EP0517179B1

EP0517179B1 - Method of making two phase Rare Earth permanent magnets

Info

Publication number: EP0517179B1
Application number: EP92109366A
Authority: EP
Inventors: Matou Kusunoki; Takehisa Monowa; Masao Yoshikawa
Original assignee: Shin Etsu Chemical Co Ltd
Current assignee: Shin Etsu Chemical Co Ltd
Priority date: 1991-06-04
Filing date: 1992-06-03
Publication date: 1995-05-17
Anticipated expiration: 2012-06-03
Also published as: DE69202515D1; DE69202515T2; EP0517179A1

Description

BACKGROUND OF THE INVENTION

The present invention relates to a method of preparing a rare earth-based permanent magnet or, more particularly, to a method of preparing a rare earth-based permanent magnet having remarkably improved magnetic properties such as the residual magnetic flux density, coercive force, maximum energy product and the like.
Several methods of preparing rare earth-based permament magnets are known in the prior art and widely used in practical applications by virtue of their very excellent magnetic properties as compared with other non- rare earth permanent magnets. Among th various methods of preparing the rare earth-based permanent magnets, those preparing rare earth-based permanent magnets from a ternary alloy of neodymium, iron and boron are highlighted. The application fields of those magnets are expanding in recent years because they can be manufactured at a relatively low cost owing to the to the abundance of neodymium among the rare earth elements and omission of expensive cobalt, which is a principal constituent in the samarium-cobalt magnets developed in the early stage of the history of rare earth-based permanent magnets, while they still have excellent magnetic properties comparable with or even better than the magnetic properties of other types of rare earth-based permanent magnets. Extensive investigations have been undertaken and many proposals and attempts have been made for the improvement of the magnetic properties and stability of these neo-dymium-iron-boron based permanent magnets. For example, an excellent neodymium-iron-boron permanent magnet can be produced by the so-called two-alloy method in which, instead of the conventional powder-metallurgical method using a powder of the single alloy having the same composition as the magnet, two alloys having different compositions are prepared and the fine powders obtained by pulverizing the two alloys are mixed together in a specified proportion to give a powder mixture which is molded into a shape of the desired magnet in a magnetic field followed by sintering. The permanent magnet prepared by this two-alloy method sometimes has much better magnetic properties than the magnets prepared by the single-alloy method even when the overall chemical compositions of the magnets are the same.
The above mentioned two-alloy method can be classified into three types depending on the procedure by which the alloy powders before blending are prepared. In the method of the first type disclosed, for example, in Japanese Patent Kokai 63-93841, 63-252403, 63-278308, 1-108707, 1-146310, 1-146309 and 1-155603, either one or both of the two alloys are prepared by the so-called liquid-quenching method so that the alloy thus produced can be an amorphous or microcrystalline alloy. It is recently reported by E. Otuki, et al. in Eleventh International Workshop on Rare Earth Magnets, Pittsuburgh, Penn., October 1990, page 328 that the rare earth-based permanent magnet prepared by using this liquid-quenching method may have an extremely high maximum energy product of 50 MGOe or even larger.
In the second-type method disclosed in Japanese Patent Kokai 61-81603,61-81504,61-71605,61-81606, 61-81607, 61-119007, 61-207546, 63-245903 and 1-177335, two different alloys of a rare earth element R each having a chemical composition of the formula R₂Fe₁₄B as an intermetallic compound are prepared by modifying the kind and content of the rare earth element or elements, one being rich and the other being lean in the content of neodymium.
In the third-type method disclosed in Japanese Patent Kokai 60-230959, 61-263201, 62-181402, 62-182248, 62-206802, 62-70746, 63-6808, 63-104406, 63-114939, 63-272006, 1-111843 and 1-146308, one of the alloy powders having a chemical composition of the formula R₂Fe₁₄B, in which R is mainly neodymium, is mixed with a second powder prepared from a certain element or alloy of a low melting point or carbide, boride, hydride and the like of a rare earth element followed by the powder metallurgical process for the preparation of a magnet.
A magnet which is produced by this third-type method is also disclosed in WO-A-9 106 107. According to this document a R₂T₁₄B phase, in which Tone or more of Fe, Co or Ni and which has a high residual flux densitiy, and a low-melting RT- and/or RTB phase are used as starting makrials.
A further two alloy-method is disclosed in JP-63 127 505. The two alloys selected have different sintering temperatures. Therefore in the sintering step a crystal grows. This crystal consists of one alloy being the kernel and the second forming the peripheral part. The resulting permament magnet has a large maximum energy product and causes no substantial reduction of the maximum energy product.
The above described two-alloy methods of the prior art have their respective advantages and disadvantages so that none of the prior art methods are quite satisfactory from the practical standpoint of view. For example, the first-type method disclosed by Otuki has a problem that the coercive force of the permanent magnet obtained thereby cannot be high enough to rarely exceed 9 kOe, which is subject to a decrease as the temperature is increased, although a considerably large value of the maximum energy product can be obtained by the method.
Even when the problem relative to the low coercive force can somehow be solved at least at room temperature, a serious problem inherent in the liquid-quenching method is the poor susceptibility of the alloy particles to magnetic orientation because, as is mentioned above, the particles as prepared by the liquid-quenching method have an amorphous or microcrystalline structure exhibiting no magnetic anisotropy so that the otherwise effective molding method of the powder in a magnetic field is hardly effective to effect magnetic orientation of such non-anisotropic particles.
In the second-type method, an alloy phase rich in the content of neodymium and a phase having a formula of Ndl,xFe4B4 co-exist with the R₂Fe₁₄B compound in the magnetic alloy while these two phases are both non-magnetic at room temperature. Accordingly, the magnetic orientation of magnetic particles is greatly disturbed by the presence of such non-magnetic particles so that the magnetic properties of the thus prepared permanent magnet cannot be fully improved. This situation of disturbed magnetic orientation is similar also in the third-type method because the additive powder is also non-magnetic to cause an increase in the demagnetizing field with a consequent decrease in the effective strength of the magnetic field so as not to give full orientation of the magnetic particles along the direction of the magnetic field.
The third-type method using a low melting-point powder is based on an idea that the particles of the low melting-point phase in the powder mixture may have an effect of removing the nucleation sites, such as the lattice defects and the oxide phase, found on the grain boundary of the R₂Fe₁₄B compound during sintering to have an effect of cleaning of the grain boundaries leading to an increase in the coercive force. Presence of such a low melting-point phase in the powder mixture, however, is an adverse condition for the improvement of the magnetic properties of the permanent magnets to cancel the advantage. When the low melting-point phase has a melting point of, for example, around 660 °C, the melt of this phase would have a considerably decreased viscosity at the temperature of sintering which is usually at about 1100 °C so that the magnetic particles which have been oriented in the molding process in a magnetic field may float in the low-viscosity melt to cause random rotation resulting in shuffling of once magnetically oriented particles in the course of sintering which proceeds by the liquid-phase sintering to effect shrinkage of the molded body. In other words, it is important in the liquid-phase sintering of the neodymium-based magnet that the melt of the low melting-point phase in the sintering process has a viscosity high enough not to allow free rotation of the magnetic particles but low enough to give a fully densified structure of the sintered body with a full cleaning effect of the grain boundaries.
In short, the two-alloy methods of the prior art, so to say, have been established without detailed consideration of the balance between the magnetic orientation of the magnetic particles to which the liquid-phase constituent of the mixture pertains and the improvement in the coercive force of the magnet leading to adjustment of the magnetic properties and the melting point of the low melting-point phase to optimize the balance above mentioned.

SUMMARY OF THE INVENTION

The present invention accordingly has an object to provide a two-alloy method of preparing a rare earth-based permanent magnet having well-balanced magnetic properties. The two-alloy methods however have been established after a fundamental reconsideration of the above discussed problems in the prior art two-alloy methods relative to the composition of each of the alloy powders.
Thus, the method of preparing a rare earth-based permanent magnet provided by the invention comprises the steps of:

a) mixing a powder of a first magnetic alloy or intermetallic compound having a ternary composition substantially represented by the formula
in which R is a rare earth element which preferably is selected from the group consisiting of neodymium, parseodymium, dysprosium and terbium and T is iron or a combination of iron and cobalt in such a proportion that the amount of cobalt does not exceed 40% by weight of the total amount of iron and cobalt, and a powder of a second magnetic alloy or intermetallic compound having a composition substantially representd by the formula
in which R has the same meaning as defined above, M is an element selected from the group consisting of gallium, aluminum, copper, zinc, indium, silicon, phosphorus, sulfur, titanium, vanadium, chromium, manganese, germanium, zirconium, niobium, molybdenum, palladium, silver, cadmium, tin, antimony, hafnium, tantalum and tungsten, the subscript a is a positive number in the range from 15 to 40, b is zero or a positive number not exceeding 80, c is a positive number in the range from 5 to 85, d is zero or a positive number not exceeding 20 and e is zero or a positive number not exceeding 20 with the proviso that a+b+c+d+e is 100, in a weight proportion of the first magnetic alloy powder to the second magnetic alloy powder in the range from 99:1 to 70:30 to give a powder mixture;
(b) shaping the powder mixture into a form of magnet by compression molding in a magnetic field; and
(c) sintering the shaped form of the powder mixture by heating in vacuum or in an atmosphere of an inert gas.

The second alloy powder is characterized by the unique metallographic structure including, besides the R₂T₁₄B phase, at least one of the phases having a chemical composition of the formulas RT₄L, RT₃, RT₂, R₂T₇ and RT₅, in which R and T each have the same meaning as defined above and L is boron or a combination of boron and the element M, e.g., gallium.

DETAILLED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As is described above, the method of preparing a rare earth-based permanent magnet of the invention is a so-called two-alloy method in which powders of two kinds of magnetic alloys having a specified but different compositions are mixted together in a specified weight proportion and the powder mixture is subjected to shaping by molding in a magnetic field to give a green body which is sintered by heating in vacuum or in an atmos- pere of an inert gas such as argon followed, usually, by an aging treatment at a temperature substantially lower than the sintering temperature.
The first of two magnetic alloys, referred to as the Alloy I hereinafter, is basically a ternary alloy mainly consisting of the metallographic phase expressed by the formula R₂T_l4B. In this formula, R is a rare earth element including yttrium but preferably selected from the group consisting of neodymium, praseodymium, dysprosium and terbium although a limited portion thereof can be replaced with other rare earth elements including yttrium and the elements having an atomic number in the range from 57 to 71 inclusive. T in the formula is iron or a combination of at least 60% by weight of iron and 40% by weight or less of cobalt. Although cobalt is an optimal ingredient, the amount thereof in the combination of iron and cobalt, when added, should be at least 0,1% by weight or, preferably, at least 1% by weight in order that the advantage obtained by the combined use of cobalt can be fully exhibited. Namely, addition of cobalt to the Alloy I is effective in increasing the Curie point of the alloy and also increasing the corrosion resistance of the magnet.
The Alloy I is prepared by melting together each a specified and weighed amount of the respective ingredients R, i.e. one or more of the rare earth elements, T, i.e. iron or iron and cobalt, and boron each in a metallic or elementary form in vacuum or in an atmosphere of an inert gas which is preferably argon. It is optional to use a ferroboron as the source material of boron and a part of the iron source in combination with an appropriate amount of elementary iron or boron to adjust the iron to boron ratio depending on the composition of the ferroboron. Although the above mentioned source materials of the respective ingredients shoud have a purity as high as possible, small amounts of impurities usually unavoidable in industrial production of the materials may have no particular adverse influences. The melt of the alloy is cast in a metal mold and cooled to give an alloy ingot mainly consisting of the phase of R₂T_l4B. Since this phase is formed by the peritective reaction between the incipient phase of a-iron and a liquid phase rich in the content of the rare earth element, it would be a possible case that the ingot of the Alloy I contains small amounts of the remaining a-iron phase, a phase rich in the content of boron and/or a phase rich in the content of the rare earth element. When the fraction of these extraneous phases are too large, the alloy ingot is subjected to a solution treatment by heating at 700 to 1200 °C for at least 1 hour in vacuum or in an atmosphere of an inert gas so as to convert these phases into the phase of R₂T_l4B, the fraction of which should desirably be as large as possible.
The ingot of the Alloy I obtained in the above described manner is finely pulverized either by a wet process or dry process using a suitable pulverizing machine. Namely, the ingot is first crushed into coarse particles which are then finely pulverized. It is essential in each method of pulverization that surface oxidation of the alloy particles, which is highly reactive with atmospheric oxygen, should be avoided as far as possible. For example, the wet-process pulverization is performed in a non-reactive organic solvent such as fluorinated hydrocarbon solvents and the dry-process pulverization is performed in an atmosphere of an inert gas such as nitrogen. When fine pulverization of the coarse particles is conducted by using a jet mill, nitrogen is used as the jet gas. The powder of the Alloy I shoud have an average particle diameter in the range from 0.5 to 20 f..lm or, preferably, in the range from 1 to 10 µm. When the average particle diameter of the particles is too large, the powder mixture of the two magnetic alloys cannot be sintered to effect full densification while the average particle diameter should not be too small because a too fine powder is highly susceptible to the surface oxidation of the particles by the atmospheric oxygen to cause serious degradation of the magnetic properties of the magnets.
The second of the two magnetic alloys, referred to as the Alloy II hereinafter, is basically a five-component alloy having a composition represented by the formula R_aF_ObC_OeB_dM_e, in which R has the same meaning as defined above for the Alloy I and M is an element selected from the group consisting of gallium, aluminum, copper, zinc, indium, silicon, phosphorus, sulfur, titanium, vanadium, chromium, manganese, germanium, zirconium, niobium, molybdenum, palladium, silver, cadmium, tin, antimony, hafnium, tantalum and tungsten or, preferably, gallium.
The subscript a in the formula is a positive number in the range from 15 to 40 or, preferably, from 25 to 35, b is zero or a positive number not exceeding 80 or, preferably, a positive number in the range from 5 to 45, c is a positive number in the range from 5 to 85 or, preferably, in the range from 15 to 65, d is zero or a positive number not exceeding 20 or, preferably, a positive number in the range from 1 to 15 and e is zero or a positive number not exceeding 20 or, preferbly, not exceeding 10 with the proviso that the sum of the subscripts a+b+c+d+e is 100. When the value of the subscript a, i.e. the atomic fraction of the rare earth element, is too small, the deficiency in the content of the rare earth element provides no sufficient amount of the liquid phase in the sintering process so that the sintered body cannot be fully densified. When the value of the subscript a is too large, on the other hand, the melting point of the Alloy II would be too low to exhibit the desired effect for the improvement of the magnetic properties of the resulting permanent magnet. When the value of the subscript c, i.e. the atomic fraction of cobalt, is too small, none of the unique metallographic phases of the formulas RT₄L, RT₃, RT₂, R₂T₇ and RT₅ described before, which can be detected and identified by the combination of the techniques using a scanning electron microscope and electron microprobe analyzer equipped with a wavelength dispersion X-ray spectrometer, could be formed in the Alloy II so that no full improvement can be obtained in the magnetic properties of the permanent magnet. When the value of e, i.e. the atomic fraction of the element denoted by M or, in particular, gallium, is too large, the saturation magnetic flux den-sity of the alloy would be decreased not to impart high magnetic properties to the permanent magnet.
The method forthe preparation of an ingot of the Alloy II or a fine powder thereof is not different in principle from that for the preparation of an ingot of the Alloy I or a fine powder thereof described above. The requirement for the average particle diameter of the Alloy II powder is also about the same as for the Alloy I powder. It is of course that the liquid-quenching method is applicable also in this case. Namely, the thin belt of the alloy formed by quenching, which is crystallographically amorphous or microcrystalline, formed by the liquid-quenching method is subjected to a heat treatment at a temperature higher than the temperature of crystallization for a certain length of time so as to cause crystallization or growth by recrystallization resulting in the appearance of the characteristic phase or phases.
The metallographic phases contained in the Alloy II include, besides the phase of the formula R₂T_l4B, in which R and T each have the meaning as defined before, and a phase rich in the content of the rare earth element or elements containing at least 35 atomic % of the rare earth element or elements, which were also the constituents of the alloys used in the prior art two-alloy method or the rare earth-boron based magnetic alloys known in the prior art, at least one of the above mentioned five kinds of the unique metallographic phases which appear as an equilibrium phase in the Alloy II as a consequence of the high cobalt content of at least 5 atomic %.
Following are the presumable reasons for the advantages obtained as a consequence of the presence of these unique metallographic phases in the Alloy II. In the first place, it is pointed out that some of these characteristic phases have a Curie point higher than room temperature as a consequence of the content of cobalt. Further, some of these phases each have magnetic crystalline anisotropy in the direction of a particular crystallographic axis. When the powders of the Alloys I and II are mixed together and the powder mixture is brought in a magnetic field for molding, accordingly, a major portion of the particles has magnetic anisotropy and to give good magnetic orientation in alignment of the easy-magnetization axis to give excellent magnetic properties of the resulting sintered permanent magnet.
In the second place, it is advantageous that these phases each have a melting point in the range from 700 to 1155 °C, which is suitable for the liquid-phase sintering of the neodymium-containing rare earth-based permanent magnet prepared by the method of sintering. Namely, the above mentioned melting point is higher than the melting point of the phase rich in the content of neodymium, i.e. 500 to 650 °C, but lower than the melting point of the R₂Fe₁₄B phase which is 1155 °C. Accordingly, presence of these characteristic phases efficiently avoids the disadvantage that the once aligned particles in the magnetic field is disturbed as a consequence of the too low viscosity of the melt in the absence of these characteristic phases while the melting point thereof is low enough to give a good cleaning effect for the grain boundaries so as to contribute to an increase in the density of the sintered body.
In addition, the relatively high resistance of the Alloy II against oxidation is a consequence of the content of cobalt therein. While otherwise the Alloy II is more susceptible to oxidation than the Alloy I due to the higher content of the rare earth element or elements than the Alloy I, addition of cobalt to the Alloy II has an effect of compensating for the increase in the oxidation susceptibility so as to contribute to the stabilization of the magnetic properties of the magnet obtained therefrom by preventing degradation due to oxidation. When gallium is contained as the element M in the Alloy II, gallium is concentrated at the grain boundaries even after sintering to exhibit an effect of increasing the coercive force of the sintered magnet.
The powders of the Alloys I and II prepared in the above described manner are mixed together in a specified weight proportion as uniformly as possible. This mixing process is also conducted in an atmosphere of an inert gas such as nitrogen in order to minimize oxidation of the particle surface. Instead of mixing the finely pulverized powders of the alloys each having an average particle diameter of 0.5 to 20 µm, it is optional that each of the alloys is first crushed into coarse particles and the coarse particles of the respective alloys are mixed together in a specified weight proportion followed by fine pulverization. The mixing proportion of the powder of the Alloy I to the powder of the Alloy II is in the range from 99:1 to 70:30 by weight or, preferably, in the range from 95:5 to 85:15 by weight. When the content of the Alloy II is too small, the density of the sintered magnet cannot be fully increased so as not to give a high coercive force while, when the content thereof is too large, the non-magnetic phases would have some predominance in the sintered body so that the saturation magnetic flux density of the magnet would be decreased.
The powder mixture of the Alloys I and II is then shaped into a green body of a desired form of the magnet by compression molding in a magnetic field. The direction of the magnetic field is usually perpendicular to the direction of compression but can be parallel. Quite satisfactory results of magnetization molding can be obtained in a magnetic field of 15 kOe and under a compression force of 1000 kgf/cm² though not particularly limitative thereto.
The green body of the powder mixture as compression-molded is subjected to sintering by heating in vacuum or in an atmosphere of an inert gas such as argon at a temperature in the range from 900 to 1200 °C for a length of time of at least 30 minutes. The sintered body is usually subjected to an aging treatment by heating at a temperature substantially lower than the sintering temperature or, usually, in the range from 400 to 800 °C for 30 minutes or longer. Advantageously, the sinterign process of the green body of the mixed powders is so efficient that the thus sintered body usually has a high density of at least 95% of the true density, i.e. the density of an alloy ingot having the same chemical as the average composition of the mixed powders, so as to exhibit a high residual magnetic flux density.
In the following, the method of preparing rare earth-based permanent magnet of the invention is described in more detail by way of inventive examples and comparative examples making comparison between the two-alloy method and the single-alloy method. Namely, two Alloys I and II were prepared in each of the inventive examples and these alloys were processed in a weight ration of 85:15 to 95:5 into a sintered permanent magnet in the above described mannerwhile the permanent magnet in the comparative example having the same number as the inventive example was prepared from a single alloy ingot having the same chemical composition as the weighted average of the Alloys I and II in the corresponding inventive example.

Inventive Exeample 1 and Comparative Example 1.

An ingot of an alloy, referred to as the Alloy 1-1 hereinbelow, having a composition of the formula 12.5.Nd-6.0B-81.5Fe in the atomic proportion, was prepared in Inventive Example 1 by melting together neodymium metal and iron metal each having a purity of at least 99.9% by weight and a ferroboron containing about 20% by weight of boron, the balance being iron, each in a calculated and weighed amount to give a composition of the above given formula under an atmosphere of argon in a high-frequency induction-furnace followed by casting of the melt. The ingot was subjected to a solution treatment by heating at 1070 °C for 20 hours under an atmosphere of argon.
Separately, an ingot of another alloy, referred to as the Alloy 11-1 hereinbelow, was prepared in substantially the same manner as above from metals of neodymium, dysprosium, iron, gallium and cobalt each having a purity of at least 99.9% by weight and a ferroboron each in a calculated and weighed amount corresponding to the composition of the formula 20.0Nd-10.0Dy-20.0Fe-6.0B-4.0Ga-40.0Co.
The Alloys 1-1 and 11-1 were separately pulverized in an atmosphere of nitrogen each into a coarse powder having a particle size to pass a 30 mesh screen and these coarse powders of the Alloy 1-1 and Alloy 11-1 were taken and blended in a weight ratio of 90:10 taking 30 minutes in a V-mixer filled with nitrogen gas to replace air. The powder blend was then finely pulverized in a jet mill using high-pressure nitrogen as the jet gas into a fine powder mixture having an average particle diameter of about 5wm.
A metal mold was filled with the fine powder mixture obtained above, which was brought in a magnetic field of 15 kOe to effect magnetic orientation of the particles and compression-molded under a compressive pressure of about 1000 kgf/cm² into a shaped green body. This green body was subjected to sintering by heating at 1070 °C for 1 hour under an atmosphere of argon in a sintering furnace followed by an aging treatment by keeping for 1 hour at a temperature of 530°C into a sintered permanent magnet, which is referred to as the Magnet 1a a hereinbelow.
In Comparative Example 1, an ingot of a further alloy, referred to as the Alloy III-1, was prepared in substantially the same manner as above from the same lots of the metals of neodymium, dysprosium, iron, gallium and cobalt and ferroboron each taken in an amount corresponding to the composition of the formula 13.1 Nd-0.8Dy-3.2Co-6.0B-0.3Ga-76.6Fe in the atomic percentage, which was equivalent to the weighted average of the two formulas 12.5Nd-6.0B-81.5Fe for the Alloy 1-1 and 20.0Nd-1 0.0Dy-20.0Fe-6.0B-4.0Ga-40.0Co for the Alloy 11-1 combined in a weight ratio of 90:10. The ingot of the Alloy III-1 was pulverized into a fine powder in the same manner as in the pulverization of the Alloy 1-1 and 11-1 and the fine powder of the Alloy III-1 was processed singly into a permanent magnet, referred to as the Magnet 1 b hereinbelow, in the same manner as above.
The Magnets 1a and 1 b prepared above were subjected to the measurements of the density p in g/cm³ and the magnetic properties including residual magnetic flux density Br in kG, coercive force iHc in kOe and maximum energy product (BH)_max in MGOe to give the results shown in Tables 4 and 5, respectively, given below.

Inventive Examples 2 to 70 and Comparative Examples 2 to 70.

The experimental procedure in each of Inventive Examples 2 to 70, in which Magnets 2a to 70a, respectively, were prepared, was substantially the same as in Inventive Example 1 described above except that the compositions of the two types of the alloys and/or mixing proportion of the alloys were different. Namely, the first magnetic alloy was selected from the Alloys 1-1 to 1-18 indicated in Table 1 below and the second magnetic alloy was selected from the Alloys 11-1 to II-58 indicated in Table 2 below.
The experimental procedure in each of Comparative Examples 2 to 70, in which Magnets 2b to 70b, respectively, were prepared was substantially the same as in Comparative Example 1 described above except that each of the Magnets 2b to 70b was prepared from Alloys III-2 to III-70, respectively, while the composition of each of these alloys was the weighted average of the first-type alloy and the second-type alloy used in the Inventive Example having the same number. For example, the Alloy III-2 prepared and used in Comparative Example 2 for the Magnet 2b had a composition equivalent to the weighted average of the Alloy 1-1 and Alloy 11-2 prepared and used in Inventive Example 2 in a weight ratio of 90:10 for the Magnet 2a. The composition of these Alloys III-2 to III-70 is shown in Table 3 below.
The Magnets 2a to 70a and 2b to 70b were each subjected to the measurements of the density and magnetic properties to give the results respectively shown in Table 4 below, which also shows the mixing ratio of the two types of the alloys by weight in Inventive Examples 2 to 70, and in Table 5 below.

Claims

1. Method of preparing a permanent magnet comprising the steps of:

a) mixing a powder of a first magnetic alloy having a ternary composition substantially represented by the formula

in which R is a rare earth element and T is iron or a combination of iron and cobalt in such a proportion that the amount of cobalt does not exceed 40% by weight of the total amount of iron and cobalt, and a powder of a second magnetic alloy having a composition substantially represented by

in which R has the same meaning as defined above, M is an element selected from the group consisting of gallium, aluminium, copper, zinc, indium, silicon, phosophorus, sulfur, titanium, vanadium, chromium, manganese, germanium, zirconium, niobium, molybdenum, palladium, silver, cadmium, tin, antimony, hafnium, tantalum and tungsten, the subscript a is a positive number in the range from 15 to 40, b is zero or a positive numer not exceeding 80, c is a positive number in the range from 5 to 85, d is zero or a positive number not exceeding 20 and e is zero or a positive number not exceeding 20 with the proviso that a+b+c+d+e is 100, in a weight proportion of the first magnetic alloy powder to the second magnetic alloy powder in the range from 99:1 to 70:30 to give powder mixture;

b) shaping the powder mixture into a form of magnet by compression molding in a magnetic field; and

c) sintering the shaped form of the powder mixture by heating in vacuum or in an atmosphere of an innert gas.

2. Method of preparing a rare earth-based permanent magnet as claimed in claim 1 in which the element denoted by M is gallium.

3. Method of preparing a rare earth-based permanent magnet as claimed in claim 1 in which the element denoted by R is a rare earth element selected from the group consisting of neodymium, praseodymium, dysprosium and terbium.

4. Method of preparing a rare earth-based permament magnet as claimed in claim 3 in which the element denoted by R is neodymium.

5. Method of preparing a rare earth-based permanent magnet as claimed in claim 1 in which the subscript a is a positive number in the range from 25 to 35, b is a positive number in the range from 5 to 45, c is a positive number in the range from 15 to 65, d is a positive number in the range from 1 to 15 and e is zero or a positive number not exceeding 10 with the proviso that a+b+c+d+e is 100.

6. Method of preparing a earth-based permanent magnet as claimed in claim 1 in which the second magnetic alloy has a metallographic structure comprising the phase of R₂T_l4B and at least on of the formulas RT₄L,RT₃,RT₂,R₂T₇ and RT₅, in which R and T each have the same meaning as defined above and L is boron or a combination of boron and the element M, which has the same meaning as defined above.