R-Fe-B sintered magnets

Abstract

To provide an R-Fe-B sintered magnet that exhibits high coercivity while keeping material costs down. [Solution] The R-Fe-B sintered magnet contains a rare earth element R including Tb, an element T consisting of Fe or a combination of Fe and Co, B, and each of Al, Cu, and Ga, with an element M consisting of a combination of Al, Cu, and Ga, and includes an RTM phase in the grain boundary phase, where the number of atoms of element M in the RTM phase is in the order of Cu > Al > Ga. Furthermore, in the RTM phase, the atomic ratio of the rare earth element R to element M is preferably 6.0 for the rare earth element R and 1.3 or more for element M.

Description

[Technical Field]

[0001] The present invention relates to R-Fe-B sintered magnets, and more particularly to R-Fe-B sintered magnets containing Al, Cu, and Ga. [Background technology]

[0002] Rare earth magnets are used in a variety of devices, including automobiles, industrial equipment, and household electrical appliances. Depending on the application, these rare earth magnets are required to have high magnetic properties, such as high coercivity. One type of rare earth magnet with high magnetic properties is the R-Fe-B sintered magnet (where R is a rare earth element). However, generally, R-Fe-B sintered magnets tend to have low coercivity among various magnetic properties. Therefore, in Patent Document 1 by one of the inventors of this invention, an improvement in coercivity is achieved by adding a predetermined amount of Al to the R-Fe-B sintered magnet and generating an RFeAl phase at the grain boundaries. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2020-077843 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] As with the RFeAl phase described in Patent Document 1, the coercivity of an R-Fe-B sintered magnet can be improved by generating a specific phase at the grain boundaries. In addition, it may be possible to further increase the coercivity by adjusting the component composition of the R-Fe-B sintered magnet. For example, the coercivity of an R-Fe-B sintered magnet with a specific phase generated at the grain boundaries may be further improved by adding heavy rare earth elements such as Dy or Ga. However, heavy rare earth elements such as Dy and Ga are expensive elements, and adding them in large quantities increases the material cost of the R-Fe-B sintered magnet. From the viewpoint of suppressing material costs, it is desirable to achieve high coercivity without adding these elements, or by keeping the amount added to a minimum.

[0005] Therefore, the objective of this invention is to provide an R-Fe-B sintered magnet that exhibits high coercivity while keeping material costs down. [Means for solving the problem]

[0006] To solve the above problems, the R-Fe-B sintered magnet according to the present invention has the following configuration.

[0007] [1] The R-Fe-B sintered magnet of the present invention contains a rare earth element R including Tb, an element T consisting of Fe or an aggregate of Fe and Co, B, and each of Al, Cu, and Ga, with an RTM phase in the grain boundary phase, where the aggregate of Al, Cu, and Ga is called element M, and the number of atoms of element M in the RTM phase is in the order of Cu > Al > Ga.

[0008] [2] In the embodiment of [1] above, it is preferable that in the RTM phase, the atomic ratio of the rare earth element R to the element M is 6.0 and the element M is 1.3 or more.

[0009] [3] In the embodiment of [1] or [2] above, the Ga content is preferably 0.5% by mass or less.

[0010] [4] In the embodiment described in [3] above, it is preferable that the mass percentages are 0.3% ≤ Al ≤ 1.0%, 0.05% ≤ Cu ≤ 0.5%, and 0.05% ≤ Ga ≤ 0.5%.

[0011] [5] In any embodiment of [1] to [4] above, the R-Fe-B sintered magnet may be free of Dy, except for unavoidable impurities.

[0012] [6] In any of the embodiments described in [1] to [5] above, the R-Fe-B sintered magnet may be manufactured by undergoing grain boundary diffusion treatment using a Tb-Cu-Al alloy. [Effects of the Invention]

[0013] The R-Fe-B sintered magnet according to the present invention, having the configuration described in [1] above, contains Al, Cu, and Ga as elements M, and has high coercivity due to the effect of including an RTM phase at the grain boundaries. Furthermore, by including amounts of Cu and Al in the RTM phase that exceed amounts of Ga, a significant improvement in coercivity can be obtained even with a low Ga content. The inclusion of Tb as a rare earth element R in the R-Fe-B sintered magnet also effectively contributes to the improvement of coercivity. In particular, when Tb is added by grain boundary diffusion treatment using an alloy containing Tb, even a small amount of Tb can effectively contribute to the improvement of coercivity. If this grain boundary diffusion treatment is performed using a Tb-Cu-Al alloy, a large amount of Cu is distributed at the grain boundaries, and a state in which the number of atoms of element M is Cu > Al > Ga is easily formed in the RTM phase at the grain boundaries. In this way, by introducing an RTM phase at the grain boundaries of an R-Fe-B sintered magnet containing Tb as the rare earth element R, and by setting the element M content in the RTM phase to Cu > Al > Ga, and especially by adding Tb through grain boundary diffusion treatment using a Tb-Cu-Al alloy, a significant improvement in coercivity can be obtained without adding large amounts of expensive elements such as Ga or Dy. In other words, it is possible to obtain an R-Fe-B sintered magnet exhibiting high coercivity while keeping material costs down.

[0014] In the embodiment described in [2] above, the RTM phase contains element M at an atomic ratio of 1.3 or higher, with rare earth element R being 6.0. Having the RTM phase with such a high concentration of element M is highly effective in improving the coercivity of R-Fe-B sintered magnets. Adding Tb by grain boundary diffusion treatment using a Tb-Cu-Al alloy increases the concentration of Cu in the RTM phase, and the overall content of element M tends to increase as well.

[0015] In the embodiment described in [3] above, the Ga content of the R-Fe-B sintered magnet as a whole is kept to 0.5 mass% or less. In the R-Fe-B sintered magnet of the present invention, by including Tb as the rare earth element R and having an RTM phase at the grain boundary where Cu>Al>Ga, a high effect on improving coercivity can be obtained even with such a small amount of Ga content. By keeping the Ga content low, the material cost of the R-Fe-B sintered magnet can be effectively suppressed.

[0016] In the embodiment described in [4] above, the Al, Cu, and Ga content of the R-Fe-B sintered magnet as a whole is set to a predetermined range. By applying these content levels, the RTM phase with Cu > Al > Ga is made to appear at the grain boundaries, which contributes to improving the coercivity.

[0017] In the embodiment described in [5] above, the R-Fe-B sintered magnet does not contain Dy except for unavoidable impurities. In the R-Fe-B sintered magnet of the present invention, by containing Tb as the rare earth element R and having an RTM phase with Cu>Al>Ga at the grain boundaries, a high effect on improving coercivity can be obtained even without the addition of Dy. By not using Dy, the material cost of the R-Fe-B sintered magnet can be effectively suppressed.

[0018] In the aspect of [6] above, the R-Fe-B sintered magnet is manufactured through a grain boundary diffusion treatment using a Tb-Cu-Al alloy. As described above, by performing the addition of Tb to the R-Fe-B sintered magnet through the grain boundary diffusion treatment, Tb can be effectively contributed to the improvement of the coercive force. Further, by performing the grain boundary diffusion treatment using a Tb-Cu-Al alloy, Cu is distributed at a high concentration in the grain boundary phase, and in the R-T-M phase of the grain boundary, the content of the element M tends to have the relationship of Cu>Al>Ga. Also, in the R-T-M phase, the content of the entire element M also tends to increase.

Brief Description of the Drawings

[0019] [Figure 1] The results of FE-EPMA observation on the sintered magnet of Example 1 are shown. (a) is a BED image, and (b) to (f) show the concentration distributions of the respective elements shown. Separately, a color photograph is submitted. [Figure 2] The results of FE-EPMA observation on the sintered magnet of Comparative Example 1 are shown. (a) is a BED image, and (b) to (f) show the concentration distributions of the respective elements shown. Separately, a color photograph is submitted. [Figure 3] The figure shows the concentration distribution of elements along the thickness direction of the evaluation sample for the sintered magnet of Example 2. (a) to (d) show the concentration distributions of the respective elements shown.

Modes for Carrying Out the Invention

[0020] Hereinafter, an R-Fe-B sintered magnet (hereinafter sometimes simply referred to as a sintered magnet) according to an embodiment of the present invention will be described in detail. In this specification, the content of each component as the entire sintered magnet is expressed in units of mass% based on the entire sintered magnet, and the alloy composition of each phase is expressed in units of atomic%. Also, various characteristics refer to the values measured at room temperature (23°C) in the air.

[0021] [Component Composition of R-Fe-B Sintered Magnet] An R-Fe-B sintered magnet according to one embodiment of the present invention contains the following elements. • Rare earth element R • Fe, or a combination of Fe and Co (let's call it element T) ·B • Al, Cu, and Ga (let's call the set of these three elements element M) R-Fe-B sintered magnets may be composed solely of the above elements, excluding unavoidable impurities, or they may also contain other elements such as Zr, as described later.

[0022] The rare earth element R includes Tb. To that extent, the specific composition of the rare earth element R is not particularly limited, but it is preferable to include light rare earth elements such as Nd and Pr, and heavy rare earth elements including Tb. The heavy rare earth element may be Tb alone, or a combination of Tb and other heavy rare earth elements, but it is preferable to keep the amount of Dy added to a minimum. For example, it is preferable to keep the Dy content to 0.5 mass% or less. Furthermore, it is preferable to not contain Dy except for unavoidable impurities. Dy is highly effective in improving the coercivity of sintered magnets, but as will be described later, the sintered magnet according to this embodiment has sufficiently high coercivity due to the contribution of the RTM phase, and it is not necessary to add a large amount of the expensive element Dy for the purpose of improving coercivity. Preferably, the heavy rare earth element is Tb alone, except for unavoidable impurities.

[0023] As described later, element M contributes to improving the coercivity of sintered magnets by forming an RTM phase at grain boundaries. The composition of element M is not particularly limited, as long as it contains all three elements: Al, Cu, and Ga. In the RTM phase described later, the number of atoms is Cu > Al > Ga, but the relationship between the content of each element in the overall composition of the sintered magnet is not particularly limited; for example, Al > Cu in mass percent. However, it is preferable that the Ga content is kept lower than that of Cu and Al in mass percent. Furthermore, Ga effectively enhances the robustness of the sintered magnet, that is, its ability to resist changes in magnetic properties such as coercivity even if the temperature of heat treatments such as aging changes to some extent during the manufacturing process. Therefore, it is undesirable to omit Ga from the sintered magnet, and it should be added, even in small amounts.

[0024] The specific composition of the sintered magnet is not particularly limited, but examples include a form containing the following elemental components, with the remainder being Fe and unavoidable impurities. The addition of Zr is optional, but its addition is effective in improving the square-to-square ratio (SQ) of the sintered magnet. • 28% ≤ R ≤ 33% (including Tb in the case of 0.1% ≤ Tb ≤ 1.0%) 0% ≤ Co ≤ 2.5% 0.9% ≤ B ≤ 1.2% 0.3% ≤ Al ≤ 1.0% 0.05% ≤ Cu ≤ 0.5% 0.05% ≤ Ga ≤ 0.5% • As an arbitrary element, 0.05% ≤ Zr ≤ 0.35%

[0025] For each component element, the following ranges can be listed as particularly preferred within the above content ranges. The content ranges, as well as the upper and lower limits, for each element can be independently adopted as preferred values. 30%≦R;R≦32%(0.2%≦Tb;Tb≦0.5%) • 0.5% ≤ Co; Co ≤ 1.5% 0.94% ≤ B; B ≤ 1.1% 0.4% ≤ Al; Al ≤ 0.7% • 0.1% ≤ Cu, and further 0.2% ≤ Cu; Cu ≤ 0.3% 0.03%≦Ga;Ga≦0.2% 0.10% ≤ Zr; Zr ≤ 0.20%

[0026] Examples of unavoidable impurities contained in the sintered magnet according to this embodiment include the following: • Cr ≤ 0.1% · Mn ≤ 0.1% • Ni ≤ 0.1% • Si ≤ 0.1% • O < 1500 ppm, preferably O < 1000 ppm • C < 2000 ppm, preferably C < 1000 ppm, more preferably C < 500 ppm • N < 2000 ppm, preferably N < 1000 ppm, more preferably N < 300 ppm Reducing the content of O, C, and N, and especially reducing the content of O, suppresses the formation of compounds between rare earth elements and their impurity elements at grain boundaries, and the incorporation of element M into these compound phases, making it easier to maintain high magnetic properties of sintered magnets, such as coercivity. To reduce the content of these elements in sintered magnets, for example, sintering and heat treatment can be carried out in a vacuum or an inert gas atmosphere, as will be described later.

[0027] [Phase composition of R-Fe-B sintered magnets] The sintered magnet according to this embodiment is mainly R2T 14 The material has a main phase composed of B and a grain boundary phase that occupies the grain boundaries between the crystal grains of the main phase. The grain boundary phase contains the RTM phase. The RTM phase is a phase composed of rare earth elements R, T, and M, excluding unavoidable impurities. In addition to the RTM phase, the grain boundary phase may also contain other phases, such as an R-rich phase containing a higher concentration of the rare earth element R than the main phase, and / or a B-rich phase containing a higher concentration of B than the main phase.

[0028] The RTM phase is R6T 13 It has a general composition denoted as M. Here, R6T13 Having a general composition of M means R6T 14-x M x (0.2 ≦ x ≦ 3.5), which refers to having such a component composition. Among the elements M, Al forms an R-Fe-Al phase composed of tetragonal R6Fe 13 Al. However, due to the substitution of a part of Fe with Co, a part of Al with Cu and Ga, and the formation of lattice defects, in the alloy composition of the R-T-M phase, the atomic ratio of R:(T + M) deviates from 6:14. As described above, R6T 14-x M x (0.2 ≦ x ≦ 3.5) is what is shown. As will be described in detail later, the R-T-M phase has an effect on improving the coercive force of the sintered magnet.

[0029] In the R-T-M phase at the grain boundary, the breakdown of the element M is, in terms of the number of atoms, in the order of Cu > Al > Ga. That is, Cu is the most, Al is the second most, and Ga is the least. As long as this relationship is satisfied, the relationship between the contents of Cu, Al, and Ga in the R-T-M phase is not particularly limited. For example, in terms of the atomic ratio, Cu may be 1.2 times or more of Al, and Ga may be 0.6 times or less of Al. Also, from the perspective of the robustness described above, in the R-T-M phase, it is preferable that Ga > 0 atomic %, and further preferably Ga > 0.5 atomic %.

[0030] Furthermore, in the R-T-M phase, it is preferable that the atomic ratio of the rare earth element R to the element M is 1.3 or more with the rare earth element R being 6.0. That is, in the component composition of the R-T-M phase with R6T 14-x M x x ≧ 1.3 is preferable. Furthermore, x ≧ 1.5 and x ≧ 1.8 are also good.

[0031] The composition of the RTM phase described above is preferably achieved throughout the entire sintered magnet having a predetermined shape and the component composed of the sintered magnet. However, as will be explained next regarding the manufacturing method of the sintered magnet, when grain boundary diffusion treatment is performed, a non-uniform distribution of the component composition of the RTM phase may occur depending on the location within the sintered magnet. In such cases, it is preferable that the RTM phase has the above composition at least at the position where the diffusion distance of the diffusion material, such as the Tb-Cu-Al alloy, is longest during the grain boundary diffusion treatment. This position with the longest diffusion distance can be identified, for example, based on the concentration distribution of Tb in the sintered magnet (see Figure 3(d)). In other words, the position where the Tb concentration is lowest, i.e., where the Tb concentration takes a minimum or local minimum value, on any straight line crossing the sintered magnet, including a straight line along the diffusion direction of the diffusion material, can be considered the position with the longest diffusion distance. As in the later examples, when grain boundary diffusion treatment is performed by contacting the diffusion material with two opposing surfaces of the sintered magnet, the position midway between those two surfaces will be the relevant position. Furthermore, while it is preferable that the composition of the RTM phase described above be satisfied for each of the RTM phases distributed at multiple locations on the observation image obtained by observing the sintered magnet with an electron microscope or electron probe microanalyzer, it may also be satisfied as the average of the RTM phases at those multiple locations.

[0032] [Manufacturing method for R-Fe-B sintered magnets] The sintered magnet according to this embodiment can be suitably manufactured by first producing a sintered body of magnetic material as a base material, and then performing grain boundary diffusion (GBD) treatment on the base material. When manufacturing a sintered magnet via grain boundary diffusion treatment on a base material, the presence or absence of RTM phase formation at the grain boundaries is mainly influenced by the component composition of the base material, and the component composition of the formed RTM phase is influenced by both the base material and the diffusion material used in the grain boundary diffusion treatment. The base material can be manufactured by crushing a raw material alloy containing each component element in a predetermined ratio to produce magnetic powder, and then molding and sintering the magnetic powder.

[0033] The component composition of the substrate can be appropriately set so that, after grain boundary diffusion treatment, a sintered magnet according to this embodiment having a predetermined component composition can be obtained. However, it is preferable that the substrate does not contain Tb, except for unavoidable impurities.

[0034] When using a Tb-Cu-Al alloy as a diffusion material in subsequent grain boundary diffusion treatment, an example of a suitable substrate composition is one containing the following elemental components, with the remainder being Fe and unavoidable impurities. · 28% ≤ R ≤ 33% 0% ≤ Co ≤ 2.5% 0.9% ≤ B ≤ 1.2% 0.3% ≤ Al ≤ 1.0% 0.05% ≤ Cu ≤ 0.5% 0.05% ≤ Ga ≤ 0.5% • As an arbitrary element, 0.05% ≤ Zr ≤ 0.3%

[0035] The magnet powder used as a raw material for the base material can be manufactured by creating an alloy ingot having a predetermined component composition, for example, by strip casting, and then crushing it. Preferably, the alloy ingot is brought into contact with hydrogen gas to absorb hydrogen molecules, which embrittles the alloy ingot, and then it is crushed. As for the crushing method, examples include mechanical coarse crushing followed by fine crushing using a jet mill or the like. The particle size of the magnet powder after crushing can be exemplified by an average particle size D50 of 5 μm or less.

[0036] Next, the magnet powder obtained in this way is molded into a predetermined shape and sintered. Sintering is preferably performed after oriented the magnet powder in a magnetic field. In particular, from the viewpoint of reducing the content of impurities in the sintered body, it is preferable to use the PLP method (press-less process), in which sintering is performed without press molding during or after orientation. For example, the magnet powder can be filled into a container corresponding to the shape of the magnet to be manufactured in a vacuum or inert gas atmosphere, the magnet powder can be oriented by applying a magnetic field from the outside, and then sintering can be performed with the magnet powder still filled in the container. An example of a sintering temperature is in the range of 900°C to 1050°C. The raw material magnet powder contains residual hydrogen molecules that were absorbed during pulverization, but these hydrogen molecules are released into the gas phase during sintering by desorption due to heating and by reaction with carbon atoms present as impurities in the magnet powder. In particular, heating in an inert gas such as Ar during the process of raising the temperature to the sintering temperature, specifically in the temperature range of 450-550°C, allows hydrogen molecules absorbed into the alloy to react more readily with carbon atoms rather than being removed in their original form, resulting in a highly effective reduction of carbon as an impurity. After reaching that intermediate temperature, the subsequent heating and sintering can be carried out in a vacuum atmosphere.

[0037] The substrate obtained as a sintered body in this way can then be subjected to grain boundary diffusion treatment. In grain boundary diffusion treatment, a diffusion material made of an alloy containing Tb is brought into contact with the surface of the substrate and heated, thereby diffusing the constituent elements of the diffusion material into the interior of the substrate. Adding Tb to a sintered magnet can increase its coercivity, but by performing this addition through grain boundary diffusion treatment, a significant improvement in coercivity can be obtained even with a small amount of Tb.

[0038] The diffusion material used in grain boundary diffusion treatment is not particularly limited as long as it is an alloy containing Tb, but Tb-Cu-Al alloy can be suitably used. By performing grain boundary diffusion treatment with Tb-Cu-Al alloy, Tb can be efficiently diffused into the substrate. When grain boundary diffusion treatment is performed with Tb-Cu-Al alloy, Tb is distributed in both the main phase and the grain boundary phase, but Cu is mainly distributed in the grain boundary phase. Therefore, Cu is distributed at a high concentration in the RTM phase, which accounts for at least a part of the grain boundary phase, and the relationship of Cu > Al > Ga in the number of atoms in the RTM phase is easily obtained. In addition to Tb-Cu-Al alloy, Tb-Ni-Al alloy is also often used as a diffusion material for adding Tb by grain boundary diffusion treatment, but Tb-Cu-Al alloy is less expensive and can be suitably used.

[0039] When using a Tb-Cu-Al alloy as the diffusion material, a suitable heating temperature for grain boundary diffusion treatment can be set to a range of 800°C or higher and 1000°C or lower. When bringing the diffusion material into contact with the surface of the substrate, the diffusion material can be in powder form and applied to the surface of the substrate either as is or dispersed in an organic solvent or organic binder. Contact with the diffusion material only needs to be made on at least a portion of the surface of the substrate, but it is preferable to bring the diffusion material into contact with at least one pair of opposing surfaces of the substrate surface to perform grain boundary diffusion treatment. The concentration of Tb to be diffused at grain boundaries can be controlled by the amount of diffusion material brought into contact with the substrate, for example, the concentration of the diffusion material dispersed in an organic solvent or organic binder.

[0040] Through grain boundary diffusion treatment, the constituent elements of the diffusion material are incorporated into the substrate, but the concentrations of these elements form a non-uniform distribution within the substrate. The concentration of these elements is higher closer to the substrate surface where the diffusion material is in contact, and decreases further away from the contact point. However, the degree of non-uniformity in the concentration distribution differs depending on the constituent elements of the diffusion material. As shown in Figure 3, when a Tb-Cu-Al alloy is used as the diffusion material, Tb shows a highly non-uniform concentration distribution, but the non-uniformity of the concentration distributions of Cu and Al is considerably smaller than that of Tb. In particular, the concentration distribution of Cu is small. If it is to avoid the effect of extremely high Tb concentration near the surface where the diffusion material is in contact, the microstructure near the surface of the sintered magnet that has undergone grain boundary diffusion treatment (and the aging treatment described below) can be removed by grinding, cutting, etc., as appropriate.

[0041] It is preferable to perform grain boundary diffusion treatment on the substrate, followed by aging treatment as a heat treatment. By undergoing aging treatment, it becomes easier to obtain a structure in a sintered magnet having a main phase and a grain boundary phase in which the grain boundary phase contains the RTM phase and a high concentration of Tb. From the viewpoint of optimizing coercivity, it is preferable to carry out the aging treatment at 400°C to 600°C. Furthermore, it is preferable to carry out the aging treatment in a vacuum or inert gas atmosphere.

[0042] [Characteristics of R-Fe-B sintered magnets] The sintered magnet according to this embodiment has high coercivity because it contains an RTM phase in the grain boundary phase. In an R-Fe-B sintered magnet, if element M is not added, the grain boundary will have an Fe-rich phase (R2T) with high saturation magnetization. 17Although a Fe-rich phase is formed, when element M is added, at least a portion of the Fe-rich phase is replaced by the RTM phase. Since the RTM phase has a lower saturation magnetization than the Fe-rich phase, when a reverse magnetic field is applied to a magnetized sintered magnet and the magnetization of some of the main phase crystal grains is reversed, the magnetization of other adjacent main phase crystal grains is less likely to be reversed due to ferromagnetic interactions via the grain boundary phase. Therefore, the sintered magnet according to this embodiment having the RTM phase exhibits high coercivity. Furthermore, the sintered magnet according to this embodiment contains Tb, and Tb also has a high effect in improving coercivity.

[0043] Of the elements M (Al, Cu, Ga), Ga is highly effective in improving coercivity, but it is an expensive element. Therefore, from the perspective of reducing the material cost of sintered magnets, it is desirable to keep the Ga content low. However, in the RTM phase formed at grain boundaries, by setting the content of element M to Cu > Al > Ga, and including more Al and Cu than Ga, even if the amount of Ga added is kept small, the coercivity improvement effect of the RTM phase can be greatly enhanced by the contribution of Al and Cu. In particular, Cu is incorporated in large quantities into the grain boundary phase when grain boundary diffusion treatment is performed using a Tb-Cu-Al alloy, so its concentration in the RTM phase tends to be high, and it effectively contributes to improving coercivity. Cu in the RTM phase also shows a high effect in improving the corrosion resistance of sintered magnets. Furthermore, in the RTM phase, if the atomic ratio is set to R = 6.0 and the element M is 1.3 or more (x ≥ 1.3), a particularly high effect on improving coercivity can be obtained.

[0044] As described above, the sintered magnet according to this embodiment can be suitably manufactured by undergoing grain boundary diffusion treatment using a Tb-Cu-Al alloy, and as a result of this grain boundary diffusion treatment, the relationship Cu>Al>Ga is easily obtained regarding the element M content of the RTM phase at the grain boundary. In this way, by utilizing grain boundary diffusion treatment, a high effect of improving coercivity can be obtained even when the amount of Tb added is kept to a small amount.

[0045] As described above, the sintered magnet according to this embodiment contains Tb and an RTM phase in the grain boundary phase, and the content of element M in the RTM phase is Cu > Al > Ga, resulting in high coercivity. Therefore, there is no need to include large amounts of Ga or Dy, which are expensive elements that are very effective in improving coercivity, in the sintered magnet for the purpose of improving coercivity. The content of Tb, which is a heavy rare earth element similar to Dy, can also be kept low. Thus, the material cost of the sintered magnet can be kept low. In particular, a sufficiently high coercivity can be obtained even without including Dy. Using a Tb-Cu-Al alloy instead of a Tb-Ni-Al alloy for grain boundary diffusion treatment also contributes to suppressing material costs. In the sintered magnet according to this embodiment, for example, a coercivity of 25 kOe or more (Hcj) can be obtained even without adding Dy, and even if the Ga content is kept below 0.2 mass%. [Examples]

[0046] Examples of the present invention are shown below. However, the present invention is not limited to these examples.

[0047] [1] Relationship between grain boundary phase and magnetic properties of sintered magnets First, we investigated the relationship between the grain boundary phase state and magnetic properties of sintered magnets with typical component compositions.

[0048] [Sample preparation] Sintered magnets were prepared for Example 1 and Comparative Example 1. The sintered magnets were prepared by sintering to create a substrate, followed by grain boundary diffusion treatment and aging treatment of the obtained substrate.

[0049] For the preparation of the base material, hydrogen molecules were first absorbed into an alloy ingot produced by the strip casting method. Then, coarse grinding by mechanical pulverization and fine grinding by jet milling were performed to obtain magnetic powder with an average particle size of 3-4 μm. This magnetic powder was filled into a container, oriented in a magnetic field, and then sintered. The sintering temperature was set to 985-1035°C, and the sintering time was set to 4-6 hours. During sintering, an Ar atmosphere was maintained from room temperature up to the intermediate temperature, and the subsequent heating and sintering were performed in a vacuum. The intermediate temperature was set in the range of 450-550°C.

[0050] The component composition of the base material is as shown in Table 1 below. The base material was in the form of a plate with a thickness of 4.8 mm.

[0051] [Table 1]

[0052] In the grain boundary diffusion treatment, a Tb-Cu-Al alloy was used as the diffusion material in Example 1, and a Tb-Ni-Al alloy was used in Comparative Example 1. In both cases, the diffusion material powder was dispersed in silicone oil and applied to both sides of the substrate in the thickness direction. In this state, the substrate was heated in a vacuum atmosphere at 900°C for 15 to 30 hours to diffuse the diffusion material into the substrate. Then, an aging treatment was performed in a vacuum at 400 to 600°C. After that, both sides in the thickness direction were uniformly ground to a thickness of 4.5 mm, which was used as the evaluation sample.

[0053] [Evaluation Method] (1) Tissue observation Microstructure observation was performed on the evaluation samples obtained from Example 1 and Comparative Example 1 as described above. Microstructure observation was performed at the center of the evaluation sample. Specifically, the cross-section of the evaluation sample was observed near the center in the thickness direction using a field emission electron probe microanalyzer (FE-EPMA; JEOL "JXA-iHP200F"). The composition of the resulting phases was then analyzed. For the analysis, the microstructure was confirmed based on backscattered electron images (BED images) in the cross-section of the sample, and the distribution of the grain boundary phase was evaluated. In addition, the concentration distribution of each element constituting the sintered magnet was evaluated, and the component composition was further analyzed at multiple representative points. Specifically, FE-EPMA mapping was acquired at a magnification of 4000x, and the concentrations of Nd, Pr, Tb, Fe, Co, Al, Cu, and Ga were quantitatively analyzed at arbitrary points in the grain boundary phase. The concentration values ​​were set so that the sum of these elements equaled 100 atomic percent.

[0054] (2) Measurement of coercivity Magnetization curves were measured for each evaluation sample. Measurements were performed using a DC BH tracer. The values ​​of coercivity (Hcj) and remanent magnetic flux density (Br) were recorded. Measurements were performed in air at room temperature.

[0055] [Test Results] Table 2 shows the overall component composition for each sample. Figure 1 shows the FE-EPMA mapping results for Example 1, and Figure 2 shows the results for Comparative Example 1. Each shows (a) the BED image and (b) the concentration distribution of the elements Fe, (c) Nd, (d) Al, (e) Cu, and (f) Tb.

[0056] [Table 2]

[0057] In the microstructure observation results for Example 1 in Figure 1, the dark areas occupying a large area in the BED image correspond to the main phase crystal grains. Furthermore, the areas shown in lighter gray than the main phase crystal grains, including the areas exemplified by the arrows, correspond to the RTM phase within the grain boundary phase. This is also indicated by the fact that the concentrations of Al and Cu in the elemental distribution image are both high in these lighter gray areas. The even brighter areas within the grain boundary phase consist of phases other than the RTM phase, such as the R-rich phase.

[0058] Thus, in Example 1, the formation of the RTM phase at the grain boundaries is clearly observed by FE-EPMA mapping. In fact, Table 3 below shows the results of quantitative analysis of the content of each element at multiple analysis points arbitrarily extracted from the region corresponding to the RTM phase in the image. At all of the analysis points shown in Table 3, the ratio of R:T:M is close to 6:13:1, and the ratio of R:(T+M) is close to 6:14. From this, it can be seen that the RTM phase is R6T 13 A general composition that can be denoted as M, i.e., R6T 14-x M x It has been confirmed that the component composition can be expressed as (0.2 ≤ x ≤ 3.5).

[0059] Looking at the component composition of the RTM phase shown in Table 3 in more detail, at every analysis point, the number of constituent atoms of element M follows the order Cu > Al > Ga. Also, the total number of atoms of element M is 1.3 or more compared to the number of atoms of rare earth element R, which is 6.0.

[0060] [Table 3]

[0061] On the other hand, in the microstructure observation of Comparative Example 1 in Figure 2, unlike in Example 1 in Figure 1, there were almost no areas where the concentrations of Al and Cu were significantly higher than in other parts of the grain boundaries between the main phases. In other words, the grain boundary phase corresponding to the RTM phase was hardly formed. In fact, quantitative analysis of the content of each element at multiple points arbitrarily extracted from the grain boundaries did not detect any points with a composition corresponding to the RTM phase.

[0062] Next, Table 4 shows the evaluation results of the magnetic properties. [Table 4]

[0063] According to Table 4, compared to Comparative Example 1, Example 1 has a lower residual magnetic flux density Br, but a higher coercivity Hcj. From this, it can be said that the coercivity of a sintered magnet can be improved by generating an RTM phase with Cu>Al>Ga at the grain boundaries, as in Example 1.

[0064] [2] Control of grain boundary phase composition by grain boundary diffusion treatment Next, we investigated the relationship between the type and amount of diffusion material used in grain boundary diffusion treatment and the composition of the RTM phase at the grain boundaries. We also examined the distribution of elemental concentrations after grain boundary diffusion treatment.

[0065] [Sample preparation] Similar to the above test [1], evaluation samples were prepared by preparing the substrate and performing grain boundary diffusion treatment. Two types of evaluation samples were prepared: Example 2 and Comparative Example 2. For both Example 2 and Comparative Example 2, the substrate used was the same as that of Example 1 in test [1], but the type and amount of diffusion material used in the grain boundary diffusion treatment were changed as follows. Other than that, the sample preparation conditions were the same as in test [1].

[0066] In Example 2, a Tb-Cu-Al alloy was used as the diffusion material, and the amount of diffusion material used was set with a target value of 0.35 mass% for the Tb content in the overall composition of the sintered magnet. In contrast, in Example 1 of the above test [1], which also used a Tb-Cu-Al alloy, the amount of diffusion material used was set with a target value of 0.55 mass% for the Tb content, indicating that the amount of diffusion material used is less in Example 2. For Comparative Example 2, a Tb-Ni-Al alloy was used as the diffusion material, and the amount of diffusion material used was set with a target value of 0.50 mass% for the Tb content.

[0067] [Evaluation Method] (1) Tissue observation For each sample, tissue observation was performed in the same manner as in Test [1]. After confirming the formation of the RTM phase, quantitative analysis of the component composition of the RTM phase was performed. However, for each evaluation sample, analysis was performed at the central position in the thickness direction, as in Test [1], and also at the surface position in the thickness direction. The average value of the analysis results at multiple analysis points was recorded for both the central and surface positions to represent the component composition of the RTM phase.

[0068] (2) Evaluation of elemental concentration distribution To confirm the concentration distribution of elements formed after grain boundary diffusion treatment, wide-area FE-EPMA measurements were performed. Specifically, FE-EPMA measurements were performed on cross-sections cut along the thickness direction, passing through the center of a plate-shaped evaluation sample, and the concentrations of Al, Cu, Ga, and Tb at each position in the thickness direction were evaluated. For each element, the average value within the sample cross-section was recorded for each position in the thickness direction.

[0069] [Test Results] Table 5 shows the overall component composition for each sample. Table 6 shows the results (average values) of the RTM phase composition analysis for the central and surface regions in the thickness direction of each sample.

[0070] [Table 5]

[0071] [Table 6]

[0072] According to Table 5, in Comparative Example 2, the Cu content in the sample remained low, corresponding to the absence of Cu in the diffusion material used for grain boundary diffusion treatment. In contrast, in Example 2, the Cu content in the sample increased, corresponding to the presence of Cu in the diffusion material. Furthermore, according to Table 6, in Example 2, where a Tb-Cu-Al alloy was used for grain boundary diffusion treatment, the breakdown of element M content in the RTM phase was Cu > Al > Ga in both the central and surface layers in the thickness direction, similar to Example 1. Also, the total number of atoms of element M was 1.3 or more compared to the number of atoms of rare earth element R, which was 6.0. These compositional characteristics are consistent with Example 1 of Test [1], in which a larger amount of Tb-Cu-Al alloy was used and Tb was added at a high concentration (see Table 3). On the other hand, when comparing the Tb concentration in the RTM phase between the average value of Example 1 in Table 3 (0.7 atomic%) and the value at the center of the sample in Example 2 in Table 6 (0.6 atomic%), the concentration is higher in Example 1, where Tb was added at a higher concentration.

[0073] From the comparison between Examples 1 and 2, it can be confirmed that the concentration of Tb in the RTM phase at the grain boundaries can be controlled by the amount of Tb-Cu-Al alloy used in the grain boundary diffusion treatment. Furthermore, it can be confirmed that even when the concentration of Tb is changed in this way, the RTM phase at the grain boundaries can be made to contain element M such that the relationship Cu>Al>Ga is satisfied, and the number of atoms of element M is 1.3 or more compared to the 6.0 atoms of rare earth element R. In Comparative Example 2, corresponding to the use of a Tb-Ni-Al alloy that does not contain Cu in the grain boundary diffusion treatment, the breakdown of element M in the RTM phase at the grain boundaries shows that Al is more abundant than Cu, and the relationship Cu>Al>Ga is not obtained.

[0074] Finally, Figure 3 shows the elemental concentration distribution along the thickness direction for Example 2. The horizontal axis represents the position in the thickness direction, and the distributions of (a) Al, (b) Cu, (c) Ga, and (d) Tb are shown.

[0075] As shown in Figure 3, corresponding to the grain boundary diffusion treatment performed by applying a diffusion material made of Tb-Cu-Al alloy to both surfaces in the thickness direction, the concentration of Tb in (d) shows a non-uniform distribution with respect to the position in the thickness direction. That is, the Tb concentration is high at both positions in the thickness direction and low in the inner region in the thickness direction. The center in the thickness direction is the lowest point of Tb concentration. Similarly, the concentration of Al in (a) is high on both sides in the thickness direction, but the degree of change in concentration is considerably smaller than that of Tb, except in the immediate vicinity of the surface. For Cu in (c), a slight tendency for the concentration to be higher on both sides in the thickness direction is also observed, but the amount of change is very small. The concentration distribution of Tb observed here is consistent with the fact that the Tb content in the RTM phase in Table 6 is higher in the surface layer than in the center. Although not shown here, a similar concentration distribution for Tb and Al was obtained for Comparative Example 2, which uses a diffusion material made of Tb-Ni-Al alloy.

[0076] The embodiments of the present invention have been described above. The present invention is not particularly limited to these embodiments, and various modifications are possible.

Claims

1. Rare earth elements R, including Tb, Fe, or an element T consisting of Fe and Co, B and, It contains Al, Cu, and Ga, Let the aggregate of Al, Cu, and Ga be element M, and the grain boundary phase includes the R-T-M phase. An R-Fe-B sintered magnet in which the number of atoms of element M in the R-T-M phase is in the order of Cu > Al > Ga.

2. The R-Fe-B sintered magnet according to claim 1, wherein in the R-T-M phase, the atomic ratio of the rare earth element R to the element M is 1.3 or more, with the rare earth element R being 6.

0.

3. The R-Fe-B sintered magnet according to claim 1 or claim 2, wherein the Ga content is 0.5% by mass or less.

4. Each is expressed in mass percent, 0.3% ≤ Al ≤ 1.0%, 0.05% ≤ Cu ≤ 0.5%, The R-Fe-B sintered magnet according to claim 3, wherein 0.05% ≤ Ga ≤ 0.5%.

5. An R-Fe-B sintered magnet according to claim 1 or claim 2, which does not contain Dy except for unavoidable impurities.

6. An R-Fe-B sintered magnet according to claim 1 or claim 2, manufactured by undergoing grain boundary diffusion treatment using a Tb-Cu-Al alloy.

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