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Microstructure, magnetic properties and diffusion mechanism of DyMg co-deposited sintered Nd-Fe-B magnets
Journal Pre-proof Microstructure, magnetic properties and diffusion mechanism of DyMg co-deposited sintered Nd-Fe-B magnets Shuwei Zhong, Yang Munan, Sajjad Ur Rehman, Lu Yaojun, Li Jiajie, Bin Yang PII:
S0925-8388(19)34248-3
DOI:
https://doi.org/10.1016/j.jallcom.2019.153002
Reference:
JALCOM 153002
To appear in:
Journal of Alloys and Compounds
Received Date: 25 July 2019 Revised Date:
1 November 2019
Accepted Date: 11 November 2019
Please cite this article as: S. Zhong, Y. Munan, S. Ur Rehman, L. Yaojun, L. Jiajie, B. Yang, Microstructure, magnetic properties and diffusion mechanism of DyMg co-deposited sintered Nd-Fe-B magnets, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.153002. 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. © 2019 Published by Elsevier B.V.
Microstructure, magnetic properties and diffusion mechanism of DyMg co-deposited sintered Nd-Fe-B magnets
Zhong Shuwei a, Yang Munan a,b,**, Sajjad Ur Rehman b, Lu Yaojun a, Li Jiajie a, b, Yang Bin a,* a
Faculty of Materials Metallurgy and Chemistry, Jiangxi University of Science and Technology,
Ganzhou 341000,China b
Jiangxi Key Laboratory for Rare Earth Magnetic Materials and Devices/Institute for Rare
Earth Magnetic Materials and Devices (IREMMD), Jiangxi University of Science and Technology, Ganzhou 341000,China
Abstract The effect of DyMg co-deposition and diffusion on magnetic properties and service stability of sintered Nd-Fe-B magnets was investigated in this paper. The coercivity of increased from 13.26 kOe for original magnet to 18.21 kOe after DyMg diffusion. As compared to single Dy diffusion the Dy consumption was reduced by 39 % in MgDy diffused magnets. The thermal stability and corrosion resistance of the DyMg-diffused magnet improved significantly. The microstructure and element distribution were observed by SEM, EPMA and TEM. Mg element or Dy-Mg alloy repaired the damaged grain boundary to form a low melting point diffusion channel in the magnet and improved the distribution uniformity of the Dy-rich shell. In addition to the increase of
magnetocrystalline anisotropy of the main phase, the optimization of the grain boundary phase is found to be a key factor for the effective improvement of coercivity. Key word: Nd-Fe-B; Grain boundary diffusion process; Co-deposited; Low melting diffusion channel; Microstructural optimization. * Corresponding author. E-mail addresses: [email protected] (Yang Bin), [email protected] (Yang Munan)
1. Introduction The sintered Nd-Fe-B magnets have been widely used in the fields of space shuttle, new energy automobile, wind power generation and high-end technology since 1983 [1]. After years of research, the remanence (Jr) and maximum magnetic energy product ((BH)max) of sintered Nd-Fe-B magnets have reached close to the theoretical values [2]. However, the coercivity (Hcj) of Nd-Fe-B magnets is only about 30 % of the theoretical value and still has a lot of space for improvement. Improving the Hcj and the stability of magnetic properties at high temperature is one of the focuses of current research [3, 4]. The addition of heavy rare earth element (HREE) Dy, Tb significantly improve the Hcj, however, the production cost is increased due to criticality and high prices of Dy and Tb [5-7]. In order to reduce the production cost of high performance magnets and balance the consumption of rare earth (RE) resources, Prak et al. [8] deposited Dy film on the
surface of magnets by sputtering and achieved excellent results in 2000. Henceforth, grain boundary diffusion process (GBDP) has been widely studied as an effective means to improve the Hcj of RE based magnets. Many studies have shown that the diffusion of HRE alloys such as hydrides [9], fluorides [10,11] and oxides [12] of Dy or Tb could improved the Hcj at the cost of marginal decrease of Jr and (BH)max. Furthermore, studies [13-16] have shown that the Hcj can be effectively improved by low melting point alloys of HRE. It is considered that core-shell structure of the 2:14:1 phase constructed by HRE (Nd,HRE)2Fe14B shell and a HRE-lean core increases the magneto-crystalline anisotropy (HA). In addition, the wettability of grain boundary (GB) was improved greatly by introducing low melting point alloys, such as Dy70Cu30 [17], Dy35Pr35Cu30 [18] or NdCu [19]. However, GBDP also has many shortcomings, such as the poor distribution uniformity, low diffusion rate and limited diffusion depth. Moreover, the alloys of Dy element undergo a series of complex physical and chemical reactions during the process [20-22], and the diffusion mechanism for different elements is still not very clear. Therefore, a proper investigation of diffusion mechanism of HRE with low melt point element is urgently needed. This study provides an idea to solve the problem of limited diffusion depth of heavy rare earth elements in sintered Nd-Fe-B magnets. Based on the physicochemical property of low melting element Mg and heavy rare earth Dy during heat treatment, the co-deposition of Dy and Mg elements on the surface has been realized by magnetron sputtering technology, and the diffusion depth and diffusion efficiency of Dy element
have been improved. The promotion effect of Mg on the diffusion of Dy element in the process was emphatically analyzed. Finally the diffusion mechanism of Dy element with low melting point metal Mg is revealed. 2. Experimental procedure In the study, N40 commercial sintered Nd-Fe-B magnet was used as the original sample. The sample was cut into cubes of 10×10×6 mm3(6 mm c-axis) by electric spark cutting. Subsequently, Dy (99.9 % purity) and Mg (99.99 % purity) were deposited on the magnet by high vacuum magnetron sputtering technique. Two separate samples were prepared by single deposition of Dy (hereafter Dy-diffused magnet) or by co-deposition of DyMg (hereafter DyMg-diffused magnet). The basic vacuum in the cavity was over 2×10-3 Pa, and the sputtering pressure and argon flow were 3-5 Pa and 50 sccm, respectively. The sputtering power for both Dy and Mg target was 80 W. The deposition time for single Dy was 45 min, while the co-deposition time for DyMg was 30 min. The mass fraction of different elements of deposited film was detected by SEM-EDS. The sample was treated at 900
for 10 h and annealed at 500
for 3 h, and
finally was air quenched to room temperature. The magnetic performance of these magnets was measured by magnetic measurement system (NIM-500C) at room temperature. The magnetic properties at elevated temperature were measured by physical property measurement system (PPMS, Quantum Design, USA) equipped with 9 T vibrating sample magnetometer (VSM). The dynamic potential polarization curve and the EIS spectra was measured by
electrochemical comprehensive test system (PARSTART 4000) in 3.5 wt.% NaCl solution. X-ray diffractometer (XRD-Panalytical Empyrean) was used to study the phase and crystal structure before and after diffusion. Scanning Electron Microscopy (SEM-MIRA3 LMH) with an energy dispersive X-ray spectrometer was used for microstructure observation. The element distribution was analyzed by electron probe microanlyzer (EPMA-JXA-8100). Field emission transmission electron microscopy (TEM-FEI Tecnai G2 F20) was used to analyze microstructure of the grain boundary (GB) phase.
3. Results and discussion 3.1. Magnetic properties Fig. 1 shows the demagnetization curves of the original and diffusion magnets, and the inset shows the schematic diagram for Dy consumption. The amount of element deposited and the magnetic properties are shown in Table 1. At room temperature, the Jr, Hcj and (BH)max of the original magnet was 12.85 kGs, 13.26 kOe and 39.75 MGOe, respectively. The Hcj increased to 17.34 kOe after Dy diffusion , while the Jr and (BH)max decreased slightly to 12.61 kGs and 38.92 MGOe, respectively. The results are similar to those obtained by Wu et al. [23]. The magnet prepared by DyMg co-deposition and diffusion magnet depicts Hcj = 18.21 kOe, Jr = 12.68 kG and (BH)max = 39.09 MGOe. The DyMg-diffused magnet show more prominent magnetic properties than the Dy-diffused magnet. In particular, the deposited film quality on the surface of
DyMg-diffused magnet is similar with the Dy-diffused magnet, which is 2.4 mg and 2.1 mg, respectively. The consumption of Dy by magnetron sputtering method is much lower than coating or electrophoretic deposition [24,25]. The consumption of Dy element is reduced to about 39% by co-deposition of DyMg. In addition, the quantity of heavy rare earth Dy is only about 47 wt.% of the total mass of the co-deposited film, whose quality is only about 1.1 mg, which is 1.8 mg lower than the Dy consumption when deposited individually. The above results indicate that the DyMg-diffused magnet not only exhibits excellent magnetic properties compared to the conventional Dy-diffused magnet, but also promise low manufacturing cost.
Fig. 1. Demagnetization curves of the original, Dy-and DyMg-diffused magnets. The inset shows the amount of Dy element deposited into the magnets and corresponding Hcj.
Table 1 The quantity of deposited layers and the microscopic composition of each element and the magnetic properties of different magnets before and after diffusion. Increased Sample
Dy
O
Mg
Bal
Jr
Hcj
（BH）max
(wt.%)
(wt.%)
(wt.%)
(wt.%)
（kGs）
（kOe）
（MGOe）
weight (mg)
Original
-
-
-
-
-
12.85
13.26
39.75
Dy-diffused
2.1
87
11
-
2
12.61
17.34
38.92
DyMg-diffused
2.4
47
24
26
3
12.68
18.21
39.09
3.2. Thermal stability In order to study the thermal stability, the magnetic properties were measured from 300 to 380 K as shown in Fig. 2. It is observed that Jr of original magnet, Dy-diffused magnet and DyMg-diffused magnet show a downward trend as the temperature rises. However, the Jr of DyMg-diffused magnet shows a smaller variation between 340 K and 380 K, whose reduction is only 0.0662 T as compared 0.0860 T for original magnet. Fig. 2 (b) shows the values of the Hcj at different temperatures. The HA is affected by high temperature, therefore the Hcj decrease dramatically [26,27]. The temperature coefficient of remanence (αJr) and the temperature coefficient of coercivity (βHcj) are used to measure the stability of the magnet at high temperature [28, 29] by using the following formulas [30,31]:
α = [Jr (T ) - Jr (T 0)] /[Jr (T 0)(T − T 0)]× 100% Jr
β = [Hcj(T ) − Hcj (T 0)] /[Hcj (T )(T − T 0)]× 100% Hcj
(1) (2)
where T0 is the room temperature, and T is the elevated temperature. The values of both αJr and βHcj are negative, the smaller the absolute value of αJr and βHcj, the better is the thermal stability [32]. The αJr of Dy-diffused magnet decreased from -0.142 %/K to -0.157 %/K, but the αJr of the DyMg-diffused magnet improved to -0.128 %/K from -0.142 %/K. It is believed that the Dy atom mainly interacts with Co atom in the main phase, and the substituted amount of DyMg-diffused magnet was more suitable than Dy-diffused magnet [33]. Moreover, the promotion of liquid phase flow by Mg element during the diffusion increases the density, which stabilizes surface energy of main phase, and the lattice distortion is thus avoided effectively when the temperature changes [34,35]. The βHcj of Dy-diffused magnet and DyMg-diffused magnet increased to -0.686 %/K and -0.678 %/K respectively from -0.836 %/K for original magnet. Compared with the Dy-diffused magnet, not only the Hcj of DyMg-diffused magnet increased, the thermal stability is also enhanced obviously. The above experimental results show that the co-diffusion of Dy and Mg has a positive effect on the stability of Jr and Hcj.
Fig. 2. Magnetic properties of the magnets at 300-380 K: (a) Jr, (b) Hcj.
3.3. Corrosion resistance Fig. 3 (a) depicts the dynamic potential polarization curves of various magnets in 3.5% NaCl solution. As shown in Table 2, the self-corrosion potential and corrosion current density of the original magnet are -0.78747 V and 1.1151×10×5 A/cm2, respectively. The self-corrosion potential and the corrosion current density of the Dy-diffused magnet show no obvious change from the original magnet. The DyMg-diffused magnet showed a greater corrosion trend whose self-corrosion current density decreased to -0.8158 V. However, the corrosion current density of DyMg-diffused magnet decreased to 1.985×10-6 A/ cm2, which is much lower than the original magnet. The corresponding result could be seen from the EIS spectra shown in Fig. 3. (b). The radius of capacitance loop of the Dy-diffused magnet is not significantly different from the original magnet. However, the radius of capacitance loop of the DyMg-diffused magnet is larger than that of the original and Dy-diffused magnet. The impedance value of DyMg-diffused magnet is also larger than original magnet and the Dy-diffused magnet showing an optimal corrosion resistance. According to these results, we can argue that the introduction of Dy and Mg elements could improve the corrosion resistance. Wherein, the addition of the Mg element induce the formation of the Nd–O–Fe–Mg phase during heat treatment [36], while the increase of O content makes the grain boundary of the magnet more stable [37,38].
Table 2 Self-corrosion potential and corrosion current density of different magnets.
Sample
Potential/V
A/cm2
Original
-0.78747
1.1151×10-5
Dy-diffused
-0.79606
7.212×10-6
DyMg-diffused
-0.85801
1.985×10-6
Fig. 3. (a) The dynamic potential polarization curves, and (b) the EIS spectra of various magnets.
3.4. XRD analysis In order to investigate the reasons for the difference in magnetic performance, thermal stability and corrosion resistance, X-ray diffraction of the diffusion surface (the vertical c-axis) of different magnets was measured. Fig. 4 shows the XRD curves of the original, Dy-diffused and DyMg-diffused magnets at different angles. The results show that most of the diffraction peaks come from Re2Fe14B phase. No new diffraction peaks were found in the diffusion magnets. It is assumed that no new phase has been introduced into the magnet during the diffusion process and the increase of magnetic
performance is presumably independent of any phase transition. The diffraction peaks of Re2Fe14B phase of Dy-diffused magnet shift towards the high angle compared with that of original magnet. The offset of DyMg-diffused magnet is slightly smaller than that of Dy-diffused magnet. Following the Bragg equation [39] 2dsinθ = nλ , the shift of the diffraction peak towards the higher angles suggests that the crystal plane spacing and lattice parameters of diffusion magnet have reduced [40]. Because of the difference in the concentration gradient, part of the Dy atom diffuses and replaces the Nd atom [41]. As a result, the lattice parameters of the main phase and the crystal plane spacing decreased [42]. The Dy content deposited on the surface of the DyMg-diffused magnet is less than that of the Dy-diffused magnet. The substitution amount of Dy element decreased, and the change of main phase cell parameters and crystal plane spacing was smaller than the Dy-diffused magnet. Therefore, the degree of deviation of diffraction peak to high angle is also small.
Fig. 4. The X-xay diffraction patterns of various magnets.
3.5. Microstructure analysis 3.5.1. SEM analysis In order to understand the enhancement of the Hcj in the DyMg-diffused magnet, SEM has been used to observe the micrographs of these magnets. Fig. 5 depicts the SEM backscattered electron images of the surface of the magnet before and after diffusion. The microstructure of the original magnet is shown in Fig. 5(a), in which the dark gray regions correspond to the Nd2Fe14B main phases and the bright regions correspond to the Nd-rich grain boundary phases. The main phase grains are in contact with each other directly and the grain boundary phase are discontinuous. A large number of Nd-rich phases are distributed in blocks, which result in a weak exchange coupling among the main phase grains [43]. Fig. 5(b) shows the micrograph of the Dy-diffused magnet. The grain boundary phase size is obviously reduced and became more continuous and smooth after diffusion. The thin GB has increased in quantity and has made the main phase grain isolated from each other. This grain boundary optimization results in a significant increase in Hcj [44]. As shown in Fig. 5(c), the number of thin GB in the DyMg-diffused magnet is more than the Dy-diffused magnet, and the grain boundary phase is more uniform and continuous. The formation of low melting point RE-Mg phase (about 650
) by Mg element and Re-rich phase in the process greatly
promoted the liquid phase flow and enhance the wettability of grain boundary during heat treatment [45]. The formation of low melting point liquid phase during treatment made the main phase grain easy to be wrapped by thin GB. Thus, the direct contact
between the 2:14:1 phases is avoided.
Fig. 5. SEM micrographs of the magnets: (a) Original, (b) Dy-diffused, (c) DyMg-diffused.
3.5.2. The EPMA mapping of diffusion layer The EPMA mapping of the cross sections of magnets at the depth of 0 - 80 µm are shown in the Fig. 6. Fig. 6(a) shows the distribution of Nd, Mg, Fe and Dy elements near surface of Dy-diffused magnet. The Fe element is mainly distributed in the grains. The RE elements (Nd,Dy) is enriched in the Nd-rich grain boundary phase at the depth of 0 - 10 µm because of the low diffusion rate [46]. In addition, there is an overlap between parts of Dy element and the Fe element distributed in the main phase. It indicates that the formation of (Nd,Dy)2Fe14B from fractional Dy element in the surface layer enter into main phase driven by thermal diffusion. However, as the diffusion depth increases, the agglomeration of the Nd and Dy deteriorates, and the content of Dy decreases significantly. The distribution of the Nd and Dy coincides with each other, which is mainly concentrated in the triple junction region. It is shown that the content of Dy element entered the main phase decreases due to the decrease of concentration
gradient. The above results agree with the distribution law of concentration gradient [47]. These results are also consistent with the XRD results. Fig. 6(b) is the distribution map of the each element of the DyMg-diffused magnet. It can be seen from the diagram that the concentration of Mg element changes from high to low with the increase of diffusion depth. The thickness of Nd and Dy enriched layer in the surface is significantly decreased under the influence of Mg element. But in the deeper part, the size of triple junction becomes smaller and more uniform with the diffusion of Mg element. The continuity of grain boundary is obviously optimized. This is mainly due to the preferential diffusion of Mg during high temperature (900
) treatment. The Dy-Mg
alloy with low melting point was formed by the reaction of residual Mg with Dy element, as shown in Supplementary Material A. The formation of Dy-Mg alloy effectively improves the wettability of grain boundary. It promotes a continues Nd-rich phase. According to Fick's second law of diffusion [48,49], the grain boundary phase with continuous and low melting point increases the diffusion rate of Dy. The concentration gradient of Dy in the surface layer of magnet is reduced and the enrichment of Dy is alleviated.
Fig. 6. EPMA maps of section drawing (Parallel to c-axis): (a) Dy-diffused magnet, and (b) DyMg-diffused magnet at the depth of 0 - 80 µm.
For further understanding the distribution of each element in the deeper diffusion areas, the elemental diagram at the depth of ~110 µm was obtained by EPMA as shown
in Fig. 7. As shown in Fig. 7(a), the distribution uniformity of Nd element is not good and Nd is agglomerated in the large triple junctions. Meanwhile, the existence of Dy element could still be detected at the depth, which coincides with Nd-rich phase in the form of agglomeration and whose concentration is much lower than that at the surface layer. However, a different microstructure is exhibited in the DyMg-diffused magnet within the same diffusion depth. The content of Mg element was further reduced in the depth of ~ 110 µm, which is enough to cause a change of GB as shown in Fig. 7 (b). The GB phase became uniform and small, which could increase the demagnetization exchange coupling between the main phase grains and thus increase the Hcj. In addition, a continuous and uniform Dy-rich shell was observed distinctly in the epitaxial layer of 2:14:1 main phase. It shows that the core-shell structure could still be formed in that range of depth and enhanced the local magnetocrystalline anisotropy field [50,51].
Fig. 7. The EPMA maps of section drawing (Parallel to c-axis): (a) Dy-diffused magnet, and (b) DyMg-diffused magnet at the depth of ~110 µm.
3.5.3. The microstructure of GB phase Fig. 8 (a) and (b) respectively show the microstructure of the Dy-diffused magnet
and the DyMg-diffused magnet observed by TEM. The darker contrast refers to the 2:14:1 main phase, and the brighter contrasts shown the RE-rich GB phase. The micro-morphology of grain boundary of the Dy-diffused magnet is shown in Fig. 8 (a), the thickness of the GB is ~17 nm. The interface between GB and main phase grains is not clear and the GB is vague. In the Dy-diffused magnet, the morphology of GB is not optimized. The thickness of GB decreases to ~8 nm after co-diffusion of DyMg. The interface between the main phase and the GB phase becomes clearer. The introduction of Mg element improves the wettability and liquidity of Nd-rich phase and promotes the formation of a large number of thin GB from triple junction of Nd-rich phase through the interface between GB and main phase grains during the secondary annealing [52,53]. The continuous thin GB phase plays an essential role in weakening the short-range exchange coupling between adjacent 2:14:1 phase grains, which in turn prevents the cascade propagation of reversed magnetic domain throughout the grains [54].
Fig. 8. TEM images of the grain boundary structure: (a) Dy-diffused magnet, and (b) DyMg-diffused magnet.
3.5.4. The diffusion mechanism analysis Based on the above test results, the thermal diffusion process of the elements is analyzed and the mechanism of microstructure evolution is presented. Figs. 9(a)-(c) depict that there are two type of Dy diffusion: grain boundary diffusion and grain lattice diffusion [55]. Firstly, the surface-deposited Dy element diffuses along the grain boundaries, the diffusion rate is slow and the diffusion depth of Dy element is shallow. Due to multiple factors such as the grain boundary continuity and the melting point of the Nd-rich phase, part of the Dy element is agglomerated at the surface part of the Nd-rich phase [26]. Secondly, under the force of high concentration gradient, Dy element starts to diffuse into the crystal lattice to replace the Nd element. As shown, Dy element is overlapped with Fe element in the 2:14:1 main phase grains. However, the agglomeration of Dy element at the surface layer leads to less content of Dy at deeper parts of the magnet. Fig. 9(d) shows that under the influence of temperature, there are four different types of reactions and diffusion behavior in the DyMg-diffused magnet. In Fig. 9(e), at the first stage, the Mg element is preferentially diffused into magnet along the Nd-rich grain boundary phase due to its low melting point characteristics and better diffusion rate [57], This improves the wettability of grain boundary and dredge the diffusion channel of Dy element [58,59]. Second, the remaining Mg element on the surface reacts with the Dy element to form Dy-Mg alloy with a lower melting point (a little higher than Mg element). This low melting point alloy is similar to the low melting heavy rare earth alloy used in the work mentioned earlier [13-16]. Dy diffuses into the
magnet along the grain boundary under the influence of temperature. The concentration gradient of Dy element decreased dramatically due to the reduction of Dy element deposition and the formation of Dy-Mg alloy at the surface diffusion source. In addition, under the repairment of low melting point metal Mg and Dy-Mg alloy on grain boundary diffusion channels, the diffusion depth and rate of Dy element are greatly increased. The distribution ratio and content of Dy element are more uniform along the depth direction of diffusion, so that the concentration of Dy element at the depth of 0 80 µm in the DyMg-diffused magnet is lower than that the Dy-diffused magnet. The final type of diffusion behavior is the grain lattice diffusion [60]. On the basis of the above three kinds of diffusion behavior, there is still a certain difference of concentration gradient in Dy element between grain boundary and grain lattice in the deeper diffusion depth. When the concentration difference is greater than the critical diffusion concentration of the Dy element, a thin Dy-rich shell forms by lattice diffusion under the action of driving force of diffusion [45], as shown in Figure7(b). The consumption of Dy element in the DyMg-diffused magnet is less than that in the Dy-diffused magnet, but the increase in Hcj was significantly high. On one hand, the Hcj enhancement depends on the optimization of Mg element and the microstructure, on the other hand, it originates from the increase of Dy element on the intrinsic magnetic properties. The existence of Mg element can improve the grain boundary morphology. The main phase grains of Nd2Fe14B is effectively isolated, which is beneficial to the improvement of the Hcj. Moreover, both the Mg element and DyMg alloy can also repair
grain boundary diffusion channels. This leads to the formation of thinner Dy-rich core-shell structure along the grain boundary diffusion direction. The existence of Dy-rich core-shell structure would enhance local magnetocrystalline anisotropy field of 2:14:1 main phase, which leads to the Hcj enhancement.
Fig. 9. The schematic diagram of diffusion mechanism.
3.6. Coercivity mechanism The Hcj of sintered Nd-Fe-B magnets is generally expressed by the formula of nucleation model [61,62]: Hcj = α HA - NeffM s , the so-called microstructural parameters α and Neff are related to the non-ideal microstructure of the real magnet. The parameter α describes the influence of the non-perfect grain surfaces on the crystal anisotropy and orientation degree. The parameter Neff describes the enhanced stray fields at the edges and corners of the grains [34]. The Hcj, Ms and HA of the magnet at different
temperatures (300 - 380 K) are measured. The values represent the change in the internal microstructure of the magnet and determine the mechanism of the Hcj enhancement. As shown in Fig. 9, the main phase is hardened and HA is strengthened with the Dy element diffusion. The α of the original magnet is increased from 0.9477 to 0.9597 for the Dy-diffused magnet and 0.9580 for the DyMg-diffused magnet. Compared with Dy-diffused magnet, the Neff of DyMg-diffused magnet decreased significantly from 4.1249 to 3.9066. The addition of Mg and Dy changed the α and Neff in such a way that the Hcj enhanced. The introduction of the Mg element improved the grain shape and size, and the adjacent grains are wrapped by thin GBs to improve their dipolar interaction [63]. The presence of thin GB in the magnet leads to a weak demagnetization coupling and enhanced the Hcj.
Fig. 10. The Hcj/ Ms vs. HA/Ms curves of various magnets.
4. Conclusions In this paper, the effect of Dy-Mg co-deposition on the magnetic properties and corrosion resistance of magnet has been studied. The Hcj increased from 13.26 kOe for original magnet to 18.21 kOe for DyMg-diffused magnets, while the Jr and (BH)max decreased only marginally. The temperature coefficient of remanence of the DyMg-diffused magnet increased from -0.142 %/K to -0.128 %/K, and temperature coefficient of coercivity increased from -0.836 %/K to -0.678%/K. The corrosion current density of the magnet decreased from 1.1151×10-5 A/cm2 to 1.985×10-6 A/cm2, and the corrosion resistance of the magnet was enhanced. In the process of diffusion, the diffusion behavior of the low melting metal Mg and Dy-Mg alloy realized the optimization of grain boundary. The optimization of grain boundary phase effectively promoted the diffusion depth of Dy element and made it uniform. Besides the improvement of the magnetocrystalline anisotropy field and the optimization of microstructure played an indispensable role in the improvement of Hcj.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51801085) , the Education Department of Jiangxi Province (Grant No. 2012215) and Jiangxi University of Science and Technology (Grant No. jxxjbs00077 & 3401223330).
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Highlights: 1. Dy and Mg are separately deposited on the Nd-Fe-B magnets. 2. The coercivity of the DyMg-diffused magnet increased from 13.26 to 18.21 kOe. 3. Both the thermal stability and corrosion resistance are improved by the diffusion of DyMg. 4. The formation of Dy-Mg alloy is beneficial to promote the distribution of the Dy-rich shell. 5. The diffusion mechanism of HRE with low melting point element is proposed.
Declaration of Interest statement It is hereby stated that there is no conflict of interest to declare by author/co-author of this manuscript.
On behalf of all authors/co-authors Prof. Yang Bin Corresponding author
S0925-8388(19)34248-3
DOI:
https://doi.org/10.1016/j.jallcom.2019.153002
Reference:
JALCOM 153002
To appear in:
Journal of Alloys and Compounds
Received Date: 25 July 2019 Revised Date:
1 November 2019
Accepted Date: 11 November 2019
Please cite this article as: S. Zhong, Y. Munan, S. Ur Rehman, L. Yaojun, L. Jiajie, B. Yang, Microstructure, magnetic properties and diffusion mechanism of DyMg co-deposited sintered Nd-Fe-B magnets, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.153002. 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. © 2019 Published by Elsevier B.V.
Microstructure, magnetic properties and diffusion mechanism of DyMg co-deposited sintered Nd-Fe-B magnets
Zhong Shuwei a, Yang Munan a,b,**, Sajjad Ur Rehman b, Lu Yaojun a, Li Jiajie a, b, Yang Bin a,* a
Faculty of Materials Metallurgy and Chemistry, Jiangxi University of Science and Technology,
Ganzhou 341000,China b
Jiangxi Key Laboratory for Rare Earth Magnetic Materials and Devices/Institute for Rare
Earth Magnetic Materials and Devices (IREMMD), Jiangxi University of Science and Technology, Ganzhou 341000,China
Abstract The effect of DyMg co-deposition and diffusion on magnetic properties and service stability of sintered Nd-Fe-B magnets was investigated in this paper. The coercivity of increased from 13.26 kOe for original magnet to 18.21 kOe after DyMg diffusion. As compared to single Dy diffusion the Dy consumption was reduced by 39 % in MgDy diffused magnets. The thermal stability and corrosion resistance of the DyMg-diffused magnet improved significantly. The microstructure and element distribution were observed by SEM, EPMA and TEM. Mg element or Dy-Mg alloy repaired the damaged grain boundary to form a low melting point diffusion channel in the magnet and improved the distribution uniformity of the Dy-rich shell. In addition to the increase of
magnetocrystalline anisotropy of the main phase, the optimization of the grain boundary phase is found to be a key factor for the effective improvement of coercivity. Key word: Nd-Fe-B; Grain boundary diffusion process; Co-deposited; Low melting diffusion channel; Microstructural optimization. * Corresponding author. E-mail addresses: [email protected] (Yang Bin), [email protected] (Yang Munan)
1. Introduction The sintered Nd-Fe-B magnets have been widely used in the fields of space shuttle, new energy automobile, wind power generation and high-end technology since 1983 [1]. After years of research, the remanence (Jr) and maximum magnetic energy product ((BH)max) of sintered Nd-Fe-B magnets have reached close to the theoretical values [2]. However, the coercivity (Hcj) of Nd-Fe-B magnets is only about 30 % of the theoretical value and still has a lot of space for improvement. Improving the Hcj and the stability of magnetic properties at high temperature is one of the focuses of current research [3, 4]. The addition of heavy rare earth element (HREE) Dy, Tb significantly improve the Hcj, however, the production cost is increased due to criticality and high prices of Dy and Tb [5-7]. In order to reduce the production cost of high performance magnets and balance the consumption of rare earth (RE) resources, Prak et al. [8] deposited Dy film on the
surface of magnets by sputtering and achieved excellent results in 2000. Henceforth, grain boundary diffusion process (GBDP) has been widely studied as an effective means to improve the Hcj of RE based magnets. Many studies have shown that the diffusion of HRE alloys such as hydrides [9], fluorides [10,11] and oxides [12] of Dy or Tb could improved the Hcj at the cost of marginal decrease of Jr and (BH)max. Furthermore, studies [13-16] have shown that the Hcj can be effectively improved by low melting point alloys of HRE. It is considered that core-shell structure of the 2:14:1 phase constructed by HRE (Nd,HRE)2Fe14B shell and a HRE-lean core increases the magneto-crystalline anisotropy (HA). In addition, the wettability of grain boundary (GB) was improved greatly by introducing low melting point alloys, such as Dy70Cu30 [17], Dy35Pr35Cu30 [18] or NdCu [19]. However, GBDP also has many shortcomings, such as the poor distribution uniformity, low diffusion rate and limited diffusion depth. Moreover, the alloys of Dy element undergo a series of complex physical and chemical reactions during the process [20-22], and the diffusion mechanism for different elements is still not very clear. Therefore, a proper investigation of diffusion mechanism of HRE with low melt point element is urgently needed. This study provides an idea to solve the problem of limited diffusion depth of heavy rare earth elements in sintered Nd-Fe-B magnets. Based on the physicochemical property of low melting element Mg and heavy rare earth Dy during heat treatment, the co-deposition of Dy and Mg elements on the surface has been realized by magnetron sputtering technology, and the diffusion depth and diffusion efficiency of Dy element
have been improved. The promotion effect of Mg on the diffusion of Dy element in the process was emphatically analyzed. Finally the diffusion mechanism of Dy element with low melting point metal Mg is revealed. 2. Experimental procedure In the study, N40 commercial sintered Nd-Fe-B magnet was used as the original sample. The sample was cut into cubes of 10×10×6 mm3(6 mm c-axis) by electric spark cutting. Subsequently, Dy (99.9 % purity) and Mg (99.99 % purity) were deposited on the magnet by high vacuum magnetron sputtering technique. Two separate samples were prepared by single deposition of Dy (hereafter Dy-diffused magnet) or by co-deposition of DyMg (hereafter DyMg-diffused magnet). The basic vacuum in the cavity was over 2×10-3 Pa, and the sputtering pressure and argon flow were 3-5 Pa and 50 sccm, respectively. The sputtering power for both Dy and Mg target was 80 W. The deposition time for single Dy was 45 min, while the co-deposition time for DyMg was 30 min. The mass fraction of different elements of deposited film was detected by SEM-EDS. The sample was treated at 900
for 10 h and annealed at 500
for 3 h, and
finally was air quenched to room temperature. The magnetic performance of these magnets was measured by magnetic measurement system (NIM-500C) at room temperature. The magnetic properties at elevated temperature were measured by physical property measurement system (PPMS, Quantum Design, USA) equipped with 9 T vibrating sample magnetometer (VSM). The dynamic potential polarization curve and the EIS spectra was measured by
electrochemical comprehensive test system (PARSTART 4000) in 3.5 wt.% NaCl solution. X-ray diffractometer (XRD-Panalytical Empyrean) was used to study the phase and crystal structure before and after diffusion. Scanning Electron Microscopy (SEM-MIRA3 LMH) with an energy dispersive X-ray spectrometer was used for microstructure observation. The element distribution was analyzed by electron probe microanlyzer (EPMA-JXA-8100). Field emission transmission electron microscopy (TEM-FEI Tecnai G2 F20) was used to analyze microstructure of the grain boundary (GB) phase.
3. Results and discussion 3.1. Magnetic properties Fig. 1 shows the demagnetization curves of the original and diffusion magnets, and the inset shows the schematic diagram for Dy consumption. The amount of element deposited and the magnetic properties are shown in Table 1. At room temperature, the Jr, Hcj and (BH)max of the original magnet was 12.85 kGs, 13.26 kOe and 39.75 MGOe, respectively. The Hcj increased to 17.34 kOe after Dy diffusion , while the Jr and (BH)max decreased slightly to 12.61 kGs and 38.92 MGOe, respectively. The results are similar to those obtained by Wu et al. [23]. The magnet prepared by DyMg co-deposition and diffusion magnet depicts Hcj = 18.21 kOe, Jr = 12.68 kG and (BH)max = 39.09 MGOe. The DyMg-diffused magnet show more prominent magnetic properties than the Dy-diffused magnet. In particular, the deposited film quality on the surface of
DyMg-diffused magnet is similar with the Dy-diffused magnet, which is 2.4 mg and 2.1 mg, respectively. The consumption of Dy by magnetron sputtering method is much lower than coating or electrophoretic deposition [24,25]. The consumption of Dy element is reduced to about 39% by co-deposition of DyMg. In addition, the quantity of heavy rare earth Dy is only about 47 wt.% of the total mass of the co-deposited film, whose quality is only about 1.1 mg, which is 1.8 mg lower than the Dy consumption when deposited individually. The above results indicate that the DyMg-diffused magnet not only exhibits excellent magnetic properties compared to the conventional Dy-diffused magnet, but also promise low manufacturing cost.
Fig. 1. Demagnetization curves of the original, Dy-and DyMg-diffused magnets. The inset shows the amount of Dy element deposited into the magnets and corresponding Hcj.
Table 1 The quantity of deposited layers and the microscopic composition of each element and the magnetic properties of different magnets before and after diffusion. Increased Sample
Dy
O
Mg
Bal
Jr
Hcj
（BH）max
(wt.%)
(wt.%)
(wt.%)
(wt.%)
（kGs）
（kOe）
（MGOe）
weight (mg)
Original
-
-
-
-
-
12.85
13.26
39.75
Dy-diffused
2.1
87
11
-
2
12.61
17.34
38.92
DyMg-diffused
2.4
47
24
26
3
12.68
18.21
39.09
3.2. Thermal stability In order to study the thermal stability, the magnetic properties were measured from 300 to 380 K as shown in Fig. 2. It is observed that Jr of original magnet, Dy-diffused magnet and DyMg-diffused magnet show a downward trend as the temperature rises. However, the Jr of DyMg-diffused magnet shows a smaller variation between 340 K and 380 K, whose reduction is only 0.0662 T as compared 0.0860 T for original magnet. Fig. 2 (b) shows the values of the Hcj at different temperatures. The HA is affected by high temperature, therefore the Hcj decrease dramatically [26,27]. The temperature coefficient of remanence (αJr) and the temperature coefficient of coercivity (βHcj) are used to measure the stability of the magnet at high temperature [28, 29] by using the following formulas [30,31]:
α = [Jr (T ) - Jr (T 0)] /[Jr (T 0)(T − T 0)]× 100% Jr
β = [Hcj(T ) − Hcj (T 0)] /[Hcj (T )(T − T 0)]× 100% Hcj
(1) (2)
where T0 is the room temperature, and T is the elevated temperature. The values of both αJr and βHcj are negative, the smaller the absolute value of αJr and βHcj, the better is the thermal stability [32]. The αJr of Dy-diffused magnet decreased from -0.142 %/K to -0.157 %/K, but the αJr of the DyMg-diffused magnet improved to -0.128 %/K from -0.142 %/K. It is believed that the Dy atom mainly interacts with Co atom in the main phase, and the substituted amount of DyMg-diffused magnet was more suitable than Dy-diffused magnet [33]. Moreover, the promotion of liquid phase flow by Mg element during the diffusion increases the density, which stabilizes surface energy of main phase, and the lattice distortion is thus avoided effectively when the temperature changes [34,35]. The βHcj of Dy-diffused magnet and DyMg-diffused magnet increased to -0.686 %/K and -0.678 %/K respectively from -0.836 %/K for original magnet. Compared with the Dy-diffused magnet, not only the Hcj of DyMg-diffused magnet increased, the thermal stability is also enhanced obviously. The above experimental results show that the co-diffusion of Dy and Mg has a positive effect on the stability of Jr and Hcj.
Fig. 2. Magnetic properties of the magnets at 300-380 K: (a) Jr, (b) Hcj.
3.3. Corrosion resistance Fig. 3 (a) depicts the dynamic potential polarization curves of various magnets in 3.5% NaCl solution. As shown in Table 2, the self-corrosion potential and corrosion current density of the original magnet are -0.78747 V and 1.1151×10×5 A/cm2, respectively. The self-corrosion potential and the corrosion current density of the Dy-diffused magnet show no obvious change from the original magnet. The DyMg-diffused magnet showed a greater corrosion trend whose self-corrosion current density decreased to -0.8158 V. However, the corrosion current density of DyMg-diffused magnet decreased to 1.985×10-6 A/ cm2, which is much lower than the original magnet. The corresponding result could be seen from the EIS spectra shown in Fig. 3. (b). The radius of capacitance loop of the Dy-diffused magnet is not significantly different from the original magnet. However, the radius of capacitance loop of the DyMg-diffused magnet is larger than that of the original and Dy-diffused magnet. The impedance value of DyMg-diffused magnet is also larger than original magnet and the Dy-diffused magnet showing an optimal corrosion resistance. According to these results, we can argue that the introduction of Dy and Mg elements could improve the corrosion resistance. Wherein, the addition of the Mg element induce the formation of the Nd–O–Fe–Mg phase during heat treatment [36], while the increase of O content makes the grain boundary of the magnet more stable [37,38].
Table 2 Self-corrosion potential and corrosion current density of different magnets.
Sample
Potential/V
A/cm2
Original
-0.78747
1.1151×10-5
Dy-diffused
-0.79606
7.212×10-6
DyMg-diffused
-0.85801
1.985×10-6
Fig. 3. (a) The dynamic potential polarization curves, and (b) the EIS spectra of various magnets.
3.4. XRD analysis In order to investigate the reasons for the difference in magnetic performance, thermal stability and corrosion resistance, X-ray diffraction of the diffusion surface (the vertical c-axis) of different magnets was measured. Fig. 4 shows the XRD curves of the original, Dy-diffused and DyMg-diffused magnets at different angles. The results show that most of the diffraction peaks come from Re2Fe14B phase. No new diffraction peaks were found in the diffusion magnets. It is assumed that no new phase has been introduced into the magnet during the diffusion process and the increase of magnetic
performance is presumably independent of any phase transition. The diffraction peaks of Re2Fe14B phase of Dy-diffused magnet shift towards the high angle compared with that of original magnet. The offset of DyMg-diffused magnet is slightly smaller than that of Dy-diffused magnet. Following the Bragg equation [39] 2dsinθ = nλ , the shift of the diffraction peak towards the higher angles suggests that the crystal plane spacing and lattice parameters of diffusion magnet have reduced [40]. Because of the difference in the concentration gradient, part of the Dy atom diffuses and replaces the Nd atom [41]. As a result, the lattice parameters of the main phase and the crystal plane spacing decreased [42]. The Dy content deposited on the surface of the DyMg-diffused magnet is less than that of the Dy-diffused magnet. The substitution amount of Dy element decreased, and the change of main phase cell parameters and crystal plane spacing was smaller than the Dy-diffused magnet. Therefore, the degree of deviation of diffraction peak to high angle is also small.
Fig. 4. The X-xay diffraction patterns of various magnets.
3.5. Microstructure analysis 3.5.1. SEM analysis In order to understand the enhancement of the Hcj in the DyMg-diffused magnet, SEM has been used to observe the micrographs of these magnets. Fig. 5 depicts the SEM backscattered electron images of the surface of the magnet before and after diffusion. The microstructure of the original magnet is shown in Fig. 5(a), in which the dark gray regions correspond to the Nd2Fe14B main phases and the bright regions correspond to the Nd-rich grain boundary phases. The main phase grains are in contact with each other directly and the grain boundary phase are discontinuous. A large number of Nd-rich phases are distributed in blocks, which result in a weak exchange coupling among the main phase grains [43]. Fig. 5(b) shows the micrograph of the Dy-diffused magnet. The grain boundary phase size is obviously reduced and became more continuous and smooth after diffusion. The thin GB has increased in quantity and has made the main phase grain isolated from each other. This grain boundary optimization results in a significant increase in Hcj [44]. As shown in Fig. 5(c), the number of thin GB in the DyMg-diffused magnet is more than the Dy-diffused magnet, and the grain boundary phase is more uniform and continuous. The formation of low melting point RE-Mg phase (about 650
) by Mg element and Re-rich phase in the process greatly
promoted the liquid phase flow and enhance the wettability of grain boundary during heat treatment [45]. The formation of low melting point liquid phase during treatment made the main phase grain easy to be wrapped by thin GB. Thus, the direct contact
between the 2:14:1 phases is avoided.
Fig. 5. SEM micrographs of the magnets: (a) Original, (b) Dy-diffused, (c) DyMg-diffused.
3.5.2. The EPMA mapping of diffusion layer The EPMA mapping of the cross sections of magnets at the depth of 0 - 80 µm are shown in the Fig. 6. Fig. 6(a) shows the distribution of Nd, Mg, Fe and Dy elements near surface of Dy-diffused magnet. The Fe element is mainly distributed in the grains. The RE elements (Nd,Dy) is enriched in the Nd-rich grain boundary phase at the depth of 0 - 10 µm because of the low diffusion rate [46]. In addition, there is an overlap between parts of Dy element and the Fe element distributed in the main phase. It indicates that the formation of (Nd,Dy)2Fe14B from fractional Dy element in the surface layer enter into main phase driven by thermal diffusion. However, as the diffusion depth increases, the agglomeration of the Nd and Dy deteriorates, and the content of Dy decreases significantly. The distribution of the Nd and Dy coincides with each other, which is mainly concentrated in the triple junction region. It is shown that the content of Dy element entered the main phase decreases due to the decrease of concentration
gradient. The above results agree with the distribution law of concentration gradient [47]. These results are also consistent with the XRD results. Fig. 6(b) is the distribution map of the each element of the DyMg-diffused magnet. It can be seen from the diagram that the concentration of Mg element changes from high to low with the increase of diffusion depth. The thickness of Nd and Dy enriched layer in the surface is significantly decreased under the influence of Mg element. But in the deeper part, the size of triple junction becomes smaller and more uniform with the diffusion of Mg element. The continuity of grain boundary is obviously optimized. This is mainly due to the preferential diffusion of Mg during high temperature (900
) treatment. The Dy-Mg
alloy with low melting point was formed by the reaction of residual Mg with Dy element, as shown in Supplementary Material A. The formation of Dy-Mg alloy effectively improves the wettability of grain boundary. It promotes a continues Nd-rich phase. According to Fick's second law of diffusion [48,49], the grain boundary phase with continuous and low melting point increases the diffusion rate of Dy. The concentration gradient of Dy in the surface layer of magnet is reduced and the enrichment of Dy is alleviated.
Fig. 6. EPMA maps of section drawing (Parallel to c-axis): (a) Dy-diffused magnet, and (b) DyMg-diffused magnet at the depth of 0 - 80 µm.
For further understanding the distribution of each element in the deeper diffusion areas, the elemental diagram at the depth of ~110 µm was obtained by EPMA as shown
in Fig. 7. As shown in Fig. 7(a), the distribution uniformity of Nd element is not good and Nd is agglomerated in the large triple junctions. Meanwhile, the existence of Dy element could still be detected at the depth, which coincides with Nd-rich phase in the form of agglomeration and whose concentration is much lower than that at the surface layer. However, a different microstructure is exhibited in the DyMg-diffused magnet within the same diffusion depth. The content of Mg element was further reduced in the depth of ~ 110 µm, which is enough to cause a change of GB as shown in Fig. 7 (b). The GB phase became uniform and small, which could increase the demagnetization exchange coupling between the main phase grains and thus increase the Hcj. In addition, a continuous and uniform Dy-rich shell was observed distinctly in the epitaxial layer of 2:14:1 main phase. It shows that the core-shell structure could still be formed in that range of depth and enhanced the local magnetocrystalline anisotropy field [50,51].
Fig. 7. The EPMA maps of section drawing (Parallel to c-axis): (a) Dy-diffused magnet, and (b) DyMg-diffused magnet at the depth of ~110 µm.
3.5.3. The microstructure of GB phase Fig. 8 (a) and (b) respectively show the microstructure of the Dy-diffused magnet
and the DyMg-diffused magnet observed by TEM. The darker contrast refers to the 2:14:1 main phase, and the brighter contrasts shown the RE-rich GB phase. The micro-morphology of grain boundary of the Dy-diffused magnet is shown in Fig. 8 (a), the thickness of the GB is ~17 nm. The interface between GB and main phase grains is not clear and the GB is vague. In the Dy-diffused magnet, the morphology of GB is not optimized. The thickness of GB decreases to ~8 nm after co-diffusion of DyMg. The interface between the main phase and the GB phase becomes clearer. The introduction of Mg element improves the wettability and liquidity of Nd-rich phase and promotes the formation of a large number of thin GB from triple junction of Nd-rich phase through the interface between GB and main phase grains during the secondary annealing [52,53]. The continuous thin GB phase plays an essential role in weakening the short-range exchange coupling between adjacent 2:14:1 phase grains, which in turn prevents the cascade propagation of reversed magnetic domain throughout the grains [54].
Fig. 8. TEM images of the grain boundary structure: (a) Dy-diffused magnet, and (b) DyMg-diffused magnet.
3.5.4. The diffusion mechanism analysis Based on the above test results, the thermal diffusion process of the elements is analyzed and the mechanism of microstructure evolution is presented. Figs. 9(a)-(c) depict that there are two type of Dy diffusion: grain boundary diffusion and grain lattice diffusion [55]. Firstly, the surface-deposited Dy element diffuses along the grain boundaries, the diffusion rate is slow and the diffusion depth of Dy element is shallow. Due to multiple factors such as the grain boundary continuity and the melting point of the Nd-rich phase, part of the Dy element is agglomerated at the surface part of the Nd-rich phase [26]. Secondly, under the force of high concentration gradient, Dy element starts to diffuse into the crystal lattice to replace the Nd element. As shown, Dy element is overlapped with Fe element in the 2:14:1 main phase grains. However, the agglomeration of Dy element at the surface layer leads to less content of Dy at deeper parts of the magnet. Fig. 9(d) shows that under the influence of temperature, there are four different types of reactions and diffusion behavior in the DyMg-diffused magnet. In Fig. 9(e), at the first stage, the Mg element is preferentially diffused into magnet along the Nd-rich grain boundary phase due to its low melting point characteristics and better diffusion rate [57], This improves the wettability of grain boundary and dredge the diffusion channel of Dy element [58,59]. Second, the remaining Mg element on the surface reacts with the Dy element to form Dy-Mg alloy with a lower melting point (a little higher than Mg element). This low melting point alloy is similar to the low melting heavy rare earth alloy used in the work mentioned earlier [13-16]. Dy diffuses into the
magnet along the grain boundary under the influence of temperature. The concentration gradient of Dy element decreased dramatically due to the reduction of Dy element deposition and the formation of Dy-Mg alloy at the surface diffusion source. In addition, under the repairment of low melting point metal Mg and Dy-Mg alloy on grain boundary diffusion channels, the diffusion depth and rate of Dy element are greatly increased. The distribution ratio and content of Dy element are more uniform along the depth direction of diffusion, so that the concentration of Dy element at the depth of 0 80 µm in the DyMg-diffused magnet is lower than that the Dy-diffused magnet. The final type of diffusion behavior is the grain lattice diffusion [60]. On the basis of the above three kinds of diffusion behavior, there is still a certain difference of concentration gradient in Dy element between grain boundary and grain lattice in the deeper diffusion depth. When the concentration difference is greater than the critical diffusion concentration of the Dy element, a thin Dy-rich shell forms by lattice diffusion under the action of driving force of diffusion [45], as shown in Figure7(b). The consumption of Dy element in the DyMg-diffused magnet is less than that in the Dy-diffused magnet, but the increase in Hcj was significantly high. On one hand, the Hcj enhancement depends on the optimization of Mg element and the microstructure, on the other hand, it originates from the increase of Dy element on the intrinsic magnetic properties. The existence of Mg element can improve the grain boundary morphology. The main phase grains of Nd2Fe14B is effectively isolated, which is beneficial to the improvement of the Hcj. Moreover, both the Mg element and DyMg alloy can also repair
grain boundary diffusion channels. This leads to the formation of thinner Dy-rich core-shell structure along the grain boundary diffusion direction. The existence of Dy-rich core-shell structure would enhance local magnetocrystalline anisotropy field of 2:14:1 main phase, which leads to the Hcj enhancement.
Fig. 9. The schematic diagram of diffusion mechanism.
3.6. Coercivity mechanism The Hcj of sintered Nd-Fe-B magnets is generally expressed by the formula of nucleation model [61,62]: Hcj = α HA - NeffM s , the so-called microstructural parameters α and Neff are related to the non-ideal microstructure of the real magnet. The parameter α describes the influence of the non-perfect grain surfaces on the crystal anisotropy and orientation degree. The parameter Neff describes the enhanced stray fields at the edges and corners of the grains [34]. The Hcj, Ms and HA of the magnet at different
temperatures (300 - 380 K) are measured. The values represent the change in the internal microstructure of the magnet and determine the mechanism of the Hcj enhancement. As shown in Fig. 9, the main phase is hardened and HA is strengthened with the Dy element diffusion. The α of the original magnet is increased from 0.9477 to 0.9597 for the Dy-diffused magnet and 0.9580 for the DyMg-diffused magnet. Compared with Dy-diffused magnet, the Neff of DyMg-diffused magnet decreased significantly from 4.1249 to 3.9066. The addition of Mg and Dy changed the α and Neff in such a way that the Hcj enhanced. The introduction of the Mg element improved the grain shape and size, and the adjacent grains are wrapped by thin GBs to improve their dipolar interaction [63]. The presence of thin GB in the magnet leads to a weak demagnetization coupling and enhanced the Hcj.
Fig. 10. The Hcj/ Ms vs. HA/Ms curves of various magnets.
4. Conclusions In this paper, the effect of Dy-Mg co-deposition on the magnetic properties and corrosion resistance of magnet has been studied. The Hcj increased from 13.26 kOe for original magnet to 18.21 kOe for DyMg-diffused magnets, while the Jr and (BH)max decreased only marginally. The temperature coefficient of remanence of the DyMg-diffused magnet increased from -0.142 %/K to -0.128 %/K, and temperature coefficient of coercivity increased from -0.836 %/K to -0.678%/K. The corrosion current density of the magnet decreased from 1.1151×10-5 A/cm2 to 1.985×10-6 A/cm2, and the corrosion resistance of the magnet was enhanced. In the process of diffusion, the diffusion behavior of the low melting metal Mg and Dy-Mg alloy realized the optimization of grain boundary. The optimization of grain boundary phase effectively promoted the diffusion depth of Dy element and made it uniform. Besides the improvement of the magnetocrystalline anisotropy field and the optimization of microstructure played an indispensable role in the improvement of Hcj.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51801085) , the Education Department of Jiangxi Province (Grant No. 2012215) and Jiangxi University of Science and Technology (Grant No. jxxjbs00077 & 3401223330).
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Highlights: 1. Dy and Mg are separately deposited on the Nd-Fe-B magnets. 2. The coercivity of the DyMg-diffused magnet increased from 13.26 to 18.21 kOe. 3. Both the thermal stability and corrosion resistance are improved by the diffusion of DyMg. 4. The formation of Dy-Mg alloy is beneficial to promote the distribution of the Dy-rich shell. 5. The diffusion mechanism of HRE with low melting point element is proposed.
Declaration of Interest statement It is hereby stated that there is no conflict of interest to declare by author/co-author of this manuscript.
On behalf of all authors/co-authors Prof. Yang Bin Corresponding author