IEEE TRANSACTIONS ON MAGNETICS, VOL. 26, NO. 5, SEPTEMBER 1990

Composition

Pr13Fe81B6

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iHc (kOe) Br (kG) &gE$ 18 7.8 11.2

THE EFFECTS OF THE a-Fe PRECIPITATION ON THE COERCIVE FORCE OF THE RAPIDLY QUENCHED Pr-Fe-B-Zr ALLOYS

W. Y. Jeung and I. K. Kang Div. of Metals, Korea Institute of Science and Technology, Seoul, Korea

and J. 0. Choi and J. J. Lee

Dept. of Metallurgical Engineering, Seoul National University, Seoul, Korea

Pr12.5Fe80.5B5.5Cu1.5

Pr12.5Fe80.5B5.5Agl.5

Absnuct - It has been found that the magnetic properties of the optimally heat-treated overquenched ribbons are inferior to those of the ribbons optimally quenched directly from the molten alloy. This paper tries to elucidate the difference in coercive force between the two states. Through the investigations, including X-ray diffraction and thermomagnetic phenomena, it has been verified that a-Fe phase is precipitated during the underquenching and the annealing after overquenching. This indicates that the degradation of coercive force of the annealed overquenched ribbons originates from the a-Fe precipitates. The existence of a-Fe is less detrimental to the coercive force than expected, suggesting that the grain boundary suppress the propagation of domain walls.

13.3 7.9 11.5

8.2 8.6 11.5

INTRODUCTION Pr12.5Fe80.5B5.5Nb1.5 The rapid solidification process is the well-established

technique to attain the magnetic properties to be used for the rare-earth permanent magnets[l]. Magnetic hardening can be achieved either by direct quenching or by subsequent annealing after overquenching. In either case, the coercive force is sensitive to the microstructures and the existing phases as well. There have been many papers to elucidate the magnetic hardening mechanism of the rapidly quenched ribbons in relation to the microstructures, especially in the grain boundary region[2,3]. Meanwhile, there exist many phases[4] which have not been fully characterized, whether they are effective on the coercivity or not, and a-Fe is the possible one. This paper tries to investigate the effects of a-Fe on the coercive force at various quenching rates and annealing conditions.

EXPERIMENTAL Rapidly quenched ribbons of the Pr-Fe-B-X system were

prepared by the melt-spinning technique. The overquenched ribbons were annealed in vacuum-sealed quartz tube in the temperature range of 55O0C-90O0C for 5-60 min.. After pulsed magnetization (37kOe), the magnetic properties were measured on the compacted powder specimen by D.C. fluxmeter. Thermomagnetic curves were obtained by VSM using Ms method and Mr method. Crystal structure and microstructure at various quenching rates and annealing conditions were examined by X-ray diffraction and TEM respectively.

13.6 7.9 10.9

RESULTS AND DISCUSSION

Coercivity and a-Fe

Table 1 shows magnetic properties at the optimal condition for the melt-spun ribbons of Pr-Fe-EX alloys. It was noted that the ribbons having maximun energy product, (BH),,, of 12-13 MGOe and remanence, Br, of 8kG were achieved by adding Zr.

Fig.1 shows the X-ray diffraction spectra of the melt-spun Pr12,5Feso,5B5,,Zrl,5 ribbons at different quenching rates. As the quenching rate increases, line broadening occurs due to grain size refinement and amorphization. Abnormally strong intensity of the matrix Pr2Fel,B (006) reflection led us to suspect the presence of a-Fe. Because of the close interplanar distance of the tetragonal (006) plane and bcc a-Fe (110) plane, the reflections were assumed to be superimposed at the normal scanning speed. But the peak

Table 1. Magnetic properties of optimally quenched ribbons with different alloying element

(36.5dsec) I

4 0 28

X-ray diffraction patterns of Prl,,5Feso,B5,5Zrl,5 ribbons at various quenching rates.

( 7 ) was split at the low scanning speed of 0.025'?min, which indicated that a-Fe has been precipitated during the underquenching. The a-Fe was, however, not detected in the optimally and the overquenched states. The lower coercivity of the underquenched ribbons was partly ascribed to the cy-Fe precipitates besides coarser grains.

Fig.2 shows the magnetic properties of the overquenched ribbons annealed at the various temperatures. As the annealing temperature increased, the coercive force developed to its peak value (iHc= 12.4kOe, (BH)max= 11.2MGOe) through crystallization of the amorphous phase. Overaging the ribbons up to 9oOOC deteriorated the coercive force, owing to the decomposition of grain boundary layer [2] with effective domain wall ,pinning. The effect of annealing time on the magnetic properties was not appreciable and is not presented here. As can be seen above, however, the

0018-9464/90/0900-1733$01.00 0 1990 IEEE

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magnetic properties of the heat-treated ribbons were inferior to those of the ribbons optimally quenched directly from the melt. This tendency was also reported elsewhere [5,6].

After annealed at 800OC the intrinsic coercivity, iHc, increased to 12kOe and the remanence, Br, to 7.8kG. Extending the annealing time up to 1 hour did not cause any appreciable change. But the remanence and intrinsic coercivity were lower by 0.8kG, 3.5kOe than obtained from the optimally quenched state.

5 I

A i H c ( k O e ) '. 0 B r ( k G )

(BH),,, (MGOe)

I 4 , , .4n n e all n g Temp er a t u r e

603 ':a eoc 900

Fig. 2. Magnetic properties of overquenched Pr12,5Fem,5B5,5Zrl,5 ribbons annealed for 30 min. at different temperatures.

Fig.3 shows the X-ray diffraction patterns obtained after annealing at various temperatures. As the temperature increased, the intensity( 7 ) increased due to the formation of Pr2Fe,,B phase and the precipitation of the a-Fe from the amorphous state. But the X-ray diffraction spectra showed that the intensity increased due to the superposition of the intensity from a-Fe (110) plane. This led to the conclusion that a-Fe phase has been precipitated during the heat treatment, which results in the lower coercive force of the annealed ribbons.

Fig. 4 shows microstructures of the optimally quenched specimens and overquenched specimens annealed at 800°C for 5 min and 30 min. Grain morphology in the optimally

CuKa (a)600"C,30min.

(4 900"C,30min. 3

I 40 28

20

Fig. 3. X-ray diffraction patterns of Pr,2,,Feso.5B,,,Zr,,5 ribbons annealed after overquenchmg.

quenched specimen was not clear and grain size was smaller than 50 nrn. But after the annealing at 800°C for 5 min, they had distinct grain boundaries and grains were grown up to 100 nrn. When annealed at 800OC for 30 min grains grown up to 200 nm were often discovered. In fig. 3, annealing caused separation of a-Fe precipitates and matrix phase from the amorphous state. It was clear that the increased a-Fe precipitates and the grain growth up to 200 nm did not cause any appreciable decrease of the coercive force.

Fig. 4. TEM micrographs of Prl2,,Fe,,,,B5,,Zr,,, ribbons.

Thermomagnetic behavior

Fig.5 shows the themomagnetic curves at different quenched states using the Ms and Mr method. In the overquenched state (a), as the temperature increased the first magnetic transition temperature appeared at 150°C (Tcl) due to the amorphous phase and the second at around 300OC(TcJ due to Pr2Fel,B primary crystallites. Subsequent heating showed that somewhat rapid increase of magnetization occured at around 50OoC (Tp) caused by a -Fe precipitation, and then magnetization disappeared at around 77OOC (T%) which is the Curie temperature of a -Fe. On cooling, magnetic transition temperature, T% and T% appeared again but Tc, disappeared because the amorphous phase has been fully crystallized.

In the optimally quenched state (b), phenomena like the Hopkinson effect [7,8] appeared in the early stage up to 100'C. It was considered that magnetic saturation did not occur at room temperature with applied field of 15kOe. But because of the ease of movement of domain walls with increasing temperature, saturation easily reached in the low field.

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(a) overquenched (36.5dsec) . --c Msmethod --• M r m t h o d

T) optimally qucnchcd (15 .Sdscc) T) optimally qucnchcd (15 .Sdscc)

, (c) underquenchcd (2.9m/sec)

- Ms method Mr method --

\ \.I+*

100 200 300 400 500 600 700 80

Subsequent heating showed the same magnetic behavior as that of the overquenched state except for the absence of T% due to the lack of the amorphous phase. In the case of the overquenched ribbons, the Hopkinson effect did not occur. This was why the overquenched ribbons did not have a phase that prevent movement of domain walls and could be saturated in the applied field of 15kOe.

As for the optimally quenched state, precipitation of a -Fe was shown at around 5000C as in the overquenched state. At around 150°C the magnetic transition by the amorhous phase was not observed and a change of curvature was observed at near 3oooC where Curie temperature of Pr,Fe,,B phase exists. It was considered that the optimally quenched ribbons consist of PrzFel,B phase only and other magnetic phases were not present.

In case of the underquenched state (c), a substantial amount of magnetization is retained at and above T% due to the primary a-Fe crystallites. These results are well accord with the X-ray diffraction results. Magnetization increased up to the looOC by the Hospkinson effect, but its effect was slighter than in the optimally quenched. In the thermomagnetic curve by Mr method, the Hopkinson effect was not observed. Taking account of the above results, the Hopkinson effect was greatly affected by the cooling rate and intensity of applied field.

CONCLUSION It was confirmed by x-ray diffraction and thermomagnetic

analysis that the precipitation of a-Fe beginning at 5000C was observed both in the optimally quenched and overquenched ribbons, and in case of the underquenched ribbons a-Fe was aleady precipitated in the quenched state. The existence of a-Fe was less detrimental to the coercive force than expected, suggesting that the grain boundary inhibit the propagation of the domain walls.

REFERENCES [I] J. J. Croat, J. H. Herbst, R. W. Lee and F. E. Pinkerton, J. Appl. Phys., 55(6), 2078 (1984). [2] R. K. Mishra, I. Magn. Magn. Mater., 54-57, 450 (1986). [3] G. C. Hadjipanayis, R. C. Dickerson and K. R. Lawless, ibid., p. 557. [4] J. Fidler and K. G. Kncch. Proceedings of The 10th Internatioal Workshop on Rare-Earth Magnets and Their Applications, Kyoto, Japan, 1989, Part 2, p. 283. [SI H. Yamamoto, M. Nakagura, Y. Ozawa and T. Katsuno, ibid., Part 1, p. 543. [6] J. Wecker and L. Schultz, J. Appl. Phys., 62(3), 990 (1987). [7] 0. Popov and M. Mikhov, J. M a p . Map. Mater., 75, 135 (1988). [8] H. Miyajima, Y.Otani, M. Kameda, H. Sakamoto, Proceedings of The 10th Intematioal Workshop on Rare- Earth Magnets and Their Applications, Kyoto, Japan, 1989, Part 1, p. 237.

Temperature ("C)

Fig. 5. Thermomagnetic curve of Prlz.5Few.5B5,5Zrl.5 ribbons at various quenching rates (heating rate ; 5Wmin.)