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Hydrogen isotopes behavior in nickel in nuclear fusion reactor conditions
International Journal of Hydrogen Energy 26 (2001) 507–509
www.elsevier.com/locate/ijhydene
Hydrogen isotopes behavior in nickel in nuclear fusion reactor conditions B. Constantinescua;∗ , F. Vasiliua , G. Alexandrub a Institute
of Atomic Physics, POB MG-6, Bucharest, Romania b ICERP, Ploiesti, Romania
Abstract The behavior of hydrogen isotopes in metals is very important for future nuclear fusion reactors. To study deuterium in1uence on helium pre-irradiated nickel, 0 –28 MeV He2+ and 7 MeV D+ beams were used. The He samples pre-implanted at 30, 100 and 300 appm (atomic parts per million) D+ (1018 ions=cm2 ) irradiated and annealed (1273 K) in high vacuum for 1, 10, 100 and 1000 h. After an electrochemical preparation (Struers jet), they were examined by transmission electron microscopy (TEM). Values for He bubble radii and densities depending on He ions doses and annealing periods are reported. Two types of bubbles were observed, namely: “small” bubbles, predominantly in the matrix (from 5 nm radius for 30 appm, 1 h annealing to 35 nm for 300 appm, 100 h annealing), and “large” bubbles, predominantly in the grain boundary regions, but even in some dislocations (from 30 nm radius for 30 appm, 1 h annealing to 120 nm for 300 appm, 100 h annealing). A unique bubble coarsening mechanism — the Ostwald ripening — is considered. The in1uence of deuterium on the “small” and “large” bubble radius values is discussed. ? 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.
1. Introduction The behavior of helium in metals has gained considerable interest, because it can cause embrittlement and degradation in materials used for the components of advanced future nuclear energy systems [1]. The Drst wall of a fusion reactor is also subject to high hydrogen ion and atom 1uxes [2]. It has been assumed that due to the higher diEusivity and release rate of hydrogen than of helium in most candidate materials, the eEect of hydrogen is minimal. However, the role of hydrogen for microstructural changes, such as void swelling, is not well understood. It is established that hydrogen may be trapped at particular defect sites in metals. The purpose of the present study is to determine combined helium–hydrogen eEects such as trapping of hydrogen around or near helium bubbles and the
∗ Corresponding author. Tel.: +40-41-780-7040; fax: +40-41423-1701. E-mail address: [email protected] (B. Constantinescu).
in1uence on size distributions of bubbles as a function of helium content and periods of annealing at 1273 K.
2. Experimental Pure nickel foils, 99.995% of 100 m thickness were annealed at 1373 K for 2 h in a vacuum better than 10−5 Pa. Homogeneous helium implantation at room temperature was achieved by varying the range of the 28 MeV alpha particles beam of KFA-Julich (Germany) Compact Cyclotron by a wheel that continuously rotated 50 Al foils of diEerent thickness through the beam [3]. Samples with nominal helium concentrations of 30, 100 and 300 appm were obtained. Subsequently, these samples were deuterium (1018 D+ 7 MeV ions=cm2 ) irradiated at room temperature by the Bucharest U-120 Classical Cyclotron [3]. Afterwards, the samples were annealed at 1273 K for 1, 10, 100 and 1000 h in vacuum. From each of the helium + deuterium implanted specimen areas 3 mm disks were punched out. The disks were
0360-3199/01/$ 20.00 ? 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 0 0 ) 0 0 0 8 5 - 9
508
B. Constantinescu et al. / International Journal of Hydrogen Energy 26 (2001) 507–509
electrochemically thinned by jet polishing using a TENUPOL STRUERS JET apparatus and two chemical ◦ solutions: perchloric acid plus ethanol at 0 C and sul◦ furic acid plus glycerin at 10 C. The specimens were investigated in a JEOL TEMSCAN 200 CX microscope operated at 200 kV and in a TESLA BS 240 microscope operated at 120 kV. Most of the micrographs were taken in phase contrast under a defocusing condition. Bubble radii, rB , were determined from histograms obtained with a semiautomatic particle-size analyzer and, partially, by direct observation, with an uncertainty of at most ±30%. For the evaluation of bubble densities, cB , the foil thickness determination was made by interference fringe observation which introduces an uncertainty of at most ±30%. 3. Results and discussion The annealing behavior of helium bubbles formed in nickel after helium implantation at room temperature which is discussed in detail in [1] seems to represent a striking example for the eEect of the internal pressure on bubble coarsening. For annealing temperatures above 900 K and high He concentrations (500 –5000 appm) the coarsening rate is found to be substantially faster close to the surface of the sample than in the bulk but comparable to the coarsening rate at low He concentrations (6 200 appm). Accordingly, two distinct coarsening branches in an Arrhenius plot of average bubble radii were identiDed: a weakly activated one (rB values from 0.6 at 800 K to 2 nm at 1400 K) for slow bubble coarsening in the bulk, and a highly activated one (rB values from 1 at 1000 K to 800 nm at 1400 K) for fast bubble coarsening close to the surface (or in the grain boundaries). This diEerence may be correlated with diEerences in the pressure within the bubbles: higher pressure with increasing He concentration in the bulk but a relaxation of an initially high pressure within bubbles suPciently close to the surface (or grain boundaries) due to vacancies provided by the sample surface (or by internal radiation damage). Consequently, the weakly activated branch (slow coarsening) may be attributed to bubbles with high internal pressure which can coarsen by migration and coalescence while Ostwald ripening is completely suppressed. The highly activated branch (fast coarsening) may be attributed to bubbles in which pressure is low or relaxes during annealing from an initially high to a Dnally low value. Such bubbles can coarsen by Ostwald ripening which seems to surpass its maximum rate when the pressure relaxes by vacancies originating from implantation damage (bulk bubbles for low He concentrations and grain boundary bubbles for high He concentrations) or from the surface. Our results are presented in Figs. 1 and 2 (unfortunately, there are some missing points, because some specimens were damaged during thinning). Apparently, there are two categories of bubbles: “small” (bulk, matrix) bubbles
Fig. 1. Mean radius rB of helium bubbles as a function of annealing period.
and “large” (grain boundaries and dislocations) bubbles. Evidently, it is a conventional classiDcation, the essential diEerence being the association of “small” bubbles strictly with the matrix and of “large” bubbles strictly with the grain boundaries and dislocations. Critical sizes (Fig. 2) for separating them are 30 nm for 1, 10 and 100 h annealing periods and 100 nm for the 1000 h annealing period, for all He concentration values. The rB values for small bubbles (6 –30 nm) are much larger than the values reported in [1] for the same type of bubbles in the case of simple He implantation (1.5 nm for 1273 K), so, we could assume an Ostwald ripening coarsening mechanism via helium, hydrogen (deuterium) and vacancy resolution and reabsorption. Probably, the relaxation of initially overpressurized He bubbles by annealing of the associated He + H (D) dislocation network is quite substantial. Concerning “large” bubbles, our values are slightly larger than in [1], Ostwald ripening being evident. Variations in the implanted helium concentrations (from 30 to 300 appm) have relatively little in1uence on the mean radius. However, we must Dnd and explain the in1uence of hydrogen (deuterium) on our data as compared to the simple helium implantation case reported for nickel in [1]. We assume that trapped H isotopes could play a role in the triggering of fast Ostwald ripening. In [4], there is an attempt
B. Constantinescu et al. / International Journal of Hydrogen Energy 26 (2001) 507–509
509
consequently, the migration and coalescence mechanism, and suggest Ostwald ripening mechanism even for matrix bubbles. We can assume a relatively important role of hydrogen isotopes trapped in grain boundaries as active vacancy sources, and even as a H (D) atom source (the internal pressure for H is lower than the internal pressure for He bubbles) contributing to Ostwald ripening for the “large” bubbles. For the “small” bubbles, this contribution is probably reduced; this fact could explain the diEerence in rB values. The cracks induced by H irradiation around the He bubbles [5] could also play a role in the coarsening phenomenon. We can conclude, for our analyzed cases, that there is only one coarsening mechanism for “small” and “large” bubbles: the Ostwald ripening, the diEerence in rB values being due to the diEerent H contribution, which even though smaller in the matrix than in the grain boundaries, is suPcient to transform the migration and coalescence coarsening mechanism in the bulk into an Ostwald ripening mechanism by the created vacancy sources. Acknowledgements
Fig. 2. Mean radius rB of helium bubbles as a function of He ion dose.
to explain the trapping of H around He bubbles and relate it to a chemisorption-like interaction at the isolated bubbles. In [5] a cracking phenomenon generated by hydrogen in the grain boundaries of nickel, as a result of the greater stress on the material due to the hydride formation, is reported. 4. Conclusion It was found that matrix bubbles (diameter values less than 1 nm) can coarsen by migration and coalescence of He atoms and vacancies, moderately reduced by the observed high internal pressure, while Ostwald ripening is completely suppressed. This is observed for grain boundary bubbles in which the pressure is low or relaxes during annealing mainly due to surface vacancies. The large diameter values for matrix bubbles (¿5 nm) exclude high internal pressure for 30 –300 appm He concentration values and,
Work supported in part by the BMFT (Germany)-IAP (Romania) bilateral agreement, Project RUM 056.2. The authors are very grateful to Prof. H. Ullmaier and his team from IFF-KFA Julich for specimen He-implantation and annealing. References [1] Chernikov V, Trinkhaus H, Jung P, Ullmaier H. The formation of helium bubbles near the surface and in the bulk in nickel during post-implantation annealing. J Nucl Mater 1990;170:31. [2] Jung P. A hydrogen problem in fusion material technology. Fusion Technol 1998;33:63. [3] Constantinescu B, Dima S, Florescu V, Ivanov E, Plostinaru D, Sarbu C. Use of 4.7 MeV alpha particles in elemental analysis and fusion reactor material studies. Nucl Instr and Meth B 1986;16:488. [4] Myers M, Follstaedt DM, Besenbacher F, Bootiger J. Defect trapping of ion-implanted deuterium in nickel. J Appl Phys 1982;53:8734. [5] SolovioE G, Abramov E, Eliezer D. The formation of H induced blisters and their growth in nickel pre-implanted with He. J Nucl Mater 1994;217:287.
www.elsevier.com/locate/ijhydene
Hydrogen isotopes behavior in nickel in nuclear fusion reactor conditions B. Constantinescua;∗ , F. Vasiliua , G. Alexandrub a Institute
of Atomic Physics, POB MG-6, Bucharest, Romania b ICERP, Ploiesti, Romania
Abstract The behavior of hydrogen isotopes in metals is very important for future nuclear fusion reactors. To study deuterium in1uence on helium pre-irradiated nickel, 0 –28 MeV He2+ and 7 MeV D+ beams were used. The He samples pre-implanted at 30, 100 and 300 appm (atomic parts per million) D+ (1018 ions=cm2 ) irradiated and annealed (1273 K) in high vacuum for 1, 10, 100 and 1000 h. After an electrochemical preparation (Struers jet), they were examined by transmission electron microscopy (TEM). Values for He bubble radii and densities depending on He ions doses and annealing periods are reported. Two types of bubbles were observed, namely: “small” bubbles, predominantly in the matrix (from 5 nm radius for 30 appm, 1 h annealing to 35 nm for 300 appm, 100 h annealing), and “large” bubbles, predominantly in the grain boundary regions, but even in some dislocations (from 30 nm radius for 30 appm, 1 h annealing to 120 nm for 300 appm, 100 h annealing). A unique bubble coarsening mechanism — the Ostwald ripening — is considered. The in1uence of deuterium on the “small” and “large” bubble radius values is discussed. ? 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.
1. Introduction The behavior of helium in metals has gained considerable interest, because it can cause embrittlement and degradation in materials used for the components of advanced future nuclear energy systems [1]. The Drst wall of a fusion reactor is also subject to high hydrogen ion and atom 1uxes [2]. It has been assumed that due to the higher diEusivity and release rate of hydrogen than of helium in most candidate materials, the eEect of hydrogen is minimal. However, the role of hydrogen for microstructural changes, such as void swelling, is not well understood. It is established that hydrogen may be trapped at particular defect sites in metals. The purpose of the present study is to determine combined helium–hydrogen eEects such as trapping of hydrogen around or near helium bubbles and the
∗ Corresponding author. Tel.: +40-41-780-7040; fax: +40-41423-1701. E-mail address: [email protected] (B. Constantinescu).
in1uence on size distributions of bubbles as a function of helium content and periods of annealing at 1273 K.
2. Experimental Pure nickel foils, 99.995% of 100 m thickness were annealed at 1373 K for 2 h in a vacuum better than 10−5 Pa. Homogeneous helium implantation at room temperature was achieved by varying the range of the 28 MeV alpha particles beam of KFA-Julich (Germany) Compact Cyclotron by a wheel that continuously rotated 50 Al foils of diEerent thickness through the beam [3]. Samples with nominal helium concentrations of 30, 100 and 300 appm were obtained. Subsequently, these samples were deuterium (1018 D+ 7 MeV ions=cm2 ) irradiated at room temperature by the Bucharest U-120 Classical Cyclotron [3]. Afterwards, the samples were annealed at 1273 K for 1, 10, 100 and 1000 h in vacuum. From each of the helium + deuterium implanted specimen areas 3 mm disks were punched out. The disks were
0360-3199/01/$ 20.00 ? 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 0 0 ) 0 0 0 8 5 - 9
508
B. Constantinescu et al. / International Journal of Hydrogen Energy 26 (2001) 507–509
electrochemically thinned by jet polishing using a TENUPOL STRUERS JET apparatus and two chemical ◦ solutions: perchloric acid plus ethanol at 0 C and sul◦ furic acid plus glycerin at 10 C. The specimens were investigated in a JEOL TEMSCAN 200 CX microscope operated at 200 kV and in a TESLA BS 240 microscope operated at 120 kV. Most of the micrographs were taken in phase contrast under a defocusing condition. Bubble radii, rB , were determined from histograms obtained with a semiautomatic particle-size analyzer and, partially, by direct observation, with an uncertainty of at most ±30%. For the evaluation of bubble densities, cB , the foil thickness determination was made by interference fringe observation which introduces an uncertainty of at most ±30%. 3. Results and discussion The annealing behavior of helium bubbles formed in nickel after helium implantation at room temperature which is discussed in detail in [1] seems to represent a striking example for the eEect of the internal pressure on bubble coarsening. For annealing temperatures above 900 K and high He concentrations (500 –5000 appm) the coarsening rate is found to be substantially faster close to the surface of the sample than in the bulk but comparable to the coarsening rate at low He concentrations (6 200 appm). Accordingly, two distinct coarsening branches in an Arrhenius plot of average bubble radii were identiDed: a weakly activated one (rB values from 0.6 at 800 K to 2 nm at 1400 K) for slow bubble coarsening in the bulk, and a highly activated one (rB values from 1 at 1000 K to 800 nm at 1400 K) for fast bubble coarsening close to the surface (or in the grain boundaries). This diEerence may be correlated with diEerences in the pressure within the bubbles: higher pressure with increasing He concentration in the bulk but a relaxation of an initially high pressure within bubbles suPciently close to the surface (or grain boundaries) due to vacancies provided by the sample surface (or by internal radiation damage). Consequently, the weakly activated branch (slow coarsening) may be attributed to bubbles with high internal pressure which can coarsen by migration and coalescence while Ostwald ripening is completely suppressed. The highly activated branch (fast coarsening) may be attributed to bubbles in which pressure is low or relaxes during annealing from an initially high to a Dnally low value. Such bubbles can coarsen by Ostwald ripening which seems to surpass its maximum rate when the pressure relaxes by vacancies originating from implantation damage (bulk bubbles for low He concentrations and grain boundary bubbles for high He concentrations) or from the surface. Our results are presented in Figs. 1 and 2 (unfortunately, there are some missing points, because some specimens were damaged during thinning). Apparently, there are two categories of bubbles: “small” (bulk, matrix) bubbles
Fig. 1. Mean radius rB of helium bubbles as a function of annealing period.
and “large” (grain boundaries and dislocations) bubbles. Evidently, it is a conventional classiDcation, the essential diEerence being the association of “small” bubbles strictly with the matrix and of “large” bubbles strictly with the grain boundaries and dislocations. Critical sizes (Fig. 2) for separating them are 30 nm for 1, 10 and 100 h annealing periods and 100 nm for the 1000 h annealing period, for all He concentration values. The rB values for small bubbles (6 –30 nm) are much larger than the values reported in [1] for the same type of bubbles in the case of simple He implantation (1.5 nm for 1273 K), so, we could assume an Ostwald ripening coarsening mechanism via helium, hydrogen (deuterium) and vacancy resolution and reabsorption. Probably, the relaxation of initially overpressurized He bubbles by annealing of the associated He + H (D) dislocation network is quite substantial. Concerning “large” bubbles, our values are slightly larger than in [1], Ostwald ripening being evident. Variations in the implanted helium concentrations (from 30 to 300 appm) have relatively little in1uence on the mean radius. However, we must Dnd and explain the in1uence of hydrogen (deuterium) on our data as compared to the simple helium implantation case reported for nickel in [1]. We assume that trapped H isotopes could play a role in the triggering of fast Ostwald ripening. In [4], there is an attempt
B. Constantinescu et al. / International Journal of Hydrogen Energy 26 (2001) 507–509
509
consequently, the migration and coalescence mechanism, and suggest Ostwald ripening mechanism even for matrix bubbles. We can assume a relatively important role of hydrogen isotopes trapped in grain boundaries as active vacancy sources, and even as a H (D) atom source (the internal pressure for H is lower than the internal pressure for He bubbles) contributing to Ostwald ripening for the “large” bubbles. For the “small” bubbles, this contribution is probably reduced; this fact could explain the diEerence in rB values. The cracks induced by H irradiation around the He bubbles [5] could also play a role in the coarsening phenomenon. We can conclude, for our analyzed cases, that there is only one coarsening mechanism for “small” and “large” bubbles: the Ostwald ripening, the diEerence in rB values being due to the diEerent H contribution, which even though smaller in the matrix than in the grain boundaries, is suPcient to transform the migration and coalescence coarsening mechanism in the bulk into an Ostwald ripening mechanism by the created vacancy sources. Acknowledgements
Fig. 2. Mean radius rB of helium bubbles as a function of He ion dose.
to explain the trapping of H around He bubbles and relate it to a chemisorption-like interaction at the isolated bubbles. In [5] a cracking phenomenon generated by hydrogen in the grain boundaries of nickel, as a result of the greater stress on the material due to the hydride formation, is reported. 4. Conclusion It was found that matrix bubbles (diameter values less than 1 nm) can coarsen by migration and coalescence of He atoms and vacancies, moderately reduced by the observed high internal pressure, while Ostwald ripening is completely suppressed. This is observed for grain boundary bubbles in which the pressure is low or relaxes during annealing mainly due to surface vacancies. The large diameter values for matrix bubbles (¿5 nm) exclude high internal pressure for 30 –300 appm He concentration values and,
Work supported in part by the BMFT (Germany)-IAP (Romania) bilateral agreement, Project RUM 056.2. The authors are very grateful to Prof. H. Ullmaier and his team from IFF-KFA Julich for specimen He-implantation and annealing. References [1] Chernikov V, Trinkhaus H, Jung P, Ullmaier H. The formation of helium bubbles near the surface and in the bulk in nickel during post-implantation annealing. J Nucl Mater 1990;170:31. [2] Jung P. A hydrogen problem in fusion material technology. Fusion Technol 1998;33:63. [3] Constantinescu B, Dima S, Florescu V, Ivanov E, Plostinaru D, Sarbu C. Use of 4.7 MeV alpha particles in elemental analysis and fusion reactor material studies. Nucl Instr and Meth B 1986;16:488. [4] Myers M, Follstaedt DM, Besenbacher F, Bootiger J. Defect trapping of ion-implanted deuterium in nickel. J Appl Phys 1982;53:8734. [5] SolovioE G, Abramov E, Eliezer D. The formation of H induced blisters and their growth in nickel pre-implanted with He. J Nucl Mater 1994;217:287.