RFeB-based magnet and method for producing RFeB-based magnet

Abstract

The present invention relates to an RFeB-based magnet in which a treatment (grain boundary diffusion treatment) for diffusing atoms of the heavy rare earth element R.sup.H is performed in a base material including an R.sup.LFeB-based sintered magnet obtained by subjecting crystal grains in a raw-material powder including a powder of an R.sup.LFeB-based alloy containing the light rare earth element R.sup.L, Fe and B to orientation in a magnetic field and then sintering the oriented raw-material powder, or an R.sup.LFeB-based hot-deformed magnet obtained by subjecting the same raw-material powder to hot pressing and then to hot deforming to thereby orient the crystal grains in the raw-material powder, and a method for producing the RFeB-based magnet.

Claims

1. A method for producing an RFeB-based magnet, the method comprising: preparing an adhesion substance containing an R.sup.HCuAl alloy which comprises a contained heavy rare earth R.sub.C.sup.H including one or a plurality of kinds of heavy rare earth elements R.sup.H, Cu, and Al, and has a composition represented by a point in an octagon with 8 coordinates, (R.sub.C.sup.H.sub.a%, Cu.sub.at%, Al.sub.at%)=(50, 40, 10), (58, 30, 12), (58, 20, 22), (48, 20, 32), (33, 24, 43), (17, 50, 33), (17, 60, 23), and (33, 58, 9), as vertexes in a ternary composition diagram with R.sub.C.sup.H, Cu, and Al as vertexes, or on a side of the octagon; adhering the adhesion substance to a surface of a base material comprising an R.sup.LFeB-based sintered magnet body which comprises a contained light rare earth R.sub.C.sup.Lincluding one or two kinds of light rare earth elements R.sup.L, Fe, and B; and heating the base material having the adhesion substance adhered thereto to a predetermined temperature at which atoms of the contained heavy rare earth R.sub.C.sup.H in the adhesion substance diffuse into the base material through grain boundaries of the base material, wherein the RFeB-base magnet has a coercive three of 22.5 kOe or more, and the predetermined temperature is in a range from 820° C. to 1,000° C.

2. The method for producing an RFeB-based magnet according to claim 1, wherein the R.sup.HCuAl alloy has a composition represented by a point in a hexagon with 6 coordinates, (R.sub.C.sup.H.sub.at%, Cu.sub.at%, Al.sub.at%)=(50, 40, 10), (50, 32, 18), (33, 24, 43), (17, 50, 33), (17, 60, 23), and (33, 58, 9), as vertexes in the ternary composition diagram, or on a side of the hexagon.

3. The method for producing an RFeB-based magnet according to claim 1, wherein, after the heating, a content by mass of the Cu in the grain boundary of the RFeB-based magnet is in a range from 3.9% to 14.0%.

4. The method for producing an RFeB-based magnet according to claim 3, wherein, after the heating, a content by mass of the Al in the grain boundary of the RFeB-based magnet is in a range from 0.09% to 1.00%.

5. The method for producing an RFeB-based magnet according to claim 1, wherein, after the heating, a content by mass of the Al in the grain boundary of the RFeB-based magnet is in a range from 0.09% to 1.00%.

6. The method for producing an RFeB-based magnet according to claim 1, wherein the one or the plurality of kinds of heavy rare earth elements RH comprises Tb, and wherein a mass ratio of the Tb to the base. material is in a range from 0.2% to 1.2%.

7. The method for producing an RFeB-based magnet according to claim 1, wherein the one or the plurality of kinds of heavy rare earth elements RH is consisted of Tb, and wherein a mass ratio of the Tb to the base material is in a range from 0.2% to 1,2%.

8. The method for producing an RFeB-based magnet according to claim 1, wherein, after the heating, a content by mass of the contained heavy rare earth R.sub.C.sup.H in a grain boundary of the RFeB-based magnet is in a range from 0.40% to 1.25.sub.C.sup.H %.

9. The method for producing an RFeB-based magnet according to claim 1, further comprising an aging treatment in which the base material is heated at a lower temperature than the predetermined temperature.

10. The method for producing an RFeB-based magnet according to claim 9, wherein the temperature of the aging treatment is about 500° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a ternary composition diagram showing a composition of an R.sup.HCuAl alloy used in a method for producing an RFeB-based magnet according to the present invention.

(2) FIGS. 2A and 2B are schematic views showing steps of one embodiment of a method for producing an RFeB-based magnet.

(3) FIG. 3 is a diagram showing an example of specifying places at which composition analysis are performed based on a sample image obtained by an EPMA device.

(4) FIG. 4 is a graph showing the measurement results of coercive force iHc for an RFeB-based magnet prepared by one example of a method for producing an RFeB-based magnet.

(5) FIG. 5 is a graph showing the measurement results of the content of Tb in a grain boundary for an RFeB-based magnet prepared by one example of a method for producing an RFeB-based magnet.

(6) FIG. 6 is a ternary composition diagram showing a composition of another R.sup.HCuAl alloy used in a method for producing an RFeB-based magnet according to the preset invention.

(7) FIG. 7 is a graph showing the results of a corrosion resistance test performed for RFeB-based magnets of Examples and Comparative Examples.

DETAILED DESCRIPTION OF THE INVENTION

(8) Embodiments of an RFeB-based magnet and a method for producing the same according to the present invention will be described with reference to FIGS. 1 to 7.

(9) (1) Embodiments of Method for Producing RFeB-Based Magnet According to Present Invention

(10) (1-1) Base Material

(11) A base material used in the embodiments of the method for producing an RFeB-based magnet includes one or two kinds of light rare earth elements R.sup.L, that is, an R.sup.LFeB-based sintered magnet body or R.sup.LFeB-based hot-deformed magnet body which contains Nd and/or Pr, Fe and B. Of these, the R.sup.LFeB-based sintered magnet body may be prepared by a press method of press-forming an R.sup.LFeB-based alloy powder as a raw material while orienting the powder by a magnetic field and then sintering the powder, or a PLP (press-less process) method of orienting an R.sup.LFeB-based alloy powder in a mold by a magnetic field without press-forming the powder and then sintering the powder as it is, as described in Patent Document 3. The PLP method is preferred in that the coercive force can be more increased, and in that the R.sup.LFeB-based sintered magnet body having a complicated shape can be prepared without performing machining. The R.sup.LFeB-based hot-deformed magnet body can be prepared by the method described in Non-Patent Document 1.

(12) (1-2) R.sup.HCuAl Alloy

(13) FIG. 1 shows a composition of an R.sup.HCuAl alloy used in the embodiments of the method for producing an RFeB-based magnet. This figure is a diagram generally called a ternary composition diagram, and one point in the diagram shows the contents of 3 kinds of elements, R.sub.C.sup.H, Cu and Al, in which R.sub.C.sup.H may be any one of Tb, Dy and Ho. In this diagram, it is assumed that R.sub.C.sup.H is one kind of element (that is, any one kind of Tb, Dy and Ho). However, in the actual R.sup.HCuAl alloy, atoms of two or three kinds of elements of Tb, Dy and Ho may be mixed.

(14) For the content of R.sub.C.sup.H, the vertex of the triangle described as “R.sub.C.sup.H” in FIG. 1 is 100 atomic %, and the opposite side to the vertex is 0 atomic %. For example, in FIG. 1, the numerical value “33” at a point at which a straight line drawn in parallel to the opposite side from a point 3 intersects a side described as “CONTENT OF R.sub.C.sup.H” indicates that the content of R.sub.C.sup.H at the point 3 is 33 atomic %. Similarly, at the point 3, the Cu content is 24 atomic %, and the Al content is 43 atomic %.

(15) The contents of the respective atoms of R.sub.C.sup.H, Cu and Al at points 1 to 9 in FIG. 1 are as shown in Table 1. In Table 1, in addition to the atomic contents, the mass contents are also described together, for the case where R.sub.C.sup.H is Dy and the case where R.sub.C.sup.H is Tb.

(16) TABLE-US-00001 TABLE 1 Composition of Composition of Composition of Point R.sup.HCuAl alloy DyCuAl alloy TbCuAl alloy in [atomic %] [mass %] [mass %] FIG. 1 R.sub.C.sup.H Cu Al Dy Cu Al Tb Cu Al 1 50 40 10 74.3 23.2 2.5 73.9 23.6 2.5 2 50 32 18 76.3 19.1 4.6 75.9 19.4 4.6 3 33 24 43 66.8 18.6 14.6 66.1 19.2 14.6 4 17 50 33 40.4 46.5 13.0 39.9 46.9 13.2 5 17 60 23 38.4 53.0 8.6 37.9 53.4 8.7 6 33 58 9 57.7 39.7 2.6 57.2 40.2 2.6 7 58 30 12 80.9 16.4 2.8 80.5 16.7 2.8 8 58 20 22 83.5 11.3 5.3 83.2 11.5 5.4 9 48 20 32 78.5 12.8 8.7 78.1 13.0 8.8

(17) In the embodiments of the method for producing an RFeB-based magnet, the R.sup.HCuAl alloy having the contents of the respective atoms of R.sub.C.sup.H, Cu and Al indicated by a point in the first hexagon (shown by hatched lines drawn from the upper left to the lower right in FIG. 1) with the points 1 to 6 as the vertexes, which is indicated by thick solid lines in FIG. 1, or on a side of the hexagon can be used in a grain boundary diffusion treatment described later. In the R.sup.HCuAl alloy having such contents, there is present a ternary R.sup.HCuAl phase (R.sup.H:Cu:Al is 1:1:1) having a larger R.sub.C.sup.H composition ratio than that of the other phases, and therefore, the coercive force of the RFeB-based sintered magnet or the RFeB-based hot-deformed magnet can be increased. The range in which the R.sup.HCuAl phase described herein is present is based on the ternary composition diagram at 573K (300° C.) shown in Non-Patent Document 2.

(18) In addition, in the embodiments of the method for producing an RFeB-based magnet, the R.sup.HCuAl alloy having the contents of the respective atoms of R.sub.C.sup.H, Cu and Al indicated by points in a second hexagon (shown by hatched lines drawn from the upper right to the lower left in FIG. 1) with points 1, 7, 8, 9, 3 and 2 as the vertexes, which is indicated by thick broken lines in FIG. 1, or on a side of the hexagon can also be used in a grain boundary diffusion treatment described later. It has been proved by experiments described later that the R.sup.HCuAl alloy having these contents exhibits action equivalent to that of R.sup.HCuAl alloy having the contents indicated by the first hexagon.

(19) Accordingly, in the embodiments of the method for producing an RFeB-based magnet, the R.sup.HCuAl alloy having the contents of the respective atoms of R.sub.C.sup.H, Cu and Al indicated by a point in the octagon formed by combining the first hexagon and the second hexagon, with the points 1, 7, 8, 9, 3, 4, 5 and 6 as the vertexes, or on a side of the octagon is used.

(20) (1-3) Adhesion Substance (R.sup.HCuAl Alloy-Containing Substance), Adhesion Substance Preparation Step

(21) An adhesion substance used in the embodiments of the method for producing an RFeB-based magnet contains the above-mentioned R.sup.HCuAl alloy. The adhesion substance may be composed of the R.sup.HCuAl alloy such as a powder or foil of the R.sup.HCuAl alloy, but may be a mixture of the powder of the R.sup.HCuAl alloy and another material as described below. The materials to be mixed with the powder of the R.sup.HCuAl alloy typically include organic solvents. Use of the organic solvent makes it possible to easily adhere the adhesion substance to a surface of the base material. Of the organic solvents, particularly, a silicone-based organic solvent including silicone grease, silicone oil or a mixture thereof can be suitably used. Use of such a silicone-based organic solvent more increases adhesiveness of the adhesion substance to the base material, and the atoms of R.sub.C.sup.H become easily transferred to the grain boundaries of the base material during the grain boundary diffusion treatment. Therefore, the coercive force of the RFeB-based magnet can be more increased. The viscosity of the adhesion substance can be adjusted by mixing the silicone grease and the silicone oil at an appropriate ratio.

(22) (1-4) Grain Boundary Diffusion Treatment

(23) Using the base material and adhesion substance prepared as described above, the grain boundary diffusion treatment is performed as follows. First, an adhesion substance 12 is adhered to a surface of a base material 11 (FIG. 2A, the adhesion substance (R.sup.HCuAl alloy-containing substance) adhering step). The adhesion substance 12 may be adhered to the entire surface of the base material 11, or may be adhered to a part of the surface. For example, the adhesion substance 12 obtained by mixing the silicone-based organic solvent can be adhered to two plate surfaces of the plate-shaped base material 11 by coating. In this case, side faces of the base material 11 are not coated with the adhesion substance 12.

(24) Then, the base material 11 coated with the adhesion substance 12 is heated to a predetermined temperature (FIG. 2B, a heating step). The predetermined temperature as used herein is a temperature at which atoms of a contained heavy rare earth R.sub.C.sup.H in the adhesion substance 12 diffuse into the base material 11 through grain boundaries of the base material 11, and typically from 700 to 1,000° C. By this heating step, the atoms of the contained heavy rare earth R.sub.C.sup.H in the adhesion substance 12 diffuse into the base material 11 through the grain boundaries of the base material 11, thereby increasing the concentration of R.sub.C.sup.H mainly in the vicinity of surfaces of crystal grains in the base material 11. On the other hand, the atoms of the contained heavy rare earth R.sub.C.sup.H are difficult to enter the inside of the grains. Therefore, the RFeB-based magnet (RFeB-based sintered magnet or RFeB-based hot-deformed magnet) in which the content of the contained heavy rare earth R.sub.C.sup.H is higher in the grain boundary than in the crystal grain is obtained by this heating step. Thereafter, an aging treatment (a treatment in which the base material is heated at a relatively low temperature of about 500° C.), a grinding treatment for removing the residue of the adhesion substance 12 remaining on the surfaces of the base material 11 and a magnet shaping treatment are performed as needed, thereby obtaining the RFeB-based magnet as a final product.

(25) The content of the contained heavy rare earth R.sub.C.sup.H in the grain boundary of the RFeB-based magnet obtained is from 0.40 to 1.25% by mass, although it depends on the content of the contained heavy rare earth R.sub.C.sup.H in the R.sup.HCuAl alloy and the kind of the contained heavy rare earth R.sub.C.sup.H of the base material 11. In addition, the content of Cu in the grain boundary of the RFeB-based magnet obtained is from 3.9 to 14.0% by mass, and the content of Al in the grain boundary thereof is from 0.09 to 1.00% by mass.

(26) (2) Examples of Method for Producing RFeB-Based Magnet According to Present Invention and Embodiments of RFeB-Based Magnet According to Present Invention

(27) Then, examples of producing the RFeB-based magnet by the embodiments of the method for producing an RFeB-based magnet and performing composition analysis in the grain boundary of the RFeB-based magnet obtained are explained, and embodiments of the RFeB-based magnet according to the present invention are explained based on the experimental results of the examples.

(28) In Example 1, a plate-shaped R.sup.LFeB-based sintered magnet body containing no R.sup.H and small amounts of Cu and Al (Cu: 0.1% by mass, Al: 0.2% by mass) and having a thickness of 5 mm was used as a base material. An R.sup.HCuAl alloy in which R.sup.H was Tb, having a Tb content of 46.00 atomic % (74.53% by mass), a Cu content of 30.00 atomic % (19.01% by mass) and an Al content of 24.00 atomic % (6.46% by mass), was prepared by a strip cast method. The contents of respective elements of this R.sup.HCuAl alloy correspond to a point marked by triangles in FIG. 1. An adhesion substance was prepared by mixing silicone grease with an R.sup.HCuAl alloy powder obtained by pulverizing this R.sup.HCuAl alloy by a hydrogen pulverization process and then removing hydrogen.

(29) The amount of the adhesion substance to be adhered to the base material was adjusted so that the mass of Tb in the adhesion substance to the mass of the base material was within a range of 0.2 to 1.4%, and a plurality of experiments different in the amount of the adhesion substance were performed. The adhesion substance was adhered to two entire plate surfaces of the plate-shaped base material, and not adhered to four side faces. The composition analysis in the grain boundary of the RFeB-based magnet obtained was performed using an EPMA device (manufactured by JEOL Ltd., JXA-8500F). In this analysis, for positions in the grain boundaries, one place was randomly designated from each one of grain-boundary triple points different from one another, that is, 7 places in total were randomly designated at positions of 2.5 mm in depth from positions corresponding to the surface of the base material (that is, positions equidistant from the both surfaces of the base material), and an average value of 5 places excluding 2 places showing the maximum and minimum Tb contents was determined. FIG. 3 shows an example in which positions (i) to (vii) in the grain-boundary triple points of 7 places were designated based on a backscattered electron image of a sample, which was obtained by the EPMA device.

(30) For the RFeB-based magnet obtained, the results of measurement of the coercive force iHc are shown in FIG. 4, and the results of measurement of the Tb content in the grain boundary are shown in FIG. 5. From FIG. 4, when the mass of Tb in the adhesion substance to the mass of the base material is within a range of 0.2 to 1.2% by mass, the coercive force increases with an increase in the mass of Tb in the adhesion substance. Meanwhile, when the mass of Tb in the adhesion substance to the mass of the base material exceeds 1.2% by mass, such an increase in the coercive force is not observed. When the mass of Tb in the adhesion substance to the mass of the base material is within a range of 0.2 to 1.2% by mass, in which the effect of increasing the coercive force has been observed as described above, the content of Tb in the grain boundary is from 0.40 to 1.25% by mass, as shown in FIG. 5.

(31) Furthermore, for TbCuAl alloys having 6 kinds of compositions corresponding to the points 1 to 6 in FIG. 1, as shown in Table 1, RFeB-based magnets were prepared under the same conditions as in Example 1 except for the TbCuAl alloy used, in each of the case where the mass of Tb in the adhesion substance to the mass of the base material was 0.2% by mass and the case where it was 1.2% by mass, and the contents of Tb, Cu and Al in the grain boundary were measured. In addition, for TbCuAl alloys having 3 kinds of compositions corresponding to the points 7 to 9 in FIG. 1, as shown in Table 1, and TbCuAl alloys having 6 kinds of compositions corresponding to points A to F in FIG. 6, as shown in Table 2, RFeB-based magnets were prepared under the same conditions as in Example 1 except for the TbCuAl alloy used, in the case where the mass of Tb in the adhesion substance to the mass of the base material was 1.0% by mass, and the contents of Tb, Cu and Al in the grain boundary were measured (the above is defined as Example 2). Here, all of the points A to F in FIG. 6 are present in the octagon described above.

(32) TABLE-US-00002 TABLE 2 Point Composition of R.sup.HCuAl Composition of TbCuAl in alloy [atomic %] alloy [mass %] FIG. 6 R.sub.C.sup.H Cu Al Tb Cu Al A 48 40 12 72.7 24.2 3.1 B 38 40 22 65.8 27.7 6.5 C 38 30 32 68.6 21.6 9.8 D 28 46 26 55.1 36.2 8.7 E 20 52 28 43.9 45.7 10.4 F 48 30 22 75.3 18.8 5.9

(33) The results of Example 2 are shown in Table 3.

(34) TABLE-US-00003 TABLE 3 Mass ratio of each element and whole Point alloy in adhesion substance in Composition of TbCuAl to base material [mass %] Composition in grain Sample FIG. 1 alloy [atomic %] TbCuAl boundary [mass %] No. or 6 Tb Cu Al Tb Cu Al alloy Tb Cu Al 1 1 50 40 10 0.20 0.06 0.01 0.27 0.41 4.00 0.09 2 2 50 32 18 0.20 0.05 0.01 0.26 0.41 3.92 0.11 3 3 33 24 43 0.20 0.06 0.04 0.30 0.41 3.96 0.18 4 4 17 50 33 0.20 0.23 0.06 0.49 0.41 5.05 0.23 5 5 17 60 23 0.20 0.28 0.04 0.52 0.41 5.34 0.18 6 6 33 58 9 0.20 0.14 0.01 0.35 0.41 4.47 0.10 7 1 50 40 10 1.20 0.38 0.04 1.62 1.23 5.97 0.17 8 2 50 32 18 1.20 0.30 0.07 1.57 1.23 5.50 0.25 9 3 33 24 43 1.20 0.33 0.26 1.80 1.23 5.71 0.71 10 4 17 50 33 1.20 1.38 0.39 2.97 1.23 12.29 1.00 11 5 17 60 23 1.20 1.66 0.27 3.13 1.23 14.02 0.72 12 6 33 58 9 1.20 0.82 0.05 2.08 1.23 8.79 0.21 13 7 58 30 12 1.00 0.21 0.04 1.24 1.14 4.90 0.07 14 8 58 20 22 1.00 0.14 0.06 1.20 1.17 4.47 0.24 15 A 48 40 12 1.00 0.33 0.04 1.38 1.13 5.70 0.18 16 9 48 20 32 1.00 0.17 0.11 1.28 1.22 4.65 0.35 17 B 38 40 22 1.00 0.42 0.10 1.52 1.24 6.25 0.32 18 C 38 30 32 1.00 0.32 0.14 1.46 1.16 5.59 0.42 19 D 28 46 26 1.00 0.66 0.16 1.82 1.15 7.77 0.45 20 E 20 52 28 1.00 1.03 0.24 2.27 1.14 10.09 0.66 21 F 48 30 22 1.00 0.25 0.06 1.33 1.23 5.17 0.27

(35) From Table 3, the content of Tb in the grain boundary of each sample was approximately equivalent to the value in Example 1. In addition, the content of Cu in the grain boundary was from 3.9 to 14.0% by mass, and the content of Al in the grain boundary was from 0.09 to 1.00% by mass.

(36) Then, using adhesion substances containing alloys having the compositions shown in Table 4, RFeB-based sintered magnets of Comparative Examples 1 to 6 were prepared in the same manner as in Examples 1 and 2. For the alloys in the adhesion substances, alloys containing Ni or Co in place of Cu were used in Comparative Examples 1 to 3, and binary alloys (containing no Al) composed of Tb and any one of Cu, Ni and Co were used in Comparative Examples 4 to 6. The amount of the adhesion substance of each example was adjusted so that the amount of Tb in the adhesion substance adhered to the base material became the same in all examples. For the RFeB-based sintered magnets of Examples 1 and 2 and Comparative Examples 1 to 6 thus obtained, samples in each of which the base material was polished by 0.15 mm from two plate surfaces thereof were prepared, and the amount of Tb in these samples was measured. The reason why such polishing is performed herein is that surface polishing is performed as finishing also in an actual product of the RFeB-based sintered magnet, and that useless Tb is removed in order to confirm the efficiency of the grain boundary diffusion treatment, because useless Tb remaining without diffusing in the base material is present in the vicinity of the surfaces of the base material. The amount of Tb in each sample is shown in Table 4 as the ratio to the amount of Tb in the adhesion substance adhered to the base material. In the term “Example 2-X” in Table 4, X is any one of 7 to 9 and A to F, and is a symbol that represents a composition of the TbCuAl alloy and is shown in Tables 1 and 2 and FIGS. 1 and 6.

(37) TABLE-US-00004 TABLE 4 Ratio of amount of Tb in sample to amount Composition of alloy used of Tb in adhesion [mass %] substance used Tb Cu Ni Co Al [mass %] Example 1 74.53  19.01 — — 6.46 77.1 Example 2-7 80.6 16.6 — — 2.9 73.3 Example 2-8 83.1 11.4 — — 5.4 74.8 Example 2-A 72.7 24.3 — — 3.1 72.4 Example 2-9 78.1 13.0 — — 8.8 78.2 Example 2-B 65.8 27.7 — — 6.6 81.0 Example 2-C 68.6 21.7 — — 9.8 74.2 Example 2-D 55.1 36.5 — — 8.6 73.7 Example 2-E 44.0 45.4 — — 10.6 73.1 Example 2-F 75.3 18.8 — — 5.9 79.1 Comparative 92.0 —  4.3 — 3.7 66.0 Example 1 Comparative 74.0 — 19.7 — 6.3 72.2 Example 2 Comparative 74.0 — — 24.3 5.7 65.4 Example 3 Comparative 85.6 14.4 — — — 54.1 Example 4 Comparative 85.0 — 15.0 — — 67.3 Example 5 Comparative 85.0 — — 17.8 — 68.5 Example 6

(38) From the results of this experiment, it was confirmed that the amount of Tb in the samples was larger in Examples 1 and 2 than in Comparative Examples 1 to 6, which made it possible to diffuse Tb more efficiently into the base materials.

(39) Then, for the samples of Example 1 and Comparative Example 1, a corrosion resistance test was performed. The results thereof are shown in FIG. 7. In this test, after the mass of the sample was measured, the sample was maintained under high-temperature and high-humidity conditions of a temperature of 120° C., a humidity of 100% and a pressure of 2 atm (saturated water vapor pressure) for 400 to 1,000 hours, and thereafter the mass of the sample was measured, thereby determining the reduction rate of the mass of the sample. The smaller absolute value of the reduction rate of the mass means the higher corrosion resistance. From FIG. 7, in Comparative Example 1, the absolute value of the reduction rate of the mass became larger, as the time for maintaining the sample under the high-temperature and high-humidity conditions became longer, whereas in Example 1, the reduction rate of the mass was approximately 0, even when the sample was maintained under the high-temperature and high-humidity conditions for 1,000 hours. By this corrosion test, it was confirmed that the sample of Example 1 was higher in corrosion resistance than the sample of Comparative Example 1. This is considered because in Example 1, the potential in the grain boundary was raised by the presence of Cu in the grain boundary to suppress elution of a rare earth-rich (Nd-rich) grain boundary phase and suppress falling off of RFeB (NdFeB) grains.

(40) Next, as embodiments of the method for producing an RFeB-based magnet according to the present invention, the results of performing the grain boundary diffusion treatment using R.sup.HCuAl alloys to R.sup.H-containing base materials are shown. In these embodiments, there were used 3 kinds of RFeB-based sintered magnet base materials containing Tb as R.sup.H in amounts of 0.20%, 4.40% and 10.0% by mass, respectively, and Cu and Al in the same amounts as in Example 1 (Cu: 0.1% by mass, Al: 0.2% by mass), and adhesion substances which were TbCuAl alloys having the compositions indicated by the points 8, B and F, respectively, in FIG. 6 as the R.sup.HCuAl alloys. The amount of the adhesion substance was adjusted so that the content of Tb in the adhesion substance to the base material became 0.20% by mass or 1.00% by mass. For 18 kinds of samples combining these 3 kinds of base materials, 3 kinds of adhesion substances and 2 kinds of Tb contents in the adhesion substance, the contents of Tb, Cu and Al in the grain boundary were measured under the same conditions as in Examples 1 and 2. In addition, the content of Tb in the crystal grain positioned at the same depth from the base material surface as that at which the contents of Tb and the like in the grain boundary were measured was also measured by the EPMA device. For positions for measuring the content of Tb in the crystal grain, one place was designated from each one of the crystal grains different from one another, that is, 7 places in total were designated, and an average value in 5 places excluding 2 places showing the maximum and minimum Tb contents was determined. FIG. 3 shows an example in which positions (A) to (G) in the crystal grains of 7 places were designated based on a backscattered electron image of a sample, which was obtained by the EPMA device.

(41) TABLE-US-00005 TABLE 5 Amount of heavy R.sub.C.sup.H (R.sub.C.sup.H amount in rare earth element Composition of TbCuAl Mass ratio of each element amount in grain boundary) − R.sup.H contained in alloy [atomic %] in adhesion substance crystal Composition in grain (R.sub.C.sup.H amount in Sample base material Point in to base material [%] grain boundary [mass %] crystal grain) No. [mass %] FIG. 6 Tb Cu Al R.sup.H Cu Al [mass %] R.sub.C.sup.H Cu Al [mass %] 1 0.20 8 58 20 22 0.20 0.03 0.01 0.20 0.68 3.98 0.11 0.48 2 0.20 B 38 40 22 0.20 0.08 0.02 0.21 0.69 4.39 0.12 0.48 3 0.20 F 48 30 22 0.20 0.05 0.02 0.21 0.67 4.21 0.11 0.46 4 0.20 8 58 20 22 1.00 0.14 0.06 0.24 1.43 4.63 0.18 1.19 5 0.20 B 38 40 22 1.00 0.42 0.10 0.22 1.45 6.61 0.17 1.23 6 0.20 F 48 30 22 1.00 0.25 0.08 0.23 1.43 5.27 0.17 1.20 7 4.40 8 58 20 22 0.20 0.03 0.01 4.43 4.88 4.01 0.11 0.45 8 4.40 B 38 40 22 0.20 0.08 0.02 4.47 4.93 4.38 0.12 0.46 9 4.40 F 48 30 22 0.20 0.05 0.02 4.46 4.90 4.01 0.10 0.44 10 4.40 8 58 20 22 1.00 0.14 0.06 4.50 5.50 4.59 0.17 1.00 11 4.40 B 38 40 22 1.00 0.42 0.10 4.57 5.64 6.56 0.17 1.07 12 4.40 F 48 30 22 1.00 0.25 0.08 4.53 5.56 5.35 0.18 1.03 13 10.00 8 58 20 22 0.20 0.03 0.01 10.08 10.52 3.94 0.10 0.44 14 10.00 B 38 40 22 0.20 0.08 0.02 10.08 10.54 4.27 0.11 0.46 15 10.00 F 48 30 22 0.20 0.05 0.02 10.10 10.52 3.93 0.11 0.42 16 10.00 8 58 20 22 1.00 0.14 0.06 10.19 11.06 4.71 0.16 0.87 17 10.00 B 38 40 22 1.00 0.42 0.10 10.22 10.95 6.34 0.18 0.73 18 10.00 F 48 30 22 1.00 0.25 0.08 10.21 10.98 5.20 0.17 0.77

(42) From Table 5, it is known that the difference between the content of Tb in the grain boundary and the content of Tb in the crystal grain falls within a range of 0.40 to 1.25% by mass, regardless of the content of Tb in the base material, although the content of Tb in the grain boundary covers a wide range of 0.67 to 11.06% by mass, depending on the content of Tb in the base material. This range is the amount of Tb (R.sup.H) supplied to the grain boundaries appropriately in the effect of improving the coercive force due to the grain boundary diffusion treatment.

(43) The present application is based on Japanese patent application No. 2017-124954 filed on Jun. 27, 2017, and Japanese patent application No. 2018-092254 filed on May 11, 2018, and the contents of which are incorporated herein by reference.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

(44) 11 Base material 12 Adhesion substance (R.sup.HCuAl alloy-containing substance)