C.A. Ribeiro

Experimental study of the AlCr equilibrium diagram

Abstract The phase diagram for the AlCr system has been experimentally revised. The main features of the diagram in the literature have been confirmed. The γ (Al1,Cr2) phase has not been found in as-cast samples, heat-treated samples or samples subjected to differential thermal analysis. A diffusion couple treated at 650°C has not shown this phase either. The reaction involving the aluminium phase has been found to be of a eutectic nature. A new version of the AlCr phase diagram is proposed, taking account of these findings and literature data.

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.

Directional solidification and characterization of binary Fe-Pr and Fe-Nd eutectic alloys

Abstract In this work, the directional solidification technique was employed in binary Fe–Pr and Fe–Nd eutectic alloys with compositions near to their respective eutectic points. Solidifying the Fe–Pr and Fe–Nd hypereutectic alloys in a vertical Bridgman crystal growth unit, we were able to obtain the two eutectic morphologies, one globular and other feathered, normally observed in as-cast samples, in a coarser form and sequentially displayed along the length of the samples. The transition temperatures nearby to the eutectic points were also determined using a Calvet type calorimeter. The phases comprising the globular and feathered morphologies were determined as being, respectively, Fe 17 R 2 and Fe 2 R (R=Pr or Nd), for both systems. These experiments also showed very clearly the peritectic formation of the Fe 2 R phases from the Fe 17 R 2 and the liquids. Thermomagnetic measurements gave the Curie temperature as being 44°C for the Fe 2 Pr phase and 252°C for the Fe 2 Nd phase.

Experimental study of the iron-tantalum equilibrium diagram

New features of the Fe-Ta equilibrium phase diagram are presented. The experimental techniques of investigation are metallography, X-ray diffraction, electron probe microanalysis, and differential thermal analysis. The solubility limits of the Fe2Ta (ε), FeTa (Μ), and (Ta) phases at 1400 ‡C are 28 to 38 at. % Ta, 49 to 59 at. % Ta, and 94 at. % Ta, respectively. The ↔ Μ+ (Ta) eutectic temperature is 1750 ‡C.

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.

Evidence for the precipitation of the Fe2Pr phase in the Fe–Pr binary system

Directional solidification experiments were employed on Fe–Pr alloys at several low cooling rates in order to observe the solidification sequence displayed along the length of the samples. Measurements of the transition temperatures around the eutectic temperature of this system were also performed using a Calvet-type calorimeter. The results show the precipitation of the Fe2Pr phase and its peritectic formation, described as Fe17Pr2+L→Fe2Pr. The calorimetric measurements give the temperature of this reaction as being 669 °C. The system presents only one eutectic reaction at 664 °C, described as L→Fe2Pr+Pr, with a feathered morphology under metallographic analysis. Thermomagnetic measurements give the Curie temperature of the Fe2Pr phase as 44 °C. A sequence of reactions during solidification of the alloys around the eutectic point for the Fe–Pr binary system is proposed.

Oxidation on the Fe-Pr system

Abstract Some observations of oxidation in the Fe-Pr system are described. It is observed that the Fe17Pr2 phase decomposes into Fe solid solution and Pr2O3. If the oxygen contamination is heavy, the whole sample decomposes. If it is small, the decomposition pattern is observed along regions of defect concentration, like grain boundaries and diffusion fronts.

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.