Structure and properties of sintered MM – Fe – B magnets

MM14Fe79.9B6.1 magnets were prepared by conventional sintering method. The Curie temperature of the sintered MM2Fe14B magnet was about 210 °C. When the sintering temperature increased from 1010 °C to 1030 °C, the density of the magnet increased from 6.85 g/cm3 to 7.52 g/cm3. After the first stage tempering at 900 °C, the (BH)max and Hcj had a slight increase. The maximum value of (BH)max = 7.6 MGOe and Hcj = 1080 Oe was obtained when sintered at 1010 °C and tempering at 900 °C, respectively. The grain size grew very large when the sintering temperature increased to 1050 °C, and the magnetic properties deteriorated rapidly. La reduced by ∼ 7.5 at. % in grains, which is almost equal to the increased percentage of Nd. That is mainly because La-Fe-B is very difficult to form the 2: 14: 1 phase.

In the work by Ko, et al. the (BH) max = 7.6 MGOe and H cj = 5.81 kOe were achieved by meltspun (MM) 12.5 Fe 78.9 B 8.6 alloys.After die-upset, the (BH) max and H cj of MM 12.5 Fe 78.9 B 8.6 alloys change to 4.1 MGOe and 1.75 kOe, respectively. 9In this work, conventional sintering method was applied for preparation of MM 14 Fe 79.9 B 6.1 magnet.The relationship between magnetic properties and sintering temperature has been researched.Microstructure of sintered magnets and the change of elements in grains are also explored.

II. EXPERIMENTAL
With nominal compositions of MM 14 Fe 79.9 B 6.1 (at.%) alloy was prepared by arc-melting in argon atmosphere.The ingot was melted five times to ensure homogenization in a vacuum furnace.The purpose of the ingot is to study the lattice constants and the theoretical density of MM 2 Fe 14 B phase.
The powders of MM 14 Fe 79.9 B 6.1 (at.%) alloy were prepared by induction melting, subsequent strip-casting (SC), hydrogen decrepitating (HD) and jet-milling (JM).The average particle size of the powders was approximately 3.0 um.Then the powders were aligned and compacted under a magnetic field of 1.8 T and a pressure of ∼ 5 MPa in a N 2 -filled glove box, followed by an isostatic pressing at ∼160MPa.The green compacts were sintered for 2 h at different temperatures from 1000 to 1080 • C, followed by a two-step tempering treatment, which was performed for 2 h at 900 • C and for 2 h at 520 • C, respectively.
The density of the magnets was measured based on Archimedes principle.The magnetic propincluding B r , H cj , and (BH) max , were measured by quasi-closed loop permanent magnetic measurement system NIM-500C.The phase component of the magnets were characterized by X-ray diffraction (XRD) using a Rigaku D/Max-2400 diffractometer with Cu Kα radiation.Curie temperature was measured by vibrating sample magnetometer (Model 4 HF-VSM).The microstructure was examined and characterized by a Phenom ProX scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectroscopy (EDS).

III. RESULTS AND DISCUSSION
Theoretical pattern was simulated by means of GSAS program for MM 14 Fe 79.9 B 6.1 ingot.The crystal parameter of refinement for the MM 14 Fe 79.9 B 6.1 sample were referred from the single crystal data of Nd 2 Fe 14 B (JCPDS PDF#39-0437).The lattice constants of MM 2 Fe 14 B were determined to be a = b = 8.780 Å, c = 12.185 Å.The fraction factors and thermal vibration parameters were refined with convergence and satisfied the reflection condition, R wp = 11 %, CHI**2 = 1.05 %, Nbos = 7.9 %.The result show that the lattice constants of MM 2 Fe 14 B are smaller than that of Nd 2 Fe 14 B (a = b = 8.792 Å, c = 12.27 Å). 10 Changes of rare earth cations in the host lattice might be the reason for the result.Specifically, a part of positions of Nd 3+ were occupied by both Ce 3+ and Ce 4+ ions, and the radius of Ce 4+ ion is small compared with Nd 3+ , which led to the decrease of lattice constants. 11Meanwhile, the theoretical density of MM 2 Fe 14 B magnet (ρ = 7.589 g/cm 3 ) was also obtained by XRD refinement.
Fig. 1 shows XRD patterns for the anisotropic sintered MM 14 Fe 79.9 B 6.1 magnets.The intensity of (004), ( 105  C, respectively. 12MM 2 Fe 14 B's Curie temperature is higher than Ce 2 Fe 14 B's and lower than the others.Curie temperature of R-Fe-B is mainly determined by the exchange interaction of Fe-Fe atom pair, the close distance between Fe-Fe would weaken theirs exchange interaction and then cause the reduction of Curie temperature. 13Massive Ce 4+ ions substitution for Nd 3+ in MM 14 Fe 79.9 B 6.1 magnets, the distances between Fe-Fe get close due to the smaller radius of Ce 4+ compared with Nd 3+ .This point can also be seen from the decrease of lattice constants of MM 2 Fe 14 B. Consequently, a sharp decrease of T c was probably due to the existence of Ce 4+ .One can find the M was not reduced to zero when the temperature exceeded 240 • C, it suggests that some soft magnetic phase may exist in the sintered MM 2 Fe 14 B magnets. 14 It may be produced in the process of strip casting, soft magnetic phase will deteriorate the intrinsic coercivity and maximum energy product of magnets.Therefore, optimizing the strip casting technique in order to restrain the production of soft magnetic phase is helpful to improve the performance of the magnet. The magnetic properties and density of the MM 14 Fe 79.9 B 6.1 magnets with different sintering and tempering temperatures are compared in Fig. 3.According to Fig. 3  .With increase of the sintering time, liquid phase decreased and grains were getting close to others, solid phase sintering occurred on the contact surfaces of grains, the result was that the grains grew very large and the grain boundaries disappeared. 15This will adversely affect the performance of the magnet.The (BH) max and H cj increased slightly after the first stage of tempering at 900 • C, it was due to the optimization of grain boundaries. 16But, both of them dropped almost in half after the second stage of tempering at 520 • C.This is mainly because of deterioration of the grain boundaries.The maximum value of (BH) max = 7.6 MGOe and H cj = 1080 Oe was obtained when sintered at 1010 • C and tempering at 900 • C, respectively.Fig. 3 (d) shows the density and its normalization of the magnets sintered at different temperatures.It can be seen that a rapid increase in the density from 91.2 % to 99.9 % when the sintering temperature increased from 1000 • C to 1030 • C, the corresponding density value increased from 6.85 g/cm 3 to 7.52 g/cm 3 .The density was not changed with the increase of sintering temperature from 1030 • C to 1060 • C. It means the magnet was densification when sintering temperature reached 1030 • C. Fig. 4 shows the SEM micrographs in backscatter mode for MM 14 Fe 79.9 B 6.1 magnets after two stage tempering under different sintering temperatures.There were no obviously continuous grain boundaries and change of grain size at lower sintering temperature from 1010 • C to 1040 • C. Comparably, the grain size grew very large (>200 µm) when the sintering temperature increased to 1050 • C. Due to the differences of the formation energy and technical conditions for R-Fe-B (R= La, Ce, Pr and Nd), the atomic ratio of La, Ce, Pr and Nd in the grains of the sintered MM 14 Fe 79.9 B 6.1 magnets was not the same.The mean percentages of La, Ce, Pr and Nd in the grains by means of EDS analysis were summarized in Table I.Compared with misch metal, the percentages of Ce and Pr only have a small change and almost don't vary with sintering temperature.And it also can be clearly seen that the percentage of La reduces by ∼ 7.5 at.%, which is almost equal to the increased  17 The reduced La, in the form of its oxide, existed in the grain boundary phase. 18

IV. CONCLUSION
The MM 14 Fe 79.9 B 6.1 magnets were prepared by conventional sintering method.R 2 Fe 14 B structure phase was confirmed by XRD patterns.Its crystal parameter and theoretical density are small compared with Nd 2 Fe 14 B. The Curie temperature of the sintered MM 2 Fe 14 B magnet was about 210 • C, that's maybe because the existence of Ce 4+ ions weakened Fe-Fe exchange interaction.When the sintering temperature reached 1030 • C, the magnet was densification.The optimum (BH) max of 7.6 MGOe and H cj of 1080 Oe was obtained when sintered at 1010 • C for 2 h and tempering at 900 • C for 2h.When the sintering temperature increased to 1050 • C, solid phase sintering occurred on the contact surfaces of grains, the grain size grew very large and magnetic properties sharply declined.In the grains, the percentage of La reduced by ∼ 7.5 at.%, that's because rigorous technical condition should be required for forming La 2 Fe 14 B phase.It might make to further improve the performance of MM 14 Fe 79.9 B 6.1 by modifying SC process.
FIG. 3. Effects of the sintering and tempering temperature on maximum energy product (a), intrinsic coercive force (b), remanence (c), and density (d) of the MM 14 Fe 79.9 B 6.1 magnet.

TABLE I .
EDS analysis of the mean percentages of La, Ce, Pr and Nd in the grains of the MM 14 Fe 79.9 B 6.1 magnets in Fig.4.That is mainly because La-Fe-B need very rigorous technical condition to form the 2: 14: 1 phase compared with Ce-Fe-B, Pr-Fe-B and Nd-Fe-B, and La 2 Fe 14 B phase is unstable.