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Strength and superconductivity of Nb3Al prepared by spark plasma sintering
Journal of Alloys and Compounds 336 (2002) 232–236
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Strength and superconductivity of Nb 3 Al prepared by spark plasma sintering a, b b b Xingguo Li *, Akihiko Chiba , Masaya Sato , Seiki Takashash b
a Department of Inorganic Materials, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China Department of Materials Science and Engineering, Faculty of Engineering, Iwate University, 4 -3 -5 Ueda, Morioka 020 -8551, Japan
Received 11 July 2001; accepted 10 September 2001
Abstract Nb 3 Al powder prepared by hydriding–dehydriding was sintered using spark plasma sintering. Experiments on high temperature strength and superconductivity show that the sintered samples have superior properties than those reported elsewhere. 2002 Elsevier Science B.V. All rights reserved. Keywords: Superconductors; Sintering; Hydrogen absorbing materials; Mechanical properties; Magnetic measurements
1. Introduction The intermetallic compound Nb 3 Al is widely investigated because of its high temperature strength, superior superconductivity and relatively small density [1–4]. As Nb 3 Al has an extremely high melting point and lack of deformability, it is impossible to prepare it by using the conventional metallurgy. The Nb tube process is a method developed for fabricating multifilamentary superconducting wire [1,5]. The complicated fabrication process, simple shape and mixture of impurity phases limit its application. Although powder metallurgy is considered an effective way to avoid these problems, industrial production of fine Nb 3 Al powder is difficult so far. Recently, however, we successfully prepared Nb 3 Al fine powder by a hydriding– dehydriding method, which leads to a narrow size distribution and to a nearby single phase with A15 structure [6–8]. In a previous study, we investigated the superconductivity of this powder and the result showed that the powder has good superconducting properties [9]. Thus, our next interest is what kind of properties it exhibits after sintering and whether it can be used in powder metallurgy. Spark plasma sintering (SPS) has recently received increasing attention as a new sintering process [10,11]. Its biggest advantage is that the dense sintering from a green compact can be completed in a short time without signifi*Corresponding author. E-mail address: [email protected] (X. Li).
cant grain growth since the oxide layer on the particle surface can be removed by spark plasma between the particles. In this study, we used SPS to sinter the Nb 3 Al powder and investigated high temperature strength and superconductivity of the sintered samples.
2. Experimental A schematic illustration of experimental equipment which was used for production of Nb 3 Al powder by hydriding–dehydriding was described previously [6–8]. Green compacts were prepared from powder under 350 mesh and then were sintered in a vacuum of |10 23 Pa at 1773 and 1873 K for 5 min under a pressure of 500 kgf / cm 2 . The heating rate is 10 K / min. Samples for optical microstructure observations were prepared by grinding on emery papers and finishing on 4000 lapping film. Samples for compression tests were cut from sintered buttons in a size of 2.532.534 mm 3 by a wire-cut spark machine and then mechanically polished with emery papers. Compression tests were carried out in a vacuum at 1773 and 1873 K by using an Instron testing machine at an initial strain rate of 1.5310 23 s 21 . Structure of samples was identified by X-ray diffraction (XRD) using monochromated Cu Ka radiation over 208#2u #908. The superconducting transition temperature was evaluated from a magnetization versus temperature curve using a superconducting quantum interference device (SQUID) in magnetic
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01864-3
X. Li et al. / Journal of Alloys and Compounds 336 (2002) 232 – 236
fields up to 50 kOe. For simplification, the samples sintered at 1773 and 1873 K are hereafter called samples A and B, respectively.
3. Results and discussion Fig. 1 shows optical micrographs before and after etching for the samples A (a, b) and B (c, d). The two samples are densely sintered though there are pinholes of less than 10 mm in size. Like the conventional methods, it is also difficult to prepare Nb 3 Al without any defects by SPS. However, the defects are fewer and smaller in this study. Moreover, samples with complicated shapes can be prepared without plastic deformation. By comparison of the sintering at 1773 and 1873 K, it can be considered that the sintering is almost completed at 1773 K and the grain growth begins when the temperature is more elevated. Grains are almost equiaxed or elongated, and the grain size is below 15 mm for the sample A and below 20 mm for the sample B, which are smaller than the results of Murayama
233
et al. who prepared Nb–Al alloys by diffusion annealing of fine Nb /Al layers [12]. This fine microstructure is attributed to a striking advantage of SPS that the whole process can be completed in a very short time. The fine microstructure is expected to result in not only a high strength but also in a high critical current density Jc in magnetic fields [13]. XRD patterns of the two samples are given in Fig. 2. The sample A consists of an Nb 3 Al matrix with a small amount of Nb 2 Al phase. Instead, the sample B consists of a single Nb 3 Al phase with A15 structure. It is usually quite difficult to prepare a sintered compact of the single Nb 3 Al phase by other methods [14–16]. Formation of the single Nb 3 Al phase in this study is due to the homogeneous nature of the powder composed of nearby single phase Nb 3 Al. It is known that the Nb 3 Al phase exists in the range 75–82 at.% Nb and the Nb 2 Al phase in the range 63–69 at.% Nb [17,18]. When considering the sample composition, it can be suggested that both the Nb 3 Al phase and the Nb 2 Al phase in sample A are an Nb-rich phase. The disappearance of the Nb 2 Al phase from sample B can
Fig. 1. Optical micrographs before and after chemical etching of the Nb 3 Al samples sintered at 1773 K (a), (b) and at 1873 K (c), (d).
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X. Li et al. / Journal of Alloys and Compounds 336 (2002) 232 – 236
Fig. 2. XRD patterns of the Nb 3 Al samples sintered at 1773 K (a) and at 1873 K (b).
be explained by assuming that the extra Nb in the Nb 3 Al phase reacts with the Nb 2 Al phase to form Nb 3 Al. The high-temperature compressive properties of samples A and B are shown in Fig. 3a and b, respectively. The testing temperatures are selected to be 1373, 1473 and 1573 K. Similar to the sintered Nb 3 Al sample obtained from a different fabrication method [12], both samples tested at 1373 K exhibit a distinct peak stress followed by a remarkable deformation softening, and reach a steady state deformation. The peak stress of sample B is |750 MPa, higher than that of sample A. A cause for the difference in the peak stress between the two samples stems from the variation in microstructure of the two samples. In general, the strength of the materials with coarse microstructure is higher than that with fine microstructure at elevated temperatures, because the deformation occurs by grain boundary sliding as well as activation of the dislocation gliding during high-temperature deformation. In fact, as illustrated in Fig. 1, the microstructure of sample B is found to be coarser than that of sample A. Thus the high-temperature strength behavior in the present samples obey the general rule of high-temperature deformation mechanisms. This indicates that the present sintered sample fabricated by spark plasma sintering using
Fig. 3. Stress–strain curves at various temperatures for the Nb 3 Al samples sintered at 1773 K (a) and 1873 K (b).
alloy powder, leads to similar high-temperature mechanical properties as a sample obtained by the conventional sintering process [12]. Figs. 4–6 show the temperature dependence of the magnetization for the samples A, B and an Nb 3 Al ingot prepared by arc melting for comparison, respectively. The common characteristic of the three samples is that the magnetization exhibits an abrupt negative increase below a critical temperature, indicating that the samples transform from the paramagnetic state into the superconducting state. The critical temperature for the samples, i.e. the superconducting transition temperature T c , is 18 K, being consistent with reported values [3,19]. The magnetization first increases with increasing magnetic field below 2 kOe and then decreases in higher fields. T c decreases gradually with
X. Li et al. / Journal of Alloys and Compounds 336 (2002) 232 – 236
Fig. 4. Temperature dependence of magnetization for the Nb 3 Al sample sintered at 1773 K.
235
Fig. 6. Temperature dependence of magnetization for the Nb 3 Al ingot.
increasing magnetic field above 10 kOe. Compared with the result in a powdered state [9], it is known that the superior superconducting property in the powdered state is further improved after sintering. The magnetization of sample B is slightly larger than that of sample A and the Nb 3 Al ingot, indicating sample B has a high critical current density Jc in magnetic fields. The superconductivity of Nb–Al alloys is sensitive to composition. The better superconductivity of sample B is attributed to its good single phase condition.
4. Conclusions Dense sintering of Nb 3 Al powder prepared by hydriding and dehydriding can be performed by SPS at 1773 and 1873 K. The sintered samples possess a high temperature strength and a superconductivity superior to those reported so far. Hence, this kind of Nb 3 Al powder is promising in powder metallurgy for fabrication of high temperature materials and superconductors.
References
Fig. 5. Temperature dependence of magnetization for the Nb 3 Al sample sintered at 1873 K.
[1] C. Laveik, J. Less-Common Metals 139 (1988) 107. [2] R. Akihama, R.J. Murphy, S. Foner, IEEE Trans. Magn. MAG-17 (1981) 274.
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X. Li et al. / Journal of Alloys and Compounds 336 (2002) 232 – 236
[3] K. Barmak, K.R. Coffer, D.A. Rudman, S. Foner, J. Appl. Phys. 67 (1990) 7313. [4] K. Schulze, G. Muller, G. Petzow, J. Less-Common Metals 139 (1988) 97. [5] C.L.H. Thieme, S. Pourrahimi, B.B. Schwartz, S. Foner, Appl. Phys. Lett. 44 (1984) 260. [6] X.G. Li, K. Ohsaki, Y. Morita, M. Uda, J. Alloys Comp. 227 (1995) 141. [7] X.G. Li, A. Chiba, K. Ohsaki, Y. Morita, M. Uda, J. Alloys Comp. 238 (1996) 202. [8] X.G. Li, K. Kikuchi, M. Sato, A. Chiba, S. Takahashi, J. Alloys Comp. 282 (1999) 291. [9] X.G. Li, K. Ohsaki, A. Chiba, S. Takahashi, J. Mater. Res. 13 (1998) 2526. [10] R.A. Dugdate, J. Mater. Sci. 1 (1966) 160. [11] K. Upadhya, Am. Ceram. Soc. Bull. 67 (1988) 1691.
[12] Y. Murayama, S. Hanada, K. Obara, Mater. Trans. JIM 34 (1993) 325. [13] K. Togano, H. Kumakura, Y. Yoshida, K. Tachikawa, IEEE Trans. Magn. MAG-21 (1985) 463. [14] H. Kohmoto, N. Murahashi, T. Kohno, J. Jpn. Powder Powder Metall. 39 (1992) 815. [15] C.L.H. Thieme, S. Pourrahimi, S. Foner, IEEE Trans. Magn. 25 (1989) 1992. [16] W. Schaper, M. Kehlenbeck et al., IEEE Trans. Magn. 25 (1989) 1988. [17] J.L. Jorda, R. Flukiger, J. Muller, J. Less-Common Metals 75 (1980) 227. [18] K. Schulze, G. Muller, G. Petzow, J. Less-Common Metals 158 (1990) 71. [19] Handbook of Metals, Version 3, Japan Institute of Metal, 1993, p. 216.
L
www.elsevier.com / locate / jallcom
Strength and superconductivity of Nb 3 Al prepared by spark plasma sintering a, b b b Xingguo Li *, Akihiko Chiba , Masaya Sato , Seiki Takashash b
a Department of Inorganic Materials, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China Department of Materials Science and Engineering, Faculty of Engineering, Iwate University, 4 -3 -5 Ueda, Morioka 020 -8551, Japan
Received 11 July 2001; accepted 10 September 2001
Abstract Nb 3 Al powder prepared by hydriding–dehydriding was sintered using spark plasma sintering. Experiments on high temperature strength and superconductivity show that the sintered samples have superior properties than those reported elsewhere. 2002 Elsevier Science B.V. All rights reserved. Keywords: Superconductors; Sintering; Hydrogen absorbing materials; Mechanical properties; Magnetic measurements
1. Introduction The intermetallic compound Nb 3 Al is widely investigated because of its high temperature strength, superior superconductivity and relatively small density [1–4]. As Nb 3 Al has an extremely high melting point and lack of deformability, it is impossible to prepare it by using the conventional metallurgy. The Nb tube process is a method developed for fabricating multifilamentary superconducting wire [1,5]. The complicated fabrication process, simple shape and mixture of impurity phases limit its application. Although powder metallurgy is considered an effective way to avoid these problems, industrial production of fine Nb 3 Al powder is difficult so far. Recently, however, we successfully prepared Nb 3 Al fine powder by a hydriding– dehydriding method, which leads to a narrow size distribution and to a nearby single phase with A15 structure [6–8]. In a previous study, we investigated the superconductivity of this powder and the result showed that the powder has good superconducting properties [9]. Thus, our next interest is what kind of properties it exhibits after sintering and whether it can be used in powder metallurgy. Spark plasma sintering (SPS) has recently received increasing attention as a new sintering process [10,11]. Its biggest advantage is that the dense sintering from a green compact can be completed in a short time without signifi*Corresponding author. E-mail address: [email protected] (X. Li).
cant grain growth since the oxide layer on the particle surface can be removed by spark plasma between the particles. In this study, we used SPS to sinter the Nb 3 Al powder and investigated high temperature strength and superconductivity of the sintered samples.
2. Experimental A schematic illustration of experimental equipment which was used for production of Nb 3 Al powder by hydriding–dehydriding was described previously [6–8]. Green compacts were prepared from powder under 350 mesh and then were sintered in a vacuum of |10 23 Pa at 1773 and 1873 K for 5 min under a pressure of 500 kgf / cm 2 . The heating rate is 10 K / min. Samples for optical microstructure observations were prepared by grinding on emery papers and finishing on 4000 lapping film. Samples for compression tests were cut from sintered buttons in a size of 2.532.534 mm 3 by a wire-cut spark machine and then mechanically polished with emery papers. Compression tests were carried out in a vacuum at 1773 and 1873 K by using an Instron testing machine at an initial strain rate of 1.5310 23 s 21 . Structure of samples was identified by X-ray diffraction (XRD) using monochromated Cu Ka radiation over 208#2u #908. The superconducting transition temperature was evaluated from a magnetization versus temperature curve using a superconducting quantum interference device (SQUID) in magnetic
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01864-3
X. Li et al. / Journal of Alloys and Compounds 336 (2002) 232 – 236
fields up to 50 kOe. For simplification, the samples sintered at 1773 and 1873 K are hereafter called samples A and B, respectively.
3. Results and discussion Fig. 1 shows optical micrographs before and after etching for the samples A (a, b) and B (c, d). The two samples are densely sintered though there are pinholes of less than 10 mm in size. Like the conventional methods, it is also difficult to prepare Nb 3 Al without any defects by SPS. However, the defects are fewer and smaller in this study. Moreover, samples with complicated shapes can be prepared without plastic deformation. By comparison of the sintering at 1773 and 1873 K, it can be considered that the sintering is almost completed at 1773 K and the grain growth begins when the temperature is more elevated. Grains are almost equiaxed or elongated, and the grain size is below 15 mm for the sample A and below 20 mm for the sample B, which are smaller than the results of Murayama
233
et al. who prepared Nb–Al alloys by diffusion annealing of fine Nb /Al layers [12]. This fine microstructure is attributed to a striking advantage of SPS that the whole process can be completed in a very short time. The fine microstructure is expected to result in not only a high strength but also in a high critical current density Jc in magnetic fields [13]. XRD patterns of the two samples are given in Fig. 2. The sample A consists of an Nb 3 Al matrix with a small amount of Nb 2 Al phase. Instead, the sample B consists of a single Nb 3 Al phase with A15 structure. It is usually quite difficult to prepare a sintered compact of the single Nb 3 Al phase by other methods [14–16]. Formation of the single Nb 3 Al phase in this study is due to the homogeneous nature of the powder composed of nearby single phase Nb 3 Al. It is known that the Nb 3 Al phase exists in the range 75–82 at.% Nb and the Nb 2 Al phase in the range 63–69 at.% Nb [17,18]. When considering the sample composition, it can be suggested that both the Nb 3 Al phase and the Nb 2 Al phase in sample A are an Nb-rich phase. The disappearance of the Nb 2 Al phase from sample B can
Fig. 1. Optical micrographs before and after chemical etching of the Nb 3 Al samples sintered at 1773 K (a), (b) and at 1873 K (c), (d).
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X. Li et al. / Journal of Alloys and Compounds 336 (2002) 232 – 236
Fig. 2. XRD patterns of the Nb 3 Al samples sintered at 1773 K (a) and at 1873 K (b).
be explained by assuming that the extra Nb in the Nb 3 Al phase reacts with the Nb 2 Al phase to form Nb 3 Al. The high-temperature compressive properties of samples A and B are shown in Fig. 3a and b, respectively. The testing temperatures are selected to be 1373, 1473 and 1573 K. Similar to the sintered Nb 3 Al sample obtained from a different fabrication method [12], both samples tested at 1373 K exhibit a distinct peak stress followed by a remarkable deformation softening, and reach a steady state deformation. The peak stress of sample B is |750 MPa, higher than that of sample A. A cause for the difference in the peak stress between the two samples stems from the variation in microstructure of the two samples. In general, the strength of the materials with coarse microstructure is higher than that with fine microstructure at elevated temperatures, because the deformation occurs by grain boundary sliding as well as activation of the dislocation gliding during high-temperature deformation. In fact, as illustrated in Fig. 1, the microstructure of sample B is found to be coarser than that of sample A. Thus the high-temperature strength behavior in the present samples obey the general rule of high-temperature deformation mechanisms. This indicates that the present sintered sample fabricated by spark plasma sintering using
Fig. 3. Stress–strain curves at various temperatures for the Nb 3 Al samples sintered at 1773 K (a) and 1873 K (b).
alloy powder, leads to similar high-temperature mechanical properties as a sample obtained by the conventional sintering process [12]. Figs. 4–6 show the temperature dependence of the magnetization for the samples A, B and an Nb 3 Al ingot prepared by arc melting for comparison, respectively. The common characteristic of the three samples is that the magnetization exhibits an abrupt negative increase below a critical temperature, indicating that the samples transform from the paramagnetic state into the superconducting state. The critical temperature for the samples, i.e. the superconducting transition temperature T c , is 18 K, being consistent with reported values [3,19]. The magnetization first increases with increasing magnetic field below 2 kOe and then decreases in higher fields. T c decreases gradually with
X. Li et al. / Journal of Alloys and Compounds 336 (2002) 232 – 236
Fig. 4. Temperature dependence of magnetization for the Nb 3 Al sample sintered at 1773 K.
235
Fig. 6. Temperature dependence of magnetization for the Nb 3 Al ingot.
increasing magnetic field above 10 kOe. Compared with the result in a powdered state [9], it is known that the superior superconducting property in the powdered state is further improved after sintering. The magnetization of sample B is slightly larger than that of sample A and the Nb 3 Al ingot, indicating sample B has a high critical current density Jc in magnetic fields. The superconductivity of Nb–Al alloys is sensitive to composition. The better superconductivity of sample B is attributed to its good single phase condition.
4. Conclusions Dense sintering of Nb 3 Al powder prepared by hydriding and dehydriding can be performed by SPS at 1773 and 1873 K. The sintered samples possess a high temperature strength and a superconductivity superior to those reported so far. Hence, this kind of Nb 3 Al powder is promising in powder metallurgy for fabrication of high temperature materials and superconductors.
References
Fig. 5. Temperature dependence of magnetization for the Nb 3 Al sample sintered at 1873 K.
[1] C. Laveik, J. Less-Common Metals 139 (1988) 107. [2] R. Akihama, R.J. Murphy, S. Foner, IEEE Trans. Magn. MAG-17 (1981) 274.
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X. Li et al. / Journal of Alloys and Compounds 336 (2002) 232 – 236
[3] K. Barmak, K.R. Coffer, D.A. Rudman, S. Foner, J. Appl. Phys. 67 (1990) 7313. [4] K. Schulze, G. Muller, G. Petzow, J. Less-Common Metals 139 (1988) 97. [5] C.L.H. Thieme, S. Pourrahimi, B.B. Schwartz, S. Foner, Appl. Phys. Lett. 44 (1984) 260. [6] X.G. Li, K. Ohsaki, Y. Morita, M. Uda, J. Alloys Comp. 227 (1995) 141. [7] X.G. Li, A. Chiba, K. Ohsaki, Y. Morita, M. Uda, J. Alloys Comp. 238 (1996) 202. [8] X.G. Li, K. Kikuchi, M. Sato, A. Chiba, S. Takahashi, J. Alloys Comp. 282 (1999) 291. [9] X.G. Li, K. Ohsaki, A. Chiba, S. Takahashi, J. Mater. Res. 13 (1998) 2526. [10] R.A. Dugdate, J. Mater. Sci. 1 (1966) 160. [11] K. Upadhya, Am. Ceram. Soc. Bull. 67 (1988) 1691.
[12] Y. Murayama, S. Hanada, K. Obara, Mater. Trans. JIM 34 (1993) 325. [13] K. Togano, H. Kumakura, Y. Yoshida, K. Tachikawa, IEEE Trans. Magn. MAG-21 (1985) 463. [14] H. Kohmoto, N. Murahashi, T. Kohno, J. Jpn. Powder Powder Metall. 39 (1992) 815. [15] C.L.H. Thieme, S. Pourrahimi, S. Foner, IEEE Trans. Magn. 25 (1989) 1992. [16] W. Schaper, M. Kehlenbeck et al., IEEE Trans. Magn. 25 (1989) 1988. [17] J.L. Jorda, R. Flukiger, J. Muller, J. Less-Common Metals 75 (1980) 227. [18] K. Schulze, G. Muller, G. Petzow, J. Less-Common Metals 158 (1990) 71. [19] Handbook of Metals, Version 3, Japan Institute of Metal, 1993, p. 216.