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Magnetic properties, phase evolution, and microstructure of directly cast Nd–Fe–Nb–Sn–B bulk magnets
Journal of Alloys and Compounds 545 (2012) 231–235
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Magnetic properties, phase evolution, and microstructure of directly cast Nd–Fe–Nb–Sn–B bulk magnets H.W. Chang a,⇑, W.C. Lin b, C.W. Shih b, C.C. Hsieh b, W.C. Chang b,⇑ a b
Department of Physics, Tunghai University, Taichung 407, Taiwan, ROC Department of Physics, National Chung Cheng University, Chia-Yi 621, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 13 July 2012 Received in revised form 9 August 2012 Accepted 10 August 2012 Available online 19 August 2012 Keywords: Rare earth magnets Cast magnets Magnetic properties
a b s t r a c t Magnetic properties, phase evolution, and microstructure of directly cast Nd9.5Fe75.5 xMxB15 (M = Nb, Sn, Nb + Sn; x = 0–6) rod magnets with 0.9 mm in diameter and 15 mm in length have been studied. Nb substitution for Fe in Nd9.5Fe75.5B15 alloys not only suppresses soft magnetic phases, i.e., orthorhombic Fe3B, and a-Fe, leading to the presence of large amount of Nd2Fe14B phase, but also refines the grain size. Meanwhile, a slight Sn substitution for Fe (x = 0.5) in Nd9.5Fe75.5B15 has the similar effect as that of Nb, but body-center-tetragonal Fe3B phase appears and the amount of it increases with increasing Sn content. Nevertheless, a proper Sn substitution for Fe in Nd9.5Fe71.5Nb4B15 may well modify phase constitution, uniformly refine the microstructure, and consequently, improve the magnetic properties of the rod magnets. The optimum magnetic properties of Br = 6.3 kG, iHc = 9.3 kOe and (BH)max = 8.2 MGOe were obtained for Nd9.5Fe71Nb4Sn0.5B15 bulk magnet. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Exchange-coupled a-Fe/R2Fe14B and Fe3B/R2Fe14B (R = rare earth elements) nanocomposite magnets have attracted much attention due to their outstanding permanent magnetic properties for the potential industrial applications [1–6]. Recently, both the devitrified annealing of the amorphous precursors [7–12] and the direct casting method [13–16] have been adopted to develop fully dense bulk nanocomposite permanent magnets to simplify the production processes and also avoid the dilution of magnetic properties by nonmagnetic polymer [7–16]. Magnetic properties of them are highly dependent upon the phase constitution and microstructure, controlled by the composition in addition to cooling rate [7–16]. For the former method, in order to acquire the amorphous precursors, high B content (P20 at.%) is quite essential. At first, lower R content of 3–4 at.% is adopted to produce cylindrical bulk magnet with the diameter of 0.5–0.6 mm. After proper annealing at 590– 670 °C, 2:14:1 phase and the considerable amount of the soft magnetic phases a-Fe and Fe3B are coexisted. Accordingly, higher energy product ((BH)max) of 7.3–12.0 MGOe is attained, but the intrinsic coercivity (iHc) is too low (2.8–3.7 kOe) to suit for high temperature application and/or thin magnet applications [7–11]. ⇑ Corresponding author. E-mail addresses: [email protected] (H.W. Chang), [email protected] (W.C. Chang). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.08.047
On the other hand, with the increase of R content to 9.6 at.%, both the remanent magnetization (Br) and (BH)max are drastically reduced to 4.4 kG and 4.1 MGOe, respectively, due to lower Fe content, even though higher iHc of 13.8 kOe can be obtained [12]. For the latter method, large volume fraction of 2:14:1 phase with fine grain size and grain boundary phase are obtained through composition modification and the multi-component refractory element substitutions, i.e., Ti, Zr, Cr, and C. Consequently, high (BH)max = 6.8–8.7 MGOe and iHc = 7.7–16.2 kOe could be attained simultaneously for Pr9.5Fe71.5Nb4B15 and Nd9.5Febal.MyB15 zCz (M = Ti, Ti + Zr, Ti + Zr + Cr; y = 3–4; z = 0.5) magnets with various diameter of 0.7–1.1 mm [13–16]. According to previous studies [17–19], Nb is the most effective element not only in reducing the grain size for Fe-B/R2Fe14B nanocompoiste ribbons [17,18] and bulk Pr-Fe-B magnets [13], but also in increasing the glass formability for bulk metallic glass (BMG) alloy systems [19]. On the other hand, of all the studied low meltingpoint metals, Sn is beneficial in improving the permanent magnetic properties of melt spun a-(Fe,Co)/Pr2Fe14B and Fe3B/Nd2Fe14B nanocomposites, respectively, mainly originated from the microstructure refinement [20,21]. Based on the above results, in this study, Nb and Sn elements are adopted to progressively substitute for Fe, in Nd9.5Fe75.5B15 alloy, respectively, to develop directly quenched Nd9.5Fe75.5 xMxB15 (M = Nb, Sn, Nb + Sn; x = 0–6) bulk magnets with a diameter of 0.9 mm and a length of 15 mm. The effect of Nb and Sn contents on the phase evolution, microstructure and magnetic properties are reported.
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2. Experiment
Nd9.5Fe75.5-xNbxB15 2:14:1
o-Fe3B
x=2
x=3
x=4
x=5
3. Results and discussion
x=6
3.1. Effect of Nb content
200 300 400 500 600 700 800 900
At first, effect of Nb content on the magnetic properties, phase evolution, and microstructure is studied. Fig. 1 shows magnetic properties of directly cast Nd9.5Fe75.5 xNbxB15 rod magnets. For ternary Nd9.5Fe75.5B15 magnet, low magnetic properties of Br = 4.7 kG, iHc = 0.8 kOe and (BH)max = 0.7 MGOe are found. Nevertheless, substitution of Nb for Fe in directly cast Nd9.5Fe75.5 xNbxB15 magnets can remarkably enhance the magnetic properties. With increasing Nb content x, iHc monotonically increases to 16.3 kOe for x = 6, and Br increases to 5.9 kG for x = 2–5, then slightly decreases to 5.7 kG for x = 6. On the other hand, (BH)max reaches the maximum value of 7.1 MGOe for x = 4 at first due to the increase of both Br and iHc, and then slightly decreases to 6.1 MGOe for x = 6. In this series alloys, the optimal magnetic properties of Br = 5.9 kG, iHc = 14.6 kOe and (BH)max = 7.1 MGOe are obtained for Nd9.5Fe71.5Nb4B15 rods. In order to identify the magnetic phases clearly inside the directly cast magnets, TMA technique is employed in this study. Fig. 2 depicts TMA scans of Nd9.5Fe75.5 xNbxB15 rods. For ternary Nd9.5Fe75.5B15 magnet, magnetically soft orthorhombic Fe3B (oFe3B), and a-Fe phases are found to coexist with magnetically hard Nd2Fe14B phase. Clearly, o-Fe3B phase is suppressed and the volume fraction of a-Fe phase is reduced to form more 2:14:1 phase with Nb substitution, resulting in the remarkable increase of coercivity. It is seen that the TC of 2:14:1 phase is slightly decreased with the increment of Nb content x from 320 °C for x = 0–311 °C
(BH)max (MGOe)
Nd9.5Fe75.5-xNbxB15 8 6 4 2 0 16
iHc (kOe)
x=0 α -Fe
x=1
Magnetic Weight (arb. unit)
Alloy ingots with nominal compositions of Nd9.5Fe75.5 xMxB15 (M = Nb, Sn, Nb + Sn; x = 0–6) were prepared by arc melting mixtures of pure Nd, Fe, Nb, Sn, and B in an argon atmosphere. A 5 wt.% excess Nd was adopted to compensate the loss during processing. The magnetic rods with a cylindrical shape of various diameters of 0.9 mm and a length of 15 mm were prepared by injecting the melt into a copper mold. The magnetic properties at room temperature were measured by a vibrating sample magnetometer (VSM). All samples were magnetized by a 50 kOe peak pulse field prior to magnetic measurement. The crystalline structures were identified by X-ray diffraction (XRD) with Cu-Ka radiation. The Curie temperatures (TC) of magnetic phases were determined by thermo-gravimetric analysis (TGA) with an externally applied magnetic field (conventionally referred as ‘‘TMA’’), at a heating rate of 20 °C/min. The microstructure was observed directly by transmission electron microscope (TEM).
12 8 4 0
Temperature (oC) Fig. 2. TMA scans of directly cast Nd9.5Fe75.5 xNbxB15 magnets.
for x = 6, revealing that small part of Nb atoms have entered the crystal structure of 2:14:1 phase. The grain size and its distribution of the magnets are very crucial, which may affect the permanent magnetic properties of the magnets effectively. Normally, the grain size of the peripheral region of the directly cast magnets is fine enough due to its high cooling rate, to ensure its high coercivity. Nevertheless, cooling rate at the core region of the magnets is lower than that at the peripheral region, and thus, to know the microstructure especially at the core region of the magnets is an important issue. Fig. 3(a) and (b) show the TEM images of the core region of directly cast Nd9.5Fe75.5B15 and Nd9.5Fe71.5Nb4B15 magnets, respectively. The extremely large grain size of 1500–2000 nm is observed for the Nd9.5Fe75.5B15 magnet, while the grain size of the magnet is refined to 50–200 nm by Nb substitution of 4 at.%. The grain refinement with Nb substitution is presumably resulted from the effect of multicomponent alloy consisting of more elements based on Inoue’s empirical rules [19], which gives rise to the improvement of coercivity and magnetic energy product in Nd9.5Fe71.5Nb4B15. Besides, grain boundary phase is observed in those two magnets. For identifying the compositions of the grain and grain boundary phase, energy dispersive X-ray analysis (EDX) was employed. The EDX results, listed in Table 1, show that Nb atoms tend to appear at the grain boundaries. It is believed that Nb prefers to react with excess boron to form Nb-boride in the grain boundary, owing to the strong affinity between Nb and B. The isolation effect of nonmagnetic grain boundary phase to 2:14:1 phase might play an important role in impeding the domain wall movement and, subsequently, in enhancing the coercivity of the magnets [22]. Nevertheless, some part of the Nb atoms have entered into the crystal structure of 2:14:1 phase, reducing both TC of 2:14:1 phase, as shown in Fig. 2, and the magnetization of magnets, as shown in Fig. 1. 3.2. Effect of Sn content
Br (kG)
6.0 5.5 5.0 4.5
0
1
2
3
4
5
6
X Fig. 1. Magnetic properties of directly cast Nd9.5Fe75.5 xNbxB15 magnets.
Effect of Sn content on the magnetic properties of directly cast Nd9.5Fe75.5 xSnxB15 rod magnets is also studied. Fig. 4 shows the variation of magnetic properties with the Sn content. Unlike the effect of Nb substitution, magnetic properties of the ternary rod magnets are only slightly improved for Sn substitution (Br = 5.4 kG, iHc = 5.0 kOe and (BH)max = 3.1 MGOe for Nd9.5Fe75Sn0.5B15 alloys). Furthermore, higher Sn content (x = 1–2) may deteriorate the magnetic properties of the magnets.
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200 nm
500 nm
(a)
(b)
(c)
(d)
Fig. 3. TEM images of the core region of directly cast (a) Nd9.5Fe75.5B15, (b) Nd9.5Fe71.5Nb4B15, (c) Nd9.5Fe75Sn0.5B15, and (d) Nd9.5Fe71Nb4Sn0.5B15 magnets.
Table 1 The average compositions of grain interior and grain boundaries of Nd9.5Fe71.5 xNb4SnxB15 bulk magnet measured by TEM-EDX analysis. x
Nd (at.%)
Fe (at.%)
Nb (at.%)
0
Grain Boundary
13.2 ± 1.1 14.7 ± 1.2
84.8 ± 1.5 80.2 ± 1.3
2.0 ± 0.1 5.1 ± 0.2
0.5
Grain Boundary
12.1 ± 1.0 14.9 ± 1.1
85.9 ± 1.5 80.3 ± 1.8
1.8 ± 0.2 4.2 ± 0.2
Fig. 5 presents TMA scans of directly cast Nd9.5Fe75.5 xSnxB15 rod magnets. It is seen that o-Fe3B phase is suppressed and the amount of a-Fe phase is decreased with 0.5 at.% Sn substitution. In contrast, the amount of 2:14:1 phase is increased, which results in a slight increase of iHc. However, a body-center-tetragonal Fe3B (bct-Fe3B) phase appears for x = 0.5, and an additional 2:23:3 phase is present for x = 1.5–2. Besides, the amount of bct-Fe3B phase increases with increasing Sn content from 0.5 to 2 at.%, which is in agreement with the results of Fe3B/R2Fe14B nanocomposite ribbons [21]. The increased volume fraction of magnetic soft phases accompanies with fewer volume fraction of 2:14:1 phase in the matrix with Sn substitution of 1–2 at.%, leading to the reduction of magnetic properties. As compared to Nd9.5Fe75.5B15 magnet, finer grain size of about 100–500 nm in the core region of Nd9.5Fe75Sn0.5B15 magnet, shown in Fig. 3(c), accounts for the improvement of magnetic properties. Nevertheless, the grain size for Sn-substituted magnets is still larger than that of Nb-substituted ones, which may explain the inferior magnetic properties in Nd9.5Fe75.5 xSnxB15 rod magnets.
Sn (at.%) 0 0 0.2 ± 0.1 0.6 ± 0.1
3.3. Effect of Nb and Sn co-substitution According to the above two results and also Inoue’s empirical rules [19], effect of Nb and Sn co-substitution on the magnetic properties is also investigated in order to further improve the magnetic properties of the rod magnets. Fig. 6 shows magnetic properties of directly cast Nd9.5Fe71.5 xNb4SnxB15 rods. Clearly, with the increase of Sn content x, iHc monotonically decreases from 14.6 kOe for x = 0–7.3 kOe for x = 2, but Br and (BH)max increase from 5.9 kG and 7.1 MGOe for x = 0–6.3 kG and 8.2 MGOe for x = 0.5 firstly, and then decreases to 5.0 kG and 2.9 MGOe for x = 2, respectively. The highest (BH)max = 8.2 MGOe (Br = 6.3 kG and iHc = 9.3 kOe) is attained for Nd9.5Fe71Nb4Sn0.5B15 magnet, while the highest iHc = 14.6 kOe (Br = 5.9 kG and (BH)max = 7.1 MGOe) is achieved for Nd9.5Fe71.5Nb4B15 magnet. Fig. 7 depicts TMA scans of directly cast Nd9.5Fe71.5 xNb4SnxB15 rods. Clearly, two magnetic phases, namely 2:14:1 and a-Fe phases, are found for Sn-free alloy. Meanwhile, the amorphous phase is present in the magnet with Sn substitution, and an
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Nd9.5Fe71.5-xNb4SnxB15 9.0
(BH)max (MGOe)
(BH)max (MGOe)
Nd9.5Fe75.5-xSnxB15 5 4 3 2 1 0
7.5 6.0 4.5 3.0 1.5 16
iHc (kOe)
iHc (kOe)
8 6 4 2 0
10 8
6.3
Br (kG)
Br (kG)
12
6
-2 5.7 5.4 5.1 4.8 4.5
14
6.0 5.7 5.4 5.1 4.8
0.0
0.5
1.0
1.5
0.0
2.0
0.5
1.0
X Fig. 4. Magnetic properties of directly cast Nd9.5Fe75.5 xSnxB15 magnets.
2.0
Fig. 6. Magnetic properties of directly cast Nd9.5Fe71.5 xNb4SnxB15 magnets.
Nd9.5Fe75.5-xSnxB15 2:14:1
o-Fe3B
Nd9.5Fe71.5-xNb4SnxB15 x=0
x=0 2:14:1
Magnetic Weight (arb. unit)
α -Fe
Magnetic Weight (arb. unit)
1.5
X
x=0.5
x=1 bct-Fe3B 2:23:3
x=1.5
α -Fe
x=0.5
Amor.
x=1
Amor. bct-Fe3B
x=1.5 Amor.
x=2
x=2 Amor.
100 200 300 400 500 600 700 800
100 200 300 400 500 600 700 800
o
o
Temperature ( C)
additional bct-Fe3B phase appears for x = 1. Besides, the amount and TC of the amorphous phase increases with the increase of x, suggesting that Sn substitution is helpful in increasing glass formation ability of Nd9.5Fe71.5 xNb4SnxB15 alloy. The appearance and increased amount of magnetically soft amorphous phase with Sn content x decrease iHc. On the other hand, the grain size in the core region of the rod is uniformly refined from 50–200 nm for Nb substitution, shown in Fig. 3(b), to 50–80 nm for Nb and Sn co-substitution, shown in Fig. 3(d). Furthermore, EDX results, also listed in Table 1, show that Sn prefers to appear in the grain boundary, which is similar to the result of melt spun Pr2(FeCo)14B/a-(FeCo) nanocomposites [20]. Although stronger pinning effect due to more nonmagnetic grain boundary phase and finer microstructure may exist for Nd9.5Fe71Nb4Sn0.5B15 magnets, the appearance of magnetically soft amorphous phase and presumably the modifications of anisotropy field of the alloy and grain morphology with Sn substitution reduce the coercivity of Nd9.5Fe71.5Nb4B15 bulk magnets. Fig. 8 depicts the applied magnetic field dependence of dM = md(H) (1–2mr(H)) [23,24], with md being the reduced magnetization and mr the reduced remanence, of Nd9.5Fe71.5 xNb4SnxB15
Fig. 7. TMA scans of directly cast Nd9.5Fe71.5 xNb4SnxB15 magnets.
Nd9.5Fe71.5-xNb4SnxB15 0.5
x=0 x = 0.5
0.4
0.3
δM
Fig. 5. TMA scans of directly cast Nd9.5Fe75.5 xSnxB15 magnets.
Temperature ( C)
0.2
0.1
0.0
-0.1
0
2
4
6
8
10
12
H (kOe) Fig. 8. dM-H curves of directly cast Nd9.5Fe71.5–xNb4SnxB15 magnets.
H.W. Chang et al. / Journal of Alloys and Compounds 545 (2012) 231–235
bulk magnets. The positive dM peak height, (dM)max, indicates that the strength of exchange-coupling interaction between magnetic grains. Clearly, the exchange coupling effect exists in those two samples, and the stronger exchange coupling effect is found for Sn-containing Nd9.5Fe71Nb4Sn0.5B15 bulk magnets due to grain refinement with Sn substitution, leading to an enhancement in Br and (BH)max. The present results suggest that proper co-substitution of refractory and low-melting point elements could well modify phase constitution, refine microstructure, and therefore markedly enhance the permanent magnetic properties of the isotropic Nd–Fe–B bulk magnets. 4. Conclusions Magnetic properties of directly cast Nd9.5Fe75.5 xMxB15 (M = Nb, Sn, Nb + Sn; x = 0–6) bulk magnets are mainly dominated by phase constitutions and microstructure. Nb substitution in Nd9.5Fe75.5B15 alloys effectively suppresses o-Fe3B and a-Fe phases and refines the grain size of the rod magnets. Phases modification and grain refinement by proper Nb substitution remarkably enhanced the permanent magnetic properties from Br = 4.7 kG, iHc = 0.8 kOe and (BH)max = 0.7 MGOe for ternary alloy to Br = 5.9 kG, iHc = 14.6 kOe and (BH)max = 7.1 MGOe for Nd9.5Fe71.5Nb4B15 alloys. On the other hand, o-Fe3B and a-Fe phases are suppressed for the magnet with a slight Sn substitution (x = 0.5). However, bct-Fe3B phase appears and its amount increases with increasing Sn content from 0.5 to 2 at.%, leaving less volume fraction of 2:14:1 phase in the matrix, which leads to a slight improvement in magnetic properties merely. Nevertheless, a proper amount of Sn substitution for Fe in Nb-substituted Nd9.5Fe71.5Nb4B15 rods uniformly refines the grain size of the bulk magnet and strengthens the exchangecoupling effect between magnetic grains, resulting in the improvement of Br and (BH)max effectively. In this study, the highest (BH)max = 8.2 MGOe (Br = 6.3 kG and iHc = 9.3 kOe) is attained for Nd9.5Fe71Nb4Sn0.5B15 magnet, while the highest iHc = 16.3 kOe (Br = 5.7 kG and (BH)max = 6.1 MGOe) is achieved for Nd9.5Fe69.5Nb6B15 magnet.
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Acknowledgement This paper was supported by National Science Council, Taiwan under Grant Nos. NSC-98-2112-M-194-005-MY3 and NSC-1002112-M-029-002-MY3. References [1] R. Coehoorn, D.B. DeMooij, C. DeWaard, J. Magn. Magn. Mater. 80 (1989) 101. [2] A. Manaf, R.A. Buckley, H.A. Davis, M. Leonowicz, J. Magn. Magn. Mater. 360 (1991) 101. [3] J. Bauer, M. Seeger, A. Zern, H. Kronmüller, J. Appl. Phys. 80 (1996) 1667. [4] K. Raviprsad, M. Funakoshi, M. Umemoto, J. Appl. Phys. 83 (1998) 921. [5] W.C. Chang, D.Y. Chiou, S.H. Wu, B.M. Ma, C.O. Bounds, Appl. Phys. Lett. 72 (1998) 121. [6] W.C. Chang, S.H. Wu, B.M. Ma, C.O. Bounds, S.Y. Yao, J. Appl. Phys. 83 (1998) 2147. [7] W. Zhang, A. Inoue, Appl. Phys. Lett. 80 (2002) 1610. [8] W. Zhang, A. Inoue, J. Appl. Phys. 91 (2002) 8834. [9] P. Pawlik, H.A. Davies, Scr. Mater. 49 (2003) 755. [10] M. Marinescu, P.C. Pawlik, H.A. Davies, H. Chiriac, J. Optoelectron. Adv. Mater. 6 (2004) 603. [11] M. Marinescu, H. Chiriac, M. Grigoras, J. Magn. Magn. Mater. 290 (2005) 1267. [12] J. Zhang, K.Y. Kim, Y.P. Feng, Y Li, Scr. Mater. 56 (2007) 943. [13] H.W. Chang, M.F. Shih, C.W. Chang, C.C. Hsieh, Y.T. Cheng, W.C. Chang, Y.D. Yao, A.C. Sun, Scr. Mater. 59 (2008) 227. [14] H.W. Chang, M.F. Shih, C.W. Chang, C.C. Hsieh, Y.K. Fang, W.C. Chang, Y.D. Yao, A.C. Sun, J. Appl. Phys. 103 (2008) 07E105. [15] H.W. Chang, Y.T. Cheng, C.W. Chang, C.C. Hsieh, Z.H. Guo, W.C. Chang, A.C. Sun, J. Appl. Phys. 105 (2009) 07A742. [16] H.W. Chang, J.Y. Gan, C.C. Hsieh, X.G. Zhao, W.C. Chang, J. Appl. Phys. 107 (2010) 09A740. [17] H.W. Chang, C.H. Chiu, C.W. Chang, C.H. Chen, W.C. Chang, Y.D. Yao, A.C. Sun, J. Alloys Compd. 407 (2006) 53. [18] I. Betancourt, H.A. Davies, Appl. Phys. Lett. 87 (2005) 162516. [19] Y. Long, W. Zhang, X. Wang, A. Inoue, J. Appl. Phys. 91 (2002) 5227. [20] Y.Z. Chen, S.I. He, H.W. Zhang, R.J. Chen, C.B. Rong, J.R. Sun, B.G. Shen, J. Phys. D: Appl. Phys. 39 (2006) 605. [21] M. Rajasekhar, D. Akhtar, M.M. Raja, S. Ram, V. Chandrasekaran, J. Phys. D: Appl. Phys. 42 (2009) 095009. [22] E. Girt, K.M. Krishnan, G. Thomas, Z. Altounian, Appl. Phys. Lett. 76 (2000) 1746. [23] P.E. Kelly, K. O’Grady, P.I. Mayo, R.W. Cantrell, IEEE Trans. Magn. 25 (1989) 388. [24] F. Vajda, E.D. Torre, J. Appl. Phys. 75 (1994) 5689.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Magnetic properties, phase evolution, and microstructure of directly cast Nd–Fe–Nb–Sn–B bulk magnets H.W. Chang a,⇑, W.C. Lin b, C.W. Shih b, C.C. Hsieh b, W.C. Chang b,⇑ a b
Department of Physics, Tunghai University, Taichung 407, Taiwan, ROC Department of Physics, National Chung Cheng University, Chia-Yi 621, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 13 July 2012 Received in revised form 9 August 2012 Accepted 10 August 2012 Available online 19 August 2012 Keywords: Rare earth magnets Cast magnets Magnetic properties
a b s t r a c t Magnetic properties, phase evolution, and microstructure of directly cast Nd9.5Fe75.5 xMxB15 (M = Nb, Sn, Nb + Sn; x = 0–6) rod magnets with 0.9 mm in diameter and 15 mm in length have been studied. Nb substitution for Fe in Nd9.5Fe75.5B15 alloys not only suppresses soft magnetic phases, i.e., orthorhombic Fe3B, and a-Fe, leading to the presence of large amount of Nd2Fe14B phase, but also refines the grain size. Meanwhile, a slight Sn substitution for Fe (x = 0.5) in Nd9.5Fe75.5B15 has the similar effect as that of Nb, but body-center-tetragonal Fe3B phase appears and the amount of it increases with increasing Sn content. Nevertheless, a proper Sn substitution for Fe in Nd9.5Fe71.5Nb4B15 may well modify phase constitution, uniformly refine the microstructure, and consequently, improve the magnetic properties of the rod magnets. The optimum magnetic properties of Br = 6.3 kG, iHc = 9.3 kOe and (BH)max = 8.2 MGOe were obtained for Nd9.5Fe71Nb4Sn0.5B15 bulk magnet. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Exchange-coupled a-Fe/R2Fe14B and Fe3B/R2Fe14B (R = rare earth elements) nanocomposite magnets have attracted much attention due to their outstanding permanent magnetic properties for the potential industrial applications [1–6]. Recently, both the devitrified annealing of the amorphous precursors [7–12] and the direct casting method [13–16] have been adopted to develop fully dense bulk nanocomposite permanent magnets to simplify the production processes and also avoid the dilution of magnetic properties by nonmagnetic polymer [7–16]. Magnetic properties of them are highly dependent upon the phase constitution and microstructure, controlled by the composition in addition to cooling rate [7–16]. For the former method, in order to acquire the amorphous precursors, high B content (P20 at.%) is quite essential. At first, lower R content of 3–4 at.% is adopted to produce cylindrical bulk magnet with the diameter of 0.5–0.6 mm. After proper annealing at 590– 670 °C, 2:14:1 phase and the considerable amount of the soft magnetic phases a-Fe and Fe3B are coexisted. Accordingly, higher energy product ((BH)max) of 7.3–12.0 MGOe is attained, but the intrinsic coercivity (iHc) is too low (2.8–3.7 kOe) to suit for high temperature application and/or thin magnet applications [7–11]. ⇑ Corresponding author. E-mail addresses: [email protected] (H.W. Chang), [email protected] (W.C. Chang). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.08.047
On the other hand, with the increase of R content to 9.6 at.%, both the remanent magnetization (Br) and (BH)max are drastically reduced to 4.4 kG and 4.1 MGOe, respectively, due to lower Fe content, even though higher iHc of 13.8 kOe can be obtained [12]. For the latter method, large volume fraction of 2:14:1 phase with fine grain size and grain boundary phase are obtained through composition modification and the multi-component refractory element substitutions, i.e., Ti, Zr, Cr, and C. Consequently, high (BH)max = 6.8–8.7 MGOe and iHc = 7.7–16.2 kOe could be attained simultaneously for Pr9.5Fe71.5Nb4B15 and Nd9.5Febal.MyB15 zCz (M = Ti, Ti + Zr, Ti + Zr + Cr; y = 3–4; z = 0.5) magnets with various diameter of 0.7–1.1 mm [13–16]. According to previous studies [17–19], Nb is the most effective element not only in reducing the grain size for Fe-B/R2Fe14B nanocompoiste ribbons [17,18] and bulk Pr-Fe-B magnets [13], but also in increasing the glass formability for bulk metallic glass (BMG) alloy systems [19]. On the other hand, of all the studied low meltingpoint metals, Sn is beneficial in improving the permanent magnetic properties of melt spun a-(Fe,Co)/Pr2Fe14B and Fe3B/Nd2Fe14B nanocomposites, respectively, mainly originated from the microstructure refinement [20,21]. Based on the above results, in this study, Nb and Sn elements are adopted to progressively substitute for Fe, in Nd9.5Fe75.5B15 alloy, respectively, to develop directly quenched Nd9.5Fe75.5 xMxB15 (M = Nb, Sn, Nb + Sn; x = 0–6) bulk magnets with a diameter of 0.9 mm and a length of 15 mm. The effect of Nb and Sn contents on the phase evolution, microstructure and magnetic properties are reported.
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2. Experiment
Nd9.5Fe75.5-xNbxB15 2:14:1
o-Fe3B
x=2
x=3
x=4
x=5
3. Results and discussion
x=6
3.1. Effect of Nb content
200 300 400 500 600 700 800 900
At first, effect of Nb content on the magnetic properties, phase evolution, and microstructure is studied. Fig. 1 shows magnetic properties of directly cast Nd9.5Fe75.5 xNbxB15 rod magnets. For ternary Nd9.5Fe75.5B15 magnet, low magnetic properties of Br = 4.7 kG, iHc = 0.8 kOe and (BH)max = 0.7 MGOe are found. Nevertheless, substitution of Nb for Fe in directly cast Nd9.5Fe75.5 xNbxB15 magnets can remarkably enhance the magnetic properties. With increasing Nb content x, iHc monotonically increases to 16.3 kOe for x = 6, and Br increases to 5.9 kG for x = 2–5, then slightly decreases to 5.7 kG for x = 6. On the other hand, (BH)max reaches the maximum value of 7.1 MGOe for x = 4 at first due to the increase of both Br and iHc, and then slightly decreases to 6.1 MGOe for x = 6. In this series alloys, the optimal magnetic properties of Br = 5.9 kG, iHc = 14.6 kOe and (BH)max = 7.1 MGOe are obtained for Nd9.5Fe71.5Nb4B15 rods. In order to identify the magnetic phases clearly inside the directly cast magnets, TMA technique is employed in this study. Fig. 2 depicts TMA scans of Nd9.5Fe75.5 xNbxB15 rods. For ternary Nd9.5Fe75.5B15 magnet, magnetically soft orthorhombic Fe3B (oFe3B), and a-Fe phases are found to coexist with magnetically hard Nd2Fe14B phase. Clearly, o-Fe3B phase is suppressed and the volume fraction of a-Fe phase is reduced to form more 2:14:1 phase with Nb substitution, resulting in the remarkable increase of coercivity. It is seen that the TC of 2:14:1 phase is slightly decreased with the increment of Nb content x from 320 °C for x = 0–311 °C
(BH)max (MGOe)
Nd9.5Fe75.5-xNbxB15 8 6 4 2 0 16
iHc (kOe)
x=0 α -Fe
x=1
Magnetic Weight (arb. unit)
Alloy ingots with nominal compositions of Nd9.5Fe75.5 xMxB15 (M = Nb, Sn, Nb + Sn; x = 0–6) were prepared by arc melting mixtures of pure Nd, Fe, Nb, Sn, and B in an argon atmosphere. A 5 wt.% excess Nd was adopted to compensate the loss during processing. The magnetic rods with a cylindrical shape of various diameters of 0.9 mm and a length of 15 mm were prepared by injecting the melt into a copper mold. The magnetic properties at room temperature were measured by a vibrating sample magnetometer (VSM). All samples were magnetized by a 50 kOe peak pulse field prior to magnetic measurement. The crystalline structures were identified by X-ray diffraction (XRD) with Cu-Ka radiation. The Curie temperatures (TC) of magnetic phases were determined by thermo-gravimetric analysis (TGA) with an externally applied magnetic field (conventionally referred as ‘‘TMA’’), at a heating rate of 20 °C/min. The microstructure was observed directly by transmission electron microscope (TEM).
12 8 4 0
Temperature (oC) Fig. 2. TMA scans of directly cast Nd9.5Fe75.5 xNbxB15 magnets.
for x = 6, revealing that small part of Nb atoms have entered the crystal structure of 2:14:1 phase. The grain size and its distribution of the magnets are very crucial, which may affect the permanent magnetic properties of the magnets effectively. Normally, the grain size of the peripheral region of the directly cast magnets is fine enough due to its high cooling rate, to ensure its high coercivity. Nevertheless, cooling rate at the core region of the magnets is lower than that at the peripheral region, and thus, to know the microstructure especially at the core region of the magnets is an important issue. Fig. 3(a) and (b) show the TEM images of the core region of directly cast Nd9.5Fe75.5B15 and Nd9.5Fe71.5Nb4B15 magnets, respectively. The extremely large grain size of 1500–2000 nm is observed for the Nd9.5Fe75.5B15 magnet, while the grain size of the magnet is refined to 50–200 nm by Nb substitution of 4 at.%. The grain refinement with Nb substitution is presumably resulted from the effect of multicomponent alloy consisting of more elements based on Inoue’s empirical rules [19], which gives rise to the improvement of coercivity and magnetic energy product in Nd9.5Fe71.5Nb4B15. Besides, grain boundary phase is observed in those two magnets. For identifying the compositions of the grain and grain boundary phase, energy dispersive X-ray analysis (EDX) was employed. The EDX results, listed in Table 1, show that Nb atoms tend to appear at the grain boundaries. It is believed that Nb prefers to react with excess boron to form Nb-boride in the grain boundary, owing to the strong affinity between Nb and B. The isolation effect of nonmagnetic grain boundary phase to 2:14:1 phase might play an important role in impeding the domain wall movement and, subsequently, in enhancing the coercivity of the magnets [22]. Nevertheless, some part of the Nb atoms have entered into the crystal structure of 2:14:1 phase, reducing both TC of 2:14:1 phase, as shown in Fig. 2, and the magnetization of magnets, as shown in Fig. 1. 3.2. Effect of Sn content
Br (kG)
6.0 5.5 5.0 4.5
0
1
2
3
4
5
6
X Fig. 1. Magnetic properties of directly cast Nd9.5Fe75.5 xNbxB15 magnets.
Effect of Sn content on the magnetic properties of directly cast Nd9.5Fe75.5 xSnxB15 rod magnets is also studied. Fig. 4 shows the variation of magnetic properties with the Sn content. Unlike the effect of Nb substitution, magnetic properties of the ternary rod magnets are only slightly improved for Sn substitution (Br = 5.4 kG, iHc = 5.0 kOe and (BH)max = 3.1 MGOe for Nd9.5Fe75Sn0.5B15 alloys). Furthermore, higher Sn content (x = 1–2) may deteriorate the magnetic properties of the magnets.
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200 nm
500 nm
(a)
(b)
(c)
(d)
Fig. 3. TEM images of the core region of directly cast (a) Nd9.5Fe75.5B15, (b) Nd9.5Fe71.5Nb4B15, (c) Nd9.5Fe75Sn0.5B15, and (d) Nd9.5Fe71Nb4Sn0.5B15 magnets.
Table 1 The average compositions of grain interior and grain boundaries of Nd9.5Fe71.5 xNb4SnxB15 bulk magnet measured by TEM-EDX analysis. x
Nd (at.%)
Fe (at.%)
Nb (at.%)
0
Grain Boundary
13.2 ± 1.1 14.7 ± 1.2
84.8 ± 1.5 80.2 ± 1.3
2.0 ± 0.1 5.1 ± 0.2
0.5
Grain Boundary
12.1 ± 1.0 14.9 ± 1.1
85.9 ± 1.5 80.3 ± 1.8
1.8 ± 0.2 4.2 ± 0.2
Fig. 5 presents TMA scans of directly cast Nd9.5Fe75.5 xSnxB15 rod magnets. It is seen that o-Fe3B phase is suppressed and the amount of a-Fe phase is decreased with 0.5 at.% Sn substitution. In contrast, the amount of 2:14:1 phase is increased, which results in a slight increase of iHc. However, a body-center-tetragonal Fe3B (bct-Fe3B) phase appears for x = 0.5, and an additional 2:23:3 phase is present for x = 1.5–2. Besides, the amount of bct-Fe3B phase increases with increasing Sn content from 0.5 to 2 at.%, which is in agreement with the results of Fe3B/R2Fe14B nanocomposite ribbons [21]. The increased volume fraction of magnetic soft phases accompanies with fewer volume fraction of 2:14:1 phase in the matrix with Sn substitution of 1–2 at.%, leading to the reduction of magnetic properties. As compared to Nd9.5Fe75.5B15 magnet, finer grain size of about 100–500 nm in the core region of Nd9.5Fe75Sn0.5B15 magnet, shown in Fig. 3(c), accounts for the improvement of magnetic properties. Nevertheless, the grain size for Sn-substituted magnets is still larger than that of Nb-substituted ones, which may explain the inferior magnetic properties in Nd9.5Fe75.5 xSnxB15 rod magnets.
Sn (at.%) 0 0 0.2 ± 0.1 0.6 ± 0.1
3.3. Effect of Nb and Sn co-substitution According to the above two results and also Inoue’s empirical rules [19], effect of Nb and Sn co-substitution on the magnetic properties is also investigated in order to further improve the magnetic properties of the rod magnets. Fig. 6 shows magnetic properties of directly cast Nd9.5Fe71.5 xNb4SnxB15 rods. Clearly, with the increase of Sn content x, iHc monotonically decreases from 14.6 kOe for x = 0–7.3 kOe for x = 2, but Br and (BH)max increase from 5.9 kG and 7.1 MGOe for x = 0–6.3 kG and 8.2 MGOe for x = 0.5 firstly, and then decreases to 5.0 kG and 2.9 MGOe for x = 2, respectively. The highest (BH)max = 8.2 MGOe (Br = 6.3 kG and iHc = 9.3 kOe) is attained for Nd9.5Fe71Nb4Sn0.5B15 magnet, while the highest iHc = 14.6 kOe (Br = 5.9 kG and (BH)max = 7.1 MGOe) is achieved for Nd9.5Fe71.5Nb4B15 magnet. Fig. 7 depicts TMA scans of directly cast Nd9.5Fe71.5 xNb4SnxB15 rods. Clearly, two magnetic phases, namely 2:14:1 and a-Fe phases, are found for Sn-free alloy. Meanwhile, the amorphous phase is present in the magnet with Sn substitution, and an
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Nd9.5Fe71.5-xNb4SnxB15 9.0
(BH)max (MGOe)
(BH)max (MGOe)
Nd9.5Fe75.5-xSnxB15 5 4 3 2 1 0
7.5 6.0 4.5 3.0 1.5 16
iHc (kOe)
iHc (kOe)
8 6 4 2 0
10 8
6.3
Br (kG)
Br (kG)
12
6
-2 5.7 5.4 5.1 4.8 4.5
14
6.0 5.7 5.4 5.1 4.8
0.0
0.5
1.0
1.5
0.0
2.0
0.5
1.0
X Fig. 4. Magnetic properties of directly cast Nd9.5Fe75.5 xSnxB15 magnets.
2.0
Fig. 6. Magnetic properties of directly cast Nd9.5Fe71.5 xNb4SnxB15 magnets.
Nd9.5Fe75.5-xSnxB15 2:14:1
o-Fe3B
Nd9.5Fe71.5-xNb4SnxB15 x=0
x=0 2:14:1
Magnetic Weight (arb. unit)
α -Fe
Magnetic Weight (arb. unit)
1.5
X
x=0.5
x=1 bct-Fe3B 2:23:3
x=1.5
α -Fe
x=0.5
Amor.
x=1
Amor. bct-Fe3B
x=1.5 Amor.
x=2
x=2 Amor.
100 200 300 400 500 600 700 800
100 200 300 400 500 600 700 800
o
o
Temperature ( C)
additional bct-Fe3B phase appears for x = 1. Besides, the amount and TC of the amorphous phase increases with the increase of x, suggesting that Sn substitution is helpful in increasing glass formation ability of Nd9.5Fe71.5 xNb4SnxB15 alloy. The appearance and increased amount of magnetically soft amorphous phase with Sn content x decrease iHc. On the other hand, the grain size in the core region of the rod is uniformly refined from 50–200 nm for Nb substitution, shown in Fig. 3(b), to 50–80 nm for Nb and Sn co-substitution, shown in Fig. 3(d). Furthermore, EDX results, also listed in Table 1, show that Sn prefers to appear in the grain boundary, which is similar to the result of melt spun Pr2(FeCo)14B/a-(FeCo) nanocomposites [20]. Although stronger pinning effect due to more nonmagnetic grain boundary phase and finer microstructure may exist for Nd9.5Fe71Nb4Sn0.5B15 magnets, the appearance of magnetically soft amorphous phase and presumably the modifications of anisotropy field of the alloy and grain morphology with Sn substitution reduce the coercivity of Nd9.5Fe71.5Nb4B15 bulk magnets. Fig. 8 depicts the applied magnetic field dependence of dM = md(H) (1–2mr(H)) [23,24], with md being the reduced magnetization and mr the reduced remanence, of Nd9.5Fe71.5 xNb4SnxB15
Fig. 7. TMA scans of directly cast Nd9.5Fe71.5 xNb4SnxB15 magnets.
Nd9.5Fe71.5-xNb4SnxB15 0.5
x=0 x = 0.5
0.4
0.3
δM
Fig. 5. TMA scans of directly cast Nd9.5Fe75.5 xSnxB15 magnets.
Temperature ( C)
0.2
0.1
0.0
-0.1
0
2
4
6
8
10
12
H (kOe) Fig. 8. dM-H curves of directly cast Nd9.5Fe71.5–xNb4SnxB15 magnets.
H.W. Chang et al. / Journal of Alloys and Compounds 545 (2012) 231–235
bulk magnets. The positive dM peak height, (dM)max, indicates that the strength of exchange-coupling interaction between magnetic grains. Clearly, the exchange coupling effect exists in those two samples, and the stronger exchange coupling effect is found for Sn-containing Nd9.5Fe71Nb4Sn0.5B15 bulk magnets due to grain refinement with Sn substitution, leading to an enhancement in Br and (BH)max. The present results suggest that proper co-substitution of refractory and low-melting point elements could well modify phase constitution, refine microstructure, and therefore markedly enhance the permanent magnetic properties of the isotropic Nd–Fe–B bulk magnets. 4. Conclusions Magnetic properties of directly cast Nd9.5Fe75.5 xMxB15 (M = Nb, Sn, Nb + Sn; x = 0–6) bulk magnets are mainly dominated by phase constitutions and microstructure. Nb substitution in Nd9.5Fe75.5B15 alloys effectively suppresses o-Fe3B and a-Fe phases and refines the grain size of the rod magnets. Phases modification and grain refinement by proper Nb substitution remarkably enhanced the permanent magnetic properties from Br = 4.7 kG, iHc = 0.8 kOe and (BH)max = 0.7 MGOe for ternary alloy to Br = 5.9 kG, iHc = 14.6 kOe and (BH)max = 7.1 MGOe for Nd9.5Fe71.5Nb4B15 alloys. On the other hand, o-Fe3B and a-Fe phases are suppressed for the magnet with a slight Sn substitution (x = 0.5). However, bct-Fe3B phase appears and its amount increases with increasing Sn content from 0.5 to 2 at.%, leaving less volume fraction of 2:14:1 phase in the matrix, which leads to a slight improvement in magnetic properties merely. Nevertheless, a proper amount of Sn substitution for Fe in Nb-substituted Nd9.5Fe71.5Nb4B15 rods uniformly refines the grain size of the bulk magnet and strengthens the exchangecoupling effect between magnetic grains, resulting in the improvement of Br and (BH)max effectively. In this study, the highest (BH)max = 8.2 MGOe (Br = 6.3 kG and iHc = 9.3 kOe) is attained for Nd9.5Fe71Nb4Sn0.5B15 magnet, while the highest iHc = 16.3 kOe (Br = 5.7 kG and (BH)max = 6.1 MGOe) is achieved for Nd9.5Fe69.5Nb6B15 magnet.
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Acknowledgement This paper was supported by National Science Council, Taiwan under Grant Nos. NSC-98-2112-M-194-005-MY3 and NSC-1002112-M-029-002-MY3. References [1] R. Coehoorn, D.B. DeMooij, C. DeWaard, J. Magn. Magn. Mater. 80 (1989) 101. [2] A. Manaf, R.A. Buckley, H.A. Davis, M. Leonowicz, J. Magn. Magn. Mater. 360 (1991) 101. [3] J. Bauer, M. Seeger, A. Zern, H. Kronmüller, J. Appl. Phys. 80 (1996) 1667. [4] K. Raviprsad, M. Funakoshi, M. Umemoto, J. Appl. Phys. 83 (1998) 921. [5] W.C. Chang, D.Y. Chiou, S.H. Wu, B.M. Ma, C.O. Bounds, Appl. Phys. Lett. 72 (1998) 121. [6] W.C. Chang, S.H. Wu, B.M. Ma, C.O. Bounds, S.Y. Yao, J. Appl. Phys. 83 (1998) 2147. [7] W. Zhang, A. Inoue, Appl. Phys. Lett. 80 (2002) 1610. [8] W. Zhang, A. Inoue, J. Appl. Phys. 91 (2002) 8834. [9] P. Pawlik, H.A. Davies, Scr. Mater. 49 (2003) 755. [10] M. Marinescu, P.C. Pawlik, H.A. Davies, H. Chiriac, J. Optoelectron. Adv. Mater. 6 (2004) 603. [11] M. Marinescu, H. Chiriac, M. Grigoras, J. Magn. Magn. Mater. 290 (2005) 1267. [12] J. Zhang, K.Y. Kim, Y.P. Feng, Y Li, Scr. Mater. 56 (2007) 943. [13] H.W. Chang, M.F. Shih, C.W. Chang, C.C. Hsieh, Y.T. Cheng, W.C. Chang, Y.D. Yao, A.C. Sun, Scr. Mater. 59 (2008) 227. [14] H.W. Chang, M.F. Shih, C.W. Chang, C.C. Hsieh, Y.K. Fang, W.C. Chang, Y.D. Yao, A.C. Sun, J. Appl. Phys. 103 (2008) 07E105. [15] H.W. Chang, Y.T. Cheng, C.W. Chang, C.C. Hsieh, Z.H. Guo, W.C. Chang, A.C. Sun, J. Appl. Phys. 105 (2009) 07A742. [16] H.W. Chang, J.Y. Gan, C.C. Hsieh, X.G. Zhao, W.C. Chang, J. Appl. Phys. 107 (2010) 09A740. [17] H.W. Chang, C.H. Chiu, C.W. Chang, C.H. Chen, W.C. Chang, Y.D. Yao, A.C. Sun, J. Alloys Compd. 407 (2006) 53. [18] I. Betancourt, H.A. Davies, Appl. Phys. Lett. 87 (2005) 162516. [19] Y. Long, W. Zhang, X. Wang, A. Inoue, J. Appl. Phys. 91 (2002) 5227. [20] Y.Z. Chen, S.I. He, H.W. Zhang, R.J. Chen, C.B. Rong, J.R. Sun, B.G. Shen, J. Phys. D: Appl. Phys. 39 (2006) 605. [21] M. Rajasekhar, D. Akhtar, M.M. Raja, S. Ram, V. Chandrasekaran, J. Phys. D: Appl. Phys. 42 (2009) 095009. [22] E. Girt, K.M. Krishnan, G. Thomas, Z. Altounian, Appl. Phys. Lett. 76 (2000) 1746. [23] P.E. Kelly, K. O’Grady, P.I. Mayo, R.W. Cantrell, IEEE Trans. Magn. 25 (1989) 388. [24] F. Vajda, E.D. Torre, J. Appl. Phys. 75 (1994) 5689.