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Effects of helium and hydrogen on radiation-induced microstructural changes in austenitic stainless steel
Nuclear Instruments and Methods in Physics Research B 359 (2015) 69–74
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Effects of helium and hydrogen on radiation-induced microstructural changes in austenitic stainless steel Hyung-Ha Jin ⇑, Eunsol Ko, Sangyeop Lim, Junhyun Kwon Nuclear Materials Safety Research Division, Korea Atomic Energy Research Institute, 989-111 Daedeok-daero, Yuseong-gu, Daejeon 305-353, Republic of Korea
a r t i c l e
i n f o
Article history: Received 11 May 2015 Received in revised form 3 July 2015 Accepted 16 July 2015 Available online 28 July 2015 Keywords: Radiation induced microstructural changes Ion irradiation TEM analysis Segregation Defect
a b s t r a c t Microstructural changes in austenitic stainless steel by helium, hydrogen, and iron ion irradiation were investigated with transmission electron microscopy. Typical radiation-induced changes, such as the formation of Frank loops in the matrix and radiation-induced segregation (RIS) or depletion at grain boundaries, were observed after ion irradiation. The helium ion irradiation led to the formation of cavities both at grain boundaries and in the matrix, as well as the development of smaller Frank loops. The hydrogen ion irradiation generated stronger RIS behavior at the grain boundaries compared to irradiation with helium and iron ions. The effects of helium and hydrogen on radiation-induced microstructural changes were discussed. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Stress corrosion cracking (SCC) by neutron irradiation is a significant concern for materials used in commercial nuclear reactors under extended operation [1–3]. Irradiation-assisted stress corrosion cracking (IASCC) is known to be closely related to microstructural changes caused by radiation, such as the formation of defects in the metallic matrix and the development of radiation-induced segregation (RIS) at grain boundaries, which depletes chromium and enriches nickel, silicon, and phosphorus. The depletion of chromium at grain boundaries was thought to be identified as a primary factor for IASCC susceptibility in commercial boiling-water nuclear reactors (BWR) [4]. However, the RIS phenomenon at the grain boundary was recognized as a supplementary or minor factor for affecting IASCC susceptibility in pressurized-water nuclear reactors (PWR) [3,5,6]. Localized deformation behavior by the existence of radiation defects is possibly a strong contributor to IASCC. Localized deformation is known to promote dislocation pileups at grain boundaries, resulting in the initiation of SCC via the rupture of the surface oxide film [7,8]. Recently, the retention of helium and hydrogen was identified as another possible factor affecting the IASCC susceptibility of nuclear internals [9,10]. In one experiment [9], austenitic stainless steels irradiated in a PWR had higher hydrogen (3500 appm)/helium (600 appm) gas concentrations and lower IASCC susceptibilities compared to those irradiated in a fast ⇑ Corresponding author. Tel.: +82 42 868 4790; fax: +82 42 868 8549. E-mail address: [email protected] (H.-H. Jin). http://dx.doi.org/10.1016/j.nimb.2015.07.086 0168-583X/Ó 2015 Elsevier B.V. All rights reserved.
breeder reactor (FBR). Based on these experimental results, it was proposed that hydrogen and helium may also affect the IASCC susceptibility of PWR core internal materials, in addition to RIS formation and radiation-induced hardening behavior. The effect of helium irradiation on radiation-induced defect microstructures and the corresponding hardening behavior in austenitic stainless steel were investigated by Lee et al. [11–13] and Hunn et al. [14]. These studies revealed that additional helium-induced hardening developed due to the formation of helium-filled cavities. Helium bubbles were found to dramatically affect the evolution of radiation-induced microstructures. These investigations focused on the effect of helium on the evolution of radiation defects and the changes in mechanical properties resulting from a certain radiation environment that produces considerable quantities of transmutation products, such as hydrogen and helium. In this work, we provide additional information on the effects of helium and hydrogen on the evolution of RIS, as well as radiation defects in austenitic stainless steel. We characterize the microstructure of austenitic stainless steels irradiated with helium, hydrogen, and iron ions. Transmission electron microscopy (TEM) was used to characterize the microstructural changes in these irradiated metal samples. 2. Experimental Commercial SS316 austenitic stainless steel was used for this work, with a composition by weight of 10.8% nickel, 16.7%
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chromium, 2.0% molybdenum, 1.3% manganese, 0.047% carbon, and 0.59% silicon, with the balance in iron. As impurities, 0.05% phosphorus and 0.001% sulfur were present. Mechanical polishing was performed using suspensions of diamond with sizes of 3 lm and 0.25 lm. Fine polishing was performed with a vibratory polisher (Vibromet 2) using colloidal silica (0.02 lm) to minimize any surface damage generated in the previous polishing steps. Ion irradiation experiments were performed using a multi-purpose ion implanter at the Korea Institute of Geoscience & Mineral Resources (KIGAM). Helium ions (He+) or hydrogen ions (H+2) were used in multiple energies ranges, from 490 keV to 50 or 100 keV, for the development of uniform radiation damage (5 displacements per atom, or dpa) and implanted ion concentrations in the ion-irradiated samples. Another ion irradiation was performed using a Tandem ion accelerator with 8 MeV iron ions (Fe4+). The ion energies and fluences for all ion irradiation experiments are shown in Table 1. The irradiation temperature was set to 400 °C. Fig. 1 shows the radiation damage profile and concentration of residual implanted ions after irradiations with helium and hydrogen ion, calculated by the Stopping Range of Ions and Matter (SRIM) computer program [15,16]. The radiation damage profile resulting from the single iron irradiation is also displayed in Fig. 1(c). Since the ion-irradiated layers were calculated to be approximately 1–2 lm in depth, depending on the irradiation media, as shown in Fig. 1, a focused ion beam (FIB) technique was applied to fabricate cross-sectional TEM samples from selected regions in the ion-irradiated layer. The TEM lamellae were treated with a low-energy Ar-ion milling after the high-energy Ga-ion milling with FIB [17]. A TEM (JEOL 2100F) equipped with an energy-dispersive spectrometry (EDS) system was used to analyze radiation-induced defects and RIS at the grain boundaries of the steel samples. To minimize the effect of specimen drift under EDS analysis, the site-lock option available in the EDS system (INCA, Oxford) was used during the analysis. To measure the thickness of the TEM samples to quantify the results of radiation defects, electron energy loss spectroscopy (EELS) was conducted during the TEM observation. Most researchers have used the reciprocal lattice rod (rel-rod) dark-field imaging method to quantify Frank loops observed in irradiated austenitic stainless steels. However, TEM bright-field and high-resolution imaging was used to analyze Frank loops in this work, because of the thinner radiation damage layer present in the TEM sample. Out-of-focus contrast condition was used to observe small cavities. We also conducted the inside–outside contrast technique in order to determine the nature of dislocation loop [18].
Fig. 1. Radiation-induced damage and residual implanted ion concentration introduced by ion bombardment as a function of depth, calculated with SRIM [15]. A displacement energy of 40 eV was used for the calculation [16]. The calculation was performed in full cascade mode.
3. Results and discussion Low-magnification TEM images of the irradiated austenitic stainless steels are shown in Fig. 2. Radiation damage is present uniformly up to a depth of approximately 1 lm in the
helium-implanted sample. The experimentally measured depth of the radiation damage is similar to that calculated by SRIM. In the case of hydrogen implantation, the radiation damage seems to be formed up to a depth of approximately 0.8 lm, somewhat less than
Table 1 Ion energies and corresponding fluences. 1
2
3
4
5
6
7
He
Energy (keV) Fluence (ion/cm2) Dose rate (dpa/s)
490 5E16 2.5E-4
400 5E16
300 5E16
200 3E16
150 3E16
100 3E16
50 3E16
H
Energy (keV) Fluence (ion/cm2) Dose rate (dpa/s)
490 7E16 4.2E-4
450 4E16
400 5E16
350 4E16
300 17E16
250 9E16
200 4E16
Fe
Energy(keV) Fluence (ion/cm2) Dose rate (dpa/s)
8000 5.8E15 3.5E-4 (at a depth of 1 lm)
8
9
150 5E16
100 4E16
H.-H. Jin et al. / Nuclear Instruments and Methods in Physics Research B 359 (2015) 69–74
71
Fig. 2. Low-magnification TEM images showing the radiation damage layer in the case of helium (a), hydrogen (b), and iron ion irradiation (c).
Fig. 3. TEM images showing Frank loops in helium- (a), hydrogen- (b), and iron- (c) irradiated samples.
Fig. 4. Bright field TEM micrographs in iron- (a and b) and hydrogen- (c and d) irradiated samples by the inside–outside contrast technique for determination of nature of the Frank loop. In (a) and (b), the zone axis (beam direction) is close to [1 1 1], and in (c) and (d) it is close to [2 3 3]. The Burgers vector of A loop in (a and b) is measured to be b= ±1/3 [111] using invisibility criterion. Since the image of A loop is larger (outside contrast) when g=[202], the upward drawn normal, n, is [111] and hence the A loop is using invisibility criterion. Since the image of ‘‘B’’ loop is larger when g=[31 1], the upward interstitial. The Burgers vector of B loop in (c and d) is analyzed to be b= ±1/3 [111] and hence the B loop is interstitial. drawn normal, n, is [111]
that calculated by SRIM. The radiation-induced defects seem to be distributed uniformly in the radiation damage layer of both irradiated samples. In the case of the single iron-ion irradiation, the population of radiation defects increases gradually up to a
depth of approximately 2 lm and there reaches its highest concentration, consistent with SRIM calculations shown in Fig. 1(c). In the austenitic stainless steels irradiated with helium, hydrogen, and Fe ions, dislocation loops are observed in all matrices, as
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Table 2 Quantitative analysis of radiation induced defects. Average diameter (nm) Loop
Cavity
Number density (/m3) Loop
Cavity
He
8.2 ± 3.2
3.0 ± 0.5
3.5E23 ± 1.5E23
4.8E23 ± 3.2E22
H
23 ± 8.5
11.7 ± 3.7
3.1E22 ± 4.8E21
1.6E21 ± 8.1E20
Fe
17 ± 0.6
–
2.0E22 ± 8.6E21
–
shown in Fig. 3. Most dislocation loops are identified as faulted Frank loops with Burgers vectors of 1/3<111>. We conducted the inside–outside contrast technique in order to determine the nature of dislocation loop. It was observed that dislocation loops in the iron-irradiated and hydrogen irradiated samples have interstitial type in nature by inside–outside contrast technique as shown in Fig. 4. The nature of Frank loops in the helium-irradiated samples was not determined because of limitation of the size of dislocation loops for inside–outside contrast techniques. The average diameters and average number densities of the Frank loops were measured from TEM images taken near the <110> zone axis and listed in Table 2. In the case of iron-ion irradiation, the Frank loops were analyzed at a depth of approximately 1 lm, where the amount of radiation damage was calculated to be approximately 5 dpa, as shown in Fig. 1(c). As previously mentioned, the samples irradiated with helium and hydrogen samples have equivalent radiation damage amounts of approximately 5 dpa in their radiation damage layers. Many cavities were observed in the helium- and hydrogen-irradiated samples, as shown in Fig. 5. In particular, the helium-irradiated sample is observed to contain nanometer-sized cavities in the steel matrix. These are observed along the grain boundaries in the helium-implanted sample. This is consistent with the results of neutron-irradiated austenitic stainless steel [9,10]. The size and number density of cavities in the irradiated samples are also shown in Table 2.
The helium-irradiated sample has a higher density of smaller Frank loops than the samples irradiated with other species. The number density of cavities is also measured to be highest in the helium irradiated sample. No cavities are observed in the iron-irradiated sample. In comparing the TEM observations shown in Figs. 3 and 5, helium irradiation is seen to cause the development of nanometer-sized cavities and smaller Frank loops. The size and density of Frank loops in austenitic stainless steel have been known to reach saturation at levels above only a few dpa. In this work, the experimental samples have radiation damage of 5 dpa with similar dose rates (10 4–10 3 dpa/s), where loop sizes and densities are expected to become stable. However, these samples exhibit different microstructural evolution patterns within their respective media despite similar levels of radiation damage. It was observed that the iron-irradiated sample contained interstitial type Frank loops in the matrix as shown in Fig. 4. This result indicates that its vacancies remained as mono-vacancies, di-vacancies or very fine vacancy clusters, which were invisible to detection by TEM. It is thought that vacancies among the point defects produced at the irradiation temperature (400 °C) have insufficient mobility to form large clusters or voids visible in TEM (such as vacancy type dislocation loops or stacking fault tetrahedral). Since the formation of voids in austenitic stainless steel has been recognized to start on the order of 101 dpa, greater amounts of radiation damage may be necessary for the formation of voids in the iron-irradiated sample. Conversely, small cavities were observed in the helium- and hydrogen-irradiated samples. The hydrogen-irradiated sample contained some cavities with larger diameters in the matrix, but its cavity density was measured to be significantly lower than that of the helium-irradiated sample. The clear development of nanometer-sized cavities in the helium-irradiated sample is attributed to the slower motion of helium atoms compared to that of hydrogen atoms in the matrix; helium atoms also stabilize vacancies during the irradiation process [19]. Thus, the implanted helium atoms form nanometer-sized cavities with vacancies in the matrix of the
Fig. 5. Bright field TEM micrographs in under focused contrast condition showing cavity formation in the helium-irradiated sample (a) and the hydrogen irradiated sample (c). High-magnification TEM micrographs (b and d) showing Frank loops in the helium-irradiated sample (b) and the hydrogen irradiated sample (d), which were taken near [1 0 1] zone axis.
H.-H. Jin et al. / Nuclear Instruments and Methods in Physics Research B 359 (2015) 69–74
73
Fig. 6. TEM images and EDS elemental analysis results across grain boundary in helium- (a), hydrogen- (b), and iron- (c) irradiated austenitic stainless steels. EDS analysis in the iron-irradiated sample was performed at a depth of approximately 1 lm.
helium-irradiated sample. It is clear that the cavities observed in both the helium- and hydrogen-irradiated samples are not voids consisting of only vacancies, but bubbles formed by helium atoms or hydrogen atoms in addition to vacancies. As previously mentioned, the microstructure of the helium-irradiated sample is dominated by a higher density of smaller interstitial Frank loops as well as cavities. One hypothesis is that the helium/vacancy filled cavities in the matrix are developed in the early stages of irradiation, before the Frank loops
become more prevalent. In the early stages, the vacancies are expected to be gradually absorbed by cavities for the formation and growth of cavities with implanted helium. Therefore, it is possible that a higher density of freely migrated interstitials is involved in the strong formation of Frank loops under helium irradiation, despite the fact that the total population of point defects (interstitials and vacancies) is expected to be the same in the helium-, hydrogen- and iron-irradiated samples. After this stage, the growth of Frank loops can be suppressed because of the
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enhanced recombination of point defects, such as vacancies and interstitials at the nanometer-sized cavities. In general, interstitials as well as vacancies have been known to annihilate at sink sites such as cavities, grain boundaries, dislocations, and precipitates. These densely placed cavities may also impede the merging behavior between adjacent Frank loops by pinning these defects at the helium cavities. Additionally, interstitial type dislocation loops may be produced by over-pressurized helium bubbles, which have been experimentally observed by TEM in molybdenum and iron alloys [20,21]. Hence, it is concluded that the formation of nanometer-sized cavities plays an important role in the unique microstructural changes that result in a high density of smaller Frank loops in austenitic stainless steel during helium irradiation. The EDS profiles in Fig. 6 indicate the depletion of chromium and enrichment of nickel at each grain boundary. According to the EDS profiles in Fig. 6, the nickel alloying element is enriched most noticeably in the case of the hydrogen-irradiated sample, reaching a peak of 30 wt% compared to 15 wt% and 18 wt% for the helium- and iron-irradiated samples, respectively. Chromium levels were measured at approximately 10 wt% in the hydrogen-irradiated sample. This strong RIS at the grain boundary is most noticeable after irradiation with hydrogen. The RIS process is known to be closely related to the flow of vacancies or interstitials to the grain boundaries [22,23]. According to TEM observations of the hydrogen-irradiated sample, larger interstitial type Frank loops were observed in hydrogen-irradiated samples than in the iron-irradiated and the helium-irradiated samples. These TEM results demonstrate that implanted hydrogen atoms promote the flow of migrated interstitials to Frank loops. Thus, it is suggested that the implanted hydrogen atoms preferentially form mobile solute complexes with nickel, which migrate as solute interstitials to develop the strong RIS observed at grain boundaries. However, further investigations are required to fully understand the effect of hydrogen on the RIS process in irradiated austenitic stainless steel.
nanometer-sized cavities is closely related with the implantation of helium and play key role in development of a high density of small Frank loops. (2) The enrichment of nickel and depletion of chromium at the grain boundaries of the austenitic stainless steel were most strongly developed under hydrogen irradiation. Although the mechanism of RIS is not well understood, the implanted hydrogen may promote the RIS process by forming solute interstitial complexes with nickel present in the steel.
Acknowledgements This work is supported by the National Research Foundation of Korea (NRF-2012M2A8A4025886) grant funded by Ministry of Science, ICT and Future Planning of the Korea government. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
[10] [11] [12] [13] [14] [15] [16]
4. Summary [17]
Radiation-induced microstructural changes in austenitic stainless steel after irradiation by helium, hydrogen, and iron ions were investigated by TEM imaging. (1) The highest density of Frank loops and cavities developed in the austenitic stainless steel under helium irradiation. The present results clearly indicate that the evolution of
[18] [19] [20] [21] [22] [23]
E.P. Simonen, S.M. Brummer, JOM 50 (1998) 52–55. E.A. Kenik, J.T. Busby, Mater. Sci. Eng. R 73 (2012) 67–83. P. Scott, J. Nucl. Mater. 211 (1994) 101–122. G.S. Was, S.M. Bruemmer, J. Nucl. Mater. 216 (1994) 326–347. H.M. Chung, W.E. Ruther, J.E. Sanecki, A. Hins, N.J. Zaluze, T.F. Kassner, J. Nucl. Mater. 239 (1996) 61–79. S.M. Bruemmer, E.P. Simonen, P.M. Scott, P.L. Andresen, G.S. Was, J.L. Nelson, J. Nucl. Mater. 274 (1999) 299–314. G.S. Was, J.T. Busby, Philos. Mag. 85 (2005) 443–465. Z. Jiao, G.S. Was, J. Nucl. Mater. 408 (2011) 246–256. K. Fujimoto, T. Yonezawa, E. Wachi, Y. Yamaguchi, M. Nakano, R.P. Shogan, J. P. Massoud, T. R. Mager, in: T.R. Allen, P.J. King, L. Nelson (Eds.), Proceedings of the 12th international Conference on Environmental Degradation of Materials in Nuclear Power system, 2005, pp. 299–310. D.J. Edwards, F.A. Garnder, S.M. Bruemmer, P. Efsing, J. Nucl. Mater. 384 (2009) 249–255. E.H. Lee, J.D. Hunn, T.S. Byun, L.K. Mansur, J. Nucl. Mater. 280 (2000) 18–24. E.H. Lee, L.K. Mansur, Metall. Trans. A 23 (1992) 1977–1986. E.H. Lee, N.H. Packan, M.B. Lewis, L.K. Mansur, Nucl. Mater. Methods B 16 (1986) 251–259. J.D. Hunn, E.H. Lee, T.S. Byun, L.K. Mansur, J. Nucl. Mater. 282 (2000) 131–136. J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. ASTM Standard E693-01, Standard practice for characterizing neutron exposure in iron and low alloy steels in terms of displacements per atom (dpa), 2001. H.-H. Jin, H.D. Cho, S.-C. Kwon, C. Shin, J. Kwon, Mater. Lett. 89 (2012) 133– 136. M.H. Loretto, R.E. Smallman, Defect Analysis in Electron Microscopy, Chapman and Hall Ltd., London, 1975. L.K. Mansur, J. Nucl. Mater. 216 (1994) 97–123. J.H. Evans, A. van Venn, L.M. Caspers, Nature 291 (1981) 310–312. J. Chen, P. Jung, W. Hoffelner, H. Ullmaier, Acta Mater. 56 (2008) 250–258. T. Allen, J.T. Busby, G.S. Was, E.A. Kenik, J. Nucl. Mater. 225 (1998) 44–58. J.M. Perks, S.M. Murphy, Materials for Nuclear Reactor Core Applications, vol. 1, BNES, London, 1987. 117.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Effects of helium and hydrogen on radiation-induced microstructural changes in austenitic stainless steel Hyung-Ha Jin ⇑, Eunsol Ko, Sangyeop Lim, Junhyun Kwon Nuclear Materials Safety Research Division, Korea Atomic Energy Research Institute, 989-111 Daedeok-daero, Yuseong-gu, Daejeon 305-353, Republic of Korea
a r t i c l e
i n f o
Article history: Received 11 May 2015 Received in revised form 3 July 2015 Accepted 16 July 2015 Available online 28 July 2015 Keywords: Radiation induced microstructural changes Ion irradiation TEM analysis Segregation Defect
a b s t r a c t Microstructural changes in austenitic stainless steel by helium, hydrogen, and iron ion irradiation were investigated with transmission electron microscopy. Typical radiation-induced changes, such as the formation of Frank loops in the matrix and radiation-induced segregation (RIS) or depletion at grain boundaries, were observed after ion irradiation. The helium ion irradiation led to the formation of cavities both at grain boundaries and in the matrix, as well as the development of smaller Frank loops. The hydrogen ion irradiation generated stronger RIS behavior at the grain boundaries compared to irradiation with helium and iron ions. The effects of helium and hydrogen on radiation-induced microstructural changes were discussed. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Stress corrosion cracking (SCC) by neutron irradiation is a significant concern for materials used in commercial nuclear reactors under extended operation [1–3]. Irradiation-assisted stress corrosion cracking (IASCC) is known to be closely related to microstructural changes caused by radiation, such as the formation of defects in the metallic matrix and the development of radiation-induced segregation (RIS) at grain boundaries, which depletes chromium and enriches nickel, silicon, and phosphorus. The depletion of chromium at grain boundaries was thought to be identified as a primary factor for IASCC susceptibility in commercial boiling-water nuclear reactors (BWR) [4]. However, the RIS phenomenon at the grain boundary was recognized as a supplementary or minor factor for affecting IASCC susceptibility in pressurized-water nuclear reactors (PWR) [3,5,6]. Localized deformation behavior by the existence of radiation defects is possibly a strong contributor to IASCC. Localized deformation is known to promote dislocation pileups at grain boundaries, resulting in the initiation of SCC via the rupture of the surface oxide film [7,8]. Recently, the retention of helium and hydrogen was identified as another possible factor affecting the IASCC susceptibility of nuclear internals [9,10]. In one experiment [9], austenitic stainless steels irradiated in a PWR had higher hydrogen (3500 appm)/helium (600 appm) gas concentrations and lower IASCC susceptibilities compared to those irradiated in a fast ⇑ Corresponding author. Tel.: +82 42 868 4790; fax: +82 42 868 8549. E-mail address: [email protected] (H.-H. Jin). http://dx.doi.org/10.1016/j.nimb.2015.07.086 0168-583X/Ó 2015 Elsevier B.V. All rights reserved.
breeder reactor (FBR). Based on these experimental results, it was proposed that hydrogen and helium may also affect the IASCC susceptibility of PWR core internal materials, in addition to RIS formation and radiation-induced hardening behavior. The effect of helium irradiation on radiation-induced defect microstructures and the corresponding hardening behavior in austenitic stainless steel were investigated by Lee et al. [11–13] and Hunn et al. [14]. These studies revealed that additional helium-induced hardening developed due to the formation of helium-filled cavities. Helium bubbles were found to dramatically affect the evolution of radiation-induced microstructures. These investigations focused on the effect of helium on the evolution of radiation defects and the changes in mechanical properties resulting from a certain radiation environment that produces considerable quantities of transmutation products, such as hydrogen and helium. In this work, we provide additional information on the effects of helium and hydrogen on the evolution of RIS, as well as radiation defects in austenitic stainless steel. We characterize the microstructure of austenitic stainless steels irradiated with helium, hydrogen, and iron ions. Transmission electron microscopy (TEM) was used to characterize the microstructural changes in these irradiated metal samples. 2. Experimental Commercial SS316 austenitic stainless steel was used for this work, with a composition by weight of 10.8% nickel, 16.7%
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H.-H. Jin et al. / Nuclear Instruments and Methods in Physics Research B 359 (2015) 69–74
chromium, 2.0% molybdenum, 1.3% manganese, 0.047% carbon, and 0.59% silicon, with the balance in iron. As impurities, 0.05% phosphorus and 0.001% sulfur were present. Mechanical polishing was performed using suspensions of diamond with sizes of 3 lm and 0.25 lm. Fine polishing was performed with a vibratory polisher (Vibromet 2) using colloidal silica (0.02 lm) to minimize any surface damage generated in the previous polishing steps. Ion irradiation experiments were performed using a multi-purpose ion implanter at the Korea Institute of Geoscience & Mineral Resources (KIGAM). Helium ions (He+) or hydrogen ions (H+2) were used in multiple energies ranges, from 490 keV to 50 or 100 keV, for the development of uniform radiation damage (5 displacements per atom, or dpa) and implanted ion concentrations in the ion-irradiated samples. Another ion irradiation was performed using a Tandem ion accelerator with 8 MeV iron ions (Fe4+). The ion energies and fluences for all ion irradiation experiments are shown in Table 1. The irradiation temperature was set to 400 °C. Fig. 1 shows the radiation damage profile and concentration of residual implanted ions after irradiations with helium and hydrogen ion, calculated by the Stopping Range of Ions and Matter (SRIM) computer program [15,16]. The radiation damage profile resulting from the single iron irradiation is also displayed in Fig. 1(c). Since the ion-irradiated layers were calculated to be approximately 1–2 lm in depth, depending on the irradiation media, as shown in Fig. 1, a focused ion beam (FIB) technique was applied to fabricate cross-sectional TEM samples from selected regions in the ion-irradiated layer. The TEM lamellae were treated with a low-energy Ar-ion milling after the high-energy Ga-ion milling with FIB [17]. A TEM (JEOL 2100F) equipped with an energy-dispersive spectrometry (EDS) system was used to analyze radiation-induced defects and RIS at the grain boundaries of the steel samples. To minimize the effect of specimen drift under EDS analysis, the site-lock option available in the EDS system (INCA, Oxford) was used during the analysis. To measure the thickness of the TEM samples to quantify the results of radiation defects, electron energy loss spectroscopy (EELS) was conducted during the TEM observation. Most researchers have used the reciprocal lattice rod (rel-rod) dark-field imaging method to quantify Frank loops observed in irradiated austenitic stainless steels. However, TEM bright-field and high-resolution imaging was used to analyze Frank loops in this work, because of the thinner radiation damage layer present in the TEM sample. Out-of-focus contrast condition was used to observe small cavities. We also conducted the inside–outside contrast technique in order to determine the nature of dislocation loop [18].
Fig. 1. Radiation-induced damage and residual implanted ion concentration introduced by ion bombardment as a function of depth, calculated with SRIM [15]. A displacement energy of 40 eV was used for the calculation [16]. The calculation was performed in full cascade mode.
3. Results and discussion Low-magnification TEM images of the irradiated austenitic stainless steels are shown in Fig. 2. Radiation damage is present uniformly up to a depth of approximately 1 lm in the
helium-implanted sample. The experimentally measured depth of the radiation damage is similar to that calculated by SRIM. In the case of hydrogen implantation, the radiation damage seems to be formed up to a depth of approximately 0.8 lm, somewhat less than
Table 1 Ion energies and corresponding fluences. 1
2
3
4
5
6
7
He
Energy (keV) Fluence (ion/cm2) Dose rate (dpa/s)
490 5E16 2.5E-4
400 5E16
300 5E16
200 3E16
150 3E16
100 3E16
50 3E16
H
Energy (keV) Fluence (ion/cm2) Dose rate (dpa/s)
490 7E16 4.2E-4
450 4E16
400 5E16
350 4E16
300 17E16
250 9E16
200 4E16
Fe
Energy(keV) Fluence (ion/cm2) Dose rate (dpa/s)
8000 5.8E15 3.5E-4 (at a depth of 1 lm)
8
9
150 5E16
100 4E16
H.-H. Jin et al. / Nuclear Instruments and Methods in Physics Research B 359 (2015) 69–74
71
Fig. 2. Low-magnification TEM images showing the radiation damage layer in the case of helium (a), hydrogen (b), and iron ion irradiation (c).
Fig. 3. TEM images showing Frank loops in helium- (a), hydrogen- (b), and iron- (c) irradiated samples.
Fig. 4. Bright field TEM micrographs in iron- (a and b) and hydrogen- (c and d) irradiated samples by the inside–outside contrast technique for determination of nature of the Frank loop. In (a) and (b), the zone axis (beam direction) is close to [1 1 1], and in (c) and (d) it is close to [2 3 3]. The Burgers vector of A loop in (a and b) is measured to be b= ±1/3 [111] using invisibility criterion. Since the image of A loop is larger (outside contrast) when g=[202], the upward drawn normal, n, is [111] and hence the A loop is using invisibility criterion. Since the image of ‘‘B’’ loop is larger when g=[31 1], the upward interstitial. The Burgers vector of B loop in (c and d) is analyzed to be b= ±1/3 [111] and hence the B loop is interstitial. drawn normal, n, is [111]
that calculated by SRIM. The radiation-induced defects seem to be distributed uniformly in the radiation damage layer of both irradiated samples. In the case of the single iron-ion irradiation, the population of radiation defects increases gradually up to a
depth of approximately 2 lm and there reaches its highest concentration, consistent with SRIM calculations shown in Fig. 1(c). In the austenitic stainless steels irradiated with helium, hydrogen, and Fe ions, dislocation loops are observed in all matrices, as
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H.-H. Jin et al. / Nuclear Instruments and Methods in Physics Research B 359 (2015) 69–74
Table 2 Quantitative analysis of radiation induced defects. Average diameter (nm) Loop
Cavity
Number density (/m3) Loop
Cavity
He
8.2 ± 3.2
3.0 ± 0.5
3.5E23 ± 1.5E23
4.8E23 ± 3.2E22
H
23 ± 8.5
11.7 ± 3.7
3.1E22 ± 4.8E21
1.6E21 ± 8.1E20
Fe
17 ± 0.6
–
2.0E22 ± 8.6E21
–
shown in Fig. 3. Most dislocation loops are identified as faulted Frank loops with Burgers vectors of 1/3<111>. We conducted the inside–outside contrast technique in order to determine the nature of dislocation loop. It was observed that dislocation loops in the iron-irradiated and hydrogen irradiated samples have interstitial type in nature by inside–outside contrast technique as shown in Fig. 4. The nature of Frank loops in the helium-irradiated samples was not determined because of limitation of the size of dislocation loops for inside–outside contrast techniques. The average diameters and average number densities of the Frank loops were measured from TEM images taken near the <110> zone axis and listed in Table 2. In the case of iron-ion irradiation, the Frank loops were analyzed at a depth of approximately 1 lm, where the amount of radiation damage was calculated to be approximately 5 dpa, as shown in Fig. 1(c). As previously mentioned, the samples irradiated with helium and hydrogen samples have equivalent radiation damage amounts of approximately 5 dpa in their radiation damage layers. Many cavities were observed in the helium- and hydrogen-irradiated samples, as shown in Fig. 5. In particular, the helium-irradiated sample is observed to contain nanometer-sized cavities in the steel matrix. These are observed along the grain boundaries in the helium-implanted sample. This is consistent with the results of neutron-irradiated austenitic stainless steel [9,10]. The size and number density of cavities in the irradiated samples are also shown in Table 2.
The helium-irradiated sample has a higher density of smaller Frank loops than the samples irradiated with other species. The number density of cavities is also measured to be highest in the helium irradiated sample. No cavities are observed in the iron-irradiated sample. In comparing the TEM observations shown in Figs. 3 and 5, helium irradiation is seen to cause the development of nanometer-sized cavities and smaller Frank loops. The size and density of Frank loops in austenitic stainless steel have been known to reach saturation at levels above only a few dpa. In this work, the experimental samples have radiation damage of 5 dpa with similar dose rates (10 4–10 3 dpa/s), where loop sizes and densities are expected to become stable. However, these samples exhibit different microstructural evolution patterns within their respective media despite similar levels of radiation damage. It was observed that the iron-irradiated sample contained interstitial type Frank loops in the matrix as shown in Fig. 4. This result indicates that its vacancies remained as mono-vacancies, di-vacancies or very fine vacancy clusters, which were invisible to detection by TEM. It is thought that vacancies among the point defects produced at the irradiation temperature (400 °C) have insufficient mobility to form large clusters or voids visible in TEM (such as vacancy type dislocation loops or stacking fault tetrahedral). Since the formation of voids in austenitic stainless steel has been recognized to start on the order of 101 dpa, greater amounts of radiation damage may be necessary for the formation of voids in the iron-irradiated sample. Conversely, small cavities were observed in the helium- and hydrogen-irradiated samples. The hydrogen-irradiated sample contained some cavities with larger diameters in the matrix, but its cavity density was measured to be significantly lower than that of the helium-irradiated sample. The clear development of nanometer-sized cavities in the helium-irradiated sample is attributed to the slower motion of helium atoms compared to that of hydrogen atoms in the matrix; helium atoms also stabilize vacancies during the irradiation process [19]. Thus, the implanted helium atoms form nanometer-sized cavities with vacancies in the matrix of the
Fig. 5. Bright field TEM micrographs in under focused contrast condition showing cavity formation in the helium-irradiated sample (a) and the hydrogen irradiated sample (c). High-magnification TEM micrographs (b and d) showing Frank loops in the helium-irradiated sample (b) and the hydrogen irradiated sample (d), which were taken near [1 0 1] zone axis.
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Fig. 6. TEM images and EDS elemental analysis results across grain boundary in helium- (a), hydrogen- (b), and iron- (c) irradiated austenitic stainless steels. EDS analysis in the iron-irradiated sample was performed at a depth of approximately 1 lm.
helium-irradiated sample. It is clear that the cavities observed in both the helium- and hydrogen-irradiated samples are not voids consisting of only vacancies, but bubbles formed by helium atoms or hydrogen atoms in addition to vacancies. As previously mentioned, the microstructure of the helium-irradiated sample is dominated by a higher density of smaller interstitial Frank loops as well as cavities. One hypothesis is that the helium/vacancy filled cavities in the matrix are developed in the early stages of irradiation, before the Frank loops
become more prevalent. In the early stages, the vacancies are expected to be gradually absorbed by cavities for the formation and growth of cavities with implanted helium. Therefore, it is possible that a higher density of freely migrated interstitials is involved in the strong formation of Frank loops under helium irradiation, despite the fact that the total population of point defects (interstitials and vacancies) is expected to be the same in the helium-, hydrogen- and iron-irradiated samples. After this stage, the growth of Frank loops can be suppressed because of the
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enhanced recombination of point defects, such as vacancies and interstitials at the nanometer-sized cavities. In general, interstitials as well as vacancies have been known to annihilate at sink sites such as cavities, grain boundaries, dislocations, and precipitates. These densely placed cavities may also impede the merging behavior between adjacent Frank loops by pinning these defects at the helium cavities. Additionally, interstitial type dislocation loops may be produced by over-pressurized helium bubbles, which have been experimentally observed by TEM in molybdenum and iron alloys [20,21]. Hence, it is concluded that the formation of nanometer-sized cavities plays an important role in the unique microstructural changes that result in a high density of smaller Frank loops in austenitic stainless steel during helium irradiation. The EDS profiles in Fig. 6 indicate the depletion of chromium and enrichment of nickel at each grain boundary. According to the EDS profiles in Fig. 6, the nickel alloying element is enriched most noticeably in the case of the hydrogen-irradiated sample, reaching a peak of 30 wt% compared to 15 wt% and 18 wt% for the helium- and iron-irradiated samples, respectively. Chromium levels were measured at approximately 10 wt% in the hydrogen-irradiated sample. This strong RIS at the grain boundary is most noticeable after irradiation with hydrogen. The RIS process is known to be closely related to the flow of vacancies or interstitials to the grain boundaries [22,23]. According to TEM observations of the hydrogen-irradiated sample, larger interstitial type Frank loops were observed in hydrogen-irradiated samples than in the iron-irradiated and the helium-irradiated samples. These TEM results demonstrate that implanted hydrogen atoms promote the flow of migrated interstitials to Frank loops. Thus, it is suggested that the implanted hydrogen atoms preferentially form mobile solute complexes with nickel, which migrate as solute interstitials to develop the strong RIS observed at grain boundaries. However, further investigations are required to fully understand the effect of hydrogen on the RIS process in irradiated austenitic stainless steel.
nanometer-sized cavities is closely related with the implantation of helium and play key role in development of a high density of small Frank loops. (2) The enrichment of nickel and depletion of chromium at the grain boundaries of the austenitic stainless steel were most strongly developed under hydrogen irradiation. Although the mechanism of RIS is not well understood, the implanted hydrogen may promote the RIS process by forming solute interstitial complexes with nickel present in the steel.
Acknowledgements This work is supported by the National Research Foundation of Korea (NRF-2012M2A8A4025886) grant funded by Ministry of Science, ICT and Future Planning of the Korea government. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
[10] [11] [12] [13] [14] [15] [16]
4. Summary [17]
Radiation-induced microstructural changes in austenitic stainless steel after irradiation by helium, hydrogen, and iron ions were investigated by TEM imaging. (1) The highest density of Frank loops and cavities developed in the austenitic stainless steel under helium irradiation. The present results clearly indicate that the evolution of
[18] [19] [20] [21] [22] [23]
E.P. Simonen, S.M. Brummer, JOM 50 (1998) 52–55. E.A. Kenik, J.T. Busby, Mater. Sci. Eng. R 73 (2012) 67–83. P. Scott, J. Nucl. Mater. 211 (1994) 101–122. G.S. Was, S.M. Bruemmer, J. Nucl. Mater. 216 (1994) 326–347. H.M. Chung, W.E. Ruther, J.E. Sanecki, A. Hins, N.J. Zaluze, T.F. Kassner, J. Nucl. Mater. 239 (1996) 61–79. S.M. Bruemmer, E.P. Simonen, P.M. Scott, P.L. Andresen, G.S. Was, J.L. Nelson, J. Nucl. Mater. 274 (1999) 299–314. G.S. Was, J.T. Busby, Philos. Mag. 85 (2005) 443–465. Z. Jiao, G.S. Was, J. Nucl. Mater. 408 (2011) 246–256. K. Fujimoto, T. Yonezawa, E. Wachi, Y. Yamaguchi, M. Nakano, R.P. Shogan, J. P. Massoud, T. R. Mager, in: T.R. Allen, P.J. King, L. Nelson (Eds.), Proceedings of the 12th international Conference on Environmental Degradation of Materials in Nuclear Power system, 2005, pp. 299–310. D.J. Edwards, F.A. Garnder, S.M. Bruemmer, P. Efsing, J. Nucl. Mater. 384 (2009) 249–255. E.H. Lee, J.D. Hunn, T.S. Byun, L.K. Mansur, J. Nucl. Mater. 280 (2000) 18–24. E.H. Lee, L.K. Mansur, Metall. Trans. A 23 (1992) 1977–1986. E.H. Lee, N.H. Packan, M.B. Lewis, L.K. Mansur, Nucl. Mater. Methods B 16 (1986) 251–259. J.D. Hunn, E.H. Lee, T.S. Byun, L.K. Mansur, J. Nucl. Mater. 282 (2000) 131–136. J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. ASTM Standard E693-01, Standard practice for characterizing neutron exposure in iron and low alloy steels in terms of displacements per atom (dpa), 2001. H.-H. Jin, H.D. Cho, S.-C. Kwon, C. Shin, J. Kwon, Mater. Lett. 89 (2012) 133– 136. M.H. Loretto, R.E. Smallman, Defect Analysis in Electron Microscopy, Chapman and Hall Ltd., London, 1975. L.K. Mansur, J. Nucl. Mater. 216 (1994) 97–123. J.H. Evans, A. van Venn, L.M. Caspers, Nature 291 (1981) 310–312. J. Chen, P. Jung, W. Hoffelner, H. Ullmaier, Acta Mater. 56 (2008) 250–258. T. Allen, J.T. Busby, G.S. Was, E.A. Kenik, J. Nucl. Mater. 225 (1998) 44–58. J.M. Perks, S.M. Murphy, Materials for Nuclear Reactor Core Applications, vol. 1, BNES, London, 1987. 117.