Magnetic properties and microstructure in NdDyFeBZr–HDDR

Journal of Magnetism and Magnetic Materials 188 (1998) 119—124

Magnetic properties and microstructure in NdDyFeBZr—HDDR P.J. McGuiness*, I. S[ kulj, A. Porenta, S. Kobe ‘Jozef Stefan+ Institute, Jamova 39, 1001 Ljubljana, Slovenia Received 22 January 1998; received in revised form 14 April 1998

Abstract Cast alloys with a range of compositions based on Nd Fe B with additions of Dy and Zr were produced by 15 77 8 a conventional casting method. Zr was added prior to casting in the form of either zirconium or zirconia. The materials were processed using the hydrogen decrepitation, disproportionation, recombination (HDDR) technique in order to develop the fine microstructures necessary for high coercivities. Magnetic measurements on bonded samples produced from the HDDR powders indicated that additions of Zr tended not to affect the recombination temperatures at which the material began to develop high coercivity, but was clearly observed to extend the temperature range over which high coercivity material could be produced. Microstructural analysis revealed that the Zr-doped samples were less prone to the explosive grain growth exhibited by the Zr-free samples at these temperatures. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: HDDR; Zr; NdFeB; Coercivity; Processing

1. Introduction In this paper we have investigated the effects on magnetic properties and microstructure during HDDR processing of conventional NdDyFeB cast alloys with Zr and ZrO additions. 2 High coercivity NdFeB powders can be produced by the hydrogenation, disproportionation, desorption, recombination (HDDR) process. The

* Corresponding author. Tel.: #386 61 177 3818; fax: #386 61 126 3126; e-mail: [email protected].

submicron grain size necessary for the development of high coercivities is formed by disproportionating the large Nd Fe B grains in a hydrogen atmo2 14 sphere to form an intimate mixture of NdH , a-Fe 2 and Fe B, in a strongly exothermic reaction. The 2 removal of the hydrogen under vacuum at high temperature, in a strongly endothermic reaction induces the NdH , a-Fe and Fe B to recombine, 2 2 but on a much finer, submicron scale. This second recombination stage has been observed to be the most critical step [1], since the formation of an even, submicron, grain structure is vital, and a strongly endothermic reaction can cause large

0304-8853/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 1 6 8 - 1

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netic phase and as a consequence enhance the coercivity. With the development of the HDDR process [14,15], investigations into the effect of Zr continued. Improvements in coercivity [16] and a degree of anisotropic growth [17] were reported, and in both cases the amounts of Zr required were found to be very critical if good properties were to be obtained. Work in this laboratory [18] has shown that the addition of zirconia can enhance the coercivity, although results tended to indicate some inhomogeneity in the samples. Zr-containing samples were also found to show greater resistance to corrosion in dry environments, however, under humid conditions the existence of Zr—B phases leads to an increased formation of local corrosion cells [19].

temperature variations within a processed batch and result in inconsistent grain sizes that can lead to poor magnetic properties. Chemical substitution is of great importance in order to improve or tailor the properties of a magnetic material. In the case of NdFeB sintered magnets Co can be added in order to enhance the Curie temperature [2], Dy substitution can enhance the intrinsic coercivity [2], as can low levels of Ga [3] and Nb [4]. Al too has been very thoroughly investigated [5—7] and although is has a tendency to reduce the anisotropy field in NdFeB, improvements in microstructure and grain isolation lead to higher levels of intrinsic coercivity. The earliest reports of Zr additions to NdFeB sintered materials [8,9] indicated that an improvement in the intrinsic coercivities of these materials could be obtained. A later study found Zr degraded the properties of sintered magnets [10] in the case of materials with 16 and 18% (atomic) neodymium; it was concluded that a coercivity enhancement would be observed with materials containing less neodymium. Investigations into the effect of Zr additions to melt spun NdFeB and NdFeCoB [11] found Zr to have a very positive effect on the coercivity of near stoichiometric ribbons, with improvements in the order of a factor of 3 being observed. Work carried out in this laboratory [12,13] on NdDyFeB sintered magnets has shown that Zr additions in the form of ZrO results in an im2 provement in the magnetic properties, temperature coefficient and corrosion resistance. The improvements in these parameters were attributed to microstructural changes and the formation of plate-like precipitates of a ZrB phase which was 2 found to suppress grain growth of the hard mag-

2. Experimental details Four alloys: NdFeB(1), NdDyFeB(2), NdDy FeBZr(3) and NdDyFeBZr(4) with compositions listed in Table 1 were produced in 5 kg batches by Less Common Metals Ltd. The compositions were chosen so as to provide a good baseline comparison for the effects of Zr on the magnetic properties and microstructure of the subsequent HDDR powder. The alloy, NdDyFeBZr(3), was produced with the addition of zirconia, whereas the alloy, NdDyFeBZr(4), was manufactured using a zirconium metal. The materials were HDDR processed in a specially constructed rotating furnace, two samples of 18 g each, crushed to (5 mm, in separate containers, positioned within the hot zone of the furnace. The rotating furnace was capable of operating between

Table 1 Chemical composition of as-cast alloys

NdFeB(1) NdDyFeB(2) NdDyFeBZr(3) NdDyFeBZr(4)

Nd (wt%)

Dy (wt%)

Fe (wt%)

B (wt%)

Zr (wt%)

Al (wt%)

32.6 32.4 31.9 32.8

— 2.46 2.43 2.48

66.1 63.85 63.97 62.42

1.33 1.29 1.36 1.34

— — 0.34 0.96

0.22 — — —

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3. Results and discussion

Fig. 1. Heat treatment/atmosphere cycle for HDDR process.

1 bar over pressure and 10~2 mbar vacuum with the temperature controlled by a conventional PID controller. The temperature/gas processing scheme used in this series of experiments is shown in Fig. 1. Hydrogen was admitted into the pre-evacuated furnace at room temperature and the pressure maintained at 1 bar until the material was fully hydrided. The furnace was then set to rotate at 10 rpm and the materials heated in a flow of hydrogen gas. The flow rate of hydrogen was adjusted manually during the process in order to accommodate the rapid uptake of hydrogen that occurred at the disproportionation temperature of &700°C. Flowing hydrogen was maintained during the 100 min disproportionation time which was followed by a switch to vacuum in order to initiate recombination of the NdH , a-Fe and Fe B. 2 2 2 g of the resulting powder was hand crushed to (200 lm before being warm pressed with a polymer in a 7 mm die under a load of 2.5 tonnes. Magnetic properties of the polymer bonded magnets were determined using a conventional permeameter set-up at room temperature. Four solid samples of the same alloys were processed separately with a 100 min disproportionation time and a 850°C recombination temperature. Each sample measured approximately 8]5] 0.5 mm, sufficiently thin so as to avoid fracture during the processing. These samples were mounted, polished and etched after HDDR processing in order that the grain size and grain size distribution could be easily determined using a JEOL 5400 SEM in SEI mode.

In order to optimise the procedure for the NdFeB(1) alloy, the disproportionation temperature and time were fixed at 820°C and 60 min. While the recombination time was fixed at 60 min and the temperature varied between 680 and 820°C in 10°C steps. The results of the study can be seen in Fig. 2. The curve indicates a peak in coercivity at 790°C. The recombination temperature was then fixed at 790°C and the disproportionation time varied between 10 and 1000 min. The graph in Fig. 3 shows how the coercivity of the bonded samples increased steadily with disproportionation time with the best properties being obtained for the sample disproportionated at 1000 min. For the purposes of the subsequent

Fig. 2. Optimisation of recombination temperature.

Fig. 3. Optimisation of coercivity varying disproportionation time.

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investigation the 1000 min dispropotionation time was considered impracticably long and a compromise time of 100 min was chosen in order to produce enough samples for the investigation. The procedure of determining the optimum recombination temperature was then rerun with the disproportionation time and the temperature fixed at 100 min and 820°C. This series of experiments was run twice, once using 18 g each of the NdFeB(1) and NdDyFeB(2) alloys and a second time with the NdDyFeBZr(3) and NdDyFeBZr(4) materials. Experiments were carried out with recombination temperatures varying between 690 and 910°C. The results of the measurements on all four materials are shown in Figs. 4—7. The baseline alloy NdFeB(1) shows the expected behaviour, observed

Fig. 6. Variation of coercivity with recombination temperature, NdDyFeBZr(3).

Fig. 7. Variation of coercivity with recombination temperature, NdDyFeBZr(4).

Fig. 4. Variation of coercivity with recombination temperature, NdFeB(1).

Fig. 5. Variation of coercivity with recombination temperature, NdDyFeB(2).

previously, with the coercivity rising to a peak at processing temperatures in the 790—830°C range offering a narrow processing window over approximately 40°C before the coercivity begins to fall significantly, to less than 2 kOe when the recombination temperature exceeds 850°C. A very similar behaviour was observed for the alloy NdDyFeB(2), which contains approximately 1% (atomic) Dy, and as a consequence shows &15% more coercivity due to a higher intrinsic anisotropy field (H ). The Zr-containing alloys, A however, are markedly different. The material containing 0.3% Zr (weight) shows a similar behaviour at lower temperatures, i.e. the increase in coercivity begining to take place at &760—770°C but a significant increase can be seen in the temperature at which the coercivity of the material begins to drop,

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magnets with coercivity less than 2 kOe were not seen at processing temperatures below 890°C. It is interesting to note that this curve does not display the same consistency of data when compared with Figs. 4 and 5; there appears to be a greater scatter in the points. The NdDyFeBZr(4) alloy containing &1% (weight) of Zr exhibits this same broadening of the range for high coercivities only to a more striking degree with a processing window extending over &100°C and the first sub 2 kOe material not measured until the processing temperature was raised above 900°C. This last graph also includes a degree of inconsistency in the data at the lower processing temperatures. There is evidence of a small peak prior to the broad peak in coercivity between 810 and 890°C, whether this is a real peak or merely the result of some variation in the material as a result of inhomogeneous distribution of Zr in the alloy is not certain. A microstructural investigation of polished and etched materials processed at 850°C using a scanning electron microscope revealed a stark contrast between the microstructures of the ‘Zr-containing’ and ‘Zr-free’ samples. At 850°C, the NdFeB(1), (see Fig. 8) and NdDyFeB(2) samples show evidence of some very large grains, well in excess of 10 lm, smaller grains in the 1—10 lm range and areas of very fine sub-lm grains. The NdDyFeBZr(3) and NdDyFeBZr(4) samples (see Fig. 9) in contrast, show an almost uniform distribution of sub-lm

grains, with only the occasional large grain being observed.

Fig. 8. Microstructure of low coercivity NdFeB(1) processed at 850°C.

Fig. 9. Microstructure of high coercivity NdDyFeBZr(3) processed at 850°C.

4. Conclusions It can be seen from the coercivity versus processing temperature graphs (Figs. 4—7) that at 850°C the NdFeB(1) and NdDyFeB(2) samples are low coercivity magnets, whereas the NdDyFeBZr(3) and NdDyFeBZr(4) samples show high coercivity, indicating that the sub-lm grain structure observed in Fig. 9 results in samples that show a strong resistance to demagnetisation and that this microstructure is a result of Zr additions to the material. The change in coercivity with recombination temperature for the materials NdDyFeBZr(3) and NdDyFeBZr(4) which contain Zr, shows a degree of inconsistency at low temperatures in comparison with the materials that do not contain any Zr. It is possible that this is due to a degree of chemical inhomogeneity in the material, with the element Zr not being evenly distributed. The study will be extended to look at materials that have been subjected to preprocessing homogenising treatments and it is planned to incorporate more compositions into the study in order that we can better understand these observed phenomenon. It is clear that additions of Zr are extremely beneficial to the ease of HDDR processing

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NdFeB-type materials. The larger processing window at a critical stage of the procedure, where significant endothermic effects are encountered, should lead to the opportunity for processing larger batches of material in the manufacturing environment.

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