PERMANENT MAGNETS

Abstract: A permanent magnet having high coercivity and energy product contains rare earth elements, boron, at least one element of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W, and a balance of Fe or Fe and Co, and consists of a primary phase of substantially tetragonal grain structure, or a mixture of such a primary phase and an amorphous or crystalline rare earth element-poor auxiliary phase wherein the volume ratio of auxiliary phase to primary phase is smaller than a specific value.

Description: CROSS-REFERENCE TO RELATED US APPLICATION This application is related to Yajima et al., Serial No. 038,195 filed April 14, 1987 for Permanent Magnet and Method of Producing Same. BACKGROUND OF THE INVENTION Field of the Invention This invention relates to high performance permanent magnets used in various electric appliances, and more particularly, to permanent magnets in the form of rapidly quenched alloy materials of Fe-R-B and Fe-Co-R-B systems wherein R is a rare earth element. Prior Art Typical of high performance rare earth magnets are Sm-Co magnets. They are mass produced by powder metallurgy and some exhibit a maximum energy product of as high as 32 MGOe. However, Sm and Co source materials are very expensive. Those rare earth elements having a relatively low atomic mass such as cerium, praseodymium, and neodymium are supplied in more plenty and thus less expensive than samarium. To take advantage of inexpensive iron, Nd-Fe-B magnets have been recently developed. Japanese Patent Application Kokai No. 59-46008 describes sintered Nd-Fe-B magnets, and Japanese Patent Application Kokai No. 60-9852 describes rapid quenching of such magnets. The conventional powder metallurgy process for the manufacture of Sm-Co magnets can be applied to the manufacture of sintered Nd-Fe-B magnets at the sacrifice of the advantage of using inexpensive source materials. The powder metallurgy process includes a step of finely dividing a Nd-Fe alloy ingot to a size of from about 2 to about 10 µm. This step is difficult to carry out because the Nd-Fe alloy ingot is readily oxidizable. In addition, the powder metallurgy process requires a number of steps including melting, casting, rough crushing of ingot, fine crushing, pressing, and sintering until a magnet is completed. On the other hand, the rapid quenching process is advantageous in that a magnet can be produced by a rather simple process without a fine pulverizing step. The rapid quenching process requires a smaller number of steps including melting, rapid quenching, rough crushing, and cold or hot pressing until a magnet is completed. Nevertheless, coercive force, energy product, and magnetizing behavior must be improved as well as cost reduction before rapidly quenched magnets can be commercially acceptable. Among the properties of rare earth element-iron-boron permanent magnets, coercivity is sensitive to temperature. Rare earth element-cobalt magnets have a temperature coefficient of coercive force (iHc) of 0.15%/°C, whereas rare earth element-iron-boron magnets have a temperature coefficient of coercive force (iHc) of 0.6 to 0.7%/°C, which is at least four times higher than the former. The rare earth element-iron-boron magnets have the likelihood of demagnetizing with an increasing temperature, limiting the design of a magnetic circuit to which the magnets are applicable. In addition, this type of magnet cannot be incorproated in parts which are mounted in an engine room of automobiles used in the tropics. As is known in the prior art, a high temperature coefficient of coercive force creates a bar when it is desired to commercially use rare earth element-iron-boron permanent magnets. There is a need for development of a magnet having a great magnitude of coercive force (see Nikkei New Material, 4-28, No. 9 (1986), page 80). Japanese Patent Application Kokai No. 60-9852 or Croat, EPA 0108474 describes how to impart high values of coercive force (iHc) and energy product to R-Fe-B alloy by rapid quenching. The composition is claimed as comprising at least 10% of rare earth element of Nd or Pr, 0.5 to 10% of B, and a balance of Fe. It was believed that the outstanding magnetic properties of R-Fe-B alloy were attributable to the Nd₂Fe₁₄B compound-phase. Accordingly, regardless of whether the method is by sintering or by rapid cooling, most prior art proposals for improving magnetic properties were based on experiments using materials having a composition in proximity to the above compound, i.e., 12-17% of R and 5-8% of B (see Japanese Patent Application Kokai Nos. 59-89401, 60-144906, 61-79749, 57-141901, and 61-73861). Since the rare earth elements are expensive, it is desired to reduce their content as low as possible. Unfortunately, coercive force (iHc) is dramatically reduced at a rare earth element content of less than 12%. As indicated in FIGS. 11 and 12 of EPA 0108474, iHc is reduced to 6 kOe or less at a rare earth element content of 10% or less. Although it is known for R-Fe-B alloys that coercivity is reduced at a rare earth element content of less than 12%, no method is known for controlling the composition and structure of an R-Fe-B alloy so as to optimize magnetic properties while preventing coercivity from decreasing. Although Nd₂Fe₁₄B compound is used as the basic compound in both the sintering method and the rapid quenching method, the magnets produced by these methods are not only different in the production method, but also belong to essentially different types of magnet with respect to alloy structure and coercivity-generating mechanism, as described in Oyobuturi (Applied Physics), Vol. 55, No. 2 (1986), page 121. More particularly, the sintered R-Fe-B magnet has a grain size of approximately 10 µm and is of the nucleation type as observed with SmCo₅ magnet in which coercivity depends on the nucleation of inverse magnetic domains, if compared to conventional SmCo magnets. On the contrary, the rapidly quenched magnet is of the pinning type as observed with Sm₂Co₁₇ magnet in which coercivity depends on the pinning of magnetic domain walls due to the extremely fine structure of fine particles of from 0.01 to 1 µm in size being surrounded by an amorphous phase which is richer in Nd than Nd₂Fe₁₄B compound (see J. Appl. Phys., 62(3), Vol. 1 (1987), pages 967-971). Thus any approach for improving the properties of these two types of magnets must first take into account the difference of coercivity-generating mechanism. We have proposed in Japanese Patent Application No. 62-90709 a permanent magnet having a composition of R x T (100-x-y-z) B y M z wherein 5.5 ≦ x ≦ 20.0 and R, T, y and z have the same meanings as defined in the present disclosure, having a fine crystalline phase or a mixture of a fine crystalline phase and an amorphous phase. This magnet is still not fully satisfactory. SUMMARY OF THE INVENTION An object of the present invention is to provide a permanent magnet exhibiting a high coercive force, a high energy product, improved magnetization, high corrosion resistance, and stable performance, thus finding commercial use. According to a first aspect of the present invention, there is provided a permanent magnet formed from a magnetically hard material having a composition represented by the formula: R x T (100-x-y-z) B y M z wherein R is at least one member selected from the rare earth elements including Y, T is Fe or a mixture of Fe and Co, B is boron, M is at least one member selected from the group consisting of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W, 5.5 ≦ x < 11.76, 2 ≦ y < 15, and z ≦ 10, and consisting of a primary phase of substantially tetragonal grain structure, or a primary phase of substantially tetragonal grain structure and at least one auxiliary phase selected from amorphous and crystalline R-poor auxiliary phases. In the latter case where the permanent magnet consists of primary and auxiliary phases, the volume ratio of auxiliary phase to primary phase, v, is smaller than the value given by the formula: [0.1176(100 - z) - x]/x. According to a second aspect of the present invention, there is provided a permanent magnet formed from a magnetically hard material having a composition represented by the formula: R x T (100-x-y-z) B y M z wherein R is at least one member selected from the rare earth elements including Y, T is Fe or a mixture of Fe and Co, B is boron, M is a mixture of at least one member selected from the group consisting of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W and at least one member selected from the group consisting of Cu, Ni, Mn and Ag, 5.5 ≦ x < 11.76, 2 ≦ y < 15, and z ≦ 10, and consisting of a primary phase of substantially tetragonal grain structure, or a primary phase of substantially tetragonal grain structure and at least one auxiliary phase selected from amorphous and crystalline R-poor auxiliary phases. In the latter case where the permanent magnet consists of primary and auxiliary phases, the volume ratio of auxiliary phase to primary phase, v, is smaller than the value given by the formula: [0.1176(100 - z) - x]/x. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features, and advantages of the present invention will be more readily understood from the following description when taken in conjunction with the accompanying drawings, in which FIG. 1 is a ternary diagram showing the composition of the permanent magnet according to the present invention; FIGS. 2 and 3 are electron photomicrographs of X50,000 and X200,000 showing the grain structure of permanent magnet sample No. 3 of Example 1; FIG. 4 is a X-ray diffraction diagram of permanent magnet sample No. 3 of Example 1; and FIG. 5 is a diagram showing the lattice constant of a permanent magnet of Example 8 as a function of the composition of its primary phase. DETAILED DESCRIPTION OF THE INVENTION Briefly stated, the permanent magnet according to the present invention has a composition represented by the formula: R x T (100-x-y-z) B y M z wherein R is at least one member selected from the rare earth elements including Y, T is Fe or a mixture of Fe and Co, B is boron, M is at least one member selected from the group consisting of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W, or a mixture of at least one member selected from the group consisting of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W and at least one member selected from the group consisting of Cu, Ni, Mn and Ag, 5.5 ≦ x < 11.76, 2 ≦ y < 15, and z ≦10. More particularly, R is at least one member selected from the rare earth elements including yttrium (Y). In the above-defined composition, the quantity x of rare earth element R ranges from 5.5 to less than 11.76. With x of less than 5.5, the magnet tends to show a low coercive force iHc. With x of 11.76 or higher, remanence Br is drastically lowered. Better results are obtained when x ranges from 5.5 to 11. Preferably, R is represented by the formula: R′ a (Ce b La 1-b ) 1-a wherein R′ is at least one member selected from the rare earth elements including yttrium (Y), but excluding cerium (Ce) and lanthanum (La), 0.80 ≦ a ≦1.00 and 0 ≦ b ≦ 1. When the value of (1-a) exceeds 0.2, maximum energy product becomes lower. R′ may further contain samarium (Sm) provided that the quantity of samarium is less than 20% of the quantity x of rare earth element R. Otherwise there results a low anisotropic constant. Most preferably, R is selected from neodymium (Nd), praseodymium (Pr), dysprosium (Dy), and mixtures thereof. The quantity y of boron B ranges from 2 to less than 15. Coercive force iHc is low with a value of y of less than 2, whereas remanence Br is low with a value of y of 15 or higher. Better results are obtained when y ranges from 2 to 14. T may be either iron (Fe) alone or a mixture of iron (Fe) and cobalt (Co). Partial replacement of Fe by Co improves the magnetic performance and Curie temperature of the magnet. Provided that T is represented by Fe 1-c Co c , the replacement quantity c should preferably range from 0 to 0.7 because coercive force becomes low with a value of c in excess of 0.7. M is at least one member selected from the group consisting of titanium (Ti), vanadium (V), chromium (Cr), zirconium (Zr), niobium (Nb), molybdenum (Mo), hafnium (Hf), thallium (Ta), and tungsten (W). Since the addition of element M controls grain growth, the coercive force of a magnet is maintained high even when it is processed at high temperatures for a long time. Part of element M may be replaced by at least one member selected from the group consisting of copper (Cu), nickel (Ni), manganese (Mn), and silver (Ag). The addition of Cu, Ni, Mn or Ag facilitates the plastic processing of magnet material without deteriorating the magnetic properties thereof. The quantity z of element M should be up to 10 because magnetization is drastically reduced with a value of z in excess of 10. A value of z of at least 0.1 is preferred to increase coercive force iHc. A value of z of at least 0.5, especially at least 1, more especially at least 1.8 is preferred to increase corrosion resistance. The addition of more than one element M is more effective in increasing coercive force iHc than the addition of element M alone. When a mixture of two or more elements M is added, the maximum quantity of the elements combined is 10% as described above. Element M will be described in more detail. Assumed that M1 represents at least one member selected from the group consisting of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W and M2 represents at least one member selected from the group consisting of Cu, Ni, Mn and Ag, the ratio of M1:M2 preferably ranges from 2:1 to 10:1, more preferably from 3:1 to 5:1. Within this range, the plastic processability of magnet material is improved without sacrificing remanence and coercive force. When up to 50% of B is replaced by Si, C, Ga, Al, P, N, Se, S, Ge, In, Sn, Sb, Te, Tl, Pb or Bi, or a mixture thereof, there is available an effect similar to the addition of B alone. To obtain a magnet having a high coercive force, it is preferred that x range from 7 to 11, more preferably from 8 to 10, y range from 2 to less than 15, more preferably from 4 to 12, most preferably from 4 to 10, c range from 0 to 0.7, more preferably from 0 to 0.6, and z range from 0.1 to 10, more preferably from 2 to 10. To obtain an isotropic magnet having a high energy product, it is preferred that x range up to less than 11, more preferably up to less than 10, y range from 2 to less than 15, more preferably from, 4 to 12, most preferably from 4 to 10, c range from 0 to 0.7, more preferably from 0 to 0.6, and z range from more than 0 to 10, more preferably from 2 to 10. To obtain an isotropic, readily magnetizable magnet having a high energy product, it is preferred that x range from 6 to 11, more preferably from 6 to less than 10, y range from 2 to less than 15, more preferably from 4 to 12, most preferably from 4 to 10, c range from 0 to 0.7, more preferably from 0 to 0.6, and z range from more than 0 to 10, more preferably from 2 to 10. To obtain an anisotropic magnet having a high energy product, it is preferred that x range from 6 to 11.76, more preferably from 6 to less than 10, y range from 2 to less than 15, more preferably from 4 to 12, most preferably from 4 to 10, c range from 0 to 0.7, more preferably from 0 to 0.6, and z range from more than 0 to 10, more preferably from 2 to 10. The composition of the magnet may be readily determined by atomic-absorption spectroscopy, fluorescent X-ray spectroscopy or gas analysis. The permanent magnet of the present invention consists of a primary or major phase of substantially tetragonal grain structure, or a primary or major phase of substantially tetragonal grain structure and at least one auxiliary or minor phase selected from amorphous and crystalline R-poor auxiliary phases. In the latter case where the permanent magnet consists of primary and auxiliary phases, the volume ratio of auxiliary phase to primary phase, v, is smaller than the stoichiometric ratio of auxiliary phase to primary phase occurring upon quasi-static cooling of a melt having the same composition which is given by the formula: [0.1176(100 - z) - x]/x. The volume ratio of auxiliary phase to primary phase, v, may be determined by an observation under an electron microscope. More particularly, the volume ratio is determined by observing a sample under a scanning electron microscope with a magnifying power of X10,000 to X200,000, sampling out about 5 to 10 visual fields at random, subjecting them to image information processing, separating primary phase areas from auxiliary phase areas in terms of gradation, and calculating the ratio of the areas. FIGS. 2 and 3 are scanning electron photomicrographs of a sample with a magnification of X50,000 and X200,000, respectively, which are used for the purpose. The stoichiometric ratio of auxiliary phase to primary phase may be derived as follows. Among R-T-B compounds, a stable tetragonal compound is represented by R₂T₁₄B wherein R = 11.76 at%, T = 82.36 at%, and B = 5.88 at%. According to the present invention, the primary phase has a substantially tetragonal grain structure and the auxiliary phase has a R-poor composition. FIG. 1 shows a ternary phase diagram of an R-T-B system in which R₂T₁₄B is designated at R (11.76, 82.36, 5.88). The area defined and surrounded by ABCD in the diagram of FIG. 1 is the range of R-T-B composition of the magnet material according to the present invention excluding element M. It is now assumed in the ternary diagram of FIG. 1 that a composition falling within the scope of the present invention is designated at point Q having coordinates, R = 100x/(100 - z), B = 100y/(100 - z), and T = 100(100 - x - y - z)/(100 - z) . When a melt having the composition of point Q is quasi-statically cooled from the melting point, the melt is separated into two phases, R (R₂T₁₄B) and P (T). For stoichiometric calculation, the atomic ratio of T/R₂T₁₄B is equal to QR/PQ. Then, QR/PQ is calculated as follows. QR/PQ = Q′R′/PQ′ = [11.76 - 100x/(100-z)]/[100x/(100 - z)] = [0.1176(100 - z) - x]/x . According to the present invention, the auxiliary-to-primary phase ratio v ranges from 0 to the value given by [0.1176(100 - z) - x]/x, that is, 0 ≦ v < [0.1176(100 - z) - x]/x. The auxiliary-to-primary phase ratio v is limited to this range because (B.H)max is reduced and iHc is markedly reduced if v exceeds the value given by [0.1176(100 - z) - x]/x. The quotient A of auxiliary-to-primary phase ratio v divided by [0.1176(100 - z) - x]/x preferably ranges from 0.15 to 0.95, more preferalby from 0.3 to 0.8. When quotient A has a value of from 0.15 to 0.95, not only coercive force iHc and remanence are stable and high, but also squareness ratio Hk/iHc is increased. As a result, maximum energy product (BH)max is further increased. Quotient A may be controlled to fall within the range by rapidly quenching magnet material. Preferred rapid quenching is melt spinning as will be later described in detail. Usually single roll melt spinning is employed. More specifically, the circumferential speed of a rotating chill roll is controlled to 2 to 50 m/sec., more preferably to 5 to 20 m/sec. There is some likelihood that at a circumferential speed of less than 2 m/sec., most of the resulting thin ribbon has crystallized to an average grain size as large as at least 3 µm. The value of quotient A becomes too high at a circumferential speed of more than 50 m/sec. Better properties including higher values of coercive force and energy product are achieved by controlling the circumferential speed within the preferred range. According to the present invention, it is also possible to first control the value of quotient A to the range of from 0.2 to 1.2 by rapid quenching and thereafter to the range of from 0.15 to 0.95 by a heat treatment. In this case, the circumferential speed of a rotating chill roll used in single roll melt spinning is controlled to 10 to 70 m/sec., more preferably to 20 to 50 m/sec. There is some likelihood that at a circumferential speed of less than 10 m/sec., most of the resulting thin ribbon has crystallized to such an extent that no crystallization or crystal growth of amorphous portions is necessary in the subsequent heat treatment. The value of quotient A becomes too high at a circumferential speed of more than 70 m/sec. The heat treatment used herein may be annealing in an inert atmosphere or vacuum at a temperature of from 400 to 850°C for about 0.01 to about 100 hours. The inert atmosphere or vacuum is used in the heat treatment to prevent oxidation of the ribbon. No crystallization or crystal growth takes place at a temperature of lower than 400°C whereas quotient A will have a value of more than 1 at a temperature of higher than 850°C. Shorter than 0.01 hour of heat treatment will be less effective whereas longer than 100 hours of heat treatment achieves no further improvement and is only an economic waste. The present invention does not necessarily require heat treatment as described above. The embodiment of the present invention which does not require heat treatment is more simple. In one embodiment, the permanent magnet of the present invention consists of a primary phase having a substantially tetragonal grain structure. This primary phase is a metastable R₂T₁₄B phase with which M forms an oversaturated solid solution and which preferably has an average grain size of 0.01 to 3 µm, more preferably 0.01 to 1 µm, most preferably at least 0.01 to less than 0.3 µm. The grain size is preferably chosen in this range because grains with a size of less than 0.01 µm are incomplete and produce little coercive force iHc whereas the coercive force and squareness is rather reduced with grains having a size of more than 3 µm. In a preferred embodiment, the permanent magnet of the present invention consists of a primary phase as defined above and at least one auxiliary phase selected from amorphous and crystalline R-poor auxiliary phases. The auxiliary phase is present as a grain boundary layer around the primary phase. The R-poor auxiliary phase includes amorphous and crystalline phases of α-Fe, Fe-M-B, Fe-B, Fe-M and M-B systems. It is preferred that the R content of the auxiliary phase is preferably up to 9/10, more preferably up to 2/3, especially , from 0 to 2/3 of that of the primary phase in atomic ratio. Most preferably, the atomic ratio of R content of the auxiliary phase to the primary phase is up to 1/2, especially from more than 0 to 1/2. Beyond the upper limit of 2/3, despite an increase of coercive force, remanence and hence, maximum energy product are lowered. The composition of the primary and auxiliary phases may be determined by a transmission type analytic electron microscope. It sometimes occurs that an auxiliary phase has smaller dimensions than the diameter of an electron radiation beam which normally ranges from 5 to 20 nm. In such a case, the influence of ingredients of the primary phase must be taken into account. The auxiliary phase has the following contents of the elements other than R. Expressed in atomic ratio, the content of T is 0 ≦ T ≦ 100, more preferably 0 < T < 100, most preferably 20 ≦ T ≦ 90, the content of boron B is 0 ≦ B ≦ 60, more preferably 0 < B ≦ 60, most preferably 10 ≦ B ≦ 50, and the content of M is 0 ≦ M ≦ 50, more preferably 0 < M ≦ 50, most preferably 10 ≦ M ≦ 40. Within this composition range, magnetic properties including coercive force iHc, remanence Br and maximum energy product (BH)max are improved. To increase the coercive force of magnet material, the content of T in the auxiliary phase is 0 ≦ T ≦ 60, more preferably 0 < T ≦ 60, most preferably 10 ≦ T ≦ 50, the content of B is 10 ≦ B ≦ 60, more preferably 20 ≦ B ≦ 50, and the content of M is 10 ≦ M ≦ 50, more preferably 20 ≦ M ≦ 40. To increase the remanence of magnet material, the content of T in the auxiliary phase is 60 ≦ T < 100, more preferably 70 ≦ T ≦ 90, the content of B is 0 < B ≦ 30, more preferably 0 < B ≦ 20, and the content of M is 0 < M ≦ 30, more preferably 0 < M ≦ 20. In this embodiment, the primary phase preferably has a content of R and M combined of from about 11 to about 13 atom%, more preferably from about 11 to about 12 atom%. Outside this range, it is difficult for the primary phase to maintain a tetragonal structure. It is preferred that the primary phase has a content of R of from 6 to 11.76 atom%, more preferably from 8 to 11.76 atom%. Coercive force is substantially reduced with an R content of less than 6 atom% whereas an R content of more than 11.76 atom% results in a reduction of remanence and maximum energy product despite an increase of coercive force. It is preferred that the content of T in the primary phase is 80 ≦ T ≦ 85, more preferably 82 ≦ T ≦ 83 and the content of B is 4 ≦ B ≦ 7, more preferably 5 ≦ B ≦ 6. Within this range, a magnet having a high energy product is obtained in spite of a low content of rare earth element. The composition of the primary and auxiliary phases may be determined by a transmission type analytic electron microscope. The auxiliary phase constituting a grain boundary layer preferably has an average width of up to 0.3 µm, more preferably from 0.001 to 0.2 µm. A grain boundary layer having a width of more than 0.3 µm results in a low coercive force iHc. The permanent magnet of the present invention is generally prepared by the so-called melt spinning method, that is, by quenching and solidifying molten Fe-R-B or Fe-Co-R-B alloy having a composition within the above-defined range at a high cooling rate. The melt spinning method is by ejecting molten alloy through a nozzle onto the surface of a rotary metal chill roll cooled with water or another coolant, obtaining a magnet material in ribbon form. Melt spinning may be carried out with a disk, a single roll or double rolls. Most preferred for the present invention is a single roll melt spinning method comprising ejecting molten alloy onto the circumferential surface of a rotating single roll. A magnet having a coercive force iHc of up to about 20,000 Oe and a magnetization σ of 65 to 150 emu/gr may be prepared by rapidly quenching and solidifying molten alloy of the above-defined composition by the single roll melt spinning method while controlling the circumferential speed of the roll within the above-defined range. In addition to the melt spinning method using a roll, various other rapid quenching methods including atomizing and spraying and a mechanical alloying method may also be applied to the present invention. The magnets thus prepared have a good temperature coefficient of their magnetic properties. More particularly, the magnets have the following coefficients of remanence (Br) and coercive force (iHc) with temperature (T): dBr/dT = -0.09 to -0.06%/°C diHc/dT = -0.48 to -0.31%/°C over the temperature range of 20°C ≦ T ≦ 120°C, for example. Since a very fine grained crystalline structure or a structure consisting of a very fine grained crystalline primary phase and a crystalline and/or amorphous auxiliary phase is formed by quenching and solidifying directly from a molten alloy, the resulting magnet exhibits excellent magnetic properties as described above. A thin film obtained in ribbon form generally has a thickness of about 20 to about 80 µm. It is preferred to form a ribbon to a thickness of from 30 to 60 µm, more preferably from 40 to 50 µm, because the distribution of grain size in film thickness direction and hence, the variation of magnetic properties due to varying grain size is minimized. Then the average values of magnetic properties are increased. The structure obtained after quenching, which will vary with quenching conditions, consists of a fine grained crystal structure or a mixture of a fine grained crystal structure and an amorphous structure. If desired, this fine crystalline or fine crystalline-amorphous structure as well as its size may be further controlled so as to provide more improved properties by a subsequent heat treatment or annealing. The magnet which is quenched and frozen by the melt spinning method may be heat treated or annealed as described above. The annealing heat treatment is effective for the quenched magnet of the composition defined by the present invention to more closely fulfil the above-mentioned requirements and to exhibit more stable properties more consistently. A compacted magnet or a bonded magnet may be prepared from the quenched magnet in ribbon form. A bulk magnet having a high density may be prepared by pulverizing a ribbon magnet, preferably to a particle size of about 30 to 500 µm, and cold or hot pressing the resulting powder into a compact of a suitable density. A bonded magnet may be obtained from the permanent magnet of the present invention by a powder bonding method. More particularly, a ribbon magnet obtained by the melt spinning method or a powder thereof is annealed and again pulverized if desired, and then mixed with a resinous binder or another suitable binder. The mixture of magnet powder and binder is then compacted into a bonded magnet. Well-known isotropic bonded magnets have a maximum energy product of at most about 10 MGOe (megaGauss Oersted). In contrast, a bonded magnet having a maximum energy product of more than 10 MGOe can be produced according to the present invention by controlling the manufacturing parameters such that the magnet has a quotient A of less than 1, more preferably from 0.15 to 0.95 and a density cf more than 6 g/cm³. Ribbon magnets obtained by the melt spinning method are disclosed in Japanese Patent Application Kokai No. 59-211549 as well as bulk magnets obtained by compacting pulverized ribbon powder and bonded magnets obtained by compacting pulverized ribbon powder with binder. In order to magnetize conventional magnets to saturation magnetization, a magnetizing field of as high as 40 kOe to 110 kOe must be applied as described in J.A.P., 60(10), vol. 15 (1986), page 3685. In contrast, the magnet alloys of the present invention containing Zr, Ti or another element M have an advantage that they can be magnetized to saturation magnetization by applying a magnetizing field of 15 kOe to 20 kOe. Differently stated, the magnets of the present invention show significantly improved magnetic properties after magnetization under a field of 15 to 20 kOe. Plastic processing of ribbon magnet obtained by the melt spinning method or magnet powder obtained by pulverizing ribbon magnet will result in an anisotropic magnet having a higher density whose magnetic properties are improved by a factor of two or three. The temperature and time conditions under which plastic processing is carried out should be chosen so as to establish a finely crystalline phase as described in conjunction with annealing while preventing the formation of coarse grains. In this respect, the inclusion of additive element M such as Nb, Zr, Ti and V has an advantage of mitigating hot plastic processing conditions. Since additive element M controls grain growth during hot plastic processing, the magnet can maintain a high coercive force even after an extended period of processing at elevated temperatures. Plastic processing may include hot pressing, extrusion, rolling, swaging, and forging. Hot pressing and extrusion will give optimum magnetic properties. Hot pressing is preferably carried out at a temperature of 550 to 1,100°C under a pressure of 200 to 5,000 kg/cm². Primary hot pressing will suffice although primary hot pressing followed by secondary hot pressing will further improve magnetic properties. Extrusion molding is preferably carried out at a temperature of 500 to 1,100°C under a pressure of 400 to 20,000 kg/cm². The magnet which is rendered anisotropic by such plastic processing may also be used in the form of bonded magnet. In the practice of the present invention, not only the melt spinning method is used, but a hot processing method such as hot pressing may also be used insofar as processing conditions are selected so as to achieve grain size control. The magnet of the present invention can be readily prepared by hot pressing because the inclusion of element M dulls the sensitivity in grain growth of the magnet to temperature and time conditions. Since a permanent magnet is prepared by rapid quenching according to the present invention, the magnet may include not only an equilibrium phase, but also a non-equilibrium phase. Even when the magnet has an R content as low as from 5.5 atom% to less than 11.76 atom% and is isotropic, it shows high values of coercivity and energy product. It is a practical high performance permanent magnet. In an embodiment wherein R is Nd, the addition of element M contributes particularly to an increase of coercivity when the Nd content is at least 10 atom%, and to an increase of maximum energy product (BH)max when the Nd content is reduced to less than 10 atom% for cost reduction purpose. Additive element M greatly contributes to coercivity improvement. This tendency is observed not only with Nd, but also with the other rare earth elements. The coercivity of the present magnet is increased because its coercivity-generating mechanism relies on a finely crystalline structure having as major phase a metastable R₂Fe₁₄B phase with which element M forms an oversaturated solid solution when the R content is within the scope of the present invention, particularly less than 10 atom%, as opposed to the coercivity-generating mechanism relying on stable tetragonal R₂Fe₁₄B compound which is observed with conventional R-Fe-B magnets. In general, up to about 2 atom% of element M can form a stable solid solution at elevated temperatures. Only rapid quenching enables more than 2 atom% of element M to form a solid solution in which element M is kept metastable. For this reason, additive element M stabilizes R₂Fe₁₄B phase even with a low R content. This stabilizing effect is available only by rapid quenching, but not available in sintered magnets. Preferably, the permanent magnet of the present invention consists of a finely crystalline primary phase and a crystalline and/or amorphous R-poor auxiliary phase. The auxiliary phase serves as a boundary layer to provide pinning sites, reinforcing the bonding between primary grains. The permanent magnet of the present invention is readily magnetizable and fully resistant to corrosion. Conventional R-T-B magnets need careful rust prevention because they contain a corrodible B-rich phase or R-rich phase or both in addition to R₂T₁₄B phase. In contrast, the permanent magnets of the present invention need little or simple rust prevention because they are composed of a primary phase consisting essentially of R₂T₁₄B and an R-poor auxiliary phase and are thus well resistant to corrosion. EXAMPLES In order that those skilled in the art will better understand the practice of the present invention, examples of the present invention are given below by way of illustration and not by way of limitation. Example 1 An alloy having a composition: 10.5Nd-6B-3Zr-1Mn-bal.Fe (designated Composition 1, hereinafter, figures represent atomic percents) was prepared by arc melting. A ribbon of 30 to 60 µm thick was formed from the alloy by melt spinning. More particularly, argon gas was applied to the molten alloy under a pressure of 0.2 to 2 kg/cm² to eject the melt through a quartz nozzle onto the surface of a chill roll rotating at a varying speed of from 10 to 30 m/sec. The melt was quenched and solidified in ribbon form. A series of samples were prepared as shown in Table 1. The volume of axiliary phase in each sample shown in Table 1 was controlled by varying a quenching parameter, that is, the rotational speed of the chill roll. The magnetic properties of each sample measured are reported in Table 1. Sample No. 3 in ribbon form was cut in a transverse direction. The fracture section was electrolytically polished and observed under a scanning electron microscope (SEM). FIGS. 2 and 3 are photomicrographs of magnification X50,000 and X200,000, respectively. The presence of an auxiliary phase is clearly observed in the photomicrographs. SEM images were taken for the remaining samples. The average grain size of the primary phase and the average thickness of the grain boundary layer that the auxiliary phase formed were determined. The results are shown in Table 1. Sample No. 3 was analyzed by X-ray diffractometry, with the result shown in FIG. 4. FIG. 4 indicates that the primary phase consists of R₂Fe₁₄B and the auxiliary phase is amorphous. The SEM images were subjected to image information processing to determine the auxiliary-to-primary phase ratio, v. The value of quotient A was calculated by dividing the auxiliary-to-primary phase ratio, v by the stoichiometric ratio given by the formula: [0.1176(100 -z) - x]/x. The measurements are shown in Table 1. For sample Nos. 2 and 4, the composition of the primary and auxiliary phases, the content (R1) of R in the auxiliary phase, and the content (R2) of R in the primary phase were determined using a transmission type analytic electron microscope. The composition and ratio R1/R2 are shown in Table 2. A series of samples having each of the following compositions were prepared by the same procedure as used in Composition 1 while varying the volume of the auxiliary phase. Equivalent results were obtained. Composition (atomic percent) 10.5Nd-6B-3Nb-1Ti-bal.Fe 10Nd-0.5Pr-6B-2.5Zr-1V-bal.Fe 10.5Nd-5B-10Co-3Nb-1Ti-bal.Fe 10.5Nd-5B-1Ti-1Mo-bal.Fe 10.5Nd-5B-1Ti-1W-bal.Fe 10.5Nd-5B-1Ti-1Mo-7Co-bal.Fe 10.5Nd-5B-1Ti-1W-7Co-bal.Fe 11Nd-6B-2Nb-1Ni-bal.Fe 10.5Nd-6B-3Zr-0.5Cr-bal.Fe 10.5Nd-6B-3Zr-1Ti-10Co-bal.Fe 11Nd-1Pr-5B-3Zr-1Ti-bal.Fe 10.5Nd-6B-2.5Nb-1.5V-bal.Fe 10Nd-1La-5B-10Co-3Nb-1Ti-bal.Fe 11Nd-5.5B-2Ti-1Ni-bal.Fe The samples were measured for magnetization by means of a vibrating magnetometer first after they were magnetized in a field of 18 kOe and then after they were magnetized in a pulsating field of 40 kOe. All the samples were found to be readily magnetizable. Example 2 A ribbon of Composition 1 alloy in Example 1 was prepared by the same procedure as in Example 1 except that the rotating speed of the roll was set to 40 m/sec. The sample was found to have a quotient A of 1.45. The sample was aged in an argon gas atmosphere at 600 to 700°C for 1 hour. The aged sample was found to have a quotient A of 0.89. The aged sample was determined for magnetic properties. The average grain size of the primary phase and the average thickness of the grain boundary layer that the auxiliary phase formed were determined. The results are shown below. Br: 8.3 kG iHc: 12.6 kOe (BH)max: 14.1 MGOe Primary phase average grain size: 0.07 µm Auxiliary phase grain boundary thickness: 0.002 µm Primary phase composition: 10.9Nd-0.8Zr-0.1Mn-5.8B-bal.Fe Auxiliary phase composition: 6.3Nd-32.2Zr-12.9Mn-7.6B-bal.Fe R1/R2 = 0.57 Example 3 A series of samples as reported in Table 2 were prepared by the same procedure as in Example 1 except that the composition used was 8.5Nd-8B-2.5Nb-1Ni-10Co-bal.Fe. The rotating speed of the roll was varied from 7.5 to 25 m/sec. As in Example 1, the samples were determined for magnetic properties, volume (in vol%) of the auxiliary phase, and quotient A. The average grain size of the primary phase and the thickness of the grain boundary that the auxiliary phase formed were also determined. The results are shown in Table 3. For sample Nos. 12 and 14, the composition of primary and auxiliary phases and R1/R2 measured are shown in Table 4. A series of samples having each of the following compositions were prepared by the same procedure as used in this example while varying the volume of the auxiliary phase. Equivalent results were obtained. Composition (atomic percent) 7.5Nd-8B-3Nb-1Ni-bal.Fe 9Nd-7.5B-3Zr-1Cu-bal.Fe 9Nd-7.5B-3Zr-1Mn-bal.Fe 9Nd-7.5B-2.5Zr-1.5Cr-bal.Fe 8Nd-8B-3Zr-1Ti-10Co-bal.Fe 7.5Nd-8B-3Zr-1Ti-10Co-bal.Fe 9Nd-7B-2Hf-2V-bal.Fe 8.5Nd-8B-2.5Nb-1Zr-0.5Ag-bal.Fe 9Nd-7B-2Zr-2Ti-10Co-bal.Fe 8.5Nd-8B-3Ti-1Cu-8Co-bal.Fe The samples were measured for magnetization by the same procedures as in Example 1. They were found to be readily magnetizable. Example 4 Sample 3 of Example 1 was finely divided to particles having a size of about 100 µm. The powder was blended with a thermosetting resin and press molded into a bonded compact having a density of about 5.80 g/cc. The compact was magnetized in a pulsating field of 40 kOe. This bonded magnet is designated sample A. Sample A was determined for magnetic properties, with the results shown below. Br: 6.4 kG iHc: 12.8 kOe (BH)max: 8.5 MGOe No difference was found between the bonded magnet and the ribbon magnet, sample No. 3 of Example 1 with respect to the average grain size of the primary phase, the thickness of the grain boundary that the auxiliary phase formed, and quotient A. Example 5 Source materials were blended so as to produce an alloy having Composition 1 of Example 1. The blend was melted by RF heating. The melt was ejected through a quartz nozzle onto the surface of a copper chill roll rotating at a circumferential speed of 30 m/sec., obtaining a ribbon of about 20 µm thick and about 5 mm wide. The ribbon was heat treated at 700°C for 30 minutes. The heat treated ribbon is designated Sample B. The heat treated ribbon was finely divided to particles having a size of about 50 to about 200 µm. The powder was hot pressed into a compact in an argon atmosphere at a temperature of about 700°C under a pressure of 2,700 kg/cm² for 10 minutes. This compact is designated Sample C. Samples B and C were determined for magnetic properties, with the results shown below. Sample B Sample C Br (kG) 8.3 8.1 iHc (kOe) 13.2 13.0 (BH)max (MGOe) 14.1 13.9 Samples B and C were measured for the average grain size of the primary phase, the average thickness of the grain boundary that the auxiliary phase formed, and quotient A. The measurements were a grain size of 0.06 µm, a thickness of 0.02 µm, and a quotient A of 0.80 for both the samples. It was found that these values remained unchanged after crushing. Example 6 The procedure of Example 1 was repeated to prepare a series of samples having the composition shown in Table 5. The samples were determined for magnetic properties by the same procedure as in Example 1. The results are shown in Table 5. The composition of the primary and auxiliary phases and R1/R2 of these samples are shown in Table 6. Example 7 A series of samples having Compositions D and E shown in Table 7 were prepared in the form of a ribbon having a thickness of 30 to 60 µm by single roll melt spinning with the rotating speed of a chill roll set to 15 m/sec. The ribbon was heat treated in an argon atmosphere at a temperature of 700°C for 30 minutes. It was then finely divided into particles having a size of about 20 to 400 µm. The powder was blended with a thermosetting resin and press molded into compacts having a varying density. Each of the bonded magnets was measured for (BH)max. The results are shown in Table 7. As seen from Table 7, the ribbon magnet of the present invention can be readily molded into a bonded magnet having a high density. Bonded magnets having a value of (BH)max of higher than 10 MGOe are obtained when the density exceeds 6 g/cm³. Example 8 Ribbons having composition (Nd (1-x) , Zr x )₁₁Fe₈₂B₈ wherein x had a value of from 0 to 6 were prepared by the same procedure as in Example 1. The ribbons were analyzed by X-ray diffractometry. The lattice constants of the primary phase along a and c axes were determined from the diffraction pattern. The composition of the primary phase was determined by means of a transmission type analytic electron microscope. FIG. 5 shows the lattice constants as a function of Zr/(Nd + Zr) of the primary phase. As seen from FIG. 5, as many as 40% of the Nd sites of Nd₂Fe₁₄B are replaced by Zr in the primary phase of the ribbon according to the present invention. Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

Claims: A permanent magnet formed from a magnetically hard material having a composition represented by the formula: R x T (100-x-y-z) B y M z wherein R is at least one member selected from the rare earth elements including Y, T is Fe or a mixture of Fe and Co, B is boron, M is at least one member selected from the group consisting of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W, 5.5 ≦ x < 11.76, 2 ≦ y < 15, and z ≦ 10, and comprising a primary phase of substantially tetragonal grain structure, or a primary phase of substantially tetragonal grain structure and at least one auxiliary phase selected from amorphous and crystalline R-poor auxiliary phases wherein the volume ratio of auxiliary phase to primary phase, v, is smaller than the value given by the formula: [0.1176(100 - z) - x]/x. The permanent magnet of claim 1 wherein 5.5 ≦ x ≦ 11. The permanent magnet of claim 1 wherein the quotient of the volume ratio of auxiliary phase to primary phase, v, divided by the value given by the formula: [0.1176(100 - z) - x]/x ranges from 0.15 to 0.95. The permanent magnet of claim 1 wherein the primary phase has an average grain size of from 0.01 to 3 µm. The permanent magnet of claim 1 wherein the auxiliary phase forms a grain boundary layer having an average width of up to 0.3 µm. The permanent magnet of claim 1 which consists of the primary and auxiliary phases wherein the R content of the auxiliary phase is up to 9/10 of that of the primary phase in atomic ratio. The permanent magnet of claim 1 wherein the primary phase has an R content of from 6 to 11.76 atom%. The permanent magnet of claim 1 in the form of a ribbon which is prepared by rapid quenching. The permanent magnet of claim 1 in the form of powder. The permanent magnet of claim 9 wherein the powder is obtained by comminuting a ribbon which is prepared by rapid quenching. The permanent magnet of claim 8 or 10 wherein the ribbon has a thickness of from 30 to 60 µm. The permanent magnet of claim 9 which is obtained by compacting the powder. The permanent magnet of claim 9 which is obtained by hot plastic processing of the powder. The permanent magnet of claim 9 which is obtained by mixing the powder with a binder. The permanent magnet of claim 1 which is obtained by rapid quenching such that the quotient of the volume ratio of auxiliary phase to primary phase, v, divided by the value given by the formula: [0.1176(100 - z) - x]/x may range from 0.15 to 0.95. The permanent magnet of claim 1 which is obtained by rapid quenching such that the quotient of the volume ratio of auxiliary phase to primary phase, v, divided by the value given by the formula: [0.1176(100 - z) - x]/x may range from 0.2 to 1.2, and then heat treating such that said quotient may range from 0.15 to 0.95. A permanent magnet formed from a magnetically hard material having a composition represented by the formula: R x T (100-x-y-z) B y M z wherein R is at least one member selected from the rare earth elements including Y, T is Fe or a mixture of Fe and Co, B is boron, M is a mixture of at least one member selected from the group consisting of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W and at least one member selected from the group consisting of Cu, Ni, Mn and Ag, 5.5 ≦ x < 11.76, 2 ≦ y < 15, and z ≦ 10, and consisting of a primary phase of substantially tetragonal grain structure, or a primary phase of substantially tetragonal grain structure and at least one auxiliary phase selected from amorphous and crystalline R-poor auxiliary phases wherein the volume ratio of auxiliary phase to primary phase, v, is smaller than the value given by the formula: [0.1176(100 - z) - x]/x. The permanent magnet of claim 17 which consists of the primary and auxiliary phases wherein the R content of the auxiliary phase is up to 9/10 of that of the primary phase in atomic ratio. The permanent magnet of claim 17 wherein the primary phase has an R content of from 6 to 11.76 atom%.

Inventors: KOHMOTO, OSAMU, YAJIMA, KOICHI, YONEYAMA, TETSUHITO
Applicants: TDK CORPORATION
Application year: 1988
Main classification: H01F1
IPC codes: C22C38, H01F1, B22F9