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Intermittent rapid motion of helium bubbles in Cu during irradiation with high energy self-ions
Nuclear Instruments and Methods in Physics Research B 206 (2003) 114–117 www.elsevier.com/locate/nimb
Intermittent rapid motion of helium bubbles in Cu during irradiation with high energy self-ions K. Ono b
a,*
, K. Arakawa a, R.C. Birtcher
b
a Department of Materials Science, Shimane University, 1060 Nishi-Kawatsu, Matsue 690-8504, Japan Materials Science Division, Argonne National Laboratory, 9750 South Cass Avenue, Argonne, IL 60439-4838, USA
Abstract The dynamical response of helium bubbles to irradiation with high energy self-ions in pure Cu has been studied by in situ electron microscopy. At enough high temperature, Brownian type motion of helium bubbles in the matrix and their easy motion along the edge dislocation are observed. At low temperatures where bubbles are thermally immobile, it is found that intermittent motion of the bubble is induced along interstitial type dislocation loops that are growing or shrinking under the irradiation. Possible mechanisms for the intermittent motion under the irradiation are discussed. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 61.72.Ff; 61.72.Ji; 61.80.Jh; 61)82.Bg Keywords: Helium bubble; Radiation damage; Copper; Electron microscopy; Ion irradiation; Defects
1. Introduction Development of fusion reactor materials requires understanding of the evolution of irradiation damage, especially of voids or bubbles, by low energy fusion plasma and high energy neutrons [1,2]. Little direct information is, however, available on the dynamic bubble behavior and the mechanism of swelling under the cascade damage. We have used in situ transmission electron microscopy to reveal the dynamical, time dependent bubble processes. We have reported results on the thermal motion of He bubbles in Al, Fe and Fealloys [3–5]. Using this technique, we present, in
*
Corresponding author. Tel.: +81-852-32-6403; fax: +81852-32-640. E-mail address: [email protected] (K. Ono).
the present work, the motion of helium bubbles introduced by irradiation with low energy helium ions in Cu and then compare with the motion observed under irradiation with high energy selfions. The main result is the observation of the intermittent motion of helium bubbles under the irradiation.
2. Experimental procedure Disk shaped ploy crystalline specimens of 99.9999 at.% purity Cu supplied by Daido-Kogyo were annealed in ultra high vacuum and then polished electrochemically to fabricate specimens for transmission electron microscopy (TEM). Helium bubbles were introduced into the specimen at around 300 °C by irradiation with 10 keV Heþ ions in an electron microscope, type JEM-2010, to
0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00695-5
K. Ono et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 114–117
which an low energy ion accelerator is attached. The specimen was then warmed stepwise to higher temperatures and the thermal motion of helium bubbles was monitored. To examine dynamical response of the helium bubbles to irradiation with high energy self-ions, 400 keV Cuþ irradiations were made using the intermediate voltage electron microscope (IVEM) facility at Argonne National Laboratory. An ion flux of 1.1 1016 ions/m2 s was used. The dynamical behavior of the bubbles during beam on and beam off periods was monitored by TEM and recorded continuously with a video recording system.
3. Results and discussion Irradiating Cu with 10 keV Heþ ions at 300 °C produced helium bubbles with diameters 1–3 nm and interstitial type dislocation loops at the depth of 20–60 nm which is close to the projected range of the Cu ions [6]. During annealing, dislocation loops start to disappear at about 430 °C and the smaller bubbles start to move at about 500 °C. Some dislocation loops or converted dislocation segments that were pinned by bubbles remained in the specimen up to higher temperatures. An illustration of the thermal motion of helium bubbles is shown in Fig. 1. In the figure, the bubble labeled A, diameter 2 nm, moved during 10 s at 587 °C. The mean square of the bubble migration distance was proportional to time indicating Brownian type motion as has been demonstrated
115
in other metals [3–5]. The diffusivity of bubble A yielded was 2.9 1019 m2 s1 at 587 °C. Bubble mobility depends on the size, shape and the temperature. Strongly faceted bubbles had a lower diffusivity than the spherical ones. The bubble labeled B, diameter 4 nm, did not move significantly during this 10 s period, while another 4 nm bubble C on a dislocation moved easily along the dislocation line converted from the dislocation loop. This indicates that the strain field of the edge dislocation modulates the surface diffusion on the bubble and enhances bubble mobility. Enhanced motion of bubbles along a general grain boundary in Al has been reported elsewhere [5,7]. To examine dynamical effects of irradiation with high energy self-ions on the bubble motion, 400 keV Cuþ ions irradiations were made on Cu specimens containing similar size helium bubbles. The energetic ions produce damages almost uniformly in the region 5–100 nm below the surface with the average damage rate about 4 103 dpa s1 [6]. The major defect clusters observed were interstitial type dislocation loops that grew or disappeared, depending on the irradiation temperature. At comparatively low temperatures where no thermal motion of helium bubbles was observed, a dynamical motion of helium bubbles in response to the irradiation with 400 keV Cuþ ions was induced. Fig. 2 shows a sequence of video frames of bubbles in Cu taken in beam off period (Fig. 2(a) and (b)) and during a beam on period (Fig. 2(c)– (f)) at 377 °C. Here, the beam irradiation started at
Fig. 1. Representative video frames taken at: (a) 0 s and (b) 10 s. These show the thermal motion of helium bubbles in Cu specimen at 587 °C. The position of the bubble C at 0 s is marked with () in (b) for easy comparison.
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K. Ono et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 114–117
Fig. 2. A sequence of video frames at: (a) 0 s; (b) 40 s; (c) 69 s; (d) 70 s; (e) 76 s and (f) 106 s. Figs. (a) and (b) are in the beam off period and (c)–(f) are in the beam on period at 377 °C. The beam irradiation started at 42 s.
42 s. During the beam off period of 40 s, there was no significant thermal motion of the bubbles. However, bubble A, for example, moved to left during 1 s as seen in Fig. 2(c) and (d), and then stayed at this position (Fig. 2(e)). This bubble moved again during next 30 s (Fig. 2(f)). The dark images in the right side of Fig. 2(c)–(f) are interstitial type dislocation loops formed during the Cuþ irradiation, although the loop image in the left side of the figure is not clearly seen due to specimen bending under irradiation. The path of the bubble B for 180 s is shown in Fig. 3. Inter-
mittent and rectilinear motion is clearly known. There were no grain boundaries in this figure area which might affect the bubble motion. As clearly demonstrated in Fig. 1, easy motion along the edge dislocation has been established. The intermittent and rectilinear motion, therefore, should be induced when the bubble interacts with a loop dislocation which is climbing under irradiation. To know the mechanism of the intermittent motion more in detail, we checked whether the bubble is mobile along the dislocation line by annealing at 377 °C, which is the temperature in
K. Ono et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 114–117
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factor that assists bubbles to follow the dislocation motion under irradiation. 4. Conclusion The dynamical response of helium bubbles to irradiation with high-energy self-ions in pure Cu has been studied by in situ electron microscopy. The thermal motion of helium bubbles in the matrix and along the dislocation has been demonstrated. During irradiation, bubble exhibited intermittent motion along the interstitial type dislocation loops at comparatively low temperatures. Possible mechanisms of the intermittent motion under the irradiation have been discussed. Acknowledgements Fig. 3. Path of the bubble B in Fig. 2, where the center position of the bubble was plotted every 2 s during 180 s. The intermittent and rectilinear motion is clearly seen.
Fig. 2. No thermal motion was evident along the dislocation line at 377 °C unless the specimen was irradiated. Therefore, we infer that there are some additional factors which make the bubble mobile along the dislocation line at lower temperatures during irradiation. It should be noticed that the self-interstitial atoms have long range interactions with the dislocation loops [8], and therefore bubbles on a loop dislocation may preferentially absorb self-interstitial atoms. This would result in an increase of the internal pressure of the bubble that can make it spherical. Our previous observations indicate that spherical bubbles move at lower temperatures than non-spherical ones. A gradient of chemical potential due to point defects near a dislocation loop, which causes climb, could be another possible
One of the authors (RCB) is supported by the U.S. Department of Energy, Office of Science, BES-Materials Science, under Contract No. W-31109-Eng-38. References [1] H. Ullmaier, Nucl. Fus. 24 (1984) 1039. [2] N. Yoshida, A. Nagao, K. Tokunaga, K. Tawara, T. Muroga, T. Fujiwara, S. Itoh, the TRIAM group, Radiat. Eff. 124 (1992) 99. [3] K. Arakawa, T. Tukamoto, K. Tadakuma, K. Yasuda, K. Ono, J. Electron. Microsco. 49 (1999) 399. [4] K. Ono, K. Arakawa, M. Oohashi, H. Kurata, K. Hojou, N. Yoshida, J. Nucl. Mater. 283–287 (2000) 210. [5] K. Ono, K. Arakawa, K. Hojou, M. Oohashi, R.C. Birtcher, S.E. Donnelly, J. Electron. Microsc. 51 (2002) s245. [6] J.P. Biersack, L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. [7] K. Ono, S. Furuno, S. Kanamitsu, K. Hojou, Philos. Mag. Lett. 75 (1997) 59. [8] M. Kiritani, Y. Maehara, H. Takata, J. Phys. Soc. Jpn. 41 (1976) 1575.
Intermittent rapid motion of helium bubbles in Cu during irradiation with high energy self-ions K. Ono b
a,*
, K. Arakawa a, R.C. Birtcher
b
a Department of Materials Science, Shimane University, 1060 Nishi-Kawatsu, Matsue 690-8504, Japan Materials Science Division, Argonne National Laboratory, 9750 South Cass Avenue, Argonne, IL 60439-4838, USA
Abstract The dynamical response of helium bubbles to irradiation with high energy self-ions in pure Cu has been studied by in situ electron microscopy. At enough high temperature, Brownian type motion of helium bubbles in the matrix and their easy motion along the edge dislocation are observed. At low temperatures where bubbles are thermally immobile, it is found that intermittent motion of the bubble is induced along interstitial type dislocation loops that are growing or shrinking under the irradiation. Possible mechanisms for the intermittent motion under the irradiation are discussed. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 61.72.Ff; 61.72.Ji; 61.80.Jh; 61)82.Bg Keywords: Helium bubble; Radiation damage; Copper; Electron microscopy; Ion irradiation; Defects
1. Introduction Development of fusion reactor materials requires understanding of the evolution of irradiation damage, especially of voids or bubbles, by low energy fusion plasma and high energy neutrons [1,2]. Little direct information is, however, available on the dynamic bubble behavior and the mechanism of swelling under the cascade damage. We have used in situ transmission electron microscopy to reveal the dynamical, time dependent bubble processes. We have reported results on the thermal motion of He bubbles in Al, Fe and Fealloys [3–5]. Using this technique, we present, in
*
Corresponding author. Tel.: +81-852-32-6403; fax: +81852-32-640. E-mail address: [email protected] (K. Ono).
the present work, the motion of helium bubbles introduced by irradiation with low energy helium ions in Cu and then compare with the motion observed under irradiation with high energy selfions. The main result is the observation of the intermittent motion of helium bubbles under the irradiation.
2. Experimental procedure Disk shaped ploy crystalline specimens of 99.9999 at.% purity Cu supplied by Daido-Kogyo were annealed in ultra high vacuum and then polished electrochemically to fabricate specimens for transmission electron microscopy (TEM). Helium bubbles were introduced into the specimen at around 300 °C by irradiation with 10 keV Heþ ions in an electron microscope, type JEM-2010, to
0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00695-5
K. Ono et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 114–117
which an low energy ion accelerator is attached. The specimen was then warmed stepwise to higher temperatures and the thermal motion of helium bubbles was monitored. To examine dynamical response of the helium bubbles to irradiation with high energy self-ions, 400 keV Cuþ irradiations were made using the intermediate voltage electron microscope (IVEM) facility at Argonne National Laboratory. An ion flux of 1.1 1016 ions/m2 s was used. The dynamical behavior of the bubbles during beam on and beam off periods was monitored by TEM and recorded continuously with a video recording system.
3. Results and discussion Irradiating Cu with 10 keV Heþ ions at 300 °C produced helium bubbles with diameters 1–3 nm and interstitial type dislocation loops at the depth of 20–60 nm which is close to the projected range of the Cu ions [6]. During annealing, dislocation loops start to disappear at about 430 °C and the smaller bubbles start to move at about 500 °C. Some dislocation loops or converted dislocation segments that were pinned by bubbles remained in the specimen up to higher temperatures. An illustration of the thermal motion of helium bubbles is shown in Fig. 1. In the figure, the bubble labeled A, diameter 2 nm, moved during 10 s at 587 °C. The mean square of the bubble migration distance was proportional to time indicating Brownian type motion as has been demonstrated
115
in other metals [3–5]. The diffusivity of bubble A yielded was 2.9 1019 m2 s1 at 587 °C. Bubble mobility depends on the size, shape and the temperature. Strongly faceted bubbles had a lower diffusivity than the spherical ones. The bubble labeled B, diameter 4 nm, did not move significantly during this 10 s period, while another 4 nm bubble C on a dislocation moved easily along the dislocation line converted from the dislocation loop. This indicates that the strain field of the edge dislocation modulates the surface diffusion on the bubble and enhances bubble mobility. Enhanced motion of bubbles along a general grain boundary in Al has been reported elsewhere [5,7]. To examine dynamical effects of irradiation with high energy self-ions on the bubble motion, 400 keV Cuþ ions irradiations were made on Cu specimens containing similar size helium bubbles. The energetic ions produce damages almost uniformly in the region 5–100 nm below the surface with the average damage rate about 4 103 dpa s1 [6]. The major defect clusters observed were interstitial type dislocation loops that grew or disappeared, depending on the irradiation temperature. At comparatively low temperatures where no thermal motion of helium bubbles was observed, a dynamical motion of helium bubbles in response to the irradiation with 400 keV Cuþ ions was induced. Fig. 2 shows a sequence of video frames of bubbles in Cu taken in beam off period (Fig. 2(a) and (b)) and during a beam on period (Fig. 2(c)– (f)) at 377 °C. Here, the beam irradiation started at
Fig. 1. Representative video frames taken at: (a) 0 s and (b) 10 s. These show the thermal motion of helium bubbles in Cu specimen at 587 °C. The position of the bubble C at 0 s is marked with () in (b) for easy comparison.
116
K. Ono et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 114–117
Fig. 2. A sequence of video frames at: (a) 0 s; (b) 40 s; (c) 69 s; (d) 70 s; (e) 76 s and (f) 106 s. Figs. (a) and (b) are in the beam off period and (c)–(f) are in the beam on period at 377 °C. The beam irradiation started at 42 s.
42 s. During the beam off period of 40 s, there was no significant thermal motion of the bubbles. However, bubble A, for example, moved to left during 1 s as seen in Fig. 2(c) and (d), and then stayed at this position (Fig. 2(e)). This bubble moved again during next 30 s (Fig. 2(f)). The dark images in the right side of Fig. 2(c)–(f) are interstitial type dislocation loops formed during the Cuþ irradiation, although the loop image in the left side of the figure is not clearly seen due to specimen bending under irradiation. The path of the bubble B for 180 s is shown in Fig. 3. Inter-
mittent and rectilinear motion is clearly known. There were no grain boundaries in this figure area which might affect the bubble motion. As clearly demonstrated in Fig. 1, easy motion along the edge dislocation has been established. The intermittent and rectilinear motion, therefore, should be induced when the bubble interacts with a loop dislocation which is climbing under irradiation. To know the mechanism of the intermittent motion more in detail, we checked whether the bubble is mobile along the dislocation line by annealing at 377 °C, which is the temperature in
K. Ono et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 114–117
117
factor that assists bubbles to follow the dislocation motion under irradiation. 4. Conclusion The dynamical response of helium bubbles to irradiation with high-energy self-ions in pure Cu has been studied by in situ electron microscopy. The thermal motion of helium bubbles in the matrix and along the dislocation has been demonstrated. During irradiation, bubble exhibited intermittent motion along the interstitial type dislocation loops at comparatively low temperatures. Possible mechanisms of the intermittent motion under the irradiation have been discussed. Acknowledgements Fig. 3. Path of the bubble B in Fig. 2, where the center position of the bubble was plotted every 2 s during 180 s. The intermittent and rectilinear motion is clearly seen.
Fig. 2. No thermal motion was evident along the dislocation line at 377 °C unless the specimen was irradiated. Therefore, we infer that there are some additional factors which make the bubble mobile along the dislocation line at lower temperatures during irradiation. It should be noticed that the self-interstitial atoms have long range interactions with the dislocation loops [8], and therefore bubbles on a loop dislocation may preferentially absorb self-interstitial atoms. This would result in an increase of the internal pressure of the bubble that can make it spherical. Our previous observations indicate that spherical bubbles move at lower temperatures than non-spherical ones. A gradient of chemical potential due to point defects near a dislocation loop, which causes climb, could be another possible
One of the authors (RCB) is supported by the U.S. Department of Energy, Office of Science, BES-Materials Science, under Contract No. W-31109-Eng-38. References [1] H. Ullmaier, Nucl. Fus. 24 (1984) 1039. [2] N. Yoshida, A. Nagao, K. Tokunaga, K. Tawara, T. Muroga, T. Fujiwara, S. Itoh, the TRIAM group, Radiat. Eff. 124 (1992) 99. [3] K. Arakawa, T. Tukamoto, K. Tadakuma, K. Yasuda, K. Ono, J. Electron. Microsco. 49 (1999) 399. [4] K. Ono, K. Arakawa, M. Oohashi, H. Kurata, K. Hojou, N. Yoshida, J. Nucl. Mater. 283–287 (2000) 210. [5] K. Ono, K. Arakawa, K. Hojou, M. Oohashi, R.C. Birtcher, S.E. Donnelly, J. Electron. Microsc. 51 (2002) s245. [6] J.P. Biersack, L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. [7] K. Ono, S. Furuno, S. Kanamitsu, K. Hojou, Philos. Mag. Lett. 75 (1997) 59. [8] M. Kiritani, Y. Maehara, H. Takata, J. Phys. Soc. Jpn. 41 (1976) 1575.