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Effects of W substitution on the magnetic properties, phase evolution and microstructure of rapidly quenched Co80Zr18B2 alloy
Journal of Magnetism and Magnetic Materials 368 (2014) 116–120
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Effects of W substitution on the magnetic properties, phase evolution and microstructure of rapidly quenched Co80Zr18B2 alloy Zhipeng Hou a, Chongli Yang b, Shifeng Xu c, Jinbao Zhang a, Dan Liu a, Feng Su a, Wenquan Wang a,n a
College of Physics, Jilin University, Changchun 130023, People's Republic of China College of Physics and Information Engineering, Hebei Normal University, Shijiazhuang 050024, People's Republic of China c College of Science, Shenyang Aerospace University, Shenyang 110136, People's Republic of China b
art ic l e i nf o
a b s t r a c t
Article history: Received 11 June 2013 Received in revised form 21 April 2014 Available online 24 May 2014
The effects of a partial substitution of W for Zr on the magnetic properties, phase evolution, and microstructure of Co80Zr18 xWxB2 (x ¼ 0, 1, 2 and 3) melt-spun ribbons are investigated in this paper. For the as-spun samples, the optimal magnetic properties of iHc ¼6.6 kOe and (BH)max ¼4.5 MGOe are achieved in the Co80Zr16W2B2. Thermomagnetic analysis (TMA) and x-ray diffraction (XRD) results suggest that the as-spun Co80Zr16W2B2 mainly comprises of hard magnetic phase. Scanning electron microscope (SEM) results indicate that W addition refines the microstructure, which results in an increase in the intergrain exchange interaction. The effects of heat treatment on the magnetic properties of as-spun Co80Zr18 xWxB2 (x ¼ 0, 1, 2 and 3) are also studied. The maximum iHc of 7.3 kOe with a (BH)max of 3.8 MGOe is obtained in the Co80Zr16W2B2 after a heat treatment at 550 1C. The origin of coercivity enhancement in Co80Zr16W2B2 is ascribed to the increase in magnetocrystalline anisotropy field (Ha) of hard magnetic phase. & 2014 Elsevier B.V. All rights reserved.
Keywords: Co-Zr-W-B alloys Melt-spun ribbons Heat treatment Coercivity
1. Introduction Last decades, the researches of new permanent magnetic materials are largely concentrated on the rare earth (RE) containing alloys [1–5]. Since the RE permanent magnets are subject to the limited RE supplies, it is of great significance to explore the RE-free hard magnetic materials which are possible for industrial applications [6,7]. Among the materials, the Co–Zr system alloy is one of the most promising candidates due to its strong magnetocrystalline anisotropy field (Ha), high Curie temperature (Tc) and excellent corrosion resistance [8]. However, the crystal structure of hard magnetic phase has not been established reliably until now [9–11]. Even the exact composition is under debate. Some authors reported that the Co11Zr2, which was also known as “Co5.1 Zr” or “CoxZr (xE5)”, was responsible for the magnetic hardness [9–14]. Ivanova et al. studied the structure of this compound and showed an orthorhombic unit cell with very long b-axis and c-axis (a¼0.471 nm, b¼1.670 nm, and c¼2.420 nm). Although the proposed orthorhombic lattice with large parameters can explain the observed XRD patterns, this does not mean the structure is really the orthorhombic. The authors pointed out that
n
Corresponding author. Tel./fax: þ 86 431 85167397. E-mail address: [email protected] (W. Wang).
http://dx.doi.org/10.1016/j.jmmm.2014.05.024 0304-8853/& 2014 Elsevier B.V. All rights reserved.
the hexagonal, rhombohedral, monoclinic and other structures could be also indexed in terms of an orthorhombic unit cell with large parameters and the actual unit cell should be refined after establishing the positions of atoms. Chang et al. investigated the magnetic properties of Co80Zr18 xMxB2 (M¼C, Cu, Ga, Al and Si; x¼0–2) melt-spun ribbons and found the existence of three phases, Co5Zr, Co23Zr6 and fcc-Co [15]. They proposed that the Co5Zr was the hard magnetic phase. It should be noted here that their results are not extraordinary and similar results are also reported by some other research groups [16–18]. Recently, most work has been focused on the investigation of magnetic properties of Co–Zr system alloy [15–21]. For instance, the Co80Zr18B2 ribbons prepared by melt spinning and subsequent heat treatment show excellent magnetic properties of the remanence sr ¼47.5 emu/g, coercivity iHc ¼4.4 kOe and maximum energy product (BH)max ¼4.7 MGOe [16]. Although the magnetic properties are low compare with those of Nd–Fe–B magnets, they are comparable to the hard ferrites [22]. In order to further improve the magnetic properties, component substitutions have been employed [15,19–22]. Cao et al. reported that a proper substitution of Si for Co improved iHc of Co80Zr18B2 significantly and a high iHc of above 6.5 kOe was achieved [22]. However, since the substitution of nonmagnetic elements for magnetic Co decreases the magnetization s, the Co–Zr–Si–B magnet exhibits
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a low (BH)max. It is known that the materials with only high iHc or high (BH)max are not very ideal for permanent magnet. This leads to an intense search for other elements which can enhance both (BH)max and iHc. W is in the VIB group elements and has an atom radius similar to Zr. It is expected that there are some positive effects of W substitution for Zr on the magnetic properties of Co80Zr18B2 alloy. In this paper, we report the results of our investigation on Co80Zr18 xWxB2 (x ¼0, 1, 2 and 3) melt-spun ribbons.
2. Experiment Co (99.9%), Zr (99.9%), W(99.9%), and Co-B alloy(Co 76.88%, B 21.72%) were melted by arc melting in an argon atmosphere of high purity for at least three times until they formed homogeneous buttons with composition Co80Zr18 xWxB2 (x¼ 0,1, 2 and 3). Then, small amounts of the alloy ingots were placed in a quartz crucible with an orifice 0.6 mm in diameter at the bottom. The Co80Zr18 xWxB2 (x ¼0, 1, 2 and 3) alloy ingots were melted in an argon atmosphere and then ejected through the orifice under argon pressure onto a copper wheel rotating at speed Vs ¼ 30 m/s. The resultant melt-spun ribbons were about 2 mm wide and 30–40 μm thick. Heat treatment for the melt-spun ribbons was performed at temperatures between 500 1C and 700 1C for 1 to 6 min using rapid thermal processing system. High temperatures were attained using four tungsten-halogen lamps. The heating rate was 10 1C/s and the temperature control precision was 71 1C. The temperature decreased from 700 1C to 400 1C in 40 s by natural cooling. The phases of the specimens were identified by x-ray diffraction (XRD) using Cu Kα radiation. Room temperature magnetization measurements were done by vibrating sample magnetometer (VSM) with an applied field up to 20 kOe. Measurements were conducted with the field along the length of thin ribbons, so the demagnetization effects were neglected in the analysis. We performed the thermomagnetic analysis (TMA) by VSM in a field of 500 Oe. Scanning electron microscope (SEM) was utilized to examine the microstructures of the specimens. The ribbons were cut off along thick direction and the typical SEM micrographs were taken in the cross-section.
Fig. 1. The XRD patterns of Co80Zr18 xWxB2 (x ¼0, 1, 2, and 3) melt-spun ribbons.
3. Results and discussion Fig. 1 shows the XRD patterns of Co80Zr18 xWxB2 (x¼0, 1, 2 and 3) melt-spun ribbons. The diffraction patterns are similar to those reported for Co–Zr based alloys and we mark the relatively significant reflections with numbers ranging from “1” to “9”. Since the crystal structure and the exact composition of hard magnetic phase have not been established reliably until now, there is no common sense on the notation of diffraction peaks [9,13,15,18–21]. Some authors believe that all the nine diffraction peaks are indexed to the hard magnetic phase [9,20,21]. However, other authors have different opinions and consider the “5” diffraction peak to be (111) reflection of fcc-Co [15,16,18]. No matter how the diffraction peaks are noted, they all believe that the melt-spun ribbons are mainly comprised of hard magnetic phase and the content of soft magnetic fcc-Co or Co23Zr6 phase is quite limited. It has been reported that Tc of hard magnetic phase is well established to be about 500 1C [9,10]. Therefore, TMA is employed to indentify the phase composition. In Fig. 2, we present the thermomagnetic curves of Co80Zr18 xWxB2 (x¼ 0, 1, 2 and 3) melt-spun ribbons. For all the four samples, only one magnetic transition which corresponds to the hard magnetic phase is found. This indicates that the small addition of W alters the phase composition little. However, it is interesting to notice that the value of Tc decreases monotonously with the increase of W content,
Fig. 2. The thermomagnetic curves of Co80Zr18 xWxB2 (x¼ 0, 1, 2 and 3) melt-spun ribbons.
strongly suggesting that the W atoms enter into the lattice of hard magnetic phase and result in the band structure change [10,23]. Fig. 3 shows sr, iHc and (BH)max of Co80Zr18 xWxB2 (x¼ 0, 1, 2 and 3) melt-spun ribbons. For x¼ 0, sr ¼38.5 emu/g, iHc ¼3.3 kOe, and (BH)max ¼ 3.5 MGOe are obtained. The introduction of W has a large influence on the magnetic properties. One can notice that all the three magnetic parameters increase first and then decrease with the increase of W content. The optimal magnetic properties of sr ¼ 41.8 emu/g, iHc ¼ 6.6 kOe, and (BH)max ¼ 4.5 MGOe are achieved in the sample with x¼ 2. Compared to the sample with x¼0, although sr is just improved by 8.6%, iHc and (BH)max are enhanced drastically by 100% and 28.6%.It has been reported that a high iHc (Z6.5) has been realized in the Co–Zr–Si–B magnet [22]. However, since the substitution of nonmagnetic elements for magnetic Co decreases the magnetization, the Co–Zr–Si–B magnet exhibits a low
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Fig. 5. Delta M plots of Co80Zr18B2 and Co80Zr16W2B2 melt-spun ribbons.
Fig. 3. The dependence of sr, iHc and (BH)max on the W content.
Fig. 4. The room temperature hysteresis loops of Co80Zr18 xWxB2 (x ¼0 and 2) melt-spun ribbons.
(BH)max. Impressively, we realize both the high iHc and (BH)max in the Co80Zr16W2B2 melt-spun ribbons. Fig. 4 displays the room temperature hysteresis loops of the asspun samples with x ¼0 and 2. It can be seen that both the demagnetization curves are smooth and exhibit the characteristic of single hard-phase behavior. Also, we can find that x ¼2 exhibits larger sr, iHc and squareness. Therefore, a larger (BH)max is obtained. The specific causes will be discussed below. It has been reported that the changes of sr and squareness are related to the intergrain exchange interaction between the grains [18,24]. In order to determine the strength of interaction, we show the Henkel plots δM of the as-spun samples with x ¼0 and 2 in Fig. 5. The expression of a Henkel plot is as follow: δM¼ [sd(H) – (sr -2sr(H))]/sr, where the remnant sr(H) is acquired after the application and the subsequent removal of a field H, sd(H) after dc saturation in one direction, and the subsequent application and the removal of a field H in the reverse direction [25]. A positive δM peak reflects the existence of intergrain exchange interaction and
the magnitude of maximum δM value represents the strength of the interaction [25]. As shown in Fig. 5, we find that a larger maximum δM is obtained in the sample with x ¼2. This suggests that the addition of W increases the strength of intergrain exchange interaction. The change tendency agrees well with that of sr and squareness. Based on the results, it is therefore concluded that a stronger intergrain exchange interaction results in a larger sr and squareness. According to the previous reports [19,20,24,25], the change of intergrain exchange interaction may be affected by the microstructure. To investigate the effects of W addition on the microstructure of Co80Zr18B2, the typical SEM micrographs of cross-section from the as-spun sample with (a) x¼ 0 and (b) x ¼2 are shown in Fig. 6. Fig. 6(a) reveals that x ¼0 consist of roughly spherical particles whose diameters range from 400 to 500 nm. Much smaller particles and more homogeneous microstructure with an average grain size of around 50–100 nm is obtained in x¼ 2, suggesting that the W addition results in a reduction in the grain size. Due to a smaller grain size, the ratio of the grain boundary area to the volume of grains increases. Therefore, the intergrain exchange interaction increases in x¼ 2. According to the previous reports [16,19,20], a suitable heat treatment has a beneficial effect on the magnetic properties improvement. In order to obtain higher magnetic properties, the Co80Zr18 xWxB2 (x ¼0, 1, 2 and 3) as-spun ribbons are annealed at temperatures ranging from 500 1C to 700 1C spaced 50 1C apart. Fig. 7 exhibits iHc and (BH)max as a function of annealing temperature. For all the four samples, heat treatment deteriorates rather than improves (BH)max. However, iHc is enhanced after a suitable heat treatment. The maximum iHc of 7.3 kOe is obtained in the sample with x¼ 2 after annealed at 550 1C. Fig. 8 displays the room temperature hysteresis loops of the samples with x ¼2 annealed at 550 1C, 600 1C, and 700 1C. For the sample annealed at 550 1C, although iHc, increases, both the sr and squareness are worsened and therefore the (BH)max decreases. In the case of the sample annealed at 600 1C, we can observe a shoulder at low fields, revealing that it is undergoing two magnetization reversal processes. Fig. 9 shows the XRD patterns of the sample with x ¼2 annealed at (a) 550 1C, (b) 600 1C and (c) 700 1C. As shown in Fig. 9(a), the XRD pattern is quite similar to that of the as-spun sample, revealing that the phase composition does not change after a heat treatment at 550 1C. For the sample annealed at 600 1C, some content of the soft magnetic Co23Zr6 and fcc-Co appear. It has been reported that the hard magnetic phase is a
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Fig. 6. The SEM micrographs of cross-section from (a) Co80Zr18B2 and (b) Co80Zr16W2B2 melt-spun ribbons.
Fig. 7. The dependence of iHc and (BH)max of Co80Zr18 xWxB2 (x ¼0, 1, 2 and 3) as-spun ribbons on the annealing temperature.
Fig. 8. The room temperature hysteresis loops of the Co80Zr16W2B2 as-spun ribbons annealed at 550 1C, 600 1C and 700 1C.
Fig. 9. The XRD patterns of Co80Zr16W2B2 ribbons annealed at (a) 550 1C, (b) 600 1C and (c) 700 1C.
metastable phase at certain intermediate temperature and can decompose into the Co23Zr6 and fcc-Co when it is allowed by the kinetics [10,12,19,20]. Hence, the two soft magnetic phases come from the decomposition of hard magnetic phase. With the increase of annealing temperature, the amount of Co23Zr6 and fcc-Co phase increases while the content of hard magnetic phase decreases, indicating that a large amount of hard magnetic phase has decomposed. Due to the increase of the content of soft magnetic phases, some of the soft grains may be partly or even completely decoupled from the neighboring grains and reverse independently, which leads to the appearance of the kink in the demagnetization curve of the sample annealed at 600 1C. As mentioned above, the as-spun sample with x ¼2 shows iHc of 6.6 kOe. After the as-spun sample is annealed at 550 1C, iHc increases to 7.3 kOe. This can be understood by the “random anisotropy model” reported by Herzer [26]. This model shows the close relationship between iHc and grain size. Within this model, if the grain size is approximately equal to the exchange length (Lex), the coercivity approaches to the maximum value.
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4. Conclusions In this paper, the effects of the substitution of W for Zr on the magnetic properties, phase evolution and microstructure of Co80Zr18 xWxB2 (x¼ 0, 1, 2 and 3) ribbons were investigated. For the as-spun samples, the optimal magnetic properties of iHc ¼ 6.6 kOe and (BH)max ¼4.5 MGOe were obtained in the Co80Zr16 W2B2 melt-spun ribbons that were produced at a wheel speed of 30 m/s. The addition of W reduced the grain size of hard magnetic phase and a refined microstructure was considered to be the main cause for the sr and squareness enhancement. Annealing investigation indicated that the hard magnetic phase was a metastable phase at certain intermediate temperature and could decompose into the soft magnetic Co23Zr6 and fcc-Co. A high coercivity of 7.3 kOe with (BH)max of 3.8 MGOe was achieved in the Co80Zr16 W2B2 ribbons after a heat treatment at 550 1C. The origin of coercivity enhancement in Co80Zr16W2B2 is ascribed to the increase in Ha of hard magnetic phase.
Acknowledgments
Fig. 10. The magnetization curves of Co80Zr18B2 and Co80Zr16W2B2 magnetic oriented powders measured perpendicular and parallel to the direction of the orienting field.
This work was supported by the National Natural Science Foundation of China (Nos. 11074092, 51073160 and 51101105), National Fund for Fostering Talents of Basic Science (No. J1103202).
Reference In our work, the maximum iHc is obtained in the sample with x¼ 2 after a suitable heat treatment. That can be attributed to the fact that the grain size of hard magnetic phase approaches to the critical size of highest iHc. On the other hand, if the grain size is below Lex, the magneto-crystalline anisotropy is supposed to be averaged out by the nanocrystalline structure and the effective magneto-crystalline anisotropy decreases. Generally speaking, if the effective magneto-crystalline anisotropy increases, it is favorable for the improvement of coercivity. In contrast, if the effective magneto-crystalline anisotropy decreases, it is unfavorable for the improvement. Since the grain size of as-spun sample is far below Lex, the decrease of iHc in the as-spun sample can be attributed to the decline of effective magnetocrystalline anisotropy [27]. In the case of the sample annealed at 600 1C and 700 1C, iHc decreases. That is caused by the decomposition of hard magnetic phase into the soft magnetic Co23Zr6 and fcc-Co. It has been reported that the highest iHc for the Co80Zr18B2 melt-spun ribbon is about 5.0 kOe [16]. However, the peak iHc is improved drastically to 7.3 kOe in the Co80Zr16W2B2. For both the samples, the grains of hard magnetic phase should approach to their critical size of highest iHc. Thus, we propose that the iHc enhancement is related to the change of magnetocrystalline anisotropy field Ha. Since the strength of Co–Zr system alloys is extremely high, it is quite difficult to grind them finely enough to single-crystal powders [10,28]. Therefore, the values of Ha determined in this paper are only estimate. Ha was measured for magnetic oriented powder in magnetic fields applied along and perpendicular to the magnetic texture and established via the field corresponding to the intersection of M (H) curves. Fig. 10 shows the magnetization curves of sample with x¼ 0 and 2 magnetic oriented powders measured perpendicular and parallel to the direction of the orienting field. It is found that the addition of W results in an increase in Ha. Accordingly, we conclude that the origin of iHc enhancement in the Co80Zr16W2B2 is due to the increase in Ha of hard magnetic phase.
[1] G.C. Hadjipanayis, J. Magn. Magn. Mater. 200 (1999) 373. [2] J.F. Herbst, Rev. Mod. Phys. 63 (1991) 819. [3] W.C. Chang, D.Y. Chiou, S.H. Wu, B.M. Ma, C.O. Bounds, Appl. Phys. Lett. 72 (1998) 121. [4] H.W. Chang, C.H. Chiu, W.C. Chang, Appl. Phys. Lett. 82 (2003) 4513. [5] H.W. Chang, C.S. Guo, C.H. Hsieh, Z.H. Guo, X.G. Zhao, W.C. Chang, J. Appl. Phys. 107 (2010) 09A710. [6] Ozan Akdogan, Wanfeng Li, George Hadjipanayis, J. Nanopart. Res. 14 (2012) 891. [7] R. Stone, Science 325 (2009) 1336. [8] A.M. Ghemawat, M. Foldeaki, R.A. Dunlap, R.C. O'Handley, IEEE Trans. Magn. 25 (1989) 3312. [9] A.M. Gabay, Y. Zhang, G.C. Hadjipanayis, J. Magn. Magn. Mater. 236 (2001) 37. [10] A.M. Gabay, N.N. Shchegoleva, V.S. Gaviko, G.V. Ivanova, Phys. Met. Metallogr. 95 (2003) 122. [11] G.V. Ivanova, N.N. Shchegoleva, A.M. Gabay, J. Alloys Compd. 432 (2007) 135. [12] H.H. Stadelmaier, T.S. Jang, E.Th. Henig, Mater. Lett. 12 (1991) 295. [13] T. Saito, M. Itakura, J. Alloys Compd. 527 (2013) 124. [14] X. Zhao, M.C. Nguyen, W.Y. Zhang, C.Z. Wang, M.J. Kramer, D.J. Sellmyer, X.Z. Li, F. Zhang, L.Q. Ke, V.P. Antropov, K.M. Ho, Phys. Rev. Lett. 112 (2014) 045502. [15] H.W. Chang, C.F. Tsai, C.C. Hsieh, C.W. Shih, W.C. Chang, C.C. Shaw, J. Magn. Magn. Mater. 346 (2013) 74. [16] T. Saito, Appl. Phys. Lett. 82 (2003) 2305. [17] T. Saito, IEEE Trans. Magn. 40 (2004) 2919. [18] L.Y. Chen, H.W. Chang, C.H. Chiu, C.W. Chang, W.C. Chang, J. Appl. Phys. 97 (2005) 10F307. [19] Z.P. Hou, S.F. Xu, J.B. Zhang, C.J. Wu, D. Liu, F. Su, W.Q. Wang, J. Alloys Compd. 555 (2013) 28. [20] Z.P. Hou, F. Su, S.F. Xu, J.B. Zhang, C.J. Wu, D. Liu, B.P. Wei, W.Q. Wang, J. Magn. Magn. Mater. 346 (2013) 124. [21] W.Y. Zhang, S. Valloppilly, X.Z. Li, Y. Liu, S. Michalski, T.A. George, R. Skomski, D.J. Sellmyer, J. Alloys Compd. 587 (2014) 578. [22] C. Gao, H. Wan, G.C. Hadjipanayis, J. Appl. Phys. 67 (1990) 4960. [23] B.G. Shen, L.Y. Yang, L. Cao, H.Q. Guo, J. Appl. Phys. 73 (1993) 5932. [24] W.C. Chang, D.M. Hsing, C. Yi, M. Hsiung, B.M. Ma, C.O. Bounds, IEEE Trans. Magn. 32 (1996) 4425. [25] H.W. Zhang, C.B. Rong, X.B. Du, J. Zhang, S.Y. Zhang, B.G. Shen, Appl. Phys. Lett. 82 (2003) 4098–4100. [26] G. Herzer, IEEE Trans. Magn. 25 (1989) 3327. [27] W.Y. Zhang, S.R. Valloppilly, X.Z. Li, R. Skomski, J.E. Shield, D.J. Sellmyer, IEEE Trans. Magn. 48 (2012) 3603. [28] T. Ishikawa, K. Ohmori, IEEE Trans. Magn. 26 (1990) 1370.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Effects of W substitution on the magnetic properties, phase evolution and microstructure of rapidly quenched Co80Zr18B2 alloy Zhipeng Hou a, Chongli Yang b, Shifeng Xu c, Jinbao Zhang a, Dan Liu a, Feng Su a, Wenquan Wang a,n a
College of Physics, Jilin University, Changchun 130023, People's Republic of China College of Physics and Information Engineering, Hebei Normal University, Shijiazhuang 050024, People's Republic of China c College of Science, Shenyang Aerospace University, Shenyang 110136, People's Republic of China b
art ic l e i nf o
a b s t r a c t
Article history: Received 11 June 2013 Received in revised form 21 April 2014 Available online 24 May 2014
The effects of a partial substitution of W for Zr on the magnetic properties, phase evolution, and microstructure of Co80Zr18 xWxB2 (x ¼ 0, 1, 2 and 3) melt-spun ribbons are investigated in this paper. For the as-spun samples, the optimal magnetic properties of iHc ¼6.6 kOe and (BH)max ¼4.5 MGOe are achieved in the Co80Zr16W2B2. Thermomagnetic analysis (TMA) and x-ray diffraction (XRD) results suggest that the as-spun Co80Zr16W2B2 mainly comprises of hard magnetic phase. Scanning electron microscope (SEM) results indicate that W addition refines the microstructure, which results in an increase in the intergrain exchange interaction. The effects of heat treatment on the magnetic properties of as-spun Co80Zr18 xWxB2 (x ¼ 0, 1, 2 and 3) are also studied. The maximum iHc of 7.3 kOe with a (BH)max of 3.8 MGOe is obtained in the Co80Zr16W2B2 after a heat treatment at 550 1C. The origin of coercivity enhancement in Co80Zr16W2B2 is ascribed to the increase in magnetocrystalline anisotropy field (Ha) of hard magnetic phase. & 2014 Elsevier B.V. All rights reserved.
Keywords: Co-Zr-W-B alloys Melt-spun ribbons Heat treatment Coercivity
1. Introduction Last decades, the researches of new permanent magnetic materials are largely concentrated on the rare earth (RE) containing alloys [1–5]. Since the RE permanent magnets are subject to the limited RE supplies, it is of great significance to explore the RE-free hard magnetic materials which are possible for industrial applications [6,7]. Among the materials, the Co–Zr system alloy is one of the most promising candidates due to its strong magnetocrystalline anisotropy field (Ha), high Curie temperature (Tc) and excellent corrosion resistance [8]. However, the crystal structure of hard magnetic phase has not been established reliably until now [9–11]. Even the exact composition is under debate. Some authors reported that the Co11Zr2, which was also known as “Co5.1 Zr” or “CoxZr (xE5)”, was responsible for the magnetic hardness [9–14]. Ivanova et al. studied the structure of this compound and showed an orthorhombic unit cell with very long b-axis and c-axis (a¼0.471 nm, b¼1.670 nm, and c¼2.420 nm). Although the proposed orthorhombic lattice with large parameters can explain the observed XRD patterns, this does not mean the structure is really the orthorhombic. The authors pointed out that
n
Corresponding author. Tel./fax: þ 86 431 85167397. E-mail address: [email protected] (W. Wang).
http://dx.doi.org/10.1016/j.jmmm.2014.05.024 0304-8853/& 2014 Elsevier B.V. All rights reserved.
the hexagonal, rhombohedral, monoclinic and other structures could be also indexed in terms of an orthorhombic unit cell with large parameters and the actual unit cell should be refined after establishing the positions of atoms. Chang et al. investigated the magnetic properties of Co80Zr18 xMxB2 (M¼C, Cu, Ga, Al and Si; x¼0–2) melt-spun ribbons and found the existence of three phases, Co5Zr, Co23Zr6 and fcc-Co [15]. They proposed that the Co5Zr was the hard magnetic phase. It should be noted here that their results are not extraordinary and similar results are also reported by some other research groups [16–18]. Recently, most work has been focused on the investigation of magnetic properties of Co–Zr system alloy [15–21]. For instance, the Co80Zr18B2 ribbons prepared by melt spinning and subsequent heat treatment show excellent magnetic properties of the remanence sr ¼47.5 emu/g, coercivity iHc ¼4.4 kOe and maximum energy product (BH)max ¼4.7 MGOe [16]. Although the magnetic properties are low compare with those of Nd–Fe–B magnets, they are comparable to the hard ferrites [22]. In order to further improve the magnetic properties, component substitutions have been employed [15,19–22]. Cao et al. reported that a proper substitution of Si for Co improved iHc of Co80Zr18B2 significantly and a high iHc of above 6.5 kOe was achieved [22]. However, since the substitution of nonmagnetic elements for magnetic Co decreases the magnetization s, the Co–Zr–Si–B magnet exhibits
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a low (BH)max. It is known that the materials with only high iHc or high (BH)max are not very ideal for permanent magnet. This leads to an intense search for other elements which can enhance both (BH)max and iHc. W is in the VIB group elements and has an atom radius similar to Zr. It is expected that there are some positive effects of W substitution for Zr on the magnetic properties of Co80Zr18B2 alloy. In this paper, we report the results of our investigation on Co80Zr18 xWxB2 (x ¼0, 1, 2 and 3) melt-spun ribbons.
2. Experiment Co (99.9%), Zr (99.9%), W(99.9%), and Co-B alloy(Co 76.88%, B 21.72%) were melted by arc melting in an argon atmosphere of high purity for at least three times until they formed homogeneous buttons with composition Co80Zr18 xWxB2 (x¼ 0,1, 2 and 3). Then, small amounts of the alloy ingots were placed in a quartz crucible with an orifice 0.6 mm in diameter at the bottom. The Co80Zr18 xWxB2 (x ¼0, 1, 2 and 3) alloy ingots were melted in an argon atmosphere and then ejected through the orifice under argon pressure onto a copper wheel rotating at speed Vs ¼ 30 m/s. The resultant melt-spun ribbons were about 2 mm wide and 30–40 μm thick. Heat treatment for the melt-spun ribbons was performed at temperatures between 500 1C and 700 1C for 1 to 6 min using rapid thermal processing system. High temperatures were attained using four tungsten-halogen lamps. The heating rate was 10 1C/s and the temperature control precision was 71 1C. The temperature decreased from 700 1C to 400 1C in 40 s by natural cooling. The phases of the specimens were identified by x-ray diffraction (XRD) using Cu Kα radiation. Room temperature magnetization measurements were done by vibrating sample magnetometer (VSM) with an applied field up to 20 kOe. Measurements were conducted with the field along the length of thin ribbons, so the demagnetization effects were neglected in the analysis. We performed the thermomagnetic analysis (TMA) by VSM in a field of 500 Oe. Scanning electron microscope (SEM) was utilized to examine the microstructures of the specimens. The ribbons were cut off along thick direction and the typical SEM micrographs were taken in the cross-section.
Fig. 1. The XRD patterns of Co80Zr18 xWxB2 (x ¼0, 1, 2, and 3) melt-spun ribbons.
3. Results and discussion Fig. 1 shows the XRD patterns of Co80Zr18 xWxB2 (x¼0, 1, 2 and 3) melt-spun ribbons. The diffraction patterns are similar to those reported for Co–Zr based alloys and we mark the relatively significant reflections with numbers ranging from “1” to “9”. Since the crystal structure and the exact composition of hard magnetic phase have not been established reliably until now, there is no common sense on the notation of diffraction peaks [9,13,15,18–21]. Some authors believe that all the nine diffraction peaks are indexed to the hard magnetic phase [9,20,21]. However, other authors have different opinions and consider the “5” diffraction peak to be (111) reflection of fcc-Co [15,16,18]. No matter how the diffraction peaks are noted, they all believe that the melt-spun ribbons are mainly comprised of hard magnetic phase and the content of soft magnetic fcc-Co or Co23Zr6 phase is quite limited. It has been reported that Tc of hard magnetic phase is well established to be about 500 1C [9,10]. Therefore, TMA is employed to indentify the phase composition. In Fig. 2, we present the thermomagnetic curves of Co80Zr18 xWxB2 (x¼ 0, 1, 2 and 3) melt-spun ribbons. For all the four samples, only one magnetic transition which corresponds to the hard magnetic phase is found. This indicates that the small addition of W alters the phase composition little. However, it is interesting to notice that the value of Tc decreases monotonously with the increase of W content,
Fig. 2. The thermomagnetic curves of Co80Zr18 xWxB2 (x¼ 0, 1, 2 and 3) melt-spun ribbons.
strongly suggesting that the W atoms enter into the lattice of hard magnetic phase and result in the band structure change [10,23]. Fig. 3 shows sr, iHc and (BH)max of Co80Zr18 xWxB2 (x¼ 0, 1, 2 and 3) melt-spun ribbons. For x¼ 0, sr ¼38.5 emu/g, iHc ¼3.3 kOe, and (BH)max ¼ 3.5 MGOe are obtained. The introduction of W has a large influence on the magnetic properties. One can notice that all the three magnetic parameters increase first and then decrease with the increase of W content. The optimal magnetic properties of sr ¼ 41.8 emu/g, iHc ¼ 6.6 kOe, and (BH)max ¼ 4.5 MGOe are achieved in the sample with x¼ 2. Compared to the sample with x¼0, although sr is just improved by 8.6%, iHc and (BH)max are enhanced drastically by 100% and 28.6%.It has been reported that a high iHc (Z6.5) has been realized in the Co–Zr–Si–B magnet [22]. However, since the substitution of nonmagnetic elements for magnetic Co decreases the magnetization, the Co–Zr–Si–B magnet exhibits a low
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Fig. 5. Delta M plots of Co80Zr18B2 and Co80Zr16W2B2 melt-spun ribbons.
Fig. 3. The dependence of sr, iHc and (BH)max on the W content.
Fig. 4. The room temperature hysteresis loops of Co80Zr18 xWxB2 (x ¼0 and 2) melt-spun ribbons.
(BH)max. Impressively, we realize both the high iHc and (BH)max in the Co80Zr16W2B2 melt-spun ribbons. Fig. 4 displays the room temperature hysteresis loops of the asspun samples with x ¼0 and 2. It can be seen that both the demagnetization curves are smooth and exhibit the characteristic of single hard-phase behavior. Also, we can find that x ¼2 exhibits larger sr, iHc and squareness. Therefore, a larger (BH)max is obtained. The specific causes will be discussed below. It has been reported that the changes of sr and squareness are related to the intergrain exchange interaction between the grains [18,24]. In order to determine the strength of interaction, we show the Henkel plots δM of the as-spun samples with x ¼0 and 2 in Fig. 5. The expression of a Henkel plot is as follow: δM¼ [sd(H) – (sr -2sr(H))]/sr, where the remnant sr(H) is acquired after the application and the subsequent removal of a field H, sd(H) after dc saturation in one direction, and the subsequent application and the removal of a field H in the reverse direction [25]. A positive δM peak reflects the existence of intergrain exchange interaction and
the magnitude of maximum δM value represents the strength of the interaction [25]. As shown in Fig. 5, we find that a larger maximum δM is obtained in the sample with x ¼2. This suggests that the addition of W increases the strength of intergrain exchange interaction. The change tendency agrees well with that of sr and squareness. Based on the results, it is therefore concluded that a stronger intergrain exchange interaction results in a larger sr and squareness. According to the previous reports [19,20,24,25], the change of intergrain exchange interaction may be affected by the microstructure. To investigate the effects of W addition on the microstructure of Co80Zr18B2, the typical SEM micrographs of cross-section from the as-spun sample with (a) x¼ 0 and (b) x ¼2 are shown in Fig. 6. Fig. 6(a) reveals that x ¼0 consist of roughly spherical particles whose diameters range from 400 to 500 nm. Much smaller particles and more homogeneous microstructure with an average grain size of around 50–100 nm is obtained in x¼ 2, suggesting that the W addition results in a reduction in the grain size. Due to a smaller grain size, the ratio of the grain boundary area to the volume of grains increases. Therefore, the intergrain exchange interaction increases in x¼ 2. According to the previous reports [16,19,20], a suitable heat treatment has a beneficial effect on the magnetic properties improvement. In order to obtain higher magnetic properties, the Co80Zr18 xWxB2 (x ¼0, 1, 2 and 3) as-spun ribbons are annealed at temperatures ranging from 500 1C to 700 1C spaced 50 1C apart. Fig. 7 exhibits iHc and (BH)max as a function of annealing temperature. For all the four samples, heat treatment deteriorates rather than improves (BH)max. However, iHc is enhanced after a suitable heat treatment. The maximum iHc of 7.3 kOe is obtained in the sample with x¼ 2 after annealed at 550 1C. Fig. 8 displays the room temperature hysteresis loops of the samples with x ¼2 annealed at 550 1C, 600 1C, and 700 1C. For the sample annealed at 550 1C, although iHc, increases, both the sr and squareness are worsened and therefore the (BH)max decreases. In the case of the sample annealed at 600 1C, we can observe a shoulder at low fields, revealing that it is undergoing two magnetization reversal processes. Fig. 9 shows the XRD patterns of the sample with x ¼2 annealed at (a) 550 1C, (b) 600 1C and (c) 700 1C. As shown in Fig. 9(a), the XRD pattern is quite similar to that of the as-spun sample, revealing that the phase composition does not change after a heat treatment at 550 1C. For the sample annealed at 600 1C, some content of the soft magnetic Co23Zr6 and fcc-Co appear. It has been reported that the hard magnetic phase is a
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Fig. 6. The SEM micrographs of cross-section from (a) Co80Zr18B2 and (b) Co80Zr16W2B2 melt-spun ribbons.
Fig. 7. The dependence of iHc and (BH)max of Co80Zr18 xWxB2 (x ¼0, 1, 2 and 3) as-spun ribbons on the annealing temperature.
Fig. 8. The room temperature hysteresis loops of the Co80Zr16W2B2 as-spun ribbons annealed at 550 1C, 600 1C and 700 1C.
Fig. 9. The XRD patterns of Co80Zr16W2B2 ribbons annealed at (a) 550 1C, (b) 600 1C and (c) 700 1C.
metastable phase at certain intermediate temperature and can decompose into the Co23Zr6 and fcc-Co when it is allowed by the kinetics [10,12,19,20]. Hence, the two soft magnetic phases come from the decomposition of hard magnetic phase. With the increase of annealing temperature, the amount of Co23Zr6 and fcc-Co phase increases while the content of hard magnetic phase decreases, indicating that a large amount of hard magnetic phase has decomposed. Due to the increase of the content of soft magnetic phases, some of the soft grains may be partly or even completely decoupled from the neighboring grains and reverse independently, which leads to the appearance of the kink in the demagnetization curve of the sample annealed at 600 1C. As mentioned above, the as-spun sample with x ¼2 shows iHc of 6.6 kOe. After the as-spun sample is annealed at 550 1C, iHc increases to 7.3 kOe. This can be understood by the “random anisotropy model” reported by Herzer [26]. This model shows the close relationship between iHc and grain size. Within this model, if the grain size is approximately equal to the exchange length (Lex), the coercivity approaches to the maximum value.
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4. Conclusions In this paper, the effects of the substitution of W for Zr on the magnetic properties, phase evolution and microstructure of Co80Zr18 xWxB2 (x¼ 0, 1, 2 and 3) ribbons were investigated. For the as-spun samples, the optimal magnetic properties of iHc ¼ 6.6 kOe and (BH)max ¼4.5 MGOe were obtained in the Co80Zr16 W2B2 melt-spun ribbons that were produced at a wheel speed of 30 m/s. The addition of W reduced the grain size of hard magnetic phase and a refined microstructure was considered to be the main cause for the sr and squareness enhancement. Annealing investigation indicated that the hard magnetic phase was a metastable phase at certain intermediate temperature and could decompose into the soft magnetic Co23Zr6 and fcc-Co. A high coercivity of 7.3 kOe with (BH)max of 3.8 MGOe was achieved in the Co80Zr16 W2B2 ribbons after a heat treatment at 550 1C. The origin of coercivity enhancement in Co80Zr16W2B2 is ascribed to the increase in Ha of hard magnetic phase.
Acknowledgments
Fig. 10. The magnetization curves of Co80Zr18B2 and Co80Zr16W2B2 magnetic oriented powders measured perpendicular and parallel to the direction of the orienting field.
This work was supported by the National Natural Science Foundation of China (Nos. 11074092, 51073160 and 51101105), National Fund for Fostering Talents of Basic Science (No. J1103202).
Reference In our work, the maximum iHc is obtained in the sample with x¼ 2 after a suitable heat treatment. That can be attributed to the fact that the grain size of hard magnetic phase approaches to the critical size of highest iHc. On the other hand, if the grain size is below Lex, the magneto-crystalline anisotropy is supposed to be averaged out by the nanocrystalline structure and the effective magneto-crystalline anisotropy decreases. Generally speaking, if the effective magneto-crystalline anisotropy increases, it is favorable for the improvement of coercivity. In contrast, if the effective magneto-crystalline anisotropy decreases, it is unfavorable for the improvement. Since the grain size of as-spun sample is far below Lex, the decrease of iHc in the as-spun sample can be attributed to the decline of effective magnetocrystalline anisotropy [27]. In the case of the sample annealed at 600 1C and 700 1C, iHc decreases. That is caused by the decomposition of hard magnetic phase into the soft magnetic Co23Zr6 and fcc-Co. It has been reported that the highest iHc for the Co80Zr18B2 melt-spun ribbon is about 5.0 kOe [16]. However, the peak iHc is improved drastically to 7.3 kOe in the Co80Zr16W2B2. For both the samples, the grains of hard magnetic phase should approach to their critical size of highest iHc. Thus, we propose that the iHc enhancement is related to the change of magnetocrystalline anisotropy field Ha. Since the strength of Co–Zr system alloys is extremely high, it is quite difficult to grind them finely enough to single-crystal powders [10,28]. Therefore, the values of Ha determined in this paper are only estimate. Ha was measured for magnetic oriented powder in magnetic fields applied along and perpendicular to the magnetic texture and established via the field corresponding to the intersection of M (H) curves. Fig. 10 shows the magnetization curves of sample with x¼ 0 and 2 magnetic oriented powders measured perpendicular and parallel to the direction of the orienting field. It is found that the addition of W results in an increase in Ha. Accordingly, we conclude that the origin of iHc enhancement in the Co80Zr16W2B2 is due to the increase in Ha of hard magnetic phase.
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