- Email: [email protected]
Comparative study on arc-melted and plasma-sintered beryllides
Journal of Alloys and Compounds 546 (2013) 171–175
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Comparative study on arc-melted and plasma-sintered beryllides Jae-Hwan Kim ⇑, Masaru Nakamichi Blanket Irradiation and Analysis Group, Fusion Research and Development Directorate, Japan Atomic Energy Agency, 2-166 Oaza-Obuchi-Aza-Omotedate, Rokkasho-mura, Kamikita-gun, Aomori 039-3212, Japan
a r t i c l e
i n f o
Article history: Received 22 June 2012 Received in revised form 14 August 2012 Accepted 14 August 2012 Available online 28 August 2012 Keywords: Beryllide Microstructure Plasma-sintering Homogenization
a b s t r a c t This study compares arc-melted and plasma-sintered beryllides, beryllium intermetallic compounds, investigating the effects of density, hardness, microstructure, and annealing on stability. From the electron probe micro-analyzer and X-ray diffraction analysis results, it was obvious that the plasma-sintered samples, including those with 3 and 5 at.% Ti, were composed of a-Be and Be12Ti phases, while Be17Ti2 and Be2Ti phases were additionally observed for the Be–7 at.% Ti sample. In case of the arc-melted beryllides, it consisted of a-Be and Be12Ti in the 3 at.% Ti and 5 at.% Ti samples, whereas the Be12Ti phase surrounded Be10Ti as a peritectic phase for Be–7 at.% Ti. Density and hardness measurements confirmed that the plasma-sintered beryllides were sintered with higher density and hardness than the arc-melted beryllides. It is suggested that the higher sinterability and greater amount of Be12Ti phase resulted in increased density and hardness, respectively. In addition, the annealing treatment at 1473 K for 5 h was carried out to investigate the homogenization of the compositional structure. From the results of observation analysis for the specimens denoted as Be–3 at.% Ti and Be–5 at.% fabricated by both methods, there was no change on compositional structure. However, for arc-melted Be–7 at.% Ti, the Be phase was almost diminished, whereas there was a slight increase in the Be12Ti phase and a slight decrease in the Be10Ti phase. In the case of the plasma-sintered Be–7 at.% Ti specimen, it was clarified that the Be17Ti2 and Be2Ti phases were homogenized into the Be12Ti phase. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Beryllium intermetallic compounds (beryllide) such as Be12Ti are the most promising advanced neutron multipliers. The advanced neutron multiplier is being developed by Japan and the European Union (EU) in the demonstration reactor research and development (DEMO R&D) of the International Fusion Energy Research Centre (IFERC) project as part of Broader Approach (BA) activities from 2007 to 2016. As the importance of safety has been increasingly recognized, beryllides, specifically Be12Ti, are increasingly being seen as an alternative to beryllium, which is considered to be a neutron multiplier in the DEMO reactor. Except for the tritium breeding rate, Be12Ti has several advantages, including lower chemical reactivity [1], lower swelling behavior [2], and greater compatibility with structural materials [3]. Moreover, from the viewpoint of safety, it has been reported that the reactivity of beryllides with water vapor, which can generate a hydrogen explosion, is much lower than that of beryllium at high temperatures over 873 K [1]. However, despite the many advantages mentioned above, there have been few studies on the beryllium intermetallic compounds, that is,
⇑ Corresponding author. Tel.: +81 071 71 6537; fax: +81 175 71 6502. E-mail address: [email protected] (J.-H. Kim). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.08.098
beryllides, because they are too brittle to fabricate the rod-type electrodes for use with rotating electrode method for pebble production. Most studies on beryllides carried out so far have been based on samples prepared by conventional methods such as arc casting and hot isostatic pressing (HIP). However, these methods have significant problems, such as being a complex process that is time-consuming, expensive, and weak against thermal shock. The plasma-sintering method was suggested as a way to fabricate not only disk-type beryllide but also rod-type of beryllide. Because this method is significantly simpler and easier to control, production on a massive scale is more appropriate than conventional methods such as arc melting, casting, and hot isostatic pressing. In comparison to the conventional method, we have obtained very significant results by investigating the applicability of plasma-sintering methods for the synthesis of beryllides and the fabrication of rod-type beryllides. In previous studies [4,5], it was reported that the plasma-sintering method can directly synthesize not only disk-type but also rodtype beryllides, and the rod fabricated on a laboratory scale was 60 mm in length and 20 mm in diameter. Herein, we report the results of a comparison between the arcmelted beryllides reported so far and newly fabricated plasma-sin-
172
J.-H. Kim, M. Nakamichi / Journal of Alloys and Compounds 546 (2013) 171–175
Fig. 1. Relative densities of samples synthesized by arc-melting and plasmasintering method.
tered beryllides. In addition to the comparison, annealing treatments were carried out with the aim of producing a homogeneous Be12Ti phase.
2. Materials and methods For the comparison between arc-melted and plasma-sintered beryllides, samples with 3, 5, and 7 at.% Ti were fabricated using both the arc-melting method [6] and the plasma-sintered method [5]. For the arc-melting method, Be–Ti specimens were fabricated from a beryllium ingot with a purity of 99.5 wt.% and a Ti sponge with a purity of 99.99%. These materials were weighed and blended into the chemical compositions denoted as Be– 3 at.% Ti, Be–5 at.% Ti, and Be–7 at.% Ti. The samples were then charged into the arc-melting furnace with a water-cooled copper mold. The master alloys were sub-
sequently melted in the vacuum induction furnace. The disks were machined to approximately 8 mm in diameter and 2 mm in thickness and their surfaces were mirror-polished. For the plasma-sintering method, the mixed powder compositions were different, 3, 5, and 7 at.% Ti. The sintering temperatures used in the experiment were 1273 K and the sintering time was 20 min, where heating and cooling rates were 100 and 200 K/min, respectively. In order to compare the sinterability of the samples, the samples were first cut into 2.7 2.7 4.0 mm3 and polished. The density of each sample was then measured ten times using a pycnometer (AccupycII 1340-1CC, Shimadzu, Japan). For evaluation of the density, the measured value was compared with the theoretical one calculated from the analytical results of area fractions for each phase. For qualitative analyses of beryllides, back-scattered electron microscopic images were obtained using an electron probe microanalyzer (EPMA, JXA-8530F, JEOL, Japan). In order to investigate the compositional variation of beryllides synthesized by the two methods, X-ray diffraction measurements (XRD, Rigaku, UltimaIV, Japan) were conducted with a scan speed of 0.01° from 10° to 100°. Measurements for the area fractions of each phase were carried out using ImageJ software (ImageJ 1.44p, National Institutes of Health, USA), gathering the average data after several measurements. In order to understand the effect of annealing treatment for homogenizing the phase to Be12Ti phase, a heat treatment at 1473 K for 5 h was carried out under Ar gas flow (purity: 99.9999%) using an atmosphere-controllable electric furnace (KEF-1600, Kaken, Japan).
3. Results and discussion Sintering density is one of parameters that can be used to evaluate sinterability. For the comparison of arc-melted vs. plasma-sintered specimens, the density was calculated from the area fraction of each phase for the samples. As can be seen in Fig. 1, the sintering density for the plasma-sintered sample was higher than that for the arc-melted sample, regardless of the atomic percent of Ti. This demonstrates that the plasma-sintered sample is synthesized with a densification process. In general, plasma sintering consists of plasma generation, resistance heating, and pressure application.
Fig. 2. Microstructures for arc-melted and plasma-sintered specimens with Be–3 at.% Ti, Be–5 at.% Ti, and Be–7 at.% Ti.
J.-H. Kim, M. Nakamichi / Journal of Alloys and Compounds 546 (2013) 171–175
Accordingly, the plasma generation and pressure application can lead to a decrease in the number of impurities and good densification, respectively. These processes may contribute to an increase of relative density for the plasma-sintered specimen. Previous work [5] demonstrated that regardless of sintering time, every sample showed high density, which means over 95% of theoretical density (% T.D.). On the other hand, it is generally known that the density of a sample synthesized by the arc-melting method shows practically 85–90% [7] of theoretical density. In addition, the density of both specimens did not seem to depend on the amount of Ti. The high relative density for the plasma-sintered specimen was confirmed from the microstructures observed using an electron probe micro-analyzer. Fig. 2 shows the microstructures of beryllides fabricated by the arc-melted method and the plasma-sintered method according to the atomic percent of Ti. The plasma-sintered samples, including those with 3 and 5 at.% Ti, were composed of a-Be and Be12Ti phases, while Be17Ti2and Be2Ti phases were additionally observed for the Be–7 at.% Ti sample. Based on the phase diagram given in Fig. 3, a peritectic reaction is expected to have occurred. However, the plasma-sintering method makes it possible to consolidate and synthesize the beryllide, which is an intermetallic compound, below its melting point. Therefore, the majority of the phase for the Be–7 at.% Ti sample was the Be12Ti phase, which is the target composition. From the results of observations, it was obvious that these samples have a typical sintered phase. However, the arc-melted sample typically indicated casting phases, a columnar structure
Fig. 3. Be–Ti phase diagram.
173
that consisted of a-Be and Be12Ti in the 3 at.% Ti and 5 at.% Ti samples, whereas the Be12Ti phase surrounded Be10Tias a peritectic phase in Be–7 at.% Ti. This is in good agreement with the phase diagram given in Fig. 2. A comparison of the area fraction for each phase was conducted using the ImageJ program. The results of calculating the area fraction using ImageJ is given in Fig. 4, which confirms that the Be–7 at.% Ti sample had a different area fraction for each phase, even though there was no difference in the area fraction for each phase in Be–3 at.% Ti and Be–5 at.% Ti. The arc-melted Be–7 at.% Ti sample was mainly composed of Be12Ti and Be10Ti phases, at 45% and 41%, respectively. This is considerably different from the plasma-sintered sample, which was composed primarily of a Be12Ti phase, at 90%, with a small fraction of Be, Be17Ti2, and Be2Ti phases. Fig. 5 shows the XRD profiles of each specimen fabricated by the arc-melting and plasma-sintering methods. In the plasma-sintering methods, the specimens denoted as Be–3 at.% Ti and Be– 5 at.% were mainly composed of Be12Ti and Be, showing that the amount of the Be12Ti phase increased according to the level rule from the phase diagram, as shown in Fig. 3. The results of the XRD profiles demonstrated that the Be–7 at.% Ti specimen was composed of four different phases, Be, Be12Ti, Be17Ti2, and Be2Ti, as reported in previous studies [4,5].On the other hand, it was confirmed that in case of the arc-melting method, a majority of the phases in both the Be–3 at.% Ti and Be–5 at.% Ti specimens were Be12Ti and Be, the same compositions as those in the plasma-sintering method. For the Be–7 at.% Ti specimen, however, peaks corresponding to Be10Ti were detected in the XRD profile. This is in good agreement with the results of the SEM observation and area fraction analysis. Although plasma-sintering is a nonconventional consolidation method for fabrication of beryllide, it is clear that beryllide can be directly and easily synthesized without variation of the area fraction at lower temperatures than the melting points for each phase, which are 1562 K for Be, 1873 K for Be12Ti, 1905 K for Be17Ti2, and 1623 K for Be2Ti phase. Fig. 6 shows the result of the Vickers hardness test. The result confirmed that the plasma-sintered sample has greater hardness than the arc-melted sample. It can be thought that the hardness of the plasma-sintered specimen was greater than that of the arc-melted sample because the arc-melted sample is composed of a minute Be12Ti phase. In addition to a comparison of hardness between two samples, the hardness of each phase was investigated in Fig. 7 for the plasma-sintered sample, and it was found that the Be12Ti and Be17Ti2 phases had the greatest hardness, followed by the Be2Ti and Be
Fig. 4. Variation of area fraction for two samples fabricated by arc-melting and plasma-sintering.
174
J.-H. Kim, M. Nakamichi / Journal of Alloys and Compounds 546 (2013) 171–175
Fig. 5. XRD profiles of Be–3 at.% Ti, Be–5 at.% Ti, and Be–7 at.% Ti specimens fabricated by the arc-melting and plasma-sintering methods.
Fig. 6. Comparison of area fraction for arc-melted and plasma-sintered samples [8].
phases. As shown in Fig. 6, the increase of hardness with increasing Ti amount was caused by both the increase of Be12Ti and Be17Ti2 and the decrease of Be. Xue et al. [9] reported that the statistical relationship between Rockwell hardness (HRB) and relative density (D) has been formulated as follows; D = 5 10 3 HRB + 0.45, where it indicates proportional correlation. However, for the Be–Ti inter-
metallic compound, it does not seem to be proportional relationship between density and hardness because the difference of hardness between Be and Be intermetallic compounds is significant and each phase is dispersed. From the viewpoint of the tritium breeder rate in a fusion reactor, the desired target composition for beryllide is the Be12Ti phase. In order to determine whether the phase stability of the arc-melted and plasma-sintered samples are affected by heat treatment or not, the treatment was conducted at 1473 K for 5 h. Fig. 8 shows the SEM images of both annealed Be–7 at.% Ti specimens, which show that the Be2Ti and Be17Ti2 phases in the plasma-sintered specimen were homogenized to a Be12Ti phase, but the 3 at.% Ti and 5 at.% Ti specimens did not show any change while there was no significant difference of phase in the specimens fabricated by arc melting. In parallel to this phase transformation based on the phase diagram, the major phase of Be10Ti in the arc-melted Be–7 at.% Ti specimen was expected to transform into a Be12Ti phase. However, the Be phase was almost diminished, whereas the Be12Ti increased slightly and the Be10Ti phase decreased slightly. In addition, it was confirmed that grain growth occurred in respect to the annealing treatment at 1473 K on the other hand, in case of the plasmasintered Be–7 at.% Ti specimen, it was clarified that the Be17Ti2 and Be2Ti phases were homogenized into a Be12Ti phase, which is stoichiometrically stable, even though a Be phase remained. This is in good agreement with the phase diagram, indicating that the Be12Ti
Fig. 7. SEM observation of samples and their hardness as determined by the Vickers hardness test. (a) Be17Ti2, (b) Be2Ti, (c) Be12Ti, and (d) Be.
J.-H. Kim, M. Nakamichi / Journal of Alloys and Compounds 546 (2013) 171–175
175
Fig. 8. Microstructures of arc-melted and plasma-sintered Be–7 at.% Ti specimens after homogenization treatment at 1473 K for 5 h.
and Be phases simultaneously exist at 7 at.% Ti, although the amount of the Be phase is small (see Fig. 3). The phase homogenization may lead to useful applications such as in the case of phase distribution due to re-melting during the rotating electrode method for granulation of beryllide, in the use of beryllide blocks in the blanket of the DEMO reactor, and so on. 4. Conclusions A comparative study of arc-melted and plasma-sintered beryllides was conducted to clarify the differences between the two using density and hardness measurements, microstructure observation, and an annealing. Density and hardness measurements confirmed that the plasma-sintered beryllides were sintered with higher density and hardness than the arc-melted beryllides. It is suggested that the higher sinterability and larger amount of Be12Ti phase resulted in the increased density and hardness, respectively. From the results of the analysis based on the phase diagram, in the plasma-sintering methods, the Be–3 at.% Ti and Be–5 at.% Ti specimens were mainly composed of Be12Ti and Be, which showed that the amount of the Be12Ti phase increased. The results of the XRD profile indicated that the Be–7 at.% Ti specimen was composed of four different phases, Be, Be12Ti, Be17Ti2, and Be2Ti. However, it was clear that arc-melting specimen included a majority of Be12Ti and Be phases in both the Be–3 at.% Ti and Be–5 at.% Ti specimens. These results were not very different from those obtained
by the plasma-sintering method. For the Be–7 at.% Ti specimen, however, peaks corresponding to Be10Ti were detected in the XRD profile. This is in good agreement with the results of SEM observation and area fraction analysis. To investigate the homogenization of the compositional structure, an annealing treatment at 1473 K for 5 h was carried out. The result of observation analysis confirmed that for the Be– 3 at.% Ti and Be–5 at.% specimens fabricated by both methods there was no change in the compositional structure. However, for arcmelted Be–7 at.% Ti, the Be phase was almost diminished, whereas there were slight changes in the Be12Ti and Be10Ti phases, increasing and decreasing, respectively. In the case of the plasma-sintered Be–7 at.% Ti specimen, it was clarified that the Be17Ti2 and Be2Ti phase were homogenized into a Be12Ti phase. References [1] [2] [3] [4] [5] [6] [7]
K.Munakata et al., Fusion Engineering and Design 75–79 (2005) 997–1002. Y. Mishima et al., Journal of Nuclear Materials 367–370 (2007) 1382–1386. K. Tsuchiya et al., Fusion Engineering and Design 81 (8–14) (2006) 1057–1063. M. Nakamichi et al., Journal of Nuclear Materials 417 (2011) 765–768. J.-H. Kim et al., Journal of Nuclear Materials, submitted for publication. M. Uchida et al., Fusion Engineering and Design 69 (2003) 491–498. P. Kurinskiy, A. Moeslang, M. Klimiankou, A.A. Goraieb, H. Harsch, Fusion Engineering and Design 82 (15–24) (2007) 2353–2358. [8] M. Kawamura et al., Nuclear Fusion 43 (2003) 675–680. [9] Y. Xue, L.H. Lang, G.L. Bu, L. Li, Science of Sintering 43 (2011) 247–260.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Comparative study on arc-melted and plasma-sintered beryllides Jae-Hwan Kim ⇑, Masaru Nakamichi Blanket Irradiation and Analysis Group, Fusion Research and Development Directorate, Japan Atomic Energy Agency, 2-166 Oaza-Obuchi-Aza-Omotedate, Rokkasho-mura, Kamikita-gun, Aomori 039-3212, Japan
a r t i c l e
i n f o
Article history: Received 22 June 2012 Received in revised form 14 August 2012 Accepted 14 August 2012 Available online 28 August 2012 Keywords: Beryllide Microstructure Plasma-sintering Homogenization
a b s t r a c t This study compares arc-melted and plasma-sintered beryllides, beryllium intermetallic compounds, investigating the effects of density, hardness, microstructure, and annealing on stability. From the electron probe micro-analyzer and X-ray diffraction analysis results, it was obvious that the plasma-sintered samples, including those with 3 and 5 at.% Ti, were composed of a-Be and Be12Ti phases, while Be17Ti2 and Be2Ti phases were additionally observed for the Be–7 at.% Ti sample. In case of the arc-melted beryllides, it consisted of a-Be and Be12Ti in the 3 at.% Ti and 5 at.% Ti samples, whereas the Be12Ti phase surrounded Be10Ti as a peritectic phase for Be–7 at.% Ti. Density and hardness measurements confirmed that the plasma-sintered beryllides were sintered with higher density and hardness than the arc-melted beryllides. It is suggested that the higher sinterability and greater amount of Be12Ti phase resulted in increased density and hardness, respectively. In addition, the annealing treatment at 1473 K for 5 h was carried out to investigate the homogenization of the compositional structure. From the results of observation analysis for the specimens denoted as Be–3 at.% Ti and Be–5 at.% fabricated by both methods, there was no change on compositional structure. However, for arc-melted Be–7 at.% Ti, the Be phase was almost diminished, whereas there was a slight increase in the Be12Ti phase and a slight decrease in the Be10Ti phase. In the case of the plasma-sintered Be–7 at.% Ti specimen, it was clarified that the Be17Ti2 and Be2Ti phases were homogenized into the Be12Ti phase. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Beryllium intermetallic compounds (beryllide) such as Be12Ti are the most promising advanced neutron multipliers. The advanced neutron multiplier is being developed by Japan and the European Union (EU) in the demonstration reactor research and development (DEMO R&D) of the International Fusion Energy Research Centre (IFERC) project as part of Broader Approach (BA) activities from 2007 to 2016. As the importance of safety has been increasingly recognized, beryllides, specifically Be12Ti, are increasingly being seen as an alternative to beryllium, which is considered to be a neutron multiplier in the DEMO reactor. Except for the tritium breeding rate, Be12Ti has several advantages, including lower chemical reactivity [1], lower swelling behavior [2], and greater compatibility with structural materials [3]. Moreover, from the viewpoint of safety, it has been reported that the reactivity of beryllides with water vapor, which can generate a hydrogen explosion, is much lower than that of beryllium at high temperatures over 873 K [1]. However, despite the many advantages mentioned above, there have been few studies on the beryllium intermetallic compounds, that is,
⇑ Corresponding author. Tel.: +81 071 71 6537; fax: +81 175 71 6502. E-mail address: [email protected] (J.-H. Kim). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.08.098
beryllides, because they are too brittle to fabricate the rod-type electrodes for use with rotating electrode method for pebble production. Most studies on beryllides carried out so far have been based on samples prepared by conventional methods such as arc casting and hot isostatic pressing (HIP). However, these methods have significant problems, such as being a complex process that is time-consuming, expensive, and weak against thermal shock. The plasma-sintering method was suggested as a way to fabricate not only disk-type beryllide but also rod-type of beryllide. Because this method is significantly simpler and easier to control, production on a massive scale is more appropriate than conventional methods such as arc melting, casting, and hot isostatic pressing. In comparison to the conventional method, we have obtained very significant results by investigating the applicability of plasma-sintering methods for the synthesis of beryllides and the fabrication of rod-type beryllides. In previous studies [4,5], it was reported that the plasma-sintering method can directly synthesize not only disk-type but also rodtype beryllides, and the rod fabricated on a laboratory scale was 60 mm in length and 20 mm in diameter. Herein, we report the results of a comparison between the arcmelted beryllides reported so far and newly fabricated plasma-sin-
172
J.-H. Kim, M. Nakamichi / Journal of Alloys and Compounds 546 (2013) 171–175
Fig. 1. Relative densities of samples synthesized by arc-melting and plasmasintering method.
tered beryllides. In addition to the comparison, annealing treatments were carried out with the aim of producing a homogeneous Be12Ti phase.
2. Materials and methods For the comparison between arc-melted and plasma-sintered beryllides, samples with 3, 5, and 7 at.% Ti were fabricated using both the arc-melting method [6] and the plasma-sintered method [5]. For the arc-melting method, Be–Ti specimens were fabricated from a beryllium ingot with a purity of 99.5 wt.% and a Ti sponge with a purity of 99.99%. These materials were weighed and blended into the chemical compositions denoted as Be– 3 at.% Ti, Be–5 at.% Ti, and Be–7 at.% Ti. The samples were then charged into the arc-melting furnace with a water-cooled copper mold. The master alloys were sub-
sequently melted in the vacuum induction furnace. The disks were machined to approximately 8 mm in diameter and 2 mm in thickness and their surfaces were mirror-polished. For the plasma-sintering method, the mixed powder compositions were different, 3, 5, and 7 at.% Ti. The sintering temperatures used in the experiment were 1273 K and the sintering time was 20 min, where heating and cooling rates were 100 and 200 K/min, respectively. In order to compare the sinterability of the samples, the samples were first cut into 2.7 2.7 4.0 mm3 and polished. The density of each sample was then measured ten times using a pycnometer (AccupycII 1340-1CC, Shimadzu, Japan). For evaluation of the density, the measured value was compared with the theoretical one calculated from the analytical results of area fractions for each phase. For qualitative analyses of beryllides, back-scattered electron microscopic images were obtained using an electron probe microanalyzer (EPMA, JXA-8530F, JEOL, Japan). In order to investigate the compositional variation of beryllides synthesized by the two methods, X-ray diffraction measurements (XRD, Rigaku, UltimaIV, Japan) were conducted with a scan speed of 0.01° from 10° to 100°. Measurements for the area fractions of each phase were carried out using ImageJ software (ImageJ 1.44p, National Institutes of Health, USA), gathering the average data after several measurements. In order to understand the effect of annealing treatment for homogenizing the phase to Be12Ti phase, a heat treatment at 1473 K for 5 h was carried out under Ar gas flow (purity: 99.9999%) using an atmosphere-controllable electric furnace (KEF-1600, Kaken, Japan).
3. Results and discussion Sintering density is one of parameters that can be used to evaluate sinterability. For the comparison of arc-melted vs. plasma-sintered specimens, the density was calculated from the area fraction of each phase for the samples. As can be seen in Fig. 1, the sintering density for the plasma-sintered sample was higher than that for the arc-melted sample, regardless of the atomic percent of Ti. This demonstrates that the plasma-sintered sample is synthesized with a densification process. In general, plasma sintering consists of plasma generation, resistance heating, and pressure application.
Fig. 2. Microstructures for arc-melted and plasma-sintered specimens with Be–3 at.% Ti, Be–5 at.% Ti, and Be–7 at.% Ti.
J.-H. Kim, M. Nakamichi / Journal of Alloys and Compounds 546 (2013) 171–175
Accordingly, the plasma generation and pressure application can lead to a decrease in the number of impurities and good densification, respectively. These processes may contribute to an increase of relative density for the plasma-sintered specimen. Previous work [5] demonstrated that regardless of sintering time, every sample showed high density, which means over 95% of theoretical density (% T.D.). On the other hand, it is generally known that the density of a sample synthesized by the arc-melting method shows practically 85–90% [7] of theoretical density. In addition, the density of both specimens did not seem to depend on the amount of Ti. The high relative density for the plasma-sintered specimen was confirmed from the microstructures observed using an electron probe micro-analyzer. Fig. 2 shows the microstructures of beryllides fabricated by the arc-melted method and the plasma-sintered method according to the atomic percent of Ti. The plasma-sintered samples, including those with 3 and 5 at.% Ti, were composed of a-Be and Be12Ti phases, while Be17Ti2and Be2Ti phases were additionally observed for the Be–7 at.% Ti sample. Based on the phase diagram given in Fig. 3, a peritectic reaction is expected to have occurred. However, the plasma-sintering method makes it possible to consolidate and synthesize the beryllide, which is an intermetallic compound, below its melting point. Therefore, the majority of the phase for the Be–7 at.% Ti sample was the Be12Ti phase, which is the target composition. From the results of observations, it was obvious that these samples have a typical sintered phase. However, the arc-melted sample typically indicated casting phases, a columnar structure
Fig. 3. Be–Ti phase diagram.
173
that consisted of a-Be and Be12Ti in the 3 at.% Ti and 5 at.% Ti samples, whereas the Be12Ti phase surrounded Be10Tias a peritectic phase in Be–7 at.% Ti. This is in good agreement with the phase diagram given in Fig. 2. A comparison of the area fraction for each phase was conducted using the ImageJ program. The results of calculating the area fraction using ImageJ is given in Fig. 4, which confirms that the Be–7 at.% Ti sample had a different area fraction for each phase, even though there was no difference in the area fraction for each phase in Be–3 at.% Ti and Be–5 at.% Ti. The arc-melted Be–7 at.% Ti sample was mainly composed of Be12Ti and Be10Ti phases, at 45% and 41%, respectively. This is considerably different from the plasma-sintered sample, which was composed primarily of a Be12Ti phase, at 90%, with a small fraction of Be, Be17Ti2, and Be2Ti phases. Fig. 5 shows the XRD profiles of each specimen fabricated by the arc-melting and plasma-sintering methods. In the plasma-sintering methods, the specimens denoted as Be–3 at.% Ti and Be– 5 at.% were mainly composed of Be12Ti and Be, showing that the amount of the Be12Ti phase increased according to the level rule from the phase diagram, as shown in Fig. 3. The results of the XRD profiles demonstrated that the Be–7 at.% Ti specimen was composed of four different phases, Be, Be12Ti, Be17Ti2, and Be2Ti, as reported in previous studies [4,5].On the other hand, it was confirmed that in case of the arc-melting method, a majority of the phases in both the Be–3 at.% Ti and Be–5 at.% Ti specimens were Be12Ti and Be, the same compositions as those in the plasma-sintering method. For the Be–7 at.% Ti specimen, however, peaks corresponding to Be10Ti were detected in the XRD profile. This is in good agreement with the results of the SEM observation and area fraction analysis. Although plasma-sintering is a nonconventional consolidation method for fabrication of beryllide, it is clear that beryllide can be directly and easily synthesized without variation of the area fraction at lower temperatures than the melting points for each phase, which are 1562 K for Be, 1873 K for Be12Ti, 1905 K for Be17Ti2, and 1623 K for Be2Ti phase. Fig. 6 shows the result of the Vickers hardness test. The result confirmed that the plasma-sintered sample has greater hardness than the arc-melted sample. It can be thought that the hardness of the plasma-sintered specimen was greater than that of the arc-melted sample because the arc-melted sample is composed of a minute Be12Ti phase. In addition to a comparison of hardness between two samples, the hardness of each phase was investigated in Fig. 7 for the plasma-sintered sample, and it was found that the Be12Ti and Be17Ti2 phases had the greatest hardness, followed by the Be2Ti and Be
Fig. 4. Variation of area fraction for two samples fabricated by arc-melting and plasma-sintering.
174
J.-H. Kim, M. Nakamichi / Journal of Alloys and Compounds 546 (2013) 171–175
Fig. 5. XRD profiles of Be–3 at.% Ti, Be–5 at.% Ti, and Be–7 at.% Ti specimens fabricated by the arc-melting and plasma-sintering methods.
Fig. 6. Comparison of area fraction for arc-melted and plasma-sintered samples [8].
phases. As shown in Fig. 6, the increase of hardness with increasing Ti amount was caused by both the increase of Be12Ti and Be17Ti2 and the decrease of Be. Xue et al. [9] reported that the statistical relationship between Rockwell hardness (HRB) and relative density (D) has been formulated as follows; D = 5 10 3 HRB + 0.45, where it indicates proportional correlation. However, for the Be–Ti inter-
metallic compound, it does not seem to be proportional relationship between density and hardness because the difference of hardness between Be and Be intermetallic compounds is significant and each phase is dispersed. From the viewpoint of the tritium breeder rate in a fusion reactor, the desired target composition for beryllide is the Be12Ti phase. In order to determine whether the phase stability of the arc-melted and plasma-sintered samples are affected by heat treatment or not, the treatment was conducted at 1473 K for 5 h. Fig. 8 shows the SEM images of both annealed Be–7 at.% Ti specimens, which show that the Be2Ti and Be17Ti2 phases in the plasma-sintered specimen were homogenized to a Be12Ti phase, but the 3 at.% Ti and 5 at.% Ti specimens did not show any change while there was no significant difference of phase in the specimens fabricated by arc melting. In parallel to this phase transformation based on the phase diagram, the major phase of Be10Ti in the arc-melted Be–7 at.% Ti specimen was expected to transform into a Be12Ti phase. However, the Be phase was almost diminished, whereas the Be12Ti increased slightly and the Be10Ti phase decreased slightly. In addition, it was confirmed that grain growth occurred in respect to the annealing treatment at 1473 K on the other hand, in case of the plasmasintered Be–7 at.% Ti specimen, it was clarified that the Be17Ti2 and Be2Ti phases were homogenized into a Be12Ti phase, which is stoichiometrically stable, even though a Be phase remained. This is in good agreement with the phase diagram, indicating that the Be12Ti
Fig. 7. SEM observation of samples and their hardness as determined by the Vickers hardness test. (a) Be17Ti2, (b) Be2Ti, (c) Be12Ti, and (d) Be.
J.-H. Kim, M. Nakamichi / Journal of Alloys and Compounds 546 (2013) 171–175
175
Fig. 8. Microstructures of arc-melted and plasma-sintered Be–7 at.% Ti specimens after homogenization treatment at 1473 K for 5 h.
and Be phases simultaneously exist at 7 at.% Ti, although the amount of the Be phase is small (see Fig. 3). The phase homogenization may lead to useful applications such as in the case of phase distribution due to re-melting during the rotating electrode method for granulation of beryllide, in the use of beryllide blocks in the blanket of the DEMO reactor, and so on. 4. Conclusions A comparative study of arc-melted and plasma-sintered beryllides was conducted to clarify the differences between the two using density and hardness measurements, microstructure observation, and an annealing. Density and hardness measurements confirmed that the plasma-sintered beryllides were sintered with higher density and hardness than the arc-melted beryllides. It is suggested that the higher sinterability and larger amount of Be12Ti phase resulted in the increased density and hardness, respectively. From the results of the analysis based on the phase diagram, in the plasma-sintering methods, the Be–3 at.% Ti and Be–5 at.% Ti specimens were mainly composed of Be12Ti and Be, which showed that the amount of the Be12Ti phase increased. The results of the XRD profile indicated that the Be–7 at.% Ti specimen was composed of four different phases, Be, Be12Ti, Be17Ti2, and Be2Ti. However, it was clear that arc-melting specimen included a majority of Be12Ti and Be phases in both the Be–3 at.% Ti and Be–5 at.% Ti specimens. These results were not very different from those obtained
by the plasma-sintering method. For the Be–7 at.% Ti specimen, however, peaks corresponding to Be10Ti were detected in the XRD profile. This is in good agreement with the results of SEM observation and area fraction analysis. To investigate the homogenization of the compositional structure, an annealing treatment at 1473 K for 5 h was carried out. The result of observation analysis confirmed that for the Be– 3 at.% Ti and Be–5 at.% specimens fabricated by both methods there was no change in the compositional structure. However, for arcmelted Be–7 at.% Ti, the Be phase was almost diminished, whereas there were slight changes in the Be12Ti and Be10Ti phases, increasing and decreasing, respectively. In the case of the plasma-sintered Be–7 at.% Ti specimen, it was clarified that the Be17Ti2 and Be2Ti phase were homogenized into a Be12Ti phase. References [1] [2] [3] [4] [5] [6] [7]
K.Munakata et al., Fusion Engineering and Design 75–79 (2005) 997–1002. Y. Mishima et al., Journal of Nuclear Materials 367–370 (2007) 1382–1386. K. Tsuchiya et al., Fusion Engineering and Design 81 (8–14) (2006) 1057–1063. M. Nakamichi et al., Journal of Nuclear Materials 417 (2011) 765–768. J.-H. Kim et al., Journal of Nuclear Materials, submitted for publication. M. Uchida et al., Fusion Engineering and Design 69 (2003) 491–498. P. Kurinskiy, A. Moeslang, M. Klimiankou, A.A. Goraieb, H. Harsch, Fusion Engineering and Design 82 (15–24) (2007) 2353–2358. [8] M. Kawamura et al., Nuclear Fusion 43 (2003) 675–680. [9] Y. Xue, L.H. Lang, G.L. Bu, L. Li, Science of Sintering 43 (2011) 247–260.