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Magnetic properties, phase and microstructure of direct cast Nd9.5Febal.Co10MB15 rod magnets
Journal of Magnetism and Magnetic Materials 326 (2013) 108–111
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic properties, phase and microstructure of direct cast Nd9.5Febal.Co10MB15 rod magnets Z. Liu a,b,c,n, C.C. Hsieh b, R.J. Chen a,c, W.C. Lin b, H.W. Chang d, W.C. Chang b,nn, A.R. Yan a,c a
Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Material Technology and Engineering, CAS, Ningbo 315201, China Department of Physics, National Chung Cheng University, Chia-Yi 621, Taiwan c Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology and Engineering, CAS, Ningbo 315201, China d Department of Physics, Tunghai University, Taichung 407, Taiwan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 June 2012 Received in revised form 22 August 2012 Available online 4 September 2012
Nd9.5Febal.Co10MB15 (M ¼ Ti2.5Zr0.5, Ti2.5Nb0.5, Zr2.5Nb0.5, Nb2.5Zr0.5) rod magnets are prepared by direct casting method. Thermo-gravimetric analysis results show that minor soft a-(Fe, Co) phase coexists with magnetically hard 2:14:1 phase in the above magnets. In addition, amorphous phase is also observed in magnets with Ti2.5Zr0.5 and Nb2.5Zr0.5 addition. According to differential scanning calorimetry results, obvious exothermic peaks are observed in the magnets with Ti2.5Zr0.5 and Nb2.5Zr0.5 addition. Meanwhile, the crystallization behavior of the amorphous phase is a two-step process. The activation energies of magnets with Ti2.5Zr0.5 and Nb2.5Zr0.5 addition are 542 kJ/mol and 558 kJ/mol, respectively. Microstructure images show that the 2:14:1 phase is surrounded by boron-rich grain boundary phase. The average grain size of 2:14:1 phase in Nb2.5Zr0.5-doped magnets is smaller than in Ti2.5Zr0.5-doped magnets. Magnetic measurements show that larger coercivity is observed in both Ti2.5Zr0.5-doped magnet and Nb2.5Zr0.5-doped magnet, while better squareness of the demagnetization curves and higher magnetic energy product are obtained in Ti2.5Nb0.5-doped magnet. & 2012 Elsevier B.V. All rights reserved.
Keywords: Cast magnet Magnetic property Amorphous phase Microstructure
1. Introduction The isotropic bonded R–Fe–B (R: rare-earth elements) permanent magnets have been developed for many years. By inducing the soft phase with considerably small size, the magnetic properties of R–Fe–B ribbons have been greatly enhanced [1–4]. Nevertheless, the disadvantages of the above bonded magnets still exist, including multifarious manufacturing processes and the dilution of magnetic properties by a nonmagnetic polymer. In order to overcome this disadvantage, R–Fe–M–B (R¼Pr, NdþDy, PrþDy; M¼Co, Cu, Mo, Nb, Ti, V and Zr) cast magnets have been developed, where the alloys were employed to produce a bulk amorphous precursor by the casting method followed by one-step annealing to optimize the permanent magnetic properties [5–9]. Although the high energy product [(BH)max] of 12.0 MGOe has been obtained, the size and intrinsic coercivity is normally very low, resulting from the existence of considerable amount of the magnetically soft phase Fe3B and the inhomogeneous microstructure, to suit for the thin magnets
application. Recently, high coercivity (iHc410 kOe) of Nd–Fe–B bulk magnets has been obtained by the addition of Zr, Nb and Ti elements. Furthermore, the diameter of rod has been increased to 1.3 mm, and the (BH)max can be as high as 6.8–8.4 MGOe [10–12]. To suit for application, the diameter and magnetic energy product of the cylindrical magnets need to be further improved. Adding some elements to improve the glass forming ability (GFA) of the alloy, for the purpose of obtaining finer grain size, is one of the methods to further improve the diameter and (BH)max. Co element has been reported to improve the GFA of the Nd–Fe–B alloy [2]. However, recent research shows that amorphous phase, which is harmful to the magnetic properties, tends to appear in directly cast Nd–Fe–B alloy with Co addition. In this article, different refractory metals are added to the Nd–FeCo–B alloys to reduce the amorphous phase and refine the microstructure. The magnetic properties, phase and microstructure evaluation of the directly cast Nd9.5Febal.Co10MB15 (M¼ Ti2.5Zr0.5, Ti2.5Nb0.5, Zr2.5Nb0.5, Nb2.5Zr0.5) rod magnets with diameter of 0.9 mm, without post-annealing, are studied in detail.
2. Experimental produces n
Corresponding author at: Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Material Technology and Engineering, CAS, Ningbo 315201, China. nn Corresponding author. E-mail addresses: [email protected] (Z. Liu), [email protected] (W.C. Chang). 0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.08.049
Alloys with compositions of Nd9.5Febal.Co10MB15 (M¼Ti2.5Zr0.5, Ti2.5Nb0.5, Zr2.5Nb0.5, Nb2.5Zr0.5) were prepared by arc melting and excess Nd of 8 wt% was added to compensate the loss during
Z. Liu et al. / Journal of Magnetism and Magnetic Materials 326 (2013) 108–111
109
processing in an argon atmosphere. The alloys were melted three times to ensure homogeneity. Magnet rods with a cylindrical shape with diameter 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 with a vibrating sample magnetometer (VSM). All the samples were magnetized by a 50 kOe peak pulse field prior to magnetic measurements. 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 1C/min. The crystallization temperatures of the magnetic phases were determined by differential scanning calorimetry (DSC). The cross section of the bulk magnets with different diameters was corroded by alcohol solution containing 10 wt% nitric acid (Nital) for 10 s and then it was observed by a scanning electron microscope (SEM). The composition of the bulk magnets is measured by energy dispersive spectrometer (EDS) attached in SEM.
3. Results and discussion Fig. 1 depicts the TMA scans of directly cast Nd9.5Febal.Co10MB15 (M¼Ti2.5Zr0.5, Ti2.5Nb0.5, Zr2.5Nb0.5, Nb2.5Zr0.5) rod magnets. It can be seen that minor soft a-(Fe, Co) phase and magnetically hard 2:14:1 phase are found in all of the rod magnets. In addition, amorphous phase is also observed in magnets with Ti2.5Zr0.5 and Nb2.5Zr0.5 addition. The peak height of 2:14:1 phase indicates that the content of 2:14:1 phase is lower in magnets with Zr2.5Nb0.5 addition than in the other magnets. In order to confirm the existence of amorphous phase, DSC curves of directly cast Nd9.5Febal.Co10MB15 (M¼Ti2.5Zr0.5, Ti2.5Nb0.5, Zr2.5 Nb0.5, Nb2.5Zr0.5) rod magnets are shown in Fig. 2. It can be seen that two obvious exothermic peaks are observed in the magnets with Ti2.5Zr0.5 and Nb2.5Zr0.5 addition, which mean the crystallization behavior is a two-step process for both alloys. According to the literature of Nd–Fe–B ribbons [13], the phase transformations of rod magnets with Ti2.5Zr0.5 and Nb2.5Zr0.5 addition during annealing take place with the similar sequences; the first crystallization temperature is about 680 1C, which corresponds to the formation of a-(Fe,Co) phase, Nd2(Fe,Co)14B phase and Nd3(Fe,Co)62B14 metastable phase from the amorphous phase, and the second one is about 720 1C, which can be related to the transformation from the Nd3(Fe,Co)62B14 metastable phase to the Nd2(Fe,Co)14B and a-(Fe,Co) phases. Minor peaks are shown in Ti2.5Nb0.5-doped magnets, yet no distinct peaks are found in magnets with Zr2.5Nb0.5 addition. Both the TMA curves and the crystallization peak height in DSC scans indicate that larger
Fig. 1. TGA traces of directly cast Nd9.5Febal.Co10MB15 (M¼ Ti2.5Zr0.5, Ti2.5Nb0.5, Zr2.5Nb0.5, Nb2.5Zr0.5) rod magnets.
Fig. 2. DSC curves of directly cast Nd9.5Febal.Co10MB15 (M¼ Ti2.5Zr0.5, Ti2.5Nb0.5, Zr2.5Nb0.5, Nb2.5Zr0.5) rod magnets.
volume of amorphous phase is formed in magnets with Nb2.5Zr0.5 and Ti2.5Zr0.5 addition, reflecting the improved glass forming ability in these alloys. Whereas, Zr2.5Nb0.5-doped alloy shows inferior ability in forming amorphous phase. The crystallization behavior of magnets with Ti2.5Zr0.5 and Nb2.5Zr0.5 are further studied by the DSC curves with different heating rates shown in Fig. 3. By measuring the peak positions of the crystallization exotherms shown in Fig. 3 at different heating rates, the average activation energies of crystallization (Ec) for the two alloys are calculated using the Kissinger method [14]. For magnets with Ti2.5Zr0.5 addition, the activation energy (ETZ) is 542 kJ/mol for the first exothermic peak and 356 kJ/mol for the second exothermic peak. For magnets with Nb2.5Zr0.5 addition, the activation energy (ENZ) is 558 kJ/mol for the first exothermic peak and 515 kJ/mol for the second exothermic peak. The higher value of ENZ, compared to ETZ, implies that it is more difficult for the grains of magnets with Nb2.5Zr0.5 (ENZ) addition to grow, as it requires larger additional energy. Thus, the growth of grains in Nb2.5Zr0.5doped alloy would be slower than that in the Ti2.5Zr0.5-doped alloy. The microstructure of rod magnets with Ti2.5Zr0.5 and Nb2.5Zr0.5 addition is further studied. Fig. 4 shows SEM microstructure of the peripheral region (lower photos) and core region (upper photos) of rod magnets with Ti2.5Zr0.5 (marked with 1) and Nb2.5Zr0.5 (marked with 2) addition. The peripheral region is the area near the boundary of rod magnets. The core region is the area at the center of rod magnets. According to the corrosion extent of rod magnets, two obviously different phases are observed in Fig. 4. In general, the phase in gray (marked with B and D) is surrounded by the phase in white (marked with A and C). In order to identify these phases, the EDS results of the area A, B, C and D are shown in Table 1. It can be seen that higher content of Nb and Ti is found in area A and C, respectively, and higher content of Zr is found in areas B and D. According to Chang’s [12] studies, larger part of Zr elements tend to enter into the 2:14:1 phase, while Nb and Ti elements tend to distribute in the boron-rich grain boundary phase. Thus, it is reasonable to presume that areas A and C are nonmagnetic boronrich phases and areas B and D are the magnetic 2:14:1 phases. According to Fig. 4, it can also be seen that the average grain size of 2:14:1 phase decreases from the core region to the peripheral region due to faster cooling rate in the area near the mold. Compared with Ti2.5Zr0.5-doped magnets, the average grain size of 2:14:1 phase in Nb2.5Zr0.5-doped magnets is much smaller. According to the DSC scans, Nb2.5Zr0.5-doped magnets have higher GFA than Ti2.5Zr0.5doped magnets. Improved GFA means increased capability of grain refinement for directly cast Nd–FeCo–B alloys, but the amorphous
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phase also easily appears in these magnets especially at the peripheral region, which is harmful to the magnetic properties of the cast magnets.
The magnetic properties of directly cast Nd9.5Febal.Co10MB15 (M¼Ti2.5Zr0.5, Ti2.5Nb0.5, Zr2.5Nb0.5, Nb2.5Zr0.5) rod magnets with diameter of 0.9 mm were investigated. Fig. 5 shows demagnetization curves of the cast magnets. Clearly, a higher coercive force larger than 950 kA/m is obtained in magnets with Ti2.5Zr0.5 and Nb2.5Zr0.5 addition due to their fine grain distribution. The coercive force of magnets with Ti2.5Nb0.5 addition has a slight decrease yet it is still higher than 800 KA/m. In contrast, the coercivity is very low for magnets with Zr2.5Nb0.5, which indicates that the grain size could be much larger than other magnets. On the other hand, although the coercive force in magnets with Ti2.5Nb0.5 addition is lower than in magnets with Ti2.5Zr0.5 and Nb2.5Zr0.5 addition, the improved squareness of the demagnetization curve and the highest remanence are obtained, which reflect an effective exchange coupling which is obtained in this rod magnet. However, an obvious kink is observed in the demagnetization curves of magnets with Ti2.5Zr0.5 and Nb2.5Zr0.5 addition, indicating a weak coupling phenomenon between the hard and soft phases. Thus, the remanence is lower than in magnets with Ti2.5Nb0.5 addition. Finally, the lowest squareness of the demagnetization curve and remanence is shown in Zr2.5Nb0.5-doped magnets, which could be due to the lower content of 2:14:1 phase and larger grain size.
4. Summary and conclusion Refractory elements were cosubstituted in Co-containing Nd9.5 Febal.Co10MB15 (M¼Ti2.5Zr0.5, Ti2.5Nb0.5, Zr2.5Nb0.5, Nb2.5Zr0.5) alloys Table 1 EDS results of rod magnets with Ti2.5Zr0.5 and Nb2.5Zr0.5 addition. Elements Magnets with Nb2.5Zr0.5 addition
Fig. 3. DSC curves of directly cast Nd9.5Febal.Co10MB15 (M ¼Ti2.5Zr0.5 (a), Nb2.5Zr0.5 (b)) rod magnets with different heating rates.
Nd Fe Co Nb Zr
A (wt%)
B (wt%)
27.96 52.51 12.75 4.69 1.09
35.43 50.35 8 6.19 0.03
Elements Magnets with Ti2.5Zr0.5 addition
Nd Fe Co Ti Zr
C (wt%)
D (wt%)
25.32 58.67 10.72 2.3 2.99
31.19 52.38 9.91 6.45 0.07
Fig. 4. SEM microstructures of the peripheral region (lower two photos) and core region (upper two photos) of rod magnets with Ti2.5Zr0.5 (marked with 1) and Nb2.5Zr0.5 (marked with 2) addition.
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Acknowledgments This paper was supported by the National Science Council of Taiwan under Grant no. NSC-98-2112-M-194-005-MY3, the State Key Program of National Natural Science Foundation of China under Grant no. 2010AA03A402, and the Science and Technology International Cooperation Projects of China under Grant no. 2010DFB53770.
References
Fig. 5. Demagnetization curves of directly cast Nd9.5Febal.Co10MB15 (M ¼Ti2.5Zr0.5, Ti2.5Nb0.5, Zr2.5Nb0.5, Nb2.5Zr0.5) rod magnets.
for making direct cast magnets. The elements M¼Ti2.5Zr0.5 and Nb2.5Zr0.5 have the capability of increasing the GFA of the cast magnets, which reduces the grain size of the magnets and enhances the coercivity effectively. Nevertheless, the appearance of amorphous phase in the peripheral region of the magnets deteriorates the squareness of the demagnetization curve. On the other hand, a better squareness of the demagnetization curve and higher magnetic energy product are obtained in Ti2.5Nb0.5-doped magnet, which arises from its highly homogeneous microstructure and is almost free from amorphous phase inside the cast magnet.
[1] A. Manaf, R.A. Buckley, H.A. Davis, Journal of Magnetism and Magnetic Materials 128 (1993) 302–306. [2] I. Panagiotoulos, L. Withanawasam, A.S. Murthy, G.C. Hadjipanayis, E.W. Singleton, D.J. Sellmyer, Journal of Applied Physics 79 (1996) 4827–4829. [3] R. Coehoorn, D.B. DeMooij, C. DeWaard, Journal of Magnetism and Magnetic Materials 80 (1989) 101–104. [4] H. Kanekiyo, M. Uehara, S. Hirosawa, IEEE Transactions on Magnetics 29 (1993) 2863–2865. [5] W. Zhang, A. Inoue, Applied Physics Letters 80 (2002) 1610–1612. [6] W. Zhang, A. Inoue, Journal of Applied Physics 91 (2002) 8834–8836. [7] P. Pawlik, H.A. Davies, Scripta Materialia 49 (2003) 755–760. [8] M. Marinescu, P.C. Pawlik, H.A. Davies, H. Chiriac, Journal of Optoelectronics and Advanced Materials 6 (2004) 603–608. [9] M. Marinescu, H. Chiriac, M. Grigoras, Journal of Magnetism and Magnetic Materials 290–291 (2005) 1267–1269. [10] H.W. Chang, J.Y. Gan, C.C. Hsieh, X.G. Zhao, W.C. Chang, Journal of Applied Physics 107 (2010) 09A740-1-09A740-3. [11] H.W. Chang, Y.T. Cheng, C.C. Hsieh, W.C. Chang, A.C. Sun, Journal of Alloys and Compounds 509 (2011) 1249–1254. [12] H.W. Chang, C.C. Hsieh, J.Y. Gan, Y.T. Cheng, M.F. Shih, W.C. Chang, Journal of Physics D: Applied Physics 44 (2011) 064002-1–064002-10. [13] G.C. Hadjipanayis, L. Withanawasam, R.F. Krause, IEEE Transactions on Magnetics 31 (1995) 3596–3601. [14] H.E. Kissinger, Analytical Chemistry 29 (1957) 1702–1706.
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic properties, phase and microstructure of direct cast Nd9.5Febal.Co10MB15 rod magnets Z. Liu a,b,c,n, C.C. Hsieh b, R.J. Chen a,c, W.C. Lin b, H.W. Chang d, W.C. Chang b,nn, A.R. Yan a,c a
Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Material Technology and Engineering, CAS, Ningbo 315201, China Department of Physics, National Chung Cheng University, Chia-Yi 621, Taiwan c Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology and Engineering, CAS, Ningbo 315201, China d Department of Physics, Tunghai University, Taichung 407, Taiwan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 June 2012 Received in revised form 22 August 2012 Available online 4 September 2012
Nd9.5Febal.Co10MB15 (M ¼ Ti2.5Zr0.5, Ti2.5Nb0.5, Zr2.5Nb0.5, Nb2.5Zr0.5) rod magnets are prepared by direct casting method. Thermo-gravimetric analysis results show that minor soft a-(Fe, Co) phase coexists with magnetically hard 2:14:1 phase in the above magnets. In addition, amorphous phase is also observed in magnets with Ti2.5Zr0.5 and Nb2.5Zr0.5 addition. According to differential scanning calorimetry results, obvious exothermic peaks are observed in the magnets with Ti2.5Zr0.5 and Nb2.5Zr0.5 addition. Meanwhile, the crystallization behavior of the amorphous phase is a two-step process. The activation energies of magnets with Ti2.5Zr0.5 and Nb2.5Zr0.5 addition are 542 kJ/mol and 558 kJ/mol, respectively. Microstructure images show that the 2:14:1 phase is surrounded by boron-rich grain boundary phase. The average grain size of 2:14:1 phase in Nb2.5Zr0.5-doped magnets is smaller than in Ti2.5Zr0.5-doped magnets. Magnetic measurements show that larger coercivity is observed in both Ti2.5Zr0.5-doped magnet and Nb2.5Zr0.5-doped magnet, while better squareness of the demagnetization curves and higher magnetic energy product are obtained in Ti2.5Nb0.5-doped magnet. & 2012 Elsevier B.V. All rights reserved.
Keywords: Cast magnet Magnetic property Amorphous phase Microstructure
1. Introduction The isotropic bonded R–Fe–B (R: rare-earth elements) permanent magnets have been developed for many years. By inducing the soft phase with considerably small size, the magnetic properties of R–Fe–B ribbons have been greatly enhanced [1–4]. Nevertheless, the disadvantages of the above bonded magnets still exist, including multifarious manufacturing processes and the dilution of magnetic properties by a nonmagnetic polymer. In order to overcome this disadvantage, R–Fe–M–B (R¼Pr, NdþDy, PrþDy; M¼Co, Cu, Mo, Nb, Ti, V and Zr) cast magnets have been developed, where the alloys were employed to produce a bulk amorphous precursor by the casting method followed by one-step annealing to optimize the permanent magnetic properties [5–9]. Although the high energy product [(BH)max] of 12.0 MGOe has been obtained, the size and intrinsic coercivity is normally very low, resulting from the existence of considerable amount of the magnetically soft phase Fe3B and the inhomogeneous microstructure, to suit for the thin magnets
application. Recently, high coercivity (iHc410 kOe) of Nd–Fe–B bulk magnets has been obtained by the addition of Zr, Nb and Ti elements. Furthermore, the diameter of rod has been increased to 1.3 mm, and the (BH)max can be as high as 6.8–8.4 MGOe [10–12]. To suit for application, the diameter and magnetic energy product of the cylindrical magnets need to be further improved. Adding some elements to improve the glass forming ability (GFA) of the alloy, for the purpose of obtaining finer grain size, is one of the methods to further improve the diameter and (BH)max. Co element has been reported to improve the GFA of the Nd–Fe–B alloy [2]. However, recent research shows that amorphous phase, which is harmful to the magnetic properties, tends to appear in directly cast Nd–Fe–B alloy with Co addition. In this article, different refractory metals are added to the Nd–FeCo–B alloys to reduce the amorphous phase and refine the microstructure. The magnetic properties, phase and microstructure evaluation of the directly cast Nd9.5Febal.Co10MB15 (M¼ Ti2.5Zr0.5, Ti2.5Nb0.5, Zr2.5Nb0.5, Nb2.5Zr0.5) rod magnets with diameter of 0.9 mm, without post-annealing, are studied in detail.
2. Experimental produces n
Corresponding author at: Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Material Technology and Engineering, CAS, Ningbo 315201, China. nn Corresponding author. E-mail addresses: [email protected] (Z. Liu), [email protected] (W.C. Chang). 0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.08.049
Alloys with compositions of Nd9.5Febal.Co10MB15 (M¼Ti2.5Zr0.5, Ti2.5Nb0.5, Zr2.5Nb0.5, Nb2.5Zr0.5) were prepared by arc melting and excess Nd of 8 wt% was added to compensate the loss during
Z. Liu et al. / Journal of Magnetism and Magnetic Materials 326 (2013) 108–111
109
processing in an argon atmosphere. The alloys were melted three times to ensure homogeneity. Magnet rods with a cylindrical shape with diameter 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 with a vibrating sample magnetometer (VSM). All the samples were magnetized by a 50 kOe peak pulse field prior to magnetic measurements. 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 1C/min. The crystallization temperatures of the magnetic phases were determined by differential scanning calorimetry (DSC). The cross section of the bulk magnets with different diameters was corroded by alcohol solution containing 10 wt% nitric acid (Nital) for 10 s and then it was observed by a scanning electron microscope (SEM). The composition of the bulk magnets is measured by energy dispersive spectrometer (EDS) attached in SEM.
3. Results and discussion Fig. 1 depicts the TMA scans of directly cast Nd9.5Febal.Co10MB15 (M¼Ti2.5Zr0.5, Ti2.5Nb0.5, Zr2.5Nb0.5, Nb2.5Zr0.5) rod magnets. It can be seen that minor soft a-(Fe, Co) phase and magnetically hard 2:14:1 phase are found in all of the rod magnets. In addition, amorphous phase is also observed in magnets with Ti2.5Zr0.5 and Nb2.5Zr0.5 addition. The peak height of 2:14:1 phase indicates that the content of 2:14:1 phase is lower in magnets with Zr2.5Nb0.5 addition than in the other magnets. In order to confirm the existence of amorphous phase, DSC curves of directly cast Nd9.5Febal.Co10MB15 (M¼Ti2.5Zr0.5, Ti2.5Nb0.5, Zr2.5 Nb0.5, Nb2.5Zr0.5) rod magnets are shown in Fig. 2. It can be seen that two obvious exothermic peaks are observed in the magnets with Ti2.5Zr0.5 and Nb2.5Zr0.5 addition, which mean the crystallization behavior is a two-step process for both alloys. According to the literature of Nd–Fe–B ribbons [13], the phase transformations of rod magnets with Ti2.5Zr0.5 and Nb2.5Zr0.5 addition during annealing take place with the similar sequences; the first crystallization temperature is about 680 1C, which corresponds to the formation of a-(Fe,Co) phase, Nd2(Fe,Co)14B phase and Nd3(Fe,Co)62B14 metastable phase from the amorphous phase, and the second one is about 720 1C, which can be related to the transformation from the Nd3(Fe,Co)62B14 metastable phase to the Nd2(Fe,Co)14B and a-(Fe,Co) phases. Minor peaks are shown in Ti2.5Nb0.5-doped magnets, yet no distinct peaks are found in magnets with Zr2.5Nb0.5 addition. Both the TMA curves and the crystallization peak height in DSC scans indicate that larger
Fig. 1. TGA traces of directly cast Nd9.5Febal.Co10MB15 (M¼ Ti2.5Zr0.5, Ti2.5Nb0.5, Zr2.5Nb0.5, Nb2.5Zr0.5) rod magnets.
Fig. 2. DSC curves of directly cast Nd9.5Febal.Co10MB15 (M¼ Ti2.5Zr0.5, Ti2.5Nb0.5, Zr2.5Nb0.5, Nb2.5Zr0.5) rod magnets.
volume of amorphous phase is formed in magnets with Nb2.5Zr0.5 and Ti2.5Zr0.5 addition, reflecting the improved glass forming ability in these alloys. Whereas, Zr2.5Nb0.5-doped alloy shows inferior ability in forming amorphous phase. The crystallization behavior of magnets with Ti2.5Zr0.5 and Nb2.5Zr0.5 are further studied by the DSC curves with different heating rates shown in Fig. 3. By measuring the peak positions of the crystallization exotherms shown in Fig. 3 at different heating rates, the average activation energies of crystallization (Ec) for the two alloys are calculated using the Kissinger method [14]. For magnets with Ti2.5Zr0.5 addition, the activation energy (ETZ) is 542 kJ/mol for the first exothermic peak and 356 kJ/mol for the second exothermic peak. For magnets with Nb2.5Zr0.5 addition, the activation energy (ENZ) is 558 kJ/mol for the first exothermic peak and 515 kJ/mol for the second exothermic peak. The higher value of ENZ, compared to ETZ, implies that it is more difficult for the grains of magnets with Nb2.5Zr0.5 (ENZ) addition to grow, as it requires larger additional energy. Thus, the growth of grains in Nb2.5Zr0.5doped alloy would be slower than that in the Ti2.5Zr0.5-doped alloy. The microstructure of rod magnets with Ti2.5Zr0.5 and Nb2.5Zr0.5 addition is further studied. Fig. 4 shows SEM microstructure of the peripheral region (lower photos) and core region (upper photos) of rod magnets with Ti2.5Zr0.5 (marked with 1) and Nb2.5Zr0.5 (marked with 2) addition. The peripheral region is the area near the boundary of rod magnets. The core region is the area at the center of rod magnets. According to the corrosion extent of rod magnets, two obviously different phases are observed in Fig. 4. In general, the phase in gray (marked with B and D) is surrounded by the phase in white (marked with A and C). In order to identify these phases, the EDS results of the area A, B, C and D are shown in Table 1. It can be seen that higher content of Nb and Ti is found in area A and C, respectively, and higher content of Zr is found in areas B and D. According to Chang’s [12] studies, larger part of Zr elements tend to enter into the 2:14:1 phase, while Nb and Ti elements tend to distribute in the boron-rich grain boundary phase. Thus, it is reasonable to presume that areas A and C are nonmagnetic boronrich phases and areas B and D are the magnetic 2:14:1 phases. According to Fig. 4, it can also be seen that the average grain size of 2:14:1 phase decreases from the core region to the peripheral region due to faster cooling rate in the area near the mold. Compared with Ti2.5Zr0.5-doped magnets, the average grain size of 2:14:1 phase in Nb2.5Zr0.5-doped magnets is much smaller. According to the DSC scans, Nb2.5Zr0.5-doped magnets have higher GFA than Ti2.5Zr0.5doped magnets. Improved GFA means increased capability of grain refinement for directly cast Nd–FeCo–B alloys, but the amorphous
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phase also easily appears in these magnets especially at the peripheral region, which is harmful to the magnetic properties of the cast magnets.
The magnetic properties of directly cast Nd9.5Febal.Co10MB15 (M¼Ti2.5Zr0.5, Ti2.5Nb0.5, Zr2.5Nb0.5, Nb2.5Zr0.5) rod magnets with diameter of 0.9 mm were investigated. Fig. 5 shows demagnetization curves of the cast magnets. Clearly, a higher coercive force larger than 950 kA/m is obtained in magnets with Ti2.5Zr0.5 and Nb2.5Zr0.5 addition due to their fine grain distribution. The coercive force of magnets with Ti2.5Nb0.5 addition has a slight decrease yet it is still higher than 800 KA/m. In contrast, the coercivity is very low for magnets with Zr2.5Nb0.5, which indicates that the grain size could be much larger than other magnets. On the other hand, although the coercive force in magnets with Ti2.5Nb0.5 addition is lower than in magnets with Ti2.5Zr0.5 and Nb2.5Zr0.5 addition, the improved squareness of the demagnetization curve and the highest remanence are obtained, which reflect an effective exchange coupling which is obtained in this rod magnet. However, an obvious kink is observed in the demagnetization curves of magnets with Ti2.5Zr0.5 and Nb2.5Zr0.5 addition, indicating a weak coupling phenomenon between the hard and soft phases. Thus, the remanence is lower than in magnets with Ti2.5Nb0.5 addition. Finally, the lowest squareness of the demagnetization curve and remanence is shown in Zr2.5Nb0.5-doped magnets, which could be due to the lower content of 2:14:1 phase and larger grain size.
4. Summary and conclusion Refractory elements were cosubstituted in Co-containing Nd9.5 Febal.Co10MB15 (M¼Ti2.5Zr0.5, Ti2.5Nb0.5, Zr2.5Nb0.5, Nb2.5Zr0.5) alloys Table 1 EDS results of rod magnets with Ti2.5Zr0.5 and Nb2.5Zr0.5 addition. Elements Magnets with Nb2.5Zr0.5 addition
Fig. 3. DSC curves of directly cast Nd9.5Febal.Co10MB15 (M ¼Ti2.5Zr0.5 (a), Nb2.5Zr0.5 (b)) rod magnets with different heating rates.
Nd Fe Co Nb Zr
A (wt%)
B (wt%)
27.96 52.51 12.75 4.69 1.09
35.43 50.35 8 6.19 0.03
Elements Magnets with Ti2.5Zr0.5 addition
Nd Fe Co Ti Zr
C (wt%)
D (wt%)
25.32 58.67 10.72 2.3 2.99
31.19 52.38 9.91 6.45 0.07
Fig. 4. SEM microstructures of the peripheral region (lower two photos) and core region (upper two photos) of rod magnets with Ti2.5Zr0.5 (marked with 1) and Nb2.5Zr0.5 (marked with 2) addition.
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Acknowledgments This paper was supported by the National Science Council of Taiwan under Grant no. NSC-98-2112-M-194-005-MY3, the State Key Program of National Natural Science Foundation of China under Grant no. 2010AA03A402, and the Science and Technology International Cooperation Projects of China under Grant no. 2010DFB53770.
References
Fig. 5. Demagnetization curves of directly cast Nd9.5Febal.Co10MB15 (M ¼Ti2.5Zr0.5, Ti2.5Nb0.5, Zr2.5Nb0.5, Nb2.5Zr0.5) rod magnets.
for making direct cast magnets. The elements M¼Ti2.5Zr0.5 and Nb2.5Zr0.5 have the capability of increasing the GFA of the cast magnets, which reduces the grain size of the magnets and enhances the coercivity effectively. Nevertheless, the appearance of amorphous phase in the peripheral region of the magnets deteriorates the squareness of the demagnetization curve. On the other hand, a better squareness of the demagnetization curve and higher magnetic energy product are obtained in Ti2.5Nb0.5-doped magnet, which arises from its highly homogeneous microstructure and is almost free from amorphous phase inside the cast magnet.
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