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Nanoscale surface morphology of tungsten materials induced by Be-seeded D-He plasma exposure
Journal of Nuclear Materials 417 (2011) 528–532
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Nanoscale surface morphology of tungsten materials induced by Be-seeded D-He plasma exposure K. Tokunaga a,⇑, M.J. Baldwin b, R.P. Doerner b, D. Nishijima b, H. Kurishita c, T. Fujiwara a, K. Araki a, Y. Miyamoto a, N. Ohno d, Y. Ueda e a
Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka 816-8580, Japan Center for Energy Research, University of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0417, USA International Research Center for Nuclear Materials Science, IMR, Tohoku University, Oarai, Ibaraki 311-1313, Japan d Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan e Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan b c
a r t i c l e
i n f o
Article history: Available online 1 February 2011
a b s t r a c t Ultra-fine grain W-(0.5, 1.5)wt%TiC alloys and stress relieved powder metallurgy W have been exposed to D-He mixed plasmas, some with added Be. The fixed exposure conditions are ion energy 60 eV and flux 3 to 6 1022/m2 s. Sample temperature is 1123 K and exposure times scanned 1000–11,000 s. Typical He/D ion is 0.2 and Be is 0.2% of the plasma. A remarkable change of the tungsten surfaces results from the plasma exposures. Formation of a nano-structured layer on the exposure surfaces is observed and believed to be related to He bubble formation. In addition, the growth rate of the nano-structured layer depends on the microstructure of the samples. In the case of a D-He plasma with Be, at 60 eV, plasma sputters away most of Be deposits with little effect on the He induced nano-structured layer formation. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Tungsten or tungsten alloys are potential candidates for the divertor armor material in ITER and the DEMO reactor due to their very low erosion rate and high-temperature properties. However, heat load, hydrogen isotopes and helium atoms from the plasma, which affect damage accumulation and mechanical properties, may reduce these properties of tungsten [1]. In particular, it is well known that the accumulation of He is much harmful to metal because of its strong interaction with lattice defects. He drastically enhances the formation of bubbles due to the strong bonding to vacancies and their clusters. As a result, local swelling and degradation of mechanical properties of bulk materials takes place as well [2–4]. In recent, low energy and high-flux He exposure experiment on W materials have been carried out. As a result, very fine morphology [5] and nano-structure on W [6–8] have been investigated after the He irradiation. These phenomena are considered to depend on such as microstructure, manufacture process and orientation of crystal grain because of high dependency of He and latent defects such as dislocation, grain boundary, impurities and residual strain in W materials. In the present work, W materials, which have been newly developed for resistance to radiation damage, low temperature embrittlement and blister formation, have been exposed to low energy and high-flux He plasma to investigate adaptability ⇑ Corresponding author. Tel.: +81 92 583 7986; fax: +81 92 583 6899. E-mail address: [email protected] (K. Tokunaga). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2011.01.078
on the divertor armor materials. In addition, exposure experiments to low energy and high-flux D and He mixed plasma, some doped with Be have been performed to investigate the synergistic effects of simultaneous implantation of D, He and Be on modification of W materials under the more relevant the ITER operation.
2. Experimental Powder metallurgy tungsten (PM-W) and ultra-fine grain W-0.5 wt%TiC and W-1.5 wt%TiC alloys produced by mechanical alloying (MA) in a purified H2 atmosphere (UFG W-0.5 wt%TiC/H2 and UFG W-1.5 wt%TiC/H2) and hot isostatic pressing (HIP) were used in the present experiments. Sizes of PM-W and UFG W-(0.5, 1.5) wt%TiC alloys were 25.4 mm diameter and 1.5 mm thickness, and 20 mm diameter and 1.5 mm thickness, respectively. The UFG W-(0.5, 1.5) wt%TiC alloys were developed for resistance to radiation damage and low temperature embrittlement [9]. The PM-W used was stress relieved powder metallurgy tungsten (SRW) from ALMT. Corp prepared by sintering followed by warm rolling to a cross section reduction of 85–95% with intermediate heat treatments, and then stress-relief (SR) annealed at 1173 K for 3.6 ks. The rolling direction of the SR-W is perpendicular to the surface resulting in high resistance to blister formation for hydrogen bombardment below the displacement threshold energy, and to degradation of heat conduction due to crack formation along grain boundaries.
K. Tokunaga et al. / Journal of Nuclear Materials 417 (2011) 528–532
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The sample was mechanically mounted on a holder, actively air cooled and irradiated in the PISCES-B linear-divertor-plasma simulator. The exposure conditions were fixed at ion energy of 60 eV and flux of 3–6 1022/m2 s. The Sample temperature was 1123 K and exposure times spanned 1000–11,000 s. The ratio of deuterium to helium ions was determined by optical spectroscopic measurement [10]. Typical ratio of He/D ion was 0.2 and Be content was 0.2%. Pure He plasma exposure experiments were also performed to compare the results with the D-He mixture plasma exposure. After exposure, surfaces and cross sections of the samples were examined with a scanning electron microscope (SEM). In addition, compositional information was obtained using energy dispersive X-ray microanalysis (EDX) and Auger electron spectroscopy analyzer (AES). Samples weight due to the exposure was also measured. 3. Results 3.1. Exposure to D-He plasma without Be Fig. 1 shows photographs of UFG W-0.5 wt%TiC/H2 before (a) and after (b) the irradiation. The sample color becomes visually black from the original metallic sliver color due to the irradiation. Fig. 2 shows SEM images of the surface of UFG W-0.5 wt%TiC/H2 and UFG W-1.5 wt%TiC/H2 after the irradiation. The surface layer consisting of interconnected nanoscale-rod or fiber-like structures found to form under exposure to D-He mixture plasma as shown in Fig. 2a. Shown in Fig. 2b is an SEM image of a cross section of the sample. The nano-scale fiber or rod dimensions have characteristic widths of the order of only a few tens nm. Compositional analyses revealed the nano-structured layer was consistent with pure W. The formation of the nano-structured layer might be related to helium bubble formation [11]. In addition, the W nano-structured layer is easily remove because the nano-rods are very delicate. The blackening of the surface by the exposure is attributed to the multiple scattering of light in the porous nano-structured layer at the surface, in addition to the diffuse reflection due to roughening of the surface. Shown in Fig. 3 is growth of the nano-structured layer of UFG W-0.5 wt%TiC/H2 with plasma exposure time. SEM cross sections of UFG-0.5 wt%W/H2 exposed at 1123 K to D-He plasmas for 2150 s, 5000 s and 10,000 s are shown. The thickness of the nano-structured layer increases with increasing exposure time and for the longest exposure time, a nano-structured layer in excess of about 4.5 lm thick was observed. Fig. 4 shows growth of thickness of the nano-structured layer of SR-W with plasma exposure time. SEM cross sections of PM-W exposed at 1123 K to D-He plasmas for 1100, 5100 and 11,000 s are included showing that the thickness of the nano-structured layer increased with increasing exposure time, similar to the result for UFG W-0.5 wt%TiC/H2. Shown in Fig. 5 is a high magnification im-
(a)
(b)
20mm Fig. 1. Images of UGF W-0.5 wt%TiC/H before (a) and after the plasma exposure (b). The exposure conditions are fixed at ion energy 60 eV and flux of 3.2–3.5 1022/ m2 s. Sample temperature is 1123 K and plasma exposure times were 2150 s. Ratio of He/D ion is 0.2.
Fig. 2. SEM images of surface of UFG W-0.5 wt%TiC/H2 (a) and cross section of UFG W-1.5 wt%TiC/H2 (b) after the plasma exposure. The exposure conditions of (a) were ion energy 60 eV and flux 3.2–3.5 1022/m2 s, sample temperature 1123 K. The exposure time was 2150 s, and ratio of He/D ion 0.2. The exposure conditions of (b) are ion energy 60 eV, flux 3.0–3.1 1022/m2 s, sample temperature 1123 K, The exposure time was 5120 s, and ratio of He/D ion 0.2.
age of the interface of a nano-structured layer and the base W of SR-W exposed to a D-He plasma at 1123 K for 11,000 s. This shows that the nano-structure is growing from the bulk W. Fig. 6 shows nano-structured layer thickness data, taken from images such as in Figs. 3 and 4 as a function of the square root of the plasma exposure time, for exposures conduced at 1123 K. Data from UFG W-1.5 wt%TiC/H2 is also added. In addition, data on thickness of the nano-structured layer of SR-W and UFG W-1.5 wt%TiC/H2 exposed to He plasma is shown. The set of data from SR-W and UFG W-0.5 wt%TiC/H2 exposed to the D-He plasma tends to have some deviations with a fitted t1/2 dependence. It is also interesting to note that the straight line fit does not pass through zero. The t1/2 axial intercepts at 400 s and 1600 s may be an indication of a short incubation or saturation time that precedes nanostructuring of SR-W and UFG W-0.5 wt%TiC/H2 alloy, respectively. In addition, there is little difference in the nano-structured layer thickness between UFG W-0.5 wt%TiC/H2 and UFG W-1.5 wt%TiC/ H2. The results from He plasma exposure are also plotted in Fig. 6. The fluence of the He plasma exposures are 2 or 3 times larger than the D-He mixture case, however, the thickness of the nano-structured layer is almost the same. This implies that the availability of diffusing He or He saturation of the W surface, contributes to growth of the nano-structured layer. In addition, in the D-He mixture case, it is speculated that the presence of D is not responsible for any reduction in the nano-structured layer growth rate at this temperature. 3.2. Exposure to D-He with Be The D-He–Be mixture plasma exposure experiments were also carried out using PISCES-B. SEM observation of surfaces and cross sections of the irradiated samples indicated nano-structured layer
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Bulk
(a)
Surface Surface
Bulk
(a)
Surface Surface
(b) (b)
(c) (c)
Fig. 3. SEM images of cross sections of the nano-structured layer on UFG W0.5 wt%TiC/H alloy with plasma exposure time. The UFG W was exposed at 1123 K to D-0.2He plasmas for 2150 s (a), 5000 s (b) and 10,000 s (c).
Fig. 4. SEM images of cross sections of the nano-structured layer on SR-W with plasma exposure time. The SR-W was exposed at 1123 K to a D-0.2He plasmas for 1,100 s (a), 5,100s (b) and 11,000 s (c).
is formed near the surface, as with exposure to the D-He plasma without Be. AES analyses of surfaces of UFG W-0.5 wt%TiC/H2 exposed to the D-He–Be plasma indicates that the surface was covered with Be and O as shown in Fig. 7. However, Be was not detected in the nano-structured material when examining the cross section of the samples, indicating that Be exists as a very thin layer near the sample surface. For the present experimental condition, most of Be deposited on the surface was eroded by sputtering and no thick Be layer formed on the surface.
4. Discussion 4.1. Microstructure dependence on surface modification Many studies have identified deleterious effects on high-temperature exposed tungsten surfaces induced by He plasmas for incident He ion energies below the displacement threshold energy. In a few instances, He blistering is observed at moderate temperature below 800 K [12], but more serious effects such as the formation of pits, holes and bubbles occur at higher temperature, above 1600 K [13–15]. The accumulation of He in defects and vacancies [16,17] is believed responsible for these effects [11] but the precise mechanisms are not well understood.
Fig. 5. High magnification SEM image of the interface between the nano-structured layer and unaffected metal of SR-W that was exposed to D-0.2He mixture plasma at 1123 K for 11,000 s.
In the temperature range 1070–1600 K, particularly relevant to the ITER ‘all metal’ scenario and to DEMO reactor, a surface layer consisting of very fine morphology [5] and nano-structure of W nano-rod-like structures [6–8] were found to form under exposure to a helium plasma in the divertor plasma simulators NAGDIS-II and PISCES-B. In the experiment in NAGDIS-II, vacuum plasma
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D
He
Be
Sputter
Be Nano-W
Sputter
He
Bulk-W
Fig. 6. Nano-structured layer thickness taken from images such as in Figs. 3 and 4 as a function of the square root of the plasma exposure time of SR-W and UFG W0.5 wt%TiC/H2. Data for UFG W-1.5 wt%TiC/H2 exposed to D-0.2He mixture plasma at 1123 K for 5120 s. In addition, thickness data for the nano-structured layer of SRW and UFG W-1.5 wt%TiC/H2 exposed to He plasma with a flux of 2–3 1022/m2 s at 1123 K is included.
Min: -13320
Max: 15370
O13.1%
dN(E)
Be 86.9%
O1 13.1 %
Be1 86.9 %
30
87
144
201
258
315
372
429
486
543
600
Kinetic energy Fig. 7. AES analyses of surface of UFG W-0.5 wt%TiC/H2 exposed to D-He plasma with Be at 1123 K for 10,100 s.
sprayed W coated graphite (VPS-W/graphite) samples were exposed to pure He plasma at temperature of 1250 and 1600 K. In addition, PISCES experiments revealed that growth of the nanostructured layer on the sheet of rolled PLANSEE 99.9999 weight% W as not limited to the near surface for He plasma exposure [7,8]. The set of data of growth of the nano-structured layer from the references [7,8] exposed to the He plasma tends to demonstrate agreement with a fitted t1/2 dependence. This is a difference with the present experiment. It is also known that the same kind of nano-structure is formed by keV energy He irradiation [3]. In the more realistic reactor PMI scenario, high-temperature exposed W will not be subject to a pure He plasma. The He fraction in the ITER reactor exhaust for example will be 0.1, with the balance being largely hydrogen isotopes. To explore the effects of the mixture, the present experiment exposed two newly developed kinds of tungsten to a simulated divertor exhaust D-0.2He mixture plasmas at 1123 K. In addition, it seems that the presence of D is not responsible for any reduction in the nano-structured layer growth rate at this temperature. Surface and cross sectional SEM analyses revealed the same surface modification by the growth of nano-structured layer. Comparing with results of growth of nano-structured layer thickness of conventional W exposed to He plasma [7,8] indicated that the growth rate depends on the kind of sample, influenced for example, by the microstructure and manufacturing process. As shown in Fig. 5, He arrives in the interface between nano-structured layer
Fig. 8. Schematic of the cross section of the sample for the present experimental condition.
and bulk SR-W through the nano-structure layer, and diffuses the bulk W materials. After that, nano-structured layer thickness grows. It seems that the growth rate of nano-structured layer thickness is considered to be due to migration and trap of He in W materials. These behavior is considered to depend on the microstructure when is formed at the time of manufacture. In particular, difference of growth rate and incubation time of the nano-structure layer between the UFG W-0.5 wt%TiC/H2, SR-W and conventional W from references [7,8] is key issue to find the mechanism. In addition, in the case of UFG W-0.5 wt%TiC/H2, long incubation time is need for the nano-structure formation. This means that this kind of W material have a resistance to formation of the nano-structure layer. The nano-structure is a potentially large sources of tungsten dust, in which hydrogen isotope can be retained, and may have a dramatic influence on the surface thermal properties. In addition, the nanoscale morphology formation on tungsten influences the lifetime of the tungsten divertor, due to remove and reformation of the nano-structured layer. Detailed understanding of the mechanism of formation and growth of the nano-structured layer is needed and will be carried out. Furthermore, R&D on methods of suppression and reduction the nano-structured layer phenomena will be needed. 4.2. Effect of Be seeding D-He–Be mixture plasma exposure experiments were also carried out. The surface was covered with Be, and a nano-structure still formed. It is believed that most of Be deposited on the surface was eroded by sputtering so a thick Be layer did not form. Fig. 8 shows a schematic of the cross section of the sample for the present experiment condition. It is conclude that He penetrates the very thin near surface Be/O layer and the nano-structure layer, and then arrives at the interface between the nano-structured layer and bulk W. After that, He diffuses in the bulk W and nanostructured layer thickness grows. As a result, the Be has little effect on the He induced nano-structured layer for the present experimental conditions. However, when energy decreases, for example, to 15 eV, a BeW alloy layer is formed that depends on the temperature, and the nano-structure is not formed [8]. 5. Conclusions Ultra-fine grain W-(0.5, 1.5)wt%TiC alloys and stress relieved powder metallurgy tungsten have been exposed to a high-flux D-
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He mixture plasma with and without Be. A remarkable change of the tungsten surfaces occurs during the plasma exposure at 1123 K. The color of samples after the exposure becomes visually black, replacing the original metallic sliver color. Formation of a nano-structured layer on the exposure surfaces is observed. The formation of the nano-structured layer might be related to He bubble formation. The growth rate of the nano-structured layer depends on the microstructure of the samples. In the case of D-He plasma with Be, at 60 eV, plasma sputters away Be deposits. Little effect is seen on the He induced nano-structured layer formation for an ion energy of 60 eV and flux of 3–6 1022/m2 s at 1123 K, for a He/D ion ratio of 0.2 and Be contents of 0.2%. At 15 eV, condition favor net Be deposition and formation of a He induced nanostructured layer is inhibited. The nano-structure is a potentially large sources of tungsten dust, in which hydrogen isotope can be retained, and may have a dramatic influence on surface thermal properties. In addition, the nano-structure morphology formation on tungsten will influence lifetime of the tungsten divertor due to remove and reformation of the nano-structured layer. Acknowledgement This work is supported by Japan–US Fusion Cooperation Program (TITAN).
References [1] M. Kaminsky (Ed.), Radiation Effects on Solid Surface, American Chemical Society, 1976, p. 112. [2] K. Tokunaga, O. Yoshikawa, K. Makise, N. Yoshida, J. Nucl. Mater. 307–311 (2002) 130. [3] K. Tokunaga, S. Tamura, N. Yoshida, K. Ezato, M. Taniguchi, K. Sato, S. Suzuki, M. Akiba, J. Nucl. Mater. 329–333 (2004) 757. [4] K. Tokunaga, T. Fujiwara, K. Ezato, S. Suzuki, M. Akiba, N. Yoshida, J. Nucl. Mater. 367–370 (2007) 812. [5] K. Tokunaga, R.P. Doerner, R. Seraydarian, N. Noda, Y. Kubota, N. Yoshida a, T. Sogabe, T. Kato, B. Schedler, J. Nucl. Mater. 313 (2003) 92. [6] S. Takamura, N. Ohno, D. Nishijima, S. Kajita, Plasma Fusion Res. 51 (2006) 1. [7] M.J. Baldwin, R.P. Doerner, Nucl. Fusion 48 (3) (2008) 035001. [8] M.J. Baldwin, R.P. Doerner, D. Nishijima, K. Tokunaga, Y. Ueda, J. Nucl. Mater. 390–391 (2009) 886. [9] H. Kurishita, Y. Amano, S. Kobayashi, K. Nakai, H. Arakawa, Y. Hiraoka, T. Takeda, K. Takabe, H. Matsui, J. Nucl. Mater. 367–370 (2007) 1453. [10] D. Nishijima, R.P. Doerner, M.J. Baldwin, E.M. Hollmann, R.P. Seraydarian, Phys. Plasmas 14 (2007) 103509. [11] H. Ullmaier, Radiat. Eff. 78 (1983) 1. [12] S. Nagata, B. Tsuchiya, T. Sugawara, N. Ohtsu, T. Shikama, J. Nucl. Mater. 307– 311 (2002) 1513. [13] D. Nishijima, M.Y. Ye, N. Ohno, S. Takamura, J. Nucl. Mater. 313–316 (2003) 97. [14] M.Y. Ye, H. Kanehara, S. Fukuta, N. Ohno, S. Takamura, J. Nucl. Mater. 313–316 (2003) 72. [15] D. Nishijima, M.Y. Ye, N. Ohno, S. Takamura, J. Nucl. Mater. 329–333 (2004) 1029. [16] R.N. Stuart, M.W. Guinan, R.J. Borg, Radiat. Eff. 30 (1976) 129. [17] V.N. Chernikov, H. Trinkaus, P. Jung, H. Ullmaier, J. Nucl. Mater. 170 (1990) 31.
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Nanoscale surface morphology of tungsten materials induced by Be-seeded D-He plasma exposure K. Tokunaga a,⇑, M.J. Baldwin b, R.P. Doerner b, D. Nishijima b, H. Kurishita c, T. Fujiwara a, K. Araki a, Y. Miyamoto a, N. Ohno d, Y. Ueda e a
Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka 816-8580, Japan Center for Energy Research, University of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0417, USA International Research Center for Nuclear Materials Science, IMR, Tohoku University, Oarai, Ibaraki 311-1313, Japan d Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan e Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan b c
a r t i c l e
i n f o
Article history: Available online 1 February 2011
a b s t r a c t Ultra-fine grain W-(0.5, 1.5)wt%TiC alloys and stress relieved powder metallurgy W have been exposed to D-He mixed plasmas, some with added Be. The fixed exposure conditions are ion energy 60 eV and flux 3 to 6 1022/m2 s. Sample temperature is 1123 K and exposure times scanned 1000–11,000 s. Typical He/D ion is 0.2 and Be is 0.2% of the plasma. A remarkable change of the tungsten surfaces results from the plasma exposures. Formation of a nano-structured layer on the exposure surfaces is observed and believed to be related to He bubble formation. In addition, the growth rate of the nano-structured layer depends on the microstructure of the samples. In the case of a D-He plasma with Be, at 60 eV, plasma sputters away most of Be deposits with little effect on the He induced nano-structured layer formation. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Tungsten or tungsten alloys are potential candidates for the divertor armor material in ITER and the DEMO reactor due to their very low erosion rate and high-temperature properties. However, heat load, hydrogen isotopes and helium atoms from the plasma, which affect damage accumulation and mechanical properties, may reduce these properties of tungsten [1]. In particular, it is well known that the accumulation of He is much harmful to metal because of its strong interaction with lattice defects. He drastically enhances the formation of bubbles due to the strong bonding to vacancies and their clusters. As a result, local swelling and degradation of mechanical properties of bulk materials takes place as well [2–4]. In recent, low energy and high-flux He exposure experiment on W materials have been carried out. As a result, very fine morphology [5] and nano-structure on W [6–8] have been investigated after the He irradiation. These phenomena are considered to depend on such as microstructure, manufacture process and orientation of crystal grain because of high dependency of He and latent defects such as dislocation, grain boundary, impurities and residual strain in W materials. In the present work, W materials, which have been newly developed for resistance to radiation damage, low temperature embrittlement and blister formation, have been exposed to low energy and high-flux He plasma to investigate adaptability ⇑ Corresponding author. Tel.: +81 92 583 7986; fax: +81 92 583 6899. E-mail address: [email protected] (K. Tokunaga). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2011.01.078
on the divertor armor materials. In addition, exposure experiments to low energy and high-flux D and He mixed plasma, some doped with Be have been performed to investigate the synergistic effects of simultaneous implantation of D, He and Be on modification of W materials under the more relevant the ITER operation.
2. Experimental Powder metallurgy tungsten (PM-W) and ultra-fine grain W-0.5 wt%TiC and W-1.5 wt%TiC alloys produced by mechanical alloying (MA) in a purified H2 atmosphere (UFG W-0.5 wt%TiC/H2 and UFG W-1.5 wt%TiC/H2) and hot isostatic pressing (HIP) were used in the present experiments. Sizes of PM-W and UFG W-(0.5, 1.5) wt%TiC alloys were 25.4 mm diameter and 1.5 mm thickness, and 20 mm diameter and 1.5 mm thickness, respectively. The UFG W-(0.5, 1.5) wt%TiC alloys were developed for resistance to radiation damage and low temperature embrittlement [9]. The PM-W used was stress relieved powder metallurgy tungsten (SRW) from ALMT. Corp prepared by sintering followed by warm rolling to a cross section reduction of 85–95% with intermediate heat treatments, and then stress-relief (SR) annealed at 1173 K for 3.6 ks. The rolling direction of the SR-W is perpendicular to the surface resulting in high resistance to blister formation for hydrogen bombardment below the displacement threshold energy, and to degradation of heat conduction due to crack formation along grain boundaries.
K. Tokunaga et al. / Journal of Nuclear Materials 417 (2011) 528–532
529
The sample was mechanically mounted on a holder, actively air cooled and irradiated in the PISCES-B linear-divertor-plasma simulator. The exposure conditions were fixed at ion energy of 60 eV and flux of 3–6 1022/m2 s. The Sample temperature was 1123 K and exposure times spanned 1000–11,000 s. The ratio of deuterium to helium ions was determined by optical spectroscopic measurement [10]. Typical ratio of He/D ion was 0.2 and Be content was 0.2%. Pure He plasma exposure experiments were also performed to compare the results with the D-He mixture plasma exposure. After exposure, surfaces and cross sections of the samples were examined with a scanning electron microscope (SEM). In addition, compositional information was obtained using energy dispersive X-ray microanalysis (EDX) and Auger electron spectroscopy analyzer (AES). Samples weight due to the exposure was also measured. 3. Results 3.1. Exposure to D-He plasma without Be Fig. 1 shows photographs of UFG W-0.5 wt%TiC/H2 before (a) and after (b) the irradiation. The sample color becomes visually black from the original metallic sliver color due to the irradiation. Fig. 2 shows SEM images of the surface of UFG W-0.5 wt%TiC/H2 and UFG W-1.5 wt%TiC/H2 after the irradiation. The surface layer consisting of interconnected nanoscale-rod or fiber-like structures found to form under exposure to D-He mixture plasma as shown in Fig. 2a. Shown in Fig. 2b is an SEM image of a cross section of the sample. The nano-scale fiber or rod dimensions have characteristic widths of the order of only a few tens nm. Compositional analyses revealed the nano-structured layer was consistent with pure W. The formation of the nano-structured layer might be related to helium bubble formation [11]. In addition, the W nano-structured layer is easily remove because the nano-rods are very delicate. The blackening of the surface by the exposure is attributed to the multiple scattering of light in the porous nano-structured layer at the surface, in addition to the diffuse reflection due to roughening of the surface. Shown in Fig. 3 is growth of the nano-structured layer of UFG W-0.5 wt%TiC/H2 with plasma exposure time. SEM cross sections of UFG-0.5 wt%W/H2 exposed at 1123 K to D-He plasmas for 2150 s, 5000 s and 10,000 s are shown. The thickness of the nano-structured layer increases with increasing exposure time and for the longest exposure time, a nano-structured layer in excess of about 4.5 lm thick was observed. Fig. 4 shows growth of thickness of the nano-structured layer of SR-W with plasma exposure time. SEM cross sections of PM-W exposed at 1123 K to D-He plasmas for 1100, 5100 and 11,000 s are included showing that the thickness of the nano-structured layer increased with increasing exposure time, similar to the result for UFG W-0.5 wt%TiC/H2. Shown in Fig. 5 is a high magnification im-
(a)
(b)
20mm Fig. 1. Images of UGF W-0.5 wt%TiC/H before (a) and after the plasma exposure (b). The exposure conditions are fixed at ion energy 60 eV and flux of 3.2–3.5 1022/ m2 s. Sample temperature is 1123 K and plasma exposure times were 2150 s. Ratio of He/D ion is 0.2.
Fig. 2. SEM images of surface of UFG W-0.5 wt%TiC/H2 (a) and cross section of UFG W-1.5 wt%TiC/H2 (b) after the plasma exposure. The exposure conditions of (a) were ion energy 60 eV and flux 3.2–3.5 1022/m2 s, sample temperature 1123 K. The exposure time was 2150 s, and ratio of He/D ion 0.2. The exposure conditions of (b) are ion energy 60 eV, flux 3.0–3.1 1022/m2 s, sample temperature 1123 K, The exposure time was 5120 s, and ratio of He/D ion 0.2.
age of the interface of a nano-structured layer and the base W of SR-W exposed to a D-He plasma at 1123 K for 11,000 s. This shows that the nano-structure is growing from the bulk W. Fig. 6 shows nano-structured layer thickness data, taken from images such as in Figs. 3 and 4 as a function of the square root of the plasma exposure time, for exposures conduced at 1123 K. Data from UFG W-1.5 wt%TiC/H2 is also added. In addition, data on thickness of the nano-structured layer of SR-W and UFG W-1.5 wt%TiC/H2 exposed to He plasma is shown. The set of data from SR-W and UFG W-0.5 wt%TiC/H2 exposed to the D-He plasma tends to have some deviations with a fitted t1/2 dependence. It is also interesting to note that the straight line fit does not pass through zero. The t1/2 axial intercepts at 400 s and 1600 s may be an indication of a short incubation or saturation time that precedes nanostructuring of SR-W and UFG W-0.5 wt%TiC/H2 alloy, respectively. In addition, there is little difference in the nano-structured layer thickness between UFG W-0.5 wt%TiC/H2 and UFG W-1.5 wt%TiC/ H2. The results from He plasma exposure are also plotted in Fig. 6. The fluence of the He plasma exposures are 2 or 3 times larger than the D-He mixture case, however, the thickness of the nano-structured layer is almost the same. This implies that the availability of diffusing He or He saturation of the W surface, contributes to growth of the nano-structured layer. In addition, in the D-He mixture case, it is speculated that the presence of D is not responsible for any reduction in the nano-structured layer growth rate at this temperature. 3.2. Exposure to D-He with Be The D-He–Be mixture plasma exposure experiments were also carried out using PISCES-B. SEM observation of surfaces and cross sections of the irradiated samples indicated nano-structured layer
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Bulk
(a)
Surface Surface
Bulk
(a)
Surface Surface
(b) (b)
(c) (c)
Fig. 3. SEM images of cross sections of the nano-structured layer on UFG W0.5 wt%TiC/H alloy with plasma exposure time. The UFG W was exposed at 1123 K to D-0.2He plasmas for 2150 s (a), 5000 s (b) and 10,000 s (c).
Fig. 4. SEM images of cross sections of the nano-structured layer on SR-W with plasma exposure time. The SR-W was exposed at 1123 K to a D-0.2He plasmas for 1,100 s (a), 5,100s (b) and 11,000 s (c).
is formed near the surface, as with exposure to the D-He plasma without Be. AES analyses of surfaces of UFG W-0.5 wt%TiC/H2 exposed to the D-He–Be plasma indicates that the surface was covered with Be and O as shown in Fig. 7. However, Be was not detected in the nano-structured material when examining the cross section of the samples, indicating that Be exists as a very thin layer near the sample surface. For the present experimental condition, most of Be deposited on the surface was eroded by sputtering and no thick Be layer formed on the surface.
4. Discussion 4.1. Microstructure dependence on surface modification Many studies have identified deleterious effects on high-temperature exposed tungsten surfaces induced by He plasmas for incident He ion energies below the displacement threshold energy. In a few instances, He blistering is observed at moderate temperature below 800 K [12], but more serious effects such as the formation of pits, holes and bubbles occur at higher temperature, above 1600 K [13–15]. The accumulation of He in defects and vacancies [16,17] is believed responsible for these effects [11] but the precise mechanisms are not well understood.
Fig. 5. High magnification SEM image of the interface between the nano-structured layer and unaffected metal of SR-W that was exposed to D-0.2He mixture plasma at 1123 K for 11,000 s.
In the temperature range 1070–1600 K, particularly relevant to the ITER ‘all metal’ scenario and to DEMO reactor, a surface layer consisting of very fine morphology [5] and nano-structure of W nano-rod-like structures [6–8] were found to form under exposure to a helium plasma in the divertor plasma simulators NAGDIS-II and PISCES-B. In the experiment in NAGDIS-II, vacuum plasma
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D
He
Be
Sputter
Be Nano-W
Sputter
He
Bulk-W
Fig. 6. Nano-structured layer thickness taken from images such as in Figs. 3 and 4 as a function of the square root of the plasma exposure time of SR-W and UFG W0.5 wt%TiC/H2. Data for UFG W-1.5 wt%TiC/H2 exposed to D-0.2He mixture plasma at 1123 K for 5120 s. In addition, thickness data for the nano-structured layer of SRW and UFG W-1.5 wt%TiC/H2 exposed to He plasma with a flux of 2–3 1022/m2 s at 1123 K is included.
Min: -13320
Max: 15370
O13.1%
dN(E)
Be 86.9%
O1 13.1 %
Be1 86.9 %
30
87
144
201
258
315
372
429
486
543
600
Kinetic energy Fig. 7. AES analyses of surface of UFG W-0.5 wt%TiC/H2 exposed to D-He plasma with Be at 1123 K for 10,100 s.
sprayed W coated graphite (VPS-W/graphite) samples were exposed to pure He plasma at temperature of 1250 and 1600 K. In addition, PISCES experiments revealed that growth of the nanostructured layer on the sheet of rolled PLANSEE 99.9999 weight% W as not limited to the near surface for He plasma exposure [7,8]. The set of data of growth of the nano-structured layer from the references [7,8] exposed to the He plasma tends to demonstrate agreement with a fitted t1/2 dependence. This is a difference with the present experiment. It is also known that the same kind of nano-structure is formed by keV energy He irradiation [3]. In the more realistic reactor PMI scenario, high-temperature exposed W will not be subject to a pure He plasma. The He fraction in the ITER reactor exhaust for example will be 0.1, with the balance being largely hydrogen isotopes. To explore the effects of the mixture, the present experiment exposed two newly developed kinds of tungsten to a simulated divertor exhaust D-0.2He mixture plasmas at 1123 K. In addition, it seems that the presence of D is not responsible for any reduction in the nano-structured layer growth rate at this temperature. Surface and cross sectional SEM analyses revealed the same surface modification by the growth of nano-structured layer. Comparing with results of growth of nano-structured layer thickness of conventional W exposed to He plasma [7,8] indicated that the growth rate depends on the kind of sample, influenced for example, by the microstructure and manufacturing process. As shown in Fig. 5, He arrives in the interface between nano-structured layer
Fig. 8. Schematic of the cross section of the sample for the present experimental condition.
and bulk SR-W through the nano-structure layer, and diffuses the bulk W materials. After that, nano-structured layer thickness grows. It seems that the growth rate of nano-structured layer thickness is considered to be due to migration and trap of He in W materials. These behavior is considered to depend on the microstructure when is formed at the time of manufacture. In particular, difference of growth rate and incubation time of the nano-structure layer between the UFG W-0.5 wt%TiC/H2, SR-W and conventional W from references [7,8] is key issue to find the mechanism. In addition, in the case of UFG W-0.5 wt%TiC/H2, long incubation time is need for the nano-structure formation. This means that this kind of W material have a resistance to formation of the nano-structure layer. The nano-structure is a potentially large sources of tungsten dust, in which hydrogen isotope can be retained, and may have a dramatic influence on the surface thermal properties. In addition, the nanoscale morphology formation on tungsten influences the lifetime of the tungsten divertor, due to remove and reformation of the nano-structured layer. Detailed understanding of the mechanism of formation and growth of the nano-structured layer is needed and will be carried out. Furthermore, R&D on methods of suppression and reduction the nano-structured layer phenomena will be needed. 4.2. Effect of Be seeding D-He–Be mixture plasma exposure experiments were also carried out. The surface was covered with Be, and a nano-structure still formed. It is believed that most of Be deposited on the surface was eroded by sputtering so a thick Be layer did not form. Fig. 8 shows a schematic of the cross section of the sample for the present experiment condition. It is conclude that He penetrates the very thin near surface Be/O layer and the nano-structure layer, and then arrives at the interface between the nano-structured layer and bulk W. After that, He diffuses in the bulk W and nanostructured layer thickness grows. As a result, the Be has little effect on the He induced nano-structured layer for the present experimental conditions. However, when energy decreases, for example, to 15 eV, a BeW alloy layer is formed that depends on the temperature, and the nano-structure is not formed [8]. 5. Conclusions Ultra-fine grain W-(0.5, 1.5)wt%TiC alloys and stress relieved powder metallurgy tungsten have been exposed to a high-flux D-
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He mixture plasma with and without Be. A remarkable change of the tungsten surfaces occurs during the plasma exposure at 1123 K. The color of samples after the exposure becomes visually black, replacing the original metallic sliver color. Formation of a nano-structured layer on the exposure surfaces is observed. The formation of the nano-structured layer might be related to He bubble formation. The growth rate of the nano-structured layer depends on the microstructure of the samples. In the case of D-He plasma with Be, at 60 eV, plasma sputters away Be deposits. Little effect is seen on the He induced nano-structured layer formation for an ion energy of 60 eV and flux of 3–6 1022/m2 s at 1123 K, for a He/D ion ratio of 0.2 and Be contents of 0.2%. At 15 eV, condition favor net Be deposition and formation of a He induced nanostructured layer is inhibited. The nano-structure is a potentially large sources of tungsten dust, in which hydrogen isotope can be retained, and may have a dramatic influence on surface thermal properties. In addition, the nano-structure morphology formation on tungsten will influence lifetime of the tungsten divertor due to remove and reformation of the nano-structured layer. Acknowledgement This work is supported by Japan–US Fusion Cooperation Program (TITAN).
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