Preparation and properties of mechanically alloyed rare earth permanent magnets

Journal of Magnetism and Magnetic Materials 80 (1989) 115-118 North-Holland, Amsterdam

115

PREPARATION AND PROPERTIES OF MECHANICALLY ALLOYED RARE EARTH PERMANENT MAGNETS L. SCHULTZ, K. S C H N I T Z K E and J. W E C K E R Siemens AG, Research Laboratories, Erlangen, Fed Rep. Germany

Mechanical alloyingis appfied to prepare Nd-Fe-B and ThMn12-typepermanent magnets. Starting from elementpowders, the hard-magnetic phases are formed by millingill a planetary ball mill and a successivesolid-state reaction at relativelylow temperatures. For Nd-Fe-B, the magneticallyisotropic particles are microcrystalline,show a high coereivity(up to 20 kOe for ternary alloys and above for Dy-substituted samples) and can be used either for making bonded magnets or compacted to dense isotropie magnets by hot uniaxial pressing. Magneticallyanisotropie samples are formed by die-upsetting. With regard to grain size, magnetic properties, compaction and formation of anisotropy, the mechanicallyalloyed material is comparable with rapidly quenched Nd-Fe-B. The mechanical alloying process has also been applied to prepare magnetic material of Fe-rich and RE-containing phases with the ThMn12 structure which show interesting intrinsic magnetic properties. Most promissing are Sm-Fe-Mo and Sm-Fe-Ti alloys, where coercivitiesup to 5 kOe have been achievedso far.

1. Introduction N d - F e - B magnets are usually produced either by the powder metallurgical [1] or by the rapid quenching process [2]. An alternative production route is mechanical alloying with a subsequent solid-state reaction [3,4]. In this contribution, we report on the formation of resin-bonded and magnetically isotropic and anisotropic compacted N d - F e - B magnets by this technique and on the first results for the new magnetic materials with the ThMn12 structure.

2. Results for N d - F e - B Mechanical alloying of N d - F e - B powder is performed in a planetary ball mill under an argon atmosphere. The elemental powders are mixed and poured into a cylindrical milling container together with 10 m m diameter steel bails. The ball milling first produces powder with a layered microstructure of Fe and Nd, The submicron boron powder remains undeformed and is caught by the colliding Fe and N d particles (which are coldwelded) and is, therefore, embedded in the F e / N d interfaces. Further milling leads to a refinement of

the layered microstructure. The crystallite size of the Fe as derived from X-ray diffraction lies in the range of 20 to 40 nm. Atomic mixing by deformation during milling does not occur [5]. There are no hints that either a crystalline or an amorphous F e - N d phase is formed during milling. Thermodynamic calculations show that for F e - N d the difference of the free enthalpies between the amorphous phase and the layered composite is positive over the whole composition range [6], thus preventing an interdiffusional reaction. Therefore, by mechanical alloying, a layered F e - N d composite is energetically favored. The hard-magnetic Nd2Fe14B phase is formed by a heat treatment. Because of the extremely fine microstructure of the milled powders, the reaction can take place at relatively low temperatures or with short reaction times at higher temperatures. Optimum coercivities are obtained after an annealing at 700 ° C for 15 to 30 min [3]. The grain size of the Nd2Fe14B phase is then about 50 nm. Fig. 1 is a schematic representation of the processing of the milled powder and the resulting magnet types. Annealing of the as-milled material results in magnetically isotropic powders which can be used to produce resin-bonded permanent magnets (MM1; MM: "Magnemilr'). Isotropic

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L Schultz et aL / Mechanically alloyed permanent magnets

resin

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Fig. 1. Preparation of Nd-Fe-B magnets from mechanically alloyed powder.

compacted magnets (MM2) can be obtained either from as-miUed or from pre-reacted powders by hot uniaxial pressing [6]. A pressure of I kbar at a temperature above 600 °C is sufficient to get fully dense material. Magnetically anisotropic samples (MM3) can be prepared from compacted isotropic magnets by texturing via hot deformation (die-upsetting; fig. 1) in a similar way to that shown for rapidly quenched material [7]. In our experiments, 5 nun diameter samples are uniaxially compressed to half their height within a 7 mm diameter die at 700 o C. Crushing of these samples can produce magnetically anisotropic powder. Fig. 2 shows the demagnetization curves of all three types of magnets produced from mechanically alloyed Ndz6Fe76B8 powder. The resinbonded sample (MM1) exhibits a coercivity of t

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15.3 kOe (the magnetization values relate only to the magnetic powder). The compacted sample (MM2) shows a similar coercivity (15.4 kOe). The hot deformation to form the anisotropic sample (MM3) reduces the coercivity to 11.5 kOe, but improves the remanence, the squareness of the magnetization loop and the energy product. The demagnetization curves parallel and perpendicular to the press direction of an anisotropic sample are shown in fig. 3. The difference in the curve shape clearly demonstrates that the sample is now magnetically anisotropic. The ratio of the remanences measured parallel and perpendicular to the press direction, which is a measure of the degree of alignment, is 1.8. Applying a further optimized process, coercivities of 17.2 kOe for the compacted isotropic and of 15.3 kOe for the anisotropic sample have been obtained. Also the degree of alignment of the anisotropic magnets can be considerably improved by increasing the deformation ratio.

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After the success of the N d - F e - B magnets, a search for further intermetallic phases which might be suitable as permanent magnets has started. So far, the most interesting phases are the iron-rich and rare-earth-containing phases with the tetragonal ThMnx2 crystal structure (1 : 12 magnets), such as RE(Fe,V)12 [8], RE(Fe,Ti)12 [9] or RE(Fe, Mo)z 2 [10]. As a general trend for these 1:12 inter-

L Schultz et a L / Mechanically alloyedpermanent magnets

metallics, the alloys with RE = Sm show by far the highest anisotropy values. Due to the promising intrinsic magnetic properties, these phases are candidates for magnets with high coercivities. Although the anisotropy field H A for SmsFes0Mo~2 (90 kOe) is higher than that for Nd2Fe14B (75 kOe), the coercivity of the samples prepared by standard powder metallurgical techniques did not exceed 1 kOe. One of the problems here is the loss of Sm during melting, since Sm has a much higher vapour pressure than the other rare-earth metals at the elevated temperatures which must be applied due to the high melting points of Mo or Ti. We therefore applied the mechanical alloying process for these materials [11], investigating especially alloys of the types S m - F e - M o and S m - F e - T i and the corresponding Co-substituted material. Fig. 4 shows the X-ray diffraction patterns of mechanically alloyed Sml0Fes0Til0 powders as-milled and after a 1 h heat treatment at temperatures between 750 and 900 °C (to prevent oxidation the powder samples were covered under a Kapton foil resulting in the increased

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intensity at low angles) in comparison with that of a bulk sample (after ref. [9]) with the 1 : 12 structure. For the as-milled powder only the broadened diffraction peaks of pure Fe are observed. The hard-magnetic 1:12 phase is formed during a diffusion heat treatment of 1 h at 850 to 900 °C at 10 -6 mbar argon. The X-ray diffraction pattern (fig. 4) of a Sml0Fes0Til0 powder annealed at 850 ° C for 1 h shows the same diffraction peaks as the bulk sample with 1 : 12 structure demonstrating that the 1 : 12 structure has been formed in the mechanically alloyed S m - F e - T i powder during annealing. Fig. 5 shows the demagnetization curve of a magnetically isotropic resin-bonded Sml0Fe70Col0Mol0 sample with a coercivity of 5.0 kOe, a remanence of 5.2 kG and an energy product of 4.9 MGOe. For Sml0Fes0Mol0 the best coercivity value obtained so far is 4.8 kOe.

4. Conclusions diffraction

angle

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ThMn12 crystal structure

Fig. 4. X-ray diffraction patterns of mechanically alloyed SmloFesoTi]0 powders as-milled and after a 1 h heat treatment a t temperatures between 750 and 900 ° C in comparison with that of a bulk sample (after ref. [9]) with 1 : 12 structure.

These results show that mechanical alloying is a useful technique to prepare N d - F e - B magnets with magnetic properties that are comparable to rapidly quenched material. Also, 1 : 12 type magnets have been produced with considerable coercivities, although these values must be further improved - maybe via a better understanding of the magnetic hardening mechanism - to make the

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1:12 magnets technologically interesting. Therefore, the mechanically alloying process has a good chance to get its share on the growing rare earth permanent magnet market.

Acknowledgement This work has been supported by the German Ministry for Research and Technology.

References [1] M. Sagawa, S. Fujimura, N. Togawa, H. Yamamoto and Y. Matsuura, J. Appl. Phys. 55 (1984) 2083. [2] J.J. Croat, J.F. Herbst, R.W. Lee and F.E. Pinkerton, Appl. Phys. Left. 44 (1984) 148.

[3] L. Schultz, J. Wecker and E. Hellstern, J. Appl. Phys. 61 (1987) 3583. [4] L. Schultz, K. Schnitzke and J. Wecker, J. Appl. Phys. 64 (1988) 5302. [5] L. Schultz, in: Proc. DGM Conf. on New Materials by Mechanical Alloying Techniques, Calw/Hirsau, October 1988, eds. E. Arzt and L. Schultz (DGM Informationsgesellschaft, Oberursel) p. 53. [6] L. Schultz and J. Weaker, Mater. Sci. Eng. 99 (1988) 127. [7] R.W. Lee, Appl. Phys. Lett. 46 (1985) 790. [8] K.H.J. Buschow, Proc. 9th Intern. Workshop on RareEarth Magnets and their Applications, Bad Soden, September 1987, Deutsche Physikalische Gesellschaft, Bad Honnef, p. 453. [9] K. Ohashi, T. Yokoyama, R. Osugi and Y. Tawara, IEEE Trans. on Magn. MAG-23 (1987) 3101. [10] A. MUller, J. Appl. Phys. 64 (1988) 249. [11] L. Schultz and J. Wecker, J. Appl. Phys. 64 (1988) 5711.