Process for making R-Fe-B permanent magnets

Abstract: A process for producing a starting powder material for use in the fabrication of high performance R-Fe-B permanent magnets comprising a specified R₂Fe₁₄B compound as the principal phase, which is characterized by adding to the said principal phase compound 70 % by weight or less of a specified alloy powder comprising an R₂Fe₁₇ compound. This process enables production of a starting alloy powder material with considerably reduced contents of the unfavorable B-rich and R-rich phases which impair the magnetic properties of the final magnet, because the starting powder blend allows the B-rich and R-rich compounds in the principal phase alloy powder to react with the R₂Fe₁₇B compound.

Description: The present invention relates to a process for producing a sintered R-Fe-B permanent magnet containing a rare earth element (R), iron (Fe) and boron (B). The symbol R as employed herein represents at least one rare earth element inclusive of yttrium. More particularly, the present invention relates to a process for producing a sintered R-Fe-B based permanent magnet (sometimes referred to hereinafter as the "starting powder material") comprising a principal phase alloy powder, i.e. a powder of an R 2 Fe 14 B principal phase, having added thereto an adjusting alloy powder, i.e. a powder containing an R 2 Fe 17 phase, and reduced in concentration of unfavorable phases which impair the magnetic properties of the resulting magnet, e.g. a B-rich phase and an R-rich phase. An R-Fe-B permanent magnet is an example of the high performance permanent magnets known at present. The excellent magnetic characteristics of an R-Fe-B permanent magnet as disclosed in JP-A-59-46008 is attributed to the composition comprising a tetragonal ternary compound as the principal phase and an R-rich phase. The R-Fe-B permanent magnet above yields an extraordinary high performance, i.e., a coercive force iHc of 25 kOe (1.99 MA/m) or higher and a maximum energy product (BH)max of 45 MGOe (3.58 GA/m) or higher, as compared with the conventional high performance rare earth-cobalt based magnets. Furthermore, a variety of R-Fe-B based permanent magnets of different compositions are proposed to meet each of the particular demands. EP-A-0447567 describes and claims a method of producing a corrosion-resistant rare earth-transition metal series magnet (RE-TM) by subjecting a mixture of powder to a compression molding and then sintering, the mixture of powder being composed mainly of an RE 2 TM 14 B phase (TM being one or more of Fe, Co and Ni) and a lower melting point powder comprising an RE-TM material, for example RE 2 TM 17 , (in which TM is Ni or a mixture of Ni and Fe or Co). To fabricate various types of R-Fe-B based permanent magnets as mentioned hereinbefore, an alloy powder having a predetermined composition should be prepared at first. The alloy powder can be prepared by an ingot-making and crushing process as disclosed in JP-A-60-63304 and JP-A-119701, which comprises melting the starting rare earth metal materials having subjected to electrolytic reduction, casting the melt in a casting mould to obtain an alloy ingot of a desired magnet composition, and then crushing the ingot into an alloy powder having the desired granularity. Otherwise, it can be prepared by a direct reduction diffusion process as disclosed in JP-A-59-21940 and JP-A-60-77943, which comprises directly preparing an alloy powder having the composition of the desired magnet from the starting materials such as rare earth metal oxides, iron powder and Fe-B alloy powder. The ingot-making and crushing process involves many steps, and, moreover, it suffers segregation of an R-rich phase and crystallization of iron (Fe) primary crystals at the step of casting the alloy ingot. According to this process, however, an alloy powder containing relatively low oxygen can be obtained, since the ingot can easily be prevented from being oxidized in a coarse grinding (primary crushing). The direct reduction diffusion process, on the other hand, is advantageous as compared with the ingot-making and crushing process above in that the steps such as melting and coarse grinding can be omitted from the process of preparing the starting alloy powder for the magnet. However, as compared to the R-rich phases in the former process, the R-rich phases being formed by this process are smaller and well dispersed, and are mostly developed at the surroundings of the principal R 2 Fe 14 B phase. The R-rich phase thus formed in this process is susceptible to oxidation, which, as a result, takes up a considerable amount of oxygen. In some kinds of magnet composition, the rare earth metal elements may be oxidized and consumed by excess oxygen, resulting in unstable magnet characteristics. It can be seen that the oxygen being incorporated in the alloy powder harms the magnet characteristics of an R-Fe-B permanent magnet. Accordingly, with the aim of reducing the oxygen content of the alloy powder, the present inventors have proposed previously, as disclosed in Japanese patent application No. 02-229685, a process which comprises first preparing an alloy powder having a composition near to that of the R 2 Fe 14 B phase by direct reduction diffusion process, while preparing separately a powder of intermetallic phases such as an R 2 (Fe,Co) 17 phase containing an R 3 Co phase [in which iron (Fe) may be present as a substitute for a part or a large part of the cobalt] by adding metallic cobalt to the R-rich alloy powder, and then mixing them to obtain an alloy material powder for an R-Fe-B permanent magnet. The proposal above is extremely effective for reducing the oxygen content of the magnet and the starting powder material in preparing the starting alloy powder material for an R-Fe-B permanent magnet. However, not only the principal R 2 Fe 14 B phase but an R-rich phase and a B-rich phase, which are known also to harm the intrinsic properties, remain in the magnet. It has been found extremely difficult to control precisely the content of these phases, and hence these phases remain as the cause for destabilizing the magnetic characteristics. An object of the present invention is to provide a process for producing various types of starting alloy powder for R-Fe-B permanent magnets in accordance with the desired magnet characteristics, which provides a magnet comprising magnetic phases increased in the principal R 2 Fe 14 B phase but considerably reduced in B-rich and R-rich phases which are unfavorable for achieving a high performance magnet, and which also provides an alloy powder of reduced oxygen content. The aforementioned object can be achieved by the present invention which provides a process for producing a sintered permanent magnet from a mixture of starting alloy powders which mixture comprises an intermetallic alloy powder I, containing a R 2 Fe 14 B phase as the principal phase, with an inherent B-rich phase and R-rich phase (wherein R is at least one element selected from the group consisting of Nd, Pr, La, Ce, Tb, Dy, Ho, Er, Eu, Sm, Gd, Pm, Tm, Yb, Lu, and Y), and an alloy powder II of the rare earth-transition metal series intermetallic compound phase, R-TM and/or an alloy powder of the rare earth-transition metal-boron series intermetallic compound phase, R-TM-B, (wherein R has the above meaning and TM is a metallic material including Fe), wherein the said powders are mixed, compacted and sintered, characterised in that the mixture comprises the R 2 Fe 14 B phase alloy powder I, which consists of 10-30 atomic % of R, 4-40 atomic % of B and the balance Fe, where Fe may be partially substituted by Co, all elements including unavoidable impurities, and an alloy powder II, containing a R 2 Fe 17 compound phase, which phase consists of 5-35 atomic % of R and the balance Fe, where Fe may be partially substituted by Co, all elements including unavoidable impurities, wherein the alloy powder II is present in the total mixture of alloy powders I + II in an amount of 70 % by weight or less, and wherein the R 2 Fe 17 compound is reacted in the sintering step, at a temperature between the vicinity of the eutectic point thereof and the sintering temperature, with the B-rich phase and the R-rich phase contained in powder I, to increase the amount of the R 2 Fe 14 B phase alloy in powder I and consequently the overall content of the R 2 Fe 14 B compound as the permanent magnetic component of the magnet. In the present invention, the alloy powder II is added in an amount of 70 % by weight or less, preferably from 0.1 - 40 % by weight, with respect to the total mixture of alloy powders I + II. Preferred amounts for the content of the element(s) R and boron in powder I are from 12 - 20 atomic % and 6 - 20 atomic %, respectively. Preferably, iron (Fe) accounts for from 30 - 84 atomic %, and more preferably from 60 to 82 atomic %, of the content of powder I. The permissible range of substitution of iron (Fe) in the principal phase alloy powder I by cobalt (Co) is 10 atomic % or less. Furthermore, when cobalt (Co) partially substitutes for iron in the principal phase alloy layer, the preferred amount of iron (Fe) therein is in the range of from 17 to 84 atomic %. In the alloy powder II , R is preferably incorporated in an amount of from 5 to 35 atomic %, and iron (Fe) is preferably contained in an amount of from 65 to 95 atomic %. The preferred amount of cobalt (Co) which can be incorporated in the alloy powder II as a partial substitute for iron (Fe) is 10 atomic % or less. The preferred amount of boron (B) as a partial substitute for iron (Fe) in the alloy powder II is 6 atomic % or less. When boron (B) replaces a part of iron (Fe) in the alloy powder II, the preferred content of iron (Fe) therein is from 59 to 89 atomic %. The present invention is described in detail below. It is known that R-Fe-B permanent magnets in general have particular textures comprising an R 2 Fe 14 B phase as a principal phase and a small amount of B-rich phase expressed by R 1.1 Fe 4 B 4 , accompanied by R-rich phases at the grain boundaries thereof. It is also known that the magnetic properties are largely influenced by such textures. When the boron (B) content in the R-Fe-B permanent magnet composition is less than 6 atomic %, an R 2 Fe 17 B phase forms within the magnet. Because this R 2 Fe 17 B intermetallic phase has its direction of easy magnetization in the crystallographic c-plane and a Curie point at the vicinity of room temperature, the formation thereof lowers the coercive force (iHc). When boron (B) is incorporated in the R-Fe-B permanent magnet in excess of 6 atomic %, on the other hand, it is known that the amount of B-rich phases is increased to lower the residual magnetization flux density (Br). The present inventors have conducted extensively studies on the fabrication of sintered R-Fe-B permanent magnets. It has been found as a result that, by sintering an R-Fe-B alloy powder (I) comprising an R 2 Fe 14 B phase as a principal phase and having added therein a specified amount of an R-Fe alloy powder containing an R 2 Fe 17 phase as an alloy powder (II) for adjusting the composition, a liquid phase having a low melting point is formed through the eutectic reaction of the R component in the intergranular R-rich phase and the R 2 Fe 17 B phase in the R-Fe alloy powder at the vicinity of the eutectic point thereof, and that this low-melting liquid phase accelerates the sintering of the R-Fe-B alloy powder. Furthermore, it has been found that the R 2 Fe 17 phase in the alloy powder II and the B-rich and R-rich phases in the alloy powder (I) undergo reaction during the sintering step so as to increase the amount of the principal R 2 Fe 14 N phase. The present invention has been accomplished based on these findings. The present inventors have conducted experiments to find that, in a case using Nd as R, for instance, an Nd-rich phase undergoes a reversible reaction with an Nd 2 Fe 17 phase at the vicinity of the eutectic point thereof, i.e., 690°C, to form a liquid phase. Accordingly, it has been found that this low-melting liquid phase accelerates the sintering of the principal phase Nd-Fe-B alloy powder. Furthermore, it has been observed that the alloy powder comprising the Nd 2 Fe 17 phase and the Nd-Fe-B alloy powder comprising the Nd 2 Fe 14 B phase undergo a chemical reaction expressed below during the sintering of the powder to effectively increase the amount of the principal Nd 2 Fe 14 B phase within the sintered magnet. 13 / 17 Nd 2 Fe 17 + 1 / 4 Nd 1.1 Fe 4 B 4 + 133 / 680 Nd → Nd 2 Fe 14 B The reaction above reads that an Nd 2 Fe 14 B phase is newly developed from the reaction between the Nd 2 Fe 17 phase of the alloy powder II and the B-rich Nd 1.1 Fe 4 B 4 phase of the principal Nd-Fe-B alloy powder I. Accordingly, the B-rich phase and the R-rich (Nd-rich) phase, which were both unfavorable for a conventional process for fabricating a sintered permanent magnet from an alloy powder material comprising the principal Nd 2 Fe 14 B phase alone, can be considerably reduced in content with respect to the principal phase by employing the process according to the present invention. Furthermore, it has been confirmed that the above reaction is not only observed for the case using Nd, but also for the case using any rare earth elements inclusive of Y. As described above, the present invention provides a process for producing a starting alloy powder material for fabricating an R-Fe-B permanent magnet, characterized in that an alloy powder II comprising an R 2 Fe 17 phase and containing 50 atomic % or less of R (as defined herein) and the balance of iron (Fe) (where cobalt (Co) may be present as a partial substitute for iron (Fe)) with unavoidable impurities is added in an amount of 70 % by weight to an alloy powder I which comprises an R 2 Fe 14 B phase as the principal phase and containing from 10 to 30 atomic % of R, from 6 to 40 atomic % of boron (B), and the balance of iron (Fe) (where cobalt (Co) may be present as a partial substitute for iron (Fe)) with unavoidable impurities. In the present invention, the alloy powders I and II are prepared by a known ingot-making and crushing process or direct reduction diffusion process. The addition of the alloy powder II to the alloy powder I is 70 % by weight or less. If the addition is in excess of 70 % by weight, the formation of the R 2 Fe 14 B phases having a uniaxial anisotropy is suppressed during the fabrication of an anisotropic magnet, which comprises sintering the starting powder material under a magnetic field. The resulting magnet then suffers weak orientation and hence a low residual magnetic flux density (Br). More preferably, the alloy powder II is added in an amount of from 0.1 to 4 0 % by weight to the alloy powder I. In the present invention, R represents rare earth elements comprising light rare earth and heavy rare earth elements inclusive of yttrium (Y). More specifically, R represents at least one element selected from the group consisting of Nd, Pr, La, Ce, Tb, Dy, Ho, Er, Eu, Sm, Gd, Pm, Tm, Yb, Lu, and Y. More preferably, R represents a light rare earth element such as Nd and Pr, or a mixture thereof. The rare earth element need not necessarily be pure and can therefore be an industrially available grade containing impurities which are unavoidably incorporated during its production. Among the starting powder materials, the alloy powder I must contain from 10 to 30 atomic % of a rare earth element R. If the amount of R is less than 10 atomic %, residual Fe portions, into which R and boron (B) would not diffuse, increase within the alloy powder. If the amount of R exceeds 30 atomic %, the R-rich phase increases and thereby increases the oxygen content. It is not possible to obtain favorable sintered permanent magnets in both cases. More preferably, the content of R is in the range of from 12 to 20 atomic %. The boron (B) content in the alloy powder I must be within the range of from 6 to 40 % by weight. If boron (B) should be contained in the powder for less than 6 atomic %, the amount of the B-rich phase (R 1.1 Fe 4 B 4 ) is too small to exhibit the aforementioned effect of the present invention even though an alloy-powder II for adjusting the composition were to be added. Then, the resulting permanent magnet suffers a low coercive force (iHc). If boron (B) is added in an amount exceeding 40 atomic %, an excess amount of B-rich phase forms and reduces the formation of the principal R 2 Fe 14 B phase. In this case, a favorable permanent magnetic properties inclusive of high residual magnetic flux density (Br) cannot be expected. More preferably, boron (B) is incorporated in the alloy powder I in an amount in the range of from 6 to 20 atomic %. The last component of the alloy powder I, iron (Fe), is preferably included in an amount of from 20 to 86 atomic %. If the amount should be less than 20 atomic %, the amount of R-rich and B-rich phases relative to the principal phase becomes too high as to impair the magnetic properties of the permanent magnet. If the amount should exceed 86 atomic %, on the other hand, relative contents of rare earth elements and boron (B) are decreased as to increase the residual Fe portion. Then, a uniform alloy powder would not result due to the residual Fe portion being incorporated at a high ratio. A more preferred content of Fe is from 60 to 82 atomic %. A partial substitution of iron (Fe) being incorporated in the alloy powder I by cobalt (Co) improves the corrosion resistance of the resulting magnet. However, an excess addition of such metal elements reduces the coercive force (iHc) of the magnet due to the substitution which occurs on the constituent iron (Fe) of the R 2 Fe 14 B phase. Accordingly, cobalt (Co) preferably accounts for an amount of 10 atomic % or less. Furthermore, the preferred amount of iron (Fe) containing cobalt (Co) as a partial substitute in the principal phase alloy is from 17 to 84 atomic %. The alloy powder II must be prepared as such that the R may not exceed 50 atomic %. If R should be contained more than 50 atomic %, problems such as unfavorable oxidation occurs during the preparation of the alloy powder. More preferably, R is incorporated in the alloy powder II in an amount of from 5 to 35 atomic %. The rest of the powder composition, iron (Fe), preferably accounts for an amount of from 65 to 95 atomic %. Similar to the case of the alloy powder I, a part of the iron (Fe) being incorporated in the alloy powder II can be substituted by cobalt (Co) in an amount as defined above for the alloy powder I. The alloy powder II may be prepared by substituting a part of the iron (Fe) being incorporated in the powder by boron (B). An addition of boron (B) in an amount of 6 atomic % or less is allowable because it results in the formation of, besides the R 2 Fe 17 phases, R 2 Fe 14 B phases in the alloy powder II. However, if the addition of boron (B) should exceed 6 atomic %, the B-rich phase which is formed within the alloy powder II is incorporated in an excess amount in the starting alloy powder material on mixing the alloy powder II with the alloy powder II. The permanent magnet which results from such a starting alloy powder material has inferior magnetic properties. The amount of iron (Fe) containing boron (B) as a partial substitute in the alloy powder II is preferably in the range of from 59 to 89 atomic %. The starting alloy powder material thus obtained by mixing the alloy powder I with the alloy powder II must be size controlled as to yield a pertinent granularity, or a permanent magnet of an inferior quality would result. In particular, only a permanent magnet having a low coercive force (iHc) can be obtained. More specifically, a starting powder material composed of grains less than 1 µm in average diameter would not result in a permanent magnet having superior magnetic properties, because the powder would be severely oxidized in each of the process steps for fabricating the permanent magnet, such as press molding, sintering, and aging steps. If the grains of the starting alloy powder should exceed 80 µm in diameter, the resulting magnet would suffer a low coercive force. It can thus be seen that the preferred grain size for the starting powder material is from 1 to 80 µm in diameter, and more preferably, from 2 to 10 µm in diameter. Furthermore, an R-Fe-B permanent magnet of a superior quality having a high residual magnetic flux density (Br) and a high coercive force (iHc) results only from a mixed starting powder material the composition of which is strictly controlled. A preferred starting powder may contain, for example, from 12 to 25 atomic % of a rare earth element R, from 4 to 10 atomic % of boron (B), from 0.1 to 10 atomic % of cobalt (Co), from 55 to 83.9 atomic % of iron (Fe), and the balance of unavoidable impurities. Furthermore, a permanent magnet having not only a further improved temperature characteristics but also high coercive force and corrosion resistance can be obtained by adding, to an alloy powder I containing an R 2 Fe 14 B phase as the principal phase and/or an alloy powder II containing an R 2 Fe 17 phase, at least one selected from the group consisting of 3.5 atomic % or less of copper (Cu), 2.5 atomic % or less of sulphur (S), 4.5 atomic % or less of titanium (Ti), 15 atomic % or less of silicon (Si), 9.5 atomic % or less of vanadium (V), 12.5 atomic % or less of niobium (Nb), 10.5 atomic % or less of tantalum (Ta), 8.5 atomic % or less of chromium (Cr), 9.5 atomic % or less of molybdenum (Mo), 9.5 atomic % or less of tungsten (W), 3.5 atomic % or less of manganese (Mn), 19.5 atomic % or less of aluminium (Al), 2.5 atomic % or less of antimony (Sb), 7 atomic % or less of germanium (Ge), 3.5 atomic % or less of tin (Sn), 5.5 atomic % % or less of zirconium (Zr), 5.5 atomic % or less of hafnium (Hf), 8.5 atomic % or less of calcium (Ca), 8.5 atomic % or less of magnesium (Mg), 7.0 atomic % or less of strontium (Sr), 7.0 atomic % or less of barium (Ba), and 7.0 atomic % or less of beryllium (Be). By an experiment, a permanent magnet having a magnetic anisotropy was obtained from a starting powder material according to the present invention, and containing, for example, from 12 to 25 atomic % of a rare earth element R, from 4 to 10 atomic % of boron (B), 30 atomic % or less of cobalt (Co), and from 35 to 84 atomic % of iron (Fe). The resulting permanent magnet yielded excellent magnetic properties such as a coercive force (iHc) higher than 5 kOe (398 kA/m), a (BH)max higher than 20 MGOe (1.59 GA/m), and a temperature coefficient of the residual magnetic flux density of 0.1 %/°C or less. Furthermore, a permanent magnet containing 50 % by weight or more of light rare earth elements as the principal component for R yields superior magnetic properties. For instance, permanent magnets containing light rare earth elements and containing from 12 to 20 atomic % of a rare earth element R, from 4 to 10 atomic % of boron (B), 20 atomic % or less of cobalt (Co), and from 50 to 84 atomic % of iron (Fe) yield extremely superior magnetic properties; in particular, a (BH)max as high as 40 MGOe (3.18 GA/m) was confirmed on those containing at least one of Nd, Pr, and Dy as the rare earth element R. As described in the foregoing, the present invention relates to a process for producing a starting powder material for use in the fabrication of sintered R-Fe-B permanent magnets, by adding 70 % by weight or less of an alloy powder II comprising an R 2 Fe 17 phase to an R-Fe-B alloy powder I comprising an R 2 Fe 14 B phase as the principal phase and a B-rich phase (R 1.1 Fe 4 B 4 ). This process enables production of a starting alloy powder material considerably reduced in contents of the unfavorable B-rich and R-rich phases which impair the magnetic properties of the final magnet, because the starting powder blend allows the B-rich and R-rich phases in the alloy powder I to react with the R 2 Fe 17 phase being incorporated in the alloy powder II. Thus, the use of the starting powder material according to the present invention not only enables fabrication of high performance sintered permanent magnets, but also, because of the decreased amount of oxygen being incorporated in the powder, facilitates the fabrication process. Furthermore, by controlling properly the composition of the starting powder blend, R-Fe-B alloy powders for permanent magnets varied in composition can be produced in accordance with diversified needs. The present invention is illustrated in greater detail with reference to non-limiting examples below. EXAMPLE 1 A principal phase alloy powder I was prepared by a direct reduction diffusion process as follows. In a stainless steel vessel was charged a powder mixture obtained by adding 264 g of 99 % pure metallic calcium (Ca) and 49.3 g of anhydrous CaCl 2 to 407 g of 98 % pure Nd 2 0 3 , 15 g of 99 % pure Dy 2 0 3 , 62 g of an Fe-B powder containing 19.1 % by weight of boron, and 604 g of 99 % pure Fe alloy powder. The powder mixture was then subjected to calcium reduction and diffusion at 1030°C for 3 hours in an argon gas flow. The resulting mixed product was cooled and washed with water to remove the residual calcium. The powder slurry thus obtained was subjected to water substitution using an alcohol and the like, and then dried by heating in vacuum to obtain about 1,000 g of a principal phase alloy powder. The resulting alloy powder was composed of grains about 20 µm in average diameter, and contained 14.0 atomic % of neodymium (Nd), 0.8 atomic % of praseodymium (Pr), 0.5 atomic % of dysprosium (Dy), 7.2 atomic % of boron (B), and the balance of iron (Fe). The oxygen content thereof was 2,000 ppm. An alloy powder II containing an R 2 Fe 17 phase was prepared by an ingot-making and crushing process as follows. The starting materials, i.e., 124 g of 98 % pure metallic neodymium (Nd) and 379 g of 99 % pure electrolytic iron were molten in a melting furnace under argon gas atmosphere, and the resulting alloy ingot was crushed by using a jaw crusher and a disk mill to obtain 450 g of an alloy powder. The alloy powder thus obtained was composed of grains 10 µm in average diameter, and contained 11 atomic % of neodymium (Nd), 0.2 atomic % of praseodymium (Pr), and the balance of iron (Fe). The oxygen content thereof was 600 ppm. The alloy powder thus obtained was confirmed by EPMA (electron probe microanalysis) and XRD (X-ray diffraction) to consist largely of an Nd 2 Fe 17 phase. The starting alloy powder materials for sintered permanent magnets were obtained from the two alloy powders I and II thus obtained, by mixing predetermined amounts of the alloy powder II with the principal alloy powder material I as shown in Table 1. Besides two types (Nos. 1B and 1C) of alloy powder material according to the present invention, an alloy powder having added therein no alloy powder II was prepared according to a conventional process for use as a comparative sample (No. 1A). The alloy powder materials thus obtained were milled by a jet mill and molded under a magnetic field of about 10 kOe (796kA/m), by applying a pressure of about 2 ton/cm 2 along a direction vertical to that of the magnetic field to obtain a green compact 15 mm x 20 mm x 8 mm in size. The green compact thus obtained was sintered at 1,070°C for 3 hours in an argon gas atmosphere and then annealed at 500°C for 2 hours to obtain a permanent magnet. The mixing ratio of the alloy powders, composition of the resulting powder material, and the magnetic properties of the permanent magnets obtained therefrom are summarized in Table 1 below. Sample No. Mixing ratio of Powders Composition Magnetic properties Principal Adjusting Br iHc (BH) max) (%) (%) (atomic %) (kOe) (kOe) (MGOe) 1A 100 0 14.ONd-0.8Pr-0.5Dy-7.2B-balFe 12.3 14.5 36.5 1B 90 10 13.7Nd-0.7Pr-0.45Dy-6.5B-balFe 13.0 14.0 40.5 1C 80 20 13.4Nd-0.7Pr-0.4Dy-5.8B-balFe 13.3 13.5 42.5 From the composition of the magnet as summarized in Table 1, the compact ratio of the phases, i.e., R 2 Fe 14 B:B-rich phase:R-rich phase (oxides included), can be calculated as follows. No. 1A (Conventional) 88 : 3 : 9, No. 1B (Present invention) 91 : 1.3 : 7.7, and No. 1C (Present invention) 93 : 0.1 : 6.9. It can be seen that the component ratio of the phases in the final sintered magnet can be controlled arbitrarily by using the alloy powder materials, obtained by adding an alloy powder II into an alloy powder I according to this present invention. Accordingly, by thus adjusting the composition of the starting powder material, the magnetic properties of the resulting sintered magnet can be considerably improved as compared with those of the magnet obtained by using the alloy powder I alone. EXAMPLE 2 A principal phase alloy powder I was prepared by an ingot-making and crushing process in the same manner as that used in preparing the alloy powder II in Example 1, using 147 g of metallic neodymium (Nd), 23 g of metallic cobalt (Co), 27.5 g of an Fe-B alloy, and 307 g of electrolytic iron. The alloy powder thus obtained contained 12.5 atomic % of neodymium (Nd), 0.2 atomic % of praseodymium (Pr), 5.0 atomic % of cobalt (Co), 6.5 atomic % of boron (B), and 75.8 atomic % of iron (Fe). The alloy powder II was prepared by a direct reduction diffusion process in the same manner as that in preparing the alloy powder I in Example 1, from 260 g of Nd 2 O 3 , 80.5 g of Dy 2 O 3 , 43 g of cobalt powder, and 665 g of iron powder, having added therein 190 g of metallic calcium and 23 g of CaCl 2 . The alloy powder thus obtained contained 10.4 atomic % of neodymium (Nd), 0.1 atomic % of praseodymium (Pr), 3.0 atomic % of dysprosium (Dy), 5.0 atomic % of cobalt (Co), and the balance of iron (Fe). Then, an R-Fe-B permanent magnet in the same procedure as that used in Example 1, except for using a starting alloy powder material obtained by adding 5 % by weight of the alloy powder II prepared above to 95 % by weight of the above-obtained alloy powder I. Thus was obtained a magnet containing 12.4 atomic % of neodymium (Nd), 0.2 atomic % of praseodymium (Pr), 0.15 atomic % of dysprosium (Dy), 5 atomic % of cobalt (Co), 6.2 atomic % of boron (B), and the balance of iron (Fe), which yielded magnetic properties such as a Br of 13.6 KG, an iHc of 11 kOe, and a (BH)max of 45.5 MGOe. Furthermore, the alloy powder I only was used for trial to fabricate a magnet, but it was found that this powder alone cannot be sintered. EXAMPLE 3 A principal phase alloy powder I was prepared by an ingot-making and crushing process in the same manner as in Example 2. The alloy powder thus obtained contained 18 atomic % of neodymium (Nd), 0.8 atomic % of praseodymium (Pr) 2.0 atomic % of dysprosium (Dy), 2 atomic % of Mo (B), and the balance of iron (Fe). Similarly, an alloy powder II comprising an R 2 Fe 17 phase was prepared by an ingot-making and crushing process. The thus obtained alloy powder II comprised an Nd 2 Fe 17 phase contained 9 atomic % of neodymium (Nd), 0.2 atomic % of praseodymium (Pr), 1.0 atomic % of dysprosium (Dy), and the balance of iron (Fe). Sintered permanent magnets as shown in Table 2 below were obtained in the same procedure as that used in Example 1, by blending and mixing predetermined amounts of the alloy powder II with the powder material I. Besides two types (Nos. 3B and 3C) of alloy powder material according to the present invention, an alloy powder having added therein no alloy powder II was prepared according to a conventional process for use as a comparative sample (No. 3A). The magnetic properties of the sintered permanent magnets thus obtained are summarized in Table 2 below. Sample No. Mixing ratio of Powders Composition Magnetic properties Principal Adjusting Br iHc (BH) max) (%) (%) (atomic%) (kOe) (kOe) (MGOe) 3A 100 0 18.ONd-0.8Pr-2.ODy-2.OMo-1OB-balFe 9.2 >25 20 3B 80 20 16.2Nd-0.7Pr-1.8Dy-1.6Mo-8B-balFe 9.9 >25 23.5 3C 60 40 14.4Nd-0.5Pr-1.6Dy-1.2Mo-6B-balFe 11.0 >25 28 Table 2 clearly reads that the magnets obtained from the powder materials according to the present invention are superior in magnetic properties Br and (BH)max as compared with a magnet obtained by a conventional process. EXAMPLE 4 About 1,000 g of a principal phase alloy powder I was prepared by a direct reduction diffusion process in the same manner as in Example 1, except for using a mixture obtained by adding 236 g of metallic calcium and 43.7 g of CaCl 2 into 400 g of Nd 2 O 3 , 14.3 g Of Dy 2 0 3 , 68 g of an Fe-B alloy powder containing 19.1 % by weight of boron, and 590 g of an Fe powder. The resulting alloy powder was composed of grains 20 µm in average diameter, and contained 15.0 atomic % of neodymium (Nd), 0.5 atomic % of praseodymium (Pr), 0.5 % by atomic of dysprosium (Dy), 8.0 atomic % of boron (B), and the balance of iron (Fe). The oxygen content thereof was 2,000 ppm. Furthermore, 450 g of an alloy powder II composed of grains 10 µm in average diameter was prepared from 133 g of metallic neodymium (Nd), 6.5 g of metallic dysprosium (Dy), 18.3 g of ferroboron, and 349 g of electrolytic iron by an ingot-making and crushing process in the same procedure as in Example 1. The alloy powder thus obtained contained 11 atomic % of neodymium (Nd), 0.3 atomic % of praseodymium (Pr), 0.5 atomic % of dysprosium (Dy), 4.0 atomic % of boron (B), and the balance of iron (Fe). The alloy powder was confirmed by EPMA and XRD to consist mainly of Nd 2 Fe 17 and Nd 2 Fe 14 B phases. The oxygen content was found to be 600 ppm. Sintered permanent magnets as shown in Table 3 below were obtained in the same procedure as that used in Example 1, by blending and mixing predetermined amounts of the alloy powder II with the alloy powder material I . Besides three types (Nos. 4B, 4C, and 4D) obtained from the alloy powder materials according to the present invention, an alloy powder having added therein no alloy powder II was prepared according to a conventional process for use as a comparative sample (No. 4A). The magnetic properties of the sintered permanent magnets thus obtained are summarized in Table 3 below. Sample No. Mixing ratio of Powders Composition Magnetic properties Principal Adjusting Br iHc (BH) max) (%) (%) (atomic%) (kOe) (kOe) (MGOe) 4A 100 0 15.ONd-0.5Pr-0.5Dy-8.OB-balFe 12.0 13.6 35.0 4B 85 15 14.4Nd-0.5Pr-0.5Dy-7.4B-balFe 12.6 13.2 38.5 4C 70 30 13.8Nd-0.4Pr-0.5Dy-6.8B-balFe 13.0 13.2 41.0 4D 50 50 13.ONd-0.4Pr-0.5Dy-6.OB-balFe 13.5 13.0 44.0 From the composition of the magnet as summarized in Table 3, the component ratio of the phases, i.e., R 2 Fe, 4 B:Brich phase:R-rich phase,can be calculated as follows. No 4A (Conventional) 85.1 : 4.4 : 10.5, No 4B (Present Invention) 87.3 : 3.3 : 8.9, No 4C (Present Invention) 90.5 : 2.1 : 7.4, and No 4D (Present Invention) 94.1 : 0.6 : 5.3. It can be seen from Table 3 that the magnets obtained from the starting powder material according to the present invention yield superior Br and (BH)max values as compared with those of a magnet obtained by a conventional process. Furthermore, it can be seen also that magnets having the desired magnetic properties can be readily obtained from the powder material according to the present invention, because the content ratio of the phases in the final sintered magnet can be controlled arbitrarily. EXAMPLE 5 A principal phase alloy powder I was prepared by an ingot-making and crushing process in the same manner as that employed in Example 1, using 128 g of metallic neodymium (Nd), 28.6 g of metallic dysprosium (Dy), 22.8 g of metallic cobalt (Co), 30.4 g of an Fe-B alloy, and 294.6 g of electrolytic iron. The alloy powder thus obtained contained 11 atomic % of neodymium (Nd), 0.3 atomic % of praseodymium (Pr), 2.2 atomic % of dysprosium (Dy), 5.0 atomic % of cobalt (Co), 7.0 atomic % of boron (B) and 74.5 atomic % of iron (Fe) An alloy powder II composed of grains 20 µm in average diameter was prepared by a direct reduction diffusion process in the same manner as that in Example 1, from 320 g of Nd 2 O 3 , 63.6 g of Dy 2 O 3 , 45.7 g of cobalt powder, 16.2 g of an Fe-B alloy powder, and 620 g of iron powder, having added therein pertinent amounts each of metallic calcium and CaCl 2 . The alloy powder thus obtained contained 12.5 atomic % of neodymium (Nd), 0.3 atomic % of praseodymium (Pr), 2.2 atomic % of dysprosium (Dy), 2.0 atomic % of boron (B), and 78 atomic % of iron (Fe). The oxygen content of the powder was 2,000 ppm. Sintered permanent magnets as shown in Table 4 below were obtained in the same procedure as that used in Example 1, by blending and mixing predetermined amounts of the alloy powder II with the alloy powder material I. Besides three types (Nos. 5B, 5C, and 5D) obtained from the alloy powder materials according to the present invention, an alloy powder having added therein no alloy powder II was prepared according to a conventional process for use as a comparative sample (No. 5A). The magnetic properties of the sintered permanent magnets thus obtained are summarized in Table 4 below. Sample No. Mixing ratio of Powders Composition Magnetic properties Principal Adjusting Br iHc (BH) max) (%) (%) (atomic%) (kOe) (kOe) (MGOe) 5A 100 0 11.ONd-0.3Pr-2.2Dy-5.OCo-7.OB-balFe 12.0 21.5 34.0 5B 95 5 11.1Nd-0.3Pr-2.2Dy-5.OCo 6.7B-balFe 12.1 22.0 35.2 5C 90 10 11.2Nd-0.3Pr-2.2Dy-5.0 Co-6.5B-balFe 12.3 22.5 36.3 5D 80 20 11.3Nd-0.3Pr-2.2Dy-5.OCo-6.OB-balFe 12.5 22.8 37.5 From the composition of the magnet as summarized in Table 4, the component ratio of the phases, i.e., R 2 Fe 14 B:B rich phase:R-rich phase, can be calculated as follows. No. 5A (Conventional) 92.9 : 2.3 : 4.8, No. 5B (Present invention) 93.1 : 1.9 : 5.0, No. 5C (Present invention) 93.4 : 1.4 : 5.2, and No. 5D (Present invention) 94.0 : 0.5 : 5.5. It can be seen from the results in Table 4 that the magnets obtained from the starting powder material according to the present invention yield superior Br, iHc, and (BH)max values as compared to those of a magnet obtained by a conventional process. Furthermore, it can be seen also that magnets having desired magnetic properties can be readily obtained from the powder material according to the present invention, because the component ratio of the phases in the final sintered magnet can be controlled arbitrarily.

Claims: A process for producing a sintered permanent magnet from a mixture of starting alloy powders which mixture comprises an intermetallic alloy powder I, containing a R 2 Fe 14 B phase as the principal phase, with an inherent B-rich phase and R-rich phase (wherein R is at least one element selected from the group consisting of Nd, Pr, La, Ce, Tb, Dy, Ho, Er, Eu, Sm, Gd, Pm, Tm, Yb, Lu, and Y), and an alloy powder II of the rare earth-transition metal series intermetallic compound phase, R-TM and/or an alloy powder of the rare earth-transition metal-boron series intermetallic compound phase, R-TM-B, (wherein R has the above meaning and TM is a metallic material including Fe), wherein the said powders are mixed, compacted and sintered, characterised in that the mixture comprises the R 2 Fe 14 B phase alloy powder I, which consists of 10-30 atomic % of R, 4-40 atomic % of B and the balance Fe, where Fe may be partially substituted by Co, all elements including unavoidable impurities, and an alloy powder II, containing a R 2 Fe 17 compound phase, which phase consists of 5-35 atomic % of R and the balance Fe, where Fe may be partially substituted by Co, all elements including unavoidable impurities, wherein the alloy powder II is present in the total mixture of alloy powders I + II in an amount of 70 % by weight or less, and wherein the R 2 Fe 17 compound is reacted in the sintering step, at a temperature between the vicinity of the eutectic point thereof and the sintering temperature, with the B-rich phase and the R-rich phase contained in powder I, to increase the amount of the R 2 Fe 14 B phase alloy in powder I and consequently the overall content of the R 2 Fe 14 B compound as the permanent magnetic component of the magnet. A process as claimed in claim 1 in which at least one of the powders is prepared by a process of making an ingot which is crushed into powder particles. A process as claimed in claim 1 in which at least one of the powders is prepared by a direct reduction diffusion process. A process as claimed in any preceding claim, wherein the powder II is present in the total mixture of alloy powders in an amount of 0.1 - 40 % by weight. A process as claimed in any preceding claim, wherein the content of the element(s) R in powder I is 12 - 20 atomic %. A process as claimed in any preceding claim, wherein the content of B in powder I is 6 - 20 atomic %. A process as claimed in any preceding claim, wherein the content of Fe in powder I is 30 - 84 atomic %. A process as claimed in claim 7, wherein the content of Fe in powder I is 60 - 82 atomic %. A process as claimed in any preceding claim, wherein Co as a partial substitute for Fe is incorporated in powder I in an amount of 10 atomic % or less. A process as claimed in any preceding claim, wherein the content in powder I of Fe containing Co as a partial substitute therefor is 17 - 84 atomic %. A process as claimed in any preceding claim, wherein the content of Fe in powder II is 65 - 95 atomic %. A process as claimed in any preceding claim, wherein Fe in powder II is partially substituted by 6 atomic % or less of B. A process as claimed in any preceding claim, wherein the content in powder II of Fe plus B as a partial substitute therefor is 59 - 89 atomic %. A process as claimed in any preceding claim, wherein at least one of powder I and powder II contains at least one of: 3.5 atomic % or less of Cu, 2.5 atomic % or less of S, 4.5 atomic % or less of Ti, 15 atomic % or less of Si, 9.5 atomic % or less of V, 12.5 atomic % or less of Nb, 10.5 atomic % or less of Ta, 8.5 atomic % or less of Cr, 9.5 atomic % or less of Mo, 7.5 atomic % or less of W, 3.5 atomic % or less of Mn, 19.5 atomic % or less of Al, 2.5 atomic % or less of Sb, 7 atomic % or less of Ge, 3.5 atomic % or less of Sn, 5.5 atomic % or less of Zr, 5.5 atomic % or less of Hf, 8.5 atomic % or less of Ca, 8.5 atomic % or less of Mg, 7.0 atomic % or less of Sr, 7.0 atomic % or less of barium Ba, and 7.0 atomic % or less of Be. A process as claimed in any preceding claim, wherein the powder mixture contains 12 - 25 atomic % of an element R (as defined in claim 1), 4 - 10 atomic % of B, 0.1 - 10 atomic % of Co, and 68 - 80 atomic % of Fe. A process as claimed in any preceding claim, wherein the powder mixture has an average granularity of 1 - 80 µm. A process as claimed in Claim 16, wherein the powder mixture has an average granularity of 2 - 10 µm.

Inventors: KANEKO YUJI, TOKUHARA KOKI, KANEKO, YUJI, TOKUHARA, KOKI
Applicants: SUMITOMO SPEC METALS, SUMITOMO SPECIAL METALS COMPANY LIMITED
Application year: 1993
Main classification: H01F1
IPC codes: H01F1