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Effect of Co addition on the microstructure and magnetic properties of Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) bulk magnets
Journal of Alloys and Compounds 538 (2012) 28–33
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effect of Co addition on the microstructure and magnetic properties of Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) bulk magnets Z. Liu a,b,⇑, W.C. Lin a, C.W. Shih a, C.C. Hsieh a, H.W. Chang d, W.C. Chang a, A.R. Yan b,c a
Department of Physics, National Chung Cheng University, Chia-Yi 621, Taiwan Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China c Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China d Department of Physics, Tunghai University, Taichung 407, Taiwan b
a r t i c l e
i n f o
Article history: Received 20 March 2012 Received in revised form 28 May 2012 Accepted 1 June 2012 Available online 12 June 2012 Keywords: Rare earth magnets Bulk magnets Size effect Co effect
a b s t r a c t Cylindrical Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) magnets with diameter of 0.9–1.5 mm have been prepared by directly casting method. TMA and DSC analysis show that doping Co may enhance the glass forming ability and refine the grain size of the directly cast magnets effectively. Nevertheless, the amorphous phase is easily to appear at the peripheral region but not at the interior region, leading to much inhomogeneous microstructure especially when too high Co content (x = 20) is added. Magnetic measurements show that higher maximum energy product and coercivity can only be obtained in Co-free magnets with smaller diameter (60.9 mm), while higher maximum energy product and coercivity could be appeared in Co-doped magnets (x = 10) with larger diameter (1.1–1.3 mm in diameter). Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The isotropic bonded R–Fe–B (R: rare-earth elements) permanent magnets have been developed for more than two decades. 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(R@Pr, Nd + Dy, Pr + Dy; M@Co, 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 high maximum energy product of (BH)max = 12.0 MGOe have been obtained, the size and intrinsic coercivity is normally very low, resulting from the existing of considerable amount of the magnetically soft phase Fe3B and the inhomogeneous microstructure, to suit for the thin magnets application. Recently, high coercivity (iHc > 10 kOe) of Nd–Fe–B bulk magnets has been obtained by the addition of Zr, Nb and Ti elements. Furthermore, the diameter of rod have been increased to 1.3 mm, and the ⇑ Corresponding author at: Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China. E-mail address: [email protected] (Z. Liu). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.06.005
(BH)max can be as high as 6.8–8.4 MGOe [10–12]. To suit for application, the diameter and maximum 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. In past, it has been reported that Co element could improve the GFA of the Nd–Fe–B alloy ribbons [5]. Therefore, in this article, the magnetic properties and microstructure evaluation of the directly cast Nd9.5 Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) cylindrical magnets, without post-annealing, with diameter from 0.9 to 1.5 mm are studied systematically.
2. Experimental Alloys with compositions of Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) were prepared by arc melting in an argon atmosphere, an excess Nd of 8 wt.% was added to compensate the loss during processing. The alloys were melted three times to ensure homogeneity. Magnets rod with a cylindrical shape of various diameters of 0.9–1.5 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). The demagnetization field effect has been considered by calculating the dimensional ratio of length and diameter for samples with a cylindrical shape. 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 °C/min. The crystallization temperatures of the magnetic phases were determined by differential scanning calorimetry (DSC) at a heating rate of 15 °C/min.
Z. Liu et al. / Journal of Alloys and Compounds 538 (2012) 28–33
Fig. 1. XRD pattern of directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 10, 20) magnets with diameter in 0.9 mm.
The cross section of the bulk magnets with different diameter 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 was measured by energy dispersive spectrometer (EDS) attached in SEM.
3. Results and discussion 3.1. Phase analysis Fig. 1 show the XRD pattern of directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 10, 20) rod magnets with diameter in 0.9 mm. It can be seen that 2:14:1 phase and a-(Fe, Co) phase are clearly observed in Co-doped magnets. In order to analysis the phase structure in detail, the TMA measurements are adopted. Fig. 2a–c depict the TMA scans of directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) rod magnets with different diameters, respectively. For magnets without Co, small amount of magnetically soft a-Fe and Fe3B phases is found to coexist with magnetically hard 2:14:1 phase. With the addition of 10 at.% Co, only minor soft a-(Fe, Co) phase and magnetically hard 2:14:1 phase are found in the rod magnets with different diameters. In addition, in magnets with 0.9 and 1.1 mm in diameter, amorphous phase is also observed. For the magnets with x = 20, amorphous, a(Fe, Co) and 2:14:1 phase are found in bulk magnets with various diameters. In order to confirm the existence of amorphous phase, DSC curves for directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) rod magnets with different diameter are shown in Fig. 3. It can be seen that obvious exothermic peaks are observed in the magnets with Co addition, but not for the magnets without Co addition. The crystallization temperature of magnets with x = 10
29
Fig. 3. DSC curves for directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) magnets with different diameter.
and 20 is about 680 and 690 °C, respectively, which is consistent with the result of Nd–Fe–B ribbons with Co addition [12]. From the variation of crystallization peak height, it indicates that the volume of amorphous phase decreases with increasing the diameter of the magnets. The amorphous phase disappears for the magnets with diameter up to 1.3 mm for x = 10, yet it persists to 1.5 mm for magnets with x = 20. These results indicate that Nd9.5Febal.CoxNb2.5Zr0.5B15 magnets with higher Co content (x P 10) may possess higher ability in forming amorphous. The phase distribution and composition of rod magnets are studied by SEM and EDS, respectively. Fig. 4a and b shows the boundary and center area of directly cast Nd9.5Febal.Co20Nb2.5 Zr0.5B15 rod magnets with 1.1 mm in diameter. According to the corrosion extent of rod magnets, three different phases are clearly observed in Fig. 4. In general, the phase in grey (marked with B) is surrounded by the phase in white (marked with A). It also can be found that some areas are not easily corroded at the peripheral region, which is marked with C. In order to identify those three phases, the composition of area A, B and C are studied in magnets with 20 at.% Co, which are shown in Table 1. Nd, Fe, Co, Zr and Nb elements are found in area A, B and C. The content of Nd shows no obviously different, indicating that the area A and B could include both 2:14:1 and boron-rich phase. The different content of Zr and Nb could result from the different volume fraction of 2:14:1 and boron-rich phase in both areas. According to the TMA analysis and the high corrosion resistivity, it is clear that area C is the amorphous phase. The highest content of Nb is found in area A, and the highest content of Zr is found in area B. According to Chang’s [13] studies, larger part of Zr elements tend to enter into the 2:14:1
Fig. 2. TMA traces for directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0 (a), 10 (b), 20 (c)) magnets with different diameter.
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Z. Liu et al. / Journal of Alloys and Compounds 538 (2012) 28–33
Fig. 4. SEM micrographs of the boundary area (a) and the center area (b) of directly cast Nd9.5Febal.Co20Nb2.5Zr0.5B15 rod magnets with 1.1 mm in diameter.
3.2. Microstructure
Table 1 The EDS results of magnets with 20 at.% Co in area A, B and C. Element
A (Wt%)
B (Wt%)
C (Wt%)
Nd Fe Co Zr Nb
23.32 48.66 20.32 1.37 6.32
22.91 48.48 20.62 3.19 4.79
22.85 47.98 21.34 2.7 5.12
phase and Nb elements tend to distribute in the boron-rich grain boundary phase. Nevertheless, boron could not be detected by our SEM–EDX analysis, it is not possible to directly prove the real composition of the grain boundary phase, nor could we judge whether it is magnetic or nonmagnetic. But if judged from the higher corrosion resistivity of area A than area B against Nital, and no extra peak of magnetic phases other than 2:14:1, Fe3B and a-Fe or a-(Fe, Co) which appeared at TMA scans, it is reasonable to presume that area A is nonmagnetic magnetic boron-rich phase and area B is 2:14:1 phase.
The grain size and its distribution in the magnets are very crucial in affecting their permanent magnetic properties. As a result, it becomes an essential issue to know more about the microstructure of the magnets with different content of Co and the diameter of the cast Nd9.5Febal.CoxNb2.5Zr0.5B15 magnets. In order to show the whole microstructure of the rod magnets, three different areas are observed from the center to the boundary, named with core, outside of core and peripheral regions after corroded with Nital, respectively. The peripheral region is the area near the boundary of the magnets. The core region is the area at the center of the magnets, while outside of core region is the area between the peripheral region and the core region. Firstly, the effects of Co addition on the microstructure of directly cast magnets are studied. Fig. 5 shows the SEM pictures of phase distribution at the core region (labeled with 1), outside of core region (labeled with 2) and the peripheral region (labeled with 3) of Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20, marked with C0, C1 and C2, respectively) rod magnets with 1.1 mm in diameter.
Fig. 5. SEM micrographs for the core region (labeled with (1), outside of core region (labeled with 2) and the peripheral region (labeled with 3) of directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) rod magnets (marked with C0, C1 and C2, respectively) with 1.1 mm in diameter.
Z. Liu et al. / Journal of Alloys and Compounds 538 (2012) 28–33
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Fig. 6. TEM micrographs of the core region in directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) rod magnets.
It is seen that the average grain size of 2:14:1 phase decreases monotonously at the peripheral region and the outside of core region with the increase of Co content. However, at the core region, boron-rich phase spreads into the 2:14:1 main phase in Co-free magnets, yet it homogeneously surrounds 2:14:1 phase in Codoped magnets. It suggests that the addition of Co might increase the melting point of boron-rich phase and restrain it deeply spreading into the 2:14:1 phase during solidification process. With increasing the Co content, the distribution of 2:14:1 phase at outside of core region is greatly different from that at the core region. For x = 10, magnets show much homogeneous distribution, however, it become inhomogeneous in x = 0 and 20. As for the microstructure at the core region, further analysis is done by TEM observation. Fig. 6a–c shows the grain distribution at the core region of directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) rod magnets, respectively. It can be seen that several grains of 2:14:1 phase are surrounded by large volume of boron-rich phase in Codoped magnets, which is in accordance with the results of SEM observation. The grain size shows no obvious change with the increase of Co content, which is about 300 nm. The only difference is that the grain boundary becomes much narrower with increas-
ing the Co content, which could be due to the less amount of liquid boron-rich phase surrounding the 2:14:1 phase during solidification. In summary, although the grain size of 2:14:1 phase and Brich grain boundary phase can be refined in the Co-doped magnets, large amount of amorphous phase may appear at the peripheral region in magnets with higher Co content (x > 10), which is detrimental to the hard magnetic properties of the directly cast magnets. In addition, the effects of diameter on the microstructure of directly cast Co-doped magnets are studied. Fig. 7 show the SEM pictures of phase distribution at the core region (labeled with 1), outside of core region (labeled with 2) and the peripheral region (labeled with 3) of directly cast Nd9.5Febal.Co10Nb2.5Zr0.5B15 rod magnets with diameter in 0.9, 1.1 and 1.3 mm. It can be seen that the grain size increases with increasing the diameter of the magnets. Although the grain size at core region is much larger than that at outside of core region for magnet with diameter of 0.9 mm, the grain size at outside of core region is similar to that at core region for magnets with diameter of 1.3 mm, which indicates that the grain size at outside of core region grow at the same tendency as that at the core region with increasing the diameter from 0.9 to
Fig. 7. SEM micrographs for the core region (labeled with 1), outside of core region (labeled with 2) and the peripheral region (labeled with 3) of directly cast Nd9.5Febal.Co10Nb2.5Zr0.5B15 rod magnets with diameter of 0.9, 1.1 and 1.3 mm.
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Z. Liu et al. / Journal of Alloys and Compounds 538 (2012) 28–33
Fig. 8. The demagnetization curves of directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 rod magnets, where (a) x = 0, (b) 10, (c) 20.
1.3 mm. Meanwhile, the amount of amorphous phase at the peripheral region is gradually reduced with increasing the diameter of the magnets. 3.3. Magnetic properties Fig. 8a–c shows the demagnetization curves of directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) rod magnets with diameters of 0.9, 1.1, 1.3 and 1.5 mm, respectively. The squareness of demagnetization curves for magnets without Co addition gradually deteriorates with increasing the diameter. For magnets with x = 10, it first ameliorates with increasing the diameter from 0.9 to 1.1 mm and then gets worse with further increasing the diameter. When the Co content is increased to 20 at.%, the squareness of demagnetization curves becomes inferior. Based on the TMA and SEM results, it is understood that the homogeneous microstructure of the magnets with x = 10, in comparison with those of x = 0 and 20, leads to the better squareness of demagnetization curves in the as-cast state. However, as for magnet with 20 at.% Co, the microstructure at the peripheral region is greatly different from that at the core region, grain inhomogeneity leads to the inferior squareness of the demagnetization curve accordingly. Fig. 9 show the variation of maximum energy product, remanence and coercivity in directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) magnets with different diameters of 0.9 to 1.5 mm. It can be seen that the highest (BH)max is obtained in Co-free magnets with diameter of 0.9 mm. However, it decreases monotonically from about 7 to 1 MGOe with increasing the diameter from / = 0.9 to 1.5 mm. Unlike the Co-free magnets, for magnets with x = 10 and 20, the (BH)max first increases and then decreases with increasing the diameter. The highest (BH)max of 6.5 MGOe is obtained at the diameter of 1.1 mm for the magnet with x = 10, while it is 4.5 MGOe at the diameter of 1.3 mm for the magnet with x = 20. According to the analysis of microstructure in Co-doped magnets, it indicates that the increase of (BH)max could be due to the decrease of volume fraction of amorphous with increasing the diameter. On the other hand, the decrease of (BH)max could be resulted from the larger grain size and the inhomogeneous microstructure. In addition, the coercivity of Co-doped magnets shows much slower decrement than Co-free magnets as the diameter is smaller than 1.3 mm, which could be due to the nonmagnetic boron-rich phase is homogeneously distributed in the boundary of the 2:14:1 phase in Co-doped magnets. The nonmagnetic grain boundary phase surrounding the 2:14:1 phase might play an important role in impeding the domain wall movement and, subsequently, in enhancing the coercivity of the magnets. From the variation of magnetic properties, it is evidenced that doping proper Co content, i.e. x = 10, could modify the phase distribution and the microstructure, which is helpful for making NdFeBbased cast magnets with larger size.
Fig. 9. Maximum energy product, coercivity and remanence of directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) rod magnets with different diameters.
4. Summary The microstructure and magnetic properties of directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) rod magnets with different diameters have been studied. Microstructure analysis evidences that amorphous phase is easily appeared at the peripheral region of Co-doped magnets, but not for Co-free magnets. The average grain size of 2:14:1 phase at the peripheral region and the outside of core region decreases with the increase of Co content, implying that adding Co has the tendency to increase the glass formability of the cast Nd9.5Febal.CoxNb2.5Zr0.5B15 magnet. Higher maximum energy product and coercivity can only be obtained in Co-free magnets with smaller diameter (60.9 mm), arisen from much homogeneous distribution of grains throughout the whole cross section. Nevertheless, higher maximum energy product and coercivity could be appeared in Co-doped magnets (x = 10) with larger diameter (1.1–1.3 mm in diameter). The appearance of amorphous phase at the peripheral region and the inhomogeneous grain size throughout the whole magnets may deteriorate the squareness of the demagnetization loop and the maximum energy product in directly cast magnets with too high Co content, i.e. x = 20.
Z. Liu et al. / Journal of Alloys and Compounds 538 (2012) 28–33
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 No. 2010AA03A402, and science and technology international cooperation projects of china No. 2010DFB53770.
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Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effect of Co addition on the microstructure and magnetic properties of Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) bulk magnets Z. Liu a,b,⇑, W.C. Lin a, C.W. Shih a, C.C. Hsieh a, H.W. Chang d, W.C. Chang a, A.R. Yan b,c a
Department of Physics, National Chung Cheng University, Chia-Yi 621, Taiwan Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China c Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China d Department of Physics, Tunghai University, Taichung 407, Taiwan b
a r t i c l e
i n f o
Article history: Received 20 March 2012 Received in revised form 28 May 2012 Accepted 1 June 2012 Available online 12 June 2012 Keywords: Rare earth magnets Bulk magnets Size effect Co effect
a b s t r a c t Cylindrical Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) magnets with diameter of 0.9–1.5 mm have been prepared by directly casting method. TMA and DSC analysis show that doping Co may enhance the glass forming ability and refine the grain size of the directly cast magnets effectively. Nevertheless, the amorphous phase is easily to appear at the peripheral region but not at the interior region, leading to much inhomogeneous microstructure especially when too high Co content (x = 20) is added. Magnetic measurements show that higher maximum energy product and coercivity can only be obtained in Co-free magnets with smaller diameter (60.9 mm), while higher maximum energy product and coercivity could be appeared in Co-doped magnets (x = 10) with larger diameter (1.1–1.3 mm in diameter). Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The isotropic bonded R–Fe–B (R: rare-earth elements) permanent magnets have been developed for more than two decades. 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(R@Pr, Nd + Dy, Pr + Dy; M@Co, 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 high maximum energy product of (BH)max = 12.0 MGOe have been obtained, the size and intrinsic coercivity is normally very low, resulting from the existing of considerable amount of the magnetically soft phase Fe3B and the inhomogeneous microstructure, to suit for the thin magnets application. Recently, high coercivity (iHc > 10 kOe) of Nd–Fe–B bulk magnets has been obtained by the addition of Zr, Nb and Ti elements. Furthermore, the diameter of rod have been increased to 1.3 mm, and the ⇑ Corresponding author at: Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China. E-mail address: [email protected] (Z. Liu). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.06.005
(BH)max can be as high as 6.8–8.4 MGOe [10–12]. To suit for application, the diameter and maximum 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. In past, it has been reported that Co element could improve the GFA of the Nd–Fe–B alloy ribbons [5]. Therefore, in this article, the magnetic properties and microstructure evaluation of the directly cast Nd9.5 Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) cylindrical magnets, without post-annealing, with diameter from 0.9 to 1.5 mm are studied systematically.
2. Experimental Alloys with compositions of Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) were prepared by arc melting in an argon atmosphere, an excess Nd of 8 wt.% was added to compensate the loss during processing. The alloys were melted three times to ensure homogeneity. Magnets rod with a cylindrical shape of various diameters of 0.9–1.5 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). The demagnetization field effect has been considered by calculating the dimensional ratio of length and diameter for samples with a cylindrical shape. 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 °C/min. The crystallization temperatures of the magnetic phases were determined by differential scanning calorimetry (DSC) at a heating rate of 15 °C/min.
Z. Liu et al. / Journal of Alloys and Compounds 538 (2012) 28–33
Fig. 1. XRD pattern of directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 10, 20) magnets with diameter in 0.9 mm.
The cross section of the bulk magnets with different diameter 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 was measured by energy dispersive spectrometer (EDS) attached in SEM.
3. Results and discussion 3.1. Phase analysis Fig. 1 show the XRD pattern of directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 10, 20) rod magnets with diameter in 0.9 mm. It can be seen that 2:14:1 phase and a-(Fe, Co) phase are clearly observed in Co-doped magnets. In order to analysis the phase structure in detail, the TMA measurements are adopted. Fig. 2a–c depict the TMA scans of directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) rod magnets with different diameters, respectively. For magnets without Co, small amount of magnetically soft a-Fe and Fe3B phases is found to coexist with magnetically hard 2:14:1 phase. With the addition of 10 at.% Co, only minor soft a-(Fe, Co) phase and magnetically hard 2:14:1 phase are found in the rod magnets with different diameters. In addition, in magnets with 0.9 and 1.1 mm in diameter, amorphous phase is also observed. For the magnets with x = 20, amorphous, a(Fe, Co) and 2:14:1 phase are found in bulk magnets with various diameters. In order to confirm the existence of amorphous phase, DSC curves for directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) rod magnets with different diameter are shown in Fig. 3. It can be seen that obvious exothermic peaks are observed in the magnets with Co addition, but not for the magnets without Co addition. The crystallization temperature of magnets with x = 10
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Fig. 3. DSC curves for directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) magnets with different diameter.
and 20 is about 680 and 690 °C, respectively, which is consistent with the result of Nd–Fe–B ribbons with Co addition [12]. From the variation of crystallization peak height, it indicates that the volume of amorphous phase decreases with increasing the diameter of the magnets. The amorphous phase disappears for the magnets with diameter up to 1.3 mm for x = 10, yet it persists to 1.5 mm for magnets with x = 20. These results indicate that Nd9.5Febal.CoxNb2.5Zr0.5B15 magnets with higher Co content (x P 10) may possess higher ability in forming amorphous. The phase distribution and composition of rod magnets are studied by SEM and EDS, respectively. Fig. 4a and b shows the boundary and center area of directly cast Nd9.5Febal.Co20Nb2.5 Zr0.5B15 rod magnets with 1.1 mm in diameter. According to the corrosion extent of rod magnets, three different phases are clearly observed in Fig. 4. In general, the phase in grey (marked with B) is surrounded by the phase in white (marked with A). It also can be found that some areas are not easily corroded at the peripheral region, which is marked with C. In order to identify those three phases, the composition of area A, B and C are studied in magnets with 20 at.% Co, which are shown in Table 1. Nd, Fe, Co, Zr and Nb elements are found in area A, B and C. The content of Nd shows no obviously different, indicating that the area A and B could include both 2:14:1 and boron-rich phase. The different content of Zr and Nb could result from the different volume fraction of 2:14:1 and boron-rich phase in both areas. According to the TMA analysis and the high corrosion resistivity, it is clear that area C is the amorphous phase. The highest content of Nb is found in area A, and the highest content of Zr is found in area B. According to Chang’s [13] studies, larger part of Zr elements tend to enter into the 2:14:1
Fig. 2. TMA traces for directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0 (a), 10 (b), 20 (c)) magnets with different diameter.
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Fig. 4. SEM micrographs of the boundary area (a) and the center area (b) of directly cast Nd9.5Febal.Co20Nb2.5Zr0.5B15 rod magnets with 1.1 mm in diameter.
3.2. Microstructure
Table 1 The EDS results of magnets with 20 at.% Co in area A, B and C. Element
A (Wt%)
B (Wt%)
C (Wt%)
Nd Fe Co Zr Nb
23.32 48.66 20.32 1.37 6.32
22.91 48.48 20.62 3.19 4.79
22.85 47.98 21.34 2.7 5.12
phase and Nb elements tend to distribute in the boron-rich grain boundary phase. Nevertheless, boron could not be detected by our SEM–EDX analysis, it is not possible to directly prove the real composition of the grain boundary phase, nor could we judge whether it is magnetic or nonmagnetic. But if judged from the higher corrosion resistivity of area A than area B against Nital, and no extra peak of magnetic phases other than 2:14:1, Fe3B and a-Fe or a-(Fe, Co) which appeared at TMA scans, it is reasonable to presume that area A is nonmagnetic magnetic boron-rich phase and area B is 2:14:1 phase.
The grain size and its distribution in the magnets are very crucial in affecting their permanent magnetic properties. As a result, it becomes an essential issue to know more about the microstructure of the magnets with different content of Co and the diameter of the cast Nd9.5Febal.CoxNb2.5Zr0.5B15 magnets. In order to show the whole microstructure of the rod magnets, three different areas are observed from the center to the boundary, named with core, outside of core and peripheral regions after corroded with Nital, respectively. The peripheral region is the area near the boundary of the magnets. The core region is the area at the center of the magnets, while outside of core region is the area between the peripheral region and the core region. Firstly, the effects of Co addition on the microstructure of directly cast magnets are studied. Fig. 5 shows the SEM pictures of phase distribution at the core region (labeled with 1), outside of core region (labeled with 2) and the peripheral region (labeled with 3) of Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20, marked with C0, C1 and C2, respectively) rod magnets with 1.1 mm in diameter.
Fig. 5. SEM micrographs for the core region (labeled with (1), outside of core region (labeled with 2) and the peripheral region (labeled with 3) of directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) rod magnets (marked with C0, C1 and C2, respectively) with 1.1 mm in diameter.
Z. Liu et al. / Journal of Alloys and Compounds 538 (2012) 28–33
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Fig. 6. TEM micrographs of the core region in directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) rod magnets.
It is seen that the average grain size of 2:14:1 phase decreases monotonously at the peripheral region and the outside of core region with the increase of Co content. However, at the core region, boron-rich phase spreads into the 2:14:1 main phase in Co-free magnets, yet it homogeneously surrounds 2:14:1 phase in Codoped magnets. It suggests that the addition of Co might increase the melting point of boron-rich phase and restrain it deeply spreading into the 2:14:1 phase during solidification process. With increasing the Co content, the distribution of 2:14:1 phase at outside of core region is greatly different from that at the core region. For x = 10, magnets show much homogeneous distribution, however, it become inhomogeneous in x = 0 and 20. As for the microstructure at the core region, further analysis is done by TEM observation. Fig. 6a–c shows the grain distribution at the core region of directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) rod magnets, respectively. It can be seen that several grains of 2:14:1 phase are surrounded by large volume of boron-rich phase in Codoped magnets, which is in accordance with the results of SEM observation. The grain size shows no obvious change with the increase of Co content, which is about 300 nm. The only difference is that the grain boundary becomes much narrower with increas-
ing the Co content, which could be due to the less amount of liquid boron-rich phase surrounding the 2:14:1 phase during solidification. In summary, although the grain size of 2:14:1 phase and Brich grain boundary phase can be refined in the Co-doped magnets, large amount of amorphous phase may appear at the peripheral region in magnets with higher Co content (x > 10), which is detrimental to the hard magnetic properties of the directly cast magnets. In addition, the effects of diameter on the microstructure of directly cast Co-doped magnets are studied. Fig. 7 show the SEM pictures of phase distribution at the core region (labeled with 1), outside of core region (labeled with 2) and the peripheral region (labeled with 3) of directly cast Nd9.5Febal.Co10Nb2.5Zr0.5B15 rod magnets with diameter in 0.9, 1.1 and 1.3 mm. It can be seen that the grain size increases with increasing the diameter of the magnets. Although the grain size at core region is much larger than that at outside of core region for magnet with diameter of 0.9 mm, the grain size at outside of core region is similar to that at core region for magnets with diameter of 1.3 mm, which indicates that the grain size at outside of core region grow at the same tendency as that at the core region with increasing the diameter from 0.9 to
Fig. 7. SEM micrographs for the core region (labeled with 1), outside of core region (labeled with 2) and the peripheral region (labeled with 3) of directly cast Nd9.5Febal.Co10Nb2.5Zr0.5B15 rod magnets with diameter of 0.9, 1.1 and 1.3 mm.
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Fig. 8. The demagnetization curves of directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 rod magnets, where (a) x = 0, (b) 10, (c) 20.
1.3 mm. Meanwhile, the amount of amorphous phase at the peripheral region is gradually reduced with increasing the diameter of the magnets. 3.3. Magnetic properties Fig. 8a–c shows the demagnetization curves of directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) rod magnets with diameters of 0.9, 1.1, 1.3 and 1.5 mm, respectively. The squareness of demagnetization curves for magnets without Co addition gradually deteriorates with increasing the diameter. For magnets with x = 10, it first ameliorates with increasing the diameter from 0.9 to 1.1 mm and then gets worse with further increasing the diameter. When the Co content is increased to 20 at.%, the squareness of demagnetization curves becomes inferior. Based on the TMA and SEM results, it is understood that the homogeneous microstructure of the magnets with x = 10, in comparison with those of x = 0 and 20, leads to the better squareness of demagnetization curves in the as-cast state. However, as for magnet with 20 at.% Co, the microstructure at the peripheral region is greatly different from that at the core region, grain inhomogeneity leads to the inferior squareness of the demagnetization curve accordingly. Fig. 9 show the variation of maximum energy product, remanence and coercivity in directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) magnets with different diameters of 0.9 to 1.5 mm. It can be seen that the highest (BH)max is obtained in Co-free magnets with diameter of 0.9 mm. However, it decreases monotonically from about 7 to 1 MGOe with increasing the diameter from / = 0.9 to 1.5 mm. Unlike the Co-free magnets, for magnets with x = 10 and 20, the (BH)max first increases and then decreases with increasing the diameter. The highest (BH)max of 6.5 MGOe is obtained at the diameter of 1.1 mm for the magnet with x = 10, while it is 4.5 MGOe at the diameter of 1.3 mm for the magnet with x = 20. According to the analysis of microstructure in Co-doped magnets, it indicates that the increase of (BH)max could be due to the decrease of volume fraction of amorphous with increasing the diameter. On the other hand, the decrease of (BH)max could be resulted from the larger grain size and the inhomogeneous microstructure. In addition, the coercivity of Co-doped magnets shows much slower decrement than Co-free magnets as the diameter is smaller than 1.3 mm, which could be due to the nonmagnetic boron-rich phase is homogeneously distributed in the boundary of the 2:14:1 phase in Co-doped magnets. The nonmagnetic grain boundary phase surrounding the 2:14:1 phase might play an important role in impeding the domain wall movement and, subsequently, in enhancing the coercivity of the magnets. From the variation of magnetic properties, it is evidenced that doping proper Co content, i.e. x = 10, could modify the phase distribution and the microstructure, which is helpful for making NdFeBbased cast magnets with larger size.
Fig. 9. Maximum energy product, coercivity and remanence of directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) rod magnets with different diameters.
4. Summary The microstructure and magnetic properties of directly cast Nd9.5Febal.CoxNb2.5Zr0.5B15 (x = 0, 10, 20) rod magnets with different diameters have been studied. Microstructure analysis evidences that amorphous phase is easily appeared at the peripheral region of Co-doped magnets, but not for Co-free magnets. The average grain size of 2:14:1 phase at the peripheral region and the outside of core region decreases with the increase of Co content, implying that adding Co has the tendency to increase the glass formability of the cast Nd9.5Febal.CoxNb2.5Zr0.5B15 magnet. Higher maximum energy product and coercivity can only be obtained in Co-free magnets with smaller diameter (60.9 mm), arisen from much homogeneous distribution of grains throughout the whole cross section. Nevertheless, higher maximum energy product and coercivity could be appeared in Co-doped magnets (x = 10) with larger diameter (1.1–1.3 mm in diameter). The appearance of amorphous phase at the peripheral region and the inhomogeneous grain size throughout the whole magnets may deteriorate the squareness of the demagnetization loop and the maximum energy product in directly cast magnets with too high Co content, i.e. x = 20.
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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 No. 2010AA03A402, and science and technology international cooperation projects of china No. 2010DFB53770.
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