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Nd5Fe64B23Mo4Y4 bulk nanocomposite permanent magnets produced by crystallizing amorphous precursors
Journal of Non-Crystalline Solids 358 (2012) 1028–1031
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Nd5Fe64B23Mo4Y4 bulk nanocomposite permanent magnets produced by crystallizing amorphous precursors Shan Tao a, c, Tianyu Ma a,⁎, Zubair Ahmad b, Hong Jian a, Mi Yan b,⁎⁎ a b c
Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China Hangzhou Vocational & Technical College, Hangzhou 310018, China
a r t i c l e
i n f o
Article history: Received 12 October 2011 Received in revised form 17 January 2012 Available online 14 February 2012 Keywords: Nanocomposite; Magnetic property; Glass forming ability
a b s t r a c t The glass forming ability and magnetic properties of Nd5Fe68 − xB23Mo4Yx (x = 0, 2, 4, 6) alloys prepared by copper mold casting technique have been studied. Amorphous rods with a diameter of 2 mm were obtained in the Nd5Fe64B23Mo4Y4 alloy. After annealing for 10 min at 1013 K, the Nd5Fe64B23Mo4Y4 alloy showed optimal hard magnetic properties with the coercivity of 764.2 kA/m, remanence of 0.6 T and maximum energy product of 57.3 kJ/m3, respectively. The enhanced magnetic properties can be ascribed to the strong exchange coupling among the magnetically soft α-Fe (25–30 nm), Fe3B (30–35 nm) and hard Nd2Fe14B (40–50 nm) grains present in the magnet microstructure. Large size bulk nanocomposite magnets with sound magnetic properties make the Nd–Fe–B–Mo–Y alloy system a promising candidate for industrial applications. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Nanocomposite permanent magnets have been widely investigated for their unique microstructure and promising magnetic properties [1–4]. Recently, the copper mold casting method has been adopted to develop bulk nanocomposite permanent magnets either through direct casting or annealing the amorphous precursors [5–7]. This method provides an economical and simple approach to produce miniature nanocomposite permanent magnets [8]. Currently, one of the main challenges of this approach is to enhance the magnetic properties as well as to enlarge the magnet size. To achieve large size bulk amorphous precursors, a sufficiently high glass forming ability (GFA) of the alloy is required. Much efforts have been carried out to enhance the GFA as well as magnetic properties through doping with refractory elements such as Co, Cu, Nb, Ti, Zr, V and Y to R3–9.5FebalB6–30 (R_Nd or Pr) alloys [9–16]. Among them, Y has been deemed as one of the most effective elements in improving GFA, stabilizing the amorphous phase and reducing the oxides during the synthesis of R–Fe–M–B alloys. Wu et al. [17] presented that the critical diameter of the Nd1Y5Fe68Mo4B22 metallic glassy rod can reach 4 mm. They also achieved a maximal coercivity of 364.1 kA/m from the 1.5 mm-diameter Nd3Y3Fe68Mo4B22 rod after subsequent crystallization. Tan et al. [18] have developed bulk sheet specimens (0.8× 10×50 mm3) in the Nd5Y4Fe68Zr2B21 alloy. The magnetic
⁎ Correspondence to: Tianyu Ma, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China. ⁎⁎ Correspondence to: Mi Yan, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China. Tel.: + 86 571 87952366; fax: + 86 571 87952366. E-mail addresses: [email protected] (T.Y. Ma), [email protected] (M. Yan). 0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2012.01.041
properties obtained in these magnetic sheets were iHc =382 kA/m and (BH)max =42.9 kJ/m3 after optimally annealing. These investigations demonstrate that Y plays an important role to enhance the GFA in Nd– Fe–B based alloys. For engineering and technical applications, however, the magnetic properties of these alloy systems still need to be improved. In the present work, attempts on Nd5Fe68 − xB23Mo4Yx (x=0, 2, 4, 6) alloy system were performed to improve both GFA and magnetic properties. A large size nanocomposite Nd5Fe64B23Mo4Y4 magnet rod with 2 mm in diameter was developed by copper mold casting and subsequent annealing, in which sound magnetic properties with 3 iHc =764.2 kA/m, Br =0.6 T and (BH)max =57.3 kJ/m were achieved. 2. Experimental The Nd5Fe68 − xB23Mo4Yx (x = 0, 2, 4, 6 at.%) ingots were prepared by arc melting the mixture of pure metals Nd (99.9 wt.%), Fe (99.8 wt.%), Mo (99.9 wt.%), Y (99.9 wt.%) and Fe–B alloy (79.58 wt.% Fe and 20.42 wt.% B) in an argon atmosphere. Cast ingots were re-melted for five times to ensure the composition homogeneity. Bulk rods with 1–3 mm in diameter and 50 mm in length were obtained by injection casting the molten alloy into copper molds. Specimens cut from the rods were sealed in a quartz tube evacuated to 1 × 10− 3 Pa, and then isothermally annealed for 10 min at temperatures ranging from 963 to1063 K. The crystal structure of the specimens was studied by means of X-ray powder diffraction method (XRD, Rigaku D/max 2550Pc) using a Cu Kα radiation with 2θ scan range of 20–80°. Transmission electron microscopy (TEM) studies and selected-area electron diffraction (SAED) pattern were carried out with a JEOL JEM 2010 TEM to observe the microstructures. Thermal stability and crystallization behaviors of
S. Tao et al. / Journal of Non-Crystalline Solids 358 (2012) 1028–1031
the as-cast alloys were examined by differential scanning calorimetry (DSC, NETZSCH DSC 404 C) using a continuous argon flow at a heating rate of 0.33 K/s. Magnetic properties were measured at room temperature by using a MPMS-XL-5 superconducting quantum interference device (SQUID) magnetometer with a maximum applied field of 5 T. The magnet density was determined by the Archimedes principle. 3. Results Fig. 1(a) shows the XRD patterns of the transverse cross sections for the as-cast Nd5Fe68 − xB23Mo4Yx (x= 0, 2, 4, 6) rods with 2 and 3 mm in diameter, respectively. For the 2 mm-diameter rods, a mixed structure consisting of an amorphous matrix together with some crystalline phases is presented in the Y-free Nd5Fe68B23Mo4 alloy. It indicates that this alloy can not be fully amorphized due to the limited GFA, which agrees well with the results of Wu et al. [17]. Nevertheless, the Nd5Fe66B23Mo4Y2 and Nd5Fe64B23Mo4Y4 alloys can be transformed into fully amorphous state because no Bragg peak, except a broad hump, is detected in the corresponding XRD patterns. This implies that proper Y addition enhances the GFA of the Nd5Fe68 − xB23Mo4Yx alloy system. While, with further increasing Y content to 6 at.%, some crystalline peaks appear besides the amorphous phase matrix. It implies that adding more Y element is not beneficial for improving the GFA of Nd5Fe68 − xB23Mo4Yx alloy system. We have also prepared 3 mmdiameter rods for the Nd5Fe66B23Mo4Y2 and Nd5Fe64B23Mo4Y4 alloys, whose XRD patterns reflect some crystalline peaks in the amorphous matrix. But, there is still a large amount of amorphous phase remaining in the Nd5Fe64B23Mo4Y4 3 mm-diameter rod. Fig. 1(b) shows the outer morphology of the Nd5Fe64B23Mo4Y4 rod with 2 mm in diameter and 45 mm in length. The sample reflects a smooth surface and metallic luster free of surface degradation, which is typical of a glassy alloy casting. Fig. 1(c) shows TEM image for the as cast Nd5Fe64B23Mo4Y4 2 mmdiameter rod, which depicts no contrast over the whole image (revealing no precipitation of the crystalline phases). The corresponding SAED pattern shows halo rings, which is a typical characteristic of an amorphous state. This is consistent with the XRD results, demonstrating that the as cast 2 mm-diameter Nd5Fe64B23Mo4Y4 rod is fully amorphous. Therefore, it reveals that the addition of 4 at.% Y is most favorable to improve the GFA of Nd5Fe68 − xB23Mo4Yx alloys. Fig. 2 illustrates DSC curves for the as-cast Nd5Fe68 − xB23Mo4Yx (x = 0, 2, 4, 6) rods with 2 mm in diameter. Each curve exhibits a
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distinct glass transition behavior followed by a wide supercooled liquid region before crystallization. The glass transition temperature Tg and the onset crystallization temperature Tx (marked by arrows, respectively) were determined from these curves and summarized in Table 1. The supercooled liquid region ΔTx ( = Tx − Tg) was also listed in Table 1. It should be noted that Tg and Tx were calculated with three attempts from the DSC curves to ensure the reliability in the datas. All these thermal parameter values were within a standard deviation of ±5 °C. It is evident that Tg decreases from 903 to 880 K with the increase of Y content to 2 at.% but increases to 929 K for the alloy with Y content of 6 at.%. Tx increases from 939 to 982 K with the increase of Y content from 0 to 6 at.%. While, ΔTx reaches to the maximum value of 92 K for x = 4 alloy, which suggests that the Nd5Fe64B23Mo4Y4 alloy has the highest GFA. In addition, the maximum peak intensity of the first crystallization peak for Nd5Fe64B23Mo4Y4 is the largest among Nd5Fe68 − xB23Mo4Yx (x = 0, 2, 4, 6) system, which could lead to a single-stage crystallization behavior as the second peak is very weak in the DSC curve. It suggests that all the crystalline phases could precipitate simultaneously at the same temperature, which might result in a higher resistance against the crystallization and subsequently improve the stability of the supercooled liquid region [19]. Based on the DSC and XRD studies, the Nd5Fe64B23Mo4Y4 alloy has the best GFA, which enables us to fabricate 2 mm-diameter bulk glassy rods. Aiming to produce large size bulk nanocomposite magnets with improved magnetic properties, the as-cast Nd5Fe64B23Mo4Y4 alloy was annealed at 963, 1013 and 1063 K for 10 min, respectively. Fig. 3 illustrates the magnetization hysteresis loops for Nd5Fe64B23Mo4Y4 samples in the as-cast state and after annealing for 10 min at 963, 1013 and 1063 K, respectively. The as-cast sample and the samples annealed at 963 K show typically soft magnetic characteristics with coercivity below 0.006 kA/m. But the samples exhibit hard magnetic behaviors after annealing at 1013 or 1063 K. Magnetic properties of Br = 0.6 T, iHc = 764.2 kA/m and (BH)max = 57.3 kJ/m 3 are obtained for the sample annealed at 1013 K. However, annealing at the higher temperature (1063 K), the alloy shows inferior magnetic properties, i.e. Br = 0.44 T, iHc = 351.8 kA/m and (BH)max = 20.6 kJ/m 3 presumably due to undesirable grain growth of the magnetic phases at higher annealing temperature. The XRD patterns of the Nd5Fe64B23Mo4Y4 rods annealed for 10 min at different temperatures are shown in Fig. 4. The sample annealed at 963 K contains major amorphous phase with minor
Fig. 1. XRD patterns for Nd5Fe68 − xB23Mo4Yx (x = 0, 2, 4, 6) as-cast rods with 2 and 3 mm in diameter (a), outer morphology (b) and TEM image (c) for the as-cast Nd5Fe64B23Mo4Y4 rod with 2 mm in diameter.
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S. Tao et al. / Journal of Non-Crystalline Solids 358 (2012) 1028–1031
Fig. 3. Magnetization hysteresis loops for the Nd5Fe64B23Mo4Y4 alloys annealed for 10 min at various temperatures.
4. Discussion
Fig. 2. DSC curves of as-cast Nd5Fe68 − xB23Mo4Yx (x = 0, 2, 4, 6) rods with 2 mm in diameter.
crystalline precipitates. When annealed at 1013 K, peaks for soft α-Fe, Fe3B and hard Nd2Fe14B phases as well as nonmagnetic Nd1.1Fe4B4 phase can be identified. The good hard magnetic properties obtained in the Nd5Fe64B23Mo4Y4 alloy annealed at 1013 K are probably due to the strong exchange interaction between magnetically soft and hard phases. With further increasing the annealing temperature to 1063 K, diffraction peaks for the same crystalline phases are observed. However, the peak intensities are increased compared to the one annealed at 1013 K, indicating that the volume fraction of each phase was slightly changed with the increasing annealing temperature. These results are consistent with Tan's work [18]. It reveals that optimizing the annealing treatment is essential to improve the magnetic properties of the magnets. The Nd5Fe64B23Mo4Y4 magnet annealed at 1013 K has the optimum magnetic properties. Hence, it was selected as a representative sample for the study of the microstructure by TEM. Fig. 5(a) shows the bright field image and the SAED pattern of the annealed Nd5Fe64B23Mo4Y4 sample at 1013 K for 10 min. It reflects a multiphase structure and the SAED pattern confirms the existence of crystalline magnetic phases. Fig. 5(b) shows a high magnification image of the magnetic phases. The energy dispersive spectroscopy (EDS) and X-ray diffraction studies revealed that the optimally annealed Nd5Fe64B23Mo4Y4 microstructure is mainly composed of magnetically soft α-Fe, Fe3B and hard Nd2Fe14B phases. The nonmagnetic Nd1.1Fe4B4 phase is difficult to determine due to its small amount and overlapping by other phases. The average grain sizes were 25–30 nm and 30–35 nm for the magnetically soft α-Fe and Fe3B phases, around 40–50 nm for the magnetically hard Nd2Fe14B phase, respectively. In addition, these nanoscale small grains adjoin with each other densely, which is beneficial to promote the exchange coupling between magnetically soft and hard phases.
In this work, the GFA of the Nd5Fe68B23Mo4 alloy was improved with an addition of 4 at.% Y element, which enables us to fabricate fully amorphous Nd5Fe64B23Mo4Y4 rods with 2 mm in diameter. The improvement in GFA of Nd–Fe–B–Mo–Y alloy system can be understood from the empirical rules proposed by Inoue et al. [20]. The atomic radii among the alloy components are 0.182 nm for Nd, 0.180 nm for Y, 0.136 nm for Mo, 0.124 nm for Fe and 0.09 nm for B, which decreases in the order of Nd > Y > Mo > Fe > B. These large atomic size mismatches among the alloy components are effective to increase the high packing density of the local structure of the alloy, which, in turn, improves the GFA. The second reason for the good GFA is the presence of large negative enthalpies of mixing among the atomic pairs with Y addition. The heat of mixing value for Y–B atomic pair is −50 kJ/mol, which is more negative than −26 kJ/mol for Fe–B atomic pair [21]. Such large negative heats of mixing for the atomic pairs ensure strong attractive interactions among the alloy constituent elements and lead to the formation of atomic network structure (known as backbone structure, which is responsible for the enhancement of GFA). Furthermore, it is reported that [22,23] Y element has an oxygen scavenging effect through the formation of yttrium oxide during the arc-melting and copper mold
Table 1 Glass transition temperature (Tg), onset crystallization temperature (Tx), and supercooled liquid region (ΔTx) for the as-cast Nd5Fe68 − xB23Mo4Yx (x = 0, 2, 4, 6) rods with 2 mm in diameter. composition
Tg (K) ± 5 K
Tx (K) ± 5 K
ΔTx (K)
Nd5Fe68B23Mo4 Nd5Fe66B23Mo4Y2 Nd5Fe64B23Mo4Y4 Nd5Fe62B23Mo4Y6
903 880 887 929
939 953 979 982
36 73 92 53
Fig. 4. XRD patterns for the Nd5Fe64B23Mo4Y4 alloys annealed for 10 min at various temperatures.
S. Tao et al. / Journal of Non-Crystalline Solids 358 (2012) 1028–1031
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Fig. 5. TEM bright field images for the 2 mm-diameter Nd5Fe64B23Mo4Y4 rod after annealing at 1013 K. Morphology of phase grains (a) and high magnification features of magnetic phase grains (b).
casting processes, which might suppress the formation of the crystalline phase by eliminating the oxygen nucleation sites and stabilizing the glass matrix phase. However, an excessive addition of Y (e.g. Y = 6 at.%) deteriorates the GFA of the alloy system because of the precipitation of primary Fe4Y crystalline phases. The magnetic properties of the Nd5Fe64B23Mo4Y4 alloy are found to be sensitive to the annealing process. For an example, the alloy shows low magnetic properties after annealing at 963 K due to nonoptimal precipitation of soft and hard magnetic phases from the matrix. This can be contributed that either Nd2Fe14B or α-Fe/Fe3B phase crystals do not grow fully due to incomplete crystallization. The enhanced magnetic properties with coercivity of 764.2 kA/m, remanence of 0.6 T and maximum energy product of 57.3 kJ/m 3 in the annealed Nd5Fe64B23Mo4Y4 alloy at 1013 K can be related to the desirable microstructure. The annealed microstructure contains nanoscale grains of soft a-Fe, Fe3B and hard Nd2Fe14B phases with a homogenous distribution. The microstructural studies also reveal that the soft magnetic grains are well contacted with the neighboring hard magnetic grains. Thus, the phase grain coherency, finer grain size as well as uniform grain distributions in the microstructure might lead to the strong exchange interactions between magnetically soft a-Fe, Fe3B and hard Nd2Fe14B grains, which, in turn, improve the magnetic properties of the Nd5Fe64B23Mo4Y4 alloy [24,25]. Thus, when such an appropriate balance (in terms of volume fraction and grain size) between soft and hard magnetic phases appeared, the magnetic properties of the alloy can be enhanced. Annealing above 1013 K turns ideal phase structure to non-ideal, which, in turn degrades the magnetic properties of the Nd5Fe64B23Mo4Y4 alloy. The magnet size and the coercivity achieved in the present study reach to 2 mm and 764.2 kA/m for the Nd5Fe64B23Mo4Y4 rod, which is larger than the reported magnetic rod size (1.5 mm) and coercivity (364.1 kA/m) of the Nd3Y3Fe68Mo4B22 alloy [17]. It suggests that the magnetic properties as well as the magnet size can be improved by adjusting the alloy composition as well as by optimizing the annealing process. In comparison with the Y-added Nd5Y4Fe68Zr2B21 magnetic sheets (0.8 × 10 × 50 mm 3) [18], the present Nd5Fe64B23Mo4Y4 alloy also exhibits improved magnetic properties. The magnetic properties of the Nd5Fe64B23Mo4Y4 alloy can be further improved by suppressing the unwanted Nd1.1Fe4B4 phase as well as by adjusting the constituent element contents [15]. 5. Conclusions Nanocomposite Nd5Fe64B23Mo4Y4 magnet rods with 2 mm in diameter were developed by annealing the amorphous precursors produced by copper mold casting. The improved GFA in this alloy system is attributed to the large atomic-size mismatch and the negative heat
of mixing among the constituent elements. Optimal magnetic properties with Br of 0.6 T, iHc of 764.2 kA/m and (BH)max of 57.3 kJ/m 3 were obtained by annealing the Nd5Fe64B23Mo4Y4 amorphous alloys for 10 min at 1013 K. The annealed microstructure consists of nanoscale magnetically soft and hard phase grains leading to strong exchange coupling in the magnet. The enhanced hard magnetic properties along with macroscopically large size can make the Nd5Fe64B23Mo4Y4 magnet a promising candidate for prominent applications.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 50971113 and 51171169), the 863 project of China (Grant No. 2010AA03A402) and the Foundation for the Author of National Excellent Doctoral Dissertation of PR China (FANEDD, Grant No. 201037).
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Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Nd5Fe64B23Mo4Y4 bulk nanocomposite permanent magnets produced by crystallizing amorphous precursors Shan Tao a, c, Tianyu Ma a,⁎, Zubair Ahmad b, Hong Jian a, Mi Yan b,⁎⁎ a b c
Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China Hangzhou Vocational & Technical College, Hangzhou 310018, China
a r t i c l e
i n f o
Article history: Received 12 October 2011 Received in revised form 17 January 2012 Available online 14 February 2012 Keywords: Nanocomposite; Magnetic property; Glass forming ability
a b s t r a c t The glass forming ability and magnetic properties of Nd5Fe68 − xB23Mo4Yx (x = 0, 2, 4, 6) alloys prepared by copper mold casting technique have been studied. Amorphous rods with a diameter of 2 mm were obtained in the Nd5Fe64B23Mo4Y4 alloy. After annealing for 10 min at 1013 K, the Nd5Fe64B23Mo4Y4 alloy showed optimal hard magnetic properties with the coercivity of 764.2 kA/m, remanence of 0.6 T and maximum energy product of 57.3 kJ/m3, respectively. The enhanced magnetic properties can be ascribed to the strong exchange coupling among the magnetically soft α-Fe (25–30 nm), Fe3B (30–35 nm) and hard Nd2Fe14B (40–50 nm) grains present in the magnet microstructure. Large size bulk nanocomposite magnets with sound magnetic properties make the Nd–Fe–B–Mo–Y alloy system a promising candidate for industrial applications. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Nanocomposite permanent magnets have been widely investigated for their unique microstructure and promising magnetic properties [1–4]. Recently, the copper mold casting method has been adopted to develop bulk nanocomposite permanent magnets either through direct casting or annealing the amorphous precursors [5–7]. This method provides an economical and simple approach to produce miniature nanocomposite permanent magnets [8]. Currently, one of the main challenges of this approach is to enhance the magnetic properties as well as to enlarge the magnet size. To achieve large size bulk amorphous precursors, a sufficiently high glass forming ability (GFA) of the alloy is required. Much efforts have been carried out to enhance the GFA as well as magnetic properties through doping with refractory elements such as Co, Cu, Nb, Ti, Zr, V and Y to R3–9.5FebalB6–30 (R_Nd or Pr) alloys [9–16]. Among them, Y has been deemed as one of the most effective elements in improving GFA, stabilizing the amorphous phase and reducing the oxides during the synthesis of R–Fe–M–B alloys. Wu et al. [17] presented that the critical diameter of the Nd1Y5Fe68Mo4B22 metallic glassy rod can reach 4 mm. They also achieved a maximal coercivity of 364.1 kA/m from the 1.5 mm-diameter Nd3Y3Fe68Mo4B22 rod after subsequent crystallization. Tan et al. [18] have developed bulk sheet specimens (0.8× 10×50 mm3) in the Nd5Y4Fe68Zr2B21 alloy. The magnetic
⁎ Correspondence to: Tianyu Ma, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China. ⁎⁎ Correspondence to: Mi Yan, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China. Tel.: + 86 571 87952366; fax: + 86 571 87952366. E-mail addresses: [email protected] (T.Y. Ma), [email protected] (M. Yan). 0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2012.01.041
properties obtained in these magnetic sheets were iHc =382 kA/m and (BH)max =42.9 kJ/m3 after optimally annealing. These investigations demonstrate that Y plays an important role to enhance the GFA in Nd– Fe–B based alloys. For engineering and technical applications, however, the magnetic properties of these alloy systems still need to be improved. In the present work, attempts on Nd5Fe68 − xB23Mo4Yx (x=0, 2, 4, 6) alloy system were performed to improve both GFA and magnetic properties. A large size nanocomposite Nd5Fe64B23Mo4Y4 magnet rod with 2 mm in diameter was developed by copper mold casting and subsequent annealing, in which sound magnetic properties with 3 iHc =764.2 kA/m, Br =0.6 T and (BH)max =57.3 kJ/m were achieved. 2. Experimental The Nd5Fe68 − xB23Mo4Yx (x = 0, 2, 4, 6 at.%) ingots were prepared by arc melting the mixture of pure metals Nd (99.9 wt.%), Fe (99.8 wt.%), Mo (99.9 wt.%), Y (99.9 wt.%) and Fe–B alloy (79.58 wt.% Fe and 20.42 wt.% B) in an argon atmosphere. Cast ingots were re-melted for five times to ensure the composition homogeneity. Bulk rods with 1–3 mm in diameter and 50 mm in length were obtained by injection casting the molten alloy into copper molds. Specimens cut from the rods were sealed in a quartz tube evacuated to 1 × 10− 3 Pa, and then isothermally annealed for 10 min at temperatures ranging from 963 to1063 K. The crystal structure of the specimens was studied by means of X-ray powder diffraction method (XRD, Rigaku D/max 2550Pc) using a Cu Kα radiation with 2θ scan range of 20–80°. Transmission electron microscopy (TEM) studies and selected-area electron diffraction (SAED) pattern were carried out with a JEOL JEM 2010 TEM to observe the microstructures. Thermal stability and crystallization behaviors of
S. Tao et al. / Journal of Non-Crystalline Solids 358 (2012) 1028–1031
the as-cast alloys were examined by differential scanning calorimetry (DSC, NETZSCH DSC 404 C) using a continuous argon flow at a heating rate of 0.33 K/s. Magnetic properties were measured at room temperature by using a MPMS-XL-5 superconducting quantum interference device (SQUID) magnetometer with a maximum applied field of 5 T. The magnet density was determined by the Archimedes principle. 3. Results Fig. 1(a) shows the XRD patterns of the transverse cross sections for the as-cast Nd5Fe68 − xB23Mo4Yx (x= 0, 2, 4, 6) rods with 2 and 3 mm in diameter, respectively. For the 2 mm-diameter rods, a mixed structure consisting of an amorphous matrix together with some crystalline phases is presented in the Y-free Nd5Fe68B23Mo4 alloy. It indicates that this alloy can not be fully amorphized due to the limited GFA, which agrees well with the results of Wu et al. [17]. Nevertheless, the Nd5Fe66B23Mo4Y2 and Nd5Fe64B23Mo4Y4 alloys can be transformed into fully amorphous state because no Bragg peak, except a broad hump, is detected in the corresponding XRD patterns. This implies that proper Y addition enhances the GFA of the Nd5Fe68 − xB23Mo4Yx alloy system. While, with further increasing Y content to 6 at.%, some crystalline peaks appear besides the amorphous phase matrix. It implies that adding more Y element is not beneficial for improving the GFA of Nd5Fe68 − xB23Mo4Yx alloy system. We have also prepared 3 mmdiameter rods for the Nd5Fe66B23Mo4Y2 and Nd5Fe64B23Mo4Y4 alloys, whose XRD patterns reflect some crystalline peaks in the amorphous matrix. But, there is still a large amount of amorphous phase remaining in the Nd5Fe64B23Mo4Y4 3 mm-diameter rod. Fig. 1(b) shows the outer morphology of the Nd5Fe64B23Mo4Y4 rod with 2 mm in diameter and 45 mm in length. The sample reflects a smooth surface and metallic luster free of surface degradation, which is typical of a glassy alloy casting. Fig. 1(c) shows TEM image for the as cast Nd5Fe64B23Mo4Y4 2 mmdiameter rod, which depicts no contrast over the whole image (revealing no precipitation of the crystalline phases). The corresponding SAED pattern shows halo rings, which is a typical characteristic of an amorphous state. This is consistent with the XRD results, demonstrating that the as cast 2 mm-diameter Nd5Fe64B23Mo4Y4 rod is fully amorphous. Therefore, it reveals that the addition of 4 at.% Y is most favorable to improve the GFA of Nd5Fe68 − xB23Mo4Yx alloys. Fig. 2 illustrates DSC curves for the as-cast Nd5Fe68 − xB23Mo4Yx (x = 0, 2, 4, 6) rods with 2 mm in diameter. Each curve exhibits a
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distinct glass transition behavior followed by a wide supercooled liquid region before crystallization. The glass transition temperature Tg and the onset crystallization temperature Tx (marked by arrows, respectively) were determined from these curves and summarized in Table 1. The supercooled liquid region ΔTx ( = Tx − Tg) was also listed in Table 1. It should be noted that Tg and Tx were calculated with three attempts from the DSC curves to ensure the reliability in the datas. All these thermal parameter values were within a standard deviation of ±5 °C. It is evident that Tg decreases from 903 to 880 K with the increase of Y content to 2 at.% but increases to 929 K for the alloy with Y content of 6 at.%. Tx increases from 939 to 982 K with the increase of Y content from 0 to 6 at.%. While, ΔTx reaches to the maximum value of 92 K for x = 4 alloy, which suggests that the Nd5Fe64B23Mo4Y4 alloy has the highest GFA. In addition, the maximum peak intensity of the first crystallization peak for Nd5Fe64B23Mo4Y4 is the largest among Nd5Fe68 − xB23Mo4Yx (x = 0, 2, 4, 6) system, which could lead to a single-stage crystallization behavior as the second peak is very weak in the DSC curve. It suggests that all the crystalline phases could precipitate simultaneously at the same temperature, which might result in a higher resistance against the crystallization and subsequently improve the stability of the supercooled liquid region [19]. Based on the DSC and XRD studies, the Nd5Fe64B23Mo4Y4 alloy has the best GFA, which enables us to fabricate 2 mm-diameter bulk glassy rods. Aiming to produce large size bulk nanocomposite magnets with improved magnetic properties, the as-cast Nd5Fe64B23Mo4Y4 alloy was annealed at 963, 1013 and 1063 K for 10 min, respectively. Fig. 3 illustrates the magnetization hysteresis loops for Nd5Fe64B23Mo4Y4 samples in the as-cast state and after annealing for 10 min at 963, 1013 and 1063 K, respectively. The as-cast sample and the samples annealed at 963 K show typically soft magnetic characteristics with coercivity below 0.006 kA/m. But the samples exhibit hard magnetic behaviors after annealing at 1013 or 1063 K. Magnetic properties of Br = 0.6 T, iHc = 764.2 kA/m and (BH)max = 57.3 kJ/m 3 are obtained for the sample annealed at 1013 K. However, annealing at the higher temperature (1063 K), the alloy shows inferior magnetic properties, i.e. Br = 0.44 T, iHc = 351.8 kA/m and (BH)max = 20.6 kJ/m 3 presumably due to undesirable grain growth of the magnetic phases at higher annealing temperature. The XRD patterns of the Nd5Fe64B23Mo4Y4 rods annealed for 10 min at different temperatures are shown in Fig. 4. The sample annealed at 963 K contains major amorphous phase with minor
Fig. 1. XRD patterns for Nd5Fe68 − xB23Mo4Yx (x = 0, 2, 4, 6) as-cast rods with 2 and 3 mm in diameter (a), outer morphology (b) and TEM image (c) for the as-cast Nd5Fe64B23Mo4Y4 rod with 2 mm in diameter.
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Fig. 3. Magnetization hysteresis loops for the Nd5Fe64B23Mo4Y4 alloys annealed for 10 min at various temperatures.
4. Discussion
Fig. 2. DSC curves of as-cast Nd5Fe68 − xB23Mo4Yx (x = 0, 2, 4, 6) rods with 2 mm in diameter.
crystalline precipitates. When annealed at 1013 K, peaks for soft α-Fe, Fe3B and hard Nd2Fe14B phases as well as nonmagnetic Nd1.1Fe4B4 phase can be identified. The good hard magnetic properties obtained in the Nd5Fe64B23Mo4Y4 alloy annealed at 1013 K are probably due to the strong exchange interaction between magnetically soft and hard phases. With further increasing the annealing temperature to 1063 K, diffraction peaks for the same crystalline phases are observed. However, the peak intensities are increased compared to the one annealed at 1013 K, indicating that the volume fraction of each phase was slightly changed with the increasing annealing temperature. These results are consistent with Tan's work [18]. It reveals that optimizing the annealing treatment is essential to improve the magnetic properties of the magnets. The Nd5Fe64B23Mo4Y4 magnet annealed at 1013 K has the optimum magnetic properties. Hence, it was selected as a representative sample for the study of the microstructure by TEM. Fig. 5(a) shows the bright field image and the SAED pattern of the annealed Nd5Fe64B23Mo4Y4 sample at 1013 K for 10 min. It reflects a multiphase structure and the SAED pattern confirms the existence of crystalline magnetic phases. Fig. 5(b) shows a high magnification image of the magnetic phases. The energy dispersive spectroscopy (EDS) and X-ray diffraction studies revealed that the optimally annealed Nd5Fe64B23Mo4Y4 microstructure is mainly composed of magnetically soft α-Fe, Fe3B and hard Nd2Fe14B phases. The nonmagnetic Nd1.1Fe4B4 phase is difficult to determine due to its small amount and overlapping by other phases. The average grain sizes were 25–30 nm and 30–35 nm for the magnetically soft α-Fe and Fe3B phases, around 40–50 nm for the magnetically hard Nd2Fe14B phase, respectively. In addition, these nanoscale small grains adjoin with each other densely, which is beneficial to promote the exchange coupling between magnetically soft and hard phases.
In this work, the GFA of the Nd5Fe68B23Mo4 alloy was improved with an addition of 4 at.% Y element, which enables us to fabricate fully amorphous Nd5Fe64B23Mo4Y4 rods with 2 mm in diameter. The improvement in GFA of Nd–Fe–B–Mo–Y alloy system can be understood from the empirical rules proposed by Inoue et al. [20]. The atomic radii among the alloy components are 0.182 nm for Nd, 0.180 nm for Y, 0.136 nm for Mo, 0.124 nm for Fe and 0.09 nm for B, which decreases in the order of Nd > Y > Mo > Fe > B. These large atomic size mismatches among the alloy components are effective to increase the high packing density of the local structure of the alloy, which, in turn, improves the GFA. The second reason for the good GFA is the presence of large negative enthalpies of mixing among the atomic pairs with Y addition. The heat of mixing value for Y–B atomic pair is −50 kJ/mol, which is more negative than −26 kJ/mol for Fe–B atomic pair [21]. Such large negative heats of mixing for the atomic pairs ensure strong attractive interactions among the alloy constituent elements and lead to the formation of atomic network structure (known as backbone structure, which is responsible for the enhancement of GFA). Furthermore, it is reported that [22,23] Y element has an oxygen scavenging effect through the formation of yttrium oxide during the arc-melting and copper mold
Table 1 Glass transition temperature (Tg), onset crystallization temperature (Tx), and supercooled liquid region (ΔTx) for the as-cast Nd5Fe68 − xB23Mo4Yx (x = 0, 2, 4, 6) rods with 2 mm in diameter. composition
Tg (K) ± 5 K
Tx (K) ± 5 K
ΔTx (K)
Nd5Fe68B23Mo4 Nd5Fe66B23Mo4Y2 Nd5Fe64B23Mo4Y4 Nd5Fe62B23Mo4Y6
903 880 887 929
939 953 979 982
36 73 92 53
Fig. 4. XRD patterns for the Nd5Fe64B23Mo4Y4 alloys annealed for 10 min at various temperatures.
S. Tao et al. / Journal of Non-Crystalline Solids 358 (2012) 1028–1031
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Fig. 5. TEM bright field images for the 2 mm-diameter Nd5Fe64B23Mo4Y4 rod after annealing at 1013 K. Morphology of phase grains (a) and high magnification features of magnetic phase grains (b).
casting processes, which might suppress the formation of the crystalline phase by eliminating the oxygen nucleation sites and stabilizing the glass matrix phase. However, an excessive addition of Y (e.g. Y = 6 at.%) deteriorates the GFA of the alloy system because of the precipitation of primary Fe4Y crystalline phases. The magnetic properties of the Nd5Fe64B23Mo4Y4 alloy are found to be sensitive to the annealing process. For an example, the alloy shows low magnetic properties after annealing at 963 K due to nonoptimal precipitation of soft and hard magnetic phases from the matrix. This can be contributed that either Nd2Fe14B or α-Fe/Fe3B phase crystals do not grow fully due to incomplete crystallization. The enhanced magnetic properties with coercivity of 764.2 kA/m, remanence of 0.6 T and maximum energy product of 57.3 kJ/m 3 in the annealed Nd5Fe64B23Mo4Y4 alloy at 1013 K can be related to the desirable microstructure. The annealed microstructure contains nanoscale grains of soft a-Fe, Fe3B and hard Nd2Fe14B phases with a homogenous distribution. The microstructural studies also reveal that the soft magnetic grains are well contacted with the neighboring hard magnetic grains. Thus, the phase grain coherency, finer grain size as well as uniform grain distributions in the microstructure might lead to the strong exchange interactions between magnetically soft a-Fe, Fe3B and hard Nd2Fe14B grains, which, in turn, improve the magnetic properties of the Nd5Fe64B23Mo4Y4 alloy [24,25]. Thus, when such an appropriate balance (in terms of volume fraction and grain size) between soft and hard magnetic phases appeared, the magnetic properties of the alloy can be enhanced. Annealing above 1013 K turns ideal phase structure to non-ideal, which, in turn degrades the magnetic properties of the Nd5Fe64B23Mo4Y4 alloy. The magnet size and the coercivity achieved in the present study reach to 2 mm and 764.2 kA/m for the Nd5Fe64B23Mo4Y4 rod, which is larger than the reported magnetic rod size (1.5 mm) and coercivity (364.1 kA/m) of the Nd3Y3Fe68Mo4B22 alloy [17]. It suggests that the magnetic properties as well as the magnet size can be improved by adjusting the alloy composition as well as by optimizing the annealing process. In comparison with the Y-added Nd5Y4Fe68Zr2B21 magnetic sheets (0.8 × 10 × 50 mm 3) [18], the present Nd5Fe64B23Mo4Y4 alloy also exhibits improved magnetic properties. The magnetic properties of the Nd5Fe64B23Mo4Y4 alloy can be further improved by suppressing the unwanted Nd1.1Fe4B4 phase as well as by adjusting the constituent element contents [15]. 5. Conclusions Nanocomposite Nd5Fe64B23Mo4Y4 magnet rods with 2 mm in diameter were developed by annealing the amorphous precursors produced by copper mold casting. The improved GFA in this alloy system is attributed to the large atomic-size mismatch and the negative heat
of mixing among the constituent elements. Optimal magnetic properties with Br of 0.6 T, iHc of 764.2 kA/m and (BH)max of 57.3 kJ/m 3 were obtained by annealing the Nd5Fe64B23Mo4Y4 amorphous alloys for 10 min at 1013 K. The annealed microstructure consists of nanoscale magnetically soft and hard phase grains leading to strong exchange coupling in the magnet. The enhanced hard magnetic properties along with macroscopically large size can make the Nd5Fe64B23Mo4Y4 magnet a promising candidate for prominent applications.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 50971113 and 51171169), the 863 project of China (Grant No. 2010AA03A402) and the Foundation for the Author of National Excellent Doctoral Dissertation of PR China (FANEDD, Grant No. 201037).
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