D.S. Edgley’s research while affiliated with University of Birmingham and other places

The effect of niobium addition on the microstructure of Nd–Fe–Nb–B alloys (with 0, 2, 4 and 5 at.% Nb) has been investigated by transmission Mössbauer spectrometry and X-ray diffraction analyses. In the as-cast alloys, the Nb26Fe32B42 (at.%) phase was characterised by a paramagnetic doublet, with an isomer shift IS=−0.04 mm s−1 relative to α-Fe, and a quadrupolar splitting QS=0.22 mm s−1. The contribution of the Nd2Fe17 phase was observed in the Mössbauer spectra of the alloys with 4 and 5 at.% Nb. Niobium additions allow a drastic reduction of the homogenisation times during annealing in the 800–1100°C temperature range, in agreement with previous studies. The kinetics of the disappearance of α-Fe in alloys with 2 at.% Nb were determined by Mössbauer spectrometry. The experimental curves were fitted with a simple theoretical model of growth of a spherical Nb–Fe–B precipitate in a spherical α-Fe matrix. The activation energy related to the α-Fe disappearance was estimated to be 112±16 kJ mol−1.

The long term high temperature oxidation properties of a Nd16.4Fe75.7B7.9 commercial sintered magnet were investigated in pure oxygen atmosphere up to 500°C and in air between 350 and 600°C. In pure oxygen atmosphere, three exothermic reactions occur, corresponding to the oxidation of the Nd-rich intergranular regions, the Nd2Fe14B matrix phase, and the α-Fe phase that forms during the oxidation of Nd2Fe14B. In air, the oxidation of Nd2Fe14B was investigated further. Instead of the oxidation proceeding along the grain boundaries, the Nd2Fe14B matrix dissociates to form an adherent grey surface layer which grows transgranularly into the magnet. This is probably due to a reaction occurring in the Nd-rich regions which prevents fast path oxygen diffusion along the grain boundaries. The main reaction is the dissociation of the Nd2Fe14B matrix into α-Fe nanocrystals which contain small precipitates of oxides of Nd. The products of this reaction form the adherent grey layer which grows transgranularly into the magnet. The activation energy and the diffusivity pre-exponential factor for this reaction were found to be 114 kJ mol-−1 and 0.7 mm2 s−1, respectively. After further oxidation of the dissociated grey layer, α-Fe is oxidised to form αFe2O3 and finally, from about 600°C, some of the α-Fe2O3 reacts with the small precipitates of oxides of Nd to form FeNdO3.

The microstructures of niobium free and niobium containing Sm2F17 cast alloys were investigated by X-ray diffraction, image analysis, quantitative electron probe microanalysis and thermomagnetic analysis. A cast ingot consisting of the Sm2Fe17 phase and paramagnetic NbFe2 Laves phase was produced at different SmFeNb compositions. An addition of 4at.%Nb was found to be sufficient to produce an as cast alloy without α-Fe phase. The aim of the present work is to study the effect of niobium addition upon the microstructure and the Sm2Fe17 phase. X-ray diffraction showed that the lattice spacings were dependent upon the niobium concentration in the alloy and this indicated some solid solubility of Nb in the Sm2Fe17 phase which was confirmed by electron probe microanalysis. Thermomagnetic analysis was carried out to support the X-ray diffraction results. Some enhancement in Curie temperature was found to be associated with the volume expansion of the Sm2Fe17 phase due to the solid solubility of Nb in the Sm2Fe17 phase.

In order to establish the role of NbFeB precipitates, generated in the microstructure upon the addition of Nb to the stoichiometric NdFeB alloy, three alloys: Nd11.8Fe82.3B5.9 (A), Nd11.8Fe81.3Nb1B5.9 (B) and Nd11.8Fe80.3Nb2B5.9 (C) were characterised by studying the changes in the microstructure, the electrical resistivity and the microhardness during homogenisation.The microstructure of the as-cast alloys contains: Nd2Fe14B matrix phase (ƒ), Nd-rich phase, free iron and in alloys B and C Nb26Fe32B42 ternary phase. When alloys B and C are homogenised they contain predominantly ƒ and Nb26Fe32B42 with only very small amounts of the Nd-rich material. Nb26Fe32B42 was found to nucleate within the free iron, providing strong evidence for the role of Nb in the removal of free iron from the microstructure.The electrical resistivity and the microhardness of the alloys were measured at regular intervals during the homogenisation process at 1000 °C. These studies revealed that, after 90 h, alloy B was homogenised completely and alloy C was homogenised after 40 h; this was in good agreement with the microstructural studies. The resistance measurements also indicated a degree of anisotropy in the as-cast material. At an intermediate stage, both alloys exhibit maxima in the resistance and microhardness curves which could indicate an age hardening process.

Niobium-modified Sm2Fe17 powders were nitrided in pure nitrogen for a range of times and investigated using thermomagnetic analysis, scanning electron microscopy, optical microscopy and electron probe microanalysis (EPMA). Thermomagnetic analysis showed that, on nitriding the Curie point of the core increases continually until the whole particle has been uniformly nitrided. Such results could be explained either by the formation of a continuous solid solution of nitrogen or by the presence of just two compositions with the nitrogen-saturated shell expanding the lattice of the Sm2Fe17 core. A shell-core diffusion pattern was imaged using both optical microscopy and backscattered electron imaging. Comparing the EPMA profiles with the backscattered electron images, a diffusion layer between the shell and core with composition Sm2Fe17Nx (0 < x < 2.7) was identified. This is consistent with the diffusion of nitrogen in the form of a continuous solid solution. The thickness of the nitrided shell varied considerably, not just from particle to particle, but also from different edges of the same particle. In some cases the difference was as large as a 1:10 ratio. Assuming an isotropic bulk diffusion mechanism propagating uniformly from the surface of the particle, such large differences cannot be explained reasonably by the non-central sectioning of the particles in question. Near the edges of some partially nitrided powder particles, the Sm2Fe17N3-δ phase decomposed and this was observed as a mottled region consisting of α-Fe and samarium nitride. The soft magnetic α-Fe can be removed by subsequent zinc bonding, which develops a reaction layer ∼ 4 μm deep for the given processing parameters.

The effect of niobium addition on the microstructure and phase compositions of an Nd2(Fe0.98Nb0.02)14B alloy and an Nd16Fe76.3Nb0.3B7.4 magnet have been investigated. The microstructure of the as-cast alloy contains four phases, the Nd2Fe14B matrix phase, an NbFeB ternary phase, Nd-rich regions and free iron. After homogenisation the alloy contains only the first two phases and very small amounts of Nd-rich phase. Quantitative electron probe microanalysis of the NbFeB phase revealed a composition of approximately Nb26Fe32B42. This ternary phase was found in the as-cast alloy, the homogenised alloy and also, to a lesser extent, in the magnet. The shape of this phase in the as-cast and the sintered magnet was hexagonal whereas, in the homogenised alloy, it was also observed in an acicular form. In the as-cast alloy the ternary phase was found to nucleate within the free iron phase and then grows at the expense of the surrounding free iron. This provides strong evidence for the role of Nb in the removal of free iron from the microstructure. The results indicate that, by controlling the solidification behaviour of the alloy, free iron may be prevented from nucleating or at least reduced to a very thin film near the surface of the ingot which can be removed by machining to produce NdFeB material without free iron.

The effects of the homogenization conditions on the microstructure of a Nd{sub 2}(Fe{sub 0.98}Nb{sub 0.02}){sub 14}B alloy have been investigated. In the as cast condition the microstructure of the alloy contains four phases, the Nd{sub 2}Fe{sub 14}B matrix phase, a Nb-containing ternary phase (Nb{sub 26}Fe{sub 32}B{sub 42}), Nd-rich regions and free iron. In contrast, the homogenized alloy contains only the first two phases and very small amounts of the Nd-rich phase. The Nb{sub 26}Fe{sub 32}B{sub 42} ternary phase was found to originate (nucleate) in the heart of the free iron phase which provides strong evidence for the role of Nb in the removal of free iron from the microstructure. The electrical resistivity of the alloy was measured at regular intervals during the homogenization process at 1,000 C and these studies revealed that, after 40 hours, the alloy was completely homogenized and this was in a good agreement with the microstructural studies. The resistance measurements also indicated a degree of anisotropy in the as-cast material and there was a maximum in resistance at an intermediate stage which could indicate an age hardening process. The maximum resistance was found to correspond to a maximum microhardness value.

The dissociation of the Nd2Fe14B matrix phase of Nd-Fe-B magnets during oxidation leads to the formation of an adherent grey surface layer, which was investigated using optical microscopy; electron probe microanalysis (EPMA) and transmission electron microscopy (TEM). This layer was observed to be composed mainly of a textured mosaic of α-iron grains with fine precipitates within. After further oxidation, the iron within the dissociated layer is itself oxidised forming another surface layer, which results in extensive cracking of the magnet. Along what was the basal plane of Nd2Fe14B, the texture consists of iron grains with a common [111] crystal axis; each crystal being rotated by 30° from its neighbours. The initial steps in the formation of this texture were observed by oxidising a TEM sample in-situ. This texture results from an epitaxial growth of α-iron from the Nd2Fe14B substrate during the oxidation process, with growth in four possible crystallographically equivalent orientations upon the basal plane.

Nd₁₆Fe₇₆B₈ sintered magnets were heated in an open furnace for times up to 29 days. The results of scanning electron microscope (SEM) observations, quantitative EDX/WDX analysis, microhardness measurements and Mossbauer spectroscopy were found to be consistent with the Nd₂Fe₁₄B phase having been disproportionated by oxygen entering via the surface or the cracks. The diffusion front between the oxidized region (oxygen amount~35 at.%) and the unaffected region was observed moving through the grains, indicating that at 400°C the corrosion behavior of the Nd₂Fe₁₄B phase is not intergranular in nature. This indicates that the corrosion resistance of Nd-Fe-B magnets at elevated temperatures could be enhanced by the presence of precipitates in the matrix