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Effect of Tb on the intrinsic coercivity and impact toughness of sintered Nd–Dy–Fe–B magnets
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 320 (2008) 1735–1738 www.elsevier.com/locate/jmmm
Effect of Tb on the intrinsic coercivity and impact toughness of sintered Nd–Dy–Fe–B magnets Z.H. Hua,b,, F.Z. Liana, M.G. Zhub, W. Lib a
Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang 110004, PR China b Division of Functional Materials, Central Iron & Steel Research Institute, Beijing 100081, PR China Received 23 November 2007; received in revised form 9 January 2008 Available online 24 January 2008
Abstract The effect of Tb on the coercivity and impact toughness of sintered Nd–Dy–Fe–B magnets has been investigated. The results showed that the addition of Tb enhanced the intrinsic coercivity, reduced the remanence and improved the impact toughness of sintered magnets. The optimum impact toughness of sintered magnets was achieved when 1.0 at% Tb was incorporated. The possible reasons for increasing the intrinsic coercivity and improving impact toughness of sintered magnets were analyzed, and the relations between the microstructure and impact toughness of sintered magnets were studied. r 2008 Elsevier B.V. All rights reserved. PACS: 75.50.Ww; 62.20.x Keywords: Impact toughness; Sintered magnets; Microstructure; Intrinsic coercivity
1. Introduction Developing ultra-high intrinsic coercivity Nd–Fe–B magnets has been an important and promising research subject in the field of rare earth transition metal permanent magnets. In this condition, heavy rare-earth (Tb, Dy) is necessary for obtaining ultra-high intrinsic coercivity Nd–Fe–B magnets. It is well known that Tb or Dy substitution can markedly enhance the intrinsic coercivity (Hci), but rapidly reduce the remanence (Br) of sintered Nd–Fe–B magnets. Therefore, as for ultra-high intrinsic coercivity Nd–Fe–B magnets, it is very important to reduce the amount of heavy rare-earth without sacrificing the intrinsic coercivity. This may be achieved, if small amount of Tb is used, because the anisotropy field (Ha) of Tb2Fe14B is higher than Dy2Fe14B [1]. Sintered Nd–Fe–B magnets are very brittle, which limit their applications. So many researchers have made an Corresponding author. Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang, 110004, PR China. Tel.: +86 10 62185854; fax: +86 10 62185125. E-mail address: [email protected] (Z.H. Hu).
0304-8853/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.01.027
effort to improve the mechanical properties of sintered Nd–Fe–B magnets, and found that adding small amounts (Nb, Cu, Al, Ga, Ti) can increase the bending strength and the impact toughness of sintered Nd–Fe–B magnets [2–5]. Simultaneously, the addition of rare-earth elements (Nd, Dy) can also improve the impact toughness of sintered Nd–Fe–B magnets [6,7]. But the effect of Tb on the impact toughness of sintered Nd–Fe–B magnets is unclear till now. In the present work, the intrinsic coercivity and impact toughness of sintered Nd–Dy–Fe–B magnets with different Tb content were investigated, and the possible reasons for the change of intrinsic coercivity and impact toughness of sintered Nd–Fe–B magnets were discussed. 2. Experiments The alloy ingots with nominal compositions Nd12xDy3TbxFe78B7 (x=0, 0.5, 1.0, 1.5) were prepared by arc melting. The powders were prepared by ball milling for 40 min, pressed and simultaneously aligned in a magnetic field of 1.8 T, then isostatically compacted under a pressure of 200 MPa. The green compact was sintered at 1320–1380 K for 2 h, followed by annealing at 1173 K for
ARTICLE IN PRESS Z.H. Hu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 1735–1738
1 h and at 873 K for 2 h. The magnetic properties of sintered magnets were measured with a NIM-2000 hysteresis loop tracer. The microstructures were investigated by a JMS-6400 scanning electronic microscope. Impact toughness of sintered Nd–Fe–B magnets was examined using a falling-weight impact tester. The hammer used in the impact test was a cylinder with a cone-shaped head and the weight of the hammer is 33.088 g. The impact specimen was a round plate with diameter 10 mm and height 4 mm. While the impact test was performed, the hammer was raised to a certain height and then freely fell to hit the magnet specimen. The falling height of hammer increased in steps from the lower to higher until the specimen fractured. The falling-weight height, where the specimen fractured, was designated as ‘‘h’’. The falling-weight potential energy m g h was defined as the impact energy. The impact energy was measured on four specimens for each composition, and the data were averaged. The error bars are about 74 103 J for the impact energy. In the present work, the results of falling-weight impact test are in dependence on the geometry of the falling weight and so on, so it can be only used to compare the relative rankings of the impact stability of the magnets.
3.0
ak (kJ/m2)
1736
2.5
2.0
1.5 0.0
0.5
1.5
1.0 Tb (at%)
Fig. 2. Relationship between ak of sintered magnets and the content of Tb.
Table 1 The main phase composition of sintered magnets with the addition of Tb by the spot analysis of EDX The content of Tb, x
0 1.0 1.5
Composition (at%) Fe
Nd
Dy
Tb
88.38 88.39 87.20
9.86 8.65 8.67
1.76 2.97 3.62
0 0 0.51
3. Results and discussion 3.1. Magnetic properties and impact toughness The magnetic properties for the samples are shown in Fig. 1. The intrinsic coercivity (Hci) of the samples drastically increases with increasing Tb content, and there exists a linear relation between the intrinsic coercivity and the Tb content when the Tb content increases from 0 to 1.0 at%, after that, the intrinsic coercivity of sintered magnets increases slowly. Simultaneously, the remanence (Br) of sintered magnets decreases with increasing Tb content because heavy rare-earth and Fe moments are antiparallel [1]. 2100 Hcj (kA/m)
2000 1900
ak ¼
A bh
where A and ak denote impact energy making a specimen fracture and impact toughness, b and h denote width and height of the specimen, respectively. So we use the formula to calculate the impact toughness of sintered Nd–Fe–B magnets. Fig. 2 shows the variation of impact toughness with the Tb content in the sintered Nd–Fe–B magnets. The impact toughness of magnets with the addition of Tb first increases, reaches a maximum, and then starts to rapidly decrease. The maximum impact toughness is 2.771 kJ/m2, which is obtained at x ¼ 1.0.
1800 1700
3.2. Microstructure
1600 1.12 Br (T)
Nd–Fe–B materials and ceramic materials are both brittle materials, and the impact toughness of ceramics is defined as [8]:
1.08 1.04 1.00 0.0
0.2
0.4
0.6
0.8 Tb (at%)
1.0
1.2
1.4
Fig. 1. Variation of the magnetic properties of sintered magnets with the Tb content.
The main phase (Nd2Fe14B) composition of sintered magnets with the addition of Tb by the spot analysis of energy-dispersive X-ray spectroscopy (EDX) is shown in Table 1. It can be seen that the content of Dy in the main phase increases with the increasing Tb content. The content of Tb in the main phase hardly exists when the Tb content increases from 0 to 1.0 at%, and only 0.51 at% Tb is obtained in the main phase when the Tb content increases to 1.5 at%. So we presume that heavy rare-earth (Tb, Dy) first exists in the Nd-rich phase.
ARTICLE IN PRESS Z.H. Hu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 1735–1738
1737
Fig. 3. SEM micrographs of the sintered magnets: (a, A) x ¼ 0.5, (b, B) x ¼ 1.0, and (c, C) x ¼ 1.5.
The effect of Tb on the microstructure of the sintered magnets is shown in Fig. 3. There exist irregular grains and the grain boundaries that two grains contact directly. The Nd-rich phase (black caves) increases, and exists more in the triple points when the content of Tb addition increases from 1.0 to 1.5 at%. Nucleation of reversed domains starts at positions with a high demagnetization field and low anisotropy, and these positions are the triple points and the grain boundaries [9]. So the intrinsic coercivity of sintered magnets increases more slowly when the Tb content increases from 1.0 to 1.5 at%. The fracture of the sintered Nd–Fe–B magnets is intergranular fracture, which is a typical brittle fracture. So the Nd-rich phase at grain boundaries plays a dominant role in improving the impact toughness of sintered Nd–Fe–B magnets. The Tb addition makes the composition of the Nd-rich phase change, which possibly improve the impact toughness of sintered Nd–Fe–B magnets. Fig. 4 shows SEM micrograph showing the fractured surface of
sintered magnets. As can be seen, the fracture of the sintered magnets is mainly intergranular brittle fracture. Simultaneously, there also exists intergranular toughness fracture (as shown by black arrows in Fig. 4), and the intergranular toughness fracture is more obviously when the content of Tb increases from 0 to 1.0 at%, and weakens dramatically when the content of Tb increases from 1.0 to 1.5 at%. Maybe, the variation of the composition of the Nd-rich phase is the main reason which makes the phenomenon occurred. A great deal of plastic deformation existing in the grain boundaries consumes a lot of deformation energy, which increases the impact toughness of sintered Nd–Fe–B magnets. 4. Conclusion Addition of Tb increases the intrinsic coercivity of sintered Nd–Dy–Fe–B magnets, and the Dy content in the main phase increases with the increasing Tb content. When
ARTICLE IN PRESS Z.H. Hu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 1735–1738
1738
Fig. 4. SEM micrograph showing the fractured surface of sintered magnets: (D) x ¼ 0, (E) x ¼ 0.5, (F) x ¼ 1.0, and (G) x ¼ 1.5.
the Tb content increase from 0 to 1.0 at%, there is hardly the Tb element existing in the main phase, and only 0.51 at% Tb is obtained at x ¼ 1.5. So increasing the heavy rare earth content in the main phase will attribute to develop ultra-high intrinsic coercivity Nd–Fe–B magnets without using much heavy rare earth. The maximum of impact toughness is obtained at x ¼ 1.0. The microstructure shows that the fracture of sintered Nd–Fe–B magnets is mainly intergranular brittle fracture. Simultaneously, there also exists intergranular toughness fracture.
Acknowledgments The work was supported by the National High Technology Research and Development Program of China (Grant no. 2007AA03Z438) and the National Natural Science Foundation of China (Grant no. 50771035).
References [1] J.F. Herbst, Rev. Mod. Phys. 63 (1991) 819. [2] J.F. Liu, P. Vora, M.H. Walmer, E. Kottcamp, S.A. Bauser, A. Higgins, S. Liu, J. Appl. Phys. 97 (2005) 10H101. [3] S. Liu, D. Cao, R. Leese, S. Bauser, G.E. Kuhl, J.F. Liu, M.H. Walmer, E. Kottcamp, Proceedings of the 17th International Workshop on Rare Earth Permanent Magnets and their Applications, Newark, USA, 2002, p. 360. [4] J.H. Jiang, Z.P. Zeng, J.S. Wu, M. Tokunaga, J. Magn. Magn. Mater. 214 (2000) 61. [5] Z.H. Hu, M.G. Zhu, W. Li, F.Z. Lian, J. Magn. Magn. Mater. 320 (2008) 96. [6] H.J. Wang, A.H. Li, M.G. Zhu, W. Li, J. Magn. Magn. Mater. 307 (2006) 268. [7] H.J. Wang, A.H. Li, W. Li, Intermetallics 15 (2006) 985. [8] Z.Z. Jin, Y.W. Bao, Characterization of Mechanical Properties for Brittle Materials and Ceramics, China Railway Publishing House, 1996, p. 146 (in Chinese). [9] T. Schrefl, H.F. Schmidts, J. Fidler, H. Kronmu¨ller, J. Appl. Phys. 73 (1993) 6510.
Journal of Magnetism and Magnetic Materials 320 (2008) 1735–1738 www.elsevier.com/locate/jmmm
Effect of Tb on the intrinsic coercivity and impact toughness of sintered Nd–Dy–Fe–B magnets Z.H. Hua,b,, F.Z. Liana, M.G. Zhub, W. Lib a
Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang 110004, PR China b Division of Functional Materials, Central Iron & Steel Research Institute, Beijing 100081, PR China Received 23 November 2007; received in revised form 9 January 2008 Available online 24 January 2008
Abstract The effect of Tb on the coercivity and impact toughness of sintered Nd–Dy–Fe–B magnets has been investigated. The results showed that the addition of Tb enhanced the intrinsic coercivity, reduced the remanence and improved the impact toughness of sintered magnets. The optimum impact toughness of sintered magnets was achieved when 1.0 at% Tb was incorporated. The possible reasons for increasing the intrinsic coercivity and improving impact toughness of sintered magnets were analyzed, and the relations between the microstructure and impact toughness of sintered magnets were studied. r 2008 Elsevier B.V. All rights reserved. PACS: 75.50.Ww; 62.20.x Keywords: Impact toughness; Sintered magnets; Microstructure; Intrinsic coercivity
1. Introduction Developing ultra-high intrinsic coercivity Nd–Fe–B magnets has been an important and promising research subject in the field of rare earth transition metal permanent magnets. In this condition, heavy rare-earth (Tb, Dy) is necessary for obtaining ultra-high intrinsic coercivity Nd–Fe–B magnets. It is well known that Tb or Dy substitution can markedly enhance the intrinsic coercivity (Hci), but rapidly reduce the remanence (Br) of sintered Nd–Fe–B magnets. Therefore, as for ultra-high intrinsic coercivity Nd–Fe–B magnets, it is very important to reduce the amount of heavy rare-earth without sacrificing the intrinsic coercivity. This may be achieved, if small amount of Tb is used, because the anisotropy field (Ha) of Tb2Fe14B is higher than Dy2Fe14B [1]. Sintered Nd–Fe–B magnets are very brittle, which limit their applications. So many researchers have made an Corresponding author. Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang, 110004, PR China. Tel.: +86 10 62185854; fax: +86 10 62185125. E-mail address: [email protected] (Z.H. Hu).
0304-8853/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.01.027
effort to improve the mechanical properties of sintered Nd–Fe–B magnets, and found that adding small amounts (Nb, Cu, Al, Ga, Ti) can increase the bending strength and the impact toughness of sintered Nd–Fe–B magnets [2–5]. Simultaneously, the addition of rare-earth elements (Nd, Dy) can also improve the impact toughness of sintered Nd–Fe–B magnets [6,7]. But the effect of Tb on the impact toughness of sintered Nd–Fe–B magnets is unclear till now. In the present work, the intrinsic coercivity and impact toughness of sintered Nd–Dy–Fe–B magnets with different Tb content were investigated, and the possible reasons for the change of intrinsic coercivity and impact toughness of sintered Nd–Fe–B magnets were discussed. 2. Experiments The alloy ingots with nominal compositions Nd12xDy3TbxFe78B7 (x=0, 0.5, 1.0, 1.5) were prepared by arc melting. The powders were prepared by ball milling for 40 min, pressed and simultaneously aligned in a magnetic field of 1.8 T, then isostatically compacted under a pressure of 200 MPa. The green compact was sintered at 1320–1380 K for 2 h, followed by annealing at 1173 K for
ARTICLE IN PRESS Z.H. Hu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 1735–1738
1 h and at 873 K for 2 h. The magnetic properties of sintered magnets were measured with a NIM-2000 hysteresis loop tracer. The microstructures were investigated by a JMS-6400 scanning electronic microscope. Impact toughness of sintered Nd–Fe–B magnets was examined using a falling-weight impact tester. The hammer used in the impact test was a cylinder with a cone-shaped head and the weight of the hammer is 33.088 g. The impact specimen was a round plate with diameter 10 mm and height 4 mm. While the impact test was performed, the hammer was raised to a certain height and then freely fell to hit the magnet specimen. The falling height of hammer increased in steps from the lower to higher until the specimen fractured. The falling-weight height, where the specimen fractured, was designated as ‘‘h’’. The falling-weight potential energy m g h was defined as the impact energy. The impact energy was measured on four specimens for each composition, and the data were averaged. The error bars are about 74 103 J for the impact energy. In the present work, the results of falling-weight impact test are in dependence on the geometry of the falling weight and so on, so it can be only used to compare the relative rankings of the impact stability of the magnets.
3.0
ak (kJ/m2)
1736
2.5
2.0
1.5 0.0
0.5
1.5
1.0 Tb (at%)
Fig. 2. Relationship between ak of sintered magnets and the content of Tb.
Table 1 The main phase composition of sintered magnets with the addition of Tb by the spot analysis of EDX The content of Tb, x
0 1.0 1.5
Composition (at%) Fe
Nd
Dy
Tb
88.38 88.39 87.20
9.86 8.65 8.67
1.76 2.97 3.62
0 0 0.51
3. Results and discussion 3.1. Magnetic properties and impact toughness The magnetic properties for the samples are shown in Fig. 1. The intrinsic coercivity (Hci) of the samples drastically increases with increasing Tb content, and there exists a linear relation between the intrinsic coercivity and the Tb content when the Tb content increases from 0 to 1.0 at%, after that, the intrinsic coercivity of sintered magnets increases slowly. Simultaneously, the remanence (Br) of sintered magnets decreases with increasing Tb content because heavy rare-earth and Fe moments are antiparallel [1]. 2100 Hcj (kA/m)
2000 1900
ak ¼
A bh
where A and ak denote impact energy making a specimen fracture and impact toughness, b and h denote width and height of the specimen, respectively. So we use the formula to calculate the impact toughness of sintered Nd–Fe–B magnets. Fig. 2 shows the variation of impact toughness with the Tb content in the sintered Nd–Fe–B magnets. The impact toughness of magnets with the addition of Tb first increases, reaches a maximum, and then starts to rapidly decrease. The maximum impact toughness is 2.771 kJ/m2, which is obtained at x ¼ 1.0.
1800 1700
3.2. Microstructure
1600 1.12 Br (T)
Nd–Fe–B materials and ceramic materials are both brittle materials, and the impact toughness of ceramics is defined as [8]:
1.08 1.04 1.00 0.0
0.2
0.4
0.6
0.8 Tb (at%)
1.0
1.2
1.4
Fig. 1. Variation of the magnetic properties of sintered magnets with the Tb content.
The main phase (Nd2Fe14B) composition of sintered magnets with the addition of Tb by the spot analysis of energy-dispersive X-ray spectroscopy (EDX) is shown in Table 1. It can be seen that the content of Dy in the main phase increases with the increasing Tb content. The content of Tb in the main phase hardly exists when the Tb content increases from 0 to 1.0 at%, and only 0.51 at% Tb is obtained in the main phase when the Tb content increases to 1.5 at%. So we presume that heavy rare-earth (Tb, Dy) first exists in the Nd-rich phase.
ARTICLE IN PRESS Z.H. Hu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 1735–1738
1737
Fig. 3. SEM micrographs of the sintered magnets: (a, A) x ¼ 0.5, (b, B) x ¼ 1.0, and (c, C) x ¼ 1.5.
The effect of Tb on the microstructure of the sintered magnets is shown in Fig. 3. There exist irregular grains and the grain boundaries that two grains contact directly. The Nd-rich phase (black caves) increases, and exists more in the triple points when the content of Tb addition increases from 1.0 to 1.5 at%. Nucleation of reversed domains starts at positions with a high demagnetization field and low anisotropy, and these positions are the triple points and the grain boundaries [9]. So the intrinsic coercivity of sintered magnets increases more slowly when the Tb content increases from 1.0 to 1.5 at%. The fracture of the sintered Nd–Fe–B magnets is intergranular fracture, which is a typical brittle fracture. So the Nd-rich phase at grain boundaries plays a dominant role in improving the impact toughness of sintered Nd–Fe–B magnets. The Tb addition makes the composition of the Nd-rich phase change, which possibly improve the impact toughness of sintered Nd–Fe–B magnets. Fig. 4 shows SEM micrograph showing the fractured surface of
sintered magnets. As can be seen, the fracture of the sintered magnets is mainly intergranular brittle fracture. Simultaneously, there also exists intergranular toughness fracture (as shown by black arrows in Fig. 4), and the intergranular toughness fracture is more obviously when the content of Tb increases from 0 to 1.0 at%, and weakens dramatically when the content of Tb increases from 1.0 to 1.5 at%. Maybe, the variation of the composition of the Nd-rich phase is the main reason which makes the phenomenon occurred. A great deal of plastic deformation existing in the grain boundaries consumes a lot of deformation energy, which increases the impact toughness of sintered Nd–Fe–B magnets. 4. Conclusion Addition of Tb increases the intrinsic coercivity of sintered Nd–Dy–Fe–B magnets, and the Dy content in the main phase increases with the increasing Tb content. When
ARTICLE IN PRESS Z.H. Hu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 1735–1738
1738
Fig. 4. SEM micrograph showing the fractured surface of sintered magnets: (D) x ¼ 0, (E) x ¼ 0.5, (F) x ¼ 1.0, and (G) x ¼ 1.5.
the Tb content increase from 0 to 1.0 at%, there is hardly the Tb element existing in the main phase, and only 0.51 at% Tb is obtained at x ¼ 1.5. So increasing the heavy rare earth content in the main phase will attribute to develop ultra-high intrinsic coercivity Nd–Fe–B magnets without using much heavy rare earth. The maximum of impact toughness is obtained at x ¼ 1.0. The microstructure shows that the fracture of sintered Nd–Fe–B magnets is mainly intergranular brittle fracture. Simultaneously, there also exists intergranular toughness fracture.
Acknowledgments The work was supported by the National High Technology Research and Development Program of China (Grant no. 2007AA03Z438) and the National Natural Science Foundation of China (Grant no. 50771035).
References [1] J.F. Herbst, Rev. Mod. Phys. 63 (1991) 819. [2] J.F. Liu, P. Vora, M.H. Walmer, E. Kottcamp, S.A. Bauser, A. Higgins, S. Liu, J. Appl. Phys. 97 (2005) 10H101. [3] S. Liu, D. Cao, R. Leese, S. Bauser, G.E. Kuhl, J.F. Liu, M.H. Walmer, E. Kottcamp, Proceedings of the 17th International Workshop on Rare Earth Permanent Magnets and their Applications, Newark, USA, 2002, p. 360. [4] J.H. Jiang, Z.P. Zeng, J.S. Wu, M. Tokunaga, J. Magn. Magn. Mater. 214 (2000) 61. [5] Z.H. Hu, M.G. Zhu, W. Li, F.Z. Lian, J. Magn. Magn. Mater. 320 (2008) 96. [6] H.J. Wang, A.H. Li, M.G. Zhu, W. Li, J. Magn. Magn. Mater. 307 (2006) 268. [7] H.J. Wang, A.H. Li, W. Li, Intermetallics 15 (2006) 985. [8] Z.Z. Jin, Y.W. Bao, Characterization of Mechanical Properties for Brittle Materials and Ceramics, China Railway Publishing House, 1996, p. 146 (in Chinese). [9] T. Schrefl, H.F. Schmidts, J. Fidler, H. Kronmu¨ller, J. Appl. Phys. 73 (1993) 6510.