A RARE EARTH-BASED ALLOY FOR PERMANENT MAGNET

Abstract: In contrast to the generally accepted understanding that no excellent magnetically anisotropic sintered permanent magnet can be obtained of a rare earth-iron alloy, rare earth-iron based permanent magnets of high magnetic performance almost equiva­ lent to the samarium-cobalt magnets can be prepared by admixing the rare earth-iron alloy with 0.1 to 10% by weight of titanium. Partial replacement of iron in the alloy with cobalt has an effect to increase the Curie point of the permanent magnet.

Description: BACKGROUND OF THE INVENTION The present invention relates to a rare earth-based alloy for permanent magnet having excellent magnetic properties and suit­ able as a component of various kinds of electric and electronic in­ struments. Various kinds of rare earth-based permanent magnet alloys have been developed hitherto and are under production in large quantities including the samarium-cobalt magnet alloys of the chemical composition SmCo₅. The magnetic properties of the per­ manent magnets of this type are so excellent that the maximum energy product (BH) max thereof exceeds 20 MGOe in the magnets manufactured under experimental conditions or is constantly in the range from 16 to 18 MGOe in the magnets manufactured as in­ dustrial products. Accordingly, these permanent magnets are widely used in a variety of applications such as speakers, electric motors, metering instruments and the like in which the perma­ nent magnets are required to exhibit high performance. One of the problems in the samarium-based permanent magnets is the high production costs as a consequence of the high content of relatively expensive cobalt metal therein up to 60% by weight or more. It would therefore be a desirable measure to replace the cobalt metal with a less expensive metal such as iron and some attempts have been made hitherto in this direction. No fruitful results, however, have yet been obtained in such an attempt presumably due to the absence of possible solid-solution formation of iron with the SmCo₅ intermetallic compound. On the other hand, several binary intermetallic compounds of rare earths and iron are known including the compounds of the formulas RFe₂, RFe₃ and R₂Fe₁₇, R being a rare earth element. These rare earth-iron intermetallic compounds, however, are not utilized as a permanent magnet due to the low value of either one of the magnetic parameters of Curie point T c , saturation magnet­ ization 4πM s and crystalline magnetic anisotropy constant K u , In contrast to the series of rare earth-cobalt intermetallic compounds including the RCo₅-type compounds having the crystalline struc­ ture of CaCu₅, of which the above mentioned SmCo₅ alloy is prac­ tically utilized as a material of permanent magnets, no RFe₅ type compounds were known for long in the series of rare earth-iron compounds, at least, in the form of a bulk body. It was only in 1984 that Cadieu, et al. reported in Journal of Applied Physics, volume 55, page 2611 (1984) that thin films of SmFe₅ and (SmTi) x FE₁₀₀ -x' in which the atomic ratio of Ti:Fe was 1:9 or 1:19, could be formed by the method of sputtering. These intermetallic compounds in the form of a thin film reportedly have a hexagonal crystalline structure of CaCu₅. These thin films formed by the sputtering method, however, were in the state of a metastable phase and it was generally understood that such an intermetallic compound could not exist as a bulk body. Therefore, the only permanent magnet based on a rare earth-iron binary compound so far report­ ed is the magnet in a metastable phase prepared by the quenched thin-film method disclosed by Croat, et al. in IEEE Transactions on Magnetics, volume MAG 18, page 1442 (November, 1982). The quenched thin-film magnet prepared by this method is isotropic and based on a metastable phase so that the magnet is not free from the problem of low stability so that the magnets of this type are not in practical use. Turning now to the recently highlighted neodymium-iron-­ boron magnets formed of a ternary compound of a chemical compo­ sition of the formula R₂F₁₄B, they are promising as a high-per­ formance permanent magnet since the base components are inex­ pensive neodymium and iron and the magnetic properties thereof are even better than those of the samarium-cobalt magnets. These neodymium-iron-boron magnets are, however, not free from a very serious problem that they are highly susceptible to rusting so that the magnets cannot be used practically without providing a protective coating. This disadvantage can hardly be overcome and no practical solution of the problem has yet been obtained to give a possibility of industrial production of the magnets of this type. SUMMARY OF THE INVENTION An object of the present invention is therefore to provide a rare earth-based permanent magnet having magnetic properties equivalent to or even better than those of the samarium-cobalt permanent magnets without using or by decreasing the amount of expensive cobalt as well as to provide a rare earth-based alloy as a base material of such a permanent magnet. Thus, the permanent magnet alloy of the present invention consists essentially of: (a) form 12 to 45% by weight of a rare earth element or a combina­ tion of rare earth elements; (b) from 0.1 to 10% by weight of titanium; and (c) the balance of iron or a combination of iron and cobalt, the amount of iron being at least 40% by weight thereof, including un­ avoidable impurities. Further, the permanent magnet of the invention is a sintered body of a powder of the above defined rare earth-based alloy hav­ ing magnetic anisotropy. BRIEF DESCRIPTION OF THE DRAWING FIGURE 1 is a graph showing the magnetization of inven­ tive and conventional samarium-based alloys as a function of tem­ perature. FIGURE 2 illustrates the X-ray diffraction diagrams of the same alloys using CuKα line. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In respect of the problem of possible existence of RFe₅ phase, the results of experiments hitherto undertaken support that no such a 1:5 phase or, in particular, SmFe₅ phase is formed even when samarium and iron are melted together and alloyed in a pro­ portion corresponding to the composition of SmFe₅ although three phases of Sm₂Fe₁₇, SmFe₂ and SmFe₃ could be found as is indicat­ ed by the results obtained in the measurement with a vibration magnetometer and in the X-ray diffractometry as is shown in FIGURES 1 and 2. With an object to obtain a rare earth-iron based permanent magnet having excellent magnetic properties equivalent to those of the imaginary SmFe₅ magnet, the inventors have conducted ex­ tensive investigations on a third additive element to be added to the rare earth-iron binary magnet alloys and arrived at discover­ ies that addition of titanium would give a quite satisfactory result and a hitherto unknown ternary intermetallic compound of sama­ rium, titanium and iron can exist in a bulky form by the optimi­ zation of the amount of substitution of titanium for the rare earth element or, in particular, samarium. Thus, a ternary alloy of sa­ marium, titanium and iron was prepared in such a proportion as to correspond to the formula of SmTiFe₁₀ and the alloy was subjected to the measurement of the magnetization as a function of tempera­ ture and X-ray diffractometry to give the results shown in FIG­ URES 1 and 2, respectively. These figures indicate that, in clear difference from those of the SmCo₅-type crystalline structure, crys­ tallographic indices approximately corresponding to those of the tetragonal crystalline structure can be allotted to the peaks in the X-ray diffractometric diagram of the ternary ally and the temper­ ature dependency of the magnetization thereof is also close to that of a single-phase alloy leading to a conclusion that the ternary compound of samarium, titanium and iron is imparted with stabi­ lity as a result of introduction of titanium into the samarium-iron binary alloy. The further continued investigations have led to con­ firmation that the above described unique phenomenon is held also for the rare earth elements in general other than samarium including yttrium. Thus, the present invention provides, as an embodiment, a ternary alloy composed of (a) from 12 to 45% by weight of a rare earth element or a combination of rare earth elements; (b) from 0.1 to 10% by wieght of titanium; and (c) the balance of iron including unavoidable impurities. Namely, the magnet alloy can be obtained by melting the component metals together and the alloy is finely pulverized followed by the powder metallurgical processing of the powder by compression molding and sintering. When the amount of the rare earth component in the alloy formulation is outside the above specified range, the ternary compound would be less stable and, therefore, any amounts thereof smaller than 12% by weight and larger than 45% may result in a disadvantageously rapid de­ crease in the coercive force i H c and saturation magnetization 4πM s , respective-ly. The above mentioned range for titanium is also critical because the ternary compound is less stable when the amount of titanium is smaller than 0.1% by weight while the frac­ tion of the phase of the ternary compound is decreased when the amount of titanium is larger than 10% by weight. The rare earth element here implied include the so-called lanthanoid elements having atomic munbers of 57 to 71 and yttrium. Any of these rare earth elements can be used either singly or as a combination of two kinds or more according to need. The rare earth-based permanent magnet of the invention prepared of the ternary alloy contains the stable phase of the ter­ nary compound as a result of the introduction of titanium so that the Curie point thereof is about 310°C when the rare earth ele­ ment is samarium which is much higher than 120°C of the Sm₂Fe₁₇ phase. In addition, the saturation magnetization is also greatly increased so that the thus obtained permanent magnet has very high magnetic properties. Moreover, the rare earth-titanium-­ iron permanent magnet of the invention can be imparted with magnetic anistropy by the powder metallurgical method so that the overall magnetic performance of the inventive permanent magnet can be almost equivalent to or even better than the sama­ rium-cobalt based magnets. Despite the high content of iron in the alloy of the inventive permanent magnet, which is formed mainly of a tetragonally crystalline phase, the inventive permanent mgnet is highly cor­ rosion-resistant and free from rusting in clear contrast to the neo­ dymium-iron based magnets. Accordingly, the inventive perma­ nent magnets can be used in practical applications without any pr­ tective coating on the surface although the corrosion resistance thereof can of course be further increased by a protective coating or surface treatment by forming a resinous layer or a metallic layer formed by electrolytic or electroless plating, vacuum vapor deposi­ tion, sputtering or ion plating. Further, the ternary alloy can be processed into a thin film having a high coercive force by the quenched thin-film method and the thin film can be finely pulverized into fine particles of which magnetically isotropic permanent magnets can be prepared. It is of course that the magnetically anisotropic sintered magnet is pul­ verized into fine particles of which anisotropic plastic magnets can be prepared. As is mentioned above, the permanent magnet of the ternary alloy of samarium, titanium and iron has a Curie point of about 310°C. Although this Curie point is well within the practically ac­ ceptable range, it is of course desirable to have a higher Curie point when comparison is made with the SmCo₅ permanent mag­ nets having a Curie point at about 740°C. In this regard, the in­ ventors have further continued extensive investigations and arriv­ ed at a discovery that a magnetic alloy suitable for the purpose can be obtained when a solid solution is formed of the above described ternary compound of rare earth, titanium and iron with cobalt. For example, an increase by about 40 to 100°C can be obtained in the Curie point of the ternary alloy when 10 atomic % of iron in the alloy is replaced with cobalt although the increment depends on the kind of the rare earth element. The Curie point T c is increased approximately linearly with the increase in the amount of replace­ ment of iron with cobalt up to 50% replacement by weight but thereafter the increment in the Curie point is relatively small with further increased replacement of iron with cobalt to finally level off. In addition, the saturation magnetization of the magnet is in­ creased as a trend though dependent on the kind of the rare earth element by the substitution of cobalt for a part of iron in the ter­ nary magnet alloy of rare earth, titanium and iron to level off with increase of the proportion of cobalt relative to iron. It should be noted, however, that replacement of iron with cobalt has some adverse effect of decreasing the coercive force of the magnet. For example, more than 40% replacement of iron with cobalt is undesirable due to the great decrease in the coercive force of the magnet. This is the reason for the limitation that more than 60% by weight or, preferably, more than 40% by weight of iron should not be replaced with cobalt. In the following, the rare earth-based alloy for permanent magnets and the sintered permanent magnet of the alloy accord­ ing to the invention are described in more detail by way of exam­ ples. Example 1. Metals of samarium, titanium and iron each having a purity of 99.9% were taken by weighing in the proportion indicated in Table 1 below and melted together in a high-frequency induction furnace. The melt was cast into a water-cooled, copper-made cast­ ing mold to form an ingot of the alloy. The ingot was crushed and then pulverized in a jet mill using nitrogen gas as the ject gas to give a fine powder having an average particle diameter in the range from 2 to 10 µm. The powder was compression-molded under a pressure of 1.5 tons/cm² with the particles oriented in a static magnetic field of 15 KOe into a green body, which was sintered by heating in an atmosphere of argon gas for 1 hour at a temperature in the range form 1000 to 1200 °C and then subjected to thermal aging for 4 hours at a temperature in the range from 500 to 900 °C followed by quenching. The thus obtained magnetically anisotropic sintered body after the thermal aging was subjected to the measurement of the density of residual magnetic flux B r , coercive force i H c and maxi­ mum energy product (BH) max to give the results shown in Table 1 for the three different formulations of the alloys No. 1, No. 2 and No. 3. For comparison, Table 1 also includes the results of the magnetic measurement of a sintered body of a samarium-iron al­ loy corresponding to SmFe₅ (No. 4) prepared in the same manner as above. As is shown in the table, this comparative sintered body had only negligibly small values of coercive force and maximum energy product. Example 2. Metals of praseodymium, samarium, cerium, titanium and iron were taken by weighing in the proportion indicated in Table 2 and magnetically anisotropic sintered bodies were prepared each in the same manner as in Example 1. The coercive force i H c of these sintered bodies was measured to give the results shown in the table which supports the conclusion that the coercive force is little affected by the replacement of samarium with other rare earth elements. Example 3. Magnetically anisotropic sintered permanent magnets No. 1 to No. 4 were prepared each in the same manner as in Example 1 except that the magnetic alloy was prepared from metals of neody­ mium, titanium, iron and cobalt each having a purity of 99.9% taken by weighing in the proportion indicated in Table 3. These sintered permanent magnets were subjected to the measurement of the density of residual magnetic flux B r , coercive force i H c and maximum energy product (BH) max as well as Curie point to give the results shown in the table, in which Δ T c means the increment of the Curie point T c in °C obtained by the replace­ ment of a part of iron with cobalt. Example 4. Magnetically anisotropic sintered permanent magnets No. 1 and No. 2 were prepared in the same manner as in the preceding examples from metals of samarium, cerium, titanium, iron and co­ balt taken by weighing in the proportion indicated in Table 4 below. These sintered permanent magnets were subjected to the measurement of the magnetic properties to give the results shown in the table.

Claims: 1. A rare earth-based alloy for permanent magnet consisting essentially of: (a) from 12 to 45% by weight of a rare earth element or a combina­ tion of rare earth elements; (b) from 0.1 to 10% by weight of titanium; and (c) the balance of iron or a combination of iron and cobalt, the amount of iron being at least 60% by weight thereof, including un­ avoidable impurities. 2. The rare earth-based alloy for permanent magnet as claimed in claim 1 wherein the rare earth element is selected form the group consisting of the elements of the atomic number of from 57 to 71 and yttrium. 3. A permanent magnet which is a sintered body of a powder of a rare earth-based alloy consisting essentially of: (a) from 12 to 45% by weight of a rare earth element or a combina­ tion of rare earth elements; (b) from 0.1 to 10% by weight of titanium; and (c) the balance of iron or a combination of iron and cobalt, the amount of iron being at least 60% by weight thereof, including unavoidable impurities.

Inventors: OHASHI, KEN, TAWARA, YOSHIO, YOKOYAMA, TOSHIKAZU
Applicants: SHIN-ETSU CHEMICAL CO., LTD.
Application year: 1987
Main classification: C22C38
IPC codes: C22C38, H01F1