Mechanical properties of sintered Ce-Fe-B magnets
The bending strength, fracture toughness, hardness and brittleness index of Ce-Fe-B magnets with different CE contents (R1-xCex)30.5’31.5Febal.B1M1 (mass fraction,%) were tested, and the micro fracture of the magnets was analyzed by SEM and EDS. The results show that the bending strength and fracture toughness of magnets containing Ce decrease with the increase of Ce content, and the Vickers hardness of magnets with Ce content (x) has no obvious change. When x = 0.15, the mechanical properties of the magnet reach a maximum, and the bending strength, fracture toughness and brittleness index of the magnet are obviously better than those of the ordinary sintered Nd-Fe-B magnet; More “flocculent” oxide phase is found in the magnet. Flocculent phase can absorb part of the energy during the crack propagation process, which can alleviate the stress concentration at the crack tip. Therefore, it plays a role in strengthening and toughening, which is beneficial to improving the mechanical properties of the magnet. When the Ce content in the magnet reaches x = 0.45, the mechanical properties of the magnet become worse obviously, because there are large grains in the magnet and the microstructure of the magnet is obviously deteriorated. The fracture mechanism of magnets with different Ce content is mainly intergranular fracture.
As an important metal functional material, rare earth permanent magnet materials are widely used in the fields of information electronics, energy-saving household appliances, new energy vehicles, wind power generation, health care, aerospace and so on. Since the invention of Nd-Fe-B material, people have tried to use cheap rare earth Ce (or mixed rare earth mm) to replace nd in magnets [1 ~ 13]. In recent years, with the increasing output of sintered Nd-Fe-B magnets, the unbalanced use of rare earth metals has been highlighted. In order to solve the problem of balanced utilization and high quality utilization of high abundance rare earth resources CE, and reduce the cost of magnet manufacturing, the research on (R, CE) – Fe-B permanent magnet materials has become a hot spot [5 ~ 13]. The new (R, CE) – Fe-B magnet and its preparation technology have been successfully industrialized in relevant magnet manufacturers, and a series of new CE containing magnet products have been formed.
The mechanical properties of sintered rare earth permanent magnet directly determine its processing performance, and affect its manufacturing cost and service reliability. Improving the mechanical properties of sintered rare earth permanent magnetic materials without reducing (or less reducing) their magnetic properties has always been an issue of close concern to researchers and magnet manufacturers in related fields [14 ~ 30]. Among them, Li Anhua et al. [17 ~ 28] conducted a comprehensive study on the mechanical properties, fracture behavior, strengthening and toughening, and impact resistance of sintered Nd-Fe-B magnets. At present, the research on (R, CE) – Fe-B magnet mainly focuses on the microstructure, magnetic properties and how to improve the coercivity of the magnet, but few on the mechanical properties of the magnet. Zhou Xiaoqing et al.  studied the substitution of a small amount of CE for Nd (CE)/ Σ Re ≤ 0.2) on the bending strength of sintered Nd-Fe-B magnets. Jin et al.  studied the mechanical properties of sintered R-Fe-B magnets added with La CE mixed metal (LA: 35%, Ce: 65%), including compressive strength, flexural strength, Vickers hardness and Young’s modulus. However, there are obvious differences in alloy composition, microstructure and properties between La CE added magnets and Ce-Fe-B magnets without la. The mechanical properties and fracture mechanism of Ce-Fe-B magnets are rarely reported. In this work, the bending strength, fracture toughness, Vickers hardness and brittleness index of Ce-Fe-B magnet (without LA) were studied. The micro fracture of the magnet was analyzed. The relationship between the mechanical properties and fracture mechanism of Ce-Fe-B magnet and its alloy composition and microstructure was discussed.
Table of Contents
- Experimental method
- Results and discussion
The effects of Ce content on the mechanical properties and machinability of sintered (R, CE) – Fe-B magnets were studied. The results were compared with those of ordinary sintered Nd-Fe-B magnets without CE. The chemical composition of the magnet is (R1-xCex)30.5’31.5Febal.B1M1 (mass fraction,%); R contains three kinds of rare earth elements: Nd, PR and a small amount of GD. The mass ratio of nd to PR is 3:1. M contains four kinds of elements: CO, Al, Cu and Nb (or Zr); X = 0, 0.15, 0.30, 0.35, 0.40, 0.45, mass fraction). Nim-2000h hysteresis loop instrument was used to test the magnetic properties of the magnet. MTS landmark electro-hydraulic servo test system was used to test the bending strength and fracture toughness of the experimental magnet. Five specimens were tested for flexural strength under each condition, and three specimens for fracture toughness under each condition. The bending test reference standard Yb / T 5349-2014, using three-point bending test method (Fig. 1a). The bending specimen size is 5mm × mm × 19mm (Note: arrow → represents the direction of easy magnetization c-axis of the magnet, the same below), the measured span moment is 14.5mm, and the loading displacement rate is 0.5mm/min. The fracture toughness KIC was measured by the single side notched beam method (Fig. 1b), referring to the standard GB / T 4161-2007, and the sample size was mm × 10mm × 45 mm, incision width < 0.2 mm, incision depth 5 mm, experimental span 40 mm, loading displacement rate 0.1 mm / min. The hardness of the magnet was tested by tukon 2500-6 automatic Vickers hardness tester. According to the standard GB / t4340.1-2009, the standard sample with a diameter of 10 mm and a length of mm was used, and the test surface was polished. Vickers hardness test load is 300N, holding time is 10s, each sample is measured at three points, and the average value of the test results is obtained. For the same kind of magnet with the same composition and magnetism, all the experimental samples are made of the same block magnet by WEDM, and the mechanical experimental samples are in thermal demagnetization state. The micro fracture morphology of the sample was observed and analyzed by JSM-7001F scanning electron microscope (SEM), and its composition was analyzed by EDs.
Fig.1 Schematics of three-point bending test (a) and fracture toughness test (b) of the magnets(Ls—span for bending test, S—span for fracture toughness test, F—loading, unit:mm)
Results and discussion
Magnetic and mechanical properties of Ce-Fe-B magnets
The magnetic properties and other physical properties of the experimental magnet are shown in Table 1. With the increase of Ce content, the density of the magnet increases because the density of ce2fe14b is higher than that of Nd2Fe14B .
Table.1 Magnetic properties and densities of sintered (R1-xCex)30.5~31.5Febal.B1M1 magnet
|x||Br / T||Hcj / (kAm-1)||Hk / (kAm-1)||(BH)max / (kJm-3)||Hk /Hcj||ρ / (gm-3)|
Note: R—containing three elements of Nd, Pr and Gd; M—containing four elements of Co, Al, Cu and Nd (or Zr); Br—remanence; Hcj—intrinsic coercivity; Hk—knee point coercivity; (BH)max—maximum energy product; Hk /Hcj—demagnetization curve squareness; ρ—density; *—sintered Nd-Fe-B reference sample
The brittleness index formula B = HV / KIC (hardness HV) was used to characterize the resistance of the material to deformation in small volume on the surface; The fracture toughness KIC is the critical stress intensity factor under the condition of open plane strain, which reflects the ability of the material to resist crack propagation). The brittleness index of magnets with different Ce content is calculated. The lower the brittleness index, the better the machinability of magnets . Figure 2 shows the typical experimental curve of the three-point bending experiment of the magnet. The mechanical properties of sintered (R1-xCex)30.5’31.5Febal.B1M1 magnet and the calculated brittleness index are listed in Table 2, and compared with the properties of ordinary sintered Nd-Fe-B magnet without CE.
Fig.2 Typical load-displacement curve of the bending tests of sintered (R, Ce)-Fe-B magnet
Table.2 Mechanical properties of sintered (R1-xCex)30.5~31.5Febal.B1M1 magnets
|x||Rbb / MPa||KIC / (MPam1/2)||Hv / MPa||Hv/KIC / μm-1/2|
Note: Rbb—bending strength, KIC—fracture toughness, Hv—Vickers hardness, Hv/KIC—brittleness index
It can be seen from Figure 2 that the maximum deformation of Ce-Fe-B magnet before bending fracture is about 0.17mm, and there is no yield plateau in the experimental curve, which indicates that there is no obvious yield deformation before the fracture of the magnet, which is a typical brittle fracture. The macroscopic observation of the fracture shows that the fracture is even and perpendicular to the maximum tensile stress, and the color of the fracture is gray. The fracture characteristics of CE containing magnets are basically the same as those of NdFeB magnets [16,17].
It can be seen from table 2 that the bending strength and fracture toughness of the magnet containing Ce decrease with the increase of Ce content, and the Vickers hardness of the magnet has no obvious change with Ce content.
When x = 0.15 in the magnet, the mechanical properties of the magnet have a maximum value, and the bending strength, fracture toughness and brittleness index of the magnet are obviously better than those of the ordinary sintered Nd-Fe-B magnet (reference magnet), which is consistent with the report in ; When x = 0.30, the bending strength and fracture toughness of CE containing magnets are slightly better than those of sintered Nd-Fe-B magnets; When x = 0.35 ~ 0.40, its mechanical properties are slightly lower than that of sintered Nd-Fe-B magnets; When x = 0.45, the mechanical properties of the magnet become worse, which may be due to the appearance of large grains in the magnet and the obvious deterioration of the microstructure of the magnet.
Micro fracture morphology of Ce-Fe-B magnet
The fracture morphology and EDS results of (R1-xCex)30.5~31.5Febal.B1M1 magnet after bending test are shown in Fig. 3 ~ 5 and table 3. There is no obvious difference between the fracture morphology of the sample with fracture toughness and that of the sample with bending strength. It can be seen from Fig. 3 that the fracture mechanism of Nd-Fe-B magnets is the same as that of ordinary sintered magnets [16,17], and the fracture mechanism of magnets with different Ce content is mainly intergranular fracture. When x ≤ 0.30, Ce-Fe-B magnet has fine and uniform microstructure, and its average grain size is 5 ~ 8 μ M (FIGS. 3a and b); When x = 0.45, abnormal grain growth occurs in Ce-Fe-B magnets, some of which are larger than 20 μ M (FIGS. 3C and D). This may be because when the Ce content in the magnet is high, it is easy to agglomerate the rare earth rich phase [12,13], which leads to the lack of isolation of the rare earth rich phase at the lamellar grain boundary between the main phase grains of r2fe14b, resulting in abnormal grain growth, which not only significantly reduces the coercivity of the magnet, but also leads to the deterioration of the strength and toughness of the magnet.
Fig.3 Typical microfractographs of sintered (R1-xCex)30.5’31.5Febal.B1M1 magnets with x=0.15 (a), x=0.30 (b) and x=0.45 (c, d) (Fig.3d shows the local enlarged view of Fig.3c)
Table.3 EDS of oxide phases in sintered (R1-xCex)30.5’31.5Febal.B1M1 and Nd-Fe-B reference sample
|x||Atomic fraction / %|
Figure 4 shows the HRSEM image of the micro fracture of the bending specimen with x = 0.15 and the best mechanical properties of the magnet. It can be seen that there are more white “flocculent phase” in the magnet. EDS analysis results show that this “flocculent phase” is an oxide phase containing Fe and CE, and the content of O is higher. The oxide phase will absorb part of the energy during the crack propagation process, relieve the stress concentration at the crack tip, strengthen and toughen the grain boundary, so it is beneficial to improve the mechanical properties of the magnet. In the fracture of x = 0.30 bending specimen, there are oxide phases with similar composition but different morphology. Compared with Ce-Fe-B magnets, the oxide phase in ordinary sintered Nd-Fe-B magnets is mostly granular, and the content of O is lower . It is reported in  that the oxidation properties of CE are quite different from those of other rare earth metals. Firstly, Ce2O3 is generated by CE oxidation, and then CeO2 is generated by further oxidation; When it is further oxidized, because the molar volume of CeO2 is smaller than that of CE and Ce2O3, loose and cracked CeO2 will be formed, which may be the reason for the formation of special “flocculent” oxide phase in Ce-Fe-B magnets.
Fig.4 HRSEM bending fracture image of the (R0.85Ce0.15)30.5~31.5Febal.B1M1 magnet
Fig.5 EDS spectra of oxide phases in sintered (R1-xCex)30.5’31.5Febal.B1M1 with x=0.15 (a), x=0.30 (b) and sintered Nd-Fe-B reference sample (c)
Fracture behavior of Ce-Fe-B magnets
Sintered (R1-xCex)30.5’31.5Febal.B1M1 magnets are brittle fracture. The fracture mechanism of magnets with different Ce content is mainly intergranular fracture, which is the same as that of ordinary sintered Nd-Fe-B magnets. In the sintered R-Fe-B magnet, there is a second phase with a thickness of several nanometers at the grain boundary, that is, the rare earth rich grain boundary phase. The melting point of grain boundary phase rich in rare earth is low and it is liquid phase in the sintering process, which plays an important role in densification and high coercivity. The reason of intergranular fracture may be as follows: (1) the strength of rare earth rich phase at grain boundary is low（ 2) Because the rich rare earth phase infiltrates the main phase grain in the form of liquid phase during the sintering process, a thin layer covering the main phase grain is formed, and the bonding force between the two phases is weak.
Sintered magnets are prepared by powder metallurgy process, which is difficult to achieve complete densification, and there are always some cavities or micro cracks. For brittle materials with micro cracks, when subjected to external stress, the crack tip is difficult to produce plastic deformation, so it is suitable for Griffith fracture strength theory. The generation of cracks reduces the elasticity of the system and increases the surface energy of the system due to the generation of new surfaces.
In the sintered (R1-xCex)30.5’31.5Febal.B1M1 magnet, the interfacial energy between the main phase and the rare earth rich phase decreases when the crack propagates along the grain boundary, which is one of the reasons why the grain boundary phase becomes the favorable position for crack propagation. In addition, there are always some holes in the sintered magnet due to technological reasons. The rare earth rich phase with low melting point is liquid phase in the sintering process, and finally exists as grain boundary phase (the non-magnetic rare earth rich grain boundary phase wraps the main phase, which plays the role of demagnetization coupling and plays an important role in improving the coercivity of the magnet). Therefore, the voids in the magnet are concentrated in the grain boundary rare earth rich phase. The combined effect of the above factors makes the crack propagation along the grain boundary in the sintered (R1 xcex) 30.5’31.5febal.b1m1 magnets, forming the micro mechanism of intergranular fracture.
When x = 0.45, irregular grain growth occurs in the magnet (Fig. 3C). The impact fracture morphology of the magnet was observed by SEM and compared with that of the metallographic sample. The results show that the grain size of the magnet is not uniform, and the rare earth rich phase agglomerates obviously at the junction of grain boundaries (Fig. 6a). The fracture morphology shows that cleavage fracture occurs in some grains of the magnet (Fig. 6b). In sintered Nd-Fe-B magnets, cleavage cracks are also found in large grains .
Fig.6 SEM images of microstructure (a) and fracture (b) of sintered (R0.55Ce0.45)30.5-31.5Febal.B1M1 magnet
In the process of crack propagation, the abnormally grown grains are almost broken by transgranular cleavage. This is because when the crack propagation meets large grains, if it breaks along the grain boundary, the crack propagation mode will have a great deflection, which will increase the total energy of the system, so it is difficult to occur. The stress at the crack tip is concentrated on the abnormally grown grains, resulting in a large tensile stress, which makes the grains cleavage. When the transgranular crack in one grain propagates to the next, it is easy to cause cleavage fracture on the same crystal plane of the latter.
Effect of Ce on phase structure and microstructure
(1) Main phase crystal structure of CE containing magnets
The main phase Nd2Fe14B of sintered Nd-Fe-B magnet belongs to tetragonal phase with space group p42 / MNM. The whole crystal can be regarded as an alternating arrangement of six atomic layers, including nd rich, B-rich and Fe rich layers . The crystal structure of ce2fe14b is the same as that of Nd2Fe14B, and CE atoms also exist in 4G and 4f sites. In alloys or metal compounds, cerium ion may be in the + 3 valence state as the conventional rare earth ion, or it may lose another f electron and be in the + 4 valence state. Moreover, Ce4 + has an ion radius obviously smaller than Ce3 +. It is reported in  that the lattice parameters of ce2fe14b decrease abnormally, which does not conform to the law of lanthanide contraction. This is because the valence state of CE in ce2fe14b is mixed valence state (+ 3.44 valence), and the 4f electron layer is lost, which makes the lattice constant of ce2fe14b decrease. The experimental results [11,36] also show that the lattice parameters of (nd, CE) 2fe14b decrease monotonously with the increase of Ce content. CE enters into the main phase and preferentially occupies 4G crystal sites (the crystal space volume is large) , which seems not to be confirmed by the experimental results. The authors consider that in (nd, CE) 2fe14b, 3-valent CE and 4-valent CE coexist, and the selective occupation of different valence ions makes CE present mixed valence state. The gap of 4G crystal site is larger, and Ce3 + with larger volume tends to occupy 4G crystal site; However, Ce4 + occupies the 4f site with smaller volume.
In conclusion, the lattice of main phase (nd, CE) 2fe14b shrinks and the crystal plane spacing of easy cleavage plane (low index crystal plane) decreases after CE replaces nd, which means that the cleavage fracture trend of magnet decreases.
(2) Microstructure of CE containing magnets
(a) Optimization of grain boundary structure. According to the phase diagram analysis, the nd rich grain boundary phase of Nd-Fe-B alloy is a eutectic structure with low melting point of T1 (main phase) + T2 (b rich phase) + nd phase. Because the lamellar grain boundary phase is only a few nanometers thick, people can not distinguish each phase in the eutectic structure, so it is collectively referred to as “rare earth rich grain boundary phase (Nd Fe phase)”; In the micro morphology of Nd-Fe-B rapid solidification alloy sheet, the re rich phase also exists in the form of homogeneous phase, and there is no difference in morphology and contrast. Because the composition of eutectic point is certain, the theoretical value of Nd: Fe in grain boundary phase is certain, and the theoretical value of Nd: Fe is 1:4 (atomic ratio). The nd rich phase with lamellar grain boundary has low melting point and good fluidity during sintering and tempering. The content of O in grain boundary phase is low.
When the content of CE in the magnet is low, CE enters into the grain boundary phase and replaces nd in the nd rich phase to form nd CE Fe phase, which reduces the melting point of the rare earth rich phase. Therefore, the magnet can be densified at a relatively low sintering temperature. At this time, the magnet has a uniform and smooth grain boundary structure .
(b) Cefe2 phase appears. In the preparation process of sintered magnets, such as milling and pressing stage, part of the rare earth rich grain boundary phase is oxidized. Due to the high melting point of the oxidized grain boundary phase, it is difficult to form liquid phase in the sintering and tempering process, so it is deposited in the triangular grain boundary to form large-scale rare earth rich grain boundary phase. The results show that with the increase of O content from low to high, the phase structures are HCP, FCC and DHCP (double layer close packed hexagonal structure). Because the junction of grain boundary is connected with the common grain boundary, it can not be ruled out that the structure and composition of grain boundary are the same as that of lamellar grain boundary.
With the increase of Ce content, when the substitution amount of CE reaches 24%, cefe2 phase begins to appear in the magnet, which makes the volume fraction of r2fe14b main phase decrease . So far, there are two kinds of grain boundary phases in the magnet, one is nd CE Fe phase rich in rare earth, the other is cefe2 phase rich in Fe. When the content of CE is high, there are two kinds of phases at the junction of grain boundaries. The white phase has a high content of RE, which is nd CE Fe phase; The content of Fe in the light gray phase is higher, and the composition is close to that of cefe2 phase, both of which contain a certain proportion of o [36,39].
(c) The rare earth rich oxide phase is called “flocculent phase”. In the milling stage, the rich rare earth phase separated from the main phase will be formed. If the rare earth phase is seriously oxidized, the rare earth oxide particles will be formed in the Nd-Fe-B alloy. Due to sample preparation, this oxide phase is rarely observed in metallographic and TEM samples, but it is more common in fracture observation . It is found that there is a “flocculent phase” in the sintered (R, CE) – Fe-B magnet (Fig. 4), which is obviously different from the granular oxide phase in the sintered Nd-Fe-B magnet. It is considered that the “flocculent phase” here has a reinforcing and toughening effect similar to that of the fiber phase in the composite. Flocculent cracking is the characteristic morphology when Ce2O3 is further oxidized to CeO2 .
Effect of Ce on the mechanical properties of sintered (R, CE) – Fe-B magnets
When x = 0.15, the mechanical properties of sintered (R, CE) – Fe-B magnets are the best, and the bending strength, fracture toughness and brittleness index (HV / KIC) of sintered (R, CE) – Fe-B magnets are obviously better than those of ordinary sintered Nd-Fe-B magnets. The reasons are analyzed as follows: (1) the magnet has uniform and fine microstructure, and its average grain size is 5 ~ 8 μ m;( 2) The results show that the magnet has an optimized grain boundary structure. The thin sheet grain boundary phase is nd CE Fe phase with low melting point. It has better wettability with the main phase during sintering and tempering, forming a uniform and smooth grain boundary structure（ 3) The “flocculent” oxide phase in the magnet has the effect of reinforcing and toughening similar to the fiber phase in the composite. The oxide phase will absorb part of the energy during the crack propagation process, relieve the stress concentration at the crack tip, strengthen and toughen the grain boundary, so it is beneficial to improve the mechanical properties of the magnet.
When x = 0.30, the magnet still has uniform and fine microstructure, but at this time, cefe2 phase has appeared in the magnet, which has a high melting point and is easy to gather at the junction of grain boundaries. The enrichment of rare earth rich phase at the junction of grain boundaries has a negative effect on the mechanical properties of magnets. Because the rare earth rich phase at the junction of grain boundaries generally contains a certain amount of o , the wettability with the main phase is not good during sintering, and the combination with the main phase is not tight. When the load is applied, initial microcracks are easy to appear here, forming stress concentration.
When the Ce content is further increased to x = 0.35 ~ 0.40, the cefe2 phase is further increased, and two forms of re rich phase, nd CE Fe phase and cefe2 phase (containing o) appear at the junction of grain boundaries. The weakening of the interface between the two phases is also easy to produce microcracks.
When x = 0.45, the phase of cefe2 increases further, the microstructure of the magnet becomes worse, and the grains grow abnormally, some of which are larger than 20 μ m。 At this time, the mechanical properties of the magnet become worse obviously, on the one hand, it is due to the enrichment of cefe2 phase at the junction of grain boundaries, which reduces the interfacial bonding force, on the other hand, it is due to the influence of grain growth. When the grains grow obviously, the bending strength of the magnet decreases sharply .
(1) The bending strength and fracture toughness of magnets containing Ce decrease with the increase of Ce content, but the hardness of magnets does not change with the increase of Ce content.
(2) The bending strength, fracture toughness and brittleness index of the magnet are better than those of the reference sintered Nd-Fe-B magnet.
(3) When x = 0.15, more “flocculent” oxide phase was found in the magnet, which absorbed part of the energy during the crack propagation process, alleviated the stress concentration at the crack tip, and strengthened and toughened the grain boundary.
(3) When x = 0.45, the mechanical properties of the magnet become worse, which is due to the appearance of large grains in the magnet and the obvious deterioration of the microstructure of the magnet.
(4) The fracture mechanism of magnets with different Ce content is mainly intergranular fracture.
The authors have declared that no competing interests exist.
Author: LI Anhua, ZHANG Yueming, FENG Haibo, ZOU Ning, LÜ Zhongshan, ZOU Xujie, LI Wei.
Source: China Permanent Magnet Manufacturer – www.rizinia.com
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