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Effect of Nb substitution on the temperature characteristics and microstructures of rapid-quenched NdFeB alloy
Journal of Alloys and Compounds 427 (2007) 78–81
Effect of Nb substitution on the temperature characteristics and microstructures of rapid-quenched NdFeB alloy Ran Zhang, Ying Liu ∗ , Jinwen Ye, Wenfeng Yang, Yilong Ma, Shengji Gao Department of Material Science and Engineering, Sichuan University, Chengdu 610064, PR China Received 11 January 2006; received in revised form 4 March 2006; accepted 7 March 2006 Available online 18 April 2006
Abstract Effect of Nb substitution on the magnetic properties, temperature characteristics and microstructure of rapid-quenched Nd11 Dy0.5 Fe82.4−n Nbn B6.1 (n = 0, 0.5, 1, 1.5, 2) permanent magnets has been investigated. It is found that Hcj increases with increasing Nb content and the irreversible flux loss decreases with that. Nb substitution makes grains smaller and leads to the homogenization and regularization of grains. Also Nb substitution can enhance exchange coupling pining field and reduce inner dispersal magnetic field. As a result, the irreversible flux loss decreases notably and the magnets can be used at higher temperature. The maximum operating temperature of Nd11 Dy0.5 Fe81.4 Nb1 B6.1 alloy is higher than 150 ◦ C. © 2006 Elsevier B.V. All rights reserved. Keywords: NdFeB; Magnetic properties; Temperature characteristics; Microstructure
1. Introduction Since the discovery of Nd2 Fe14 B compound in the early 1980s [1,2], NdFeB permanent magnet has been developed in an essential engineering material in the industrial applications. However, one of the major drawbacks of NdFeB magnet is its poor thermal stability, which limits the application temperature below 120 ◦ C. To improve the thermal stability of NdFeB magnet, extensive efforts have been made via the substitution of other elements. It has been found that, among all the substitutions studied, Nb substitution appears to be an effective way to improve the thermal stability and refine microstructure in sintered NdFeB magnets [3,4] and in nanocomposite ␣-Fe/Nd2 Fe14 B magnets [5]. Also in the case of [NdFeBx /Nbz ]n multiplayer films, the Nb spacer layer enriched the Nd2 Fe14 B grain boundary and reduced the grain size effectively [6,7]. Recent studies on nanocrystalline ␣-Fe/Nd2 Fe14 B-based magnets [8,9] indicate that Nb substitution appears to refine grain structure, possibly by increasing the nucleation density and stabilizing the remaining amorphous phase in the melt-spun ribbons. Research on the effect of Nb substitution in Nd2 Fe14 Bbased magnets also shows that the presence of Nb suppresses
∗
Corresponding author. Tel.: +86 28 85405332. E-mail address: [email protected] (Y. Liu).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.03.034
the thermally induced formation of poorly crystallized Fe-rich regions [4] and forms stable NbFeB phase in the homogenized state [10]. However, the effect of Nb substitution on the nanocrystalline Nd2 Fe14 B alloy has not been well understood yet, especially its role in improving the thermal stability and microstructure evolution. Thus, the present study is focused on the above two points on melt-spun Nd11 Dy0.5 Fe82.4−n Nbn B6.1 (n = 0, 0.5, 1, 1.5, 2) nanocrystalline magnets, aiming at understanding the role of Nb substitution in microstructure control and the thermal stability of the nanocrystalline magnets, which will shed some lights on the future design of the nanocrystalline magnets for scientific inquiry and industrial applications. 2. Experimental Alloys ingots with nominal composition of Nd11 Dy0.5 Fe82.4−n Nbn B6.1 were prepared by induction melting under Ar atmosphere, where n = 0, 0.5, 1, 1.5, 2 in atomic%. The NdFeB alloys were melt from Nd, Dy, Fe, Nb–Fe, B–Fe constituent elements. The ingots were broken into small pieces and melt-spun onto a molybdenum wheel from a crucible at a rolling speed of 28 m·s−1 in argon atmosphere. The melt-spun ribbons were annealed at 670–720 ◦ C for 30–50 min in a vacuum heat-treatment furnace to improve their magnetic properties. The microstructure of the ribbons was characterized with atomic force microscopy (AFM). The mixture, combined the magnetic powder with epoxy resin in the proportion of 100:2.5, was pressed into cylinders with diameter, height and density of 10, 10 mm and 6.1 g cm−3 or so, respectively. Then the bonded magnets were solidified at 120 ◦ C for 1 h. Finally, the magnetic properties of the bonded
R. Zhang et al. / Journal of Alloys and Compounds 427 (2007) 78–81
79
Table 1 Dependence of magnetic properties on Nb content for Nd11 Dy0.5 Fe82.4−n Nbn B6.1 magnets Alloy
Nb/(at%)
Br /(mT)
Hcj /(kA/m)
(BH)m /(kJ/m3 )
Nd11 Dy0.5 Fe82.4 B6.1 Nd11 Dy0.5 Fe81.9 Nb0.5 B6.1 Nd11 Dy0.5 Fe81.4 Nb1 B6.1 Nd11 Dy0.5 Fe80.9 Nb1.5 B6.1 Nd11 Dy0.5 Fe80.4 Nb2 B6.1
0 0.5 1 1.5 2
663.6 660.8 655.9 650.9 646.2
895 940 974 1003 1028
75 73 73 69 61
magnets were measured by AMT-4 automatic measuring instrument of magnetization characteristic (manufactured by Mianyang Shuangji electronic company, China) with a maximum applied field of 2 T at room temperature. The flux aging loss was measured with a Helmholtz coil on bonded magnets.
of their flux due to an exposure at an elevated temperature, Tex , was measured by the sample-extraction method as a function of the exposure time, ta . The irreversible flux loss, hirr , due to the exposure was calculated from:
3. Results and discussions
hirr =
3.1. Magnetic property enhancement The effects of Nb substitution on the room temperature magnetic properties, remanence (Br ), coercivity (Hcj ) and energy product (BHmax ), of annealed Nd11 Dy0.5 Fe82.4−n Nbn B6.1 (n = 0, 0.5, 1, 1.5, 2) bonded magnets samples are listed in Table 1. It can be seen that Nb substitution leads to a significant increase in the coercivity, but the remanence is slightly decreased. In comparison with alloy Nd11 Dy0.5 Fe82.4 B6.1 without Nb, the coercivity of alloy Nd11 Dy0.5 Fe81.4 Nb1 B6.1 (with a content of 1 at% Nb) is increased by 8.8%, but the remanence of alloy Nd11 Dy0.5 Fe81.4 Nb1 B6.1 is decreased only by 1.7%. That is to say, a little Nb substitution cannot decrease remanence remarkably but promote coercivity noticeably. At the same time, with the increasing Nb content, the energy product decreases gradually first, and then decreases considerably when increasing Nb to 1.5 at%. It’s all because that Nb substitution will give birth to nonmagnetic phase [11,12]. The nonmagnetic phase exists among grains, which can not only dilute the magnetic properties of the whole bonded magnet, but also separate soft phase grains and hard phase grains, weak the exchange coupled interaction between grains and decrease the magnetic properties. As a result, overfull Nb substitution will influence general magnetic properties of bonded magnets seriously. With a content of 1 at% Nb, the optimium properties of bonded magnets is achieved: Br = 655.9 mT, Hci = 974 kA m−1 and (BH)m = 73 kJ m−3 .
where φ(T0 ) and φ (T0 ) are the flux value measured at room temperature before and after the exposure. We studied the irreversible flux losses of Nd11 Dy0.5 Fe82.4−n Nbn B6.1 (n = 0, 0.5, 1, 1.5) bonded magnets (L/D = 1) exposed for 2 h at temperature 80, 100, 120, 150 and 170 ◦ C, respectively. The result is listed in Table 2. It can be seen from Table 2 that, as Nb substitution increases, the irreversible flux losses of all bonded magnets above decreases remarkably, and operating temperature is promoted gradually. When Nb substitution reaches 1 at%, the beneficial effect comes to the maximum, and the irreversible flux loss of Nd11 Dy0.5 Fe81.4 Nb1 B6.1 bonded magnets (L/D = 1) exposed at temperature 170 ◦ C for 2 h is only 3.81%. When continuing to increase the Nb substitution, the irreversible flux loss doesn’t decline any more. The irreversible flux losses of Nd11 Dy0.5 Fe81.4 Nb1 B6.1 alloy and Nd11 Dy0.5 Fe80.9 Nb1.5 B6.1 alloy exposed at same temperature are basically same. We can conclude from above explanation that Nb substitution can notably improve the temperature resistance of rapid-quenched NdFeB magnets, and make it possible to work at a higher temperature for a longer time. We used the same test method to study the irreversible flux loss of Nd11 Dy0.5 Fe81.4 Nb1 B6.1 bonded magnet (L/D = 0.7). We found that the irreversible flux loss is only 4.35% at 150 ◦ C for 2 h. Here we can consider that the maximum operating temperature of Nd11 Dy0.5 Fe81.4 Nb1 B6.1 alloy is higher than 150 ◦ C.
3.2. Temperature characteristics improvement
3.3. Microstructure evaluation
Temperature characteristics of permanent magnetic materials are usually manifested by irreversible flux loss (hirr ). The bonded magnets were magnetized under a pulse field, and the variation
Since magnetic properties and temperature characteristics significantly change with Nb substitution, and there is strong connection between magnetic properties, temperature
φ (T0 ) − φ(T0 ) × 100% φ(T0 )
Table 2 Effect of Nb content on irreversible flux loss under different temperature Alloy
Nb (at%)
hirr (%) 80 ◦ C
hirr (%) 100 ◦ C
hirr (%) 120 ◦ C
hirr (%) 150 ◦ C
hirr (%) 170 ◦ C
Nd11 Dy0.5 Fe82.4 B6.1 Nd11 Dy0.5 Fe81.9 Nb0.5 B6.1 Nd11 Dy0.5 Fe81.4 Nb1 B6.1 Nd11 Dy0.5 Fe80.9 Nb1.5 B6.1
0 0.5 1 1.5
0.87 0.89 0.91 0.94
2.69 0.92 0.93 0.96
4.34 2.78 1.85 1.93
5.39 4.56 2.83 2.82
7.23 5.59 3.81 3.77
80
R. Zhang et al. / Journal of Alloys and Compounds 427 (2007) 78–81
characteristics and microstructure of the nanocrystalline alloy, it is very necessary to understand the details of microstructure evolution to further understand the effect of Nb substitution in the Nd11 Dy0.5 Fe82.4−n Nbn B6.1 alloys. Zhou and Dong considers that [13] magnetization reversal process of nanocrystalline NdFeB alloy includes the nucleation process in soft phase field and the domain-wall movement passing through grain boundary from soft phase field to hard phase field. As the exchange coupling pining field of irreversible domain-wall movement is larger than the nucleation field in soft phase field, the coercivity of nanocrystalline NdFeB alloy is determined by the exchange coupling pining field. The exchange coupling pining field, i.e. coercivity, can be calculated from: Hp =
2K1H δH × B − Neff Ms μ0 M s πr0
where K1H is the magnetocrystalline anisotropic constant of hard phase and r0 is the grain size of soft phase. It is obviously that the exchange coupling pining field, i.e. coercivity, is in proportion to anisotropic field HAH =
2K1H μ0 Ms
, and in inverse proportion to
the grain size of soft phase r0 . The AFM photos of Nd11 Dy0.5 Fe82.4−n Nbn B6.1 (n = 0, 0.5, 1) are shown in Fig. 1. It can be seen that the microstructure of Nbfree alloy isn’t homogeneous, and grains are bigger than those of the Nb-containing alloys, about 50–100 nm. Bigger grain structure can minish the volume fraction of grain boundary region, which participate the exchange coupling interaction. As a result, the interaction is weakened and coercivity is decreased. The grains of 0.5 at% Nb-containing alloy is smaller than those of the Nb-free alloy, about 30–50 nm. However, the microstructure is still not very homogeneous. Some bigger grains and smaller grains exist together. As for 1 at% Nb-containing alloy, the microstructure is homogeneous and grains are the smallest, about 30 nm. The small, homogeneous microstructure is favored by exchange coupling interaction. On the other hand, as the grains size is close to the critical size of Nd2 Fe14 B singledomain grain, the magnetization reversal process is magnetic moments turning. Consequently, 1 at% Nb-containing alloy gets the optimal coercivity [14]. In general, small and homogeneous grain structure can result in a better general magnetic property. Compared to the Nb-containing alloy, the microstructure of the Nb-free alloy shows much difference. There are many grains of different size and different shape, and also some grains of irregular shape. Contact between grains is less. In contrast, the grains of Nb-containing alloy are homogeneous and regular. Most of them are circular shape. Bar or cuneiform shape grains decrease or even disappear in Nb-containing alloy, as well as grains of bigger and smaller size. The contact between grains is getting more compact. Also the distribution of grains become more homogeneous. We can considered that this process is the main reason of improving coercivity and temperature characteristics. It is noticeable that the shape of grains is smooth, without sharp-angled ones, which can reduce inner dispersal magnetic field. It is also the key reason of improving coercivity and temperature characteristics.
Fig. 1. AFM photo of Nd11 Dy0.5 Fe82.4−n Nbn B6.1 (n = 0, 0.5, 1): (a) Nd11 Dy0.5 Fe82.4 B6.1 ; (b) Nd11 Dy0.5 Fe81.9 Nb0.5 B6.1 ; (c) Nd11 Dy0.5 Fe81.4 Nb1 B6.1 .
4. Conclusion In summary, effect of Nb substitution on the magnetic properties, temperature characteristics and microstructure of rapidquenched Nd11 Dy0.5 Fe82.4−n Nbn B6.1 (n = 0, 0.5, 1, 1.5, 2) permanent magnets has been investigated in the present study. The following conclusions can be drawn:
R. Zhang et al. / Journal of Alloys and Compounds 427 (2007) 78–81
(1) A little Nb substitution cannot decrease remanence remarkably but promote coercivity noticeably. With a content of 1 at% Nb, the optimium properties of bonded magnets are achieved: Br = 655.9 mT, Hci = 974 kA m−1 , (BH)m = 73 kJ m−3 . (2) As Nb substitution increases, the irreversible flux losses of bonded magnets decrease remarkably, and operating temperature is promoted gradually. When Nb substitution reaches 1 at%, the beneficial effect comes to the maximum, and the irreversible flux loss of Nd11 Dy0.5 Fe81.4 Nb1 B6.1 bonded magnets exposed at temperature 170 ◦ C for 2 h is only 3.81%. When continuing to increase the Nb substitution, the irreversible flux loss doesn’t decline any more. (3) Nb substitution makes grains smaller and leads to the homogenization and regularization of grains. Also Nb substitution can enhance exchange coupling pining field and reduce inner dispersal magnetic field. As a result, the irreversible flux loss decreases notably and the magnets can be used at a higher temperature. The maximum operating temperature of Nd11 Dy0.5 Fe81.4 Nb1 B6.1 alloy is higher than 150 ◦ C. Acknowledgements The work was supported by New century excellent person support program of China (Grant No. NCET-04-0873), Science found for distinguished young scholars of Sichuan province
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(Grant No. 03ZQ026-006) and Major science plan of Sichuan province (Grant No. 03GG009-006). References [1] J.J. Croat, J.F. Herbst, R.W. Lee, F.E. Pinkerton, Appl. Phys. Lett. 44 (1984) 148. [2] M. Sagawa, S. Fujimura, N. Togawa, H. Yamamoto, Y. Matsuura, J. Appl. Phys. 55 (1984) 2083. [3] L.H. Lewis, K. Gallagher, V. Panchanathan, J. Appl. Phys. 85 (1999) 5926. [4] W.H. Chen, W. Li, C.J. Li, X.M. Li, J. Alloys Compd. 319 (2001) 280. [5] T. Nishio, S. Koyama, Y. Kasai, V. Panchanathan, J.E. Shield, J. Appl. Phys. 81 (1997) 4447. [6] J.L. Tsai, T.S. Chin, Y.D. Yao, A. Melsheimer, S. Fisher, T. Drogen, M. Kelsch, H. Kronmuller, J. Appl. Phys. 93 (2003) 6915. [7] J.L. Tsai, T.S. Chin, Y.D. Yao, A. Melsheimer, S. Fisher, T. Drogen, M. Kelsch, H. Kronmuller, Phys. B 327 (2003) 283–286. [8] T.J. Konno, M. Uehara, S. Hirosawa, K. Sumiyama, K. Suzuki, J. Alloys Compd. 268 (1998) 278. [9] Y.Q. Wu, D.H. Ping, K. Hono, M. Hamano, A. Inoue, J. Appl. Phys. 87 (2000) 8658. [10] K. Konno, M. Matsuura, N. Ikuta, M. Sakurai, C. Mishima, J. Appl. Phys. 91 (2002) 7875. [11] Z. Chen, Y.Q. Wu, M.J. Kramer, B.R. Smith, B.M. Ma, M.Q. Huang, J. Magn. Magn. Mater. 268 (2004) 105. [12] M. Yamasaki, M. Hamano, H. Mizuguchi, T. Kobayashi, K. Hono, H. Yamamoto, A. Inoue, Scripta Mater. 44 (2001) 1375–1378. [13] S.Z. Zhou, Q.F. Dong, Super Permanent magnets-Permanent Magnetic Material of Rare-earth and Iron System, Metallurgy Industry Publishing, Beijing, 1999, p. 407. [14] M.G. Zhu, W. Pan, W. Li, Acta Phys. Sin. 51 (2002) 1608.
Effect of Nb substitution on the temperature characteristics and microstructures of rapid-quenched NdFeB alloy Ran Zhang, Ying Liu ∗ , Jinwen Ye, Wenfeng Yang, Yilong Ma, Shengji Gao Department of Material Science and Engineering, Sichuan University, Chengdu 610064, PR China Received 11 January 2006; received in revised form 4 March 2006; accepted 7 March 2006 Available online 18 April 2006
Abstract Effect of Nb substitution on the magnetic properties, temperature characteristics and microstructure of rapid-quenched Nd11 Dy0.5 Fe82.4−n Nbn B6.1 (n = 0, 0.5, 1, 1.5, 2) permanent magnets has been investigated. It is found that Hcj increases with increasing Nb content and the irreversible flux loss decreases with that. Nb substitution makes grains smaller and leads to the homogenization and regularization of grains. Also Nb substitution can enhance exchange coupling pining field and reduce inner dispersal magnetic field. As a result, the irreversible flux loss decreases notably and the magnets can be used at higher temperature. The maximum operating temperature of Nd11 Dy0.5 Fe81.4 Nb1 B6.1 alloy is higher than 150 ◦ C. © 2006 Elsevier B.V. All rights reserved. Keywords: NdFeB; Magnetic properties; Temperature characteristics; Microstructure
1. Introduction Since the discovery of Nd2 Fe14 B compound in the early 1980s [1,2], NdFeB permanent magnet has been developed in an essential engineering material in the industrial applications. However, one of the major drawbacks of NdFeB magnet is its poor thermal stability, which limits the application temperature below 120 ◦ C. To improve the thermal stability of NdFeB magnet, extensive efforts have been made via the substitution of other elements. It has been found that, among all the substitutions studied, Nb substitution appears to be an effective way to improve the thermal stability and refine microstructure in sintered NdFeB magnets [3,4] and in nanocomposite ␣-Fe/Nd2 Fe14 B magnets [5]. Also in the case of [NdFeBx /Nbz ]n multiplayer films, the Nb spacer layer enriched the Nd2 Fe14 B grain boundary and reduced the grain size effectively [6,7]. Recent studies on nanocrystalline ␣-Fe/Nd2 Fe14 B-based magnets [8,9] indicate that Nb substitution appears to refine grain structure, possibly by increasing the nucleation density and stabilizing the remaining amorphous phase in the melt-spun ribbons. Research on the effect of Nb substitution in Nd2 Fe14 Bbased magnets also shows that the presence of Nb suppresses
∗
Corresponding author. Tel.: +86 28 85405332. E-mail address: [email protected] (Y. Liu).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.03.034
the thermally induced formation of poorly crystallized Fe-rich regions [4] and forms stable NbFeB phase in the homogenized state [10]. However, the effect of Nb substitution on the nanocrystalline Nd2 Fe14 B alloy has not been well understood yet, especially its role in improving the thermal stability and microstructure evolution. Thus, the present study is focused on the above two points on melt-spun Nd11 Dy0.5 Fe82.4−n Nbn B6.1 (n = 0, 0.5, 1, 1.5, 2) nanocrystalline magnets, aiming at understanding the role of Nb substitution in microstructure control and the thermal stability of the nanocrystalline magnets, which will shed some lights on the future design of the nanocrystalline magnets for scientific inquiry and industrial applications. 2. Experimental Alloys ingots with nominal composition of Nd11 Dy0.5 Fe82.4−n Nbn B6.1 were prepared by induction melting under Ar atmosphere, where n = 0, 0.5, 1, 1.5, 2 in atomic%. The NdFeB alloys were melt from Nd, Dy, Fe, Nb–Fe, B–Fe constituent elements. The ingots were broken into small pieces and melt-spun onto a molybdenum wheel from a crucible at a rolling speed of 28 m·s−1 in argon atmosphere. The melt-spun ribbons were annealed at 670–720 ◦ C for 30–50 min in a vacuum heat-treatment furnace to improve their magnetic properties. The microstructure of the ribbons was characterized with atomic force microscopy (AFM). The mixture, combined the magnetic powder with epoxy resin in the proportion of 100:2.5, was pressed into cylinders with diameter, height and density of 10, 10 mm and 6.1 g cm−3 or so, respectively. Then the bonded magnets were solidified at 120 ◦ C for 1 h. Finally, the magnetic properties of the bonded
R. Zhang et al. / Journal of Alloys and Compounds 427 (2007) 78–81
79
Table 1 Dependence of magnetic properties on Nb content for Nd11 Dy0.5 Fe82.4−n Nbn B6.1 magnets Alloy
Nb/(at%)
Br /(mT)
Hcj /(kA/m)
(BH)m /(kJ/m3 )
Nd11 Dy0.5 Fe82.4 B6.1 Nd11 Dy0.5 Fe81.9 Nb0.5 B6.1 Nd11 Dy0.5 Fe81.4 Nb1 B6.1 Nd11 Dy0.5 Fe80.9 Nb1.5 B6.1 Nd11 Dy0.5 Fe80.4 Nb2 B6.1
0 0.5 1 1.5 2
663.6 660.8 655.9 650.9 646.2
895 940 974 1003 1028
75 73 73 69 61
magnets were measured by AMT-4 automatic measuring instrument of magnetization characteristic (manufactured by Mianyang Shuangji electronic company, China) with a maximum applied field of 2 T at room temperature. The flux aging loss was measured with a Helmholtz coil on bonded magnets.
of their flux due to an exposure at an elevated temperature, Tex , was measured by the sample-extraction method as a function of the exposure time, ta . The irreversible flux loss, hirr , due to the exposure was calculated from:
3. Results and discussions
hirr =
3.1. Magnetic property enhancement The effects of Nb substitution on the room temperature magnetic properties, remanence (Br ), coercivity (Hcj ) and energy product (BHmax ), of annealed Nd11 Dy0.5 Fe82.4−n Nbn B6.1 (n = 0, 0.5, 1, 1.5, 2) bonded magnets samples are listed in Table 1. It can be seen that Nb substitution leads to a significant increase in the coercivity, but the remanence is slightly decreased. In comparison with alloy Nd11 Dy0.5 Fe82.4 B6.1 without Nb, the coercivity of alloy Nd11 Dy0.5 Fe81.4 Nb1 B6.1 (with a content of 1 at% Nb) is increased by 8.8%, but the remanence of alloy Nd11 Dy0.5 Fe81.4 Nb1 B6.1 is decreased only by 1.7%. That is to say, a little Nb substitution cannot decrease remanence remarkably but promote coercivity noticeably. At the same time, with the increasing Nb content, the energy product decreases gradually first, and then decreases considerably when increasing Nb to 1.5 at%. It’s all because that Nb substitution will give birth to nonmagnetic phase [11,12]. The nonmagnetic phase exists among grains, which can not only dilute the magnetic properties of the whole bonded magnet, but also separate soft phase grains and hard phase grains, weak the exchange coupled interaction between grains and decrease the magnetic properties. As a result, overfull Nb substitution will influence general magnetic properties of bonded magnets seriously. With a content of 1 at% Nb, the optimium properties of bonded magnets is achieved: Br = 655.9 mT, Hci = 974 kA m−1 and (BH)m = 73 kJ m−3 .
where φ(T0 ) and φ (T0 ) are the flux value measured at room temperature before and after the exposure. We studied the irreversible flux losses of Nd11 Dy0.5 Fe82.4−n Nbn B6.1 (n = 0, 0.5, 1, 1.5) bonded magnets (L/D = 1) exposed for 2 h at temperature 80, 100, 120, 150 and 170 ◦ C, respectively. The result is listed in Table 2. It can be seen from Table 2 that, as Nb substitution increases, the irreversible flux losses of all bonded magnets above decreases remarkably, and operating temperature is promoted gradually. When Nb substitution reaches 1 at%, the beneficial effect comes to the maximum, and the irreversible flux loss of Nd11 Dy0.5 Fe81.4 Nb1 B6.1 bonded magnets (L/D = 1) exposed at temperature 170 ◦ C for 2 h is only 3.81%. When continuing to increase the Nb substitution, the irreversible flux loss doesn’t decline any more. The irreversible flux losses of Nd11 Dy0.5 Fe81.4 Nb1 B6.1 alloy and Nd11 Dy0.5 Fe80.9 Nb1.5 B6.1 alloy exposed at same temperature are basically same. We can conclude from above explanation that Nb substitution can notably improve the temperature resistance of rapid-quenched NdFeB magnets, and make it possible to work at a higher temperature for a longer time. We used the same test method to study the irreversible flux loss of Nd11 Dy0.5 Fe81.4 Nb1 B6.1 bonded magnet (L/D = 0.7). We found that the irreversible flux loss is only 4.35% at 150 ◦ C for 2 h. Here we can consider that the maximum operating temperature of Nd11 Dy0.5 Fe81.4 Nb1 B6.1 alloy is higher than 150 ◦ C.
3.2. Temperature characteristics improvement
3.3. Microstructure evaluation
Temperature characteristics of permanent magnetic materials are usually manifested by irreversible flux loss (hirr ). The bonded magnets were magnetized under a pulse field, and the variation
Since magnetic properties and temperature characteristics significantly change with Nb substitution, and there is strong connection between magnetic properties, temperature
φ (T0 ) − φ(T0 ) × 100% φ(T0 )
Table 2 Effect of Nb content on irreversible flux loss under different temperature Alloy
Nb (at%)
hirr (%) 80 ◦ C
hirr (%) 100 ◦ C
hirr (%) 120 ◦ C
hirr (%) 150 ◦ C
hirr (%) 170 ◦ C
Nd11 Dy0.5 Fe82.4 B6.1 Nd11 Dy0.5 Fe81.9 Nb0.5 B6.1 Nd11 Dy0.5 Fe81.4 Nb1 B6.1 Nd11 Dy0.5 Fe80.9 Nb1.5 B6.1
0 0.5 1 1.5
0.87 0.89 0.91 0.94
2.69 0.92 0.93 0.96
4.34 2.78 1.85 1.93
5.39 4.56 2.83 2.82
7.23 5.59 3.81 3.77
80
R. Zhang et al. / Journal of Alloys and Compounds 427 (2007) 78–81
characteristics and microstructure of the nanocrystalline alloy, it is very necessary to understand the details of microstructure evolution to further understand the effect of Nb substitution in the Nd11 Dy0.5 Fe82.4−n Nbn B6.1 alloys. Zhou and Dong considers that [13] magnetization reversal process of nanocrystalline NdFeB alloy includes the nucleation process in soft phase field and the domain-wall movement passing through grain boundary from soft phase field to hard phase field. As the exchange coupling pining field of irreversible domain-wall movement is larger than the nucleation field in soft phase field, the coercivity of nanocrystalline NdFeB alloy is determined by the exchange coupling pining field. The exchange coupling pining field, i.e. coercivity, can be calculated from: Hp =
2K1H δH × B − Neff Ms μ0 M s πr0
where K1H is the magnetocrystalline anisotropic constant of hard phase and r0 is the grain size of soft phase. It is obviously that the exchange coupling pining field, i.e. coercivity, is in proportion to anisotropic field HAH =
2K1H μ0 Ms
, and in inverse proportion to
the grain size of soft phase r0 . The AFM photos of Nd11 Dy0.5 Fe82.4−n Nbn B6.1 (n = 0, 0.5, 1) are shown in Fig. 1. It can be seen that the microstructure of Nbfree alloy isn’t homogeneous, and grains are bigger than those of the Nb-containing alloys, about 50–100 nm. Bigger grain structure can minish the volume fraction of grain boundary region, which participate the exchange coupling interaction. As a result, the interaction is weakened and coercivity is decreased. The grains of 0.5 at% Nb-containing alloy is smaller than those of the Nb-free alloy, about 30–50 nm. However, the microstructure is still not very homogeneous. Some bigger grains and smaller grains exist together. As for 1 at% Nb-containing alloy, the microstructure is homogeneous and grains are the smallest, about 30 nm. The small, homogeneous microstructure is favored by exchange coupling interaction. On the other hand, as the grains size is close to the critical size of Nd2 Fe14 B singledomain grain, the magnetization reversal process is magnetic moments turning. Consequently, 1 at% Nb-containing alloy gets the optimal coercivity [14]. In general, small and homogeneous grain structure can result in a better general magnetic property. Compared to the Nb-containing alloy, the microstructure of the Nb-free alloy shows much difference. There are many grains of different size and different shape, and also some grains of irregular shape. Contact between grains is less. In contrast, the grains of Nb-containing alloy are homogeneous and regular. Most of them are circular shape. Bar or cuneiform shape grains decrease or even disappear in Nb-containing alloy, as well as grains of bigger and smaller size. The contact between grains is getting more compact. Also the distribution of grains become more homogeneous. We can considered that this process is the main reason of improving coercivity and temperature characteristics. It is noticeable that the shape of grains is smooth, without sharp-angled ones, which can reduce inner dispersal magnetic field. It is also the key reason of improving coercivity and temperature characteristics.
Fig. 1. AFM photo of Nd11 Dy0.5 Fe82.4−n Nbn B6.1 (n = 0, 0.5, 1): (a) Nd11 Dy0.5 Fe82.4 B6.1 ; (b) Nd11 Dy0.5 Fe81.9 Nb0.5 B6.1 ; (c) Nd11 Dy0.5 Fe81.4 Nb1 B6.1 .
4. Conclusion In summary, effect of Nb substitution on the magnetic properties, temperature characteristics and microstructure of rapidquenched Nd11 Dy0.5 Fe82.4−n Nbn B6.1 (n = 0, 0.5, 1, 1.5, 2) permanent magnets has been investigated in the present study. The following conclusions can be drawn:
R. Zhang et al. / Journal of Alloys and Compounds 427 (2007) 78–81
(1) A little Nb substitution cannot decrease remanence remarkably but promote coercivity noticeably. With a content of 1 at% Nb, the optimium properties of bonded magnets are achieved: Br = 655.9 mT, Hci = 974 kA m−1 , (BH)m = 73 kJ m−3 . (2) As Nb substitution increases, the irreversible flux losses of bonded magnets decrease remarkably, and operating temperature is promoted gradually. When Nb substitution reaches 1 at%, the beneficial effect comes to the maximum, and the irreversible flux loss of Nd11 Dy0.5 Fe81.4 Nb1 B6.1 bonded magnets exposed at temperature 170 ◦ C for 2 h is only 3.81%. When continuing to increase the Nb substitution, the irreversible flux loss doesn’t decline any more. (3) Nb substitution makes grains smaller and leads to the homogenization and regularization of grains. Also Nb substitution can enhance exchange coupling pining field and reduce inner dispersal magnetic field. As a result, the irreversible flux loss decreases notably and the magnets can be used at a higher temperature. The maximum operating temperature of Nd11 Dy0.5 Fe81.4 Nb1 B6.1 alloy is higher than 150 ◦ C. Acknowledgements The work was supported by New century excellent person support program of China (Grant No. NCET-04-0873), Science found for distinguished young scholars of Sichuan province
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(Grant No. 03ZQ026-006) and Major science plan of Sichuan province (Grant No. 03GG009-006). References [1] J.J. Croat, J.F. Herbst, R.W. Lee, F.E. Pinkerton, Appl. Phys. Lett. 44 (1984) 148. [2] M. Sagawa, S. Fujimura, N. Togawa, H. Yamamoto, Y. Matsuura, J. Appl. Phys. 55 (1984) 2083. [3] L.H. Lewis, K. Gallagher, V. Panchanathan, J. Appl. Phys. 85 (1999) 5926. [4] W.H. Chen, W. Li, C.J. Li, X.M. Li, J. Alloys Compd. 319 (2001) 280. [5] T. Nishio, S. Koyama, Y. Kasai, V. Panchanathan, J.E. Shield, J. Appl. Phys. 81 (1997) 4447. [6] J.L. Tsai, T.S. Chin, Y.D. Yao, A. Melsheimer, S. Fisher, T. Drogen, M. Kelsch, H. Kronmuller, J. Appl. Phys. 93 (2003) 6915. [7] J.L. Tsai, T.S. Chin, Y.D. Yao, A. Melsheimer, S. Fisher, T. Drogen, M. Kelsch, H. Kronmuller, Phys. B 327 (2003) 283–286. [8] T.J. Konno, M. Uehara, S. Hirosawa, K. Sumiyama, K. Suzuki, J. Alloys Compd. 268 (1998) 278. [9] Y.Q. Wu, D.H. Ping, K. Hono, M. Hamano, A. Inoue, J. Appl. Phys. 87 (2000) 8658. [10] K. Konno, M. Matsuura, N. Ikuta, M. Sakurai, C. Mishima, J. Appl. Phys. 91 (2002) 7875. [11] Z. Chen, Y.Q. Wu, M.J. Kramer, B.R. Smith, B.M. Ma, M.Q. Huang, J. Magn. Magn. Mater. 268 (2004) 105. [12] M. Yamasaki, M. Hamano, H. Mizuguchi, T. Kobayashi, K. Hono, H. Yamamoto, A. Inoue, Scripta Mater. 44 (2001) 1375–1378. [13] S.Z. Zhou, Q.F. Dong, Super Permanent magnets-Permanent Magnetic Material of Rare-earth and Iron System, Metallurgy Industry Publishing, Beijing, 1999, p. 407. [14] M.G. Zhu, W. Pan, W. Li, Acta Phys. Sin. 51 (2002) 1608.