C.C. Colucci

Study of thermal decomposition mechanism of the Fe/sub 17/Sm/sub 2/N/sub 3/ phase

We studied the high temperature decomposition mechanism for the Fe/sub 17/Sm/sub 2/N phase using several different experimental techniques, as thermomagnetic and thermogravimetric analyses. Our results show that the decomposition occurs in a two step scheme. In the first reaction we hare the formation of SmN and a solid solution of nitrogen in iron. In the second step, this solid solution degases, and we observe the evolution of nitrogen over a broad temperature range.

Nitrogen diffusion patterns on RE/sub 2/Fe/sub 17/ compounds, RE=Pr, Nd, Sm and Gd

The nitrogen diffusion patterns for several RE/sub 2/Fe/sub 176/ intermetallic compounds are studied. The nitriding process was done in both slabs and powder samples, which were characterized and analyzed thermomagnetically and metallographically. Thermographic analysis 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 diffusion mechanisms: along preferential paths at low temperature and according to the shell-core model at higher temperatures. >

Diffusion patterns of nitrogen in the Fe17Nd2 compound as a function of temperature

Abstract Nitrogen diffusion patterns were obtained for the compound Fe 17 Nd 2 at temperatures of 400, 500 and 575 °C. The formation of a nitrogen solid solution was not observed, and the fully nitrided phase precipitated from the phase free from nitrogen. At 400 °C the shell-core diffusion structure was not observed, and diffusion occurred preferentially through extended defects such as grain and phase boundaries and dislocations. At higher temperatures, the bulk diffusion coefficient was large enough for bulk diffusion to compete with the faster diffusion through defects, so that the shell-core structure was observed. For all temperatures colour differences were observed in the fully nitrided phase, indicating fluctuations in nitrogen concentration. For higher temperatures, precipitation of NdN was observed.

Phase characterization of Fe17Nd2Nx samples with different nitrogen contents

Abstract Samples of Fe 17 Nd 2 N x have been prepared for x =0.0, 0.5, 1.0, 1.5, 2.0 and 2.1 nitrogen atoms per formula unit, at 400 °C. They were analysed by metallography, thermomagnetic analysis, electron microprobe and X-ray diffraction. The measurements showed that all the samples contained the phase Fe 17 Nd 2 , free of nitrogen, together with the Fe 17 Nd 2 N 2.3 saturated phase.

Diffusion patterns of formation of the phase Fe17Nd2Nx at 400°C

Abstract Samples of Fe 17 Nd 2 N x have been prepared for x =0.0, 0.5, 1.0, 1.5, 2.0 and 2.1 nitrogen atoms per formula unit, at 400°C. They were analysed by metallography, thermomagnetic analysis, electron microprobe and X-ray diffraction. The measurements show that all the samples contained the pure Fe 17 Nd 2 phase alongside the saturated Fe 17 Nd 2 N 2.3 phase. The measurements also show that the diffusion mainly proceeds along extended defects as grain and phase boundaries, dislocations and dislocations arrays. The expected shell-core diffusion model is not observed. Heavily nitrided samples shows the presence of striae depicting fast diffusion paths from the exterior to the interior of the powder particles.

/sup 57/Fe Mossbauer investigation of R/sub 2/(Fe/sub 1-x/Co/sub x/)/sub 17/N/sub y/ (R=rare earth, x<or=0.4, y<or=2.2)

The effects of different contents of nitrogen in both Pr/sub 2/Fe/sub 17/ and Nd/sub 2/Fe/sub 17/ and of cobalt in both Nd/sub 2/Fe/sub 17/ and Nd/sub 2/Fe/sub 17/N/sub 2.2/ are investigated by means of Mossbauer spectroscopy and thermal magnetic analysis. The results confirm that either nitrogen or cobalt increases the hyperfine field (B/sub hf/) as well as the Curie temperature (T/sub c/) of the material. The hyperfine parameters for nominal contents of nitrogen above y=1.5 are the same as those observed in fully nitrided samples. The data also reveal that the combination of N and Co produces an additional increase of T/sub c/. The same behavior is observed in B/sub hf/ of Nd/sub 2/(Fe/sub 1-x/Co/sub x/)/sub 17/N/sub 2.2/ in the range 0 >

A detailed study of nitride precipitates in Nd2Fe17

We report electron microprobe measurements of the diffusion profile in partially nitrided samples of the Fe17Nd2 phase. The results are fully consistent with a step‐like profile. This confirms that the nitrogenation process is one of precipitating a fully nitrided phase from a nitrogen‐free phase. We report also x‐ray diffraction patterns for the partially nitrided samples as a superposition of the pure and nitrided patterns, with displacements and broadenings of lines due to mutual stresses between the phases.

Stress-induced magnetization in Pr2Fe17Nx compounds

Abstract The dependence on x of the phases present in Pr 2 Fe 17 N x powder particles nitrided at 400°C is studied by means of Mossbauer spectroscopy. The results are compared with thermomagnetic analysis data and confirm the formation of a nitrogen-saturated core, as discussed in the literature, which is attributed to the diffusion of nitrogen through extended defects. The occurrence of three distinct Mossbauer subspectra is associated with the presence of the nitrogen-saturated phase with x = (2.8 ± 0.2), the original binary phase and a volume fraction of the Pr 2 Fe 17 phase which becomes magnetic at room temperature due to the lattice expansion provoked by a stress-strain field within the particles. A model for the partially nitrided particles is employed successfully to account for the changes in the subspectral areas as well as the approach to saturation observed in the average hyperfine field of samples with nitrogen contents as low as x = 1.5; it also produces a preliminary value for the Poisson ratio (μ = 0.2) of the Pr 2 Fe 17 phase.

A detailed study of the nitrogenation of the Fe/sub 17/R/sub 2/ phases (R=Sm, Gd, Ho and Tb)

Early studies in the 17:2 phases based on Pr, Nd and Sm for relatively low absorption temperature (400/spl deg/C) showed that the diffusion pattern is different from the shell-core model. Instead, the diffusion occurs at phase and grain boundaries. The precipitation of the saturated phase occurs along these boundaries. In this work we show, using different characterization methods, that the same behavior is observed for the 17:2 phases based on Sm, Gd, Ho, Tb.

Characterization of Fe17R2 phases (R=Pr and Sm) oxidized at 200 °C

The results of oxygen absorption in temperatures ranging from 200 to 500 °C are reported for the phases Fe17R2, R=Pr and Sm. It was observed that the oxygen saturation limit is temperature dependent, being of 3 at/fu at 200 °C, 12.8 at/fu at 300 °C, 20.3 at/fu at 400 °C, and 28.5 at/fu at 500 °C. This implies that above 200 °C the phase is deeply altered by oxygen absorption. Samples with absorption at 200 °C were studied as prepared and heat treated at 500 °C/40 h. The results show that the heat‐treated samples consist of Fe17R2, Fe, and R2O3. Metallographic examination, susceptibility, and Mossbauer spectroscopy data support the hypothesis that the as‐prepared samples consist of a mixture of Fe17R2 and ordered interstitial Fe17R2Ox.