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Sinterability and mechanical properties of plasma-sintered beryllides with different Ti contents
Journal of Alloys and Compounds 556 (2013) 292–295
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Sinterability and mechanical properties of plasma-sintered beryllides with different Ti contents Jae-Hwan Kim ⇑, Masaru Nakamichi Breeding Functional Materials Development Group, Division of Blanket Research and Development, 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 27 November 2012 Received in revised form 27 December 2012 Accepted 28 December 2012 Available online 3 January 2013 Keywords: Plasma-sintered beryllide Sinterability Mechanical property Ti content Grain size
a b s t r a c t The synthesis of plasma-sintered beryllide with different Ti contents was carried out to investigate the sinterability and mechanical properties as a function of the phase composition. The electron probe microanalysis clarified that the area fraction of the Be12Ti phase increased up to a Be-7.7 at.% Ti and then sharply decreased while the area fraction of Be decreased. Specifically, in the Be-6 at.% Ti, Be17Ti2 and Be2Ti phases began to be formed. For the Be-10.5 at.% Ti, a large amount of the Be17Ti2 phase was detected. These results are in good agreement with the X-ray diffraction profiles obtained for the plasma-sintered beryllide with different Ti contents. In addition, the Vickers micro-hardness test results clearly prove that with increasing Ti content, the hardness increased but then became saturated near Be-7.7 at.% Ti. This hardness variation does not seem to be related to the grain size effect because the grain size increased with increasing Ti content. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction As a candidate for use in advance neutron multipliers in DEMO reactors, beryllides (beryllium intermetallic compounds) have been proven to have a lower chemical reactivity [1], lower swelling against helium [2], higher compatibility with structure materials [2], and lower tritium inventory compared to pure beryllium [3]. Among the elements which can generate intermetallic compounds, Be–Ti, Be–V, and Be–Mo are actively researched from the viewpoint of their high melting point and low activation. However, a relatively low melting point is also necessary to maintain the balance with Be from the viewpoint of the melting point (melting points of Be = 1551 K, Ti = 1933 K, V = 2163 K, and Mo = 2890 K). Therefore, Be–Ti beryllide is regarded as the most promising multiplier and has been extensively investigated. The advanced neutron multiplier is being developed by Japan and the EU in the DEMO R&D of the International Fusion Energy Research Centre (IFERC) project as part of Broader Approach (BA) activities from 2007 to 2016. Since the beryllides are considered to be loaded into blanket as pebble type in the DEMO reactor, in principle, it is inevitably necessary to fabricate not only rod type but pebble type of beryllides. However, fabrication of the beryllide is considerably difficult owing to its brittleness. For fabrication of this beryllide, vacuum casting [2], arc melting [4], and hot isostatic pressing methods [5] are generally applied. The plasma sintering method for the syn⇑ 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 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.12.149
thesis of beryllide [6] and optimization of the sintering conditions based on the effects of the sintering time [7] and temperature [6] on the sinterability of plasma-sintered beryllide have been suggested. However, there have been few studies investigating the effect of Ti content in plasma-sintered beryllides. Therefore, in this study, the effect of the Ti content on the sinterability of binary Be–Ti beryllide was investigated.
2. Materials and methods For the synthesis for beryllides, Be and Ti powders with high purities (>99.0% and >99.9%, respectively) were purchased from Materion (US) and Kojundo Chemical Lab. Co., Ltd. (Japan), respectively. The beryllides used in this study were prepared by a plasma sintering method [6,7]. To investigate the effect of the content of Ti on the sinterability of plasma-sintered beryllide, starting powders with different stoichiometric Ti contents of 3 at.%, 4 at.%, 5 at.%, 6 at.%, 7 at.%, 7.7 at.%, 9.1 at.%, and 10.5 at.% were blended based on phase diagram as shown in Fig. 1 for 60 min using a RM200 (Retsch, Germany) and loaded into highly strengthened graphite punches and dies [6]. After uniaxial pressure was applied for cold compaction, an alternating current of 500 A was applied for not only the creation of the plasma environment but also activation of the particle surfaces. The powder compact was resistance-heated by a direct current while the uniaxial pressure was applied to the material. Based on several reproducible fabrications, the optimized conditions for the plasma sintering were determined, as shown in Table 1. For the sintering conditions, the sintering temperature, time, and pressure were fixed at 1273 K, 20 min, and 50 MPa, respectively, while the alternating current, holding time, and frequency were set at 500 A, 30 s, and 20.2 Hz, respectively. The block-type plasmasintered beryllide was cut to 3.0 3.0 4.0 mm3 and polished using polishing disc with 15 lm SiC particles for the measurements. The density of the prepared beryllide was measured by gas a pycnometer (AccupycII 1340-1CC, Shimadzu, Japan) 10
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Fig. 1. Be–Ti phase diagram.
Table 1 Conditions applied in the plasma sintering method. Plasma sintering conditions Raw material Powder purity Powder size Mixture ratio Sintering temperature Sintering pressure Sintering time
Be and Ti powder >99% <45 lm 3, 4, 5, 6, 7, 7.7, 9.1, 10.5 at.% Ti 1273 K 50 MPa 20 min
times and averaged. The relative density was calculated by comparing the measured density and theoretical density calculated by considering the area fractions of each phase. Scanning electron microscopy images with back-scattered electrons were obtained using an electron probe micro analyzer (JXA-8530F, JEOL, Japan) for qualitative analyses of the beryllides. In addition, X-ray diffraction measurements were carried out at a scan speed of 0.01° from 10° to 100° to confirm the compositional variations of the beryllides with different Ti contents.
3. Results and discussion It has been previously reported [7] that beryllide fabrication using the plasma sintering method resulted in a significantly high sinterability. The relative density is one of the parameters which can be considered as sinterability. Herein, the relative density
Fig. 2. Densities of the plasma-sintered beryllide with different Ti contents.
293
was evaluated using a He gas pycnometer. Fig. 2 shows both the density and relative density calculated by considering the area fractions of each phase and the empirical density. Regardless of the Ti content, the relative density for all beryllides was over 97%, which is considerably high. For comparison, data for beryllide fabricated by an arc melting method are also shown in Fig. 2 [8]. This comparative result demonstrates that the densities of the plasma-sintered beryllides were higher than those of arc-melted beryllides. The plasma sintering activates the surface of the powder to improve the consolidation to the target composition. In addition to this result, the cross-sectional images demonstrate that there were a few pores in the plasma-sintered beryllides. No correlation was found between the Ti content and the relative density. Fig. 3 shows SEM images of the beryllides synthesized by the plasma sintering method at 1273 K using back scattered electrons for Be–X with X = 3, 4, 5, 6, 7, 7.7, 9.1, and 10.5 at.% Ti. The sintering temperature used in this study was maximized to prevent the formation of beryllium carbide due to reaction between the graphite punch and die. The EPMA analyses of the beryllides sintered at 1273 K clarified that the black, gray, light gray, and white areas correspond to Be, Be12Ti, Be17Ti2, and Be2Ti phases, respectively. For the plasma-sintered beryllide with 3 at.% Ti, a large fraction of the Be phase was observed. Based on the Be–Ti phase diagram shown in Fig. 1, with an increase of the Ti content from 3 at.% to 5 at.%, the Be12Ti phase increased while the Be phase decreased. This is in good accordance with the lever rule. However, in the case of beryllide with over 6 at.% Ti, a peritectic reaction, L + Be17Ti2 ? Be12Ti, is expected to occur based on the phase diagram. From the SEM observations of the beryllides with Ti contents ranging from 6 at.% to 10.5 at.%, not only the Be17Ti2 phase but also the Be2Ti phase was identified because the sintering temperature of 1273 K was not high enough for the mass transfer related to consolidation. This insufficient consolidation can be aided by increasing the sintering time as well as utilizing an annealing treatment, as reported in previous studies [8,9], where a larger area fraction of the Be12Ti phase was present in the beryllide samples with 7.7 at.% Ti as the sintering time was increased. For the synthesis of either a single Be10Ti phase or Be17Ti2 phase, beryllides were plasma-sintered stoichiometrically with 9.1 at.% or 10.5 at.% Ti, respectively. However, the Be10Ti phase newly reported in the phase diagram was not confirmed while a large fraction of the Be17Ti2 phase was obtained in the beryllide sintered with 10.5 at.% Ti. The results of the area fractions analyzed by an image program are given in Fig. 4. The area fraction of the Be12Ti phase in the Be-7.7 at.% Ti sample increased up to 88% and then sharply decreased while that of the Be phase decreased. For the beryllide with 10.5 at.% Ti, a large amount of the Be17Ti2 phase, approximately 50%, was detected. This is in good agreement with the XRD profiles of the plasma-sintered beryllide with different contents of Ti shown in Fig. 5. To investigate the mechanical properties as a function of the Ti content, micro-hardness testing was conducted. Fig. 6 shows the Vickers micro-hardness as a function of the content of Ti including the reference arc-melted beryllide. It is obvious that with increasing Ti content, the Vickers micro-hardness increases but becomes saturated near 7.7 at.% Ti. This result seems to be associated with the fraction of Be in the beryllide. Because the hardness of Be is the lowest (approximately 500 Hv) [8] among the phases in the plasma-sintered beryllide, the beryllide with the largest area fraction of Be possesses the lowest hardness. On the contrary, in the beryllides containing 7 at.% to 10.5 at.% Ti, the hardness is mainly dominated by Be12Ti (approximately 1200 Hv) and Be17Ti2 (approximately 1200 Hv) phases because they are composed of small fractions of Be and Be2Ti (approximately 900 Hv), as shown in Fig. 4.
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Fig. 3. SEM images of beryllides sintered at 1273 K for 20 min with (a) 3, (b) 4, (c) 5, (d) 6, (e) 7, (f) 7.7, (g) 9.1, and (h) 10.5 at.% Ti.
Fig. 6. Vickers micro-hardness and phase composition as a function of content of Ti. Fig. 4. Area fractions of each phase of the beryllide sintered at 1273 K as a function of the content of Ti.
Fig. 5. XRD profiles of the beryllide sintered at 1273 K.
It is necessary to consider not only the phase distribution but also the effect of the grain size on the hardness of the plasma-sin-
tered beryllide. Grain size is known to be an important factor in many fields because it influences many properties. From the viewpoint of tritium release, it is considered that the grain size of beryllide plays an important role to influence the release of tritium. Kurinskiy et al. reported that at low temperatures, the smaller the grain size of beryllide, the better tritium release properties it possesses [9]. In terms of mechanical properties, the Hall–Pitch equation, which defines the relationship between the hardness and grain size, has been applied to evaluate the properties of many materials. Herein, to investigate the grain size of the beryllides with different Ti contents, optical observations of the beryllides after chemical etching were carried out. Fig. 7 shows optical images of the beryllides, demonstrating that with increasing Ti content, the grain size gradually increases. For the beryllides with less than 7.7 at.% Ti, grains of Be12Ti and Be were clearly observed, corresponding to large round grains and either small round or small randomly shaped grains, respectively. On the other hand, the optical images of the beryllides with 9.1 at.% and 10.5 at.% Ti exhibited dissimilar grain growth tendencies. Fig. 8 depicted the variation of grain size for the beryllides, demonstrating that with increase of Ti content, the grain size increased up to 7.7 at.% Ti and then decreased. Since the size of the Be17Ti2 phase, which is mainly located between the Be12Ti and
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Fig. 7. Optical images of the beryllides with (a) 3, (b) 4, (c) 5, (d) 6, (e) 7, (f) 7.7, (g) 9.1, and (h) 10.5 at.% Ti.
decreases with increasing Ti content. In particular, in the beryllides with 6 at.% Ti, Be17Ti2 and Be2Ti phases began to be formed because of the peritectic reaction and insufficient consolidation, respectively. This is in good agreement with the XRD profiles obtained for the plasma-sintered beryllide with different Ti contents. In addition, the results of the Vickers micro-hardness tests obviously exhibited that the hardness increased but became saturated at around 7.7 at.% Ti as the Ti content increased. This is associated with a decrease of the Be phase, which has a relatively lower hardness than the other phases. Based on the grain size observations, this hardness variation does not seem to be attributed to the grain size effect because with increasing Ti content, the grain size increased up to 7 at.% Ti and then decreased. References Fig. 8. Grain size as a function of the content of Ti.
Be2Ti phases, is relatively small, it is suggested that the grain size decreased. 4. Conclusions To investigate the effect of the Ti content on the synthesis of plasma-sintered beryllides, beryllides with 3 at.% to 10.5 at.% Ti were fabricated. Regardless of the Ti content, the beryllides were sintered with a high relative density. The EPMA results demonstrate that for Be-3 at.% Ti to Be-5 at.%, Be12Ti increases while Be
[1] K. Munakata, H. Kawamura, M. Uchida, Fus. Eng. Des. 75–79 (2005) 997–1002. [2] Y. Mishima, N. Yoshida, H. Kawamura, K. Ishida, Y. Hatano, T. Shibayama, K. Munakata, Y. Sato, M. Uchida, K. Tsuchiya, S. Tanaka, J. Nucl. Mater. 367–370 (2007) 1382–1386. [3] H. Iwakiri, K. Yasunaga, N. Yoshida, M. Uchida, H. Kawamura, J. Nucl. Mater. 329–333 (2004) 880–884. [4] P. Kurinskiy, A. Moeslang, M. Klimiankou, A.A. Goraieb, J. Nucl. Mater. Part B 367–370 (2007) 1069–1072. [5] H. Kawamura, H. Takahashi, N. Yoshida, Y. Mishima, K. Ishida, T. Iwadachi, A. Cardella, J.G. van der Laan, M. Uchida, K. Munakata, Y. Sato, V. Shestakov, S. Tanaka, J. Nucl. Mater. 329–333 (2004) 112–118. [6] M. Nakamichi, K. Yonehara, J. Nucl. Mater. 417 (2011) 765–768. [7] J.-H. Kim, M. Nakamichi, J. Nucl. Mater., in press, http://dx.doi.org/10.1016/ j.jnucmat.2012.10.013. [8] J.-H. Kim, M. Nakamichi, J. Alloys Compd. 546 (2013) 171–175. [9] P. Kurinskiy, V. Chakin, A. Moeslang, R. Rolli, A.A. Goraieb, H. Harsch, E. Alvest, N. Franco, Fus. Eng. Des. 84 (2009) 1136–1139.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Sinterability and mechanical properties of plasma-sintered beryllides with different Ti contents Jae-Hwan Kim ⇑, Masaru Nakamichi Breeding Functional Materials Development Group, Division of Blanket Research and Development, 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 27 November 2012 Received in revised form 27 December 2012 Accepted 28 December 2012 Available online 3 January 2013 Keywords: Plasma-sintered beryllide Sinterability Mechanical property Ti content Grain size
a b s t r a c t The synthesis of plasma-sintered beryllide with different Ti contents was carried out to investigate the sinterability and mechanical properties as a function of the phase composition. The electron probe microanalysis clarified that the area fraction of the Be12Ti phase increased up to a Be-7.7 at.% Ti and then sharply decreased while the area fraction of Be decreased. Specifically, in the Be-6 at.% Ti, Be17Ti2 and Be2Ti phases began to be formed. For the Be-10.5 at.% Ti, a large amount of the Be17Ti2 phase was detected. These results are in good agreement with the X-ray diffraction profiles obtained for the plasma-sintered beryllide with different Ti contents. In addition, the Vickers micro-hardness test results clearly prove that with increasing Ti content, the hardness increased but then became saturated near Be-7.7 at.% Ti. This hardness variation does not seem to be related to the grain size effect because the grain size increased with increasing Ti content. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction As a candidate for use in advance neutron multipliers in DEMO reactors, beryllides (beryllium intermetallic compounds) have been proven to have a lower chemical reactivity [1], lower swelling against helium [2], higher compatibility with structure materials [2], and lower tritium inventory compared to pure beryllium [3]. Among the elements which can generate intermetallic compounds, Be–Ti, Be–V, and Be–Mo are actively researched from the viewpoint of their high melting point and low activation. However, a relatively low melting point is also necessary to maintain the balance with Be from the viewpoint of the melting point (melting points of Be = 1551 K, Ti = 1933 K, V = 2163 K, and Mo = 2890 K). Therefore, Be–Ti beryllide is regarded as the most promising multiplier and has been extensively investigated. The advanced neutron multiplier is being developed by Japan and the EU in the DEMO R&D of the International Fusion Energy Research Centre (IFERC) project as part of Broader Approach (BA) activities from 2007 to 2016. Since the beryllides are considered to be loaded into blanket as pebble type in the DEMO reactor, in principle, it is inevitably necessary to fabricate not only rod type but pebble type of beryllides. However, fabrication of the beryllide is considerably difficult owing to its brittleness. For fabrication of this beryllide, vacuum casting [2], arc melting [4], and hot isostatic pressing methods [5] are generally applied. The plasma sintering method for the syn⇑ 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 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.12.149
thesis of beryllide [6] and optimization of the sintering conditions based on the effects of the sintering time [7] and temperature [6] on the sinterability of plasma-sintered beryllide have been suggested. However, there have been few studies investigating the effect of Ti content in plasma-sintered beryllides. Therefore, in this study, the effect of the Ti content on the sinterability of binary Be–Ti beryllide was investigated.
2. Materials and methods For the synthesis for beryllides, Be and Ti powders with high purities (>99.0% and >99.9%, respectively) were purchased from Materion (US) and Kojundo Chemical Lab. Co., Ltd. (Japan), respectively. The beryllides used in this study were prepared by a plasma sintering method [6,7]. To investigate the effect of the content of Ti on the sinterability of plasma-sintered beryllide, starting powders with different stoichiometric Ti contents of 3 at.%, 4 at.%, 5 at.%, 6 at.%, 7 at.%, 7.7 at.%, 9.1 at.%, and 10.5 at.% were blended based on phase diagram as shown in Fig. 1 for 60 min using a RM200 (Retsch, Germany) and loaded into highly strengthened graphite punches and dies [6]. After uniaxial pressure was applied for cold compaction, an alternating current of 500 A was applied for not only the creation of the plasma environment but also activation of the particle surfaces. The powder compact was resistance-heated by a direct current while the uniaxial pressure was applied to the material. Based on several reproducible fabrications, the optimized conditions for the plasma sintering were determined, as shown in Table 1. For the sintering conditions, the sintering temperature, time, and pressure were fixed at 1273 K, 20 min, and 50 MPa, respectively, while the alternating current, holding time, and frequency were set at 500 A, 30 s, and 20.2 Hz, respectively. The block-type plasmasintered beryllide was cut to 3.0 3.0 4.0 mm3 and polished using polishing disc with 15 lm SiC particles for the measurements. The density of the prepared beryllide was measured by gas a pycnometer (AccupycII 1340-1CC, Shimadzu, Japan) 10
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Fig. 1. Be–Ti phase diagram.
Table 1 Conditions applied in the plasma sintering method. Plasma sintering conditions Raw material Powder purity Powder size Mixture ratio Sintering temperature Sintering pressure Sintering time
Be and Ti powder >99% <45 lm 3, 4, 5, 6, 7, 7.7, 9.1, 10.5 at.% Ti 1273 K 50 MPa 20 min
times and averaged. The relative density was calculated by comparing the measured density and theoretical density calculated by considering the area fractions of each phase. Scanning electron microscopy images with back-scattered electrons were obtained using an electron probe micro analyzer (JXA-8530F, JEOL, Japan) for qualitative analyses of the beryllides. In addition, X-ray diffraction measurements were carried out at a scan speed of 0.01° from 10° to 100° to confirm the compositional variations of the beryllides with different Ti contents.
3. Results and discussion It has been previously reported [7] that beryllide fabrication using the plasma sintering method resulted in a significantly high sinterability. The relative density is one of the parameters which can be considered as sinterability. Herein, the relative density
Fig. 2. Densities of the plasma-sintered beryllide with different Ti contents.
293
was evaluated using a He gas pycnometer. Fig. 2 shows both the density and relative density calculated by considering the area fractions of each phase and the empirical density. Regardless of the Ti content, the relative density for all beryllides was over 97%, which is considerably high. For comparison, data for beryllide fabricated by an arc melting method are also shown in Fig. 2 [8]. This comparative result demonstrates that the densities of the plasma-sintered beryllides were higher than those of arc-melted beryllides. The plasma sintering activates the surface of the powder to improve the consolidation to the target composition. In addition to this result, the cross-sectional images demonstrate that there were a few pores in the plasma-sintered beryllides. No correlation was found between the Ti content and the relative density. Fig. 3 shows SEM images of the beryllides synthesized by the plasma sintering method at 1273 K using back scattered electrons for Be–X with X = 3, 4, 5, 6, 7, 7.7, 9.1, and 10.5 at.% Ti. The sintering temperature used in this study was maximized to prevent the formation of beryllium carbide due to reaction between the graphite punch and die. The EPMA analyses of the beryllides sintered at 1273 K clarified that the black, gray, light gray, and white areas correspond to Be, Be12Ti, Be17Ti2, and Be2Ti phases, respectively. For the plasma-sintered beryllide with 3 at.% Ti, a large fraction of the Be phase was observed. Based on the Be–Ti phase diagram shown in Fig. 1, with an increase of the Ti content from 3 at.% to 5 at.%, the Be12Ti phase increased while the Be phase decreased. This is in good accordance with the lever rule. However, in the case of beryllide with over 6 at.% Ti, a peritectic reaction, L + Be17Ti2 ? Be12Ti, is expected to occur based on the phase diagram. From the SEM observations of the beryllides with Ti contents ranging from 6 at.% to 10.5 at.%, not only the Be17Ti2 phase but also the Be2Ti phase was identified because the sintering temperature of 1273 K was not high enough for the mass transfer related to consolidation. This insufficient consolidation can be aided by increasing the sintering time as well as utilizing an annealing treatment, as reported in previous studies [8,9], where a larger area fraction of the Be12Ti phase was present in the beryllide samples with 7.7 at.% Ti as the sintering time was increased. For the synthesis of either a single Be10Ti phase or Be17Ti2 phase, beryllides were plasma-sintered stoichiometrically with 9.1 at.% or 10.5 at.% Ti, respectively. However, the Be10Ti phase newly reported in the phase diagram was not confirmed while a large fraction of the Be17Ti2 phase was obtained in the beryllide sintered with 10.5 at.% Ti. The results of the area fractions analyzed by an image program are given in Fig. 4. The area fraction of the Be12Ti phase in the Be-7.7 at.% Ti sample increased up to 88% and then sharply decreased while that of the Be phase decreased. For the beryllide with 10.5 at.% Ti, a large amount of the Be17Ti2 phase, approximately 50%, was detected. This is in good agreement with the XRD profiles of the plasma-sintered beryllide with different contents of Ti shown in Fig. 5. To investigate the mechanical properties as a function of the Ti content, micro-hardness testing was conducted. Fig. 6 shows the Vickers micro-hardness as a function of the content of Ti including the reference arc-melted beryllide. It is obvious that with increasing Ti content, the Vickers micro-hardness increases but becomes saturated near 7.7 at.% Ti. This result seems to be associated with the fraction of Be in the beryllide. Because the hardness of Be is the lowest (approximately 500 Hv) [8] among the phases in the plasma-sintered beryllide, the beryllide with the largest area fraction of Be possesses the lowest hardness. On the contrary, in the beryllides containing 7 at.% to 10.5 at.% Ti, the hardness is mainly dominated by Be12Ti (approximately 1200 Hv) and Be17Ti2 (approximately 1200 Hv) phases because they are composed of small fractions of Be and Be2Ti (approximately 900 Hv), as shown in Fig. 4.
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Fig. 3. SEM images of beryllides sintered at 1273 K for 20 min with (a) 3, (b) 4, (c) 5, (d) 6, (e) 7, (f) 7.7, (g) 9.1, and (h) 10.5 at.% Ti.
Fig. 6. Vickers micro-hardness and phase composition as a function of content of Ti. Fig. 4. Area fractions of each phase of the beryllide sintered at 1273 K as a function of the content of Ti.
Fig. 5. XRD profiles of the beryllide sintered at 1273 K.
It is necessary to consider not only the phase distribution but also the effect of the grain size on the hardness of the plasma-sin-
tered beryllide. Grain size is known to be an important factor in many fields because it influences many properties. From the viewpoint of tritium release, it is considered that the grain size of beryllide plays an important role to influence the release of tritium. Kurinskiy et al. reported that at low temperatures, the smaller the grain size of beryllide, the better tritium release properties it possesses [9]. In terms of mechanical properties, the Hall–Pitch equation, which defines the relationship between the hardness and grain size, has been applied to evaluate the properties of many materials. Herein, to investigate the grain size of the beryllides with different Ti contents, optical observations of the beryllides after chemical etching were carried out. Fig. 7 shows optical images of the beryllides, demonstrating that with increasing Ti content, the grain size gradually increases. For the beryllides with less than 7.7 at.% Ti, grains of Be12Ti and Be were clearly observed, corresponding to large round grains and either small round or small randomly shaped grains, respectively. On the other hand, the optical images of the beryllides with 9.1 at.% and 10.5 at.% Ti exhibited dissimilar grain growth tendencies. Fig. 8 depicted the variation of grain size for the beryllides, demonstrating that with increase of Ti content, the grain size increased up to 7.7 at.% Ti and then decreased. Since the size of the Be17Ti2 phase, which is mainly located between the Be12Ti and
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Fig. 7. Optical images of the beryllides with (a) 3, (b) 4, (c) 5, (d) 6, (e) 7, (f) 7.7, (g) 9.1, and (h) 10.5 at.% Ti.
decreases with increasing Ti content. In particular, in the beryllides with 6 at.% Ti, Be17Ti2 and Be2Ti phases began to be formed because of the peritectic reaction and insufficient consolidation, respectively. This is in good agreement with the XRD profiles obtained for the plasma-sintered beryllide with different Ti contents. In addition, the results of the Vickers micro-hardness tests obviously exhibited that the hardness increased but became saturated at around 7.7 at.% Ti as the Ti content increased. This is associated with a decrease of the Be phase, which has a relatively lower hardness than the other phases. Based on the grain size observations, this hardness variation does not seem to be attributed to the grain size effect because with increasing Ti content, the grain size increased up to 7 at.% Ti and then decreased. References Fig. 8. Grain size as a function of the content of Ti.
Be2Ti phases, is relatively small, it is suggested that the grain size decreased. 4. Conclusions To investigate the effect of the Ti content on the synthesis of plasma-sintered beryllides, beryllides with 3 at.% to 10.5 at.% Ti were fabricated. Regardless of the Ti content, the beryllides were sintered with a high relative density. The EPMA results demonstrate that for Be-3 at.% Ti to Be-5 at.%, Be12Ti increases while Be
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