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Coercivity and corrosion resistance enhancement of multi-main-phase Nd-Ce-Fe-B sintered magnets by the grain boundary diffusion process using Pr81.5Ga19.5 and Pr81.5Ga14.5Cu5 alloys
Journal Pre-proofs Coercivity and corrosion resistance enhancement of multi-main-phase Nd-CeFe-B sintered magnets by the grain boundary diffusion process using Pr81.5Ga19.5 and Pr81.5Ga14.5Cu5 alloys Jiahui Wang, Gang Wang, Dechang Zeng PII: DOI: Reference:
S0304-8853(19)33872-7 https://doi.org/10.1016/j.jmmm.2020.166639 MAGMA 166639
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
12 November 2019 12 February 2020 18 February 2020
Please cite this article as: J. Wang, G. Wang, D. Zeng, Coercivity and corrosion resistance enhancement of multimain-phase Nd-Ce-Fe-B sintered magnets by the grain boundary diffusion process using Pr81.5Ga19.5 and Pr81.5Ga14.5Cu5 alloys, Journal of Magnetism and Magnetic Materials (2020), doi: https://doi.org/10.1016/ j.jmmm.2020.166639
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Coercivity and corrosion resistance enhancement of multi-main-phase Nd-Ce-Fe-B sintered magnets by the grain boundary diffusion process using Pr81.5Ga19.5 and Pr81.5Ga14.5Cu5 alloys
Jiahui Wang, Gang Wang*, Dechang Zeng School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, China
In this paper, grain boundary diffusion was performed on multi-main-phase Nd-Ce-Fe-B magnets using two different diffusion sources. The coercivity of the magnet was increased from 928 kA/m to 1036 kA/m and 1174 kA/m by diffusing Pr81.5Ga19.5 (PG) and Pr81.5Ga14.5Cu5 (PGC5), respectively, and the corrosion resistance of the magnet was also improved after the above diffusion process. Two kinds of continuous grain boundaries with different contrast were observed after the PG and PGC5 alloy diffusion process, and the results of line scan and quantitative analysis indicate that they originate from different intergranular phases that have been melted during the heat treatment. The distribution of Ga element after grain boundary diffusion and its effects on the coercivity of the magnet were especially investigated. It was found that the coercivity enhancement of these diffused magnets is mainly attributed to the formation of continuous grain boundary and non-ferromagnetic grain boundary phase to weaken the magnetic exchange coupling between the 2:14:1 grains, as well as the formation of the Pr-rich shell with slightly higher magnetocrystalline anisotropy due to the partial substitution of the Nd atoms by the Pr atoms, which compensates for the coercivity drop by chemical homogenization due to the heat treatment. Keywords: Grain boundary diffusion, Nd-Ce-Fe-B sintered magnet, coercivity, corrosion resistance.
1. Introduction
Nd2Fe14B sintered magnets are indispensable for the applications in the hybrid vehicles, medical devices, electronics, etc. [1, 2]. The critical rare earth (RE) elements in Nd-Fe-B sintered magnets, such as Nd/Pr/Dy/Tb, are largely consumed with the continually growing demand, while the light RE element Ce with high abundance and low cost is rarely utilized. Introducing Ce into Nd-Fe-B sintered magnets can balance the utilization of RE resources and reduce the cost of magnets [3-7]. The conventional addition of Ce element to the Nd-Fe-B sintered magnet leads to severe magnetic dilution to deteriorate the coercivity, since the intrinsic magnetic properties of Ce2Fe14B are inferior to Nd2Fe14B [8]. Many efforts have been made to prepare high-performance Ce-containing Nd-Fe-B sintered magnets. The multi-main-phase sintered Nd-Ce-Fe-B magnets that have appeared in recent years can still maintain good magnetic properties even with high Ce content. A multi-main-phase structure in which Ce atoms are distributed heterogeneously within the 2:14:1 grains is created in magnet by sintering a certain proportion of mixed Ce-containing and Ce-free alloy powders [9, 10]. This structure alleviates the magnetic dilution caused by the addition of Ce because the magnetization reversal of the Ce-rich 2:14:1 grains is hindered by the Ce-lean grains [11-14]. But the coercivity at a high Ce substitution is still far lower than the level expected for applications to traction motors or hybrid vehicles. Therefore, it is necessary to develop a competitive permanent magnet rich in Ce element. Grain boundary diffusion is one approach to effectively enhance the coercivity of magnets. There have been some researches on the grain boundary diffusion of Nd-Ce-Fe-B magnets. Tang et al. [15] applied Nd80-xDyxAl20 (x = 0, 20, 40, 60, 80 at. %) eutectic alloys to the Nd-Ce-Fe-B sintered magnets, obtaining the highest coercivity of 1.72 T at room temperature using Nd60Dy20Al20 as diffusion source. Zhou et al. [16] reported the coercivity of Nd-Ce-Fe-B sintered magnets can be largely enhanced from 0.86 T to 1.27 T by infiltrating Nd80Al10Cu10 eutectic alloy. The grain boundary diffusion of sintered Nd-Fe-B magnets is carried out by using low-melting alloys such as Pr-Cu, Pr-Nd-Cu, Pr-Al-Cu, etc. [17-20], which can well isolate the 2:14:1 grains to weaken magnetic exchange coupling. It is well known that
alloying elements such as Al, Ga, and Cu can improve the wettability of the Nd-rich phase [21-23]. A study on the Ga-doped sintered Nd-Fe-B magnet was reported by Sasaki et al. [24], which showed that the increase in coercivity is attributed to the formation of non-ferromagnetic grain boundary phases around the 2:14:1 grains. Liu et al. [25] reported that Nd80Ga15Cu5 and Nd62Fe14Ga20Cu4 were used as diffusion sources to obtain a coercivity of 2.2 T in hot-deformed Nd-Fe-B magnets. In this work, we used Pr81.5Ga19.5 and Pr81.5Ga14.5Cu5 alloys as diffusion sources to improve the coercivity of multi-main-phase Nd-Ce-Fe-B magnet. The effects of grain boundary diffusing Pr-Ga and Pr-Ga-Cu alloys on the microstructure and magnetic properties of multi-main-phase Nd-Ce-Fe-B magnets were investigated. The corrosion resistance of diffused magnets was also explored.
2. Experimental A commercial multi-main-phase Nd-Ce-Fe-B sintered magnet with the nominal composition
of
Nd17.11Pr3.34Ce7.39Fe70.30Co0.41Nb0.28Dy0.26Mn0.12Al0.42Si0.13Cu0.17Ti0.04Zr0.02Bbal (wt. %) (Ce in the total RE is 26 wt. %) and size of 5×5×2 mm3 after wire cutting was used as the original magnet. The ingots with compositions of Pr81.5Ga19.5 (at. %) (hereafter named as PG) and Pr81.5Ga14.5Cu5 (at. %) (hereafter named as PGC5) were prepared by vacuum arc melting. Then these ingots were cut into 5×5×0.4 mm flakes as diffusion source. The cut magnets and flakes were polished by abrasive papers and cleaned by an ultrasonic cleaner in alcohol. The original magnets, covered by flakes of diffusion source on the upper and lower surfaces, were wrapped in Nb foil. Then these magnets were placed in the quartz tube and were performed the diffusion treatment at 800 °C for 5 h following subsequent annealing at 450 °C for 3 h with the protection of high vacuum following high-purity argon. The weight ratio of the diffusion source accounts for about 17 wt. % of the original magnet. In order to analyze the changes of the magnet before and after diffusion, we also made a set of samples with only heat treatment and no diffusion as a comparison. The magnetic properties of the magnets were measured by a vibrating sample
magnetometer (VSM, PPMS-9, Quantum Design Co.) with maximum applied field of 5.0 T at room temperature. Differential scanning calorimetric curves (DSC) were measured to determine the melting point of the diffusion source alloys. X-ray diffraction (XRD) analysis was carried out on a PANalytical X-ray diffractometer (X’Pert Pro) using Cu Kα radiation. The microstructure was observed by field emission scanning electron microscopy (FESEM, Nano430, FEI Co.). The energy dispersive X-ray spectroscopy (EDS) was conducted to analyze the distribution of elements. The cathodic polarization curves were investigated by PGSTAT302N electrochemical workstation, and the experiments were performed at 25 °C in 3.5 wt. % NaCl aqueous solutions, which consisting of working electrode, saturated calomel reference electrode and Pt counter electrode.
3. Results and discussion Fig. 1(a) shows the DSC curves of PG and PGC5 diffusion source alloys. The melting points of two alloys were determined as 574.4 °C and 567.1 °C, respectively. The melting point of the PG alloy is close to the eutectic point (580 °C) of the Pr-Ga phase diagram. There are some more peaks in the DSC curves for both PG and PGC5 alloys, which means that new phases may be formed. In order to understand the phase constituents of diffusion source alloys, we performed XRD analysis on these diffusion source alloys and the results are shown in Fig. 1(b). According to the Pr-Ga phase diagram [26], it can be seen that Pr and GaPr2 are the phases constituting the eutectic structure, but the presence of α-Pr in the PG alloy indicates that the sample composition deviates from the eutectic point and belongs to the hypereutectic composition, thus the DSC curve of the PG alloy appears two endothermic peaks. XRD pattern shows that there was no new phase except Pr and GaPr2 appeared in the PGC5 alloy, which seems to be inconsistent with the result of the DSC curve, but compared with the PG sample, the shift of the peak positions of GaPr2 to the right was observed in PGC5 sample, indicating that Cu atoms enter the GaPr2 phase. We think that the amount of new phases formed in PGC5 alloy may be too small to be detected by XRD. In this study, the diffusion temperature is set as 800 °C to ensure that all
phases in the diffusion source alloys can be melted completely during the heat treatment process.
Fig. 1. DSC curves (a) and XRD patterns (b) of PG and PGC5 diffusion source alloys. 3.1. Phase constituents and magnetic properties The XRD patterns of original magnet, heat treated only magnet, PG and PGC5 diffused magnets are shown in Fig. 2. All samples contain RE2Fe14B phase, RE-rich intergranular phase and REFe2 phase. Another new phase appears in the PGC5 diffused sample, which we are not sure to identify but will mention it in the following discussion. The peaks of the Nd-rich intergranular phase and the REFe2 phase are enhanced in the PGC5 diffused sample.
Fig. 2. XRD patterns of original, heat treated only, PG and PGC5 diffused magnets. Fig. 3(a) shows the room temperature demagnetization curves of the original magnet, heat treated only magnet, PG and PGC5 diffused magnets. It can be seen that the coercivity increased from 928 kA/m for the original magnet to 1036 kA/m, 1171 kA/m for the PG and PGC5 diffused magnets, respectively. Meanwhile, the remanences changed negligibly. But the coercivity of the heat treated only sample decreased from 928 kA/m to 757 kA/m. Due to the compositional inhomogeneity nature of the multi-main-phase magnets, the remanences of the magnets fluctuated slightly. Here we only focus on the coercivity. The reason for the above results will be explained by observing the microstructure of the magnets. Fig. 3(b) shows the results of the temperature dependence of Hc and Jr for the original and PGC5 diffused magnets. The temperature coefficient of coercivity (β) was improved from -0.666%/K for the original magnet to -0.632%/K for PGC5 diffused magnet. The temperature coefficients of remanence (α) were calculated to be -0.136%/K and -0.144%/K, respectively, which seem to not change much. The results
indicate that the thermal stability of the magnet gets improved through the diffusion process.
Fig. 3. Demagnetization curves (a) and temperature dependence of Hc and Jr (b) of the original and treated magnets. 3.2. Microstructure and elements distribution Fig. 4 shows back-scattered electron (BSE) images of the original (a), heat treated only (b), near surface (c) and center regions (d) of PG diffused magnet, near surface (e) and center regions (f) of PGC5 diffused magnet. In the BSE-SEM images, the dark contrast refers to the 2:14:1 main phase, and the bright and gray contrast are the RE-rich intergranular phase and REFe2 phase, respectively. Due to the uniform microstructure, the surface and center regions of the original magnet have the same morphology, and so does the heat treated only magnet. For the original magnet, there is almost no continuous grain boundary between the 2:14:1 grains, and most of the intergranular phases are congregated at the triple junctions. Compared to the original magnet, continuous grain boundaries are formed for the heat treated only, PG and PGC5 diffused magnets, but continuous grain boundaries were observed only on the near surface, not inside the PG diffused magnet, which means that the diffusion efficiency of PG alloy is limited at this temperature. From the DSC results, it can be known that all phases contained in the diffusion sources have all melted under the 800 °C heat treatment, so the difference in diffusion efficiency should be attributed to the different diffusion coefficients of diverse elements. It can be inferred from the microstructure morphology that Cu has a higher diffusion coefficient in the magnet. The near surface continuous grain boundary morphology of PG diffused magnet is less obvious than the heat treated only magnet. We think that it may be caused by the difference in the diffusion paths of elements during the formation of continuous grain boundaries in these two kinds of magnets. For the heat treated only magnet, Ce atoms in the main phase tend to segregate at the grain boundary during the heat treatment
[27, 28], which reduces the melting point of the Nd-rich phase to form continuous grain boundaries. For the PG diffused magnet, the PG alloy diffuses from the magnet surface to the interior along the molten Nd-rich phase during the heat treatment, in which the infiltrated PG alloy lowers the melting point of the Nd-rich phase to form continuous grain boundaries. The diffusion distance of Ce atoms that diffuse only between adjacent main phase grains is much shorter than the diffusion distance of PG alloy that diffuses from the surface of the magnet to the center. Therefore, the continuous grain boundary morphology of heat treated only magnets is more apparent than that of the near surface of PG diffused magnet. Based on the difference in brightness, we observed clearly two kind of continuous grain boundaries with different contrast in the PGC5 diffused magnet as shown in Fig. 4(f).
Fig. 4. Low-magnification BSE-SEM images of the original magnet (a), heat treated only magnet (b), near surface (c) and center regions (d) of PG diffused magnet, near surface (e) and center regions (f) of PGC5 diffused magnet. Fig. 5 shows high-magnification BSE-SEM and element mapping images of the original (a), heat treated only (b), PG diffused (c), PGC5 diffused (d) samples to clearly describes microstructure characteristics. Table 1 lists the compositions of various intergranular phases in the original, heat treated only, PG and PGC5 diffused magnets for the corresponding regions in Fig. 4 and 5. For the original magnet, in addition to the REFe2 phase (region A in Fig. 4(a)) and the main phase, RE-rich phases with different oxygen contents (region B in Fig. 4(a) and region C in Fig. 5(a)) were observed as shown by the white arrow in Fig. 5(a). For the heat treated only magnet, the types of intergranular phase (region A and B in Fig. 5(b)) do not change. The result of line scan analysis of the continuous grain boundary for the corresponding region 1 in Fig. 5(b) is shown in Fig. 6(a). It can be found that Ce accumulates on continuous grain boundaries, indicating that the additional heat treatment will cause the Ce element tend to concentrate around the 2:14:1 grains, which will lower the local magnetocrystalline anisotropy and thus reduce the
coercivity, which is consistent with that reported by Li et al. [27] and Zhao et al. [28]. This is one aspect of the reduction of the coercivity after heat treatment in heat treated only magnet. But there is no aggregation of Ce atoms at the grain boundaries after diffusion process because the diffusion alloys enter the grain boundary phase to function as a dilution, as shown in Fig. 7. For the PG diffused and PGC5 diffused magnets, the distribution of Pr, Ga and Cu elements was investigated. Most of the Pr atoms are distributed at the grain boundaries, and Pr atoms substitute the Nd atoms on the surface of the 2:14:1 grains to form Pr2Fe14B with slightly higher magnetocrystalline anisotropy [29], which will positively contribute a part to the coercivity. The Ga atoms are enriched at the grain boundary, but the distribution at the intergranular phase is uneven. We found that the intergranular phase with high Ga content is accompanied by the high concentration of Pr (region C in Fig. 5(c) and (d)) as shown by the red arrow in Fig. 5(c) and (d). This brighter intergranular phase does not exist in the original magnet. This kind of intergranular phase is less generated in PG diffused magnet, so it is not detected by XRD, but more abundant in PGC5 diffused magnet and has a higher Cu content, which is derived from the diffusion source alloy. We consider it is non-ferromagnetic based on its low iron content. Liu et al. [25] reported that the intergranular phases formed in the hot-deformed Nd-Fe-B magnet after diffusion through Nd-Ga-Cu and Nd-Fe-Ga-Cu alloys are non-ferromagnetic. The composition of the intergranular phase formed in our work is somewhat different from that reported by Liu et al. [25], but it indicates that Ga will participate in the formation of non-ferromagnetic intergranular phase, which would weaken the exchange coupling between 2:14:1 grains. The remaining Ga is unevenly distributed in the continuous grain boundary and intergranular phase as shown by the green and white arrows in Fig. 5 (c) and (d). In order to understand these two types of continuous grain boundaries with different contrast, we performed line scan analysis on the corresponding regions 3 and 4 in Fig. 5(d), and the results are shown in Fig. 6(c) and (d). Both types of continuous grain boundaries have the enrichment of Pr and Ga elements, and it seems that the main difference lies in the content of Cu element. These two continuous grain
boundaries are not too different in composition, but why do they show different contrast? We assume that these continuous grain boundaries originate from different intergranular phases that have been melted during the heat treatment. Therefore, we quantitatively analyzed the adjacent intergranular phases of these two different continuous grain boundaries, and the results are shown in Table 1. The intergranular phase adjacent to the continuous grain boundary with gray contrast (region A in Fig. 5(d)) contains a small amount of Ga element, and the composition ratio of RE:Fe is 1:2, it can be confirmed that the phase is the REFe2 phase. Since the mixing enthalpy between Cu and Fe is positive [30], the Cu element hardly enters the REFe2 phase. The entry of Pr and Ga elements after grain boundary diffusion may change this Fe-rich phase. Since Ga has a melting point as low as room temperature, we speculate that some Ga atoms occupy the position of Fe atoms in the REFe2 phase, lowering the melting point and improving the wettability of the REFe2 phase which can easily penetrate the 2:14:1 grains to form continuous grain boundaries, hence the peak enhancement of the REFe2 phase appears in the XRD pattern. The intergranular phase adjacent to the continuous grain boundary with bright contrast is a RE-rich phase containing a small amount of Ga. Ga also diffuses into the RE-rich phase and increases the wettability of the intergranular phase at the triple junctions to form a continuous grain boundary, which is consistent with that reported by Bernardi et al. [31]. Two types of continuous grain boundaries with different contrast are also observed in PG diffused magnet as shown in Fig. 5(c). The line scan result of the continuous grain boundary corresponding to region 2 in Fig. 5(c) is similar to that of the PGC5 magnet, as shown in Fig. 6(b). This explains why two different brightness continuous grain boundaries are formed. The PGC5 diffused magnet has higher coercivity than PG diffused magnet. The introduction of Cu element further lowers the melting point of the Nd-rich intergranular phase and further improves the wettability of the Nd-rich intergranular phase at the triple junctions to form more continuous grain boundaries, which is consistent with that reported by Bernardi et al. [23]. The continuous grain boundaries are more beneficial to provide an exchange decoupling between the 2:14:1 grains.
The coercivity obtained in our work is not as good as the coercivity obtained by the diffusion of Pr-Cu alloy reported by Tang et al. [20]. In the previous studies on the grain boundary diffusion of Pr-Cu alloys for sintered Nd-Fe-B magnets [20, 32, 33], it was found that Pr-Cu alloy has a lower melting point and higher diffusion efficiency and that Cu atoms were uniformly distributed in the intergranular phase. In this study, Ga atoms not only enter the RE-rich phase and REFe2 phase during the diffusion process, but also form a non-ferromagnetic phase with relatively high Ga content, which lead to the uneven distribution of Ga element in the grain boundary. The heterogeneously distribution of Ga element in the intergranular phase in our work means that different intergranular phases have different wetting ability, resulting in the inability to form uniform continuous grain boundaries around each 2:14:1 grain. The demagnetization coupling provided is not as significant as the Pr-Cu diffused magnet, thus the coercivity enhancement effect is inferior to that of the Pr-Cu diffused magnet.
Fig. 5. High-magnification BSE-SEM and element mapping images of the original (a), heat treated only (b), PG diffused (c), and PGC5 diffused (d) magnets.
Fig. 6. Line-scan profiles for the corresponding regions of the heat treated only (a), PG diffused (b) and PGC5 diffused (c, d) magnets. Table 1 EDS analysis of various intergranular phases in Fig. 4 and 5. Fig. 7 shows SEM and the Ce element mapping images of the original (a), heat treated only (b), PG diffused (c), and PGC5 diffused (d) magnets. For the original magnet, obvious Ce concentration difference was observed within the 2:14:1 grains as shown in Fig. 7(a). It can be found that the Ce-lean region within the 2:14:1 grains was shrunk and the Ce element distribution was more dispersed as shown in Fig. 7(b), (c) and (d). In order to show this trend more clearly, we performed line scan analysis on the corresponding regions of Fig. 7 as shown in Fig. 8, the component fluctuation
range of Ce in the 2:14:1 grains decreased after heat treatment and diffusion. The additional heat treatment promoted homogenization of the 2:14:1 grain composition in the original magnet, which is consistent with that reported by Zhang et al. [13]. The homogenization of Ce destroys the long-range magnetostatic interaction in the multi-main-phase magnet, which is achieved by suppressing the magnetization reversal of the adjacent Ce-rich 2:14:1 grains by the Ce-lean grains [11-14]. This is another aspect that will negatively contribute to coercivity.
Fig. 7. SEM and the Ce element mapping images of the original (a), heat treated only (b), PG diffused (c), and PGC5 diffused (d) magnets.
Fig. 8. Elemental distributions across different grains along the lines drawn in Fig. 7: the original (a), heat treated only (b), PG diffused (c) and PGC5 diffused (d) magnets. 3.3. Corrosion resistance of magnets Potentiodynamic polarization curves for the original, PG diffused and PGC5 diffused magnets in 3.5 wt. % NaCl electrolytes are shown in Fig. 9. Their corresponding corrosion potential Ecorr and corrosion current density icorr were acquired from the polarization curves by the Tafel slope extrapolation method and are summarized in Table 2. It can be found that the corrosion potential Ecorr increased and the corrosion current density icorr decreased both in PG and PGC5 diffused magnet, which means that the corrosion resistance of the magnet was improved by the above grain boundary diffusion. From the above elemental analysis results, the distribution of Ga and Cu elements in the intergranular phase makes the intergranular phase have a higher electrode potential, which is consistent with Zhou et al. [34] and Wang et al. [35]. Since Cu has a higher electrode potential than the Ga element, the corrosion potential Ecorr exhibited by the PGC5 diffused magnet is slightly higher.
Fig. 9. Potentiodynamic polarization curves of the original, PG diffused and PGC5 diffused magnets in 3.5 wt. % NaCl.
Table 2 Ecorr and icorr values of the original, PG diffused and PGC5 diffused magnets in 3.5wt. % NaCl solution.
4. Conclusions In summary, the coercivity of the multi-main-phase Nd-Ce-Fe-B magnet was enhanced by grain boundary diffusion of Pr81.5Ga19.5 and Pr81.5Ga14.5Cu5 alloys. There were two kinds of continuous grain boundaries with different contrast in the SEM images of PG and PGC5 diffused magnets. The line scan and quantitative analysis results attest that these two types of continuous grain boundaries with different contrast originated from the REFe2 phase and the Nd-rich phase, respectively. Based on SEM and element mapping characterization, Ga element was heterogeneously distributed in the intergranular phase. Most of the Ga atoms combined with a large amount of Pr to form a non-ferromagnetic intergranular phase. The remaining Ga atoms not only diffused into the Nd-rich phase but also entered the REFe2 phase to replace part of Fe atoms to lower the melting point of the REFe2 phase and improve the wettability to form a continuous grain boundary. Compared with the uniform distribution of Cu in the intergranular phase, due to the uneven distribution of Ga element, intergranular phases with different wetting ability could not form uniform continuous grain boundaries around each 2:14:1 grain, thus the coercivity enhancement effect was not as good as that of the Pr-Cu alloy diffused magnet. In addition, heat treatment would lead to the homogenization of Ce in the 2:14:1 grains of the original magnet, which destroys the long-range magnetostatic interaction and counteracts partially the enhanced coercivity. The corrosion resistance of the magnet was also improved after the grain boundary diffusion process due to the fact that the Ga and Cu elements have a more positive electrode potential at the grain boundary.
Acknowledgement This work was financially supported by Guangdong Natural Science Foundation
(2019A1515010857).
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Table Captions: Table 1. EDS analysis of various intergranular phases in Fig. 4 and 5. Table 2. Ecorr and icorr values of the original, PG diffused and PGC5 diffused magnets in 3.5wt. % NaCl solution.
Figure Captions: Fig. 1. DSC curves (a) and XRD patterns (b) of PG and PGC5 diffusion source alloys. Fig. 2. XRD patterns of original, heat treated only, PG and PGC5 diffused magnets. Fig. 3. Demagnetization curves (a) and temperature dependence of Hc and Jr (b) of the original and treated magnets. Fig. 4. Low-magnification BSE-SEM images of the original magnet (a), heat treated only magnet (b), near surface (c) and center regions (d) of PG diffused magnet, near surface (e) and center regions (f) of PGC5 diffused magnet. Fig. 5. High-magnification BSE-SEM and element mapping images of the original (a), heat treated only (b), PG diffused (c), and PGC5 diffused (d) magnets. Fig. 6. Line-scan profiles for the corresponding regions of the heat treated only (a), PG diffused (b) and PGC5 diffused (c, d) magnets. Fig. 7. SEM and the Ce element mapping images of the original (a), heat treated only (b), PG diffused (c), and PGC5 diffused (d) magnets. Fig. 8. Elemental distributions across different grains along the lines drawn in Fig. 7: the original (a), heat treated only (b), PG diffused (c) and PGC5 diffused (d) magnets. Fig. 9. Potentiodynamic polarization curves of the original, PG diffused and PGC5 diffused magnets in 3.5 wt. % NaCl. Highlights: 1. Grain boundary diffusion was performed on multi-main-phase Nd-Ce-Fe-B magnets using two different diffusion source alloys.
2. The distribution of Ga element after grain boundary diffusion and its effects on the coercivity of the magnet were especially investigated. 3. Comparison was done with other previous study on diffusing Pr-Cu alloys.
Table 1 EDS analysis of various intergranular phases in Fig. 4 and 5. Sample
original
Heat treated only PG diffused
PGC5 diffused
Composition (at. %)
r
Phase
egion
Nd
Ce
Pr
Fe
Ga
Cu
Al
O
A
6.54
26.65
2.33
64.48
—
—
—
—
REFe2
B
23.92
7.76
6.67
11.90
—
—
—
49.75
Nd-rich
C
27.81
7.65
8.32
28.27
—
0.44
2.11
11.80
Nd-rich
A
6.30
24.23
—
67.80
—
—
1.67
—
REFe2
B
18.01
6.41
5.47
1.90
—
—
—
68.22
Nd-rich
A
4.86
3.24
17.17
66.22
5.81
—
2.70
—
REFe2
B
20.83
6.62
11.74
8.33
—
—
—
52.48
Nd-rich
C
7.38
4.52
37.72
10.92
16.13
1.79
—
18.22
undefined
A
4.91
2.42
17.49
58.82
4.24
0.59
3.10
8.44
REFe2
B
13.80
4.68
13.09
8.65
—
—
—
59.78
Nd-rich
C
—
3.59
41.07
11.36
16.85
6.84
1.34
10.43
undefined
Table 2 Ecorr and icorr values of the original, PG diffused and PGC5 diffused magnets in 3.5wt. % NaCl solution. Solution
3.5 wt.% NaCl
Magnet
Ecorr (V)
icorr (μA/cm2)
Original
-0.933
19.45
PG diffused
-0.862
12.96
PGC5 diffused
-0.858
17.52
S0304-8853(19)33872-7 https://doi.org/10.1016/j.jmmm.2020.166639 MAGMA 166639
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
12 November 2019 12 February 2020 18 February 2020
Please cite this article as: J. Wang, G. Wang, D. Zeng, Coercivity and corrosion resistance enhancement of multimain-phase Nd-Ce-Fe-B sintered magnets by the grain boundary diffusion process using Pr81.5Ga19.5 and Pr81.5Ga14.5Cu5 alloys, Journal of Magnetism and Magnetic Materials (2020), doi: https://doi.org/10.1016/ j.jmmm.2020.166639
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© 2020 Published by Elsevier B.V.
Coercivity and corrosion resistance enhancement of multi-main-phase Nd-Ce-Fe-B sintered magnets by the grain boundary diffusion process using Pr81.5Ga19.5 and Pr81.5Ga14.5Cu5 alloys
Jiahui Wang, Gang Wang*, Dechang Zeng School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, China
In this paper, grain boundary diffusion was performed on multi-main-phase Nd-Ce-Fe-B magnets using two different diffusion sources. The coercivity of the magnet was increased from 928 kA/m to 1036 kA/m and 1174 kA/m by diffusing Pr81.5Ga19.5 (PG) and Pr81.5Ga14.5Cu5 (PGC5), respectively, and the corrosion resistance of the magnet was also improved after the above diffusion process. Two kinds of continuous grain boundaries with different contrast were observed after the PG and PGC5 alloy diffusion process, and the results of line scan and quantitative analysis indicate that they originate from different intergranular phases that have been melted during the heat treatment. The distribution of Ga element after grain boundary diffusion and its effects on the coercivity of the magnet were especially investigated. It was found that the coercivity enhancement of these diffused magnets is mainly attributed to the formation of continuous grain boundary and non-ferromagnetic grain boundary phase to weaken the magnetic exchange coupling between the 2:14:1 grains, as well as the formation of the Pr-rich shell with slightly higher magnetocrystalline anisotropy due to the partial substitution of the Nd atoms by the Pr atoms, which compensates for the coercivity drop by chemical homogenization due to the heat treatment. Keywords: Grain boundary diffusion, Nd-Ce-Fe-B sintered magnet, coercivity, corrosion resistance.
1. Introduction
Nd2Fe14B sintered magnets are indispensable for the applications in the hybrid vehicles, medical devices, electronics, etc. [1, 2]. The critical rare earth (RE) elements in Nd-Fe-B sintered magnets, such as Nd/Pr/Dy/Tb, are largely consumed with the continually growing demand, while the light RE element Ce with high abundance and low cost is rarely utilized. Introducing Ce into Nd-Fe-B sintered magnets can balance the utilization of RE resources and reduce the cost of magnets [3-7]. The conventional addition of Ce element to the Nd-Fe-B sintered magnet leads to severe magnetic dilution to deteriorate the coercivity, since the intrinsic magnetic properties of Ce2Fe14B are inferior to Nd2Fe14B [8]. Many efforts have been made to prepare high-performance Ce-containing Nd-Fe-B sintered magnets. The multi-main-phase sintered Nd-Ce-Fe-B magnets that have appeared in recent years can still maintain good magnetic properties even with high Ce content. A multi-main-phase structure in which Ce atoms are distributed heterogeneously within the 2:14:1 grains is created in magnet by sintering a certain proportion of mixed Ce-containing and Ce-free alloy powders [9, 10]. This structure alleviates the magnetic dilution caused by the addition of Ce because the magnetization reversal of the Ce-rich 2:14:1 grains is hindered by the Ce-lean grains [11-14]. But the coercivity at a high Ce substitution is still far lower than the level expected for applications to traction motors or hybrid vehicles. Therefore, it is necessary to develop a competitive permanent magnet rich in Ce element. Grain boundary diffusion is one approach to effectively enhance the coercivity of magnets. There have been some researches on the grain boundary diffusion of Nd-Ce-Fe-B magnets. Tang et al. [15] applied Nd80-xDyxAl20 (x = 0, 20, 40, 60, 80 at. %) eutectic alloys to the Nd-Ce-Fe-B sintered magnets, obtaining the highest coercivity of 1.72 T at room temperature using Nd60Dy20Al20 as diffusion source. Zhou et al. [16] reported the coercivity of Nd-Ce-Fe-B sintered magnets can be largely enhanced from 0.86 T to 1.27 T by infiltrating Nd80Al10Cu10 eutectic alloy. The grain boundary diffusion of sintered Nd-Fe-B magnets is carried out by using low-melting alloys such as Pr-Cu, Pr-Nd-Cu, Pr-Al-Cu, etc. [17-20], which can well isolate the 2:14:1 grains to weaken magnetic exchange coupling. It is well known that
alloying elements such as Al, Ga, and Cu can improve the wettability of the Nd-rich phase [21-23]. A study on the Ga-doped sintered Nd-Fe-B magnet was reported by Sasaki et al. [24], which showed that the increase in coercivity is attributed to the formation of non-ferromagnetic grain boundary phases around the 2:14:1 grains. Liu et al. [25] reported that Nd80Ga15Cu5 and Nd62Fe14Ga20Cu4 were used as diffusion sources to obtain a coercivity of 2.2 T in hot-deformed Nd-Fe-B magnets. In this work, we used Pr81.5Ga19.5 and Pr81.5Ga14.5Cu5 alloys as diffusion sources to improve the coercivity of multi-main-phase Nd-Ce-Fe-B magnet. The effects of grain boundary diffusing Pr-Ga and Pr-Ga-Cu alloys on the microstructure and magnetic properties of multi-main-phase Nd-Ce-Fe-B magnets were investigated. The corrosion resistance of diffused magnets was also explored.
2. Experimental A commercial multi-main-phase Nd-Ce-Fe-B sintered magnet with the nominal composition
of
Nd17.11Pr3.34Ce7.39Fe70.30Co0.41Nb0.28Dy0.26Mn0.12Al0.42Si0.13Cu0.17Ti0.04Zr0.02Bbal (wt. %) (Ce in the total RE is 26 wt. %) and size of 5×5×2 mm3 after wire cutting was used as the original magnet. The ingots with compositions of Pr81.5Ga19.5 (at. %) (hereafter named as PG) and Pr81.5Ga14.5Cu5 (at. %) (hereafter named as PGC5) were prepared by vacuum arc melting. Then these ingots were cut into 5×5×0.4 mm flakes as diffusion source. The cut magnets and flakes were polished by abrasive papers and cleaned by an ultrasonic cleaner in alcohol. The original magnets, covered by flakes of diffusion source on the upper and lower surfaces, were wrapped in Nb foil. Then these magnets were placed in the quartz tube and were performed the diffusion treatment at 800 °C for 5 h following subsequent annealing at 450 °C for 3 h with the protection of high vacuum following high-purity argon. The weight ratio of the diffusion source accounts for about 17 wt. % of the original magnet. In order to analyze the changes of the magnet before and after diffusion, we also made a set of samples with only heat treatment and no diffusion as a comparison. The magnetic properties of the magnets were measured by a vibrating sample
magnetometer (VSM, PPMS-9, Quantum Design Co.) with maximum applied field of 5.0 T at room temperature. Differential scanning calorimetric curves (DSC) were measured to determine the melting point of the diffusion source alloys. X-ray diffraction (XRD) analysis was carried out on a PANalytical X-ray diffractometer (X’Pert Pro) using Cu Kα radiation. The microstructure was observed by field emission scanning electron microscopy (FESEM, Nano430, FEI Co.). The energy dispersive X-ray spectroscopy (EDS) was conducted to analyze the distribution of elements. The cathodic polarization curves were investigated by PGSTAT302N electrochemical workstation, and the experiments were performed at 25 °C in 3.5 wt. % NaCl aqueous solutions, which consisting of working electrode, saturated calomel reference electrode and Pt counter electrode.
3. Results and discussion Fig. 1(a) shows the DSC curves of PG and PGC5 diffusion source alloys. The melting points of two alloys were determined as 574.4 °C and 567.1 °C, respectively. The melting point of the PG alloy is close to the eutectic point (580 °C) of the Pr-Ga phase diagram. There are some more peaks in the DSC curves for both PG and PGC5 alloys, which means that new phases may be formed. In order to understand the phase constituents of diffusion source alloys, we performed XRD analysis on these diffusion source alloys and the results are shown in Fig. 1(b). According to the Pr-Ga phase diagram [26], it can be seen that Pr and GaPr2 are the phases constituting the eutectic structure, but the presence of α-Pr in the PG alloy indicates that the sample composition deviates from the eutectic point and belongs to the hypereutectic composition, thus the DSC curve of the PG alloy appears two endothermic peaks. XRD pattern shows that there was no new phase except Pr and GaPr2 appeared in the PGC5 alloy, which seems to be inconsistent with the result of the DSC curve, but compared with the PG sample, the shift of the peak positions of GaPr2 to the right was observed in PGC5 sample, indicating that Cu atoms enter the GaPr2 phase. We think that the amount of new phases formed in PGC5 alloy may be too small to be detected by XRD. In this study, the diffusion temperature is set as 800 °C to ensure that all
phases in the diffusion source alloys can be melted completely during the heat treatment process.
Fig. 1. DSC curves (a) and XRD patterns (b) of PG and PGC5 diffusion source alloys. 3.1. Phase constituents and magnetic properties The XRD patterns of original magnet, heat treated only magnet, PG and PGC5 diffused magnets are shown in Fig. 2. All samples contain RE2Fe14B phase, RE-rich intergranular phase and REFe2 phase. Another new phase appears in the PGC5 diffused sample, which we are not sure to identify but will mention it in the following discussion. The peaks of the Nd-rich intergranular phase and the REFe2 phase are enhanced in the PGC5 diffused sample.
Fig. 2. XRD patterns of original, heat treated only, PG and PGC5 diffused magnets. Fig. 3(a) shows the room temperature demagnetization curves of the original magnet, heat treated only magnet, PG and PGC5 diffused magnets. It can be seen that the coercivity increased from 928 kA/m for the original magnet to 1036 kA/m, 1171 kA/m for the PG and PGC5 diffused magnets, respectively. Meanwhile, the remanences changed negligibly. But the coercivity of the heat treated only sample decreased from 928 kA/m to 757 kA/m. Due to the compositional inhomogeneity nature of the multi-main-phase magnets, the remanences of the magnets fluctuated slightly. Here we only focus on the coercivity. The reason for the above results will be explained by observing the microstructure of the magnets. Fig. 3(b) shows the results of the temperature dependence of Hc and Jr for the original and PGC5 diffused magnets. The temperature coefficient of coercivity (β) was improved from -0.666%/K for the original magnet to -0.632%/K for PGC5 diffused magnet. The temperature coefficients of remanence (α) were calculated to be -0.136%/K and -0.144%/K, respectively, which seem to not change much. The results
indicate that the thermal stability of the magnet gets improved through the diffusion process.
Fig. 3. Demagnetization curves (a) and temperature dependence of Hc and Jr (b) of the original and treated magnets. 3.2. Microstructure and elements distribution Fig. 4 shows back-scattered electron (BSE) images of the original (a), heat treated only (b), near surface (c) and center regions (d) of PG diffused magnet, near surface (e) and center regions (f) of PGC5 diffused magnet. In the BSE-SEM images, the dark contrast refers to the 2:14:1 main phase, and the bright and gray contrast are the RE-rich intergranular phase and REFe2 phase, respectively. Due to the uniform microstructure, the surface and center regions of the original magnet have the same morphology, and so does the heat treated only magnet. For the original magnet, there is almost no continuous grain boundary between the 2:14:1 grains, and most of the intergranular phases are congregated at the triple junctions. Compared to the original magnet, continuous grain boundaries are formed for the heat treated only, PG and PGC5 diffused magnets, but continuous grain boundaries were observed only on the near surface, not inside the PG diffused magnet, which means that the diffusion efficiency of PG alloy is limited at this temperature. From the DSC results, it can be known that all phases contained in the diffusion sources have all melted under the 800 °C heat treatment, so the difference in diffusion efficiency should be attributed to the different diffusion coefficients of diverse elements. It can be inferred from the microstructure morphology that Cu has a higher diffusion coefficient in the magnet. The near surface continuous grain boundary morphology of PG diffused magnet is less obvious than the heat treated only magnet. We think that it may be caused by the difference in the diffusion paths of elements during the formation of continuous grain boundaries in these two kinds of magnets. For the heat treated only magnet, Ce atoms in the main phase tend to segregate at the grain boundary during the heat treatment
[27, 28], which reduces the melting point of the Nd-rich phase to form continuous grain boundaries. For the PG diffused magnet, the PG alloy diffuses from the magnet surface to the interior along the molten Nd-rich phase during the heat treatment, in which the infiltrated PG alloy lowers the melting point of the Nd-rich phase to form continuous grain boundaries. The diffusion distance of Ce atoms that diffuse only between adjacent main phase grains is much shorter than the diffusion distance of PG alloy that diffuses from the surface of the magnet to the center. Therefore, the continuous grain boundary morphology of heat treated only magnets is more apparent than that of the near surface of PG diffused magnet. Based on the difference in brightness, we observed clearly two kind of continuous grain boundaries with different contrast in the PGC5 diffused magnet as shown in Fig. 4(f).
Fig. 4. Low-magnification BSE-SEM images of the original magnet (a), heat treated only magnet (b), near surface (c) and center regions (d) of PG diffused magnet, near surface (e) and center regions (f) of PGC5 diffused magnet. Fig. 5 shows high-magnification BSE-SEM and element mapping images of the original (a), heat treated only (b), PG diffused (c), PGC5 diffused (d) samples to clearly describes microstructure characteristics. Table 1 lists the compositions of various intergranular phases in the original, heat treated only, PG and PGC5 diffused magnets for the corresponding regions in Fig. 4 and 5. For the original magnet, in addition to the REFe2 phase (region A in Fig. 4(a)) and the main phase, RE-rich phases with different oxygen contents (region B in Fig. 4(a) and region C in Fig. 5(a)) were observed as shown by the white arrow in Fig. 5(a). For the heat treated only magnet, the types of intergranular phase (region A and B in Fig. 5(b)) do not change. The result of line scan analysis of the continuous grain boundary for the corresponding region 1 in Fig. 5(b) is shown in Fig. 6(a). It can be found that Ce accumulates on continuous grain boundaries, indicating that the additional heat treatment will cause the Ce element tend to concentrate around the 2:14:1 grains, which will lower the local magnetocrystalline anisotropy and thus reduce the
coercivity, which is consistent with that reported by Li et al. [27] and Zhao et al. [28]. This is one aspect of the reduction of the coercivity after heat treatment in heat treated only magnet. But there is no aggregation of Ce atoms at the grain boundaries after diffusion process because the diffusion alloys enter the grain boundary phase to function as a dilution, as shown in Fig. 7. For the PG diffused and PGC5 diffused magnets, the distribution of Pr, Ga and Cu elements was investigated. Most of the Pr atoms are distributed at the grain boundaries, and Pr atoms substitute the Nd atoms on the surface of the 2:14:1 grains to form Pr2Fe14B with slightly higher magnetocrystalline anisotropy [29], which will positively contribute a part to the coercivity. The Ga atoms are enriched at the grain boundary, but the distribution at the intergranular phase is uneven. We found that the intergranular phase with high Ga content is accompanied by the high concentration of Pr (region C in Fig. 5(c) and (d)) as shown by the red arrow in Fig. 5(c) and (d). This brighter intergranular phase does not exist in the original magnet. This kind of intergranular phase is less generated in PG diffused magnet, so it is not detected by XRD, but more abundant in PGC5 diffused magnet and has a higher Cu content, which is derived from the diffusion source alloy. We consider it is non-ferromagnetic based on its low iron content. Liu et al. [25] reported that the intergranular phases formed in the hot-deformed Nd-Fe-B magnet after diffusion through Nd-Ga-Cu and Nd-Fe-Ga-Cu alloys are non-ferromagnetic. The composition of the intergranular phase formed in our work is somewhat different from that reported by Liu et al. [25], but it indicates that Ga will participate in the formation of non-ferromagnetic intergranular phase, which would weaken the exchange coupling between 2:14:1 grains. The remaining Ga is unevenly distributed in the continuous grain boundary and intergranular phase as shown by the green and white arrows in Fig. 5 (c) and (d). In order to understand these two types of continuous grain boundaries with different contrast, we performed line scan analysis on the corresponding regions 3 and 4 in Fig. 5(d), and the results are shown in Fig. 6(c) and (d). Both types of continuous grain boundaries have the enrichment of Pr and Ga elements, and it seems that the main difference lies in the content of Cu element. These two continuous grain
boundaries are not too different in composition, but why do they show different contrast? We assume that these continuous grain boundaries originate from different intergranular phases that have been melted during the heat treatment. Therefore, we quantitatively analyzed the adjacent intergranular phases of these two different continuous grain boundaries, and the results are shown in Table 1. The intergranular phase adjacent to the continuous grain boundary with gray contrast (region A in Fig. 5(d)) contains a small amount of Ga element, and the composition ratio of RE:Fe is 1:2, it can be confirmed that the phase is the REFe2 phase. Since the mixing enthalpy between Cu and Fe is positive [30], the Cu element hardly enters the REFe2 phase. The entry of Pr and Ga elements after grain boundary diffusion may change this Fe-rich phase. Since Ga has a melting point as low as room temperature, we speculate that some Ga atoms occupy the position of Fe atoms in the REFe2 phase, lowering the melting point and improving the wettability of the REFe2 phase which can easily penetrate the 2:14:1 grains to form continuous grain boundaries, hence the peak enhancement of the REFe2 phase appears in the XRD pattern. The intergranular phase adjacent to the continuous grain boundary with bright contrast is a RE-rich phase containing a small amount of Ga. Ga also diffuses into the RE-rich phase and increases the wettability of the intergranular phase at the triple junctions to form a continuous grain boundary, which is consistent with that reported by Bernardi et al. [31]. Two types of continuous grain boundaries with different contrast are also observed in PG diffused magnet as shown in Fig. 5(c). The line scan result of the continuous grain boundary corresponding to region 2 in Fig. 5(c) is similar to that of the PGC5 magnet, as shown in Fig. 6(b). This explains why two different brightness continuous grain boundaries are formed. The PGC5 diffused magnet has higher coercivity than PG diffused magnet. The introduction of Cu element further lowers the melting point of the Nd-rich intergranular phase and further improves the wettability of the Nd-rich intergranular phase at the triple junctions to form more continuous grain boundaries, which is consistent with that reported by Bernardi et al. [23]. The continuous grain boundaries are more beneficial to provide an exchange decoupling between the 2:14:1 grains.
The coercivity obtained in our work is not as good as the coercivity obtained by the diffusion of Pr-Cu alloy reported by Tang et al. [20]. In the previous studies on the grain boundary diffusion of Pr-Cu alloys for sintered Nd-Fe-B magnets [20, 32, 33], it was found that Pr-Cu alloy has a lower melting point and higher diffusion efficiency and that Cu atoms were uniformly distributed in the intergranular phase. In this study, Ga atoms not only enter the RE-rich phase and REFe2 phase during the diffusion process, but also form a non-ferromagnetic phase with relatively high Ga content, which lead to the uneven distribution of Ga element in the grain boundary. The heterogeneously distribution of Ga element in the intergranular phase in our work means that different intergranular phases have different wetting ability, resulting in the inability to form uniform continuous grain boundaries around each 2:14:1 grain. The demagnetization coupling provided is not as significant as the Pr-Cu diffused magnet, thus the coercivity enhancement effect is inferior to that of the Pr-Cu diffused magnet.
Fig. 5. High-magnification BSE-SEM and element mapping images of the original (a), heat treated only (b), PG diffused (c), and PGC5 diffused (d) magnets.
Fig. 6. Line-scan profiles for the corresponding regions of the heat treated only (a), PG diffused (b) and PGC5 diffused (c, d) magnets. Table 1 EDS analysis of various intergranular phases in Fig. 4 and 5. Fig. 7 shows SEM and the Ce element mapping images of the original (a), heat treated only (b), PG diffused (c), and PGC5 diffused (d) magnets. For the original magnet, obvious Ce concentration difference was observed within the 2:14:1 grains as shown in Fig. 7(a). It can be found that the Ce-lean region within the 2:14:1 grains was shrunk and the Ce element distribution was more dispersed as shown in Fig. 7(b), (c) and (d). In order to show this trend more clearly, we performed line scan analysis on the corresponding regions of Fig. 7 as shown in Fig. 8, the component fluctuation
range of Ce in the 2:14:1 grains decreased after heat treatment and diffusion. The additional heat treatment promoted homogenization of the 2:14:1 grain composition in the original magnet, which is consistent with that reported by Zhang et al. [13]. The homogenization of Ce destroys the long-range magnetostatic interaction in the multi-main-phase magnet, which is achieved by suppressing the magnetization reversal of the adjacent Ce-rich 2:14:1 grains by the Ce-lean grains [11-14]. This is another aspect that will negatively contribute to coercivity.
Fig. 7. SEM and the Ce element mapping images of the original (a), heat treated only (b), PG diffused (c), and PGC5 diffused (d) magnets.
Fig. 8. Elemental distributions across different grains along the lines drawn in Fig. 7: the original (a), heat treated only (b), PG diffused (c) and PGC5 diffused (d) magnets. 3.3. Corrosion resistance of magnets Potentiodynamic polarization curves for the original, PG diffused and PGC5 diffused magnets in 3.5 wt. % NaCl electrolytes are shown in Fig. 9. Their corresponding corrosion potential Ecorr and corrosion current density icorr were acquired from the polarization curves by the Tafel slope extrapolation method and are summarized in Table 2. It can be found that the corrosion potential Ecorr increased and the corrosion current density icorr decreased both in PG and PGC5 diffused magnet, which means that the corrosion resistance of the magnet was improved by the above grain boundary diffusion. From the above elemental analysis results, the distribution of Ga and Cu elements in the intergranular phase makes the intergranular phase have a higher electrode potential, which is consistent with Zhou et al. [34] and Wang et al. [35]. Since Cu has a higher electrode potential than the Ga element, the corrosion potential Ecorr exhibited by the PGC5 diffused magnet is slightly higher.
Fig. 9. Potentiodynamic polarization curves of the original, PG diffused and PGC5 diffused magnets in 3.5 wt. % NaCl.
Table 2 Ecorr and icorr values of the original, PG diffused and PGC5 diffused magnets in 3.5wt. % NaCl solution.
4. Conclusions In summary, the coercivity of the multi-main-phase Nd-Ce-Fe-B magnet was enhanced by grain boundary diffusion of Pr81.5Ga19.5 and Pr81.5Ga14.5Cu5 alloys. There were two kinds of continuous grain boundaries with different contrast in the SEM images of PG and PGC5 diffused magnets. The line scan and quantitative analysis results attest that these two types of continuous grain boundaries with different contrast originated from the REFe2 phase and the Nd-rich phase, respectively. Based on SEM and element mapping characterization, Ga element was heterogeneously distributed in the intergranular phase. Most of the Ga atoms combined with a large amount of Pr to form a non-ferromagnetic intergranular phase. The remaining Ga atoms not only diffused into the Nd-rich phase but also entered the REFe2 phase to replace part of Fe atoms to lower the melting point of the REFe2 phase and improve the wettability to form a continuous grain boundary. Compared with the uniform distribution of Cu in the intergranular phase, due to the uneven distribution of Ga element, intergranular phases with different wetting ability could not form uniform continuous grain boundaries around each 2:14:1 grain, thus the coercivity enhancement effect was not as good as that of the Pr-Cu alloy diffused magnet. In addition, heat treatment would lead to the homogenization of Ce in the 2:14:1 grains of the original magnet, which destroys the long-range magnetostatic interaction and counteracts partially the enhanced coercivity. The corrosion resistance of the magnet was also improved after the grain boundary diffusion process due to the fact that the Ga and Cu elements have a more positive electrode potential at the grain boundary.
Acknowledgement This work was financially supported by Guangdong Natural Science Foundation
(2019A1515010857).
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Table Captions: Table 1. EDS analysis of various intergranular phases in Fig. 4 and 5. Table 2. Ecorr and icorr values of the original, PG diffused and PGC5 diffused magnets in 3.5wt. % NaCl solution.
Figure Captions: Fig. 1. DSC curves (a) and XRD patterns (b) of PG and PGC5 diffusion source alloys. Fig. 2. XRD patterns of original, heat treated only, PG and PGC5 diffused magnets. Fig. 3. Demagnetization curves (a) and temperature dependence of Hc and Jr (b) of the original and treated magnets. Fig. 4. Low-magnification BSE-SEM images of the original magnet (a), heat treated only magnet (b), near surface (c) and center regions (d) of PG diffused magnet, near surface (e) and center regions (f) of PGC5 diffused magnet. Fig. 5. High-magnification BSE-SEM and element mapping images of the original (a), heat treated only (b), PG diffused (c), and PGC5 diffused (d) magnets. Fig. 6. Line-scan profiles for the corresponding regions of the heat treated only (a), PG diffused (b) and PGC5 diffused (c, d) magnets. Fig. 7. SEM and the Ce element mapping images of the original (a), heat treated only (b), PG diffused (c), and PGC5 diffused (d) magnets. Fig. 8. Elemental distributions across different grains along the lines drawn in Fig. 7: the original (a), heat treated only (b), PG diffused (c) and PGC5 diffused (d) magnets. Fig. 9. Potentiodynamic polarization curves of the original, PG diffused and PGC5 diffused magnets in 3.5 wt. % NaCl. Highlights: 1. Grain boundary diffusion was performed on multi-main-phase Nd-Ce-Fe-B magnets using two different diffusion source alloys.
2. The distribution of Ga element after grain boundary diffusion and its effects on the coercivity of the magnet were especially investigated. 3. Comparison was done with other previous study on diffusing Pr-Cu alloys.
Table 1 EDS analysis of various intergranular phases in Fig. 4 and 5. Sample
original
Heat treated only PG diffused
PGC5 diffused
Composition (at. %)
r
Phase
egion
Nd
Ce
Pr
Fe
Ga
Cu
Al
O
A
6.54
26.65
2.33
64.48
—
—
—
—
REFe2
B
23.92
7.76
6.67
11.90
—
—
—
49.75
Nd-rich
C
27.81
7.65
8.32
28.27
—
0.44
2.11
11.80
Nd-rich
A
6.30
24.23
—
67.80
—
—
1.67
—
REFe2
B
18.01
6.41
5.47
1.90
—
—
—
68.22
Nd-rich
A
4.86
3.24
17.17
66.22
5.81
—
2.70
—
REFe2
B
20.83
6.62
11.74
8.33
—
—
—
52.48
Nd-rich
C
7.38
4.52
37.72
10.92
16.13
1.79
—
18.22
undefined
A
4.91
2.42
17.49
58.82
4.24
0.59
3.10
8.44
REFe2
B
13.80
4.68
13.09
8.65
—
—
—
59.78
Nd-rich
C
—
3.59
41.07
11.36
16.85
6.84
1.34
10.43
undefined
Table 2 Ecorr and icorr values of the original, PG diffused and PGC5 diffused magnets in 3.5wt. % NaCl solution. Solution
3.5 wt.% NaCl
Magnet
Ecorr (V)
icorr (μA/cm2)
Original
-0.933
19.45
PG diffused
-0.862
12.96
PGC5 diffused
-0.858
17.52