Hard magnetic properties of NdFeB formed by mechanical alloying and solid state reaction

Materials Science and Engineering, 99 (1988) 127-130

127

Hard Magnetic Properties of Nd-Fe-B Formed by Mechanical Alloying and Solid State Reaction* L. SCHULTZ and J. WECKER Siemens AG., Research Laboratories, Erlangen (F.R.G.)

Abstract Magnetically &otropic N d - F e - B powder is prepared from the elemental powders by mechanical alloying and a solid state reaction. Because of extremely short reaction time the particles have an ultrafine microstructure comparable with that of rapidly quenched samples. For samples with a composition of NdzxFelo o _ 3 x B x (4.5 <~x <~ I0) studied in this contribution the coercivity varies from 4 to 20 kOe, showing the influence of a secondary phase. The intrinsic coercivity of the fine-grained Nd2Fe14B phase was determined to be about 4kOe. Additional ct-Fe at low neodymium contents does not further reduce this value. Substituting cobalt for iron has a detrimental effect on coercivity in these samples. 1. Introduction N d - F e - B permanent magnets are usually produced either by the powder metallurgical [1] or by the rapid quenching process [2]. We recently reported on mechanical alloying and a subsequent solid state reaction as a third route to form N d - F e - B [3]. The first experiments showed that the material properties are similar to those of rapidly quenched samples; this is especially shown in the coercivity values at higher temperatures [4]. In this contribution, we describe the composition dependence of the magnetic properties of ternary N d - F e - B samples and the detrimental effect of cobalt additions. Mechanical alloying has been developed as a new technique of combining metals [5]. It circumvents many of the limitations of conventional alloying and creates true alloys of metals or metal-non-metal composites that are difficult or impossible to combine by other means. It is performed in a high energy ball mill in an inert atmosphere. The powder particles are trapped by the colliding balls, heavily deformed and cold welded. Characteristically layered particles are

*Paper presented at the Sixth International Conference on Rapidly Quenched Metals, Montr6al, August 3-7, 1987. 0025-5416/88/$3.50

formed. Further milling refines the microstructure increasingly. Finally, in many cases, true alloying takes place, which quite often results in the formation of metastable phases. A large number of amorphous alloys have been formed by this technique [4, 6-8] (for a recent review see ref. 9). Mechanical alloying of boron-containing alloys was first studied for Fe-Zr-B [ 10]. The experiments showed that the boron which is introduced as a submicron amorphous powder is not alloyed during the milling but is embedded into Fe-Zr particles which become amorphous. 2. Experiments and remits For the experiments, we used pure elemental powders (iron and cobalt, 5-40/~ m; neodymium, less than 0.5 mm; amorphous boron, less than 1/2m). The powders were mixed to give the desired composition and sealed under argon (less than 1 ppm 02 and H20) in a cylindrical steel container. The milling was performed in a conventional planetary ball mill (Fritsch, Pulverisette 5) without cooling. As previous microstructural investigations [3] have shown, the boron powder was finely dispersed within Fe-Nd particles, as for F e - Z r B. However, contrary to this, the Fe-Nd particles were not amorphous and were not even alloyed as X-ray investigations have revealed [3]. This can be explained by thermodynamic arguments. Figure I shows the difference AG between the free enthalpy of the amorphous phase and that of the layered composite, as calculated using the Miedema model [ I 1], for both Fe-Nd and Fe-Zr. Contrary to AG for FeZr, AG for Fe-Nd is always positive. Therefore a layered Fe-Nd composite is energetically favoured compared with the amorphous phase. After mechanical alloying the hard magnetic Nd2Fe14B phase is formed by a solid state reaction. Because of the extremely fine microstructure of the milled powders, the reaction can take place at relatively low temperatures or for short reaction times. As a detailed study of the reaction kinetics for Nd15Fe77Bs [3] showed, a reaction at 700 °C for 30 min is sufficient for optimum coercivity. Under © Elsevier Sequoia/Printed in The Netherlands

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these conditions, excessive grain coarsening does not occur, according to results obtained for overquenched material [12]. In order to study the composition dependence of the magnetic properties and the effect of interfacial phases, we chose samples of the composition Nd2xFe,oo_ 3xBx (4.5 ~ 5.9. Performing the solid state reaction below or above this temperature did not give any marked effect (see also ref. 3). The magnetic measurements were performed in a vibrating-sample magnetometer at 20 °C within the room temperature bore of an 8 T superconducting magnet. The magnetically isotropic powder is embedded in an epoxy resin after the heat treatment. The magnetization values are related to the amount of magnetic powder, neglecting the dilutional effect of the resin. Figure 2 shows the magnetization curves of some of the Nd2~Fe,oo_3xBx samples after a solid state reaction of 30 min at 700 °C. These curves show a strong composition dependence of the coercivity ~H¢ which is plotted in Fig. 3. Whereas for x = 10 a coercivity of 20 kOe is achieved, it drops to about 4 kOe at x~<6. X-ray diffraction investigations performed in a Siemens D 500 diffractometer using Cu K~t radiation

Fig. 2. Demagnetization curves of magnetically isotropic Nd2xFe]oo _ 3xBx samples prepared by mechanical alloying (re-

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mainly show the diffraction pattern of the 4) phase for all samples (Fig. 4). For samples with x ~< 6, additional ~-Fe peaks are observed. For neodymium-rich samples (7 ~< x ~< I0) the expected diffraction peaks of a secondary phase are not revealed but, because the diffraction peaks of the 4) phase are not shifted with increasing neodymium content, it must be concluded that a second, probably very thin interfacial phase is present in these samples, as is found for rapidly quenched samples. With increasing neodymium content the amount of the secondary phase increases, which results in higher coercivities. An almost singlephase material shows a relatively low coercivity of about 4 kOe which is the intrinsic coercivity of the fine-grained 4) phase. (It should be noted that the reaction conditions are optimized for Nd,~Fe77Bs and not for the single-phase material.) A similar value has been measured for melt-spun samples [ 12]. The presence of ~-Fe at a low neodymium content does not

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20 {degree ) Fig. 4. X-ray diffraction patterns of mechanically alloyed Nd2~Fel0o _ 3~B~samples (reacted for 30 min at 700 °C). reduce this intrinsic coercivity, but it increases the saturation magnetization and, to a smaller extent, also the remanence. Also, the ct-Fe does not lead to a twostep demagnetization curve. For the neodymium-rich samples the increasing amount of a secondary phase reduces the saturation magnetization and the remanence considerably, as is obvious from Fig. 2. It should be noted here that the Nd2xFe]oo 3xBx sam-

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ples with x >~ 7 do not show any ~t-Fe, whereas Schneider et al. [ 13] found that the limit of primary iron was about 77 at.% Fe (x = 7.67) for melt-alloyed samples. With respect to an improved coercivity at higher temperatures, it is certainly interesting to increase the Curie temperature by substituting cobalt for iron [ 14]. For rapidly quenched samples a beneficial effect has been found [ 15]. Figure 5 shows the coercivity vs. cobalt substitution for mechanically alloyed Nd15 (Fel ~Cox)77Bs samples which have been reacted for 30 min at 600 or 700 °C. A 50% substitution of cobalt for iron leads to an almost vanishing coercivity (much smaller than the intrinsic coercivity of the ~ phase). Also smaller substitutions decrease the coercivity substantially. It seems that for cobalt-containing mechanically alloyed samples the solid state reaction leads to a microstructure which is completely different from what is observed for rapidly quenched samples, even if they are formed by crystallizing originally amorphous material [ 16]. 3. Conclusions

These results show that, by mechanical alloying and a subsequent solid state reaction, N d - F e - B magnets with interesting magnetic properties can be produced. The material seems to be comparable with rapidly quenched material in many respects. For ternary alloys, the coercivity can be increased up to about 20 kOe by increasing the neodymium content (20 at.%) (in a similar way as for rapidly quenched samples [12]) which seems to be due to an increasing amount of interfacial phases. In contrast, increasing the neodymium content reduces the saturation magnetization and the remanence. Although cobalt substitutions are detrimental, a further improvement in coercivity is achieved by small additions of dysprosium [ 17]. The results presented here are only for magnetically isotropic samples. For many applications, it will be necessary to prepare magnetically anisotropic samples which might be possible by similar methods as for rapidly quenched samples.

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The authors thank F. Gaube for technical assistance, D. Keilholz for X-ray diffractometry and F. Friedrich for performing some of the magnetic measurements. They gratefully acknowledge helpful discussions with K. Wohlleben and E. Hellstern. This work has been supported by the German Ministry for Research and Technology.

130

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