- Email: [email protected]
Effect of grain size on the sinterability and reactivity of vanadium-beryllium intermetallic compounds
Fusion Engineering and Design 144 (2019) 93–96
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Effect of grain size on the sinterability and reactivity of vanadium-beryllium intermetallic compounds
T
⁎
Jae-Hwan Kim , Masaru Nakamichi Fusion Energy Research and Development Directorate, National Institutes for Quantum and Radiological Science and Technology, QST, 2-166 Obuchi, Omotedate, Rokkasho, Aomori 039-3212, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Pulverization Intermetallic compounds Reactivity Grain size
Vanadium beryllium intermetallic compounds (Be12V beryllides) are attractive materials for refractory and fusion applications at high temperatures. In this study, to investigate the effect of grain size on sinterability and reactivity of beryllides at high temperatures (expecting operation temperatures), single-phase vanadium beryllides with different grain sizes were fabricated via pulverization of homogenized powders and plasma sintering. While evaluating sintered Be12V, as the ball-milling time increased, the powder size decreased. As this grain size deceased, density and hardness increased. With respect to thermal expansion, the coefficient of thermal expansion slightly increased with decreasing grain size. As results of reactivity and the scanning electron microscopic observations, it indicated that grain boundary oxidation is predominant at 1273 K, whereas intergranular oxidation prevails at 1473 K. Accordingly, as the grain size increased up to 10 μm, the weight gain and the H2 generation amount decreased at 1273 K and 1473 K, respectively.
1. Introduction Beryllium intermetallic compounds (beryllides), Be12Ti, Be12V, and Be13Zr, have attracted great interest as refractory materials for hightemperature application, advanced neutron multipliers for demonstration (DEMO) fusion application [1], because of their good stability at high temperatures. As refractory materials, these beryllides exhibit exceptional mechanical properties at high temperatures. They present the highest breaking strength at 1644 K among tungsten, molybdenum, boron carbide, silicon carbide, and other refractory materials [2]. The mechanical behavior of Be12V at high temperatures was reported by Nieh [3], suggesting that deformation was contributed by dislocation climb while fracture was initiated from grain boundaries subjected to tensile stresses as a result of microstructural observation. Regarding DEMO fusion application, the beryllides, Be12Ti [4], Be12V [5], Be12Nb [6], and Be13Zr [7], and the ternary beryllides, Be-TiV [8], and Be-Ti-Zr [9], were successfully fabricated to disk and pebble type via plasma sintering or by combing plasma sintering with a rotating electrode process. In parallel to these advances in fabrication processes, many characterizations have been conducted to clarify the superior properties of beryllides over those of beryllium, including high stability with water vapor at high temperatures [10], good inventory ⁎
for hydrogen isotopes [5], and low swelling [11] at high temperatures after irradiation. In particular, Be12V, a vanadium–beryllium intermetallic compound, is a promising candidate for advanced neutron multipliers owing to the easy fabrication process of single-phase pebbles and its low reactivity with water vapor at high temperatures [5]. However, few studies on the beryllide fabrication technologies and characterizations have been reported because the materials are difficult to manipulate. Moreover, grain size affects the properties of vanadium beryllides in ways that are unclear, and the current knowledge is insufficient to allow adopting beryllides in the applications mentioned above. In the present study, we investigate the grain size effects on the sintering and thermal properties and on the reactivity of vanadium beryllide at 1273 and 1473 K for high temperature applications. 2. Experiments and materials Beryllium and vanadium powders with size distribution less than 45 μm and purity higher than 99.5% were used as raw materials. The starting powders were mixed with a stoichiometric value of Be12V using a mortar for 1 h and homogenized by annealing treatment at 1473 K for 10 h. After the treatment, the homogenized powders were slight
Corresponding author. E-mail address: [email protected] (J.-H. Kim).
https://doi.org/10.1016/j.fusengdes.2019.04.081 Received 13 December 2018; Received in revised form 5 April 2019; Accepted 23 April 2019 Available online 03 May 2019 0920-3796/ © 2019 Elsevier B.V. All rights reserved.
Fusion Engineering and Design 144 (2019) 93–96
J.-H. Kim and M. Nakamichi
successfully fabricated. The averaged grain sizes (based on the ASTM) of the 0, 10-min, 1-h, 5-h, and 10-h milled specimens were 20, 7.7, 5.9, 4.8, and 3.8 μm, respectively. Because the description of particle and grain is different, depicting that the each particle may contain more than one grain at least, these values between the particle size and grain size, seem to be dissimilar. An observational result in this figure, moreover, indicates that relatively wide grain size distribution which caused by wide particle size distribution, identified.
grinded by the mortar again for 1 h to avoid powder consolidation. To make powders finer, they were pulverized with a planetary ball-milling machine for different time-intervals, 10 min, 1 h, 5 h, and 10 h. The powder size was investigated using a size distribution machine (SALD3100, Shimadzu) and through SEM observations (JXA-8530F, JEOL). The homogenized powders with different particle size distributions were plasma sintered under the same conditions: temperature 1273 K, time 20 min, and pressure 50 MPa. Crystallographic analysis based on the ASTM E 112 (A grain boundary intersection count method is determined by the number of times at a test line cut cross) were performed to determine the grain size of the plasma-sintered Be12V disk. At the same time, SEM and optical observations were conducted after chemical etching using a mixture (corresponding to 0.83 M solution of H2SO4) of pure water (100 ml) and H2SO4 solution (97%, 5 ml). For characterization of the sintered Be12V, the sintered density was measured with a He gas pycnometer (Accupyc II 1340-1CC, Shimadzu) to investigate sinterability of the sintered Be12V disk. Moreover, microVickers hardness (HM-221, Mitsutoyo) of the samples was examined with a load of 24.52N. Additionally, thermal expansion (TD5000SE, Netzsch) was evaluated at 373, 773, and 1273 K. Weight gain (TG-8110, Rigaku) as well as hydrogen generation (CP4900, Agilent) when Be12V disks were exposed to 15% water vapor/Ar at 1273 and 1473 K were investigated to clarify the effect of grain size on the reactivity of Be12V at high temperatures.
3.2. Sintering properties of Be12V The density and micro-hardness of the sintered specimens were investigated to evaluate the sinterability of Be12V (Fig. 3). As shown in this figure, longer ball-milling times (the finer powders and the narrower particle size distribution) lead to higher densities. In other words, with increasing grain size, density decreases. It is well known that the sintering density of Be12V strongly depends on the particle size distribution. It was analytically [8] and experimentally [9] clarified in cases of some compounds that powders with the narrow particle size distribution resulted in higher final relative density. Accordingly, this decreased density can be explained by speculation that the wide distribution results in the decrease of density because as ball-milling time increases, the grain size decreases while the particle size distribution narrows. With respect to hardness, the K value, which defined by a slope of curve for Be12V, was 1531.4 Hv μm−1/2 (see Fig. 4), which was determined by the Hall-Petch relation, whereas that for Be was 208 Hv μm−1/2 [15]. Despite of density difference and inhomogeneous grain size, it shows a linearly proportional relationship between hardness and grain size.
3. Results and discussion 3.1. Particle size of pulverized Be12V powders In the present study, homogenized Be12V powders were pulverized via planetary ball milling at different times to investigate the effect of grain size on Be12V properties. Fig. 1 shows the particle size distribution and SEM images of the powders, indicating that the total particle size distribution shifted to finer particle sizes with increasing milling time. However, when milling was performed for a long time-period, the effect on the particle size distribution became insignificant. In particular, the differences in the minimum sizes (left curve of each distribution) between 5- and 10-h milling are not significant, even though the average particle size still decreases with increasing ball-milling time. In general materials during sintering process, it is well known that sintering bulk density is depending on particle size and particle size distribution [12–14]. It is considerably apparent that the particle size distribution becomes narrower as ball-milling time increases. By using the powders with different particle sizes, sintered Be12Vs was fabricated by plasma sintering under the same conditions. Fig. 2 shows the SEM images of the sintered Be12V and indicates that the single-phase Be12V specimens with different grain sizes were
3.3. Thermal expansion Fig. 5 shows the thermal expansion coefficient of Be12V. It depicts that as the grain size increases, thermal expansion slightly decreases at temperatures of 373, 773, and 1273 K. Since density decreases while porosity increases with grain size increases (Fig. 3), the increased porosity may result in a decrease in the coefficient of thermal expansion (CTE). It is in good agreement with a result in the previous study [16], demonstrating that the effective thermal expansion coefficient decreases with increasing the porosity according to the micromechanics modelling. 3.4. Reactivity with water vapor Regarding high-temperature applications, reactivity with water vapor would be critical because water coolant systems are generally applied. Not only the neutron multiplier in the fusion field but also neutron targets in accelerator BCNT fields require materials that are more stable at high temperatures, even under water vapor. In this sense, we investigated the grain size effect on the reactivity, specifically on the weight gain and on the H2 generation amount, of Be12V. The weight gain and the integrated H2 amount with increasing Be12V reactivity with water vapor at high temperatures appear in Fig. 6. It depicts a significant difference between the oxidation behavior at 1273 and 1473 K. Although, in general, the weight gain decreases with increasing grain size, in this case, the weight gain induced by oxidation at 1473 K increases with increasing grain size. The result of the integrated H2 amount generated by Be12V and H2O is consistent with that of the weight gain. On account of the oxidation behavior at 1273 K, the weight gain increases with the grain size decreases. In contrast, as temperature increases to 1473 K, the predominant oxidation behavior changes to intergranular oxidation, suggesting that the grain boundary may play a role to disturb metal and oxide ion diffusion for oxidation. This suggestion is well-supported by our SEM observations (Fig. 7),
Fig. 1. Particle size distribution and SEM images of the Be12V powders with different ball-milling times. 94
Fusion Engineering and Design 144 (2019) 93–96
J.-H. Kim and M. Nakamichi
Fig. 2. SEM images of the sintered Be12V powders ball milled for different times (a) 0, (b) 10 min, (c) 1 h, (d) 5 h, and (e) 10 h.
Fig. 3. Density (a) and hardness (b) of the sintered Be12Vs with different grain sizes.
whereas intergranular oxidation at 1273 K is predominant. The above results on reactivity suggest that the grain size effect is significant for Be12V, which is beryllide whose grain size is smaller than 10 μm, whereas it is not as important when the grain size becomes larger than 10 μm. 4. Conclusion Beryllium intermetallic compounds (beryllides) are among the most promising candidates not only as neutron multipliers for fusion application but also as refractory materials for high-temperature applications. To clarify the grain size effect on the sinterability and reactivity of beryllides, single-phase vanadium beryllides with different grain sizes were successfully fabricated and its characterizations were investigated. The obtained results are given as following; Fig. 4. Hall-Petch relation of Be12V.
(1) We observed that both the density as well as the hardness decrease with increasing grain size. It can be thought that the narrower particle size distribution may lead to lower porosity and higher density. Moreover, the hardness tendency is consistent with the Hall-Petch equation. (2) In terms of the effect of grain size on thermal expansion, the CTE at 373, 773, and 1273 K slightly decreased with increasing grain size. The porosity increase may result in a decrease of the CTE because density is inversely correlated with grain size. (3) With regard to the oxidation behavior with water vapor at high temperatures, we observed the different temperature tendency on weight gain and H2 generation amount. In particular, the grain boundary oxidation at 1273 K is so predominant that the smaller the grain size (the greater grain boundary), the larger the weight gain as well as the corresponding H2 generation. However, when temperature is increased to 1473 K, the predominant oxidation behavior changed to intergranular oxidation. This suggested that the weight gain as well as the H2 generation amount decreased with the grain size because the grain boundary disturbs metal and oxide ion diffusion for oxidation. This oxidation behavior could be explained for beryllide with a grain size smaller than 10 μm because the effect of grain sizes less than 10 μm is not as significant as that for a grain size larger than 10 μm.
Fig. 5. Coefficient of thermal expansion (CTE) of beryllides at high temperature.
demonstrating that grain boundary areas near the surface were intensively oxidized at 1273 K whereas entire areas near the surface were randomly oxidized at 1473 K. This is considerably consistent with the oxidation of beryllium [15], depicting that oxidation at 1073 K occurred at a grain boundary, 95
Fusion Engineering and Design 144 (2019) 93–96
J.-H. Kim and M. Nakamichi
Fig. 6. Weight gain (a) and H2 amount (b) of Be12V as a function of grain size at 1273 and 1473 K.
Fig. 7. Cross-sectional SEM images of Be12Vs with different grain size of (a) 20 and (b) 3.8 μm after oxidation test at 1273 and with that of (c) 20 and (d) 3.8 μm after oxidation test at 1473 K.
Acknowledgements
[7] M. Nakamichi, J.-H. Kim, K. Ochiai, Beryllide pebble fabrication of Be-Zr compositions as advanced neutron multipliers, Fusion Eng. Des. 109–111 (2016) 1719–1723. [8] J.-H. Kim, M. Nakamichi, Synthesis and characteristics of ternary Be-Ti-V beryllide pebbles as advanced neutron multipliers, Fusion Eng. Des. 109–111 (2016) 1764–1768. [9] J.-H. Kim, M. Nakamichi, Fabrication and characterization of Be-Zr-Ti ternary beryllide pebbles, Fusion Eng. Des. (2017) to be submitted. [10] J.-H. Kim, M. Nakamichi, Reactivity of plasma-sintered beryllium-titanium intermetallic compounds with water vapor, J. Nucl. Mater. 455 (2014) 26–30. [11] Y. Mishima, et al., J. Nucl. Mater. 367–370 (2007) 1382–1386. [12] F.S. Shiau, T.T. Fang, T.H. Leu, Effect of particle size distribution on the microstructural evolution in the intermediate stage of sintering, J. Am. Ceram. Soc. 80 (2) (1997) 286–290. [13] T.S. Yeh, M.D. Sacks, Effect of particle size distribution on the sintering of alumina, J. Am. Ceram. Soc. 71 (12) (1988) 484–487. [14] J.M. Ting, R.Y. Lin, Effect of particle size distribution on the sintering. Part II. Sintering of alumina, J. Mater. Sci. 30 (9) (1995) 2382–2389. [15] J.-H. Kim, M. Nakamichi, Effect of grain size on the hardness and reacitivity of plasma-sintered beryllium, J. Nucl. Mater. 453 (2014) 22–26. [16] S. Ghabezloo, Micromechanical analysis of the effect of porosity on the thermal expansion coefficient of heterogeneous porous materials, Int. J. Rock Mech. Min. Sci. 55 (2013) 97–101.
This paper has been prepared within the framework of the Broader Approach Activities between the Government of Japan and the European Atomic Energy Community. References [1] M. Abdou, N.B. Morley, S. Smolentsev, A. Ying, S. Malang, A. Rowcliffe, M. Ulrickson, Blanket/first wall challenges and required R&D on the pathway to DEMO, Fusion Eng. Des. 100 (2015) 2–43. [2] A.W. Kenneth, Beryllium Chemistry and Processing, (2009), p. 136. [3] T.G. Nieh, J. Wadsworth, F.C. Grensing, J.M. Yang, Mechanical properties of vanadium beryllides, VBe12, J. Mater. Sci. 27 (1992) 2660–2664 10. [4] J.-H. Kim, M. Nakamichi, Sinterability and mechanical properties of plasma-sintered beryllides with different Ti contents, J. Alloys Compd. 556 (2013) 292–296. [5] J.-H. Kim, M. Nakamichi, Comparative study of sinterability and thermal stability in plasma-sintered niobium and vanadium beryllides, J. Alloys Compd. 638 (2015) 277–281. [6] M. Nakamichi, J.-H. Kim, M. Miyamoto, Fabrication and characterization of advanced neutron multipliers for DEMO blanket, Nucl. Mater. Energy 9 (2016) 55–58.
96
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Effect of grain size on the sinterability and reactivity of vanadium-beryllium intermetallic compounds
T
⁎
Jae-Hwan Kim , Masaru Nakamichi Fusion Energy Research and Development Directorate, National Institutes for Quantum and Radiological Science and Technology, QST, 2-166 Obuchi, Omotedate, Rokkasho, Aomori 039-3212, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Pulverization Intermetallic compounds Reactivity Grain size
Vanadium beryllium intermetallic compounds (Be12V beryllides) are attractive materials for refractory and fusion applications at high temperatures. In this study, to investigate the effect of grain size on sinterability and reactivity of beryllides at high temperatures (expecting operation temperatures), single-phase vanadium beryllides with different grain sizes were fabricated via pulverization of homogenized powders and plasma sintering. While evaluating sintered Be12V, as the ball-milling time increased, the powder size decreased. As this grain size deceased, density and hardness increased. With respect to thermal expansion, the coefficient of thermal expansion slightly increased with decreasing grain size. As results of reactivity and the scanning electron microscopic observations, it indicated that grain boundary oxidation is predominant at 1273 K, whereas intergranular oxidation prevails at 1473 K. Accordingly, as the grain size increased up to 10 μm, the weight gain and the H2 generation amount decreased at 1273 K and 1473 K, respectively.
1. Introduction Beryllium intermetallic compounds (beryllides), Be12Ti, Be12V, and Be13Zr, have attracted great interest as refractory materials for hightemperature application, advanced neutron multipliers for demonstration (DEMO) fusion application [1], because of their good stability at high temperatures. As refractory materials, these beryllides exhibit exceptional mechanical properties at high temperatures. They present the highest breaking strength at 1644 K among tungsten, molybdenum, boron carbide, silicon carbide, and other refractory materials [2]. The mechanical behavior of Be12V at high temperatures was reported by Nieh [3], suggesting that deformation was contributed by dislocation climb while fracture was initiated from grain boundaries subjected to tensile stresses as a result of microstructural observation. Regarding DEMO fusion application, the beryllides, Be12Ti [4], Be12V [5], Be12Nb [6], and Be13Zr [7], and the ternary beryllides, Be-TiV [8], and Be-Ti-Zr [9], were successfully fabricated to disk and pebble type via plasma sintering or by combing plasma sintering with a rotating electrode process. In parallel to these advances in fabrication processes, many characterizations have been conducted to clarify the superior properties of beryllides over those of beryllium, including high stability with water vapor at high temperatures [10], good inventory ⁎
for hydrogen isotopes [5], and low swelling [11] at high temperatures after irradiation. In particular, Be12V, a vanadium–beryllium intermetallic compound, is a promising candidate for advanced neutron multipliers owing to the easy fabrication process of single-phase pebbles and its low reactivity with water vapor at high temperatures [5]. However, few studies on the beryllide fabrication technologies and characterizations have been reported because the materials are difficult to manipulate. Moreover, grain size affects the properties of vanadium beryllides in ways that are unclear, and the current knowledge is insufficient to allow adopting beryllides in the applications mentioned above. In the present study, we investigate the grain size effects on the sintering and thermal properties and on the reactivity of vanadium beryllide at 1273 and 1473 K for high temperature applications. 2. Experiments and materials Beryllium and vanadium powders with size distribution less than 45 μm and purity higher than 99.5% were used as raw materials. The starting powders were mixed with a stoichiometric value of Be12V using a mortar for 1 h and homogenized by annealing treatment at 1473 K for 10 h. After the treatment, the homogenized powders were slight
Corresponding author. E-mail address: [email protected] (J.-H. Kim).
https://doi.org/10.1016/j.fusengdes.2019.04.081 Received 13 December 2018; Received in revised form 5 April 2019; Accepted 23 April 2019 Available online 03 May 2019 0920-3796/ © 2019 Elsevier B.V. All rights reserved.
Fusion Engineering and Design 144 (2019) 93–96
J.-H. Kim and M. Nakamichi
successfully fabricated. The averaged grain sizes (based on the ASTM) of the 0, 10-min, 1-h, 5-h, and 10-h milled specimens were 20, 7.7, 5.9, 4.8, and 3.8 μm, respectively. Because the description of particle and grain is different, depicting that the each particle may contain more than one grain at least, these values between the particle size and grain size, seem to be dissimilar. An observational result in this figure, moreover, indicates that relatively wide grain size distribution which caused by wide particle size distribution, identified.
grinded by the mortar again for 1 h to avoid powder consolidation. To make powders finer, they were pulverized with a planetary ball-milling machine for different time-intervals, 10 min, 1 h, 5 h, and 10 h. The powder size was investigated using a size distribution machine (SALD3100, Shimadzu) and through SEM observations (JXA-8530F, JEOL). The homogenized powders with different particle size distributions were plasma sintered under the same conditions: temperature 1273 K, time 20 min, and pressure 50 MPa. Crystallographic analysis based on the ASTM E 112 (A grain boundary intersection count method is determined by the number of times at a test line cut cross) were performed to determine the grain size of the plasma-sintered Be12V disk. At the same time, SEM and optical observations were conducted after chemical etching using a mixture (corresponding to 0.83 M solution of H2SO4) of pure water (100 ml) and H2SO4 solution (97%, 5 ml). For characterization of the sintered Be12V, the sintered density was measured with a He gas pycnometer (Accupyc II 1340-1CC, Shimadzu) to investigate sinterability of the sintered Be12V disk. Moreover, microVickers hardness (HM-221, Mitsutoyo) of the samples was examined with a load of 24.52N. Additionally, thermal expansion (TD5000SE, Netzsch) was evaluated at 373, 773, and 1273 K. Weight gain (TG-8110, Rigaku) as well as hydrogen generation (CP4900, Agilent) when Be12V disks were exposed to 15% water vapor/Ar at 1273 and 1473 K were investigated to clarify the effect of grain size on the reactivity of Be12V at high temperatures.
3.2. Sintering properties of Be12V The density and micro-hardness of the sintered specimens were investigated to evaluate the sinterability of Be12V (Fig. 3). As shown in this figure, longer ball-milling times (the finer powders and the narrower particle size distribution) lead to higher densities. In other words, with increasing grain size, density decreases. It is well known that the sintering density of Be12V strongly depends on the particle size distribution. It was analytically [8] and experimentally [9] clarified in cases of some compounds that powders with the narrow particle size distribution resulted in higher final relative density. Accordingly, this decreased density can be explained by speculation that the wide distribution results in the decrease of density because as ball-milling time increases, the grain size decreases while the particle size distribution narrows. With respect to hardness, the K value, which defined by a slope of curve for Be12V, was 1531.4 Hv μm−1/2 (see Fig. 4), which was determined by the Hall-Petch relation, whereas that for Be was 208 Hv μm−1/2 [15]. Despite of density difference and inhomogeneous grain size, it shows a linearly proportional relationship between hardness and grain size.
3. Results and discussion 3.1. Particle size of pulverized Be12V powders In the present study, homogenized Be12V powders were pulverized via planetary ball milling at different times to investigate the effect of grain size on Be12V properties. Fig. 1 shows the particle size distribution and SEM images of the powders, indicating that the total particle size distribution shifted to finer particle sizes with increasing milling time. However, when milling was performed for a long time-period, the effect on the particle size distribution became insignificant. In particular, the differences in the minimum sizes (left curve of each distribution) between 5- and 10-h milling are not significant, even though the average particle size still decreases with increasing ball-milling time. In general materials during sintering process, it is well known that sintering bulk density is depending on particle size and particle size distribution [12–14]. It is considerably apparent that the particle size distribution becomes narrower as ball-milling time increases. By using the powders with different particle sizes, sintered Be12Vs was fabricated by plasma sintering under the same conditions. Fig. 2 shows the SEM images of the sintered Be12V and indicates that the single-phase Be12V specimens with different grain sizes were
3.3. Thermal expansion Fig. 5 shows the thermal expansion coefficient of Be12V. It depicts that as the grain size increases, thermal expansion slightly decreases at temperatures of 373, 773, and 1273 K. Since density decreases while porosity increases with grain size increases (Fig. 3), the increased porosity may result in a decrease in the coefficient of thermal expansion (CTE). It is in good agreement with a result in the previous study [16], demonstrating that the effective thermal expansion coefficient decreases with increasing the porosity according to the micromechanics modelling. 3.4. Reactivity with water vapor Regarding high-temperature applications, reactivity with water vapor would be critical because water coolant systems are generally applied. Not only the neutron multiplier in the fusion field but also neutron targets in accelerator BCNT fields require materials that are more stable at high temperatures, even under water vapor. In this sense, we investigated the grain size effect on the reactivity, specifically on the weight gain and on the H2 generation amount, of Be12V. The weight gain and the integrated H2 amount with increasing Be12V reactivity with water vapor at high temperatures appear in Fig. 6. It depicts a significant difference between the oxidation behavior at 1273 and 1473 K. Although, in general, the weight gain decreases with increasing grain size, in this case, the weight gain induced by oxidation at 1473 K increases with increasing grain size. The result of the integrated H2 amount generated by Be12V and H2O is consistent with that of the weight gain. On account of the oxidation behavior at 1273 K, the weight gain increases with the grain size decreases. In contrast, as temperature increases to 1473 K, the predominant oxidation behavior changes to intergranular oxidation, suggesting that the grain boundary may play a role to disturb metal and oxide ion diffusion for oxidation. This suggestion is well-supported by our SEM observations (Fig. 7),
Fig. 1. Particle size distribution and SEM images of the Be12V powders with different ball-milling times. 94
Fusion Engineering and Design 144 (2019) 93–96
J.-H. Kim and M. Nakamichi
Fig. 2. SEM images of the sintered Be12V powders ball milled for different times (a) 0, (b) 10 min, (c) 1 h, (d) 5 h, and (e) 10 h.
Fig. 3. Density (a) and hardness (b) of the sintered Be12Vs with different grain sizes.
whereas intergranular oxidation at 1273 K is predominant. The above results on reactivity suggest that the grain size effect is significant for Be12V, which is beryllide whose grain size is smaller than 10 μm, whereas it is not as important when the grain size becomes larger than 10 μm. 4. Conclusion Beryllium intermetallic compounds (beryllides) are among the most promising candidates not only as neutron multipliers for fusion application but also as refractory materials for high-temperature applications. To clarify the grain size effect on the sinterability and reactivity of beryllides, single-phase vanadium beryllides with different grain sizes were successfully fabricated and its characterizations were investigated. The obtained results are given as following; Fig. 4. Hall-Petch relation of Be12V.
(1) We observed that both the density as well as the hardness decrease with increasing grain size. It can be thought that the narrower particle size distribution may lead to lower porosity and higher density. Moreover, the hardness tendency is consistent with the Hall-Petch equation. (2) In terms of the effect of grain size on thermal expansion, the CTE at 373, 773, and 1273 K slightly decreased with increasing grain size. The porosity increase may result in a decrease of the CTE because density is inversely correlated with grain size. (3) With regard to the oxidation behavior with water vapor at high temperatures, we observed the different temperature tendency on weight gain and H2 generation amount. In particular, the grain boundary oxidation at 1273 K is so predominant that the smaller the grain size (the greater grain boundary), the larger the weight gain as well as the corresponding H2 generation. However, when temperature is increased to 1473 K, the predominant oxidation behavior changed to intergranular oxidation. This suggested that the weight gain as well as the H2 generation amount decreased with the grain size because the grain boundary disturbs metal and oxide ion diffusion for oxidation. This oxidation behavior could be explained for beryllide with a grain size smaller than 10 μm because the effect of grain sizes less than 10 μm is not as significant as that for a grain size larger than 10 μm.
Fig. 5. Coefficient of thermal expansion (CTE) of beryllides at high temperature.
demonstrating that grain boundary areas near the surface were intensively oxidized at 1273 K whereas entire areas near the surface were randomly oxidized at 1473 K. This is considerably consistent with the oxidation of beryllium [15], depicting that oxidation at 1073 K occurred at a grain boundary, 95
Fusion Engineering and Design 144 (2019) 93–96
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Fig. 6. Weight gain (a) and H2 amount (b) of Be12V as a function of grain size at 1273 and 1473 K.
Fig. 7. Cross-sectional SEM images of Be12Vs with different grain size of (a) 20 and (b) 3.8 μm after oxidation test at 1273 and with that of (c) 20 and (d) 3.8 μm after oxidation test at 1473 K.
Acknowledgements
[7] M. Nakamichi, J.-H. Kim, K. Ochiai, Beryllide pebble fabrication of Be-Zr compositions as advanced neutron multipliers, Fusion Eng. Des. 109–111 (2016) 1719–1723. [8] J.-H. Kim, M. Nakamichi, Synthesis and characteristics of ternary Be-Ti-V beryllide pebbles as advanced neutron multipliers, Fusion Eng. Des. 109–111 (2016) 1764–1768. [9] J.-H. Kim, M. Nakamichi, Fabrication and characterization of Be-Zr-Ti ternary beryllide pebbles, Fusion Eng. Des. (2017) to be submitted. [10] J.-H. Kim, M. Nakamichi, Reactivity of plasma-sintered beryllium-titanium intermetallic compounds with water vapor, J. Nucl. Mater. 455 (2014) 26–30. [11] Y. Mishima, et al., J. Nucl. Mater. 367–370 (2007) 1382–1386. [12] F.S. Shiau, T.T. Fang, T.H. Leu, Effect of particle size distribution on the microstructural evolution in the intermediate stage of sintering, J. Am. Ceram. Soc. 80 (2) (1997) 286–290. [13] T.S. Yeh, M.D. Sacks, Effect of particle size distribution on the sintering of alumina, J. Am. Ceram. Soc. 71 (12) (1988) 484–487. [14] J.M. Ting, R.Y. Lin, Effect of particle size distribution on the sintering. Part II. Sintering of alumina, J. Mater. Sci. 30 (9) (1995) 2382–2389. [15] J.-H. Kim, M. Nakamichi, Effect of grain size on the hardness and reacitivity of plasma-sintered beryllium, J. Nucl. Mater. 453 (2014) 22–26. [16] S. Ghabezloo, Micromechanical analysis of the effect of porosity on the thermal expansion coefficient of heterogeneous porous materials, Int. J. Rock Mech. Min. Sci. 55 (2013) 97–101.
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