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Improvement of microstructure and coercivity for Nd-Fe-B sintered magnets by boundary introducing low melting point alloys
Journal Pre-proof Improvement of microstructure and coercivity for Nd-Fe-B sintered magnets by boundary introducing low melting point alloys Shuai Cao, Xiaoqian Bao, Jiheng Li, Haijun Yu, Kunyuan Zhu, Xuexu Gao PII:
S1002-0721(19)30025-0
DOI:
https://doi.org/10.1016/j.jre.2019.04.021
Reference:
JRE 616
To appear in:
Journal of Rare Earths
Received Date: 14 January 2019 Revised Date:
10 April 2019
Accepted Date: 11 April 2019
Please cite this article as: Cao S, Bao X, Li J, Yu H, Zhu K, Gao X, Improvement of microstructure and coercivity for Nd-Fe-B sintered magnets by boundary introducing low melting point alloys, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.04.021. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © [Copyright year] Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.
Improvement of microstructure and coercivity for Nd-Fe-B sintered magnets by boundary introducing low melting point alloys Shuai Cao, Xiaoqian Bao, Jiheng Li, Haijun Yu, Kunyuan Zhu, Xuexu Gao*. State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, 30Xue Yuan Road, Beijing 100083, People's Republic of China * Corresponding author. E-mail address: [email protected] (Xuexu Gao).
Abstract Different from the grain boundary diffusion process (GBDP), which is suitable for modifying thin magnet, a green-pressing agents permeation process (GAPP) that uses low melting point alloys was applied to the Nd-Fe-B green compact with a thickness over 15 mm to reconstruct the boundary microstructure of a sintered Nd-Fe-B magnet. The coercivity increased from 12.3 kOe for the sample free of Pr80Al20 to 16.8 kOe for the sample with 2 wt% Pr80Al20. By further increasing the Pr80Al20 content to 3 wt%, the coercivity increased slightly, but the remanence and Hk/Hcj deteriorated obviously. The optimal comprehensive properties of Hcj=16.8 kOe, Br=13.4 kG and Hk/Hcj=0.975 were obtained at 2 wt% Pr80Al20, since matrix phase grains were separated by relatively continuous thin grain boundary layers, which weakened the magnetic coupling between adjacent grains. The coercivities of the samples from the GAPP that use 2 wt% Pr80Al20, Pr70Cu30 and Pr60Tb20Al20 alloys, respectively, could be enhanced to a large extent. However, the coercivity of the magnet reconstructed with Pr80Al20 was lower than that of the sample with Pr60Tb20Al20 but was higher than that of the sample reconstructed with Pr70Cu30 alloy. Moreover, the coercivity of the sample from the GAPP using 2 wt% Pr80Al20 was much higher than that of the sample from the GBDP, which was due to a nearly uniform boundary microstructure from the surface to the interior of the thick magnet from the GAPP, thus providing new insights into the fabrication of thick and bulky permanent magnets with high coercivity. Keywords: Nd-Fe-B sintered magnets, green-pressing agents permeation, grain boundary diffusion, Pr-Al alloys, thick and bulky magnets
1. Introduction Sintered NdFeB rare earth (RE) magnets with outstanding permanent magnetic properties at room temperature have found a wide range of applications, especially for the traction motors of hybrid electric vehicles and wind generators, which need not only high magnetic properties but also thermal and mechanical stability [1,2]. Recently, thick and bulky magnets with high performance have become urgently needed to meet the design requirements of large device components. In recent years, a new method named grain boundary diffusion process (GBDP) has been reported to improve magnetic properties effectively, by allocating some elements just to the grain boundary region to modify the intergranular structure. This process involves coating some diffusion sources on the surface of magnets, followed by diffusion heat treatment [3–5]. So far, diffusion sources with RE pure metals, eutectic alloys, fluorides, oxides or metal vapors have been extensively studied in the last few years. By this method, the coercivity of the magnet may be enhanced significantly without considerable reduction in the remanence [6–10]; however, because of the insufficient driving force of the diffusion agents on the grain boundary of NdFeB sintered magnets, the effective diffusion depth is very limited, and this technology can only be applied to very thin bulk magnets [11]. Moreover, the traditional GBDP usually ☆Foundation
item: Project supported by the National Natural Science Foundation of China (No. 51401021) and the State Key Laboratory Advanced Metals and Materials (No. 2016Z-14). * Corresponding author. E-mail address: [email protected] (Xuexu Gao).
inevitably causes uneven distribution of diffusion agents or decreases the squareness of the diffusion processed magnets. For the thin effective diffusion depth, deterioration of the squareness or heterogeneity in structures of sintered NdFeB magnets, industrial production and application could sometimes be limited by GBDP [12]. In addition, as for another traditional method, the dual alloys method could produce bulky magnets with a good effect on enhancing coercivity [13–15]; however, it is hard work to control remanence. In this method, the amounts of secondary phases powders always need to reach a certain quantity due to agglomeration and oxidation, which would lead to the excessive introduction of secondary phases and a considerable reduction in remanence. Moreover, secondary phase particles need to obtain a smaller size with a more uniform size distribution through additional processes in to obtain better mixing effects with main phase particles, and at this point, main phase particles with different morphologies or series need to match complementary particles of extremely small or different sizes, which means that more complex or targeted processes are required for different kinds of magnets when using the dual alloys methods. In this work, thick and bulky magnets are research samples, that have a dimensional size of Ø10 × 12 mm3 after sintering and densification. In addition, we propose a green-pressing agents permeation process (GAPP for short) for sintered NdFeB magnets that use eutectic alloys with a low melting point, in which green-pressing is the pressed compact after isostatic pressing rather than the high-density magnet after sintering. This study involved coating certain alloy ribbons with the compositions of Pr80Al20 (at%) that were prepared by the melt-spinning technique on the surface of the thick green-pressing followed by a series of heat treatments, and then the method and its influences on both microstructure and magnetic properties of the magnets have been investigated. Coercivity of the restructured magnet is found to increase with previous Pr80Al20 covering contents, while squareness is optimal with coating at 2 wt%. Meanwhile, the microstructure is fairly uniform along permeation direction. The GAPP is more similar to a method that combines the GBDP and the dual alloys method, and it could be a good way to make bulky magnets with high coercivities.
2. Experimental Magnetic powders for commercial NdFeB magnets with the composition of Nd22.35Pr7.75Co0.45Mn0.12Al0.29Ga0.11Cu0.12Zr0.11B1.09Febal (wt%) were made for green-pressing, and the green-pressing had a dimensional size of Ø12 × 16 mm3 via orientation in magnetic field and cold isostatic pressing in a certain mold. The alloy ribbons with the nominal compositions of Pr80Al20 (at%) were produced by the melt-spinning technique using the high vacuum quenching system with a copper roller speed of approximately 15 m/s, of which the differential scanning calorimeter (DSC) curves are shown in Fig. 1(a). One group of green-pressings were covered by pieces of ribbons on the upper and lower surfaces and were set in the ceramic crucibles, in which the alloy ribbons were perpendicular to the orientation of the green-pressing (c axis), as is schematically shown in Fig. 1(b), and the green-pressings were covered with Pr80Al20 alloys ribbons at mass fractions of 0%, 1.0%, 2.0%, and 3.0%, respectively. Then, these samples were treated successively at 700 for 2 h and at 1060 for 2 h, subsequently annealing at 900 and 490 after quenching. In this process, they were all protected by a high vacuum following high purity argon.
Fig. 1. The DSC curves of alloy ribbons with the compositions of Pr80Al20 (a), and a schematic diagram of the
layout and treatment mode (b).
The room-temperature magnetic properties were measured by a NIM-2000 magnetic measurement device. Microstructural and compositional analyses were conducted by backscattered electron scanning electron microscopy (SEM, Carl Zeiss, Supra55), a field emission transmission electron microscope (FE-TEM, JEOL, JEM-2200FS), and an electron probe microanalyzer (EPMA, JEOL, JXA-8230). Densities of samples were measured by drainage, and porosities of samples were measured by the automatic surface area and a porosity analyzer (ASAP, Quantachrome, QUADRASORB SI).
3. Results and discussion 3.1. Characteristics of the magnets produced by GAPP with Pr80Al20 alloys Fig. 1(a) shows the DSC results for the permeation source of Pr80Al20 alloys. There are two exothermal peaks in the curve. It is a phase transition temperature point at 584 °C, which just corresponds to the Pr-Al phase diagram. The melting points of Pr80Al20 alloys are determined to be 643 °C. Thus, a heat treatment temperature higher than 643 °C was employed for the GAPP process to ensure complete melting of the alloys and the benefit of the permeation process. Fig. 2 shows the room temperature demagnetization curves for the magnets treated with nothing and those treated with 1, 2 and 3 wt% Pr80Al20 alloys ribbons, respectively. The inset shows the magnetic properties Hcj, Br and Hk/Hcj as a function of Pr80Al20 covering amounts. All the magnets have a high squareness factor over 94% and a large density above 7.56 g/cm3. The magnet free of agents possesses a coercivity Hcj of 12.3 kOe, a remanence Br of 13.8 kG and a squareness factor Hk/Hcj of 0.960. With the increase of Pr80Al20 covering amounts, Hcj increases gradually to 14.0 kOe at 1 wt%, 16.8 kOe at 2 wt% and 16.9 kOe at 3 wt%. The coercivity increment is accompanied with a certain reduction in Br, which is mainly attributed to the introduction of secondary components and an increase of the boundary phases. It is noteworthy that Hk/Hcj achieves the highest with 0.975 when treated with 2 wt% Pr80Al20 alloy ribbons. However, a further increase of the Pr80Al20 alloy to 3 wt% can substantially reduce Hk/Hcj to 0.941 with a slight enhancement in Hcj and a greater reduction in Br, which is mainly due to the excessive introduction of agents resulting in the uneven distribution of compositions and microstructures. It implies that a proper covering level of Pr80Al20 alloys must be controlled to obtain high coercivity, high remanence and high squareness factor. Under the present condition in this study, the optimal covering level is 2 wt%. Thus, a detailed microstructure evolution by GAPP treated with the 2 wt% Pr80Al20 alloy is investigated as described below.
Fig. 2. Demagnetization curves for the magnets treated with different amounts of Pr80Al20 alloys. The inset shows the dependencies of coercivity, remanence and squareness on the Pr80Al20 covering contents.
To examine the overall microstructures change, the microstructures of the magnets free of agents and treated with 2 wt% Pr80Al20 alloys were first observed by SEM, as shown in Fig. 3. Both samples consist of a bright contrast, which indicates that the RE-rich phase and a dark contrast showing the matrix Nd2Fe14B phase. For the magnet treated with nothing, the grain boundaries are very thin and are
hardly observed in some areas, as shown in Fig. 3(a) and Fig. 3(b), where short-range exchange coupling could exist for such local grains, which is not beneficial to obtain high coercivity [1,16,17]. When Pr80Al20 alloys are added, the distribution of RE-rich phase changes apparently, as shown in Fig. 3(c), and the intergranular RE-rich phase becomes clearer and more continuous, thus isolating well the adjacent grains. Such distribution of the RE-rich GB phase can eliminate the surface defects and is beneficial to reduce the possible reversal nucleation sites [9], which is one important contribution to the enhanced coercivity. More significantly, even at 6000 µm from the surface (Fig. 3(d)), the regions have very similar structures as those at 50 µm (Fig. 3(c)), which have shown continuous GB phases. Pr and Al element concentrations were analyzed by energy dispersive X-ray spectroscopy (EDS), and 12 measured surfaces of different positions were obtained by cutting 11 equal sections along the permeation direction of the sample. The integration results of Pr and Al concentrations from top to bottom of the sample are shown in Fig. 4. It can be seen that the distributions of both elements’ concentrations are basically homogeneous in the magnet, except for the gradients at the bottom. There is no significant difference in the concentrations, even though the existence of the gradients remain between 4.30 and 4.37 at% in the Pr concentration, and are 0.92 and 1.08 at% in the Al concentration. The formation of the concentration gradients is mainly because small amounts of agents on the lower surface of the sample did not permeate into the interior completely at a low temperature, and then entered the magnet by diffusion at a relatively high temperature, when there was no fast channel to introduce agents, just as in GBDP. The homogeneous distribution of Pr and Al concentrations explains the uniformity of compositions in magnets produced by GAPP with Pr-Al alloys. Thus, it indicates that GAPP could procure more effective thickness, and the thick magnets produced by GAPP would have nearly uniform structures and compositions from the surface to the interior.
Fig. 3. SEM images of the magnet treated with nothing ((a) 50 µm, (b) 6000 µm) and regions at different depths from the surface of the magnet treated with 2 wt% Pr80Al20 alloys by GAPP ((c) 50 µm, (d) 6000 µm).
Fig. 4. Pr and Al elements concentrations distribution from top to bottom of the sample treated with 2 wt% Pr80Al20 alloys by GAPP.
Fig. 5 depicts TEM images of the grain boundary regions for magnets treated with nothing and 2 wt% Pr80Al20 alloys ribbons. The interface between the grain boundary phase and the matrix 2:14:1 phase is not very well defined in the magnet added with nothing, and the grain boundaries are relatively thin and discontinuous, and the two matrix phases appear to be indirect contact in Fig. 5(a). For the magnet treated with 2 wt% Pr80Al20 alloys, however, the interface becomes clearer and smooth, as shown in Fig. 5(b), which confirms that the grain boundary layers can be continuous as a result of GAPP. Meanwhile, some grain boundary phases of the restructured magnet are observed in TEM, as shown in Fig. 5(c), which has a long axis length of 100 nm and a short axis length of 23 nm approximately, which may have good effect on the pinning domain wall and contribute to coercivity.
Fig. 5. TEM characterization of typical regions for the magnets. (a) A TEM image of the grain boundary region for magnet added with nothing; (b, c) TEM images of the grain boundary regions for magnets treated with 2 wt% Pr80Al20 alloys by GAPP.
Fig. 6 shows the EPMA mapping images of Pr (a) and Al (b) elements for the magnet treated with 2 wt% Pr80Al20 alloys by GAPP. Most of the Pr-rich phases show a network surrounding the matrix phase grains, although there are also some agglomerating phases enriched with Pr in the triple junctions. A color level adjustment is used to show the morphology and distribution of Al-rich phases more clearly (inset in Fig. 6(b)), because of the low average contents of Al elements in each region for the magnet, and in the picture, the red auxiliary lines have marked the areas where Al elements are concentrated. It is not difficult to find that Al elements are enriched surrounding the matrix phase grains, even though some have entered into the matrix phase. These core shell structures are very similar to the surface structures of magnets in GBDP, as previously reported. The partial substitution of Pr in Nd2Fe14B compound is beneficial for high coercivity due to a higher HA of Pr2Fe14B than that of Nd2Fe14B [2]. It should be noted that the modified boundary phase isolating 2:14:1 grains by Al is helpful for coercivity, although the partial substitution of Al in Nd2Fe14B decreases the
magnetocrystalline anisotropy field (HA) [18,19]. Thus, it means that Pr-rich and Al-rich shells are formed on the surface of the matrix phase grains for a magnet produced by GAPP, and these structures are good for coercivity enhancement.
Fig. 6. EPMA mapping images of Pr (a) and Al (b) for the magnet treated with 2 wt% Pr80Al20 alloys by GAPP. The inset in (b) is exporting the picture after color level adjustment, which shows that the Al elements are enriched surrounding the matrix phase grains.
3.2. Characteristics of the magnets treated with different types of alloys and methods In addition to Pr80Al20, 2 wt% alloy ribbons with the nominal compositions of Pr70Cu30 and Pr60Tb20Al20 (at%) were also used in GAPP. The specific heat treatment processes differ slightly according to the melting point of different alloys. All the thick and bulky magnets achieved uniform structures, similar to those of the sample with Pr80Al20 alloys. Demagnetization curves for the magnets treated with nothing and different types of alloys are shown in Fig. 7. It can be seen that the coercivities of the samples processed by Pr70Cu30 and Pr60Tb20Al20 alloys increase substantially to 16.2 kOe and 17.9 kOe compared with the sample free of agents at a coercivity of 12.3 kOe. Notably, there is a large increase in coercivity, while there is also a large reduction in remanence when using heavy earth elements, and this is very close to the effect created by the dual alloys method. It is emphasized that there are massive Tb elements diffusion from the grain boundary into the grain, since densification is carried out at a high temperature. The coercivities of the samples increase with the addition of Tb elements because the substitution of Tb elements for Nd in GAPP and Tb2Fe14B phases have much higher HA than that of Nd2Fe14B or Pr2Fe14B [2], and the reduction in remanence is because of the antiferromagnetic coupling between Fe and Tb. Thus, it is may not be appropriate to use heavy rare earth elements as permeation agents in GAPP, considering the further reduction of remanence. Magnets processed by GBDP were also studied in this work. The densified N48 magnet was covered as the same mode by 2 wt% Pr80Al20 alloy ribbons, which was then followed by optimal diffusion heat treatment. The coercivity of the GBDP sample reached to 14.6 kOe, as shown in Fig. 7 (green dotted line). This coercivity is lower than the result of traditional GBDP that was reported previously, because the sample used in this experiment is a thick magnet that has a height over 12 mm, and the thin effective diffusion depth is not enough to improve the performance of the whole magnet greatly. Shown in Fig. 8 are EPMA maps of the above magnets processed by GBDP and GAPP. It could be clearly observed that there are distribution gradients of elements in the diffusion direction for the sample GBDP (Fig. 8(a–c)), especially for Al elements in Fig. 8(b). However, for the sample GAPP, elements are generally uniform in the effective thickness, as shown in Fig. 8(d–f). This once again reveals the reasons why GAPP increases coercivity more than GBDP does for a thick magnet, and magnet processed by GAPP has a better squareness factor, as shown in Fig. 7.
Fig. 7. Demagnetization curves for the magnets treated with different types of alloys and methods.
Fig. 8. EPMA maps of the samples processed by GBDP (a–c) and GAPP (d–f) from the surface close to the agent sources.
Magnetic properties at higher temperatures were measured in the study. Fig. 9 shows the temperature dependence of remanence Br and the coercivity Hc of the magnets treated with nothing and magnets treated with different types of alloys. The temperature coefficients at 20–80 °C and 20–170 °C for the different magnets are listed in the inset table. It can be seen that the temperature coefficients of remanence α for magnets treated with alloys reduced slightly compared to the magnet without GAPP treatment at 20–80 °C, and the reason is attributed to the increased nonmagnetic phases in the magnets. However, the temperature coefficients of coercivity β show different behavior compared to coefficients of α. At the low temperature range of 20–80 °C, the value of β is –0.77 %/°C for magnets without GAPP treatment, –0.74 %/°C for magnets treated with Pr80Al20 alloys, –0.75 %/°C for magnets treated with Pr70Cu30 alloys and –0.73 %/°C for magnet the treated with Pr60Tb20Al20 alloys. The improved temperature stability of coercivity at low temperature is mainly attributed to the continuous distribution of RE-rich GB phases and the core-shell structures in magnets, as shown in Figs. 3 and 6. While at the high temperature range of 20–170 °C, there is no significant difference between the values of α and β for all the magnets, which is a combined result of the changes of intrinsic property HA and the extrinsic property c/Neff in this study.
Fig. 9. Temperature dependent of remanence Br and coercivity Hc of the magnets treated with nothing and different types of alloys. The inset table shows the temperature coefficients at 20–80 °C and 20–170 °C for the different magnets.
3.3. Discussion on the mechanism of GAPP It is widely accepted that GBDP follows Fick's law [20,21]; however, the mechanism of GBDP is far different from that of GAPP, which is more complicated, and it experiences permeation behavior at a relatively low temperature and diffusion homogenization at a relatively high temperature. In a nutshell, GAPP involves permeating liquid low melting point alloys into magnetic powders, which are compressed and porous at lower temperatures, and then conducts sintering densification at higher temperatures to finally obtain magnets with generally even distribution of desired agents. GAPP can obtain more homogenous microstructures because the introduction of target elements not only relies on the diffusion process that depends on the concentration gradient as that in GBDP, it can also be described as follows. With the melting of alloys during heating, the liquid alloys preferred to permeate through larger gaps among powder particles of the green-pressing at a relatively low temperature. In addition, the permeation channels were gradually distributed throughout the green-pressing with the increasing holding time. Then, the magnet became densified at a relatively high temperature, and the above channels that filled with agents became diffusion sources at that time. Subsequently, liquid phases with agents flowed and diffused into grain boundaries from the above channels. Finally, the matrix phase particles were being greatly coated with desired agents after sintering densification and annealing, so the desired agents can enter the magnet more sufficiently. To explain the permeation behavior of the liquid alloy in GAPP better, the basic theory of fluid mechanics will be applied.
Fig. 10. Variation curves of density and the total pore volume in the sample with temperature rising. Statistical pore sizes range from 2 nm to 1 µm.
It is not difficult to determine that the Reynolds number (Re) of the molten alloy flow is very small; in other words, the liquid alloy flow in GAPP has viscous flow and is slowly permeated. At this point, the permeation behavior of the liquid alloy in GAPP follows Darcy's law [22–25], which can be calculated by Equation (1): Q=k
A∆ p µL
(1)
where Q is the volume flow rate of fluid permeated through porous medium; k is the permeability rate that is closely related to the porosity of the porous medium; A is the cross-sectional area of the pore; ∆p is the pressure differential of fluid between the beginning and end of the permeation process, which is a certain number in this study; µ is viscosity of fluid that reflects fluidity of liquid; and L is the length of effective permeation. Obviously, it is essential to control Q and L in GAPP to introduce the appropriate mass of agents for further improvement of remanence, while k and A of a certain given green-pressing are extremely relevant to the pore volume that changed by temperature. Fig. 10 shows the variation curves of density and the total pore volume in the used magnet with temperature rising, and statistical pore sizes range from 2 nm to 1 µm. It can be seen that the density of the magnet increases dramatically from 680 to 780 and then reaches its maximum at 1060 . Meanwhile, there has also been a significant reduction in the pore volume since 680 . It means that the permeation rate is higher before 680 and smaller after 780 for a certain liquid alloy, assuming that the viscosity (µ) does not change with temperatures. Actually, the viscosity of liquid alloys usually decreases with increasing temperatures, and liquid alloys with lower melting points have higher fluidities at this range of temperatures, which is the advantage of the low melting point alloys in GAPP. For example, the Pr80Al20 alloys melted at 643 °C in this study, as shown in the DSC of Fig. 1(a), so the heat treatment temperature for the permeation process was set as 700 °C, which ensures a higher fluidity of the alloys and a larger porosity of the green compact. Thus, the effects of GAPP are the result of the used temperatures and alloys for permeation. For the different requirements of the secondary phase introduction, introducing more agents requires a lower melting point of alloys and lower temperatures in the permeation process, while introducing fewer agents requires relatively higher melting point alloys and higher temperatures that are below 1060 , theoretically. This relationship means that the introduction of the secondary phases can also be controlled by adjusting the temperatures in the permeation process, and it is different from that of the dual alloys method, in which a certain amount of second phases are introduced at one time before the heat treatments. There are still many alloys that can be used for permeation in GAPP, and it provides a way of thinking for further research.
4. Conclusions
In summary, thick and bulky Nd-Fe-B sintered magnets were prepared by the GAPP with Pr80Al20 alloys ribbons. The coercivity of the restructured magnet is found to increase with Pr80Al20 covering contents (0, 1.0, 2.0 and 3.0 wt%). The coercivity increment is accompanied by a certain reduction in Br, which is mainly attributed to the introduction of secondary components and an increase of the boundary phases. Optimal comprehensive performance shows Hcj of 16.8 kOe, Br of 13.4 kG and Hk/Hcj of 0.975 at 2.0 wt% covering content. Microstructural investigations show that the coercivity enhancement of the sample is mainly attributed to the magnetic strengthening of shells on the surfaces of Nd2Fe14B grains and continuous grain boundary layers along the matrix phase grains, which can weaken magnetic coupling between adjacent grains. Meanwhile, it has nearly uniform structures from the surface to the interior. In addition, it is clear that the control of permeation temperatures and the different melt points of introduced alloys become critical factors that affect the results of GAPP due to its characteristics during the process. The GAPP is more similar to a method that combines GBDP and the dual alloys method in a sense, for it has better uniformity than that of GBDP and a more controllable second-phase introduction process than that of the dual alloys method. In all, it shed new insights into the fabrication of thick and bulky permanent magnets with high performance, which will meet the design requirements of large device components in modern production.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51401021) and the State Key Laboratory Advanced Metals and Materials (No. 2016Z-14).
References [1] [2] [3] [4] [5] [6] [7]
[8] [9] [10] [11] [12] [13]
[14]
Hono K, Sepehri-Amin H. Strategy for high-coercivity Nd–Fe–B magnets. Scr Mater. 2012, 67(6): 530. Hirosawa S, Matsuura Y, Yamamoto H, Fujimura S, Sagawa M. Magnetization and magnetic anisotropy of R2Fe14B measured on single crystals. J Appl Phys. 1986, 59(3): 873. Kohashi T, Motai K, Nishiuchi T, Hirosawa S. Magnetism in grain-boundary phase of a NdFeB sintered magnet studied by spin-polarized scanning electron microscopy. Appl Phys Lett. 2014, 104(23): 232408. Xu F, Wang J, Dong X P, Zhang L T, Wu J S. Grain boundary microstructure in DyF3-diffusion processed Nd–Fe–B sintered magnets. J Alloy Compd. 2011, 509(30): 7909. Liu X L, Wang X J, Liang L P, Zhang P, Jin J Y, Zhang Y J, Ma T Y, Yan M. Rapid coercivity increment of Nd–Fe–B sintered magnets by Dy69Ni31 grain boundary restructuring. J Magn Magn Mater. 2014, 370: 76. Hamada N, Mishima C, Mitarai H, Honkura Y. Development of Nd-Fe-B anisotropic bonded magnet with 27 MGOe. IEEE Trans Magn. 2003, 39(5): 2953. Sepehri-Amin H, Ohkubo T, Nishiuchi T, Hirosawa S, Hono K. Coercivity enhancement of hydrogenation– disproportionation–desorption–recombination processed Nd–Fe–B powders by the diffusion of Nd–Cu eutectic alloys. Scr Mater. 2010, 63(11): 1124. Wang C G, Yue M, Zhang D T, Liu W Q, Zhang J X. Structure and magnetic properties of hot deformed Nd2Fe14B magnets doped with DyHx nanoparticles. J Magn Magn Mater. 2016, 404: 64. Sepehri-Amin H, Ohkubo T, Hono K. The mechanism of coercivity enhancement by the grain boundary diffusion process of Nd–Fe–B sintered magnets. Acta Mater. 2013, 61(6): 1982. Vial F, Joly F, Nevalainen E, Sagawa M, Hiraga K, Park K T. Improvement of coercivity of sintered NdFeB permanent magnets by heat treatment. J Magn Magn Mater. 2002, 242-245: 1329. Hirota K, Nakamura H, Minowa T, Honshima M. Coercivity Enhancement by the Grain Boundary Diffusion Process to Nd–Fe–B Sintered Magnets. IEEE Trans Magn. 2006, 42(10): 2909. Nakamura H, Hirota K, Ohashi T, Minowa T. Coercivity distributions in Nd–Fe–B sintered magnets produced by the grain boundary diffusion process. J Phys D Appl Phys. 2011, 44: 064003. Wan F M, Zhang Y F, Han J Z, Liu S Q, Liu T, Zhou L, Fu J B, Zhou D, Zhang X D, Yang J B, Yang Y C, Chen J, Deng Z W. Coercivity enhancement in Dy-free Nd–Fe–B sintered magnets by using Pr-Cu alloy. J Appl Phys. 2014, 115(20): 203910. Zhou L, Li J, Cheng X H, Liu T, Yu X J, Li B. Dy gradient and coercivity in grain boundary diffusion processed Nd-Fe-B magnet. J Rare Earths, 2017, 35(6): 559.
[15] Liang L P, Ma T Y, Wu C, Zhang P, Liu X L, Yan M. Coercivity enhancement of Dy-free Nd–Fe–B sintered magnets by intergranular adding Ho63.4Fe36.6 alloy. J Magn Magn Mater. 2016, 397: 139. [16] Woodcock T G, Zhang Y, Hrkac G, Ciuta G, Dempsey N M, Schrefl T, Gutfleisch O, Givord D. Understanding the microstructure and coercivity of high performance NdFeB-based magnets. Scr Mater. 2012, 67(6): 536. [17] Li W F, Ohkubo T, Hono K. Effect of post-sinter annealing on the coercivity and microstructure of Nd–Fe–B permanent magnets. Acta Mater. 2009, 57(5): 1337. [18] Rodewald W, Fernengel W. Properties of sintered Nd-Fe-TM-B magnets. IEEE Trans Magn. 1988, 24(2): 1638. [19] Oono N, Sagawa M, Kasada R, Matsui H, Kimura A. Production of thick high-performance sintered neodymium magnets by grain boundary diffusion treatment with dysprosium–nickel–aluminum alloy. J Magn. Magn Mater. 2011, 323(3-4): 297. [20] Loewe K, Benke D, Kübel C, Lienig T, Skokov K P, Gutfleisch O. Grain boundary diffusion of different rare earth elements in Nd-Fe-B sintered magnets by experiment and FEM simulation. Acta Mater. 2017, 124: 421. [21] Hu S Q, Peng K, Chen H. Influence of annealing temperature on the Dy diffusion process in NdFeB magnets. J Magn Magn Mater. 2017, 426: 340. [22] Othman M R, Helwani Z, Martunusa. Simulated fractal permeability for porous membranes. Appl Math Model. 2010, 34(9): 2452. [23] Turcio M, Reyes J M, Camacho R, Lira-Galeana C, Vargas R O, Manero O. Calculation of effective permeability for the BMP model in fractal porous media. J Petrol Sci Eng. 2013, 103: 51. [24] Koponen A, Kataja M, Timonen J. Tortuous flow in porous media. Phys Rev E. 1996, 54(1): 406. [25] Chen Y, Shen C, Lu P, Huang Y. Role of pore structure on liquid flow behaviors in porous media characterized by fractal geometry. Chem Eng Process. 2015, 87: 75.
TOC:
Demagnetization curves for the magnets treated with different types of alloys and methods. (GAPP, Green-pressing agents permeation process. GBDP, Grain boundary diffusion process)
Highlights: 1. Significant coercivity enhancement of thick and bulky Nd-Fe-B sintered magnets was achieved by boundary introducing low melting point alloys, which was named as green-pressing agents permeation process (GAPP). 2. GAPP procured deeper effective thickness and more uniform structure for thick and bulky magnets. 3. Optimal comprehensive performance shows Hcj of 16.8 kOe, Br of 13.4 kG and Hk/Hcj of 0.975 at 2 wt.% covering content by GAPP with Pr80Al20 alloys. 4. Pr80Al20 (at.%) alloys are the optimal choice for thick and bulky magnets as permeation sources in GAPP, comparing with Pr70Cu30, and Pr60Tb20Al20 alloys. 5. The critical factors to affect the results of GAPP are the used temperatures and alloys in permeation processes, which were obtained by discussion about the mechanism of GAPP.
Conflict of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled
S1002-0721(19)30025-0
DOI:
https://doi.org/10.1016/j.jre.2019.04.021
Reference:
JRE 616
To appear in:
Journal of Rare Earths
Received Date: 14 January 2019 Revised Date:
10 April 2019
Accepted Date: 11 April 2019
Please cite this article as: Cao S, Bao X, Li J, Yu H, Zhu K, Gao X, Improvement of microstructure and coercivity for Nd-Fe-B sintered magnets by boundary introducing low melting point alloys, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.04.021. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © [Copyright year] Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.
Improvement of microstructure and coercivity for Nd-Fe-B sintered magnets by boundary introducing low melting point alloys Shuai Cao, Xiaoqian Bao, Jiheng Li, Haijun Yu, Kunyuan Zhu, Xuexu Gao*. State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, 30Xue Yuan Road, Beijing 100083, People's Republic of China * Corresponding author. E-mail address: [email protected] (Xuexu Gao).
Abstract Different from the grain boundary diffusion process (GBDP), which is suitable for modifying thin magnet, a green-pressing agents permeation process (GAPP) that uses low melting point alloys was applied to the Nd-Fe-B green compact with a thickness over 15 mm to reconstruct the boundary microstructure of a sintered Nd-Fe-B magnet. The coercivity increased from 12.3 kOe for the sample free of Pr80Al20 to 16.8 kOe for the sample with 2 wt% Pr80Al20. By further increasing the Pr80Al20 content to 3 wt%, the coercivity increased slightly, but the remanence and Hk/Hcj deteriorated obviously. The optimal comprehensive properties of Hcj=16.8 kOe, Br=13.4 kG and Hk/Hcj=0.975 were obtained at 2 wt% Pr80Al20, since matrix phase grains were separated by relatively continuous thin grain boundary layers, which weakened the magnetic coupling between adjacent grains. The coercivities of the samples from the GAPP that use 2 wt% Pr80Al20, Pr70Cu30 and Pr60Tb20Al20 alloys, respectively, could be enhanced to a large extent. However, the coercivity of the magnet reconstructed with Pr80Al20 was lower than that of the sample with Pr60Tb20Al20 but was higher than that of the sample reconstructed with Pr70Cu30 alloy. Moreover, the coercivity of the sample from the GAPP using 2 wt% Pr80Al20 was much higher than that of the sample from the GBDP, which was due to a nearly uniform boundary microstructure from the surface to the interior of the thick magnet from the GAPP, thus providing new insights into the fabrication of thick and bulky permanent magnets with high coercivity. Keywords: Nd-Fe-B sintered magnets, green-pressing agents permeation, grain boundary diffusion, Pr-Al alloys, thick and bulky magnets
1. Introduction Sintered NdFeB rare earth (RE) magnets with outstanding permanent magnetic properties at room temperature have found a wide range of applications, especially for the traction motors of hybrid electric vehicles and wind generators, which need not only high magnetic properties but also thermal and mechanical stability [1,2]. Recently, thick and bulky magnets with high performance have become urgently needed to meet the design requirements of large device components. In recent years, a new method named grain boundary diffusion process (GBDP) has been reported to improve magnetic properties effectively, by allocating some elements just to the grain boundary region to modify the intergranular structure. This process involves coating some diffusion sources on the surface of magnets, followed by diffusion heat treatment [3–5]. So far, diffusion sources with RE pure metals, eutectic alloys, fluorides, oxides or metal vapors have been extensively studied in the last few years. By this method, the coercivity of the magnet may be enhanced significantly without considerable reduction in the remanence [6–10]; however, because of the insufficient driving force of the diffusion agents on the grain boundary of NdFeB sintered magnets, the effective diffusion depth is very limited, and this technology can only be applied to very thin bulk magnets [11]. Moreover, the traditional GBDP usually ☆Foundation
item: Project supported by the National Natural Science Foundation of China (No. 51401021) and the State Key Laboratory Advanced Metals and Materials (No. 2016Z-14). * Corresponding author. E-mail address: [email protected] (Xuexu Gao).
inevitably causes uneven distribution of diffusion agents or decreases the squareness of the diffusion processed magnets. For the thin effective diffusion depth, deterioration of the squareness or heterogeneity in structures of sintered NdFeB magnets, industrial production and application could sometimes be limited by GBDP [12]. In addition, as for another traditional method, the dual alloys method could produce bulky magnets with a good effect on enhancing coercivity [13–15]; however, it is hard work to control remanence. In this method, the amounts of secondary phases powders always need to reach a certain quantity due to agglomeration and oxidation, which would lead to the excessive introduction of secondary phases and a considerable reduction in remanence. Moreover, secondary phase particles need to obtain a smaller size with a more uniform size distribution through additional processes in to obtain better mixing effects with main phase particles, and at this point, main phase particles with different morphologies or series need to match complementary particles of extremely small or different sizes, which means that more complex or targeted processes are required for different kinds of magnets when using the dual alloys methods. In this work, thick and bulky magnets are research samples, that have a dimensional size of Ø10 × 12 mm3 after sintering and densification. In addition, we propose a green-pressing agents permeation process (GAPP for short) for sintered NdFeB magnets that use eutectic alloys with a low melting point, in which green-pressing is the pressed compact after isostatic pressing rather than the high-density magnet after sintering. This study involved coating certain alloy ribbons with the compositions of Pr80Al20 (at%) that were prepared by the melt-spinning technique on the surface of the thick green-pressing followed by a series of heat treatments, and then the method and its influences on both microstructure and magnetic properties of the magnets have been investigated. Coercivity of the restructured magnet is found to increase with previous Pr80Al20 covering contents, while squareness is optimal with coating at 2 wt%. Meanwhile, the microstructure is fairly uniform along permeation direction. The GAPP is more similar to a method that combines the GBDP and the dual alloys method, and it could be a good way to make bulky magnets with high coercivities.
2. Experimental Magnetic powders for commercial NdFeB magnets with the composition of Nd22.35Pr7.75Co0.45Mn0.12Al0.29Ga0.11Cu0.12Zr0.11B1.09Febal (wt%) were made for green-pressing, and the green-pressing had a dimensional size of Ø12 × 16 mm3 via orientation in magnetic field and cold isostatic pressing in a certain mold. The alloy ribbons with the nominal compositions of Pr80Al20 (at%) were produced by the melt-spinning technique using the high vacuum quenching system with a copper roller speed of approximately 15 m/s, of which the differential scanning calorimeter (DSC) curves are shown in Fig. 1(a). One group of green-pressings were covered by pieces of ribbons on the upper and lower surfaces and were set in the ceramic crucibles, in which the alloy ribbons were perpendicular to the orientation of the green-pressing (c axis), as is schematically shown in Fig. 1(b), and the green-pressings were covered with Pr80Al20 alloys ribbons at mass fractions of 0%, 1.0%, 2.0%, and 3.0%, respectively. Then, these samples were treated successively at 700 for 2 h and at 1060 for 2 h, subsequently annealing at 900 and 490 after quenching. In this process, they were all protected by a high vacuum following high purity argon.
Fig. 1. The DSC curves of alloy ribbons with the compositions of Pr80Al20 (a), and a schematic diagram of the
layout and treatment mode (b).
The room-temperature magnetic properties were measured by a NIM-2000 magnetic measurement device. Microstructural and compositional analyses were conducted by backscattered electron scanning electron microscopy (SEM, Carl Zeiss, Supra55), a field emission transmission electron microscope (FE-TEM, JEOL, JEM-2200FS), and an electron probe microanalyzer (EPMA, JEOL, JXA-8230). Densities of samples were measured by drainage, and porosities of samples were measured by the automatic surface area and a porosity analyzer (ASAP, Quantachrome, QUADRASORB SI).
3. Results and discussion 3.1. Characteristics of the magnets produced by GAPP with Pr80Al20 alloys Fig. 1(a) shows the DSC results for the permeation source of Pr80Al20 alloys. There are two exothermal peaks in the curve. It is a phase transition temperature point at 584 °C, which just corresponds to the Pr-Al phase diagram. The melting points of Pr80Al20 alloys are determined to be 643 °C. Thus, a heat treatment temperature higher than 643 °C was employed for the GAPP process to ensure complete melting of the alloys and the benefit of the permeation process. Fig. 2 shows the room temperature demagnetization curves for the magnets treated with nothing and those treated with 1, 2 and 3 wt% Pr80Al20 alloys ribbons, respectively. The inset shows the magnetic properties Hcj, Br and Hk/Hcj as a function of Pr80Al20 covering amounts. All the magnets have a high squareness factor over 94% and a large density above 7.56 g/cm3. The magnet free of agents possesses a coercivity Hcj of 12.3 kOe, a remanence Br of 13.8 kG and a squareness factor Hk/Hcj of 0.960. With the increase of Pr80Al20 covering amounts, Hcj increases gradually to 14.0 kOe at 1 wt%, 16.8 kOe at 2 wt% and 16.9 kOe at 3 wt%. The coercivity increment is accompanied with a certain reduction in Br, which is mainly attributed to the introduction of secondary components and an increase of the boundary phases. It is noteworthy that Hk/Hcj achieves the highest with 0.975 when treated with 2 wt% Pr80Al20 alloy ribbons. However, a further increase of the Pr80Al20 alloy to 3 wt% can substantially reduce Hk/Hcj to 0.941 with a slight enhancement in Hcj and a greater reduction in Br, which is mainly due to the excessive introduction of agents resulting in the uneven distribution of compositions and microstructures. It implies that a proper covering level of Pr80Al20 alloys must be controlled to obtain high coercivity, high remanence and high squareness factor. Under the present condition in this study, the optimal covering level is 2 wt%. Thus, a detailed microstructure evolution by GAPP treated with the 2 wt% Pr80Al20 alloy is investigated as described below.
Fig. 2. Demagnetization curves for the magnets treated with different amounts of Pr80Al20 alloys. The inset shows the dependencies of coercivity, remanence and squareness on the Pr80Al20 covering contents.
To examine the overall microstructures change, the microstructures of the magnets free of agents and treated with 2 wt% Pr80Al20 alloys were first observed by SEM, as shown in Fig. 3. Both samples consist of a bright contrast, which indicates that the RE-rich phase and a dark contrast showing the matrix Nd2Fe14B phase. For the magnet treated with nothing, the grain boundaries are very thin and are
hardly observed in some areas, as shown in Fig. 3(a) and Fig. 3(b), where short-range exchange coupling could exist for such local grains, which is not beneficial to obtain high coercivity [1,16,17]. When Pr80Al20 alloys are added, the distribution of RE-rich phase changes apparently, as shown in Fig. 3(c), and the intergranular RE-rich phase becomes clearer and more continuous, thus isolating well the adjacent grains. Such distribution of the RE-rich GB phase can eliminate the surface defects and is beneficial to reduce the possible reversal nucleation sites [9], which is one important contribution to the enhanced coercivity. More significantly, even at 6000 µm from the surface (Fig. 3(d)), the regions have very similar structures as those at 50 µm (Fig. 3(c)), which have shown continuous GB phases. Pr and Al element concentrations were analyzed by energy dispersive X-ray spectroscopy (EDS), and 12 measured surfaces of different positions were obtained by cutting 11 equal sections along the permeation direction of the sample. The integration results of Pr and Al concentrations from top to bottom of the sample are shown in Fig. 4. It can be seen that the distributions of both elements’ concentrations are basically homogeneous in the magnet, except for the gradients at the bottom. There is no significant difference in the concentrations, even though the existence of the gradients remain between 4.30 and 4.37 at% in the Pr concentration, and are 0.92 and 1.08 at% in the Al concentration. The formation of the concentration gradients is mainly because small amounts of agents on the lower surface of the sample did not permeate into the interior completely at a low temperature, and then entered the magnet by diffusion at a relatively high temperature, when there was no fast channel to introduce agents, just as in GBDP. The homogeneous distribution of Pr and Al concentrations explains the uniformity of compositions in magnets produced by GAPP with Pr-Al alloys. Thus, it indicates that GAPP could procure more effective thickness, and the thick magnets produced by GAPP would have nearly uniform structures and compositions from the surface to the interior.
Fig. 3. SEM images of the magnet treated with nothing ((a) 50 µm, (b) 6000 µm) and regions at different depths from the surface of the magnet treated with 2 wt% Pr80Al20 alloys by GAPP ((c) 50 µm, (d) 6000 µm).
Fig. 4. Pr and Al elements concentrations distribution from top to bottom of the sample treated with 2 wt% Pr80Al20 alloys by GAPP.
Fig. 5 depicts TEM images of the grain boundary regions for magnets treated with nothing and 2 wt% Pr80Al20 alloys ribbons. The interface between the grain boundary phase and the matrix 2:14:1 phase is not very well defined in the magnet added with nothing, and the grain boundaries are relatively thin and discontinuous, and the two matrix phases appear to be indirect contact in Fig. 5(a). For the magnet treated with 2 wt% Pr80Al20 alloys, however, the interface becomes clearer and smooth, as shown in Fig. 5(b), which confirms that the grain boundary layers can be continuous as a result of GAPP. Meanwhile, some grain boundary phases of the restructured magnet are observed in TEM, as shown in Fig. 5(c), which has a long axis length of 100 nm and a short axis length of 23 nm approximately, which may have good effect on the pinning domain wall and contribute to coercivity.
Fig. 5. TEM characterization of typical regions for the magnets. (a) A TEM image of the grain boundary region for magnet added with nothing; (b, c) TEM images of the grain boundary regions for magnets treated with 2 wt% Pr80Al20 alloys by GAPP.
Fig. 6 shows the EPMA mapping images of Pr (a) and Al (b) elements for the magnet treated with 2 wt% Pr80Al20 alloys by GAPP. Most of the Pr-rich phases show a network surrounding the matrix phase grains, although there are also some agglomerating phases enriched with Pr in the triple junctions. A color level adjustment is used to show the morphology and distribution of Al-rich phases more clearly (inset in Fig. 6(b)), because of the low average contents of Al elements in each region for the magnet, and in the picture, the red auxiliary lines have marked the areas where Al elements are concentrated. It is not difficult to find that Al elements are enriched surrounding the matrix phase grains, even though some have entered into the matrix phase. These core shell structures are very similar to the surface structures of magnets in GBDP, as previously reported. The partial substitution of Pr in Nd2Fe14B compound is beneficial for high coercivity due to a higher HA of Pr2Fe14B than that of Nd2Fe14B [2]. It should be noted that the modified boundary phase isolating 2:14:1 grains by Al is helpful for coercivity, although the partial substitution of Al in Nd2Fe14B decreases the
magnetocrystalline anisotropy field (HA) [18,19]. Thus, it means that Pr-rich and Al-rich shells are formed on the surface of the matrix phase grains for a magnet produced by GAPP, and these structures are good for coercivity enhancement.
Fig. 6. EPMA mapping images of Pr (a) and Al (b) for the magnet treated with 2 wt% Pr80Al20 alloys by GAPP. The inset in (b) is exporting the picture after color level adjustment, which shows that the Al elements are enriched surrounding the matrix phase grains.
3.2. Characteristics of the magnets treated with different types of alloys and methods In addition to Pr80Al20, 2 wt% alloy ribbons with the nominal compositions of Pr70Cu30 and Pr60Tb20Al20 (at%) were also used in GAPP. The specific heat treatment processes differ slightly according to the melting point of different alloys. All the thick and bulky magnets achieved uniform structures, similar to those of the sample with Pr80Al20 alloys. Demagnetization curves for the magnets treated with nothing and different types of alloys are shown in Fig. 7. It can be seen that the coercivities of the samples processed by Pr70Cu30 and Pr60Tb20Al20 alloys increase substantially to 16.2 kOe and 17.9 kOe compared with the sample free of agents at a coercivity of 12.3 kOe. Notably, there is a large increase in coercivity, while there is also a large reduction in remanence when using heavy earth elements, and this is very close to the effect created by the dual alloys method. It is emphasized that there are massive Tb elements diffusion from the grain boundary into the grain, since densification is carried out at a high temperature. The coercivities of the samples increase with the addition of Tb elements because the substitution of Tb elements for Nd in GAPP and Tb2Fe14B phases have much higher HA than that of Nd2Fe14B or Pr2Fe14B [2], and the reduction in remanence is because of the antiferromagnetic coupling between Fe and Tb. Thus, it is may not be appropriate to use heavy rare earth elements as permeation agents in GAPP, considering the further reduction of remanence. Magnets processed by GBDP were also studied in this work. The densified N48 magnet was covered as the same mode by 2 wt% Pr80Al20 alloy ribbons, which was then followed by optimal diffusion heat treatment. The coercivity of the GBDP sample reached to 14.6 kOe, as shown in Fig. 7 (green dotted line). This coercivity is lower than the result of traditional GBDP that was reported previously, because the sample used in this experiment is a thick magnet that has a height over 12 mm, and the thin effective diffusion depth is not enough to improve the performance of the whole magnet greatly. Shown in Fig. 8 are EPMA maps of the above magnets processed by GBDP and GAPP. It could be clearly observed that there are distribution gradients of elements in the diffusion direction for the sample GBDP (Fig. 8(a–c)), especially for Al elements in Fig. 8(b). However, for the sample GAPP, elements are generally uniform in the effective thickness, as shown in Fig. 8(d–f). This once again reveals the reasons why GAPP increases coercivity more than GBDP does for a thick magnet, and magnet processed by GAPP has a better squareness factor, as shown in Fig. 7.
Fig. 7. Demagnetization curves for the magnets treated with different types of alloys and methods.
Fig. 8. EPMA maps of the samples processed by GBDP (a–c) and GAPP (d–f) from the surface close to the agent sources.
Magnetic properties at higher temperatures were measured in the study. Fig. 9 shows the temperature dependence of remanence Br and the coercivity Hc of the magnets treated with nothing and magnets treated with different types of alloys. The temperature coefficients at 20–80 °C and 20–170 °C for the different magnets are listed in the inset table. It can be seen that the temperature coefficients of remanence α for magnets treated with alloys reduced slightly compared to the magnet without GAPP treatment at 20–80 °C, and the reason is attributed to the increased nonmagnetic phases in the magnets. However, the temperature coefficients of coercivity β show different behavior compared to coefficients of α. At the low temperature range of 20–80 °C, the value of β is –0.77 %/°C for magnets without GAPP treatment, –0.74 %/°C for magnets treated with Pr80Al20 alloys, –0.75 %/°C for magnets treated with Pr70Cu30 alloys and –0.73 %/°C for magnet the treated with Pr60Tb20Al20 alloys. The improved temperature stability of coercivity at low temperature is mainly attributed to the continuous distribution of RE-rich GB phases and the core-shell structures in magnets, as shown in Figs. 3 and 6. While at the high temperature range of 20–170 °C, there is no significant difference between the values of α and β for all the magnets, which is a combined result of the changes of intrinsic property HA and the extrinsic property c/Neff in this study.
Fig. 9. Temperature dependent of remanence Br and coercivity Hc of the magnets treated with nothing and different types of alloys. The inset table shows the temperature coefficients at 20–80 °C and 20–170 °C for the different magnets.
3.3. Discussion on the mechanism of GAPP It is widely accepted that GBDP follows Fick's law [20,21]; however, the mechanism of GBDP is far different from that of GAPP, which is more complicated, and it experiences permeation behavior at a relatively low temperature and diffusion homogenization at a relatively high temperature. In a nutshell, GAPP involves permeating liquid low melting point alloys into magnetic powders, which are compressed and porous at lower temperatures, and then conducts sintering densification at higher temperatures to finally obtain magnets with generally even distribution of desired agents. GAPP can obtain more homogenous microstructures because the introduction of target elements not only relies on the diffusion process that depends on the concentration gradient as that in GBDP, it can also be described as follows. With the melting of alloys during heating, the liquid alloys preferred to permeate through larger gaps among powder particles of the green-pressing at a relatively low temperature. In addition, the permeation channels were gradually distributed throughout the green-pressing with the increasing holding time. Then, the magnet became densified at a relatively high temperature, and the above channels that filled with agents became diffusion sources at that time. Subsequently, liquid phases with agents flowed and diffused into grain boundaries from the above channels. Finally, the matrix phase particles were being greatly coated with desired agents after sintering densification and annealing, so the desired agents can enter the magnet more sufficiently. To explain the permeation behavior of the liquid alloy in GAPP better, the basic theory of fluid mechanics will be applied.
Fig. 10. Variation curves of density and the total pore volume in the sample with temperature rising. Statistical pore sizes range from 2 nm to 1 µm.
It is not difficult to determine that the Reynolds number (Re) of the molten alloy flow is very small; in other words, the liquid alloy flow in GAPP has viscous flow and is slowly permeated. At this point, the permeation behavior of the liquid alloy in GAPP follows Darcy's law [22–25], which can be calculated by Equation (1): Q=k
A∆ p µL
(1)
where Q is the volume flow rate of fluid permeated through porous medium; k is the permeability rate that is closely related to the porosity of the porous medium; A is the cross-sectional area of the pore; ∆p is the pressure differential of fluid between the beginning and end of the permeation process, which is a certain number in this study; µ is viscosity of fluid that reflects fluidity of liquid; and L is the length of effective permeation. Obviously, it is essential to control Q and L in GAPP to introduce the appropriate mass of agents for further improvement of remanence, while k and A of a certain given green-pressing are extremely relevant to the pore volume that changed by temperature. Fig. 10 shows the variation curves of density and the total pore volume in the used magnet with temperature rising, and statistical pore sizes range from 2 nm to 1 µm. It can be seen that the density of the magnet increases dramatically from 680 to 780 and then reaches its maximum at 1060 . Meanwhile, there has also been a significant reduction in the pore volume since 680 . It means that the permeation rate is higher before 680 and smaller after 780 for a certain liquid alloy, assuming that the viscosity (µ) does not change with temperatures. Actually, the viscosity of liquid alloys usually decreases with increasing temperatures, and liquid alloys with lower melting points have higher fluidities at this range of temperatures, which is the advantage of the low melting point alloys in GAPP. For example, the Pr80Al20 alloys melted at 643 °C in this study, as shown in the DSC of Fig. 1(a), so the heat treatment temperature for the permeation process was set as 700 °C, which ensures a higher fluidity of the alloys and a larger porosity of the green compact. Thus, the effects of GAPP are the result of the used temperatures and alloys for permeation. For the different requirements of the secondary phase introduction, introducing more agents requires a lower melting point of alloys and lower temperatures in the permeation process, while introducing fewer agents requires relatively higher melting point alloys and higher temperatures that are below 1060 , theoretically. This relationship means that the introduction of the secondary phases can also be controlled by adjusting the temperatures in the permeation process, and it is different from that of the dual alloys method, in which a certain amount of second phases are introduced at one time before the heat treatments. There are still many alloys that can be used for permeation in GAPP, and it provides a way of thinking for further research.
4. Conclusions
In summary, thick and bulky Nd-Fe-B sintered magnets were prepared by the GAPP with Pr80Al20 alloys ribbons. The coercivity of the restructured magnet is found to increase with Pr80Al20 covering contents (0, 1.0, 2.0 and 3.0 wt%). The coercivity increment is accompanied by a certain reduction in Br, which is mainly attributed to the introduction of secondary components and an increase of the boundary phases. Optimal comprehensive performance shows Hcj of 16.8 kOe, Br of 13.4 kG and Hk/Hcj of 0.975 at 2.0 wt% covering content. Microstructural investigations show that the coercivity enhancement of the sample is mainly attributed to the magnetic strengthening of shells on the surfaces of Nd2Fe14B grains and continuous grain boundary layers along the matrix phase grains, which can weaken magnetic coupling between adjacent grains. Meanwhile, it has nearly uniform structures from the surface to the interior. In addition, it is clear that the control of permeation temperatures and the different melt points of introduced alloys become critical factors that affect the results of GAPP due to its characteristics during the process. The GAPP is more similar to a method that combines GBDP and the dual alloys method in a sense, for it has better uniformity than that of GBDP and a more controllable second-phase introduction process than that of the dual alloys method. In all, it shed new insights into the fabrication of thick and bulky permanent magnets with high performance, which will meet the design requirements of large device components in modern production.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51401021) and the State Key Laboratory Advanced Metals and Materials (No. 2016Z-14).
References [1] [2] [3] [4] [5] [6] [7]
[8] [9] [10] [11] [12] [13]
[14]
Hono K, Sepehri-Amin H. Strategy for high-coercivity Nd–Fe–B magnets. Scr Mater. 2012, 67(6): 530. Hirosawa S, Matsuura Y, Yamamoto H, Fujimura S, Sagawa M. Magnetization and magnetic anisotropy of R2Fe14B measured on single crystals. J Appl Phys. 1986, 59(3): 873. Kohashi T, Motai K, Nishiuchi T, Hirosawa S. Magnetism in grain-boundary phase of a NdFeB sintered magnet studied by spin-polarized scanning electron microscopy. Appl Phys Lett. 2014, 104(23): 232408. Xu F, Wang J, Dong X P, Zhang L T, Wu J S. Grain boundary microstructure in DyF3-diffusion processed Nd–Fe–B sintered magnets. J Alloy Compd. 2011, 509(30): 7909. Liu X L, Wang X J, Liang L P, Zhang P, Jin J Y, Zhang Y J, Ma T Y, Yan M. Rapid coercivity increment of Nd–Fe–B sintered magnets by Dy69Ni31 grain boundary restructuring. J Magn Magn Mater. 2014, 370: 76. Hamada N, Mishima C, Mitarai H, Honkura Y. Development of Nd-Fe-B anisotropic bonded magnet with 27 MGOe. IEEE Trans Magn. 2003, 39(5): 2953. Sepehri-Amin H, Ohkubo T, Nishiuchi T, Hirosawa S, Hono K. Coercivity enhancement of hydrogenation– disproportionation–desorption–recombination processed Nd–Fe–B powders by the diffusion of Nd–Cu eutectic alloys. Scr Mater. 2010, 63(11): 1124. Wang C G, Yue M, Zhang D T, Liu W Q, Zhang J X. Structure and magnetic properties of hot deformed Nd2Fe14B magnets doped with DyHx nanoparticles. J Magn Magn Mater. 2016, 404: 64. Sepehri-Amin H, Ohkubo T, Hono K. The mechanism of coercivity enhancement by the grain boundary diffusion process of Nd–Fe–B sintered magnets. Acta Mater. 2013, 61(6): 1982. Vial F, Joly F, Nevalainen E, Sagawa M, Hiraga K, Park K T. Improvement of coercivity of sintered NdFeB permanent magnets by heat treatment. J Magn Magn Mater. 2002, 242-245: 1329. Hirota K, Nakamura H, Minowa T, Honshima M. Coercivity Enhancement by the Grain Boundary Diffusion Process to Nd–Fe–B Sintered Magnets. IEEE Trans Magn. 2006, 42(10): 2909. Nakamura H, Hirota K, Ohashi T, Minowa T. Coercivity distributions in Nd–Fe–B sintered magnets produced by the grain boundary diffusion process. J Phys D Appl Phys. 2011, 44: 064003. Wan F M, Zhang Y F, Han J Z, Liu S Q, Liu T, Zhou L, Fu J B, Zhou D, Zhang X D, Yang J B, Yang Y C, Chen J, Deng Z W. Coercivity enhancement in Dy-free Nd–Fe–B sintered magnets by using Pr-Cu alloy. J Appl Phys. 2014, 115(20): 203910. Zhou L, Li J, Cheng X H, Liu T, Yu X J, Li B. Dy gradient and coercivity in grain boundary diffusion processed Nd-Fe-B magnet. J Rare Earths, 2017, 35(6): 559.
[15] Liang L P, Ma T Y, Wu C, Zhang P, Liu X L, Yan M. Coercivity enhancement of Dy-free Nd–Fe–B sintered magnets by intergranular adding Ho63.4Fe36.6 alloy. J Magn Magn Mater. 2016, 397: 139. [16] Woodcock T G, Zhang Y, Hrkac G, Ciuta G, Dempsey N M, Schrefl T, Gutfleisch O, Givord D. Understanding the microstructure and coercivity of high performance NdFeB-based magnets. Scr Mater. 2012, 67(6): 536. [17] Li W F, Ohkubo T, Hono K. Effect of post-sinter annealing on the coercivity and microstructure of Nd–Fe–B permanent magnets. Acta Mater. 2009, 57(5): 1337. [18] Rodewald W, Fernengel W. Properties of sintered Nd-Fe-TM-B magnets. IEEE Trans Magn. 1988, 24(2): 1638. [19] Oono N, Sagawa M, Kasada R, Matsui H, Kimura A. Production of thick high-performance sintered neodymium magnets by grain boundary diffusion treatment with dysprosium–nickel–aluminum alloy. J Magn. Magn Mater. 2011, 323(3-4): 297. [20] Loewe K, Benke D, Kübel C, Lienig T, Skokov K P, Gutfleisch O. Grain boundary diffusion of different rare earth elements in Nd-Fe-B sintered magnets by experiment and FEM simulation. Acta Mater. 2017, 124: 421. [21] Hu S Q, Peng K, Chen H. Influence of annealing temperature on the Dy diffusion process in NdFeB magnets. J Magn Magn Mater. 2017, 426: 340. [22] Othman M R, Helwani Z, Martunusa. Simulated fractal permeability for porous membranes. Appl Math Model. 2010, 34(9): 2452. [23] Turcio M, Reyes J M, Camacho R, Lira-Galeana C, Vargas R O, Manero O. Calculation of effective permeability for the BMP model in fractal porous media. J Petrol Sci Eng. 2013, 103: 51. [24] Koponen A, Kataja M, Timonen J. Tortuous flow in porous media. Phys Rev E. 1996, 54(1): 406. [25] Chen Y, Shen C, Lu P, Huang Y. Role of pore structure on liquid flow behaviors in porous media characterized by fractal geometry. Chem Eng Process. 2015, 87: 75.
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Demagnetization curves for the magnets treated with different types of alloys and methods. (GAPP, Green-pressing agents permeation process. GBDP, Grain boundary diffusion process)
Highlights: 1. Significant coercivity enhancement of thick and bulky Nd-Fe-B sintered magnets was achieved by boundary introducing low melting point alloys, which was named as green-pressing agents permeation process (GAPP). 2. GAPP procured deeper effective thickness and more uniform structure for thick and bulky magnets. 3. Optimal comprehensive performance shows Hcj of 16.8 kOe, Br of 13.4 kG and Hk/Hcj of 0.975 at 2 wt.% covering content by GAPP with Pr80Al20 alloys. 4. Pr80Al20 (at.%) alloys are the optimal choice for thick and bulky magnets as permeation sources in GAPP, comparing with Pr70Cu30, and Pr60Tb20Al20 alloys. 5. The critical factors to affect the results of GAPP are the used temperatures and alloys in permeation processes, which were obtained by discussion about the mechanism of GAPP.
Conflict of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled