2578 IEEE TRANSACTIONS ON MAGNETICS, VOL. 28, NO. 5, S E m M B E R 1992

- ~ _ _ _ _ _ ~ NITROGEN DIFFUSION PATTERNS ON RE2Fe17 COMPOUNDS. R=Pr, Nd, Sm AND Gd

C. C. COLUCCI, S. GAMA and F. A. 0. CABRAL LABORAT6RIO de MATERIAIS e BAIXAS TEMPERATURAS

INSTITUTO de FfSICA GLEB WATAGHIN UNIVERSIDADE ESTADUAL de CAMPINAS

13081-CAMPINAS, S. P., BRASIL

Abstract-The nitrogen diffusion patterns on several RE2Fe17 intermetallic compounds are studied. The nitriding process was done in both slabs and powder samples, which were characterized and analyzed thermomagnetically and metallographically. The diffusion process occurs by both mechanisms: shell-core model and along preferential paths. A broadening caused by stress o r by dilution was detected in pure phase transition.

INTRODUCTION

The binary rare earth-iron intermetallic compounds appear to offer little attraction for use as permanent magnets because they have a low Curie temperature and relatively low magnetocrystaline anisotropy? t 2

The recently discovered nitrided 2: 17 phase, showing very interesting and promising magnetic properties, is the object of many studies utilizing several experimental techniques. The interstitial solutions of nitrogen in these compounds drastically change the magnetic properties of materials without major changes of their crystal structure. 3’

However, little attention has been paid to the detailed mechanism of nitrogen diffusion. Usually it is believed that, as happens with other gases, such as oxygen, the diffusion pattern is such that the gas penetrates the grains of the pure phase from the exterior to the interior, forming a shell of the nitrided phase. If not enough time is given for completion of the nitriding process, a core of the pure phase 2:17 must exist inside the material. It is also believed that the nitrogen is forming a continuous solid solution, with a Curie temperature varying with the nitrogen content.

This report presents some experimental results on nitriding of the 2:17 phase of the systems Fe-R, R = Pr, Nd, Sm and Gd in order to get information about the kinetics of nitrogenation.

We present the results obtained in both nitrided slabs and powders.

EXPERIMENTAL

The polycrystalline alloys were prepared from constituent elements of purity at least 99.9% from Johnson Matthey and melted in an arc furnace under Ar atmosphere. For homogenization the samples were remelted four times and in the Sm2Fe17 case, we have added Sm to compensate evaporation losses. These master alloys were wrapped in Ta foil, encapsulated in an Ar filled quartz ampoule and heat treated at 1370 K about one week.

From each sample we cut a 1.0 mm thick slab for nitriding in a special chamber at 773 K under a constant flow of ultra pure nitrogen. Part Of the sample was ground mechanically ( grain size < 100 pm and nitrided in a conventional Sievert device. The reactor was inside a resistive furnace and the temperature was controlled inside the reactor with a type K thermocouple. For each complete cycle of absorption the temperature was

kept constant at 673 K. Only in the Nd2Fe17 case were three. isothermal absorptions done, at 673 K, 773 K and 873 K. The final nitrogen concentration after each step was determined by the pressure difference between initial and final stages, measured with a membrane gauge from MKS. Thus, we may vary the concentration and we obtain x = 0.5, 1.0, 1.5, 2.0 and x = 2.2. ( x = nitrogen atoms per formula unit 1 for several compounds we have studied .

The nitrided samples were characterized by optical metallography and by thermal magnetic analysis ( TMA 1. The metallographic preparation of the powder samples comprised the mixture of the nitrided powder with a copper containing resin powder submitted to high uniaxial pressure. This pellet was then hot embedded in a metallographic press, using the same copper containing resin. This procedure allowed the usual metallographic sample preparation to be applied to the powder samples. Most samples were examined in a Neophot optical microscope without chemical etching.

The TMA apparatus consists of a set of two copp5r wire coils connected in series opposition and the sample is placed inside one of them. Both coils are inside a primary copper wire coil, fed by the function generator of a lock-in amplifier. We used a sinusoidal wave form of 5 kHz and field amplitude around 2 Oe. The output of the pick-up coils was connected to the lock-in amplifier. The sample holder was in the interior of a vertical quartz tube furnace, with argon protective atmosphere.

RESULTS AND DISCUSSION

I-Nitrided slabs

The TMA has shown two sharp transitions: one corresponding to the 2:17 pure phase and another corresponding to the 2:17 nitrided phase. The Curie temperatures, TC , for these nitrided compounds, are 723 K for Pr2Fel7Nx, 728 K for NdZFel7Nx, 748 K for Sm2Fel7Nx and 757 K for Gd2Fel7Nx.

The amount of absorbed gas after the nitriding process of the slab samples was measured by weighing the sample. By this simple technique we have detected that under the same conditions of temperature and pressure, the Pr2Fe17 sample absorbs much more nitrogen than the other 2:17 systems studied, under the same conditions of temperature, pressure and time of absorption.

Metallographic analysis of the nitrided slabs revealed a similar diffusion pattern for Nd, Sm and Gd samples. This pattern is quite similar to the ones observed in diffusion couples, and consists of an external layer of the nitrided phase. Clear indication of grain boundary short circuit diffusion can be observed in figures 1 and 2.

In the Pr case a completely different pattern is observed. Although there is still an external layer as for the other RE, a much greater grain boundary diffusion is observed, appearing over all the sample. This causes cracking of the sample, making the diffusion process much more effective, as can be seen in figure 3.

OO18-9464/92$03.00 0 1992 IEEE

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slab-l4h/773 K

Fig. 2- Microstructure of Nd2Fe17 nitrided slab-l4h/773 K

11-Nitrided oowder

Samples of RE2Fel7Nx have been prepared for x= 0.0, 0.5, 1.0, 1.5, 2.0 and until the saturation ( x 2.3 1. In the Nd2Fe17 case the isothermal nitridation Was performed at three different temperatures: 673 K, 773 K and 873 K. For the other compounds the nitriding process was done at 673 K. Only the powder below 100 pm was used in these experiments.

The samples were thermomagnetically analyzed and the results indicate the samples comprise two phases: a pure R2Fe17, without nitrogen with Tc = 323 K, and a fully nitrited R2Fe17N2.3 with Tc around 723 K. ( Fig 4 1. A broadening in the transition of the pure 2:17 susceptibility signal is observed. Figure 4 shows this broadening around 328 K for the samples of Nd2Fe17N0.5 and NdZFe17N1.5; as the nitrogen concentration increases, the signal corresponding to the pure phase decreases while the one corresponding to the saturated phase increases ( Tc around 723 K 1. These variations on the height of the transitions are fully consistent with the metallographic observations, as we will see bellow. The broadening of the transition corresponding to the 2:17 pure phase is caused either by stress and strain, induced in the pure phase by the volume dilatation due to the nitrogen up-taking, or by the fact that under high nitrogen charges a dilution of the pure phase occurs. It was also thought that the grinding process could be responsible for the broadening of the transitions.

1

50 450 T I O C I

Fig.4- Thermomagnetic signal of nitrided Nd2Fel7 samples.

We have an experimental indication that: a) the grinding process causes no changes in the pure phase transition and b) a dilution of the 2:17 ferromagnetic powder into the diamagnetic matrix makes the signal corresponding to the pure phase broadens.

Metallographic analysis of the nitrided powder has shown several peculiarities and we present some remarks about the diffusion for the nitrided Nd2Fe17 samples. The expected pattern was one corresponding to the shell-core model, in which the internal rim of the ring of the nitrided phase shrinks to the center with time dependence according to Fick's law. Also it is well-known that grain

Fig. 3- Microstructure of Pr2Fe17 nitrided boundaries are regions of high diffusivity, so that these diffusion channels could diminish the time for completion of grain nitridation.

slab-l4h/773

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We observed patterns that are in disagreement .with this model. Figure 5 shows the nitrided sample at 673 K with x= 1.5 a/f.u.. Some regions of the grains are forming a layer in the surface, but this is a rare event. Mostly, it is observed that the nitrogen is diffusing along phase and grain boundaries and extended defects. From these preferential paths the gas is diffusing perpendicularly int: the bulk of the grain of the pure Nd2Fe17 phase.

regions are the 2:17 pure phase and the dark ones are the nitrided phase.

CONCLUSIONS

Fig.5-Microstructure of the Nd2Fe17N1.5. The white regions are the 2:17 pure phase and the dark ones are the nitride phase.

Figure 6 shows one nitrided grain at 773 K with x = 1.0 a/f.u.. It presents the same feature as discussed above. A layer of the nitrided phase can be fairly seen in the border of the grain and a large high diffusivity path appears on the left bottom corner.

TMA shows two phases:the pure 2:17 and the fully nitrided one for several nitrogen concentrations in all samples studied. Metallographic analyses confirm the presence of two phases.

A broadening caused by stress or by dilution was detected for the pure phase transition.

The nitriding process occurs by both diffusion mechanisms: along preferential paths at low temperatures and shell-core model at higher temper a tur es .

Acknowledgement- We wish to thank C. Barnabe for her invaluable assistance on metallographic techniques.

We also thank Conselho de Aperfeiqoamento de Pessoal de Nivel Superior ( CAPES 1, FundaCkio de Amparo a Pesquisa do Estado de S l o Paulo ( Fapesp and Conselho Nacional de Pesquisa ( CNPq )for financial support.

REFERENCES

1-J. M. D. Coey and H. Sun- Journal of Magnetism and Magnetic Materials 87 (1990) L251-L254. 2-K. H. J. Buschow, R. Coehoorn, D. B. de Mooij, K. de Waard and T. H. Jacobs- Journal of Magnetism and Magnetic Materials 92 (1990) L35-L38. 3-M. Katter, J. Wecker, L Schultz and R. Grossinger- Journal of Magnetism and Magnetic Materials 92 (1990) L14-L18. 4-T. H. Jacobs, Gary J. Long, 0. A. Pringle, F.

Fig. 6- Microstructure of the Nd2Fe17N1.5. The white Granjean and K. H. J. Buschow- Journal of Applied region is 2:17 pure phase and the dark one is Physics 70 (10) 1991. the nitrided phase. 5-C. C. Colucci, S. Gama, L. C. Labaki and C. A.

Ribeiro-to be published.

Figure 7 shows nitrided grains at 873 K with x = 0.5 a/f.u.. It is noted that a pattern close to the shell-core accompanies a diffusion pattern along the preferential path.