Method for Manufacturing a Permanent Magnet Material From Starting Components in Powder Form

This invention relates to a method for manufacturing a permanent magnet material of a metal/metal/metalloid system, in which at least one powdered starting component of the metals, together with a powdered starting component of elemental boron or a boron compound or alloy, is mixed, optionally compacted, and finally subjected to an annealing treatment for forming the permanent magnet material.

A method of the type described above is disclosed, for instance, in the publication Journal of Applied Physics, Vol. 57, No. 1, Apr. 15, 1985, pages 4149 to 4151. For some time, new permanent magnet materials of metal/metal/metalloid systems have been known which exceed by far all materials known to date with respect to the most important hard-magnetic variable, namely, the energy product. Preferred materials of these systems have substantially the composition Nd.sub.2 Fe.sub.14 B, wherein a partial substitution of the mentioned elements or slight deviations from the stoichiometry of this tetragonal phase are possible in order to thus optimize the microstructure of the materials. For the manufacturer of such permanent magnet materials on a large technical scale, in particular, two methods are used. According to the method of European Patent EP 0 126 802 A1, an alloy of the desired composition is first melted, subsequently comminuted to form a fine pulver, magnetically oriented in a magnetic field, and then compacted by a pressure and sintering treatment. In the method disclosed in European Patent EP 0 144 112 A 1, an intermediate product is first produced by fast quenching from the melt of the starting components, which is then compacted by hot pressing, and thereafter oriented in a further process step, the so-called "die-upsetting", in the preferred magnetic direction. See, for example, Applied Physics Letters, Vol. 46, No. 8, Apr. 15, 1985, pages 790 and 791. Materials which have been produced according to these two methods differ primarily with respect to their microstructure. While in the method of EP 0 126 802 A1, a relatively coarse grain structure with several foreign phases develops, the samples which were quenched fast according to the EP 0 144 112 A1 method are characterized by an extremely fine grain structure which in this case causes the anchoring of the Bloch walls, responsible for a magnetization reversal.

Besides these two methods, it is known from the Journal of Applied Physics, Vol. 57, No. 1, pages 4149 to 4151 (1985) that for the manufacture of permanent magnet material to use as the starting components Fe--, Fe.sub.2 B-- and Nd-- powder which are subsequently compacted and sintered. In the process, the desired phase is formed by diffusion. In order to obtain a magnetically anisotropic material, however, this sintered material must subsequently be comminuted again and, after magnetic orientation, compacted and sintered again. Thus, this prior art method for the manufacture of permanent-magnet materials with magnetic anisotrophy is relatively expensive.

It is therefore an object of the present invention to improve the previously discussed prior art in such a manner that a powder of the discussed material system which has an extremely fine microstructure, similar to that of fast-quenched material, can be produced in a simple manner and can optionally be compacted to form a body of magnetically oriented material by methods known per se. These and other objects of the present invention will become apparent from the following description and claims.

According to the present invention, the powder mixture of the starting components is first subjected to a milling process in the manner of mechanical alloying, wherein a mixed powder of at least one metallic starting component with embedded or adsorbed or attached fine particles of the boron component is formed. Powders are understood here to also include, very generally, bodies, particles, etc., such as fillings, which have forms similar to powder.

The advantages connected with this embodiment of the method of the present invention are, in particular, that the powder mixture so obtained can be compacted without difficulty in a manner known per se and subjected to an annealing treatment at a relatively low temperature for forming the desired magnetically hard phase. A preceding sintering or melting process with subsequent comminution of the material is therefore not necessary. Nevertheless, extremely fine powders can be obtained by the milling process.

This invention will be explained in further detail particularly with reference to the production of a special magnetically hard metal/metal/boron (M.sub.1 M.sub.2 B) alloy.

In this type of alloy, M.sub.2 is chosen from the group of the late transition metals of the periodic system of the elements. M.sub.1 is a rare earth metal or an actinide. The corresponding metallic starting component should be in powder form or at least have a powder-like appearance, wherein they can preferably be present in elemental form or optionally also in the form of alloys or compounds. M.sub.1 and M.sub.2 can be in particular the metals neodymium (Nd) and iron (Fe). An example of one embodiment of the present invention employing the ternary alloy NdFeB will be described.

For manufacturing powder of these NdFeB alloys, powders of the two metallic starting components Fe and Nd as well as B-powder are placed together with hardened steel balls in a suitable milling cup. The mass ratio of the three types of powder of this powder mixture is determined by the predetermined resulting atom concentration of the material desired to be produced from these powders. Thus, the mass ratio of the three elemental types of powder of this powder mixture can be chosen so that after a diffusion reaction is carried out, the composition Nd.sub.15 Fe.sub.77 B.sub.8 has been produced. Generally the content of Nd can be between 10 and 20 atom percent and that of B, between 2 and 10 atom percent, the Fe content essentially accounting for the remainder.

While the size of the individual powder can be arbitrary, a similar size distribution of the two participating metallic starting components in a range between about 5 .mu.m and about 1 mm and preferably between about 20 .mu.m and about 0.5 mm is advantageous. In this example, Fe powder with a particle size under 40 .mu.m and Nd fillings with a particle size under 0.5 mm are used. In addition, the B-powder should be as fine as possible. Advantageously, the B-powder particles have an extent of less than about 10 .mu.m and preferably less than about 1 .mu.m. Suitably, this may be largely amorphous B powder. In accordance with the present invention, these three powders with corresponding particle sizes are then subjected to a milling process such as is generally known in processes of mechanical alloying. See, for example, Metallurgical Transactions Vol. 5, August 1974, pages 1929 to 1934, or Scientific American, Vol. 234, 1976, page 40 to 48. Accordingly, the three starting components in powder form are thus placed in a planetary ball mill, e.g., Trade Mark Fritsch: Type "Pulverisette 5", which may have, for example, 100 steel balls with diameters of 10 mm each. The duration of the milling process depends on the desired fineness of the mixture powder as well as on the milling parameters. Important parameters during the milling are the ball diameter, the number of balls, as well as the material of the milling crucible and the ball material. Also, the milling rate, the ratio of the steel balls, and the amount of powder are further parameters which determine the required milling duration. In order to prevent surface oxidation of the particles, the milling vessel, fabricated from steel, is kept in a protective gas atmosphere such as argon or helium and is opened again only after the milling process is completed.

During the milling process, finely stratified powder grains, comprising Fe and Nd layers, are already formed after a milling time of about two hours. In the process, the boron particles are embedded at the Fe/Nd boundary surfaces as well as in the elemental metal or adsorbed thereon. With advancing milling time, this layer structure gets finer and finer until, after a milling time of about 10 to 30 hours, it can no longer be resolved by a light microscope. Thus, powder particles of a mixture powder have then been generated which comprise an intimate mixture of Fe and Nd with embedded or adsorbed or attached boron particles, the size of which is distinctly smaller than 1 .mu.m. The powder particles themselves have a diameter of about 1 to 200 .mu.m. In X-ray examinations of this mixture powder, only a greatly widened intensity maxima of Fe can be seen. There are no references to the formation of amorphous FeNd or an FeNd phase.

The subsequent reaction anneal must likewise take place in a protective gas or in a vacuum. The anneal can be performed at one or several different temperatures. The annealing may suitably take place at a temperature between about 400.degree. C. and about 640.degree. C. A continuous temperature change is also possible. With an annealing treatment of, for instance, 1 hour at 600.degree. C., the desired Nd.sub.2 Fe.sub.14 B phase which has excellent magnetically hard properties is formed by a diffusion reaction. Thus, after embedment in plastic, the reacted powder exhibits a coercivity of more than 10 kOe.

A significant advantage of the method according to the present invention is in that an extremely good mixture of the participating elements exists with the milling process in the manner of mechanical alloying. Thus, only very short diffusion paths which can be overcome at relatively low temperatures or short times are required in the subsequent diffusion reaction. It is therefore possible to obtain an extremely fine microstructure of the Nd.sub.2 Fe.sub.14 B phase which corresponds, for instance, to that of a fast quenched material. The magnetic hardening of this material accordingly takes place by Bloch wall anchoring. It is a particular advantage there that the annealing can take place at temperatures below about 640.degree. C., the lowest eutectic temperature in the binary FeNd phase diagram. A rapid grain increase would take place above this temperature because of the existence of a liquid phase. For the mentioned ternary magnetically hard material, a reaction temperature between about 400.degree. C. and 640.degree. C. seems best.

Optionally, an anneal at higher temperatures such as 900.degree. C. for one hour, likewise leads to good values of the coercivity. The powder formed in this process, however, is relatively coarse-grained; it has foreign phases at the grain boundaries and exhibits a magnetic hardening mechanism which is influenced by the inhibited domain seed formation. It thus resembles the material which is prepared in accordance with the referenced EP 0 126 802 A1 and can then be processed further in a manner known per se to form an anisotropic magnet. The temperature treatments known from the European Patent can also be used to advantage for this purpose.

The compacting and adjustment of the magnetic anisotropy of the NdFeB particles which, according to the present invention, are formed at relatively low temperatures and the structure of which corresponds to that of rapidly quenched NdFeB, can be performed in the manner known per se by the processes developed for these materials.

However, this powder also can be used without compacting, for example, as a plastic-bonded isotropic magnet.

The composition of the material on which the example was based can deviate, when the materials are being weighed out, from the stoichiometric composition Nd.sub.2 Fe.sub.14 B, for instance, in such a manner as is customary for the methods known from the referenced publications. In addition, one or more of the three participating elements can be substituted partially or optionally, even completely, by other elements. Thus, for instance, Nd can be particularly substituted partially by an element of the heavy rate earths such as Dy or Tb or, for example, completely by Pr. Instead of Fe, another element of the late transition metals such as Co or Ni can be used. Partial substitution by Al is also possible. B can be partially substituted by another metalloid. The starting powders used depend on the desired compositions. With respect to the diffusion process, it is particularly advantageous for thermodynamic reasons if elemental powders are used because there, the driving force for the diffusion reaction is a maximum. For the same reason, the use of amorphous B-powder is particularly advantageous. In addition, the participating elements can also be added in the form of pre-alloyed powder, for instance, as Fe.sub.2 B or as an NdFe phase or an NdFe alloy with 20 to 40 atom percent Fe.

In selecting the pre-alloys, the metastable phases are again to be preferred over the equilibrium phase for the stated thermodynanic reasons.

According to the selected embodiment, it was assumed that at least two metallic starting components M.sub.1 and M.sub.2 are provided in powder shape, wherein each of these two components comprises a metallic (chemical) element or an alloy of several compounds of this element. It is also possible, however, to start off with only one single powder-shaped alloy of the two starting metals M.sub.1 and M.sub.2, i.e., the alloy M.sub.1 -M.sub.2 alone then supplies the two metallic components of the permanent magnet material. In the case of Nd.sub.2 Fe.sub.14 B, this could be the alloy Nd.sub.16 Fe.sub.84 in powder form, which together with the B-powder, constitutes the powder mixture to be milled.

Although preferred embodiments of the present invention have been discussed in detail, it will be appreciated by one skilled in the art that modifications may be made all within the spirit and the scope of the present invention as defined in the claims.