Study on Ce₂Fe₁₄B-based sintered magnets with Ce/RE> 80%

Abstract

Up to now, the development of Ce/RE> 80% sintered permanent magnets still faces a challenge, owing to their seriously deteriorated microstructure. In this article, the Ce_25.5∼31.5Nd_0∼5Fe_bal.B_1.25M_1.15 (wt.%) sintered magnets with Ce/RE > 80% were investigated, the roles of minor Nd substitution were explored. The typical island-like phase common existed in the grain-boundary (GB) regions of Ce₂Fe₁₄B-based SC alloys (which destroys the continuity of RE-rich GB phase), was confirmed as a tetragonal B-rich phase (fct-RE₅Fe₁₈B₁₈), and probably generated by the peritectic reaction of “L + Ce₂Fe₁₄B → CeFe₂ + B-rich” at 797 °C. It was found that: the deteriorated microstructures of high Ce sintered magnets were hardly improved (Nd element was not enriched in the main phase as it was expected), and the coercivity increments were far below expectations by directly adding 3–5% Nd in alloy designs. However, the GB phase distribution was more uniform and continuous, Nd-rich shells formed, and magnetic properties were remarkably promoted by blending 2% NdH_x powders into the Ce_27.5Nd₃Fe_bal.B_1.25M_1.15 (Ce_27.5Nd₃) magnet during JM milling, compared with the Ce_25.5Nd₅Fe_bal.B_1.25M_1.15 (Ce_25.5Nd₅) magnet with similar composition. The good comprehensive magnetic properties of H_cj = 1.714 kOe, B_r = 9.395 kG, and (BH)_max = 11.16 MGOe have been reached in the Ce_27.5Nd₃+2% (NdH_x) magnet (Ce accounted for 84.5 wt% of total rare earth). The present work deepens our understanding of metallurgical behavior, process/microstructure designing for Ce/RE> 80% sintered magnets, and shed a light on the large-scale utilization of Ce element in a permanent magnet.

Highlights

• •The development of Ce/RE> 80% sintered permanent magnets is facing a great challenge.
• •Island-like phase common existed in high Ce alloys is confirmed as a B-rich phase.
• • Poor microstructures of high Ce magnets were hardly improved by directly adding Nd.
• •Microstructures and coercivity were remarkably promoted by blending addition of Nd.
• •It shed a light on process/microstructure designing for Ce/RE> 80% sintered magnets.

1. Introduction

The demands for permanent magnets will keep quickly increasing in the foreseeable future, in order to help reduce carbon emission and achieve green economic development. It is well known that Ce₂Fe₁₄B is far inferior to Nd₂Fe₁₄B with regard to intrinsic magnetic properties [1]. But, in the recent 10 years, the high abundance Ce-based magnet becomes a research highlight due to its low cost and stable price [[2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]]. A question has arisen: the phase constitution and microstructure deteriorated significantly and deviated from the normal state accompanying with α-Fe precipitation and CeFe₂ aggregation, with increasing Ce content in RE-Fe-B sintered magnets [9]. The energy product ((BH)_max) and the coercivity (H_cj) of a ternary Ce–Fe–B sintered magnet have fallen to almost zero [9,13]. Therefore, the researches on high abundance sintered magnets mainly focused on the composition range of Ce (or Ce + La + Y)/RE < 50%. Metallurgical behaviors of 25% Ce/La/Y substituted Nd-Fe-B sintered magnets have been reported [24]. The fundamental research is seriously insufficient for supporting the large-scale utilization of high abundance Ce element in a permanent magnet. For PrNd-free Ce–Fe–B sintered magnets, the microstructure and H_cj has been promoted by co-substituting Y and La [19], substituting Gd [20], and doping low-melting point alloys [[21], [22], [23]], but their remanence (B_r) was rather low (about 7.3–8.3 kGs). For the hot deformed Ce-based magnets, a clear orientation is obtained by partly substituting La for Ce, which is attributed to the reduced amount of CeFe₂ phase, and the formation of RE-rich intergranular phases in the (Ce, La)–Fe–B alloy [26]. The GB microstructures and magnetic properties of the hot deformed Ce-based magnet were improved by a two-step diffusion process (Nd–Cu doping followed by Nd–Cu diffusion), and the B_r of 0.68 kGs, and (BH)_max of 9.1 MGOe were achieved in the hot deformed (Ce, La, Y)–Fe–B magnet [27].

In a word, the development of high Ce-content sintered magnets with good performance faces a challenge. Up to now, the research on effects of minor Nd substitution on magnetic properties and microstructures of Ce–Fe–B magnets was reported rarely in the publications. In this article, the Ce–Fe–B magnets with different Nd substitution were prepared, the roles of minor Nd substitution in phase constitution and microstructure revolution of the magnets have been explored, in order to upgrade comprehensive performance of sintered magnets with high Ce content.

2. Experimental

The Ce₂Fe₁₄B-based sintered magnets of Ce_31.5Fe_bal.B_1.25M_1.15, Ce_27.5Nd₃Fe_bal.B_1.25- M_1.15, Ce_25.5Nd₅Fe_bal.B_1.25M_1.15 (wt.%; M = Co + Al + Cu + Nb) (abbreviated as Ce_31.5, Ce_27.5Nd₃, and Ce_25.5Nd₅, respectively), were prepared adopting a traditional powder metallurgy sintering method including strip-casting (SC), hydrogen decrepitating (HD), jet milling (JM), powder pressing in a magnetic field, compact sintering and annealing. The JM powders (SMD≈3.0 μm) were orientated and compacted in an applied magnetic field, then sintered at 960–1000 °C for 2 h. The Ce_27.5Nd₃ + 2% (NdH_x) sintered magnets were also fabricated by blending 2% NdH_x hydrogen decrepitated powders into the Ce_27.5Nd₃ alloy during the JM process, whose composition was close to the Ce_25.5Nd₅ magnet. The magnetic properties of the magnets were tested by a NIM-2000 hysteresis loop meter.

Emission scanning electron microscopy (SEM, JSM-7200F) and transmission electron microscopy (TEM, FEI Talos F200X) were employed to observe the microstructures. Energy dispersive X-ray spectrometer (EDS, Oxford X-Max) was adopted to analyze the phase compositions, and the phase characterization was carried out by X-ray diffraction (XRD, Bruker-D8) with Co Kα radiation. We used a simultaneous thermal analyzer (NETZSCH STA 449 F3 Jupiter) to identify the magnetic phases in the experimental Ce-based magnets. An electron probe micro-analyzer (EPMA, JXA-iHP200F) was adopted to investigate the elemental concentration mappings of the magnets.

3. Results and discussion

The microstructure morphologies of the Ce_31.5, Ce_27.5Nd₃, and Ce_25.5Nd₅ SC alloys were analyzed and observed by SEM. Fig. 1 is the backscattered electron SEM (BE-SEM) images of Ce_31.5, Ce_27.5Nd₃, and Ce_25.5Nd₅ SC alloys. It can be seen that: for the Nd-free Ce_31.5 SC alloy, the main-phase grains coarsened, the crystalline orientation of grains along the cooling direction is rather poor, and the grain boundary (GB) phases are in-homogeneously distributed between grains, dark island-like phases were found in the GB regions (see Fig. 1(a) and (d)); with 3% Nd substitution for Ce, the crystalline orientation of grains were obviously improved, the lamellar main-phase grains were refined in the Ce_27.5Nd₃ SC alloy (see Fig. 1(b) and (e)); as further increasing Nd content to 5%, the main-phase grains became coarse and the island-like phases are grown up (see Fig. 1(c) and (f)). The corresponding EDS results of typical phases in these Ce-based SC alloys are listed in Table 1. It suggests that all the alloys consist of RE₂Fe₁₄B, REFe₂, and some island-like phases found in RE-rich phase regions, which agree with the previous research findings [20]. The island-like phases were deeply analyzed and discussed later in this article.

Fig. 1.

SEM microstructure images of the experimental SC alloy flakes. (a), (d) Ce_31.5.5; (b), (e) Ce_27.5Nd₃; (c), (f) Ce_25.5Nd₅.

Table 1.

EDS results of typical phases in different SC alloy flakes.

Alloys	Spots	Element/at.%			Phase analysis
		Ce	Nd	Fe
Fig. 1(d) Ce31.5	1	13.3	/	86.7	Ce2Fe14B
	2	36.0	/	64.0	CeFe2
	3	21.7	/	78.3	B-rich*
Fig. 1(e) Ce27.5Nd3	1	11.5	1.5	87	RE2Fe14B
	2	29.2	1.5	69.3	REFe2
	3	22.2	1.3	76.5	B-rich
	4	21.2	1.9	76.9	B-rich
Fig. 1(f) Ce25.5Nd5	1	10.5	2.7	86.8	RE2Fe14B
	2	31.8	2.4	65.8	REFe2
	3	20.1	3.0	76.9	B-rich
	4	19.7	3.2	77.1	B-rich

Note: The phase was confirmed as a tetragonal B-rich phase by later EPMA and TEM analyses.

The phase constitutions of four Ce-based sintered magnets of Ce_31.5, Ce_27.5Nd₃, Ce_25.5Nd₅, Ce_27.5Nd₃ + 2% (NdH_x) were characterized by XRD with Co Kα radiation. Fig. 2 shows the XRD patterns of the powder samples of the magnets. It indicated that the Ce-based sintered magnets mainly consisted of RE₂Fe₁₄B and REFe₂. There was a weak α-Fe peak appeared in all these magnet samples. With increasing Nd content, the positions of characteristic peaks of both RE₂Fe₁₄B and α-Fe phases have shifted leftward (see the inset in Fig. 3), which was probably because some Nd atoms gradually went into the RE₂Fe₁₄B, and caused a lattice expansion.

Fig. 2.

XRD patterns of the powder samples for four different types of Ce-based sintered magnets.

Fig. 3.

TMA curves (a) and their differential curves (b) of the experimental Ce-based sintered magnets with different Nd substitution.

The thermo-magnetic analyses (TMA, also called as TGA) of the Ce-based sintered magnets were performed using a simultaneous thermal analyzer with an externally applied magnetic field. As shown in Fig. 3, there are the two weight-loss steps appeared in the measured TMA curves, which were corresponding to the Curie temperatures (T_c) of RE₂Fe₁₄B and α-Fe, respectively (see Fig. 3(a)). It shows that the T_c values of main phase RE₂Fe₁₄B have enhanced from 156 °C to 170 °C with 3% Nd substitution (Ce_27.5Nd₃), and reached 183 °C with 5% Nd substitution (Ce_27.5Nd₅). For the Ce_27.5Nd₃+2% (NdH_x) magnet, the T_c of RE₂Fe₁₄B (∼175 °C) was slightly lower than the Ce_27.5Nd₅ magnet. It indicated that the Nd substituted obviously promoted the T_c of the Ce-based magnets. A very small amount of f α-Fe, corresponding to the weight-loss step at ∼770 °C, was detected in the Ce-based magnets. It seems that the α-Fe has been successfully suppressed by a high B composition design (B = 1.25 at. %) in this study, and was not found in SEM observations; however, it was not completely eliminated, and was still detectable by the XRD and TMA analyses. Despite all this, it is believed that a trace amount, fine dispersed α-Fe isn't the main cause for a low coercivity of the Ce-based sintered magnets, when the exchange coupling effect between hard/soft grains is taken into account.

The SEM microstructure images and EDS results of the Ce_31.5, Ce_27.5Nd₃, and Ce_25.5Nd₅ sintered magnets are shown in Fig. 4. The microstructures of the high Ce-content magnets were rather poor, RE-rich phases aggregated, and large grains formed due to lack of thin grain-boundary phases. As shown in Fig. 4 (b) and (e), some dark phases with Fe content of ∼76 at. % (RE: Fe ~ 1: 3) were found in the magnets, whose compositions were very close to that of the island-like phases appeared in the SC alloys (see Fig. 1, and Table 1). It suggested that the dark phases in sintered magnets were inherited from the island-like phases in the SC alloys. It is worth noting that the phases with ∼76% Fe have higher RE and lower Fe contents, but darker contrast (in BE-SEM mode) than RE₂Fe₁₄B main phase, therefore, we speculate that these dark phases should contain some chemical elements with a low atomic number (such as B, O, etc.). Elemental concentration mapping was performed later using EPMA with wavelength dispersive X-ray spectrometer (WDS). Fig. 5 shows the SEM images and EDS results of the Ce_27.5Nd₃+2% (NdH_x) sintered magnet. It shows that the distribution of grain boundary RE-rich phases became more uniform and continuous, the direct contact phenomena of main-phase grains was suppressed; the dark phases with Fe content of ∼76% were almost disappeared. The microstructure of Ce-based sintered magnets has been significantly improved by blending additions of 2% NdH_x, compared with Ce_27.5Nd₃ and Ce_25.5Nd₅ magnets. It suggested blending NdH_x during the milling, rather than directly adding Nd during the melting, was a more effective method for optimizing the microstructure of Ce-based magnets.

Fig. 4.

SEM microstructure images and EDS analysis results of Ce-based sintered magnets with different Nd substitution. (a–c) Ce_31.5, (d–f) Ce_27.5Nd₃, (g–i) Ce_25.5Nd₅.

Fig. 5.

SEM microstructure images and EDS analysis results of the Ce_27.5Nd₃ + 2% (NdHx) sintered magnet.

The magnetic properties of the optimal prepared magnet samples are listed in Table 2. For the Ce_31.5 magnet, the H_cj, as well as the B_r and (BH)_max, was very low; the H_cj slightly increased from 0.317 kOe to 0.768–1.203 kOe by 3∼5% Nd substituting in the magnet alloys. The H_cj increments were far below expectations by directly adding Nd in the Ce-based alloys, which is because minor Nd substitution hasn't obviously improved the deteriorated microstructures of Ce-based magnets (see Fig. 4). The magnetic properties were remarkably enhanced by blending 2% NdH_x HD powders into the Ce_27.5Nd₃ alloy during milling. A H_cj of 1.714 kOe, B_r of 9.395 kG, and (BH)_max of 11.16 MGOe have been obtained in the Ce_27.5Nd₃ + 2% (NdH_x) magnet, which may be attributed to the microstructure improvement (see Fig. 5). It indicates that blending addition of Nd was a more effective method for upgrading the magnetic performance of sintered Ce-based magnets.

Table 2.

Magnetic properties of the experimental Ce-based sintered magnets.

Samples	Br (kGs)	Hcj (kOe)	(BH)max (MGOe)
Ce31.5	5.802	0.317	0.643
Ce27.5Nd3	9.67	0.768	5.035
Ce25.5Nd5	9.082	1.203	5.232
Ce27.5Nd3 + 2% (NdHx)	9.395	1.714	11.16

Further study on the microstructure and elemental distribution characteristics of the Ce-based sintered magnets were carried out. Fig. 6, Fig. 7 show SEM micrographs and corresponding EPMA elemental maps (for Ce, Fe, B, and Nd) of the Ce_31.5 magnet, and the Nd-substituted magnets (Ce_27.5Nd₃, Ce_25.5Nd₅, and Ce_27.5Nd₃+2% (NdH_x), respectively. In particular, the elemental concentration mapping of B and O with low atomic number was also performed, in order to answer the question arisen from “the dark phases” in Fig. 4. As shown in Fig. 6, for the Ce_31.5 magnet, RE-rich phase (CeFe₂) was mainly aggregated at the triangular grain boundaries (TGB), and the intergranular thin Nd-rich phase is relatively few. The dark phases, inherited from the island-like phases in the SC alloys, were rich in Ce and B elements. It seems that: the dark phases should be characterized as a B-rich phase, rather than an intermediate product caused by the incomplete peritectic reaction of L + Ce₂Fe₁₇ → Ce₂Fe₁₄B + CeFe₂ [19,21], despite the dark phases are with atomic ratio of RE: Fe ~ 1: 3 (between 2: 14 and 1: 2). The solidification path of the Ce–Fe–B magnetic alloys has been summarized in the previous literature [14]. The B-rich phase was probably generated by the peritectic reaction of “L + Ce₂Fe₁₄B → CeFe₂ + B-rich” at 797 °C [14].

Fig. 6.

BE-SEM image and corresponding EPMA elemental mappings of the Ce_31.5 sintered magnet. The white contrast in (a) is REFe₂ phase, and the gray contrast is RE₂Fe₁₄B main phase. The black triangles represent the REFe₂ phase, while the white triangles represent the dark B-rich phase.

Fig. 7.

BE-SEM images and corresponding EPMA elemental mappings of (a1-a5) Ce_27.5Nd₃, (b1-b5) Ce_25.5Nd₅, (c1-c5) Ce_27.5Nd₃ + 2% (NdHx) sintered magnets.

It is reasonable that there is a higher degree of B-rich phases in the experimental Ce-based sintered magnets, considering a high B composition design (B = 1.25 at. %) was adopted in this study, in order to suppress the precipitation of α-Fe (the initial precipitated temperature of α-Fe will decrease with increasing B content [14]). The B-rich phase in the Ce-based magnets was also a nonmagnetic impurity phase, as that in Nd-Fe-B magnets, which has been confirmed by TMA results, since the TMA analyses can only detect two magnetic phases of 2:14:1 and α-Fe in the Ce-based magnets (see Fig. 3).

The dark B-rich phases still existed, the distribution of grain boundary RE-rich phases was still aggregated and non-uniform in the Ce_27.5Nd₃, Ce_25.5Nd₅ magnets. It was found that Nd element was not enriched in the main phase of RE₂Fe₁₄B, as it was expected since of Nd₂Fe₁₄B has lower formation energy than Ce₂Fe₁₄B [24] (see Fig. 7(a1) - (a5), and (b1) - (b5)). For the Ce_27.5Nd₃+2% (NdH_x) magnet, the distribution of grain boundary RE-rich phases was more uniform and continuous; here the B-rich phases has a lower B concentration, and lighter contrast (Fig. 7(c4)); it seems that Nd element has went into RE₂Fe₁₄B grains from the grain boundaries, and formed thicker shells (Fig. 7(c4)). The microstructures and GB phase element distributions of the Ce_27.5Nd₃+2% (NdH_x) magnet have been improved remarkably, which is one main reason that the good magnetic properties have been obtained in it.

Fig. 8 shows TEM characterizations of the Ce_31.5 magnet. Fig. 8 (a) is an overview picture, including typical RE-rich phase, main phase grains G1/G2 and B-rich phase B-rich 1/B-rich 2. The selected area electron diffraction (SAED) patterns of B-rich 1 (red dashed circle region) and RE-rich phase (blue dashed circle region), are shown in Fig. 8 (b), and (c), respectively. Fig. 8 (d) is the high-resolution TEM (HRTEM) image of the junction area of B-rich phase and RE-rich phase (green square region). An overview high angle annular dark field (HAADF) image and scanning TEM (STEM)-EDS mappings are show in Fig. 8 (e), in which the orange dashed square region is corresponding to Fig. 8 (a). SEAD patterns and HRTEM images in Fig. 8(b)–(d) indicate the tetragonal RE₅Fe₁₈B₁₈ phase (with a spatial group of Pccn (56), a = b = 7.117 Å, c = 35.07 Å) for B-rich dark phase, CeFe₂ phase for RE-rich GB phase, respectively. The TEM results further confirm the foregoing analyses of SEM, EDS, and EPMA. Notably, the indexed results in our study show that the B-rich phase is a tetragonal phase with the chemical formula of RE₅Fe₁₈B₁₈, rather than CeFe₂B₂ reported in Ref. [14], namely, the atomic ratio of RE: Fe not being 1: 2 for the B-rich phase.

Fig. 8.

TEM characterizations for the Ce_31.5 magnet. (a) Overview Bright field image; SAED patterns of B-rich phase (b), and RE-rich phase (c), marked by a red dashed circle and blue dashed circle in (a), respectively; (d) HRTEM of the junction area of B-rich phase and RE-rich phase (marked by green square region); (e) HAADF image, STEM-EDS mappings of Ce, Fe and O of the typical region, the orange dashed frame region corresponds to the BFI of (a). SEAD patterns and the HRTEM images indicate the fct-RE₅Fe₁₈B₁₈ phase for B-rich phase, CeFe₂ phase for RE-rich phase. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

4. Conclusions

The research on effects of minor Nd substitution on magnetic properties and microstructures of Ce–Fe–B sintered magnets were carried out. It was found that for the Nd-free Ce-based SC alloy, the main-phase grains with poor crystalline orientation were coarsened, and the island-like phases were found in the grain-boundary regions. The Ce-based sintered magnet with different Nd substitution mainly consisted of RE₂Fe₁₄B and REFe₂, a trace amount of α-Fe, and some dark phases with RE: Fe being around 1: 3. The dark phases were enriched in B element, which inherited from the island-like phases in the SC alloys, were indexed as the fct-RE₅Fe₁₈B₁₈ phase by TEM analyses. The deteriorated microstructures of high Ce magnets were hardly improved by directly adding minor Nd. The H_cj and (BH)_max were slightly increased by 3∼5% Nd substituting directly in the Ce-based alloys. Meanwhile, the microstructures and magnetic properties were remarkably enhanced by blending 2% NdH_x HD powders into the Ce_27.5Nd₃ alloy during JM milling. A H_cj of 1.714 kOe, B_r of 9.395 kG, and (BH)_max of 11.16 MGOe were achieved in the Ce_27.5Nd₃+2% (NdH_x) magnet.

Ethical statement

This study did not involve human or animal subjects, and thus, no ethical approval was required. The study protocol adhered to the guidelines established by the journal.

Data availability statement

The data that support the findings of this study will be made available on request.

Prime novalty statement

Up to now, the development of Ce/RE> 80% sintered permanent magnets with good properties still faces a challenge. The microstructure and magnetic properties of Ce₂Fe₁₄B-based sintered magnets with high Ce content are significantly deteriorated. For the ternary Ce–Fe–B magnet, the intrinsic coercivity (H_cj) was very low, and the maximum energy product ((BH) _max) of the magnet was nearly zero. Therefore, the researches on high abundance RE-Fe-B sintered magnets mainly focused on the composition range of Ce (or Ce + La + Y)/RE < 50%. The fundamental research is seriously insufficient for supporting the large-scale utilization of high abundance Ce element in a permanent magnet.

For PrNd-free Ce–Fe–B sintered magnets, to an extent, the microstructure and coercivity has been improved by co-substituting Y and La, substituting Gd, and doping low-melting point alloys, but with an obvious sacrifice in the remanence. Up to now, the research on effects of minor Nd substitution on microstructures and magnetic properties of Ce₂Fe₁₄B-based magnets is still rarely reported.

In this article, the roles of minor Nd substitution in phase compositions and microstructure revolution of Ce₂Fe₁₄B-based sintered magnets has been explored. It was found that: the typical island-like phase common existed in high Ce alloys is confirmed as a B-rich phase (RE₅Fe₁₈B₁₈); the deteriorated microstructures of high Ce magnets were hardly improved by directly adding minor Nd; however, the microstructures and coercivity were remarkably promoted by blending addition of Nd for the magnet with a similar composition as the former.

CRediT authorship contribution statement

Dengyue Wang: Writing – original draft, Investigation, Formal analysis, Data curation. Anhua Li: Writing – review & editing, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization. Haibo Feng: Methodology, Formal analysis. Wei Li: Project administration, Funding acquisition, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

The following is the Supplementary data to this article: