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Magnetic properties of melt-spun (Nd,Dy)2Fe14(B,C)
Journal of Alloys and Compounds 316 (2001) 290–295
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Magnetic properties of melt-spun (Nd,Dy) 2 Fe 14 (B,C) ¨ Jinbo Yang, A. Handstein*, A. Kirchner, O. Gutfleisch, K.-H. Muller ¨ Festkorper¨ Institut f ur und Werkstofforschung Dresden, P.O.B. 270116, D-01171 Dresden, Germany Received 6 November 2000; accepted 30 November 2000
Abstract Hard magnetic materials based on the Nd 2 Fe 14 C, (Nd,Dy) 2 Fe 14 C and (Nd,Dy) 2 Fe 14 (B,C) phases have been prepared by vacuum annealing of melt-spun ribbons. A coercivity m 0J H c of about 2.5 T can be obtained directly by melt-spinning Nd 13 Dy 2 Fe 77 C 6 B 2 . All as-quenched ribbons consist mainly of three phases: (Nd,Dy) 2 Fe 17 C x (2:17), (Nd,Dy) 2 Fe 14 (B,C) (2:14:1) and a-Fe. Due to short-time annealing, the 2:17 phase transforms into the 2:14:1 phase. Substitution of carbon by boron accelerates the formation of 2:14:1 phase in the as-spun ribbons. Dy substitution for Nd increases the coercivity of Nd–Fe–C based magnets. After optimization an energy product of (BH) max 576 kJ / m 3 , a remanence of B r 50.71 T, and a coercivity of m 0J H c 52.0 T were achieved for annealed Nd 13 Dy 2 Fe 77 C 6 B 2 ribbons. Fully dense, textured (Nd,Dy) 2 Fe 14 (B,C) magnets with (BH) max of up to 137 kJ / m 3 could be produced by hot pressing coarsely ground ribbons and subsequent die-upsetting. 2001 Elsevier Science B.V. All rights reserved. Keywords: Transition metal compounds; Rare earth compounds; Permanent magnets; Rapid quenching; Magnetic measurements PACS: 75.50Ww; 75.50Vv; 75.50Kj; 81.40Vw
1. Introduction Permanent magnet materials based on the tetragonal Nd 2 Fe 14 B (2:14:1) phase have been extensively studied due to their excellent magnetic properties [1–3]. The Nd 2 Fe 14 C compound is isostructural with Nd 2 Fe 14 B and shows similar intrinsic magnetic properties [4]. The magnetocrystalline anisotropy field of Nd 2 Fe 14 C was found to be even larger than that of the boride [5]. However, it is very difficult, especially for the light rare earths, to prepare single phase alloys of R 2 Fe 14 C (R — rare earth) by annealing of ingots due to the very sluggish solid-state transformation from the rhombohedral Th 2 Zn 17 -type structure formed during casting to the tetragonal 2:14:1-type structure. Buschow et al. [6] succeeded in the preparation of Nd 2 Fe 14 C by means of a 21-day heat treatment at 1150 K. A coercivity m 0J H c of about 1.2 T can be obtained without resorting to powder metallurgy for as-cast Nd 2 Fe 14 C [7] and Dy 2 Fe 14 C [8] (‘ingot magnets’). The *Corresponding author. Tel.: 149-351-4659-526; fax: 149-351-4659537. E-mail address: [email protected] (A. Handstein).
preparation of Pr 2 Fe 14 C involves several months of annealing and cooling [9]. A coercivity as large as 2.75 T can be obtained in cast Dy–Fe–C after appropriate heat-treatment [10]. It was found that boron and copper additions accelerate the formation of the R 2 Fe 14 C phase [11,12]. Despite the difficulty in forming the tetragonal 2:14:1-type phase the preparation of melt-spun R 2 Fe 14 C materials was successful also for light rare earth elements. Some of the light rare-earth carbides, for example those with Y, Ce [13] or Pr [14], can be obtained through annealing of melt-spun ribbons. Coehoorn et al. [15] have produced Nd–Fe–C ribbons by melt spinning, consisting mainly of Nd 2 Fe 14 C, with a coercivity of over 1.0 T and remanence of B r 50.72 T. Recently, (Nd,Dy) 2 Fe 14 C based magnets could be prepared from powders produced by mechanical alloying [16] or the hydrogenation-disproportionation-desorptionrecombination process [17]. However, up to now, the magnetic properties achieved for Nd–Fe–C based alloys are comparatively poor and not yet suitable for technological application. In this paper we report that high-coercivity magnetic materials based on Nd 2 Fe 14 C can be prepared by melt spinning and subsequent short-time annealing of the melt-
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spun ribbons. The influence of dysprosium and boron addition on the phase transformations and magnetic properties is investigated.
2. Experimental The starting ingots of Nd 15 Fe 77 C 8 , Nd 13 Dy 2 Fe 77 C 8 , Nd 13 Dy 2 Fe 77 C 6 B 2 , and Nd 13 Dy 2 Fe 73 C 10 B 2 were prepared by arc-melting using 99.9% pure elements. Rapidly quenched ribbons were obtained by melt spinning onto a copper wheel in argon gas atmosphere. The wheel velocity was 40 m / s. The ribbons were annealed in a vacuum of 5*10 25 mbar in a temperature range from T a 5600 to 8508C for 15 min. The crystal structure of these ribbons was determined by X-ray diffraction (XRD) with Co Ka radiation. For the preparation of textured magnets, crashed ribbon powders were hot pressed at a temperature of 7258C and a pressure of 150 MPa in vacuum (10 22 Pa). Dieupsetting was carried out subsequently with strain rates between 1310 23 s 21 and 3.3310 23 s 21 at 7508C in argon [18]. Grain sizes after hot deformation were determined by scanning electron microscopy (S.E.M.) using fracture surfaces. Magnetic measurements were performed with a vibrating sample magnetometer (VSM) in fields up to 8 T at room temperature. The Curie temperatures of these compounds were determined by thermomagnetic measurements in an applied field of m 0 H50.1 T.
3. Results and discussion
3.1. Phase transformation of as-quenched ribbons Fig. 1 shows the XRD patterns of (a) Nd 15 Fe 77 C 8 , (b) Nd 13 Dy 2 Fe 77 C 8 , (c) Nd 13 Dy 2 Fe 77 C 6 B 2 , and (d) Nd 13 Dy 2 Fe 73 C 10 B 2 ribbons before and after annealing at different temperatures. Generally, three phases can be observed in the as-quenched ribbons: (Nd,Dy) 2 Fe 17 C x (2:17), a-Fe and (Nd,Dy) 2 Fe 14 (C,B) (2:14:1). The amounts of the different phases in the as-spun ribbons depend on the composition of the alloys. For example, a large amount of 2:17 carbides and a-Fe is present in Nd 15 Fe 77 C 8 and Nd 13 Dy 2 Fe 77 C 8 ribbons. From Fig. 1c it is easy to see that Nd 13 Dy 2 Fe 77 C 6 B 2 ribbons consist primarily of the 2:14:1 phase in the as-quenched state. This is consistent with the observation that the addition of boron accelerates the formation of the 2:14:1 phase [11]. The XRD pattern of melt-spun Nd 13 Dy 2 Fe 73 C 10 B 2 ribbons shows line broadening indicating a very fine grain size. Some amorphous phases are present in the Nd 13 Dy 2 Fe 73 C 10 B 2 ribbons which may be due to the higher concentration of carbon. The typical grain size in these ribbons is of 20 nm in the as-quenched states according to transmission electron microscopy observation published recently [19]. A phase transformation from 2:17
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to 2:14:1 phase occurs in all ribbons during annealing. The phase transformation starts at T a 56508C, and the 2:14:1 phase can be formed by annealing in a range from 650 to 8508C for 15 min. This is different from the behavior of bulk alloys, in which the single Nd 2 Fe 14 C phase can be only formed by annealing in a narrow temperature window from 830 to 8808C for several weeks due to a very slow solid-state transformation [6].
3.2. Magnetic properties of ribbons All as-quenched ribbons show a very low coercivity except for Nd 13 Dy 2 Fe 77 C 6 B 2 which shows a coercivity of 2.5 T. This is due to the fact that as-quenched Nd 13 Dy 2 Fe 77 C 6 B 2 ribbons consist almost entirely of the 2:14:1 phase (‘direct quenching’). There is a significant amount of 2:14:1 phase in the Nd 13 Dy 2 Fe 73 C 10 B 2 hard magnetic ribbons, but the coercivity is low due to the existence of amorphous and 2:17-type phases. In the asquenched Nd 15 Fe 77 C 8 and Nd 13 Dy 2 Fe 77 C 8 ribbons which consist mainly of 2:17 and a-Fe soft phases, very low coercivities with values #0.1 T were observed. The coercivity of nanocrystalline materials is sensitive to the grain size which can be modified by vacuum annealing. Fig. 2 shows the dependence of coercivities on the annealing temperature in (Nd,Dy)–Fe–(C,B) ribbons. After annealing at 6508C the low coercivities of Nd 15 Fe 77 C 8 , Nd 13 Dy 2 Fe 77 C 8 and Nd 13 Dy 2 Fe 73 C 10 B 2 ribbons can be attributed to the presence of soft magnetic 2:17-type phases. A sharp increase of their coercivity values was found after annealing at 7008C for 15 min. After this procedure, all ribbons show according to the XRD patterns the 2:14:1 phase as main phase, which gives rise to high coercivities. The coercivities of Nd 15 Fe 77 C 8 , Nd 13 Dy 2 Fe 77 C 8 and Nd 13 Dy 2 Fe 77 C 6 B 2 decrease rapidly, if the annealing temperature is higher than 8008C which is due to grain growth. This is confirmed by the reduced peak width in the XRD patterns as compared with those of as-quenched ribbons. The coercivities of Nd 13 Dy 2 Fe 77 C 6 B 2 ribbons decrease with increasing annealing temperatures. However, in the temperature range from 700 to 7658C they remain relatively constant around 1.82 T. The coercivity of Nd 13 Dy 2 Fe 73 C 10 B 2 increases slightly with the annealing temperature, indicating that this alloy is very stable against grain growth. The addition of Dy drastically increases the coercivity by about 0.8 T, compared with the Dy free sample series Nd 15 Fe 77 C 8 . Substitution of boron for carbon also increases the coercivity. However, the Dy substitution and the increase of carbon content in the ribbons decrease the values of magnetic polarization due to the antiparallel coupling and the dilution of the total magnetic moment, respectively, as observed for Nd 13 Dy 2 Fe 73 C 10 B 2 (cf. Fig. 5). Fig. 3 shows typical hysteresis loops of ribbons before and after annealing. As described before, the as-quenched Nd 13 Dy 2 Fe 77 C 8 ribbons show soft magnetic behavior due
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Fig. 1. X-ray diffraction patterns of melt spun Nd(Dy)–Fe–C(B) ribbons before and after annealing at different temperatures, (a) Nd 15 Fe 77 C 8 , (b) Nd 13 Dy 2 Fe 77 C 8 , (c) Nd 13 Dy 2 Fe 77 C 6 B 2 , and (d) Nd 13 Dy 2 Fe 73 C 10 B 2 .
to their inhomogeneous phase composition. During annealing the coercivity increases due to the formation of 2:14:1type phase as shown for the case of T a 57308C for 15 min. The hysteresis loop of the as-quenched Nd 13 Dy 2 Fe 77 C 6 B 2 ribbon shows a kink, which can be also observed for all samples if the annealing temperature is not optimized with respect to temperature and time. It is caused by the low switching fields of soft magnetic phases. The best magnetic properties were observed for Nd 13 Dy 2 Fe 77 C 6 B 2 ribbons after annealing at 6508C for 15 min: coercivity m 0J H c 52.0 T, remanence B r 50.71 T, energy product (BH) max 576 kJ / m 3 , and Curie temperature T c 5590 K. These magnetic properties are comparable, if not better, to MQI-type (Magnequench) magnetic powders which are frequently used for permanent magnet production. Typical
virgin magnetization curves of ribbons after annealing at 7308C for 15 min are shown in Fig. 4. The shape of the virgin magnetization curves reveals an inhomogeneous magnetization behavior. Fig. 5 gives the dependence of the remanence B r and polarization J(8T) (measured at 8 T) of (Nd,Dy)–Fe– (B,C) ribbons on annealing temperature. The polarization has not reached saturation in an applied field of 8 T. This should mainly be caused due to the high anisotropy field of about m 0 H a 514.8 T [4]. The polarization of ribbons shows no significant dependence on the annealing temperatures from 700 to 8508C. This is consistent with the observation that the 2:14:1 phase is the main phase after annealing at 7008C for 15 min. Initially, the remanence values of Nd 15 Fe 77 C 8 and Nd 13 Dy 2 Fe 77 C 8 are small and increase
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Fig. 4. Virgin magnetization curves of (Nd,Dy)–Fe–(C,B) ribbons after annealing at 7308C for 15 min.
3.3. Die-upset magnets Fig. 2. Dependence of coercivities on annealing temperatures of various (Nd,Dy)–Fe–(C,B) ribbons.
with increasing T a . This is caused by the slower transformation kinetics of these two compounds compared with the ribbons with additional boron content as Nd 13 Dy 2 Fe 77 C 6 B 2 and Nd 13 Dy 2 Fe 73 C 10 B 2 . The phase transformation is not complete in Nd 15 Fe 77 C 8 and Nd 13 Dy 2 Fe 77 C 8 ribbons until 8008C. This can be confirmed from the demagnetization curves which have a kink in the temperature range from 650 to 8008C. The substitution of Nd by Dy decreases the magnetization due to the antiparallel coupling between heavy rare-earth Dy and Fe magnetic moments. Therefore, the substitution with a smaller amount of Dy should be the preferred option for producing high performance magnets. Substitution of carbon by boron increases the magnetization of ribbons which is attributed to the higher saturation magnetization of Nd 2 Fe 14 B.
Fig. 3. Hysteresis loops of Nd 13 Dy 2 Fe 77 C 8 and Nd 13 Dy 2 Fe 77 C 6 B 2 ribbons before and after annealing at 7008C for 15 min.
Textured, bulk magnets could be produced by hot pressing and subsequent die upsetting of crushed ribbon powders. Fig. 6 shows the demagnetization curves of Nd 13 Dy 2 Fe 77 C 8 and Nd 13 Dy 2 Fe 77 C 6 B 2 die-upset magnets measured parallel and perpendicular to the pressing direction. The results show that the (Nd,Dy) 2 Fe 14 (B,C) materials can be textured in the same manner as observed for the Nd–Fe–B based magnets [20] where the easy magnetization direction (EMD) is also parallel to the pressure direction, i.e. the EMD is parallel to the c-axis of 2:14:1phase grains. The relatively low remanences of B r 50.83 and 0.86 T of Nd 13 Dy 2 Fe 77 C 8 and Nd 13 Dy 2 Fe 77 C 6 B 2
Fig. 5. Dependence of remanence and magnetization (measured at 8 T) of various (Nd,Dy)–Fe–(C,B) ribbons on the annealing temperature.
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size larger than 500 nm by S.E.M. on fracture surfaces of a Nd 13 Dy 2 Fe 77 C 6 B 2 die-upset magnet as shown in Fig. 7.
4. Conclusions
Fig. 6. Demagnetization curves of die-upset Nd 13 Dy 2 Fe 77 C 8 and Nd 13 Dy 2 Fe 77 C 6 B 2 magnets measured parallel and perpendicular to the pressure direction.
((BH) max |137 kJ / m 3 ) show, however, that the degree of alignment is not comparable with that of corresponding 2:14:1 borides. Slower anisotropic grain growth of the 2:14:1-type phase and the presence of soft magnetic impurity phases have been suggested previously [21] in order to explain such a behavior. The present study has shown that the formation of platelet-shaped grains is less pronounced (compare the large number of almost equiaxed grains in Fig. 7) and also revealed that the hot workability of the 2:14:1 carbides is much worse than that of the corresponding 2:14:1 borides. Analysis of the stress–strain relations showed that the deformation stresses required for a certain degree of deformation are about eight times as high as in MQ-type ribbons. The relatively low coercivity of e.g. m 0J H c ~1 T for Nd 13 Dy 2 Fe 77 C 6 B 2 compared with the results shown in Fig. 2 point to rapid grain growth during the procedure of hot pressing and deforming. This is confirmed by the occasional observation of grains with a
Fig. 7. S.E.M. picture of the fracture surface of a Nd 13 Dy 2 Fe 77 C 6 B 2 magnet.
Magnetic soft phases, 2:17 carbides and a-Fe, together with the 2:14:1 phase were found in melt-spun ribbons of Nd 2 Fe 14 C, (Nd,Dy) 2 Fe 14 C, and (Nd,Dy) 2 Fe 14 (B,C). The soft magnetic phases can be fully transformed into 2:14:1 hard magnetic phase after annealing for optimum temperature and time. Addition of boron accelerates the formation of the 2:14:1 carbide phase in the melt spun ribbons. Nd 13 Dy 2 Fe 77 C 6 B 2 shows a very high coercivity of about 2.5 T after direct quenching. After optimum annealing magnetically hard Nd 13 Dy 2 Fe 77 C 6 B 2 ribbons with a coercivity of about 2.0 T, remanence of 0.71 T, and energy product of 76 kJ / m 3 were obtained. The increase of carbon content in the ribbons leads to a smaller grain size, and impedes grain growth during annealing. The production of textured magnets made from melt spun (Nd,Dy) 2 Fe 14 (B,C) materials by die-upsetting is in principle possible, however, with a smaller degree of alignment compared with Nd–Fe–B based materials.
Acknowledgements J.B. Yang is very grateful to the Alexander von Humboldt-Foundation and A. Kirchner to the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 463) for financial support.
References [1] J.J. Croat, J.F. Herbst, R.W. Lee, P.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] J. Herbst, J.J. Croat, F.E. Pinkerton, W.B. Yelon, Phys. Rev. B29 (1984) 4176. [4] K.H.J. Buschow, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 10, Elsevier Science B.V, 1997, p. 529. [5] W. Lui, Z. Zhang, X.K. Sun, Y.C. Chuang, F. Yang, F.R. de Boer, Solid State Commun. 76 (1990) 1375. [6] K.H.J. Buschow, D.B. de Mooij, C.J.M. Denissen, J. Less-Common. Met. 141 (1988) L15. [7] N.C. Liu, H.H. Stadelmaier, G. Schneider, J. Appl. Phys. 61 (1987) 3574. [8] N.C. Liu, H.H. Stadelmaier, Mater. Lett. 4 (1986) 377. ¨ [9] M. Gueramian, A. Bezinge, K. Yvon, J. Muller, Solid State Commun. 64 (1987) 639. [10] R. Van Mens, D.B. de Mooij, K.H.J. Buschow, J. Appl. Phys. 64 (1988) 5708. [11] B. Grieb, K. Fritz, E.Th. Henig, J. Appl. Phys. 70 (1991) 6447. [12] T.S. Jang, H.H. Stadelmaier, Mater. Lett. 9 (1990) 483. [13] D. Fuerst, J.F. Herbst, J. Appl. Phys. 69 (1991) 7727. [14] G.C. Hadjipanayis, Mater. Sci. Eng. B 3 (1989) 431.
J. Yang et al. / Journal of Alloys and Compounds 316 (2001) 290 – 295 [15] R. Coehoorn, J.P.W.B. Duchateau, C.J.M. Denissen, J. Appl. Phys. 65 (1989) 704. [16] Y.C. Sui, Z.D. Zhang, Q.F. Xiao, W. Liu, X.G. Zhao, T. Zhao, Y.C. Chuang, J. Phys.: Condens. Matter. 8 (1996) 11231. [17] Y.C. Sui, Z.D. Zhang, W. Liu, Q.F. Xiao, X.G. Zhao, T. Zhao, Y.C. Chuang, J. Phys.: Condens. Matter. 10 (1998) 363. ¨ ¨ [18] W. Grunberger, D. Hinz, A. Kirchner, K.-H. Muller, L. Schultz, J. Alloys Comp. 257 (1997) 293.
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¨ [19] J.B. Yang, O. Gutfleisch, A. Handstein, D. Eckert, K.-H. Muller, Appl. Phys. Lett. 76 (2000) 3627. ¨ ¨ [20] A. Kirchner, W. Grunberger, O. Gutfleisch, V. Neu, K.-H. Muller, L. Schultz, J. Phys. D: Appl. Phys. 31 (1998) 1660. [21] C.D. Fuerst, E.G. Brewer, J. Appl. Phys. 70 (1991) 6444.
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Magnetic properties of melt-spun (Nd,Dy) 2 Fe 14 (B,C) ¨ Jinbo Yang, A. Handstein*, A. Kirchner, O. Gutfleisch, K.-H. Muller ¨ Festkorper¨ Institut f ur und Werkstofforschung Dresden, P.O.B. 270116, D-01171 Dresden, Germany Received 6 November 2000; accepted 30 November 2000
Abstract Hard magnetic materials based on the Nd 2 Fe 14 C, (Nd,Dy) 2 Fe 14 C and (Nd,Dy) 2 Fe 14 (B,C) phases have been prepared by vacuum annealing of melt-spun ribbons. A coercivity m 0J H c of about 2.5 T can be obtained directly by melt-spinning Nd 13 Dy 2 Fe 77 C 6 B 2 . All as-quenched ribbons consist mainly of three phases: (Nd,Dy) 2 Fe 17 C x (2:17), (Nd,Dy) 2 Fe 14 (B,C) (2:14:1) and a-Fe. Due to short-time annealing, the 2:17 phase transforms into the 2:14:1 phase. Substitution of carbon by boron accelerates the formation of 2:14:1 phase in the as-spun ribbons. Dy substitution for Nd increases the coercivity of Nd–Fe–C based magnets. After optimization an energy product of (BH) max 576 kJ / m 3 , a remanence of B r 50.71 T, and a coercivity of m 0J H c 52.0 T were achieved for annealed Nd 13 Dy 2 Fe 77 C 6 B 2 ribbons. Fully dense, textured (Nd,Dy) 2 Fe 14 (B,C) magnets with (BH) max of up to 137 kJ / m 3 could be produced by hot pressing coarsely ground ribbons and subsequent die-upsetting. 2001 Elsevier Science B.V. All rights reserved. Keywords: Transition metal compounds; Rare earth compounds; Permanent magnets; Rapid quenching; Magnetic measurements PACS: 75.50Ww; 75.50Vv; 75.50Kj; 81.40Vw
1. Introduction Permanent magnet materials based on the tetragonal Nd 2 Fe 14 B (2:14:1) phase have been extensively studied due to their excellent magnetic properties [1–3]. The Nd 2 Fe 14 C compound is isostructural with Nd 2 Fe 14 B and shows similar intrinsic magnetic properties [4]. The magnetocrystalline anisotropy field of Nd 2 Fe 14 C was found to be even larger than that of the boride [5]. However, it is very difficult, especially for the light rare earths, to prepare single phase alloys of R 2 Fe 14 C (R — rare earth) by annealing of ingots due to the very sluggish solid-state transformation from the rhombohedral Th 2 Zn 17 -type structure formed during casting to the tetragonal 2:14:1-type structure. Buschow et al. [6] succeeded in the preparation of Nd 2 Fe 14 C by means of a 21-day heat treatment at 1150 K. A coercivity m 0J H c of about 1.2 T can be obtained without resorting to powder metallurgy for as-cast Nd 2 Fe 14 C [7] and Dy 2 Fe 14 C [8] (‘ingot magnets’). The *Corresponding author. Tel.: 149-351-4659-526; fax: 149-351-4659537. E-mail address: [email protected] (A. Handstein).
preparation of Pr 2 Fe 14 C involves several months of annealing and cooling [9]. A coercivity as large as 2.75 T can be obtained in cast Dy–Fe–C after appropriate heat-treatment [10]. It was found that boron and copper additions accelerate the formation of the R 2 Fe 14 C phase [11,12]. Despite the difficulty in forming the tetragonal 2:14:1-type phase the preparation of melt-spun R 2 Fe 14 C materials was successful also for light rare earth elements. Some of the light rare-earth carbides, for example those with Y, Ce [13] or Pr [14], can be obtained through annealing of melt-spun ribbons. Coehoorn et al. [15] have produced Nd–Fe–C ribbons by melt spinning, consisting mainly of Nd 2 Fe 14 C, with a coercivity of over 1.0 T and remanence of B r 50.72 T. Recently, (Nd,Dy) 2 Fe 14 C based magnets could be prepared from powders produced by mechanical alloying [16] or the hydrogenation-disproportionation-desorptionrecombination process [17]. However, up to now, the magnetic properties achieved for Nd–Fe–C based alloys are comparatively poor and not yet suitable for technological application. In this paper we report that high-coercivity magnetic materials based on Nd 2 Fe 14 C can be prepared by melt spinning and subsequent short-time annealing of the melt-
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 00 )01502-4
J. Yang et al. / Journal of Alloys and Compounds 316 (2001) 290 – 295
spun ribbons. The influence of dysprosium and boron addition on the phase transformations and magnetic properties is investigated.
2. Experimental The starting ingots of Nd 15 Fe 77 C 8 , Nd 13 Dy 2 Fe 77 C 8 , Nd 13 Dy 2 Fe 77 C 6 B 2 , and Nd 13 Dy 2 Fe 73 C 10 B 2 were prepared by arc-melting using 99.9% pure elements. Rapidly quenched ribbons were obtained by melt spinning onto a copper wheel in argon gas atmosphere. The wheel velocity was 40 m / s. The ribbons were annealed in a vacuum of 5*10 25 mbar in a temperature range from T a 5600 to 8508C for 15 min. The crystal structure of these ribbons was determined by X-ray diffraction (XRD) with Co Ka radiation. For the preparation of textured magnets, crashed ribbon powders were hot pressed at a temperature of 7258C and a pressure of 150 MPa in vacuum (10 22 Pa). Dieupsetting was carried out subsequently with strain rates between 1310 23 s 21 and 3.3310 23 s 21 at 7508C in argon [18]. Grain sizes after hot deformation were determined by scanning electron microscopy (S.E.M.) using fracture surfaces. Magnetic measurements were performed with a vibrating sample magnetometer (VSM) in fields up to 8 T at room temperature. The Curie temperatures of these compounds were determined by thermomagnetic measurements in an applied field of m 0 H50.1 T.
3. Results and discussion
3.1. Phase transformation of as-quenched ribbons Fig. 1 shows the XRD patterns of (a) Nd 15 Fe 77 C 8 , (b) Nd 13 Dy 2 Fe 77 C 8 , (c) Nd 13 Dy 2 Fe 77 C 6 B 2 , and (d) Nd 13 Dy 2 Fe 73 C 10 B 2 ribbons before and after annealing at different temperatures. Generally, three phases can be observed in the as-quenched ribbons: (Nd,Dy) 2 Fe 17 C x (2:17), a-Fe and (Nd,Dy) 2 Fe 14 (C,B) (2:14:1). The amounts of the different phases in the as-spun ribbons depend on the composition of the alloys. For example, a large amount of 2:17 carbides and a-Fe is present in Nd 15 Fe 77 C 8 and Nd 13 Dy 2 Fe 77 C 8 ribbons. From Fig. 1c it is easy to see that Nd 13 Dy 2 Fe 77 C 6 B 2 ribbons consist primarily of the 2:14:1 phase in the as-quenched state. This is consistent with the observation that the addition of boron accelerates the formation of the 2:14:1 phase [11]. The XRD pattern of melt-spun Nd 13 Dy 2 Fe 73 C 10 B 2 ribbons shows line broadening indicating a very fine grain size. Some amorphous phases are present in the Nd 13 Dy 2 Fe 73 C 10 B 2 ribbons which may be due to the higher concentration of carbon. The typical grain size in these ribbons is of 20 nm in the as-quenched states according to transmission electron microscopy observation published recently [19]. A phase transformation from 2:17
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to 2:14:1 phase occurs in all ribbons during annealing. The phase transformation starts at T a 56508C, and the 2:14:1 phase can be formed by annealing in a range from 650 to 8508C for 15 min. This is different from the behavior of bulk alloys, in which the single Nd 2 Fe 14 C phase can be only formed by annealing in a narrow temperature window from 830 to 8808C for several weeks due to a very slow solid-state transformation [6].
3.2. Magnetic properties of ribbons All as-quenched ribbons show a very low coercivity except for Nd 13 Dy 2 Fe 77 C 6 B 2 which shows a coercivity of 2.5 T. This is due to the fact that as-quenched Nd 13 Dy 2 Fe 77 C 6 B 2 ribbons consist almost entirely of the 2:14:1 phase (‘direct quenching’). There is a significant amount of 2:14:1 phase in the Nd 13 Dy 2 Fe 73 C 10 B 2 hard magnetic ribbons, but the coercivity is low due to the existence of amorphous and 2:17-type phases. In the asquenched Nd 15 Fe 77 C 8 and Nd 13 Dy 2 Fe 77 C 8 ribbons which consist mainly of 2:17 and a-Fe soft phases, very low coercivities with values #0.1 T were observed. The coercivity of nanocrystalline materials is sensitive to the grain size which can be modified by vacuum annealing. Fig. 2 shows the dependence of coercivities on the annealing temperature in (Nd,Dy)–Fe–(C,B) ribbons. After annealing at 6508C the low coercivities of Nd 15 Fe 77 C 8 , Nd 13 Dy 2 Fe 77 C 8 and Nd 13 Dy 2 Fe 73 C 10 B 2 ribbons can be attributed to the presence of soft magnetic 2:17-type phases. A sharp increase of their coercivity values was found after annealing at 7008C for 15 min. After this procedure, all ribbons show according to the XRD patterns the 2:14:1 phase as main phase, which gives rise to high coercivities. The coercivities of Nd 15 Fe 77 C 8 , Nd 13 Dy 2 Fe 77 C 8 and Nd 13 Dy 2 Fe 77 C 6 B 2 decrease rapidly, if the annealing temperature is higher than 8008C which is due to grain growth. This is confirmed by the reduced peak width in the XRD patterns as compared with those of as-quenched ribbons. The coercivities of Nd 13 Dy 2 Fe 77 C 6 B 2 ribbons decrease with increasing annealing temperatures. However, in the temperature range from 700 to 7658C they remain relatively constant around 1.82 T. The coercivity of Nd 13 Dy 2 Fe 73 C 10 B 2 increases slightly with the annealing temperature, indicating that this alloy is very stable against grain growth. The addition of Dy drastically increases the coercivity by about 0.8 T, compared with the Dy free sample series Nd 15 Fe 77 C 8 . Substitution of boron for carbon also increases the coercivity. However, the Dy substitution and the increase of carbon content in the ribbons decrease the values of magnetic polarization due to the antiparallel coupling and the dilution of the total magnetic moment, respectively, as observed for Nd 13 Dy 2 Fe 73 C 10 B 2 (cf. Fig. 5). Fig. 3 shows typical hysteresis loops of ribbons before and after annealing. As described before, the as-quenched Nd 13 Dy 2 Fe 77 C 8 ribbons show soft magnetic behavior due
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Fig. 1. X-ray diffraction patterns of melt spun Nd(Dy)–Fe–C(B) ribbons before and after annealing at different temperatures, (a) Nd 15 Fe 77 C 8 , (b) Nd 13 Dy 2 Fe 77 C 8 , (c) Nd 13 Dy 2 Fe 77 C 6 B 2 , and (d) Nd 13 Dy 2 Fe 73 C 10 B 2 .
to their inhomogeneous phase composition. During annealing the coercivity increases due to the formation of 2:14:1type phase as shown for the case of T a 57308C for 15 min. The hysteresis loop of the as-quenched Nd 13 Dy 2 Fe 77 C 6 B 2 ribbon shows a kink, which can be also observed for all samples if the annealing temperature is not optimized with respect to temperature and time. It is caused by the low switching fields of soft magnetic phases. The best magnetic properties were observed for Nd 13 Dy 2 Fe 77 C 6 B 2 ribbons after annealing at 6508C for 15 min: coercivity m 0J H c 52.0 T, remanence B r 50.71 T, energy product (BH) max 576 kJ / m 3 , and Curie temperature T c 5590 K. These magnetic properties are comparable, if not better, to MQI-type (Magnequench) magnetic powders which are frequently used for permanent magnet production. Typical
virgin magnetization curves of ribbons after annealing at 7308C for 15 min are shown in Fig. 4. The shape of the virgin magnetization curves reveals an inhomogeneous magnetization behavior. Fig. 5 gives the dependence of the remanence B r and polarization J(8T) (measured at 8 T) of (Nd,Dy)–Fe– (B,C) ribbons on annealing temperature. The polarization has not reached saturation in an applied field of 8 T. This should mainly be caused due to the high anisotropy field of about m 0 H a 514.8 T [4]. The polarization of ribbons shows no significant dependence on the annealing temperatures from 700 to 8508C. This is consistent with the observation that the 2:14:1 phase is the main phase after annealing at 7008C for 15 min. Initially, the remanence values of Nd 15 Fe 77 C 8 and Nd 13 Dy 2 Fe 77 C 8 are small and increase
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Fig. 4. Virgin magnetization curves of (Nd,Dy)–Fe–(C,B) ribbons after annealing at 7308C for 15 min.
3.3. Die-upset magnets Fig. 2. Dependence of coercivities on annealing temperatures of various (Nd,Dy)–Fe–(C,B) ribbons.
with increasing T a . This is caused by the slower transformation kinetics of these two compounds compared with the ribbons with additional boron content as Nd 13 Dy 2 Fe 77 C 6 B 2 and Nd 13 Dy 2 Fe 73 C 10 B 2 . The phase transformation is not complete in Nd 15 Fe 77 C 8 and Nd 13 Dy 2 Fe 77 C 8 ribbons until 8008C. This can be confirmed from the demagnetization curves which have a kink in the temperature range from 650 to 8008C. The substitution of Nd by Dy decreases the magnetization due to the antiparallel coupling between heavy rare-earth Dy and Fe magnetic moments. Therefore, the substitution with a smaller amount of Dy should be the preferred option for producing high performance magnets. Substitution of carbon by boron increases the magnetization of ribbons which is attributed to the higher saturation magnetization of Nd 2 Fe 14 B.
Fig. 3. Hysteresis loops of Nd 13 Dy 2 Fe 77 C 8 and Nd 13 Dy 2 Fe 77 C 6 B 2 ribbons before and after annealing at 7008C for 15 min.
Textured, bulk magnets could be produced by hot pressing and subsequent die upsetting of crushed ribbon powders. Fig. 6 shows the demagnetization curves of Nd 13 Dy 2 Fe 77 C 8 and Nd 13 Dy 2 Fe 77 C 6 B 2 die-upset magnets measured parallel and perpendicular to the pressing direction. The results show that the (Nd,Dy) 2 Fe 14 (B,C) materials can be textured in the same manner as observed for the Nd–Fe–B based magnets [20] where the easy magnetization direction (EMD) is also parallel to the pressure direction, i.e. the EMD is parallel to the c-axis of 2:14:1phase grains. The relatively low remanences of B r 50.83 and 0.86 T of Nd 13 Dy 2 Fe 77 C 8 and Nd 13 Dy 2 Fe 77 C 6 B 2
Fig. 5. Dependence of remanence and magnetization (measured at 8 T) of various (Nd,Dy)–Fe–(C,B) ribbons on the annealing temperature.
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size larger than 500 nm by S.E.M. on fracture surfaces of a Nd 13 Dy 2 Fe 77 C 6 B 2 die-upset magnet as shown in Fig. 7.
4. Conclusions
Fig. 6. Demagnetization curves of die-upset Nd 13 Dy 2 Fe 77 C 8 and Nd 13 Dy 2 Fe 77 C 6 B 2 magnets measured parallel and perpendicular to the pressure direction.
((BH) max |137 kJ / m 3 ) show, however, that the degree of alignment is not comparable with that of corresponding 2:14:1 borides. Slower anisotropic grain growth of the 2:14:1-type phase and the presence of soft magnetic impurity phases have been suggested previously [21] in order to explain such a behavior. The present study has shown that the formation of platelet-shaped grains is less pronounced (compare the large number of almost equiaxed grains in Fig. 7) and also revealed that the hot workability of the 2:14:1 carbides is much worse than that of the corresponding 2:14:1 borides. Analysis of the stress–strain relations showed that the deformation stresses required for a certain degree of deformation are about eight times as high as in MQ-type ribbons. The relatively low coercivity of e.g. m 0J H c ~1 T for Nd 13 Dy 2 Fe 77 C 6 B 2 compared with the results shown in Fig. 2 point to rapid grain growth during the procedure of hot pressing and deforming. This is confirmed by the occasional observation of grains with a
Fig. 7. S.E.M. picture of the fracture surface of a Nd 13 Dy 2 Fe 77 C 6 B 2 magnet.
Magnetic soft phases, 2:17 carbides and a-Fe, together with the 2:14:1 phase were found in melt-spun ribbons of Nd 2 Fe 14 C, (Nd,Dy) 2 Fe 14 C, and (Nd,Dy) 2 Fe 14 (B,C). The soft magnetic phases can be fully transformed into 2:14:1 hard magnetic phase after annealing for optimum temperature and time. Addition of boron accelerates the formation of the 2:14:1 carbide phase in the melt spun ribbons. Nd 13 Dy 2 Fe 77 C 6 B 2 shows a very high coercivity of about 2.5 T after direct quenching. After optimum annealing magnetically hard Nd 13 Dy 2 Fe 77 C 6 B 2 ribbons with a coercivity of about 2.0 T, remanence of 0.71 T, and energy product of 76 kJ / m 3 were obtained. The increase of carbon content in the ribbons leads to a smaller grain size, and impedes grain growth during annealing. The production of textured magnets made from melt spun (Nd,Dy) 2 Fe 14 (B,C) materials by die-upsetting is in principle possible, however, with a smaller degree of alignment compared with Nd–Fe–B based materials.
Acknowledgements J.B. Yang is very grateful to the Alexander von Humboldt-Foundation and A. Kirchner to the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 463) for financial support.
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