects should be considered. To date, the effects of He atoms on the nucleation of dislocation loops and cavities are still unknown in SIMP steel. In this study, the microstructural evolution of SIMP steel during irradiation at 350$^\circ\!$C and 400$^\circ\!$C are investigated by the in-situ method via high voltage electron irradiation. The temperatures that we have chosen are within th temperature range used in CIADS. The material analyzed in the present study was Fe-11 wt.% Cr, a model alloy of RAFM steels. Its main chemical compositions are Fe, 11.2 Cr, 1.43 W, 0.2 V, 0.15 Ta, 0.53 Mn, 1.4 Si, 0.01 Nb and 0.32 C in wt.%. The heat treatments were normalized at 1050$^\circ\!$C with 30 min followed by air cool, and tempered at 760$^\circ\!$C for 90 min followed by air cool. The detailed fabrication processing was described in Ref. [14]. The specimens were prepared by standard techniques for the transmission electron microscopy (TEM) studies. The specimens were mechanically ground to approximately 130$\,µ$m in thickness and punched out to disks with a diameter of 3 mm. Afterwards, the specimens were polished using a twin-jet electro-polisher with a polishing solution of 5% HClO$_{4}$ + 95% C$_{2}$H$_{5}$OH compound at $-15^\circ\!$C. The specimens were irradiated with two methods: (1) simple electron irradiation, (2) helium implantation and then electron irradiation. After electro-polishing, the specimens were checked by JEOL-200X. Ion implantation was then performed followed by electron beam irradiation when the specimens were suitable for TEM observations. Helium ion implantation with an energy of 100 keV with a fluence of $4 \times 10^{15}$/cm$^{2}$ at 400$^\circ\!$C was carried out in a 300 kV ion accelerator. The damage and concentration profiles were simulated via the Monte–Carlo code Stopping and Range of Ions in Matter (SRIM-2008) quick cascade simulations using the displacement energy of Fe of 40 eV.[15] This gets the damage peak of 0.2 displacements per atom (dpa) and the projected range of 320 + 130 nm. Afterwards, electron beam irradiation was carried out at 350$^\circ\!$C and 400$^\circ\!$C in a JEM-ARM 1300, with 1250 kV accelerating voltage installed at Hokkaido University, Japan. The damage rate was approximately $1.22 \times 10^{-4} $ dpa/s. The specimen thickness of irradiation area was selected to be approximately 400 nm to avoid surface effect on the formation of irradiation point defects. A low index crystal plane (100) was chosen for electron-beam irradiation. The behaviors of dislocation loops and cavities were studied in situ during the process of electron irradiation.

Fig. 1. In-situ observation of the growth process of the dislocation loop in the Fe-11 wt.% Cr model alloy implanted with helium under electron irradiation at 350$^\circ\!$C (dpa: displacements per atom). The dislocation loop is indicated by an arrow.

Fig. 2. In-situ observation of the growth process of the dislocation loop in the Fe-11 wt.% Cr model alloy under simple electron irradiation at 350$^\circ\!$C. The dislocation loop is indicated by an arrow.

In situ electron irradiation experiments were performed on the He ion-implanted Fe-11 wt.% Cr specimens. Figure 1 shows the growth process of the dislocation loops with electron irradiation in the helium-implanted foils at 350$^\circ\!$C. In-situ microstructural observation reveals that the dislocation loop identified by an arrow in Fig. 1 grew with increasing irradiation dose. Huang et al.[16] argued that the observed dislocation loop was recognized to be interstitial-type. At 350$^\circ\!$C, the dislocation loops grew slowly due to the relatively slow interstitial migration rate. For comparison, the specimen was simply electron irradiated at 350$^\circ\!$C, and the results are shown in Fig. 2. The size of the dislocation loops increased with increasing the irradiation dose. Proliferation of the dislocation loops occurred upon electron irradiation.

Fig. 3. In-situ observation of the growth process of the cavities in the Fe-11 wt.% Cr model alloy implanted with helium ions under electron irradiation up to 8.78 dpa at 400$^\circ\!$C. The cavities are indicated by arrows.

Fig. 4. In-situ observation of the growth process of the cavities in the Fe-11 wt.% Cr model alloy irradiated with electron up to 10.25 dpa at 400$^\circ\!$C. The cavities are indicated by arrows.

Figure 3 shows the growth process of cavities for the electron irradiation in the He-implanted specimen up to 10.25 dpa at 400$^\circ\!$C. Cavities were formed at 1.68 dpa and their size increased as the irradiation dose increased. The cavities show a faceted feature, where cavity growth is fast along a plane of low surface energy.[17,18] Figure 4 shows the growth process of cavities for the specimen with simple electron irradiation up to 8.78 dpa at 400$^\circ\!$C. Cavities were formed at 1.9 dpa and their sizes increased with increasing the irradiation dose. The cavities show a spherical feature. The number density of observed cavities is significantly larger in the He pre-injected specimen than that of simple electron irradiated specimen. This indicates that the helium atoms can enhance cavity nucleation.

Fig. 5. (a) Dose dependence of the loop size in Fe-11 wt.% Cr model alloy irradiated with two methods at 350$^\circ\!$C, (b) dose dependence of the cavity mean size in Fe-11 wt.% Cr model alloy irradiated with two methods at 400$^\circ\!$C.

Figure 5(a) presents the size of the dislocation loops as a function of the electron irradiation dose. For the electron irradiation in the helium-implanted specimen, the size of the loops increased linearly with increasing the irradiation dose. The growth rate of the dislocation loops was only 0.2 nm/min. For the electron-irradiated specimen, the size of the loops increased monotonically with increasing the irradiation dose at a low electron irradiation dose ($ < $0.6 dpa) and then approached saturation. The growth rate of the dislocation loops was approximately 1.6 nm/min at the beginning of the electron irradiation. This result is consistent with the report of Liu et al.[19] The growth rate of dislocation loops during electron irradiation is higher than that of pre-implantation of He ions and subsequent electron irradiation. Kiritani et al.[20] mentioned that the migration energy of vacancy, $E_{m}$, is related to the growth speed of interstitial loops ($dR/dt$) according to $dR/dt=C\cdot \exp (-E_{m}/2kT)$, where $k$ is the Boltzmann constant and $C$ is an experimental constant determined by production rate of free Frenkel pairs, concentration of permanent sinks for interstitials and vacancies. Due to the low solubility of helium atoms in SIMP steel, helium atoms are preferential to accumulate into vacancies to reduce the mobility of the vacancies. Consequently, the migration energy of vacancies increases when the vacancies contain helium atoms. Therefore, the growth rate of dislocation loops decreases during the presence of a small amount of helium atoms in the specimen. Seto et al.[21] have investigated electron and helium co-irradiated Fe-8 wt.% Cr steel and argued that the increase in the migration energy of vacancies is due to trapping helium atoms. Figure 5(b) shows the mean size of cavities as a function of the electron irradiation dose. The mean size of cavities increased with increasing the irradiation dose. With the same irradiation temperature and dose, the mean size of cavities during pre-implantation with helium followed by electron irradiation is smaller than that of simple electron irradiation. This indicates that the growth of cavities is strongly affected by the presence of a small amount of helium atoms, which is in good agreement with the report of Ohnuki et al.[22] This is attributed to a large fraction of small cavities formed due to the increase of cavity nucleation by helium atoms. It is reasonable to judge that the lattice swelling in the He pre-injected specimen is lower than that of a simple electron irradiated specimen. Therefore, the lattice swelling at different temperatures should be investigated in future. In summary, SIMP steels have been irradiated by single-beam (electron) and dual-beam (helium and electron) irradiation at 350 and 400$^\circ\!$C. Dislocation growth and cavity formation are characterized by in-situ HVEM observations. The main results are summarized as follows: (1) under the condition of pre-injection of helium followed by electron irradiation, the growth rate of the dislocation loops is only 0.2 nm/min, whereas the simple electron irradiation has a much higher growth rate of approximately 1.6 nm/min; and (2) the mean size of observed cavities decreases due to the presence of the pre-injection of helium. References Overview of materials research for fusion reactorsFerritic/martensitic steels – overview of recent resultsMaterials challenges in nuclear energyReduction method of DBTT shift due to irradiation for reduced-activation ferritic/martensitic steelsEmbrittlement behavior of neutron irradiated RAFM steelsEffect of experimental sample size on local Weibull assessment of cleavage fracture for steelAssessment of neutron irradiation effects on RAFM steelsDislocation Loop Formation and Growth under In Situ Laser and/or Electron IrradiationSwelling of CLAM steel irradiated by electron/helium to 17.5dpa with 10appm He/dpaHelium effects on EUROFER97 martensitic steel irradiated by dual-beam from 1 to 50 dpa at 250 and 300 °C with 10 He appm/dpaPositron annihilation Doppler broadening spectroscopy study on Fe-ion irradiated NHS steelCharacterization of Microstructure and Stability of Precipitation in SIMP Steel Irradiated with Energetic Fe Ions ^*Helium-Implantation-Induced Damage in NHS Steel Investigated by Slow-Positron Annihilation SpectroscopyTransmission electron microscopy and high-resolution electron microscopy studies of structural defects induced in Si single crystals implanted by helium ions at 600 °CEquilibrium shapes and surface selection of nanostructures in 6H-SiCEffects of deuterium implantation and subsequent electron irradiation on the microstructure of Fe–10Cr model alloyMicrostructure evolution during irradiationEffects of multi-beam irradiation on defect formation in Fe–Cr alloysEffect of electron/helium dual-beam irradiation on cavity formation in ferritic stainless steel

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