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Effect of diffusing TbF3 powder on magnetic properties and microstructure transformation of sintered Nd-Fe-Cu-B magnets
Journal of Magnetism and Magnetic Materials 443 (2017) 179–183
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Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
Effect of diffusing TbF3 powder on magnetic properties and microstructure transformation of sintered Nd-Fe-Cu-B magnets Xiao Yang a,b, Shuai Guo a,⇑, Guangfei Ding a,b, Xuejing Cao a, Jiling Zeng a,b, Jie Song a, Don Lee c, Aru Yan a,⇑ a
Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China University of Chinese Academy of Sciences, Beijing 100049, China c University of Dayton, Dayton, OH 45469, USA b
a r t i c l e
i n f o
Article history: Received 25 May 2017 Received in revised form 12 July 2017 Accepted 20 July 2017 Available online 21 July 2017 Keywords: Cu migration Grain growth area Sintered Nd-Fe-B magnets Grain boundary diffusion
a b s t r a c t The coercivity of sintered Nd-Fe-Cu-B magnets is markedly enhanced from 12.57 to 21.70 kOe while the remanence decreases from 13.80 to 13.49 kGs by grain boundary diffusion of TbF3 powder for 2 h. Microstructure analysis suggests that, during the diffusion process, F diffuses into the magnets easily and forms a new F-rich phase. The enrichment of F in grain boundary near the surface leads to the Cu movement into the interior and the Cu reduction in the surface of magnets. Diffusion of Tb leads to an increase of local total rare earth elements content. Under the combined effect of Cu reduction and increase of local total rare earth elements content, grain growth area is formed and further diffusion is suppressed. That excessive Tb diffuses into matrix phase leads to a decrease in remanence. When the grain growth area is removed, the deterioration of remanence recovers to 13.80 kGs without any reduction of coercivity. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Nd-Fe-B based sintered magnets have been widely applied in traction motors of electric vehicles and hybrid electric vehicles because of their excellent magnetic properties [1]. But the intrinsic coercivity of commercial magnets is still insufficient to satisfy the demands for electric vehicles [2]. Partial substitution of heavy rare earth (HRE) elements like Dy and Tb for Nd is proved an efficient method to significantly increase the coercivity of magnets by enhancing magnetocrystalline anisotropy [3]. However, the remanence and the energy product reduced inevitably due to the antiferromagnetic coupling between the HRE atoms and Fe atoms. Furthermore, the abundance of HRE in the earth’s crust is limited, so an available method for enhancing coercivity with none or slight HRE elements introduction is desired. Grain boundary diffusion process (GBDP) has been proved to be a promising way to enhance coercivity without sacrifice of remanence and energy product. In these processes, magnets are coated with HRE elements compounds in the forms of oxides [4], fluorides [5,6], sulfides [7], pure
⇑ Corresponding authors at: Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China. E-mail addresses: [email protected] (X. Yang), [email protected] (S. Guo), [email protected] (A. Yan). http://dx.doi.org/10.1016/j.jmmm.2017.07.071 0304-8853/Ó 2017 Elsevier B.V. All rights reserved.
metal [8] or low melting eutectics [9,10], followed by the heat treatment for HRE elements diffusion. In GBDP technology, the formation of (Nd,HRE)2Fe14B shell structure surrounding Nd2Fe14B matrix phase is supposed to be the source of the enhancement of coercivity because coercivity of sintered Nd-Fe-B magnets is mainly controlled by the nucleation of reversal domains from the surface of matrix phase. HRE fluorides like DyF3 and TbF3 are common diffusion source in laboratory. We know that DyF3 additives reacted with the grain boundary phase and Nd-O-F phase was formed during sintering [11]. And Kim et al. [12] reported that the DyH2 dipped magnets and the DyF3 dipped magnets had different chemical composition of the grain boundary phases due to F introduction. After diffusion, the magnets had different magnetic properties. Therefore, F has huge influence on the chemical composition properties of the Nd-rich grain boundary as well as the diffusion behavior. On the other hand, Cu is one kind of most common elements in sintered Nd-Fe-B magnets and has huge influence on the microstructure and magnetic properties. Cu segregates in grain boundary phase because of the positive mixing enthalpy of Fe and Cu [13]. Cu addition is in favor of the continuous Nd–rich thin layer along the grain boundary and the decoupling between the neighboring matrix grains [14]. However, the effect of F on the Cu distribution, the related microstructure transformation and the corresponding change of magnetic properties of Nd-Fe-Cu-B magnets have not been revealed completely.
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In this study, Nd-Fe-Cu-B magnets with different diffusion time were obtained by electrophoretic diffusing TbF3 powder. The elements distribution and resultant microstructural changes were investigated thoroughly. Based on these results, we clarified the effect of F on chemical composition in grain boundary phase and the evolution mechanism of grain growth area. 2. Experimental procedure Alloys with a nominal composition Nd32.5Cu0.1FebalB0.92 (wt%) were prepared by strip cast (SC) technique. The strips were subjected to subsequent hydrogen decrepitation (HD) process and further jet milling (JM) in a nitrogen atmosphere to refine the powder with an average particle size of 1.9 lm. The prepared powders were compacted and aligned under a magnetic field of 2.25 T and followed by the cold isostatic compacting under a pressure of 150 MPa. The green compacts were sintered at 980 °C for 2 h in vacuum followed by gas quenching. Then the as-sintered magnets were annealed at 900 °C and 500 °C for 2 h in sequence. The final annealed samples were machined to cylinders with a diameter of 10 mm and a height of 4 mm for diffusing. In electrophoretic deposition (EPD) technology, the cut magnets as anode and the steel plate as cathode were both immersed in ethanol-based TbF3 suspension at a constant applied voltage of 60 V for 2 min. After coated process, the coated samples were removed from the suspension and dried in hot air. Then the coated samples were diffused at 900 °C for 2 h, 4 h, 10 h and 15 h respectively and subsequently annealed at 500 °C for 2 h in vacuum. In dipping technology, the cut magnets were directly immersed in ethanol-based TbHx suspension for 5 s and then dried in N2 atmosphere. The TbHx coated magnets were diffused at 900 °C for 15 h and then annealed at 500 °C for 2 h in vacuum. Magnetic properties were measured by NIM-500 C hysteresigraph analyzer system. The microstructures and elemental distribution of the magnets were observed by scanning electron microscope (SEM) (Quanta FEG 250) equipped with an energy dispersive X-ray spectrometer (EDS) (Oxford Aztec system). The phase constituents of diffused magnets were examined by X-ray diffraction (XRD) with the Cu-Ka radiation. The surface depth profiles were determined by glow discharge atomic emission spectrometer (GDA) (Spectruma Analytik GMBH 750HP). 3. Results and discussion Fig. 1 shows Nd, Tb, total rare earth and Cu relative concentration versus distance from the surface of the diffused magnets with different diffusion time. After diffusion, a large amount of Tb is
enriched at the magnet surface, and Tb concentration first rapidly decreases and then gradually becomes stable while Nd concentration shows a contrary tendency. The width of Tb enrichment region increases as diffusion time increases. About 20 lm away from the surface, Nd concentration reaches a maximum value, which is higher than the level of the original magnets, and that is because Nd is partly substituted by Tb and the redundant Nd migrates into deeper region. The total rare earth concentration curves which consist of Tb and Nd are also showed. That the total rare earth content of diffused magnets is lower than the original magnets at the surface might be caused by a little of residual TbF3. Due to the diffusion of Tb, the total rare earth concentration in the region near the surface is higher than that of the original magnets. The Cu concentration variation curve has three distinct stages, and the level of the two stages near the surface is below the original magnets while the level of the third stage is upon the original magnets. With the increase of diffusion time, the width of the Cu-lean region increase gradually, that might be derived from the Cu migration from surface to interior. Tb concentration variation of EPD-processed and dipped samples diffusing for 15 h is shown in Fig. 2. In the region of headmost 50 lm, Tb concentration of the EPD-processed sample is higher than the sample dipped with TbHx powders. But in the region deeper than 50 lm, the Tb concentration shows a contrary relationship. The cross-sectional Second Electron (SE) SEM image of the diffused magnets for 15 h diffusion is shown in Fig. 2 inset (a). A grain growth area is developed at the surface while the internal grains still keep initial morphology. It is well known that Cu segregates in the grain boundary phase. The formation of grain growth area extrudes the grain boundary phase and the grain boundary phase moves into deeper region, so the Cu concentration in the grain growth area is lower than the adjacent region. A compared experiment using TbHx as diffusate was carried out to prove the grain growth area is the primary barricade of diffusion. Fig. 2 inset (b) shows the SE-SEM image of magnets dipped with TbHx powders after 15 h diffusion. There is only a small amount of grown grains occur in the region near the surface, and the grain boundary, i.e., the diffusion channel still exists. It indicates that more Tb atoms can diffuse into the interior without the grain growth area. Noted that the grain boundary is supposed to be the main diffusion channel of the diffusion, the formation of the grain growth area would obstruct the diffusion channel and prevent the further diffusion. As a result, Tb is confined in the grain growth area and difficult to diffuse into the interior. The cross-sectional back scattered electron (BSE) SEM images of the diffused magnets with different diffusion time are shown in Fig. 3. In all diffused magnets, the typical Tb-rich shell structure
Fig. 1. Nd, Tb, Cu and total rare earth elements (labeled as RE) relative concentration, which is equal to respective concentration of diffused samples minus the concentration of original samples, versus depth from the surface of the diffused magnets with (a) 2 h and (d) 15 h diffusion.
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Fig. 2. Tb concentration variation of EPD-processed and dipped samples diffusing for 15 h. The insets show SE-SEM images of (a) EPD-processed and (b) TbHx dipped samples treated for 15 h.
which corresponds to grey area surrounding the dark Nd2Fe14B phase is clearly visible in the region near the surface. In addition, it is obvious that the grain boundary phase consists of two phases with different contrasts. Fig. 4 shows the typical microstructure of the grain boundary phase in the diffused magnets. We can find that Nd is enriched and Fe is lean in the grain boundary phase. However, F and Cu mapping exhibit contrary distribution, which F concentrates in area 2 and Cu concentrates in area 1. According to EDS analysis shown in Table 1, we can conclude that the grey contrast corresponds to the F-rich phase marked as green arrows in Fig. 3 while the bright contrast corresponds to the Cu-rich phase marked as red arrows in Fig. 3. As shown in Fig. 3, in all diffused magnets, the distribution of newly formed F-rich phase occupies the triple junction area near the surface and then decreases gradually as depth increases. But the Cu-rich phase exists more in the corners of the tripe junction area and the inner region. With the increase of diffusion time, there is no obvious difference in the distribution of F-rich phase and Cu-rich phase. Integrating the previous analysis, Fig. 5 (a) shows the schematic diagrams of the microstructure evolution of F diffusion. Noted that the size of F atom is smaller than Tb atom, F diffuses much more easily than Tb atom. 2 h are enough for F atoms to diffuse sufficiently, and this is why there is no obvious difference in the distribution of F-rich phase with the increase of diffusion time. During the diffusion process, F diffuses into the magnets along grain boundary and reacts with the grain boundary phase. F-rich phase is formed and occupies the triple junction phase. Because the Frich phase is stoichiometrically stable [12] and the solubility of Cu in F-rich phase is negligible, Cu separates out from the grain boundary phase and migrate into the interior of magnets. This is why a small amount of Cu-rich phase exists in the corner of triple junction phase and Cu concentration in the interior is higher in the GDA curves. At the same time, as shown in Fig. 5(b), Tb diffuses into magnets and causes higher total rare earth content in the region near the surface. The sinter process of Nd-Fe-B based sintered magnets is a liquid phase sintering process. The diffusion of Tb into the magnets causes the increase of the liquid phase and promote the sintering process for full density. So the grains are easier to grow in the case of higher rare earth content. On the other hand, The magnets are more susceptible to densification in the case of lower Cu content [15]. During the GBDP, on the magnets
Fig. 3. BSE-SEM images for diffused magnets with (a) 2 h, (b) 4 h, (c) 10 h and (d) 15 h diffusion.
surface, the liquid phase increases because of the diffusion of Tb into magnets while Cu content reduces because of Cu migration caused by F diffusion. Under the combined influence of the two factors, the grains of the surface of the magnets begin to grow. Eventually, the grain growth area is formed and the diffusion is suppressed. So the concentration of Tb in grain growth area is far higher than that in the interior. Magnetic properties of the TbF3 EPD-treated sintered magnets as a function of different diffusion time are shown in Fig. 6(a). In general, the coercivity of diffused magnets first increases and then becomes stable gradually. With 2 h diffusion, coercivity increases from 12.52 to 21.70 kOe and remanence decrease from 13.80 to 13.41 kGs. The enhancement of coercivity and the reduction of remanence can be attributed to the partial substitution of Tb for Nd in the surface of matrix phase. Because the grain growth area suppresses more Tb diffusion, with further increasing the diffusion time, coercivity and remanence do not show obvious change and tend to be stable. When the diffusion time is more than 10 h, coercivity does not change markedly. So the optimal diffusion time of the TbF3 EPD magnets should be 10 h. As shown in Fig. 6(b), there is a slight abnormal reduction in demagnetization curves of the diffused magnets when the external field is small. The reason for the
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Fig. 4. BSE image and corresponding EDS elemental distributions mapping of Nd, Fe, F and Cu of diffused magnets with 2 h.
Table 1 Chemical analysis of area 1 and area 2 in Fig. 4. Area
Nd(wt%)
Fe(wt%)
O(wt%)
F(wt%)
Cu(wt%)
1 2
77.38 80.61
10.85 7.73
0.75 3.31
0 8.29
11.01 0.06
Fig. 5. Schematic diagram of microstructure evolution of (a) F diffusion and (b) Tb diffusion in the diffusion process. GB refers to grain boundary.
abnormal reduction is that the grain growth area has extremely low coercivity and demagnetizes easily. When removed the grain growth area (50 lm), the slight abnormal reduction disappears and remanence recovers from 13.59 to 13.80 kGs and coercivity slightly increases at the same time. The recovery of remanence is due to the removal of the region containing excessive Tb in matrix phase.
4. Conclusion In summary, the magnetic properties and microstructure evolution of TbF3 diffused sintered Nd-Fe-Cu-B magnets with different diffusion time using EPD were investigated. After diffusion with different time, the coercivity increases first and then becomes stable. Two kinds of Cu migration are observed in the diffused
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Fig. 6. (a) Magnetic properties of untreated and diffused magnets with different diffusion time. Hcj and Br refer to coercivity and remanence respectively. (b) Demagnetization curves of diffused and polished magnets.
magnets, which are caused by the reaction between F and the grain boundary phase and the formation of the grain growth area respectively. The trace Tb that diffuses into the interior of magnets is the source of the coercivity enhancement. However, excessive Tb concentrates in the surface and makes local total rare earth content increase. Under the combined effect of Cu reduction and increase of local total rare earth elements content, the grain growth area is formed. The grain growth area obstructs the diffusion channel and prevents further diffusion. When the grain growth area is removed, the deterioration of remanence recovers to original level without any reduction of coercivity.
Acknowledgements This work is supported by the National Natural Science Foundation of China (No. 51501211), Ningbo Major Industrial Technology Innovation Projects (2016B10007), the Program of Ningbo International Corporation (No. 2015D10019) and the Program of Ningbo Innovation Team (No. 2012B81001), Guangxi Science and technology projects (No. 2016AD05041) and Major projects of Inner Mongolia.
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[3] S. Hirosawa, Y. Matsuura, H. Yamamoto, S. Fujimura, M. Sagawa, H. Yamauchi, Magnetization and magnetic anisotropy of R2Fe14B measured on single crystals, J. Appl. Phys. 59 (1986) 873–879. [4] K.-H. Bae, T.-H. Kim, S.-R. Lee, S. Namkung, T.-S. Jang, Effects of DyHx and Dy2O3 powder addition on magnetic and microstructural properties of Nd-FeB sintered magnets, J. Appl. Phys. 112 (2012) 093912. [5] K.-H. Bae, T.-H. Kim, S.-R. Lee, H.-J. Kim, M.-W. Lee, T.-S. Jang, Magnetic and microstructural characteristics of DyF3/DyHx dip-coated Nd–Fe–B sintered magnets, J. Alloys Compd. 612 (2014) 183–188. [6] X. Cao, L. Chen, S. Guo, R. Chen, G. Yan, A. Yan, Impact of TbF3 diffusion on coercivity and microstructure in sintered Nd–Fe–B magnets by electrophoretic deposition, Scr. Mater. 116 (2016) 40–43. [7] A.M. Gabay, M. Marinescu, W.F. Li, J.F. Liu, G.C. Hadjipanayis, Dysprosiumsaving improvement of coercivity in Nd-Fe-B sintered magnets by Dy2S3 additions, J. Appl. Phys. 109 (2011) 083916. [8] H. Sepehri-Amin, T. Ohkubo, K. Hono, The mechanism of coercivity enhancement by the grain boundary diffusion process of Nd–Fe–B sintered magnets, Acta Mater. 61 (2013) 1982–1990. [9] S. Sawatzki, A. Dirks, B. Frincu, K. Löwe, O. Gutfleisch, Coercivity enhancement in hot-pressed Nd-Fe-B permanent magnets with low melting eutectics, J. Appl. Phys. 115 (2014) 17A705. [10] M. Tang, X. Bao, K. Lu, L. Sun, J. Li, X. Gao, Boundary structure modification and magnetic properties enhancement of Nd–Fe–B sintered magnets by diffusing (PrDy)–Cu alloy, Scr. Mater. 117 (2016) 60–63. [11] T.-H. Kim, S.-R. Lee, H.-J. Kim, M.-W. Lee, T.-S. Jang, Magnetic and microstructural modification of the Nd–Fe–B sintered magnet by mixed DyF3/DyHx powder doping, J. Appl. Phys. 115 (2014) 17A763. [12] T.-H. Kim, S.-R. Lee, H.-J. Kim, M.-W. Lee, T.-S. Jang, Simultaneous application of Dy–X (X = F or H) powder doping and dip-coating processes to Nd–Fe–B sintered magnets, Acta Mater. 93 (2015) 95–104. [13] A.K. Niessen, F.R. de Boer, R. Boom, P.F. de Châtel, W.C.M. Mattens, A.R. Miedema, Model predictions for the enthalpy of formation of transition metal alloys II, Calphad 7 (1983) 51–70. [14] W.F. Li, T. Ohkubo, T. Akiya, H. Kato, K. Hono, The role of Cu addition in the coercivity enhancement of sintered Nd-Fe-B permanent magnets, J. Mater. Res. 24 (2011) 413–420. [15] O.M. Ragg, I.R. Harris, A study of the effects of the addition of various amounts of Cu to sintered Nd-Fe-B magnets, J. Alloys Compd. 256 (1997) 252–257.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
Effect of diffusing TbF3 powder on magnetic properties and microstructure transformation of sintered Nd-Fe-Cu-B magnets Xiao Yang a,b, Shuai Guo a,⇑, Guangfei Ding a,b, Xuejing Cao a, Jiling Zeng a,b, Jie Song a, Don Lee c, Aru Yan a,⇑ a
Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China University of Chinese Academy of Sciences, Beijing 100049, China c University of Dayton, Dayton, OH 45469, USA b
a r t i c l e
i n f o
Article history: Received 25 May 2017 Received in revised form 12 July 2017 Accepted 20 July 2017 Available online 21 July 2017 Keywords: Cu migration Grain growth area Sintered Nd-Fe-B magnets Grain boundary diffusion
a b s t r a c t The coercivity of sintered Nd-Fe-Cu-B magnets is markedly enhanced from 12.57 to 21.70 kOe while the remanence decreases from 13.80 to 13.49 kGs by grain boundary diffusion of TbF3 powder for 2 h. Microstructure analysis suggests that, during the diffusion process, F diffuses into the magnets easily and forms a new F-rich phase. The enrichment of F in grain boundary near the surface leads to the Cu movement into the interior and the Cu reduction in the surface of magnets. Diffusion of Tb leads to an increase of local total rare earth elements content. Under the combined effect of Cu reduction and increase of local total rare earth elements content, grain growth area is formed and further diffusion is suppressed. That excessive Tb diffuses into matrix phase leads to a decrease in remanence. When the grain growth area is removed, the deterioration of remanence recovers to 13.80 kGs without any reduction of coercivity. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Nd-Fe-B based sintered magnets have been widely applied in traction motors of electric vehicles and hybrid electric vehicles because of their excellent magnetic properties [1]. But the intrinsic coercivity of commercial magnets is still insufficient to satisfy the demands for electric vehicles [2]. Partial substitution of heavy rare earth (HRE) elements like Dy and Tb for Nd is proved an efficient method to significantly increase the coercivity of magnets by enhancing magnetocrystalline anisotropy [3]. However, the remanence and the energy product reduced inevitably due to the antiferromagnetic coupling between the HRE atoms and Fe atoms. Furthermore, the abundance of HRE in the earth’s crust is limited, so an available method for enhancing coercivity with none or slight HRE elements introduction is desired. Grain boundary diffusion process (GBDP) has been proved to be a promising way to enhance coercivity without sacrifice of remanence and energy product. In these processes, magnets are coated with HRE elements compounds in the forms of oxides [4], fluorides [5,6], sulfides [7], pure
⇑ Corresponding authors at: Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China. E-mail addresses: [email protected] (X. Yang), [email protected] (S. Guo), [email protected] (A. Yan). http://dx.doi.org/10.1016/j.jmmm.2017.07.071 0304-8853/Ó 2017 Elsevier B.V. All rights reserved.
metal [8] or low melting eutectics [9,10], followed by the heat treatment for HRE elements diffusion. In GBDP technology, the formation of (Nd,HRE)2Fe14B shell structure surrounding Nd2Fe14B matrix phase is supposed to be the source of the enhancement of coercivity because coercivity of sintered Nd-Fe-B magnets is mainly controlled by the nucleation of reversal domains from the surface of matrix phase. HRE fluorides like DyF3 and TbF3 are common diffusion source in laboratory. We know that DyF3 additives reacted with the grain boundary phase and Nd-O-F phase was formed during sintering [11]. And Kim et al. [12] reported that the DyH2 dipped magnets and the DyF3 dipped magnets had different chemical composition of the grain boundary phases due to F introduction. After diffusion, the magnets had different magnetic properties. Therefore, F has huge influence on the chemical composition properties of the Nd-rich grain boundary as well as the diffusion behavior. On the other hand, Cu is one kind of most common elements in sintered Nd-Fe-B magnets and has huge influence on the microstructure and magnetic properties. Cu segregates in grain boundary phase because of the positive mixing enthalpy of Fe and Cu [13]. Cu addition is in favor of the continuous Nd–rich thin layer along the grain boundary and the decoupling between the neighboring matrix grains [14]. However, the effect of F on the Cu distribution, the related microstructure transformation and the corresponding change of magnetic properties of Nd-Fe-Cu-B magnets have not been revealed completely.
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In this study, Nd-Fe-Cu-B magnets with different diffusion time were obtained by electrophoretic diffusing TbF3 powder. The elements distribution and resultant microstructural changes were investigated thoroughly. Based on these results, we clarified the effect of F on chemical composition in grain boundary phase and the evolution mechanism of grain growth area. 2. Experimental procedure Alloys with a nominal composition Nd32.5Cu0.1FebalB0.92 (wt%) were prepared by strip cast (SC) technique. The strips were subjected to subsequent hydrogen decrepitation (HD) process and further jet milling (JM) in a nitrogen atmosphere to refine the powder with an average particle size of 1.9 lm. The prepared powders were compacted and aligned under a magnetic field of 2.25 T and followed by the cold isostatic compacting under a pressure of 150 MPa. The green compacts were sintered at 980 °C for 2 h in vacuum followed by gas quenching. Then the as-sintered magnets were annealed at 900 °C and 500 °C for 2 h in sequence. The final annealed samples were machined to cylinders with a diameter of 10 mm and a height of 4 mm for diffusing. In electrophoretic deposition (EPD) technology, the cut magnets as anode and the steel plate as cathode were both immersed in ethanol-based TbF3 suspension at a constant applied voltage of 60 V for 2 min. After coated process, the coated samples were removed from the suspension and dried in hot air. Then the coated samples were diffused at 900 °C for 2 h, 4 h, 10 h and 15 h respectively and subsequently annealed at 500 °C for 2 h in vacuum. In dipping technology, the cut magnets were directly immersed in ethanol-based TbHx suspension for 5 s and then dried in N2 atmosphere. The TbHx coated magnets were diffused at 900 °C for 15 h and then annealed at 500 °C for 2 h in vacuum. Magnetic properties were measured by NIM-500 C hysteresigraph analyzer system. The microstructures and elemental distribution of the magnets were observed by scanning electron microscope (SEM) (Quanta FEG 250) equipped with an energy dispersive X-ray spectrometer (EDS) (Oxford Aztec system). The phase constituents of diffused magnets were examined by X-ray diffraction (XRD) with the Cu-Ka radiation. The surface depth profiles were determined by glow discharge atomic emission spectrometer (GDA) (Spectruma Analytik GMBH 750HP). 3. Results and discussion Fig. 1 shows Nd, Tb, total rare earth and Cu relative concentration versus distance from the surface of the diffused magnets with different diffusion time. After diffusion, a large amount of Tb is
enriched at the magnet surface, and Tb concentration first rapidly decreases and then gradually becomes stable while Nd concentration shows a contrary tendency. The width of Tb enrichment region increases as diffusion time increases. About 20 lm away from the surface, Nd concentration reaches a maximum value, which is higher than the level of the original magnets, and that is because Nd is partly substituted by Tb and the redundant Nd migrates into deeper region. The total rare earth concentration curves which consist of Tb and Nd are also showed. That the total rare earth content of diffused magnets is lower than the original magnets at the surface might be caused by a little of residual TbF3. Due to the diffusion of Tb, the total rare earth concentration in the region near the surface is higher than that of the original magnets. The Cu concentration variation curve has three distinct stages, and the level of the two stages near the surface is below the original magnets while the level of the third stage is upon the original magnets. With the increase of diffusion time, the width of the Cu-lean region increase gradually, that might be derived from the Cu migration from surface to interior. Tb concentration variation of EPD-processed and dipped samples diffusing for 15 h is shown in Fig. 2. In the region of headmost 50 lm, Tb concentration of the EPD-processed sample is higher than the sample dipped with TbHx powders. But in the region deeper than 50 lm, the Tb concentration shows a contrary relationship. The cross-sectional Second Electron (SE) SEM image of the diffused magnets for 15 h diffusion is shown in Fig. 2 inset (a). A grain growth area is developed at the surface while the internal grains still keep initial morphology. It is well known that Cu segregates in the grain boundary phase. The formation of grain growth area extrudes the grain boundary phase and the grain boundary phase moves into deeper region, so the Cu concentration in the grain growth area is lower than the adjacent region. A compared experiment using TbHx as diffusate was carried out to prove the grain growth area is the primary barricade of diffusion. Fig. 2 inset (b) shows the SE-SEM image of magnets dipped with TbHx powders after 15 h diffusion. There is only a small amount of grown grains occur in the region near the surface, and the grain boundary, i.e., the diffusion channel still exists. It indicates that more Tb atoms can diffuse into the interior without the grain growth area. Noted that the grain boundary is supposed to be the main diffusion channel of the diffusion, the formation of the grain growth area would obstruct the diffusion channel and prevent the further diffusion. As a result, Tb is confined in the grain growth area and difficult to diffuse into the interior. The cross-sectional back scattered electron (BSE) SEM images of the diffused magnets with different diffusion time are shown in Fig. 3. In all diffused magnets, the typical Tb-rich shell structure
Fig. 1. Nd, Tb, Cu and total rare earth elements (labeled as RE) relative concentration, which is equal to respective concentration of diffused samples minus the concentration of original samples, versus depth from the surface of the diffused magnets with (a) 2 h and (d) 15 h diffusion.
X. Yang et al. / Journal of Magnetism and Magnetic Materials 443 (2017) 179–183
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Fig. 2. Tb concentration variation of EPD-processed and dipped samples diffusing for 15 h. The insets show SE-SEM images of (a) EPD-processed and (b) TbHx dipped samples treated for 15 h.
which corresponds to grey area surrounding the dark Nd2Fe14B phase is clearly visible in the region near the surface. In addition, it is obvious that the grain boundary phase consists of two phases with different contrasts. Fig. 4 shows the typical microstructure of the grain boundary phase in the diffused magnets. We can find that Nd is enriched and Fe is lean in the grain boundary phase. However, F and Cu mapping exhibit contrary distribution, which F concentrates in area 2 and Cu concentrates in area 1. According to EDS analysis shown in Table 1, we can conclude that the grey contrast corresponds to the F-rich phase marked as green arrows in Fig. 3 while the bright contrast corresponds to the Cu-rich phase marked as red arrows in Fig. 3. As shown in Fig. 3, in all diffused magnets, the distribution of newly formed F-rich phase occupies the triple junction area near the surface and then decreases gradually as depth increases. But the Cu-rich phase exists more in the corners of the tripe junction area and the inner region. With the increase of diffusion time, there is no obvious difference in the distribution of F-rich phase and Cu-rich phase. Integrating the previous analysis, Fig. 5 (a) shows the schematic diagrams of the microstructure evolution of F diffusion. Noted that the size of F atom is smaller than Tb atom, F diffuses much more easily than Tb atom. 2 h are enough for F atoms to diffuse sufficiently, and this is why there is no obvious difference in the distribution of F-rich phase with the increase of diffusion time. During the diffusion process, F diffuses into the magnets along grain boundary and reacts with the grain boundary phase. F-rich phase is formed and occupies the triple junction phase. Because the Frich phase is stoichiometrically stable [12] and the solubility of Cu in F-rich phase is negligible, Cu separates out from the grain boundary phase and migrate into the interior of magnets. This is why a small amount of Cu-rich phase exists in the corner of triple junction phase and Cu concentration in the interior is higher in the GDA curves. At the same time, as shown in Fig. 5(b), Tb diffuses into magnets and causes higher total rare earth content in the region near the surface. The sinter process of Nd-Fe-B based sintered magnets is a liquid phase sintering process. The diffusion of Tb into the magnets causes the increase of the liquid phase and promote the sintering process for full density. So the grains are easier to grow in the case of higher rare earth content. On the other hand, The magnets are more susceptible to densification in the case of lower Cu content [15]. During the GBDP, on the magnets
Fig. 3. BSE-SEM images for diffused magnets with (a) 2 h, (b) 4 h, (c) 10 h and (d) 15 h diffusion.
surface, the liquid phase increases because of the diffusion of Tb into magnets while Cu content reduces because of Cu migration caused by F diffusion. Under the combined influence of the two factors, the grains of the surface of the magnets begin to grow. Eventually, the grain growth area is formed and the diffusion is suppressed. So the concentration of Tb in grain growth area is far higher than that in the interior. Magnetic properties of the TbF3 EPD-treated sintered magnets as a function of different diffusion time are shown in Fig. 6(a). In general, the coercivity of diffused magnets first increases and then becomes stable gradually. With 2 h diffusion, coercivity increases from 12.52 to 21.70 kOe and remanence decrease from 13.80 to 13.41 kGs. The enhancement of coercivity and the reduction of remanence can be attributed to the partial substitution of Tb for Nd in the surface of matrix phase. Because the grain growth area suppresses more Tb diffusion, with further increasing the diffusion time, coercivity and remanence do not show obvious change and tend to be stable. When the diffusion time is more than 10 h, coercivity does not change markedly. So the optimal diffusion time of the TbF3 EPD magnets should be 10 h. As shown in Fig. 6(b), there is a slight abnormal reduction in demagnetization curves of the diffused magnets when the external field is small. The reason for the
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Fig. 4. BSE image and corresponding EDS elemental distributions mapping of Nd, Fe, F and Cu of diffused magnets with 2 h.
Table 1 Chemical analysis of area 1 and area 2 in Fig. 4. Area
Nd(wt%)
Fe(wt%)
O(wt%)
F(wt%)
Cu(wt%)
1 2
77.38 80.61
10.85 7.73
0.75 3.31
0 8.29
11.01 0.06
Fig. 5. Schematic diagram of microstructure evolution of (a) F diffusion and (b) Tb diffusion in the diffusion process. GB refers to grain boundary.
abnormal reduction is that the grain growth area has extremely low coercivity and demagnetizes easily. When removed the grain growth area (50 lm), the slight abnormal reduction disappears and remanence recovers from 13.59 to 13.80 kGs and coercivity slightly increases at the same time. The recovery of remanence is due to the removal of the region containing excessive Tb in matrix phase.
4. Conclusion In summary, the magnetic properties and microstructure evolution of TbF3 diffused sintered Nd-Fe-Cu-B magnets with different diffusion time using EPD were investigated. After diffusion with different time, the coercivity increases first and then becomes stable. Two kinds of Cu migration are observed in the diffused
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Fig. 6. (a) Magnetic properties of untreated and diffused magnets with different diffusion time. Hcj and Br refer to coercivity and remanence respectively. (b) Demagnetization curves of diffused and polished magnets.
magnets, which are caused by the reaction between F and the grain boundary phase and the formation of the grain growth area respectively. The trace Tb that diffuses into the interior of magnets is the source of the coercivity enhancement. However, excessive Tb concentrates in the surface and makes local total rare earth content increase. Under the combined effect of Cu reduction and increase of local total rare earth elements content, the grain growth area is formed. The grain growth area obstructs the diffusion channel and prevents further diffusion. When the grain growth area is removed, the deterioration of remanence recovers to original level without any reduction of coercivity.
Acknowledgements This work is supported by the National Natural Science Foundation of China (No. 51501211), Ningbo Major Industrial Technology Innovation Projects (2016B10007), the Program of Ningbo International Corporation (No. 2015D10019) and the Program of Ningbo Innovation Team (No. 2012B81001), Guangxi Science and technology projects (No. 2016AD05041) and Major projects of Inner Mongolia.
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