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The influence of dislocation and hydrogen on thermal helium desorption behavior in Fe9Cr alloys
Journal of Nuclear Materials 495 (2017) 244e248
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Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
The influence of dislocation and hydrogen on thermal helium desorption behavior in Fe9Cr alloys Te Zhu a, b, Shuoxue Jin a, **, Yihao Gong a, b, Eryang Lu a, Ligang Song a, Qiu Xu c, Liping Guo d, Xingzhong Cao a, *, Baoyi Wang a, b a
Multi-discipline Research Center, Institute of High Energy Physics, CAS, Beijing 100049, China School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100039, China Research Reactor Institute, Kyoto University, Kumatori-cho, Osaka 590-0494, Japan d School of Physics and Technology, Wuhan University, Wuhan 430072, China b c
h i g h l i g h t s Synchronous desorption of helium/hydrogen and the microstructure states were observed. Effect of dislocation on thermal helium desorption were analyzed. The existence of hydrogen will strongly affect the thermal helium desorption which could be reflected in the TDS spectrum.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 January 2017 Received in revised form 5 August 2017 Accepted 15 August 2017 Available online 16 August 2017
Transmutation helium may causes serious embrittlement which is considered to be due to helium from clustering as a bubble in materials. Suppression of transmutation helium can be achieved by introducing trapping sites such as dislocations and impurities in materials. Here, effects of intentionally-induced dislocations and hydrogen on helium migrate and release behaviors were investigated using thermal desorption spectrometry (TDS) technique applied to well-annealed and cold-worked Fe9Cr alloys irradiated by energetic helium/hydrogen ions. Synchronous desorption of helium and hydrogen was observed, and the microstructure states during helium release at different temperatures were analyzed. High thermally stable HenD type complexes formed in cold-worked specimens, resulting in the retardation of helium migration and release. The existence of hydrogen will strongly affect the thermal helium desorption which could be reflected in the TDS spectrum. It was confirmed that hydrogen retained in the specimens can result in obvious delay of helium desorption. © 2017 Elsevier B.V. All rights reserved.
Keywords: Thermal desorption Defect Helium/hydrogen Ion-irradiation
1. Introduction There is a great interest in the behavior of helium migration and retention in Plasma-facing-materials (PFMs) and structural materials for the nuclear energy systems. It is expected that the materials in nuclear reactors will suffer severe irradiation damage, and transmutation helium in the materials may accelerate undesirable materials degradation such as the swelling owe to the formation of
* Corresponding author. Multi-discipline Research Center, Institute of High Energy Physics, CAS, Beijing 100049, China. ** Corresponding author. Multi-discipline Research Center, Institute of High Energy Physics, CAS, Beijing 100049, China. E-mail addresses: [email protected] (S. Jin), [email protected] (X. Cao). http://dx.doi.org/10.1016/j.jnucmat.2017.08.027 0022-3115/© 2017 Elsevier B.V. All rights reserved.
helium bubbles [1e3]. An important feature of the irradiation process in nuclear system is the formation of large displacement cascades, in which primary knock-on atoms and secondary particles formed by nuclear reactions generate a considerable number of defects such as dislocations, vacancies and spallation products [4e6]. More importantly, the generation of insoluble helium impurities induces substantial bubble formation in materials at enough high temperatures, especially the materials for Acceleratordriven systems (ADS) where the ratio of helium generated to displacement per atom produced is more than 10 times higher than that in fusion reactors [7,8]. It is well known that helium retained in the metals significantly decreases the diffusion, permeation and release of hydrogen. Hydrogen trapping is remarkably enhanced when helium pre-
T. Zhu et al. / Journal of Nuclear Materials 495 (2017) 244e248
existed within the materials [9e11], however, we know little about the effects of hydrogen on helium release behavior up to know. Generally, the solubility for hydrogen in iron-base alloy is expected to be very low, but intrinsic and radiation-induced defects can retain a significant amount of hydrogen. Therefore, the influence of hydrogen on helium retention/desorption in materials is an important issue for nuclear reactor systems. Transmutation helium sometime causes serious embrittlement which is considered to be due to formation of helium bubbles in materials. Suppression of transmutation helium can be achieved by introducing trapping sites such as dislocations and Nano-sized oxide particles in materials [12e15]. The 9Cr-ODS steels and Ferritic-martensitic steels are leading candidates for fusion systems, advanced fission reactors and possibly for long-term spallation neutron sources used for transmutation of spallation products. Such steels, which are based on FeCr, have rather resistant to irradiation, low activation and suppress helium embrittlement [16,17]. In this study, TDS was measured for Fe9Cr alloys to investigate the influence of intentionally-induced dislocations and pre-existed hydrogen on helium retention/release behaviors. 2. Experimental procedure Fe9Cr binary alloys used in this study were made from the high purity Fe (99.99%) and Cr (99.99%) metals by arc melting process at General Research Institute for Nonferrous Metals. The bulk materials were first cut to a thickness of 0.2 mm in 10 mm 10 mm square sheets followed by well-annealed at 1100 K for 2 h in vacuum (~105 Pa). To introduce dislocations, the well-annealed specimens were respective cold rolling to 10% and 20% of their original thickness at room temperature and then annealed at 673 K for 1 h in a high vacuum to annihilate vacancies and vacancy clusters. The samples were mechanically polishing with SiC sandpapers from 600 up to 4000 Grit, followed by electro-polished with a 25%-HClO4 and 75%-CH3COOH solution to remove surface contamination before every annealing treatment. The wellannealed sample is shown in Fig. 1(a). Bright-field micrograph of the annealed specimens after cold-worked (hereafter denominated ‘CW’) are shown in Fig. 1(b) and (c), and the beam directions were close to [111]. For the CW samples, a number of dislocations induced by cold rolling were observed and more dislocations were produced in CW-20% sample. The cold-worked (CW) specimens and well-annealed specimens were irradiated with 5 keV Heþ ions using an Omegatron gun, in which mono-energetic Heþ ions were collimated and massanalyzed, at a flux of 5 1017 Heþ/m2s to the same nominal dose of 1 1020 Heþ/m2 at room temperature. The damage depth profile was calculated with the Stopping and Range of Ions in Matter (SRIM-2013) program [18] in “Detailed Calculation” mode and the
245
displacements were acquired from the vacancy.txt file. A flux of 5 keV Heþ normal to Fe9Cr alloy produces a damage profile between 1 nm and 60 nm, with a damage peak at about 10 nm. Helium concentration mainly distributed in the region from 0 to 65 nm and peak at ~25 nm (shown in Fig. 2). In addition, to investigate the effects of hydrogen on helium desorption from Fe9Cr alloys, ion-irradiation experiments were carried out: (i) Heþ irradiation, (ii) Hþ irradiation and (iii) Heþ irradiation followed by Hþ irradiation, at room temperature. Well-annealed sample was used in the irradiation experiment. The beam (with the dimension of 20 20 mm2) was scanned in both the horizontal and vertical directions to maintain the uniformity of the irradiation dose. The dose of 100 keV Heþ and 60 keV Hþ were 1 1020 Heþ/m2 (corresponding to the peak dose of 0.043 dpa) and 5 1020 Hþ/m2 (the peak dose of 0.014 dpa), respectively. The peaks for both helium and hydrogen concentration were overlap at approximately 330 nm according to the calculation of SRIM code, as shown in Fig. 3. The damage dose was up to about 0.043 dpa at the peak with the damage rate of 4.2 104 dpa/s. TDS was performed by heating the samples at 1 K/s to 1523 K using infrared irradiation. During heating, the helium release was monitored by a quadrupole mass analyzer. The pressure within the TDS chamber was reduced to below 1.0 105 Pa by vacuum pump before heating the samples.
3. Results and discussion 3.1. Effect of dislocation on thermal helium desorption Fig. 4 shows the TDS spectrum of specimen irradiated with
Fig. 2. Profiles of vacancies and helium atoms distribution in Fe9Cr alloy implanted with 5 keV He-ions calculated with SRIM.
Fig. 1. (a) TEM micrographs of the well-annealed sample, a small number of defects were observed. The image of the annealed specimens after cold-worked, a number of dislocations induced by cold rolling to 10% (b) and 20% (c) of their original thickness were observed. All the dislocations were obtained in a bright-field, with the foil tilted to [111] pole.
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T. Zhu et al. / Journal of Nuclear Materials 495 (2017) 244e248
Fig. 3. Profiles of damage and atoms distribution in Fe9Cr irradiated with 100 keV Heþ and 60 keV Hþ calculated with SRIM.
Fig. 4. Thermal helium desorption spectrum of As-annealed specimen after 5 keV Heþ implantation to 1 1020 Heþ/m2.
5 keV to a dose of 1 1020 Heþ/m2. Five desorption groups were observed at about 380e700 K, 700e1000 K, 1000e1200 K, 1200e1320 K, and greater than 1320 K, respectively. In our previous research [19], these peaks can be ascribed helium desorbing from a vacancy in the neighborhood of surface, as well as overpressurized HenV (2 n 6) clusters, dislocation networks (HenD), the influence of phase transformation from bcc to fcc, larger HenVm (n < m) clusters, respectively. Here the word ‘group’ is used instead of ‘peak’ because each ‘group’ requires more than one single dissociation event to reasonably reproduce the peak width and thus, may involve multiple dissociation mechanisms [20]. The peak at bcc/fcc transition temperature is a good example. A sharp helium desorption peak was observed for this phase change point in most of previous studies [19,20]. But the sharp desorption peaks can also be affected by many factors, such as re-adsorption, irradiation dose, becoming wide peaks (‘group’) and even shifting. In the present work, the positions of bcc/fcc transition peak 3 in Figs. 5, 6 and 7 are different. The reason is that there is a high density of dislocations in the deformed samples. Heating can promote localizes dislocation activity significantly, leading to locally enhanced vacancy formation from dislocations. These new vacancies and residual dislocation
lead to re-trapping of helium atoms, so the subsequent desorption processes are affected. Thermal helium desorption measurements were also performed with cold-worked specimens that contains high density of dislocations to investigate the effect of dislocations on thermal helium desorption. Figs. 5 and 6 show thermal helium desorption spectra from CW-10% and CW-20% samples. The groups (or peaks) in both spectra were classified into Group1, Group2, Peak3, Peak4 and Bubble, respectively. These groups are considered to be ascribed to helium desorbing from multiply filled and singly filled vacancies and their clusters, dislocations, and helium bubbles, as discussed above. The spectra counts were less desorption events for asannealed specimen than for cold-worked specimens at temperatures lower than ~1100 K, with only a weak signals observed. A comparison of helium desorption spectra obtained from CW-10% and CW-20% samples shows that the Group2 shifted to higher temperature at higher deformation. These dislocations induced by cold rolling, acted as the trapping sites, could capture the helium atoms and form HenD type complexes below 800 K [21]. Therefore, more high thermal stable HenD type complexes formed in CW-20% specimen, resulting in the retardation of helium release. Moreover, the amount of He detected in a desorption peak is associated with defect concentration and defect dissociation energy. The calculated total amounts of He (excluding the background) desorbed are approximately 8.54%, 23.79% and 8.80% of the implanted dose for As-annealed, CW-10% and CW-20%, respectively. Helium release rate for Group2 (HenD type complexes) are about 5.81%, 6.23% of the total amounts for CW-10% and CW-20%, respectively. These results indicates helium retention/release increases with the increase of the dislocation density, and it is in good agreement with the one explained theoretically by Y.X. Wang et al. [22]. Helium release rate for ‘Bubble’ are about 22.3%, 11.8%, 5.5% of the total amounts for As-annealed, CW-10% and CW-20%, respectively. It clearly shows that the more the dislocation density, the less helium bubbles are formed. In other words, dislocations can impede the migration and aggregation of helium atoms, thereby inhibiting the formation of helium bubbles. One general feature of the desorption spectra can be noticed in Fig. 6, which is all peaks except Group1 shifted to higher temperature in comparison with CW-10% sample. To explain this, it must be stated that helium cannot be completely desorbed after a thermal desorption test (i.e. the defects in the sample are not fully recovered after a thermal desorption test).
Fig. 5. Thermal helium desorption spectrum of CW-10% specimen after 5 keV Heþ implantation to 1 1020 Heþ/m2.
T. Zhu et al. / Journal of Nuclear Materials 495 (2017) 244e248
Fig. 6. Thermal helium desorption spectrum of CW-20% specimen after 5 keV Heþ implantation to 1 1020 Heþ/m2.
Heating can promote localizes dislocation activity significantly, leading to locally enhanced vacancy formation from dislocations. These new vacancies and residual dislocation lead to re-trapping of helium atoms, therefore, the motion of dislocation also influences the subsequent desorption processes (Peak4). 3.2. Effect of pre-exited hydrogen on thermal helium desorption Hydrogen binding to trap sites associated with helium is via chemisorption mechanism on HenVm clusters or helium bubbles which is much stronger than irradiated defects [23]. The existence of hydrogen will strongly affect the thermal helium desorption which can be reflected in the TDS spectrum. Fig. 7 indicates the thermal helium desorption spectrum of the specimen irradiated by 100 keV Heþ and the He þ H sequential irradiated sample. Two specimens exhibit a set of higher temperature helium peaks in the range of ~1000e1200 K, more importantly, and the positions of these higher temperature peaks are
Fig. 7. Thermal helium desorption spectra of the specimen irradiated by 100 keV Heþ (black line) and the He þ H sequential irradiated sample (red line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
247
almost identical for the two specimens, where the a-g phase transformation related helium desorption from the g phase occurs in Fe9Cr alloys. However, the He þ H sequential irradiated sample displays a delaying desorption peak than the single Heþ irradiated sample at temperatures lower than ~1000 K (i.e. the peak of helium desorption shifted to higher temperatures). The calculated total amounts of He (excluding the background) released are approximately 45.8% and 39.3% of the implanted dose for the single Heþ irradiated sample and the He þ H sequential irradiated sample, respectively. Typical thermal hydrogen desorption spectra for the sample irradiated by 60 keV Hþ and the He þ H sequential irradiated one are shown in Fig. 8. When comparing between the TDS spectra, both curves are characterized by one main desorption peak at 876 K and 1034 K. During sequential 100 keV Heþ and Hþ irradiations, the preliminary creation of damage did cause significant changes in TDS spectra except for some redistribution of peak intensities and the appearance of new small desorption groups at temperatures lower than ~687 K (Fig. 8, blue curve). More importantly, the single Hþ irradiated specimen displays much stronger signals than the He þ H sequential irradiated sample at temperatures higher than ~923 K. In our previous research [24], slow positron beam results revealed that He/H-ions implanted into the samples would combine with vacancies to form (He, H)-V clusters, and the synergistic effect of post-irradiation of H ions on HenVm clusters was helping them to form He-H-V clusters. For the thermal helium desorption spectra of the He þ H specimen, the temperature range of 835e946 K is related to the formation of larger clusters HenVm, which caused by the much lower helium release. Implantation of hydrogen into the samples pre-irradiated by Heþ can result in obvious enhancement of clusters size and retardation of helium release which can be regarded as a consequence of hydrogen being trapped by HenVm type complexes. Thus, the preexistence of hydrogen in the specimens may result in obvious delay of helium release. The calculated total amounts of hydrogen (excluding the background) released are approximately 52.6% and 37.1% of the implanted dose for the single Hþ irradiated sample and the He þ H sequential irradiated sample, respectively. We can clearly find that the amount of helium and hydrogen released from the He þ H sequential irradiated sample is less than that desorbed from the single He/H-ions irradiated sample. This also indicates that hydrogen and helium combine with each other during thermal desorption to inhibit their release.
Fig. 8. Typical thermal hydrogen desorption spectra for the sample irradiated by 60 keV Hþ (black line) and the He þ H sequential irradiated sample (blue line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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4. Conclusion Thermal desorption measurements were performed to examined the influence of dislocation and hydrogen on helium releases from Fe9Cr binary alloys. Synchronous desorption of helium/ hydrogen and the microstructure states were analyzed by TDS with the different helium release temperatures. High thermally stable HenD type complexes formed in cold-worked specimens, resulting in the retardation of helium migrate/release. The existence of hydrogen in the specimens pre-irradiated by Hþ can result in obvious delay of helium release. Acknowledgments All authors acknowledge support from the National Natural Science foundation of China Grant Nos. 11475193, 11505205 and 11505192. References [1] H. Schroeder, P.J. Batfalsky, J. Nucl. Mater 117 (1983) 287. [2] L.K. Mansur, W.A. Coghlan, J. Nucl. Mater 119 (1) (1983) 1. [3] V.N. Chernikov, A.P. Zakharov, P.R. Kazansky, J. Nucl. Mater 155e157 (1988) 1142. [4] T. Yoshiie, Y. Satoh, M. Kawai, in: J. Wagemans, H.A. Abderrahim, P. D'hondt, C.D. Raedt (Eds.), Proceedings of the 11th International Symposium on Reactor Dosimetry, Aug. 18-23, 2002, p. 93. Brussels, Belgium. [5] Q. Xu, T. Ishizaki, K. Sato, T. Yoshiie, S. Nagata, Mater. Trans. 47 (2006) 2885.
[6] X.Z. Cao, Q. Xu, K. Sato, T. Yoshiie, J. Nucl. Mater 412 (2011) 165. [7] Y. Dai, J. Henry, Z. Tong, X. Averty, J. Malaplate, B. Long, J. Nucl. Mater 415 (2011) 306. [8] K. Nishihara, K. Kikuchi, J. Nucl. Mater 377 (2008) 298. [9] G.D. Tolstolutskaya, V.V. Ruzhytskiy, I.E. Kopanets, S.A. Karpov, V.V. Bryk, V. Voyevodin, F.A. Garner, J. Nucl. Mater 356 (2006) 136. [10] F.A. Garner, E.P. Simonen, B. Oliver, L. Greenwood, M.L. Grossbeck, W.G. Wolfer, P.M. Scott, J. Nucl. Mater 356 (2006) 122. [11] F.A. Garner, B. Oliver, L. Greenwood, M.R. James, P.D. Ferguson, S.A. Maloy, W. Sommer, J. Nucl. Mater 296 (2001) 66. [12] A. Kimura, R. Kasada, R. Sugano, A. Hasegawa, H. Matsui, J. Nucl. Mater 283e287 (2000) 827. [13] A. Kimura, T. Morimura, R. Kasada, H. Matsui, A. Hasegawa, K. Abe, ASTM STP 1366 (2000) 626. [14] R. Kasada, T. Morimura, H. Matsui, M. Narui, A. Kimura, ASTM STP 1366 (2000) 448. [15] A. Kimura, R. Kasada, K. Morishita, R. Sugano, A. Hasegawa, K. Abe, T. Yamamoto, H. Matsui, N. Yoshida, B.D. Wirth, T.D. Rubia, J. Nucl. Mater 307e311 (2002) 521. [16] S. Ukai, T. Nishida, T. Okuda, T. Yoshitake, J. Nucl. Mater 258e263 (1998) 1745. [17] R.L. Klueh, D.S. Gelles, S. Jitsukawa, A. Kimura, G.R. Odette, B. Vander Schaaf, M. Victoria, J. Nucl. Mater 307e311 (2002) 455. [18] ASTM E521, Annual Book of ASTM Standards, Standard Practice for Neutron Radiation Damage Simulation by Charged Particle Irradiation, 12.02, American Society for Testing and Materials, Philadelphia, PA, 1989. p. D-9. [19] T. Zhu, X.Z. Cao, S.X. Jin, J.P. Wu, Y.H. Gong, E.Y. Lu, B.Y. Wang, R.S. Yu, L. Wei, J. Nucl. Mater 466 (2015) 522. [20] D. Xu, B.D. Wirth, J. Nucl. Mater 386e388 (2009) 395. [21] R. Sugano, K. Morishita, H. Iwakiri, N. Yoshida, J. Nucl. Mater 307e311 (2002) 941. [22] Y.X. Wang, Q. Xu, T. Yoshiie, Z.Y. Pan, J. Nucl. Mater 376 (2008) 133. [23] S.R. Lee, S.M. Myers, R.G. Spulak, J. Appl. Phys. 66 (3) (1989) 1137. [24] Te Zhu, Shuoxue Jin, Liping Guo, Yuanchao Hu, Eryang Lu, Jianping Wu, Baoyi Wang, Long Wei, Xingzhong Cao, Philos. Mag. 96 (2016) 253.
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
The influence of dislocation and hydrogen on thermal helium desorption behavior in Fe9Cr alloys Te Zhu a, b, Shuoxue Jin a, **, Yihao Gong a, b, Eryang Lu a, Ligang Song a, Qiu Xu c, Liping Guo d, Xingzhong Cao a, *, Baoyi Wang a, b a
Multi-discipline Research Center, Institute of High Energy Physics, CAS, Beijing 100049, China School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100039, China Research Reactor Institute, Kyoto University, Kumatori-cho, Osaka 590-0494, Japan d School of Physics and Technology, Wuhan University, Wuhan 430072, China b c
h i g h l i g h t s Synchronous desorption of helium/hydrogen and the microstructure states were observed. Effect of dislocation on thermal helium desorption were analyzed. The existence of hydrogen will strongly affect the thermal helium desorption which could be reflected in the TDS spectrum.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 January 2017 Received in revised form 5 August 2017 Accepted 15 August 2017 Available online 16 August 2017
Transmutation helium may causes serious embrittlement which is considered to be due to helium from clustering as a bubble in materials. Suppression of transmutation helium can be achieved by introducing trapping sites such as dislocations and impurities in materials. Here, effects of intentionally-induced dislocations and hydrogen on helium migrate and release behaviors were investigated using thermal desorption spectrometry (TDS) technique applied to well-annealed and cold-worked Fe9Cr alloys irradiated by energetic helium/hydrogen ions. Synchronous desorption of helium and hydrogen was observed, and the microstructure states during helium release at different temperatures were analyzed. High thermally stable HenD type complexes formed in cold-worked specimens, resulting in the retardation of helium migration and release. The existence of hydrogen will strongly affect the thermal helium desorption which could be reflected in the TDS spectrum. It was confirmed that hydrogen retained in the specimens can result in obvious delay of helium desorption. © 2017 Elsevier B.V. All rights reserved.
Keywords: Thermal desorption Defect Helium/hydrogen Ion-irradiation
1. Introduction There is a great interest in the behavior of helium migration and retention in Plasma-facing-materials (PFMs) and structural materials for the nuclear energy systems. It is expected that the materials in nuclear reactors will suffer severe irradiation damage, and transmutation helium in the materials may accelerate undesirable materials degradation such as the swelling owe to the formation of
* Corresponding author. Multi-discipline Research Center, Institute of High Energy Physics, CAS, Beijing 100049, China. ** Corresponding author. Multi-discipline Research Center, Institute of High Energy Physics, CAS, Beijing 100049, China. E-mail addresses: [email protected] (S. Jin), [email protected] (X. Cao). http://dx.doi.org/10.1016/j.jnucmat.2017.08.027 0022-3115/© 2017 Elsevier B.V. All rights reserved.
helium bubbles [1e3]. An important feature of the irradiation process in nuclear system is the formation of large displacement cascades, in which primary knock-on atoms and secondary particles formed by nuclear reactions generate a considerable number of defects such as dislocations, vacancies and spallation products [4e6]. More importantly, the generation of insoluble helium impurities induces substantial bubble formation in materials at enough high temperatures, especially the materials for Acceleratordriven systems (ADS) where the ratio of helium generated to displacement per atom produced is more than 10 times higher than that in fusion reactors [7,8]. It is well known that helium retained in the metals significantly decreases the diffusion, permeation and release of hydrogen. Hydrogen trapping is remarkably enhanced when helium pre-
T. Zhu et al. / Journal of Nuclear Materials 495 (2017) 244e248
existed within the materials [9e11], however, we know little about the effects of hydrogen on helium release behavior up to know. Generally, the solubility for hydrogen in iron-base alloy is expected to be very low, but intrinsic and radiation-induced defects can retain a significant amount of hydrogen. Therefore, the influence of hydrogen on helium retention/desorption in materials is an important issue for nuclear reactor systems. Transmutation helium sometime causes serious embrittlement which is considered to be due to formation of helium bubbles in materials. Suppression of transmutation helium can be achieved by introducing trapping sites such as dislocations and Nano-sized oxide particles in materials [12e15]. The 9Cr-ODS steels and Ferritic-martensitic steels are leading candidates for fusion systems, advanced fission reactors and possibly for long-term spallation neutron sources used for transmutation of spallation products. Such steels, which are based on FeCr, have rather resistant to irradiation, low activation and suppress helium embrittlement [16,17]. In this study, TDS was measured for Fe9Cr alloys to investigate the influence of intentionally-induced dislocations and pre-existed hydrogen on helium retention/release behaviors. 2. Experimental procedure Fe9Cr binary alloys used in this study were made from the high purity Fe (99.99%) and Cr (99.99%) metals by arc melting process at General Research Institute for Nonferrous Metals. The bulk materials were first cut to a thickness of 0.2 mm in 10 mm 10 mm square sheets followed by well-annealed at 1100 K for 2 h in vacuum (~105 Pa). To introduce dislocations, the well-annealed specimens were respective cold rolling to 10% and 20% of their original thickness at room temperature and then annealed at 673 K for 1 h in a high vacuum to annihilate vacancies and vacancy clusters. The samples were mechanically polishing with SiC sandpapers from 600 up to 4000 Grit, followed by electro-polished with a 25%-HClO4 and 75%-CH3COOH solution to remove surface contamination before every annealing treatment. The wellannealed sample is shown in Fig. 1(a). Bright-field micrograph of the annealed specimens after cold-worked (hereafter denominated ‘CW’) are shown in Fig. 1(b) and (c), and the beam directions were close to [111]. For the CW samples, a number of dislocations induced by cold rolling were observed and more dislocations were produced in CW-20% sample. The cold-worked (CW) specimens and well-annealed specimens were irradiated with 5 keV Heþ ions using an Omegatron gun, in which mono-energetic Heþ ions were collimated and massanalyzed, at a flux of 5 1017 Heþ/m2s to the same nominal dose of 1 1020 Heþ/m2 at room temperature. The damage depth profile was calculated with the Stopping and Range of Ions in Matter (SRIM-2013) program [18] in “Detailed Calculation” mode and the
245
displacements were acquired from the vacancy.txt file. A flux of 5 keV Heþ normal to Fe9Cr alloy produces a damage profile between 1 nm and 60 nm, with a damage peak at about 10 nm. Helium concentration mainly distributed in the region from 0 to 65 nm and peak at ~25 nm (shown in Fig. 2). In addition, to investigate the effects of hydrogen on helium desorption from Fe9Cr alloys, ion-irradiation experiments were carried out: (i) Heþ irradiation, (ii) Hþ irradiation and (iii) Heþ irradiation followed by Hþ irradiation, at room temperature. Well-annealed sample was used in the irradiation experiment. The beam (with the dimension of 20 20 mm2) was scanned in both the horizontal and vertical directions to maintain the uniformity of the irradiation dose. The dose of 100 keV Heþ and 60 keV Hþ were 1 1020 Heþ/m2 (corresponding to the peak dose of 0.043 dpa) and 5 1020 Hþ/m2 (the peak dose of 0.014 dpa), respectively. The peaks for both helium and hydrogen concentration were overlap at approximately 330 nm according to the calculation of SRIM code, as shown in Fig. 3. The damage dose was up to about 0.043 dpa at the peak with the damage rate of 4.2 104 dpa/s. TDS was performed by heating the samples at 1 K/s to 1523 K using infrared irradiation. During heating, the helium release was monitored by a quadrupole mass analyzer. The pressure within the TDS chamber was reduced to below 1.0 105 Pa by vacuum pump before heating the samples.
3. Results and discussion 3.1. Effect of dislocation on thermal helium desorption Fig. 4 shows the TDS spectrum of specimen irradiated with
Fig. 2. Profiles of vacancies and helium atoms distribution in Fe9Cr alloy implanted with 5 keV He-ions calculated with SRIM.
Fig. 1. (a) TEM micrographs of the well-annealed sample, a small number of defects were observed. The image of the annealed specimens after cold-worked, a number of dislocations induced by cold rolling to 10% (b) and 20% (c) of their original thickness were observed. All the dislocations were obtained in a bright-field, with the foil tilted to [111] pole.
246
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Fig. 3. Profiles of damage and atoms distribution in Fe9Cr irradiated with 100 keV Heþ and 60 keV Hþ calculated with SRIM.
Fig. 4. Thermal helium desorption spectrum of As-annealed specimen after 5 keV Heþ implantation to 1 1020 Heþ/m2.
5 keV to a dose of 1 1020 Heþ/m2. Five desorption groups were observed at about 380e700 K, 700e1000 K, 1000e1200 K, 1200e1320 K, and greater than 1320 K, respectively. In our previous research [19], these peaks can be ascribed helium desorbing from a vacancy in the neighborhood of surface, as well as overpressurized HenV (2 n 6) clusters, dislocation networks (HenD), the influence of phase transformation from bcc to fcc, larger HenVm (n < m) clusters, respectively. Here the word ‘group’ is used instead of ‘peak’ because each ‘group’ requires more than one single dissociation event to reasonably reproduce the peak width and thus, may involve multiple dissociation mechanisms [20]. The peak at bcc/fcc transition temperature is a good example. A sharp helium desorption peak was observed for this phase change point in most of previous studies [19,20]. But the sharp desorption peaks can also be affected by many factors, such as re-adsorption, irradiation dose, becoming wide peaks (‘group’) and even shifting. In the present work, the positions of bcc/fcc transition peak 3 in Figs. 5, 6 and 7 are different. The reason is that there is a high density of dislocations in the deformed samples. Heating can promote localizes dislocation activity significantly, leading to locally enhanced vacancy formation from dislocations. These new vacancies and residual dislocation
lead to re-trapping of helium atoms, so the subsequent desorption processes are affected. Thermal helium desorption measurements were also performed with cold-worked specimens that contains high density of dislocations to investigate the effect of dislocations on thermal helium desorption. Figs. 5 and 6 show thermal helium desorption spectra from CW-10% and CW-20% samples. The groups (or peaks) in both spectra were classified into Group1, Group2, Peak3, Peak4 and Bubble, respectively. These groups are considered to be ascribed to helium desorbing from multiply filled and singly filled vacancies and their clusters, dislocations, and helium bubbles, as discussed above. The spectra counts were less desorption events for asannealed specimen than for cold-worked specimens at temperatures lower than ~1100 K, with only a weak signals observed. A comparison of helium desorption spectra obtained from CW-10% and CW-20% samples shows that the Group2 shifted to higher temperature at higher deformation. These dislocations induced by cold rolling, acted as the trapping sites, could capture the helium atoms and form HenD type complexes below 800 K [21]. Therefore, more high thermal stable HenD type complexes formed in CW-20% specimen, resulting in the retardation of helium release. Moreover, the amount of He detected in a desorption peak is associated with defect concentration and defect dissociation energy. The calculated total amounts of He (excluding the background) desorbed are approximately 8.54%, 23.79% and 8.80% of the implanted dose for As-annealed, CW-10% and CW-20%, respectively. Helium release rate for Group2 (HenD type complexes) are about 5.81%, 6.23% of the total amounts for CW-10% and CW-20%, respectively. These results indicates helium retention/release increases with the increase of the dislocation density, and it is in good agreement with the one explained theoretically by Y.X. Wang et al. [22]. Helium release rate for ‘Bubble’ are about 22.3%, 11.8%, 5.5% of the total amounts for As-annealed, CW-10% and CW-20%, respectively. It clearly shows that the more the dislocation density, the less helium bubbles are formed. In other words, dislocations can impede the migration and aggregation of helium atoms, thereby inhibiting the formation of helium bubbles. One general feature of the desorption spectra can be noticed in Fig. 6, which is all peaks except Group1 shifted to higher temperature in comparison with CW-10% sample. To explain this, it must be stated that helium cannot be completely desorbed after a thermal desorption test (i.e. the defects in the sample are not fully recovered after a thermal desorption test).
Fig. 5. Thermal helium desorption spectrum of CW-10% specimen after 5 keV Heþ implantation to 1 1020 Heþ/m2.
T. Zhu et al. / Journal of Nuclear Materials 495 (2017) 244e248
Fig. 6. Thermal helium desorption spectrum of CW-20% specimen after 5 keV Heþ implantation to 1 1020 Heþ/m2.
Heating can promote localizes dislocation activity significantly, leading to locally enhanced vacancy formation from dislocations. These new vacancies and residual dislocation lead to re-trapping of helium atoms, therefore, the motion of dislocation also influences the subsequent desorption processes (Peak4). 3.2. Effect of pre-exited hydrogen on thermal helium desorption Hydrogen binding to trap sites associated with helium is via chemisorption mechanism on HenVm clusters or helium bubbles which is much stronger than irradiated defects [23]. The existence of hydrogen will strongly affect the thermal helium desorption which can be reflected in the TDS spectrum. Fig. 7 indicates the thermal helium desorption spectrum of the specimen irradiated by 100 keV Heþ and the He þ H sequential irradiated sample. Two specimens exhibit a set of higher temperature helium peaks in the range of ~1000e1200 K, more importantly, and the positions of these higher temperature peaks are
Fig. 7. Thermal helium desorption spectra of the specimen irradiated by 100 keV Heþ (black line) and the He þ H sequential irradiated sample (red line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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almost identical for the two specimens, where the a-g phase transformation related helium desorption from the g phase occurs in Fe9Cr alloys. However, the He þ H sequential irradiated sample displays a delaying desorption peak than the single Heþ irradiated sample at temperatures lower than ~1000 K (i.e. the peak of helium desorption shifted to higher temperatures). The calculated total amounts of He (excluding the background) released are approximately 45.8% and 39.3% of the implanted dose for the single Heþ irradiated sample and the He þ H sequential irradiated sample, respectively. Typical thermal hydrogen desorption spectra for the sample irradiated by 60 keV Hþ and the He þ H sequential irradiated one are shown in Fig. 8. When comparing between the TDS spectra, both curves are characterized by one main desorption peak at 876 K and 1034 K. During sequential 100 keV Heþ and Hþ irradiations, the preliminary creation of damage did cause significant changes in TDS spectra except for some redistribution of peak intensities and the appearance of new small desorption groups at temperatures lower than ~687 K (Fig. 8, blue curve). More importantly, the single Hþ irradiated specimen displays much stronger signals than the He þ H sequential irradiated sample at temperatures higher than ~923 K. In our previous research [24], slow positron beam results revealed that He/H-ions implanted into the samples would combine with vacancies to form (He, H)-V clusters, and the synergistic effect of post-irradiation of H ions on HenVm clusters was helping them to form He-H-V clusters. For the thermal helium desorption spectra of the He þ H specimen, the temperature range of 835e946 K is related to the formation of larger clusters HenVm, which caused by the much lower helium release. Implantation of hydrogen into the samples pre-irradiated by Heþ can result in obvious enhancement of clusters size and retardation of helium release which can be regarded as a consequence of hydrogen being trapped by HenVm type complexes. Thus, the preexistence of hydrogen in the specimens may result in obvious delay of helium release. The calculated total amounts of hydrogen (excluding the background) released are approximately 52.6% and 37.1% of the implanted dose for the single Hþ irradiated sample and the He þ H sequential irradiated sample, respectively. We can clearly find that the amount of helium and hydrogen released from the He þ H sequential irradiated sample is less than that desorbed from the single He/H-ions irradiated sample. This also indicates that hydrogen and helium combine with each other during thermal desorption to inhibit their release.
Fig. 8. Typical thermal hydrogen desorption spectra for the sample irradiated by 60 keV Hþ (black line) and the He þ H sequential irradiated sample (blue line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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4. Conclusion Thermal desorption measurements were performed to examined the influence of dislocation and hydrogen on helium releases from Fe9Cr binary alloys. Synchronous desorption of helium/ hydrogen and the microstructure states were analyzed by TDS with the different helium release temperatures. High thermally stable HenD type complexes formed in cold-worked specimens, resulting in the retardation of helium migrate/release. The existence of hydrogen in the specimens pre-irradiated by Hþ can result in obvious delay of helium release. Acknowledgments All authors acknowledge support from the National Natural Science foundation of China Grant Nos. 11475193, 11505205 and 11505192. References [1] H. Schroeder, P.J. Batfalsky, J. Nucl. Mater 117 (1983) 287. [2] L.K. Mansur, W.A. Coghlan, J. Nucl. Mater 119 (1) (1983) 1. [3] V.N. Chernikov, A.P. Zakharov, P.R. Kazansky, J. Nucl. Mater 155e157 (1988) 1142. [4] T. Yoshiie, Y. Satoh, M. Kawai, in: J. Wagemans, H.A. Abderrahim, P. D'hondt, C.D. Raedt (Eds.), Proceedings of the 11th International Symposium on Reactor Dosimetry, Aug. 18-23, 2002, p. 93. Brussels, Belgium. [5] Q. Xu, T. Ishizaki, K. Sato, T. Yoshiie, S. Nagata, Mater. Trans. 47 (2006) 2885.
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