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Thermal helium desorption behavior in advanced ferritic steels
Journal of Physics and Chemistry of Solids 66 (2005) 504–508 www.elsevier.com/locate/jpcs
Thermal helium desorption behavior in advanced ferritic steels Akihiko Kimuraa,*, R. Suganob, Y. Matsushitab, S. Ukaic a Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Graduate School of Energy Science, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan c Japan Nuclear Cycle Development Institute, Tokai, Japan
b
Accepted 9 July 2004
Abstract Thermal helium desorption measurements were performed to investigate the difference in the helium trapping and accumulation behavior among a reduced activation ferritic (RAF) steel and oxide dispersion strengthening (ODS) steels after implantation of HeC ions at room temperature. Thermal helium desorption spectra (THDS) were obtained during annealing to 1200 8C at a heating rate of 1 8C/s. The THDS of the ODS steels are very similar to that of the RAF steel, except for the presence of the peak in the temperature range from 800 to 1000 8C, where the a–g transformation related helium desorption from the g-phase is considered to occur in the 9Cr-ODS martensitic steels. The fraction of helium desorption becomes larger at higher temperatures, and this trend is increased with the amount of implanted helium. In the 9Cr-ODS steels, the fraction of helium desorption by bubble migration mechanism was smaller than that in the RAF steel. This suggests that the bubble formation was suppressed in the ODS steels. In the 12Cr-ODS steel, the fraction of helium desorption by bubble migration reached more than 90%, suggesting that the trapping capacity of martensite phase in the 9Cr-ODS steel is rather large. q 2004 Elsevier Ltd. All rights reserved. Keywords: D. Defects; A. Alloys; I. Iron; N. Nono-particles
1. Introduction It is expected that the structural materials in fusion reactors will suffer severe irradiation embrittlement, and transmutation helium in the materials may accelerate the embrittlement [1]. Among the candidate fusion structural materials, reduced activation ferritic (RAF) steels have been considered to be a prime candidate structural material, because they are rather resistant to irradiation and helium embrittlement, although the high temperature strength is limited [2]. Oxide dispersion strengthening (ODS) steels have been developed for fuel claddings of fast breeder reactor where the temperature of the claddings is expected to elevate up to 700 8C [3]. In order to increase thermal efficiency of fusion reactor, improvement of high temperature strength of ferritic steels has been demanded. The application of the ODS steels for fusion blanket is considered to be very * Corresponding author. Tel.: C81 774 38 3476; fax: C81 774 38 3479. E-mail address: [email protected] (A. Kimura). 0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.07.012
promising, because they are extremely resistant to neutron irradiation [4]. Transmutation helium sometime causes severe embrittlement in metallic materials as well as ceramic materials. The helium-induced embrittlement is considered to be due to formation of helium bubbles, namely heliumvacancy clusters, in materials. In the temperature range where single vacancy is almost immobile, transmutation helium is trapped by single vacancy and helium bubbles are not formed. At higher temperatures, however, vacancies and helium atoms become mobile and aggregate into helium bubbles [5–8]. Suppression of helium bubble formation namely helium embrittlement can be achieved by introducing trapping sites for helium in materials. The 9Cr-ODS steels consist of martensitic structure and high density of nano-sized oxide particles, which are considered to be effective trapping sites for helium atoms as well as vacancies, and resultantly suppress helium embrittlement [5–8]. In this work, thermal helium desorption spectra (THDS) were measured for a RAFS and ODS steels to investigate
– 0.0033 0.0025 – 0.06 0.06 – 0.35 0.24 0.0277 0.010 0.010 0.0030 – – – 0.20 0.30 0.098 – – 0.20 – – 1.96 1.94 2.01 – 0.022 0.034 8.97 8.99 11.95 0.051 0.02 0.05 JLF-1CB M12(9Cr-ODS) F11(12Cr-ODS)
0.089 0.120 0.024
0.50 0.036 0.046
!0.002 0.003 0.002
0.0004 0.004 0.003
W Ni Cr S P Mn Si C
Table 1 Chemical compositions of the RAF steel and the ODS ferritic (12Cr) and martensitric (9Cr) steel (mass%)
V
Ta
Ti
B
N
Y2O3
Ex. O
Ar
A. Kimura et al. / Journal of Physics and Chemistry of Solids 66 (2005) 504–508
505
helium trapping behavior and evaluate the performance of the steels under fusion relevant environment. 2. Experimental The materials used in this work were a RAF steel and two sorts of ODS steels, an ODS ferritic steel and an ODS martensitic steel. The ODS ferritic steel, F11, contains 12 wt% Cr and no martensite phase exists, while the martensitic steel, M12, contains 9 wt% Cr and 0.12 wt% C, and consists of martensitic structure. The amounts of the other alloy elements were almost similar for both the steels. The chemical compositions of the steels are shown in Table 1. Two different heat treatments were performed on the 9Cr-ODS steel: (1) normalization and tempering (NT) and (2) furnace cooling (FC). Specimens for THDS measurements, which measure 10 mm by 5 mm coupon with 0.25 mm thickness, were fabricated from thick steel bars, and finally they were chemically polished in a solution of hydrofluoric acid. Helium trapping behavior in the steels was investigated by means of thermal desorption method after implantation with collimated, mass-analyzed beams of mono-energetic HeC ions at room temperature. The incident energies were 8 keV, by which atomic displacement damage takes place in iron. The helium implantation was limited in near surface area about 100 nm in depth. During the THDS measurements, the temperature was elevated up to 1200 8C at a heating-up rate of 1 8C/s. 3. Results 3.1. Implantation dose dependence Typical examples of the thermal desorption spectrum of a RAF steel (broken line) and the ODS martensitic steels-FC (dotted line) and NT (solid line) are shown in Fig. 1 after bombardment of 8 keV helium ions up to 2!1018, 2!1019 and 2!1020 HeC/m2. Generally, there were six main peaks in the THDS of each steel: peak I (100 8C), peak II (450 8C), peak III (550 8C), peak IV (800 8C), peak V (900 8C) and peak VI (1100 8C). It is worthy of notice that the peak IV was only observed in the ODS steels but not in the RAFS. This trend became much significant with increasing the amount of helium. The peak position appeared to be independent of amount of helium. According to the previous work on the THDS of pure iron [5,8], the peak in the THDS was characterized as follows: (1) peak I: desorption of helium from the surface (2) peak II: detrapping of helium from helium-vacancy pairs (3) peak III: detrapping of helium from dislocations (4) peak V: a–g transformation related desorption (5) peak VI: desorption by bubble migration
506
A. Kimura et al. / Journal of Physics and Chemistry of Solids 66 (2005) 504–508
Fig. 1. Typical examples of the thermal desorption spectrum of a RAF steel (broken line) and the ODS martensitic steels-FC (dotted line) and NT (solid line).
As for the peak IV, the mechanism will be discussed in the discussion session. In order to make clear the helium trapping strength or capacity of each steel, the fraction of helium desorption was measured for each temperature range, as shown in Fig. 2, in the rage of helium implantation from 2!1018 to 2!1021 HeC/m2. There are three characteristic features: (1) The fraction of helium desorption became larger at higher temperature range. (2) With increasing the amount of helium, the above tendency was strengthened. (3) The helium desorption at temperature range from 800 to 1000 8C was remarkably larger in the ODS steels than RAF steel. Amount of desorbed helium in each temperature range is shown in Fig. 3. Below 600 8C, amount of the desorbed helium in all the steels increases with proportional to the square root of the implantation dose, suggesting that
the weak trapping sites were easily saturated with helium. There is no difference in the desorption behavior among the steels below 600 8C. With increasing the desorption temperature, however, the dose dependence turned to a liner relationship as shown in the case between 800 and 1000 8C. This trend was accelerated in the ODS steels that show a liner dependence even at lower temperature range (600–800 8C) at a lower doses. Above 1000 8C, all the steels showed an almost liner relationship. 3.2. Effects of martensitic structure The 12Cr-ODS ferritic steel does not contain martensite structure. The THDS of the 12Cr-ODS steel implanted with 8 keV helium ions up to 2!1021 HeC/m2 is compared with those of the 9Cr-ODS steels in Fig. 4(a). The fraction of desorbed helium is shown in Fig. 4(b). As clearly shown in these figures, the spectrum of 12Cr-ODS steel is different from those of the 9Cr-ODS steels and very similar to that of the RAF steel. The helium desorption in the temperature
Fig. 2. The fraction of helium desorption measured in each temperature range at helium implantation from 2!1018 to 2!1021 HeC/m2.
A. Kimura et al. / Journal of Physics and Chemistry of Solids 66 (2005) 504–508
507
Fig. 3. Dependence of the amount of desorbed helium on the implantation dose in each temperature range in a RAF steel (dotted line) and the ODS martensitic steels-FC (broken line) and NT (solid line).
range between 800 and 1000 8C is so limited and more than 90% of helium does not desorb in the 12Cr-ODS steel until the temperature elevates above 1000 8C.
4. Discussion In the previous works, the peak I and II were interpreted in terms of surface desorption and detrapping helium from vacancies, respectively [9,10]. Since the peak I is around 100 8C, it has been considered that the helium absorbed on the specimen surface desorbed with water evaporation. The peak II was related with the vacancy migration. In the steels, vacancies are trapped at carbon atoms to form V–C pairs, which are stable at temperatures below 200 8C but decomposes into a vacancy and a carbon atom above it. It is expected that in the existence of helium, V–C–He complexes can be formed during helium implantation, and they may be stable up to around 450 8C, since our previous work [5] revealed that the helium bubble formation and growth in a RAF steel occurred only after the post-helium implantation anneal at 450 8C. This suggests that the helium trapped by V–C pairs can decompose at above 450 8C. Also, in our previous work on THDS of deformed iron, the peak III was identified as desorption from dislocations [11]. The peak V, which was observed in both the ODS steels and the RAF steel, is considered to be related with a–g transformation. In the 12Cr-ODS steel, no such a peak was observed, because the transformation does not occur in the steel. The peak VI is believed to be due to helium bubble migration. Because of the strong binding of helium in large heliumvacancy clusters, it is considered that helium is unable to desorb until the bubbles migrate to the surface. Helium bubble migration was directly observed by TEM for a thinned specimen of iron–chromium binary alloy during in situ heating in the microscope [12]. It is considered that the similar behavior was observed in the shallow area of
Fig. 4. (a) THDS of 9Cr-ODS-FC (dotted line) martensitic steel, 9Cr-ODSNT martensitic steel (broken line) and 12Cr-ODS ferritic steel (solid line) after helium implantation up to 1!1021 HeC/m2. (b) The fraction of desorbed helium in each temperature range in 9Cr-ODS-FC steel, 9Cr-ODS-NT steel and 12Cr-ODS steel after helium implantation up to 1!1021 HeC/m2.
508
A. Kimura et al. / Journal of Physics and Chemistry of Solids 66 (2005) 504–508
the bulk specimen surface as thinned TEM specimen, of which the thickness was 100 nm. The peak IV was only observed in the 9Cr-ODS steels, and not in the RAF steel and even in the 12Cr-ODS steel. Since the 12Cr-ODS steel also contains a high density of oxide particles, the peak IV is not directly attributed to the oxide particles. The martensitic structure contains carbides, precipitates, dislocations, lath boundaries, packet boundaries and grain boundaries. In RAF steels, the martensite phase becomes unstable above 600 8C, while the martensite phase in the 9Cr-ODS steels is much more stable than that in RAF steel [13]. Therefore, it is considered that the peak IV is due to helium desorption from g-phase in which the defects still remain and play a role in trapping helium to suppress the growth of helium bubbles. Finally, in the 9Cr-ODS steels, the fraction of helium desorption by bubble migration mechanism was smaller than in the RAF steel.
5. Conclusion Thermal desorption measurements were performed on a RAF steel and ODS steels after implantation with 8 keV HeC ions at room temperature. The obtained main results are: (1) The THDS of the ODS steels are very similar to that of the RAF steel, except for one peak in the temperature range from 800 to 1000 8C, where the a–g transformation related helium desorption from the g-phase occurs in the 9Cr-ODS martensitic steels. (2) The fraction of helium desorption becomes larger at higher temperatures, and this trend is strengthened by increasing the amount of implanted helium. In the 9Cr-ODS steels, the fraction of helium desorption
by bubble migration mechanism was remarkably smaller than in the RAF steel. This suggests that the bubble growth was suppressed in the 9Cr-ODS steels. (3) In the 12Cr-ODS ferritic steel, the fraction of helium desorption by bubble migration reached more than 90%, suggesting that the trapping capacity is rather large in the martensite phase of the 9Cr-ODS steel.
References [1] H. Schroeder, P.J. Batfalsky, J. Nucl. Mater. 117 (1983) 287–294. [2] R.L. Klueh, D.S. Gelles, S. Jitsukawa, A. Kimura, G.R. Odette, B. Van der Schaaf, M. Victoria, J. Nucl. Mater. 307–311 (2002) 455–465. [3] S. Ukai, T. Nishida, H. Okada, T. Okuda, M. Fujiwara, K. Asabe, J. Nucl. Sci. Technol. 34 (3) (1997) 256. [4] S. Ukai, T. Nishida, T. Okuda, T. Yoshitake, J. Nucl. Mater. 258–263 (1998) 1745. [5] A. Kimura, R. Kasada, R. Sugano, A. Hasegawa, H. Matsui, J. Nucl. Mater. 283–287 (2000) 827–831. [6] A. Kimura, T. Morimura, R. Kasada, H. Matsui, A. Hasegawa, K. Abe, ASTM STP 1366 (2000) 626–641. [7] R. Kasada, T. Morimura, H. Matsui, M. Narui, A. Kimura, ASTM STP 1366 (2000) 448–459. [8] 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. 307–311 (2002) 521–526. [9] R. Sugano, K. Morishita, A. Kimura, Proceedings of 15th Topical Meeting on the Technology of Fusion Energy (TOFE-15) Fusion Sci. Tech. 44 (2) (2003) 446–449. [10] R. Sugano, K. Morishita, H. Iwakiri, N. Yoshida, J. Nucl. Mater. 307– 311 (2002) 941–945. [11] A. Kimura, R. Kasada, K. Morishita, R. Sugano, A. Kohyama, T. Yamamoto, H. Matsui, A. Hasegaawa, K. Abe, N. Yamamoto, Proc. Fourth Pacific Rim Int. Conf. Adv. Mater. Process. 1 (2001) 1431. [12] K. Ono, K. Arakawa, K. Hojou, J. Nucl. Mater. 307–311 (2002) 1507. [13] S. Yamashita, K. Oka, S. Ohnuki, N. Akasaka, S. Ukai, J. Nud. Mater. 307–311 (2002) 283.
Thermal helium desorption behavior in advanced ferritic steels Akihiko Kimuraa,*, R. Suganob, Y. Matsushitab, S. Ukaic a Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Graduate School of Energy Science, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan c Japan Nuclear Cycle Development Institute, Tokai, Japan
b
Accepted 9 July 2004
Abstract Thermal helium desorption measurements were performed to investigate the difference in the helium trapping and accumulation behavior among a reduced activation ferritic (RAF) steel and oxide dispersion strengthening (ODS) steels after implantation of HeC ions at room temperature. Thermal helium desorption spectra (THDS) were obtained during annealing to 1200 8C at a heating rate of 1 8C/s. The THDS of the ODS steels are very similar to that of the RAF steel, except for the presence of the peak in the temperature range from 800 to 1000 8C, where the a–g transformation related helium desorption from the g-phase is considered to occur in the 9Cr-ODS martensitic steels. The fraction of helium desorption becomes larger at higher temperatures, and this trend is increased with the amount of implanted helium. In the 9Cr-ODS steels, the fraction of helium desorption by bubble migration mechanism was smaller than that in the RAF steel. This suggests that the bubble formation was suppressed in the ODS steels. In the 12Cr-ODS steel, the fraction of helium desorption by bubble migration reached more than 90%, suggesting that the trapping capacity of martensite phase in the 9Cr-ODS steel is rather large. q 2004 Elsevier Ltd. All rights reserved. Keywords: D. Defects; A. Alloys; I. Iron; N. Nono-particles
1. Introduction It is expected that the structural materials in fusion reactors will suffer severe irradiation embrittlement, and transmutation helium in the materials may accelerate the embrittlement [1]. Among the candidate fusion structural materials, reduced activation ferritic (RAF) steels have been considered to be a prime candidate structural material, because they are rather resistant to irradiation and helium embrittlement, although the high temperature strength is limited [2]. Oxide dispersion strengthening (ODS) steels have been developed for fuel claddings of fast breeder reactor where the temperature of the claddings is expected to elevate up to 700 8C [3]. In order to increase thermal efficiency of fusion reactor, improvement of high temperature strength of ferritic steels has been demanded. The application of the ODS steels for fusion blanket is considered to be very * Corresponding author. Tel.: C81 774 38 3476; fax: C81 774 38 3479. E-mail address: [email protected] (A. Kimura). 0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.07.012
promising, because they are extremely resistant to neutron irradiation [4]. Transmutation helium sometime causes severe embrittlement in metallic materials as well as ceramic materials. The helium-induced embrittlement is considered to be due to formation of helium bubbles, namely heliumvacancy clusters, in materials. In the temperature range where single vacancy is almost immobile, transmutation helium is trapped by single vacancy and helium bubbles are not formed. At higher temperatures, however, vacancies and helium atoms become mobile and aggregate into helium bubbles [5–8]. Suppression of helium bubble formation namely helium embrittlement can be achieved by introducing trapping sites for helium in materials. The 9Cr-ODS steels consist of martensitic structure and high density of nano-sized oxide particles, which are considered to be effective trapping sites for helium atoms as well as vacancies, and resultantly suppress helium embrittlement [5–8]. In this work, thermal helium desorption spectra (THDS) were measured for a RAFS and ODS steels to investigate
– 0.0033 0.0025 – 0.06 0.06 – 0.35 0.24 0.0277 0.010 0.010 0.0030 – – – 0.20 0.30 0.098 – – 0.20 – – 1.96 1.94 2.01 – 0.022 0.034 8.97 8.99 11.95 0.051 0.02 0.05 JLF-1CB M12(9Cr-ODS) F11(12Cr-ODS)
0.089 0.120 0.024
0.50 0.036 0.046
!0.002 0.003 0.002
0.0004 0.004 0.003
W Ni Cr S P Mn Si C
Table 1 Chemical compositions of the RAF steel and the ODS ferritic (12Cr) and martensitric (9Cr) steel (mass%)
V
Ta
Ti
B
N
Y2O3
Ex. O
Ar
A. Kimura et al. / Journal of Physics and Chemistry of Solids 66 (2005) 504–508
505
helium trapping behavior and evaluate the performance of the steels under fusion relevant environment. 2. Experimental The materials used in this work were a RAF steel and two sorts of ODS steels, an ODS ferritic steel and an ODS martensitic steel. The ODS ferritic steel, F11, contains 12 wt% Cr and no martensite phase exists, while the martensitic steel, M12, contains 9 wt% Cr and 0.12 wt% C, and consists of martensitic structure. The amounts of the other alloy elements were almost similar for both the steels. The chemical compositions of the steels are shown in Table 1. Two different heat treatments were performed on the 9Cr-ODS steel: (1) normalization and tempering (NT) and (2) furnace cooling (FC). Specimens for THDS measurements, which measure 10 mm by 5 mm coupon with 0.25 mm thickness, were fabricated from thick steel bars, and finally they were chemically polished in a solution of hydrofluoric acid. Helium trapping behavior in the steels was investigated by means of thermal desorption method after implantation with collimated, mass-analyzed beams of mono-energetic HeC ions at room temperature. The incident energies were 8 keV, by which atomic displacement damage takes place in iron. The helium implantation was limited in near surface area about 100 nm in depth. During the THDS measurements, the temperature was elevated up to 1200 8C at a heating-up rate of 1 8C/s. 3. Results 3.1. Implantation dose dependence Typical examples of the thermal desorption spectrum of a RAF steel (broken line) and the ODS martensitic steels-FC (dotted line) and NT (solid line) are shown in Fig. 1 after bombardment of 8 keV helium ions up to 2!1018, 2!1019 and 2!1020 HeC/m2. Generally, there were six main peaks in the THDS of each steel: peak I (100 8C), peak II (450 8C), peak III (550 8C), peak IV (800 8C), peak V (900 8C) and peak VI (1100 8C). It is worthy of notice that the peak IV was only observed in the ODS steels but not in the RAFS. This trend became much significant with increasing the amount of helium. The peak position appeared to be independent of amount of helium. According to the previous work on the THDS of pure iron [5,8], the peak in the THDS was characterized as follows: (1) peak I: desorption of helium from the surface (2) peak II: detrapping of helium from helium-vacancy pairs (3) peak III: detrapping of helium from dislocations (4) peak V: a–g transformation related desorption (5) peak VI: desorption by bubble migration
506
A. Kimura et al. / Journal of Physics and Chemistry of Solids 66 (2005) 504–508
Fig. 1. Typical examples of the thermal desorption spectrum of a RAF steel (broken line) and the ODS martensitic steels-FC (dotted line) and NT (solid line).
As for the peak IV, the mechanism will be discussed in the discussion session. In order to make clear the helium trapping strength or capacity of each steel, the fraction of helium desorption was measured for each temperature range, as shown in Fig. 2, in the rage of helium implantation from 2!1018 to 2!1021 HeC/m2. There are three characteristic features: (1) The fraction of helium desorption became larger at higher temperature range. (2) With increasing the amount of helium, the above tendency was strengthened. (3) The helium desorption at temperature range from 800 to 1000 8C was remarkably larger in the ODS steels than RAF steel. Amount of desorbed helium in each temperature range is shown in Fig. 3. Below 600 8C, amount of the desorbed helium in all the steels increases with proportional to the square root of the implantation dose, suggesting that
the weak trapping sites were easily saturated with helium. There is no difference in the desorption behavior among the steels below 600 8C. With increasing the desorption temperature, however, the dose dependence turned to a liner relationship as shown in the case between 800 and 1000 8C. This trend was accelerated in the ODS steels that show a liner dependence even at lower temperature range (600–800 8C) at a lower doses. Above 1000 8C, all the steels showed an almost liner relationship. 3.2. Effects of martensitic structure The 12Cr-ODS ferritic steel does not contain martensite structure. The THDS of the 12Cr-ODS steel implanted with 8 keV helium ions up to 2!1021 HeC/m2 is compared with those of the 9Cr-ODS steels in Fig. 4(a). The fraction of desorbed helium is shown in Fig. 4(b). As clearly shown in these figures, the spectrum of 12Cr-ODS steel is different from those of the 9Cr-ODS steels and very similar to that of the RAF steel. The helium desorption in the temperature
Fig. 2. The fraction of helium desorption measured in each temperature range at helium implantation from 2!1018 to 2!1021 HeC/m2.
A. Kimura et al. / Journal of Physics and Chemistry of Solids 66 (2005) 504–508
507
Fig. 3. Dependence of the amount of desorbed helium on the implantation dose in each temperature range in a RAF steel (dotted line) and the ODS martensitic steels-FC (broken line) and NT (solid line).
range between 800 and 1000 8C is so limited and more than 90% of helium does not desorb in the 12Cr-ODS steel until the temperature elevates above 1000 8C.
4. Discussion In the previous works, the peak I and II were interpreted in terms of surface desorption and detrapping helium from vacancies, respectively [9,10]. Since the peak I is around 100 8C, it has been considered that the helium absorbed on the specimen surface desorbed with water evaporation. The peak II was related with the vacancy migration. In the steels, vacancies are trapped at carbon atoms to form V–C pairs, which are stable at temperatures below 200 8C but decomposes into a vacancy and a carbon atom above it. It is expected that in the existence of helium, V–C–He complexes can be formed during helium implantation, and they may be stable up to around 450 8C, since our previous work [5] revealed that the helium bubble formation and growth in a RAF steel occurred only after the post-helium implantation anneal at 450 8C. This suggests that the helium trapped by V–C pairs can decompose at above 450 8C. Also, in our previous work on THDS of deformed iron, the peak III was identified as desorption from dislocations [11]. The peak V, which was observed in both the ODS steels and the RAF steel, is considered to be related with a–g transformation. In the 12Cr-ODS steel, no such a peak was observed, because the transformation does not occur in the steel. The peak VI is believed to be due to helium bubble migration. Because of the strong binding of helium in large heliumvacancy clusters, it is considered that helium is unable to desorb until the bubbles migrate to the surface. Helium bubble migration was directly observed by TEM for a thinned specimen of iron–chromium binary alloy during in situ heating in the microscope [12]. It is considered that the similar behavior was observed in the shallow area of
Fig. 4. (a) THDS of 9Cr-ODS-FC (dotted line) martensitic steel, 9Cr-ODSNT martensitic steel (broken line) and 12Cr-ODS ferritic steel (solid line) after helium implantation up to 1!1021 HeC/m2. (b) The fraction of desorbed helium in each temperature range in 9Cr-ODS-FC steel, 9Cr-ODS-NT steel and 12Cr-ODS steel after helium implantation up to 1!1021 HeC/m2.
508
A. Kimura et al. / Journal of Physics and Chemistry of Solids 66 (2005) 504–508
the bulk specimen surface as thinned TEM specimen, of which the thickness was 100 nm. The peak IV was only observed in the 9Cr-ODS steels, and not in the RAF steel and even in the 12Cr-ODS steel. Since the 12Cr-ODS steel also contains a high density of oxide particles, the peak IV is not directly attributed to the oxide particles. The martensitic structure contains carbides, precipitates, dislocations, lath boundaries, packet boundaries and grain boundaries. In RAF steels, the martensite phase becomes unstable above 600 8C, while the martensite phase in the 9Cr-ODS steels is much more stable than that in RAF steel [13]. Therefore, it is considered that the peak IV is due to helium desorption from g-phase in which the defects still remain and play a role in trapping helium to suppress the growth of helium bubbles. Finally, in the 9Cr-ODS steels, the fraction of helium desorption by bubble migration mechanism was smaller than in the RAF steel.
5. Conclusion Thermal desorption measurements were performed on a RAF steel and ODS steels after implantation with 8 keV HeC ions at room temperature. The obtained main results are: (1) The THDS of the ODS steels are very similar to that of the RAF steel, except for one peak in the temperature range from 800 to 1000 8C, where the a–g transformation related helium desorption from the g-phase occurs in the 9Cr-ODS martensitic steels. (2) The fraction of helium desorption becomes larger at higher temperatures, and this trend is strengthened by increasing the amount of implanted helium. In the 9Cr-ODS steels, the fraction of helium desorption
by bubble migration mechanism was remarkably smaller than in the RAF steel. This suggests that the bubble growth was suppressed in the 9Cr-ODS steels. (3) In the 12Cr-ODS ferritic steel, the fraction of helium desorption by bubble migration reached more than 90%, suggesting that the trapping capacity is rather large in the martensite phase of the 9Cr-ODS steel.
References [1] H. Schroeder, P.J. Batfalsky, J. Nucl. Mater. 117 (1983) 287–294. [2] R.L. Klueh, D.S. Gelles, S. Jitsukawa, A. Kimura, G.R. Odette, B. Van der Schaaf, M. Victoria, J. Nucl. Mater. 307–311 (2002) 455–465. [3] S. Ukai, T. Nishida, H. Okada, T. Okuda, M. Fujiwara, K. Asabe, J. Nucl. Sci. Technol. 34 (3) (1997) 256. [4] S. Ukai, T. Nishida, T. Okuda, T. Yoshitake, J. Nucl. Mater. 258–263 (1998) 1745. [5] A. Kimura, R. Kasada, R. Sugano, A. Hasegawa, H. Matsui, J. Nucl. Mater. 283–287 (2000) 827–831. [6] A. Kimura, T. Morimura, R. Kasada, H. Matsui, A. Hasegawa, K. Abe, ASTM STP 1366 (2000) 626–641. [7] R. Kasada, T. Morimura, H. Matsui, M. Narui, A. Kimura, ASTM STP 1366 (2000) 448–459. [8] 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. 307–311 (2002) 521–526. [9] R. Sugano, K. Morishita, A. Kimura, Proceedings of 15th Topical Meeting on the Technology of Fusion Energy (TOFE-15) Fusion Sci. Tech. 44 (2) (2003) 446–449. [10] R. Sugano, K. Morishita, H. Iwakiri, N. Yoshida, J. Nucl. Mater. 307– 311 (2002) 941–945. [11] A. Kimura, R. Kasada, K. Morishita, R. Sugano, A. Kohyama, T. Yamamoto, H. Matsui, A. Hasegaawa, K. Abe, N. Yamamoto, Proc. Fourth Pacific Rim Int. Conf. Adv. Mater. Process. 1 (2001) 1431. [12] K. Ono, K. Arakawa, K. Hojou, J. Nucl. Mater. 307–311 (2002) 1507. [13] S. Yamashita, K. Oka, S. Ohnuki, N. Akasaka, S. Ukai, J. Nud. Mater. 307–311 (2002) 283.