Since Nd–Fe–B magnets were invented in 1983,^[1] they are widely used in many fields due to their high magnetic performance, such as hybrid electric vehicles (HEV), wind turbines, voice-coil motor (VCM) devices, magnetic resonance imaging (MRI), etc.^[2–5] In light rare earth, not only the abundance of Nd is less than La and Ce but also its price is much higher,^[6] but the magnetic performances of La–Fe–B and Ce–Fe–B are much lower than Nd–Fe–B.^[7] Therefore, the substitution of La, Ce for Nd in Nd–Fe–B to find a balance between magnetic performance and cost is an important research domain.^[8–10] Okada et al.^[11] studied (Ce Pr Nd Fe B sintered magnets, and obtained the best magnetic propertied as MGOe and KOe. Zhu et al.^[12] reported the remarkable values of MGOe and KOe for sintered (Nd Ce (Fe,TM) B magnet. For nanocrystalline magnets, Tang et al.^[13] obtained MGOe and KOe in [(La Ce Nd ] Fe B magnet. Misch metal (MM, typically consisting of 28.0 at.% La, 52.0 at.% Ce, 5.1 at.% Pr, 14.7 at.% Nd, 0.2 at.% others) is an intermediate product in rare earth extraction process, and it does not need the separation of La, Ce, Pr, and Nd elements, which reduces the cost.^[14] In the work by Ko et al.,^[15] GOe and KOe were achieved by melt-spun MM Fe B alloy. Zhang et al.^[16] found MGOe and KOe for melt-spun MM Fe B ribbon. There are few reports on sintered MM–Fe–B. Zhu et al.^[12] deemed that the dual-main-phase alloy method can realize the adjustment of the composition and combination of main phases of the magnets and there is almost no decrease in remanence. Moreover, the increasing discrepancy of the magnetocrystalline fields between the dual main phases will enhance the coercivity of the magnets. Based on the above studies, in this work, the double main phase alloy method was used to fabricate different ratios of MM Fe B /Nd Fe B magnets. The magnetic properties, Curie temperature, and change of elements in the grains have been studied in detail.

An alloy with near-stoichiometric composition of MM Fe B (at.%) was prepared by induction melting and subsequent strip-casting (SC). Though hydrogen decrepitating (HD) and jet-milling (JM), approximately 3.0 μm powders were prepared. The powders of MM Fe B were defined as M. Powders of Nd Fe B were prepared by the same way and defined as N. Then M and N were mixed evenly according to the proportion of M: 0 Wt.%, 10 Wt.%, 30 Wt.%, 40 Wt.%, 50 Wt.%, 100 Wt.%, respectively. Then the mixed powders were aligned and compacted under a magnetic field of 1.8 T and a pressure of ~ 5 MPa in an N -filled glove box, followed by an isostatic pressing at ~ 160 MPa. The green compacts were sintered at 1040 °C for 2 h, followed by a two-step tempering treatment, which was performed at 900 °C for 2 h and at 520 °C for 2 h, respectively. The atomic ratios of LaCe/R and the atomic compositions of the samples are shown in Table 1 (R represents the total rare earth).

Table 1.

Table 1.

Table 1. Compositions of different samples. . Sample (M/(M+N))/wt.% (MM/R)/at.% (LaCe/R)/at.% Atomic composition M 0 0 0 Nd Fe B M 10 10.3 8.2 La . Ce Pr Nd Fe B M 30 30.3 24.3 La Ce Pr Nd Fe B M 40 40.5 32.4 La Ce Pr Nd Fe B M 50 50.6 40.5 La Ce Pr Nd Fe B M 100 100 80.2 La Ce Pr Nd Fe B

Table 1. Compositions of different samples. .

The density of the magnets was measured based on Archimedes principle. The Curie temperature was measured by Model 4 HF-VSM. The phase component of the magnets were characterized by x-ray diffraction (XRD) using a Rigaku D/Max-2400 diffractometer with Cu radiation. The magnetic properties of the magnets, including the remanence , the intrinsic coercivity , and the maximum energy product , were measured by quasi-closed loop permanent magnetic measurement system NIM-500C. The microstructure of the magnets was examined and characterized by a XL30 S-FEG scanning electron microscope (SEM) equipped with an energy dispersive x-ray spectroscopy (EDS).

Figure 1 shows XRD patterns of the surface perpendicular to the alignment direction of the magnets with different compositions. From the pattern, it is not obvious that the magnets are composed of two main phases. Meanwhile, the intensities and 2θ values of the diffraction peaks have no change with the increase of the MM content. That is because the lattice constants of La Fe B, Ce Fe B, Pr Fe B, and Nd Fe B are very close to each other.^[7] It is hard to distinguish them from XRD patterns.

Fig. 1.

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	Fig. 1. (color online) XRD patterns of the magnets with different compositions.

In order to ascertain whether the sintered magnets are composed of two main phases or not, the magnetization dependence on temperature of the sintered magnets was measured at the magnetic field of 500 Oe. The curves of the magnetization and are shown in Fig. 2. It can be seen that the curves of for M , M , and M magnets have two peaks, which correspond to the two Curie temperatures. In order to facilitate the following discussion, the lower Curie temperature is defined as and the higher one is defined as , and they are summarized in Table 2. It also can be seen that M magnet has only one peak, which may be due to the lack of MM Fe B and MM Fe B diffused seriously in the sintering process. is not a constant and decreases from 276.5 °C to 256.6 °C with the content of MM Fe B increasing from 30 Wt.% to 50 Wt.%. That is mainly because that M , M , and M magnets are not dual main phase but multiple main phase. Specifically, in the sintering process, the rare earth atoms of MM Fe B and Nd Fe B ineluctably diffused to each other, the atomic ratio of La, Ce, Pr, and Nd in the grains was not the same to others. Finally, the ratio formed a continuous distribution and this result is shown in Table 3. corresponds to the main phase which contains more LaCe, and corresponds to the main phase which contains more PrNd. The Curie temperatures of La Fe B and Ce Fe B are 257 °C and 157 °C, respectively.^[7] Therefore, with the increase of LaCe in the main phase, decreases. is 296.6 °C and almost does not change when the content of MM Fe B increases from 10 Wt.% to 50 Wt.%. The reason for this may be that Nd diffuses more readily into the grains of MM Fe B compared to La and Ce into Nd Fe B at the sintering temperature of 1040 °C. It is generally known that La–Fe–B is hard to form 2:14:1 phase and Ce–Fe–B to form 2:14:1 phase needs a lower sintering temperature.^[17,18]

Fig. 2.

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	Fig. 2. (color online) Temperature dependence of magnetization and of the sintered magnets: (a) M , (b) M , (c) M , (d) M , (e) M , (f) M .

Table 2.

Table 2.

Table 2. Curie temperatures of the sintered magnets. . Sample /°C /°C M – 326.4 M – 296.6 M 276.5 296.6 M 271.6 296.6 M 256.6 296.6 M 210 –

Table 2. Curie temperatures of the sintered magnets. .

The magnetic properties of the magnets and their variations as functions of MM/R are shown in Fig. 3. The densities of the magnets are about 7.53 g/cm and almost do not change with the increase of MM content in Fig. 3(a). It means that the magnets are completely densified at the sintering temperature of 1040 °C. The values of , , and for the magnets decrease drastically with the increase of MM content. The for La Fe B and Ce Fe B are 1.38 T and 1.17 T, respectively,^[7] which are lower than those of Nd Fe B and Pr Fe B. That is why decreases with increasing MM content. The for Nd Fe B and MM Fe B are 13.79 KGs and 10.28 KGs, respectively, and are connected by a red line. It can be clearly seen that the values of for the multiple-main-phase magnets are all above the red line, indicating that there exists inter-grain exchange coupling in the sintered magnets.^[19] In Figs. 3(c) and 3(d), decreases from 11.96 KOe to 0.65 KOe, meanwhile, decreases from 47.38 MGOe to 2.72 MGOe with the MM content increasing from 0 at.% to 100 at.%.

Fig. 3.

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	Fig. 3. (color online) Dependence of density and magnetic properties on MM of the sintered magnets (red line connects corresponding points of MM/R = 0 and MM/R = 100).

The decline of is due to the lower intrinsic magnetocrystalline anisotropy field of La Fe B and Ce Fe B than that of Nd Fe B.^[7] The values of for the multiple-main-phase magnets are all under the red line, this can be attributed to the changes of microstructure and phase component. Compared with Nd, La and Ce are easier to diffuse to grain boundaries in the sintering process, and they will cause deterioration of grain boundaries.^[13,20] The values of for the multiple-main-phase magnets are all above the red line, this is due to the changes of . When MM/ at.%, the magnetic properties can reach the level of KGs, KOe, and MGOe. The and are larger than 34 MGOe and 4.84 KOe, respectively, when MM/R is up to 50 at.%. If a small quantity of elements, such as Al or Cu, are added to the magnets, the will get a further increase.^[21] These magnets can be applied in the field where high coercivity is not required, and also have a cost advantage.

Back-scattered electron images of the polished magnets after two stage tempering are shown in Fig. 4. There are no obviously continuous grain boundaries between main phase grains, and a great quantity of grains merge each other. This will enhance the exchange coupling between them and lead to the increase of and the decrease of . Most grain sizes are less than 10 μm, and evenly distributed. It also can be seen that it is very hard to distinguish different grains in the M magnet compared with others because of the serious diffusion of La and Ce. This has been discussed in our previous work.^[22] Due to the diffusion between the grains, the atomic ratio of La, Ce, Pr, and Nd in each grain is different. The percentages of La, Ce, Pr, and Nd of ten grains which are randomly selected from the M magnet are analyzed by EDS. In order to facilitate observation, the results are listed in Table 3 according to the percentage of Nd. With the increase of Nd, the percentages of La and Ce decrease. The percentage of Nd in all grains is greater than that in misch metal (14.7 at.%), which indicates that La and Ce diffuse more easily than Nd. The percentages of La, Ce, Pr, and Nd in some grains are very close to each other, such as 5# and 6#. It may be due to that their surroundings are very close. Therefore, it can be concluded that the M magnet is not dual-main-phase magnet but multiple-main-phase magnet. The magnets M , M , M , and M are also multiple-main-phase magnets.

Fig. 4.

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	Fig. 4. SEM-back scattered micrographs of the sintered magnets: (a) M , (b) M , (c) M , (d) M , (e) M , (f) M .

Table 3.

Table 3.

Table 3. ESD analysis of the percentages of La, Ce, Pr, and Nd in the grains of the M magnet. . Number Nd Ce La Pr 1# 23.71 49.48 23.71 3.09 2# 52.08 32.29 13.54 2.08 3# 60.19 25.93 12.04 1.85 4# 72.95 16.39 9.02 1.64 5# 82.61 11.30 6.09 0.00 6# 83.05 10.17 5.93 0.85 7# 86.24 10.09 3.67 0.00 8# 87.62 8.57 2.86 0.95 9# 95.37 2.78 1.85 0.00 10# 96.15 1.92 1.92 0.00

Table 3. ESD analysis of the percentages of La, Ce, Pr, and Nd in the grains of the M magnet. .

MM Fe B /Nd Fe B multiple-main-phase magnets of different ratios were fabricated by conventional sintering method. It is hard to distinguish them from XRD patterns. For the curves derived from the magnetization dependence on temperature, M magnet has only one peak, which may be due to that MM Fe B diffused seriously in the sintering process. decreases from 276.5 °C to 256.6 °C with the content of MM Fe B increasing from 30 Wt.% to 50 Wt.%. But is 296.6 °C and almost has no change. The densities of the magnets are about 7.53 g/cm and completely densified. The , , and of the magnets decrease drastically with the increase of MM content. When MM/ at.%, the magnetic properties can reach the level of KOe and MGOe. The atomic ratio of La, Ce, Pr, and Nd in each grain is different and the percentage of Nd in all grains is greater than that in misch metal. The magnets M , M , M , M , and M are not dual-main-phase magnets but are multiple-main-phase magnets.