Rare Metals2020年第4期

Low-cost Sm_0.7Y_0.3Co₅ sintered magnet produced by traditional powder metallurgical techniques

Dong-Tao Zhang Nai-Xing Cai Rong-Chun Zhu Wei-Qiang Liu Ming Yue

College of Materials Science and Engineering,Key Laboratory of Advanced Functional Materials,Ministry of Education of China,Beijing University of Technology

作者简介：*Ming Yue e-mail:yueming@bjut.edu.cn;

收稿日期：16 October 2018

基金：financially supported by the State Key Program of Natural Science Foundation of China (Nos. 51331003 and 51871005);the International S&T Cooperation Program of China (No.2015DFG52020);

Low-cost Sm_0.7Y_0.3Co₅ sintered magnet produced by traditional powder metallurgical techniques

Dong-Tao Zhang Nai-Xing Cai Rong-Chun Zhu Wei-Qiang Liu Ming Yue

College of Materials Science and Engineering,Key Laboratory of Advanced Functional Materials,Ministry of Education of China,Beijing University of Technology

Abstract：

RCo₅(R=rare earth) sintered magnets have good temperature stability,so it is still widely used in high temperature field.In this paper,by the method of adding liquid phase SmCo_1.7 to the main phase,Sm_0.7Y_0.3Co₅ magnet was prepared by traditional powder metallurgical process.The results show the presence of a main phase RCo₅,a minor phase R₂ Co₇,and a R-rich phase in the magnet.Contrasting the results of the XRD(X-ray diffraction) in random and oriented directions,the magnet has a well-aligned(00l) orientated texture,which is consistent with the result of the electron backscattered diffraction(EBSD).The Sm_0.7Y_0.3Co₅ sintered magnet has good magnetic properties as remanence(Br) is 0.96 T,the coercivity(H_cj) is 1201.96 kA·m^-1,and maximum magnetic energy product((BH)_max) is 175.16 kJ m^-3.

Keyword：

Sm_0.7Y_0.3Co₅; Sintered magnets; Low-cost; Magnetic properties; RCo₅;

Received： 16 October 2018

1 Introduction

RCo₅ (R=rare earth) alloys have favorable intrinsic magnetic properties for high temperature application [ 1, 2, 3, 4, 5, 6] .As the first generation of rare earth permanent magnet,SmCo₅ magnet has a high Curie temperature of 727℃,and relatively high energy product [ 7, 8, 9, 10, 11] .In particular,it has the highest magnetocrystalline anisotropy of31,840 kA·m^-1,which is much higher than that of Sm₂Co₁₇ and other SmCo compounds [ 12, 13, 14, 15, 16, 17, 18] .In addition,compared with NdFeB magnet,SmCo₅ magnet has better corrosion resistance and temperature stability [ 19, 20, 21] .However,the cost of Sm and Co is relatively high,so decreasing the cost of SmCo₅ magnet is an important consideration.The rare earth Y (225 RMB·kg^-1) is cheaper than Sm (450 RMB·kg^-1),and YCo₅ has the same crystal structure with SmCo₅ [ 22, 23, 24] .The magnet properties of sintered SmCo₅ magnet is 191.1 kJ·m^-3 [ 25] .The YCo₅ magnet also has high theoretical maximum energy product ((BH)_max) of 224 kJ·m^-3 at room temperature [ 6, 26, 27] .Therefore,by partial replacement of Sm by Y atoms,Sm_1-xY_xCo₅ magnet with high performance can be prepared,and the cost of the magnet can be decreased.However,since the density of the YCo₅ compound is low(7.6 g·cm^-3),and the anisotropy field of H_A=10,348 kA·m^-1 is not as high as that of SmCo₅compound (31,840 kA·m^-1) [ 12] ,it will not be easy to obtain high density and maintain high coercivity for the Sm_1-xY_xCo₅ magnet.

Therefore,the major motivation for this work is to obtain high performance Sm_1-xY_xCo₅ magnet with lower cost and further understand the relationship between magnetic properties and microstructure of Sm_1-xY_xCo₅magnet after doping Y.As the density and H_A of the YCo₅compound are low,so the replacement amount of Sm by Y atoms can not be high.In this paper,Sm_0.7Y_0.3Co₅ magnet was prepared by traditional powder metallurgical techniques.Moreover,SmCo_1.7 alloy as a liquid phase was also added into the Sm_0.7Y_0.3Co₅ magnet,which can improve the density of the magnet and decrease the sintering temperature during processing.

2 Experimental

The Sm_0.7Y_0.3Co_4.8 and SmCo_1.7 ingots were prepared by induction melting in high-purity argon atmosphere.Excess Sm of 5 wt%and Y of 5 wt%were added to compensate for weight loss due to evaporation.The two ingots were crushed into powders with a size of 3-5μm by a jaw crusher,a disk mill and a rolling ball mill,respectively.The weight ratio of the balls to the powders was 5:1,the milling time was 7 h,and the ball-mill medium was gasoline.The Sm_0.7Y_0.3Co_4.8 powders,mixed with the SmCo_1.7 powders of 6 wt%,were orientated in a magnetic field of 2 T,then isostatic ally cold-pressed under a pressure of 220 MPa.Finally,the sample was sintered at 1150℃in argon for 1 h,cooled down to 850℃at a rate of1℃·min^-1 and held for 1.5 h,then fast cooled in air to room temperature.This process is schematically shown in Fig.1.

The crystal structure of the sample was analyzed by X-ray diffraction (XRD,Rigaku Dmax-C) using Cu Kαradiation.The sample with a size of 7 mm×7 mm×15 mm was magnetized in a 10-T magnetic field along the easy magnetization axis,and measured by the NIM-500C BH looper.The elemental distribution of the sample was identified by the electron probe micro-analyzer (EPMA,EPMA-1720H).The microstructure of the sample was analyzed by scanning electron microscopy (SEM,FEI NANO200) with energy-dispersive spectrometer (EDS)and transmission electron microscope (TEM,JEOL-200C).The density of the sample was measured by the Archimedes method.The electron backscattered diffraction(EBSD) data was collected by EBSD detector (Hikari ED AX) incorporated in SEM (FEI Quanta 250,USA),and the measurement surface of the samples is perpendicular to the easy axis of the magnet.The EBSD data were analyzed by the TSL OIM Analysis 5.3 software (EDAX Inc,USA).The multiple of random distribution (MRD) of inverse pole figure (IPF) maps and pole figures (PF) revealed the orientation of the sample.Magnetic domains of the sample during the magnetization and demagnetization processes were observed by a magneto-optical Kerr optical microscope (MOKE,BH-786IP-PK).

Fig.1 Sintering process curve of Sm_0.7Y_0.3Co₅ magnet

3 Results and discussion

Figure 2 shows the random and oriented XRD patterns of the Sm_0.7Y_0.3Co₅ magnets,the data of the random magnet are from the crushed powders and the data of the oriented magnet are from the aligned magnet of the Sm_0.7Y_0.3Co₅sintered magnet.For the random magnet,main phase of RCo₅,minor phase of R₂Co₇ and a trace of R-rich phase exist in the magnet.After orientation,the (00l) peaks become very strong,while the (101) and (111) peaks become weak and other peaks almost disappear in the oriented magnet.The intensity ratio of (002) to (111)peaks,I₍₀₀₂₎/I₍₁₁₁₎,can be used to characterize texture degree in the Sm_0.7Y_0.3Co₅ magnet.The value ofI₍₀₀₂₎/I₍₁₁₁₎ changes from 0.2 to 60.2 before and after orientation,indicating an excellent orientation texture in the magnet.

Figure 3 shows the demagnetization curves of the magnet in a temperature range of 25-200℃,and the magnetic properties are listed in Table 1.It can be seen that the demagnetization curve still maintains an acceptable square shape at 25℃for the Y-containing magnet.The maximum magnetic energy product((BH)_max),remanence (B_r) and coercivity (H_cj) of the magnet are175.16 kJ·m^-3,0.96 T and 1201.96 kA·m^-1 at 25℃.The density of the sintered magnet is 8.0 g·cm^-3,but the theoretical density of the Sm_0.7Y_0.3Co₅ magnet is 8.2 g·cm^-3,so the density,remanence and magnetic energy product of the magnet still have potential to be improved.As the temperature increases,the (BH)_max,B_r and H_cj of the magnet decreases to 113.9 kJ·m^-3,0.86 T and437.8 kA·m^-1 at 200℃.Therefore,the remanence temperature coefficient (a) and the coercivity temperature coefficient (β) are-0.06%.℃^-1 and-0.37%·℃^-1 in a temperature range of 25-200℃,respectively.However,along with the rise in temperature,the B-H curves are not straight,because there are 2:7 phases in the magnet and they are easier to demagnetize and nucleate than 1:5 phase at high temperature,indicating that the magnetic properties of the magnet need to be improved.

Fig.2 XRD patterns of random and oriented Sm_0.7Y_0.3Co₅ magnets

Fig.3 Demagnetization curves of Sm_0.7Y_0.3 Co₅ magnet in a tem-perature range of 25-200℃

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Table 1 Magnetic properties of Sm_0.7Y_0.3Co₅ magnet in a tempera-ture range of 25-200℃

Figure 4 shows morphology images of the Sm_0.7Y_0.3Co₅magnet.It can be seen in Fig.4a that the magnet is mainly composed of three phases.It can be seen from Fig.4b that the dark gray areas (Area 1) are the main phase,the light gray areas (Area 2) are the secondary phase,and the dark areas (Area 3) are the R-rich phase which are easy to fall out,and that the distributions of each phase in the magnet are uniform.EDS results at different areas in Fig.4b are shown in Table 2.It can be concluded from Fig.4b that the large dark gray areas marked as 1 are the main RCo₅ phase,the light gray areas marked as 2 are the minor R₂Co₇ phase,and the black hole areas marked as 3 are the R-rich phase,in agreement with XRD results.It is not good that there are excessive R₂Co₇ phases in the magnet.According to the phase diagram,an excess of Sm is required to avoid the formation of the Sm₂Co₁₇ phase [ 28] ;then,the R₂Co₇phase is easy to appear in the magnet.Excessive R₂Co₇phase can reduce the magnetic energy and the density,so it is necessary to control the proportion of Sm and Co to reduce the amount of Sm₂Co₇ phase.The distribution of the elements in the Sm_0.7Y_0.3Co₅ magnet is shown in Fig.5.In Fig.5b,it can be seen that compared with that of Area 1 (1:5 phase),the content of Sm in Area 2 (2:7 phase)has increased,and the content of Sm in Area 3 (R-rich phase) is reduced.In Fig.5c,it can be seen that the content of Y in Area 1 is the same as that of Area 2,while the content of Y at Area 3 is higher than the former two.In Fig.5d,compared with the content of Co in Area 1,that of Area 2 is less,and that of Area 3 is much less.The result is also consistent with XRD results.

The results of XRD and EPMA show that there are three phases:the 1:5 main phase,the 2:7 minor phase and the R-rich phase in the magnet.The partial replacement of Y by Sm atoms does not change the structure.Therefore,EBSD analysis was conducted by setting SmCo₅ and Sm₂Co₇ as the identification phases,as shown in Fig.6.The orientation of the grains is based on the orientation of the hexagonal symmetry of the RCo₅ phase.The oriented grain images can distinguish grains with different orientations.Compared with a standard map,it can be seen that the diffraction patterns of both 1:5 phase (Area 1) and 2:7phase (Area 2) are similar to red,indicating that the grain orientation is very close to (00l),which is in accordance with the results of XRD.The high orientation degree is very helpful in improving the remanence of the magnets [ 29] .With EBSD data,the IPF and PF were used to quantitatively analyze and calculate the crystal texture.From Fig.7a,c,it can be seen that the maximum multiple of random distribution (MRD) of the 1:5 phase in PF and IPF are 36.0 and 30.8,respectively,while the values of 2:7phase are 34.9 and 19.0,respectively,as shown in Fig.7b,d.Thus,the orientation level of 1:5 phase is better than that of 2:7 phase,and it is clear that the grain of the magnet has a better (0001) orientation,which is important in improving the remanence of the magnet [ 30, 31, 32] .

Fig.4 BSE images of Sm_0.7Y_0.3Co₅ magnet with different magnifications

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Table 2 Elemental content at different areas of Sm_0.7Y_0.3Co₅ magnet(at%)

Fig.5 EPMA results of Sm_0.7Y_0.3Co₅ magnet:a SEM image,b Sm,c Y,and d Co

Fig.6 IPF maps of Sm_0.7Y_0.3Co₅ magnet:a BSE of a selected region and b 1:5 and 2:7 with orientation legend for hexagonal symmetries

Fig.7 (0001) pole figure and inverse pole figures of normal direction (ND) of a,c 1:5 phase and b,d 2:7 phase

Figure 8 shows the magnetic domain and backscattered electron morphology (BSEM) in situ observations by MOKE and SEM,respectively.The direction of themagnetic field is consistent with c-axis.In Fig.8a,the dark gray phase (Area 1) and light gray phase (Area 2) are the1:5 and 2:7 phases,respectively.Figure 8b indicates the magnetic domains of the magnet at thermal demagnetization state.The domains in the dark gray areas are pided into two directions.The dark part is in one direction,the light part is in the reverse direction,and the magnetic domain is a maze-like domain (Area 1);the width of the domain is about 5μm.However,in the light gray 2:7 phase areas,for example,Area 2,there is no obvious domain.The magnetic domain,at remanent magnetization state after magnetization by a 10-T magnetic field,is shown in Fig.8c.All the domains in the 1:5 phase areas move to the same direction,and there are no domains in the field of vision.In Fig.8d,partial magnetic domain reversal occurs in the 1:5 phase areas when a-875.6 kA·m^-1 reverse field is applied.For the 2:7 phase areas,in the thermal demagnetization state,remanent magnetization state or applying a reversal magnetic field,there is no change of magnetic domain.The reason needs to be further clarified.The magnetic domain evolution during demagnetization process is shown in Fig.9,and the magnetic reversal field is 0,-398,-716.4,and-1114.4 kA·m^-1.The direction of the magnetic field is consistent with c-axis.When the magnetization field is 0 kA·m^-1 (remanence),the magnetic domain is a single domain state.When the reverse magnetization field is-398 kA·m^-1,a small number of reverse domains start to appear.With further increase in the reverse field (-716.4 and-1114.4 kA·m^-1),new reverse domains appear and their number increases.Moreover,the appearance of the reverse domains is irreversible.

Fig.8 a Magnetic domain SEM image of Sm_0.7Y_0.3Co₅ magnet in situ;MOKE images b at thermal demagnetization state,c at remanent magnetization state after magnetization by a magnetic field of 10 T and d under a 875.6 kA·m^-1 reverse magnetic field (analyzed surface perpendicular to orientation direction of magnet)

Fig.9 MOKE images of Sm_0,7Y_0,3Co₅ magnet at applied filed of a 0 kA·m^-1,b-398 kA·m^-1,c-716.4 kA·m^-1 and d-1114.4 kA·m^-1during demagnetization process

Fig.10 a TEM image of Sm_0.7Y_0.3Co₅ magnet,b typical high-resolution TEM image of Area 1 and c typical high-resolution TEM image ofArea 2

TEM analysis of the Sm_0.7Y_0.3Co₅ magnet is shown in Fig.10.Figure 10a is a typical bright-field TEM morphology.There is no complete grain in the TEM vision because the grain size is about 8μm.Figure 10b shows the high-resolution image of Area 1,which is identified as SmCo₅ phase with hexagonal structure.The high-resolution image of Area 2 in Fig.10c indicates that it is Sm₂O₃with the monoclinic structure.The grain size of the Sm₂O₃is about 100 nm.

4 Conclusion

The Sm_0.7Y_0.3Co₅ magnet was successfully prepared by traditional powder metallurgical process.The Sm_0.7Y_0.3Co₅ magnet bears optimal magnetic properties with B_r=0.96 T,H_cj=1201.96 kA·m^-1 and(BH)_max=175.16 kJ·m^-3.There are three coexisting phases composed of 1:5 main phase,2:7 minor phase and a R-rich phase in the magnet.The magnet has a well-aligned(00l) orientation texture.The magnetic domain of the 1:5phase is a maze domain.There is no obvious magnetic domain in the 2:7 phase.

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