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Effects of Nb on the coercivity and impact toughness of sintered Nd–Fe–B magnets
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 320 (2008) 96–99 www.elsevier.com/locate/jmmm
Effects of Nb on the coercivity and impact toughness of sintered Nd–Fe–B magnets Z.H. Hua,b,, M.G. Zhub, W. Lib, F.Z. Liana 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 13 April 2007; received in revised form 14 May 2007 Available online 24 May 2007
Abstract The effects of Nb on the coercivity and impact toughness of sintered Nd–Fe–B magnets have been investigated. The results show that the addition of Nb enhances the coercivity of sintered magnets, improves the microstructure and impact toughness of sintered magnets. The optimum impact toughness of sintered magnets was achieved when 1.5 at% Nb was incorporated. The reasons for improving the coercivity and impact toughness of sintered magnets are analyzed. r 2007 Elsevier B.V. All rights reserved. PACS: 75.50.Ww; 62.20.-x Keywords: Nd–Fe–B; Coercivity; Impact toughness; Sintered magnet
1. Introduction Developing high-coercivity Nd–Fe–B magnets without using much Dy is practically interesting and promising research subject in the field of rare earth transition metal permanent magnets [1]. In this condition, it is necessary to study effects of some alloy additions (Nb, Al, Ga, Cu) on the coercivity of sintered Nd–Fe–B magnets. Especially, the addition of Nb is favored over other elements for realizing maximum enhancement in Hci and rectangularity of the demagnetization curve [2–4]. Most of the research effort was focused on the magnetic properties and microstructure on Nd–Fe–B magnets in the past 20 years, but now more and more work has been done to improve the mechanical properties of Nd–Fe–B magnets in order to extend their applications. Liu et al. [5] studied the effect of adding small amounts of Al, Ga, Cu and Nb on impact toughness of sintered Nd–Fe–B magnets and concluded that small precipitates may contribute to the 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 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.05.010
improvement of toughness. Wang et al. [6,7] studied the effect of Nd, Pr and Dy substitution on the impact resistance of sintered Nd–Fe–B magnets, and concluded that the impact resistance of Nd–Fe–B magnets improves monotonously with the increasing Nd and Dy content, and reduces with increasing Pr content. In this paper, the variation of the coercivity and impact toughness of sintered Nd–Fe–B magnets with the Nb content is investigated, and the possible reasons for the change of the coercivity and impact toughness are discussed. 2. Experimental The alloy ingots with nominal compositions Nd15Fe77xNbxB8 and Nd15Fe72xCo5NbxB8 (x ¼ 0, 0.5, 1.0, 1.5, 2.0, 2.5) were prepared by arc melting, The powders were prepared by ball milling for 40 min, pressed and simultaneously aligned in a magnetic field of 1.5 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 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
ARTICLE IN PRESS Z.H. Hu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 96–99
were investigated using a JMS-6400 scanning electronic microscope. Impact toughness of the Nd–Fe–B magnets was examined using a falling-weight impact tester. The hammer used in the impact test was a cylinder with a coneshaped head. The radius of the cone-shaped head is 2 mm and the weight of the hammer is 48.335 g. The impact specimen was a round plate with diameter 10 mm and height 4 mm without being coated. 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 are averaged. The error bars are about 75 103 J for the impact energy. 3. Results and discussion 3.1. Magnetic properties
1100 1000 900 800 700 600 500 400 300
1.0 at%, (BH)max increases and the maximum can be obtained at 0.5 at.% Nb addition. Hci increases slowly with the increase of Nb content up to 2.0 at%, and after that, Hci increases rapidly. 3.2. Impact toughness Nd–Fe–B materials and ceramic materials are both brittle materials, and the impact toughness of ceramics is defined as [8] 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, 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 Nb content in the No. 1 and No. 2 magnets. As can be seen, the impact toughness of magnets with the content of Nb first increases, and reaches a maximum, then starts to decrease. As for the No. 1 magnets, impact toughness ak of magnets changes rapidly with increasing Nb content, and the maximum impact toughness is obtained at x ¼ 1.5. As for the No. 2 magnets, impact toughness ak of magnets does not change much with increasing Nb content, the maximum is obtained at x ¼ 1.0, and all the specimens have lower impact toughness ak than the specimens of the No. 1 magnets.
ak ¼
3.3. Microstructure The effect of Nb on the microstructure of the sintered magnets is shown in Fig. 3. A compound of NbFe is found within the grains (as shown by black arrows in Fig. 3). The ability of Nb to form a compound with Fe in the sintered magnets is viewed as beneficial for the development of high Hci and also for the near rectangularity of the intrinsic demagnetization curve [2]. The Nb addition makes the grain size of the main phase become regular, and the grain boundaries become clear and smooth [4]. So the intrinsic
4.0 NdFeNbB NdFeCoNbB
3.5 3.0 ak (kJ/m2)
(BH)max (kJ/m3)
Hci (kA/m)
Magnet samples with nominal compositions of Nd15Fe77xNbxB8 and Nd15Fe72xCo5NbxB8 (x ¼ 0, 0.5, 1.0, 1.5, 2.0, 2.5) will be called No. 1 and No. 2 magnets, respectively. Fig. 1 shows the effects of Nb on the magnetic properties of the No. 1 and No. 2 magnets. From Fig. 1 it can be seen that (BH)max, Hci of sintered magnets regularly change with the content of Nb. As for the No. 1 magnets, when the content of Nb increases from 0 to 2.0 at%, the maximum energy product (BH)max decreases slowly, while intrinsic coercivity Hci of the sintered magnets increases. When the content of Nb increases from 2.0 to 2.5 at%, (BH)max and Hci decreases dramatically. As for the No. 2 magnets, when the content of Nb increases from 0 to
97
280 260 240
2.5 2.0 1.5 1.0
220
NdFeNbB NdFeCoNbB
0.5
200
0.0
180 0.0
0.5
1.0
1.5
2.0
2.5
at.% Nb
Fig. 1. Variation of the magnetic properties of sintered magnets with the Nb content.
0.0
0.5
1.0
1.5
2.0
2.5
at.% Nb Fig. 2. Relationship between ak of sintered magnets and the content of Nb.
ARTICLE IN PRESS 98
Z.H. Hu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 96–99
Fig. 3. SEM micrographs of the sintered magnets: (a) and (A) Nd15Fe77B8, (b) and (B) Nd15Fe75.5Nb1.5B8, (c) and (C) Nd15Fe71Co5Nb1.0B8.
coercivity Hci of the sintered magnets is increased. Simultaneously, as for No. 1 magnets, there exist the grain boundaries where two grains contact directly, and this phenomenon is more obviously with the increase of Nb, which possibly leads to decrease the intrinsic coercivity Hci of the sintered magnets. But for No. 2 magnets, this phenomenon that two grains contact directly hardly appears, so the intrinsic coercivity Hci of the sintered magnets increases with increasing Nb content. The fracture of the sintered NdFeB magnets is intergranular fracture, which is a typical brittle fracture. The addition of Nb forms the compound of NbFe within the grains, which may contribute to the improvement of impact toughness. The grain boundaries where the two grains contact directly make intergranular fracture difficult. Furthermore, the Nb addition widens the grain boundaries of sintered magnets and changes the shape of the grain boundaries. All of these may be the reasons for improving the impact toughness of the sintered magnets.
4. Conclusions Addition of Nb improves the intrinsic coercivity of the No. 1 and No. 2 magnets. For the No. 1 magnets, the maximum of intrinsic coercivity is obtained with 2.0 at%, after that, the magnetic properties drop dramatically. The impact toughness first increases and then decreases with the increasing Nb content, the maximum is obtained with 1.5 at%. For the No. 2 magnets, the maximum of intrinsic coercivity is obtained with 2.5 at%, and the maximum of impact toughness is obtained with 1.0 at%. References [1] M. Sagawa. in: Proceeding of 18th International Workshop on High Performance Magnets and their Applications, Annecy, France, vol. 1, 2004, p. 7. [2] S. Pandian, V. Chandrasekaran, G. Markandeyulu, K.J.L. lyer, K.V.S. Rama Rao, J. Appl. Phys. 92 (2002) 6082. [3] L.Q. Yu, Y.H. Wen, M. Yan, J. Magn. Magn. Mater. 283 (2006) 353.
ARTICLE IN PRESS Z.H. Hu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 96–99 [4] W.H. Cheng, W. Li, C.J. Li, X.M. Li, J. Magn. Magn. Mater. 319 (2001) 280. [5] J.F. Liu, P. Vora, M.H. Walmer, E. Kottcamp, S.A. Bauser, A. Higgins, S. Liu, J. Appl. Phys. 97 (2005) 10H101. [6] H.J. Wang, A.H. Li, M.G. Zhu, W. Li, J. Magn. Magn. Mater. 307 (2006) 268.
99
[7] H.J. Wang, A.H. Li, W. Li, Intermetal. xx (2006) 1. [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).
Journal of Magnetism and Magnetic Materials 320 (2008) 96–99 www.elsevier.com/locate/jmmm
Effects of Nb on the coercivity and impact toughness of sintered Nd–Fe–B magnets Z.H. Hua,b,, M.G. Zhub, W. Lib, F.Z. Liana 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 13 April 2007; received in revised form 14 May 2007 Available online 24 May 2007
Abstract The effects of Nb on the coercivity and impact toughness of sintered Nd–Fe–B magnets have been investigated. The results show that the addition of Nb enhances the coercivity of sintered magnets, improves the microstructure and impact toughness of sintered magnets. The optimum impact toughness of sintered magnets was achieved when 1.5 at% Nb was incorporated. The reasons for improving the coercivity and impact toughness of sintered magnets are analyzed. r 2007 Elsevier B.V. All rights reserved. PACS: 75.50.Ww; 62.20.-x Keywords: Nd–Fe–B; Coercivity; Impact toughness; Sintered magnet
1. Introduction Developing high-coercivity Nd–Fe–B magnets without using much Dy is practically interesting and promising research subject in the field of rare earth transition metal permanent magnets [1]. In this condition, it is necessary to study effects of some alloy additions (Nb, Al, Ga, Cu) on the coercivity of sintered Nd–Fe–B magnets. Especially, the addition of Nb is favored over other elements for realizing maximum enhancement in Hci and rectangularity of the demagnetization curve [2–4]. Most of the research effort was focused on the magnetic properties and microstructure on Nd–Fe–B magnets in the past 20 years, but now more and more work has been done to improve the mechanical properties of Nd–Fe–B magnets in order to extend their applications. Liu et al. [5] studied the effect of adding small amounts of Al, Ga, Cu and Nb on impact toughness of sintered Nd–Fe–B magnets and concluded that small precipitates may contribute to the 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 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.05.010
improvement of toughness. Wang et al. [6,7] studied the effect of Nd, Pr and Dy substitution on the impact resistance of sintered Nd–Fe–B magnets, and concluded that the impact resistance of Nd–Fe–B magnets improves monotonously with the increasing Nd and Dy content, and reduces with increasing Pr content. In this paper, the variation of the coercivity and impact toughness of sintered Nd–Fe–B magnets with the Nb content is investigated, and the possible reasons for the change of the coercivity and impact toughness are discussed. 2. Experimental The alloy ingots with nominal compositions Nd15Fe77xNbxB8 and Nd15Fe72xCo5NbxB8 (x ¼ 0, 0.5, 1.0, 1.5, 2.0, 2.5) were prepared by arc melting, The powders were prepared by ball milling for 40 min, pressed and simultaneously aligned in a magnetic field of 1.5 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 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
ARTICLE IN PRESS Z.H. Hu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 96–99
were investigated using a JMS-6400 scanning electronic microscope. Impact toughness of the Nd–Fe–B magnets was examined using a falling-weight impact tester. The hammer used in the impact test was a cylinder with a coneshaped head. The radius of the cone-shaped head is 2 mm and the weight of the hammer is 48.335 g. The impact specimen was a round plate with diameter 10 mm and height 4 mm without being coated. 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 are averaged. The error bars are about 75 103 J for the impact energy. 3. Results and discussion 3.1. Magnetic properties
1100 1000 900 800 700 600 500 400 300
1.0 at%, (BH)max increases and the maximum can be obtained at 0.5 at.% Nb addition. Hci increases slowly with the increase of Nb content up to 2.0 at%, and after that, Hci increases rapidly. 3.2. Impact toughness Nd–Fe–B materials and ceramic materials are both brittle materials, and the impact toughness of ceramics is defined as [8] 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, 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 Nb content in the No. 1 and No. 2 magnets. As can be seen, the impact toughness of magnets with the content of Nb first increases, and reaches a maximum, then starts to decrease. As for the No. 1 magnets, impact toughness ak of magnets changes rapidly with increasing Nb content, and the maximum impact toughness is obtained at x ¼ 1.5. As for the No. 2 magnets, impact toughness ak of magnets does not change much with increasing Nb content, the maximum is obtained at x ¼ 1.0, and all the specimens have lower impact toughness ak than the specimens of the No. 1 magnets.
ak ¼
3.3. Microstructure The effect of Nb on the microstructure of the sintered magnets is shown in Fig. 3. A compound of NbFe is found within the grains (as shown by black arrows in Fig. 3). The ability of Nb to form a compound with Fe in the sintered magnets is viewed as beneficial for the development of high Hci and also for the near rectangularity of the intrinsic demagnetization curve [2]. The Nb addition makes the grain size of the main phase become regular, and the grain boundaries become clear and smooth [4]. So the intrinsic
4.0 NdFeNbB NdFeCoNbB
3.5 3.0 ak (kJ/m2)
(BH)max (kJ/m3)
Hci (kA/m)
Magnet samples with nominal compositions of Nd15Fe77xNbxB8 and Nd15Fe72xCo5NbxB8 (x ¼ 0, 0.5, 1.0, 1.5, 2.0, 2.5) will be called No. 1 and No. 2 magnets, respectively. Fig. 1 shows the effects of Nb on the magnetic properties of the No. 1 and No. 2 magnets. From Fig. 1 it can be seen that (BH)max, Hci of sintered magnets regularly change with the content of Nb. As for the No. 1 magnets, when the content of Nb increases from 0 to 2.0 at%, the maximum energy product (BH)max decreases slowly, while intrinsic coercivity Hci of the sintered magnets increases. When the content of Nb increases from 2.0 to 2.5 at%, (BH)max and Hci decreases dramatically. As for the No. 2 magnets, when the content of Nb increases from 0 to
97
280 260 240
2.5 2.0 1.5 1.0
220
NdFeNbB NdFeCoNbB
0.5
200
0.0
180 0.0
0.5
1.0
1.5
2.0
2.5
at.% Nb
Fig. 1. Variation of the magnetic properties of sintered magnets with the Nb content.
0.0
0.5
1.0
1.5
2.0
2.5
at.% Nb Fig. 2. Relationship between ak of sintered magnets and the content of Nb.
ARTICLE IN PRESS 98
Z.H. Hu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 96–99
Fig. 3. SEM micrographs of the sintered magnets: (a) and (A) Nd15Fe77B8, (b) and (B) Nd15Fe75.5Nb1.5B8, (c) and (C) Nd15Fe71Co5Nb1.0B8.
coercivity Hci of the sintered magnets is increased. Simultaneously, as for No. 1 magnets, there exist the grain boundaries where two grains contact directly, and this phenomenon is more obviously with the increase of Nb, which possibly leads to decrease the intrinsic coercivity Hci of the sintered magnets. But for No. 2 magnets, this phenomenon that two grains contact directly hardly appears, so the intrinsic coercivity Hci of the sintered magnets increases with increasing Nb content. The fracture of the sintered NdFeB magnets is intergranular fracture, which is a typical brittle fracture. The addition of Nb forms the compound of NbFe within the grains, which may contribute to the improvement of impact toughness. The grain boundaries where the two grains contact directly make intergranular fracture difficult. Furthermore, the Nb addition widens the grain boundaries of sintered magnets and changes the shape of the grain boundaries. All of these may be the reasons for improving the impact toughness of the sintered magnets.
4. Conclusions Addition of Nb improves the intrinsic coercivity of the No. 1 and No. 2 magnets. For the No. 1 magnets, the maximum of intrinsic coercivity is obtained with 2.0 at%, after that, the magnetic properties drop dramatically. The impact toughness first increases and then decreases with the increasing Nb content, the maximum is obtained with 1.5 at%. For the No. 2 magnets, the maximum of intrinsic coercivity is obtained with 2.5 at%, and the maximum of impact toughness is obtained with 1.0 at%. References [1] M. Sagawa. in: Proceeding of 18th International Workshop on High Performance Magnets and their Applications, Annecy, France, vol. 1, 2004, p. 7. [2] S. Pandian, V. Chandrasekaran, G. Markandeyulu, K.J.L. lyer, K.V.S. Rama Rao, J. Appl. Phys. 92 (2002) 6082. [3] L.Q. Yu, Y.H. Wen, M. Yan, J. Magn. Magn. Mater. 283 (2006) 353.
ARTICLE IN PRESS Z.H. Hu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 96–99 [4] W.H. Cheng, W. Li, C.J. Li, X.M. Li, J. Magn. Magn. Mater. 319 (2001) 280. [5] J.F. Liu, P. Vora, M.H. Walmer, E. Kottcamp, S.A. Bauser, A. Higgins, S. Liu, J. Appl. Phys. 97 (2005) 10H101. [6] H.J. Wang, A.H. Li, M.G. Zhu, W. Li, J. Magn. Magn. Mater. 307 (2006) 268.
99
[7] H.J. Wang, A.H. Li, W. Li, Intermetal. xx (2006) 1. [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).