Microstructure and corrosion resistance of Nd-Fe-B magnets containing additives

Abstract

The oxidation behaviour of the phase in Nd-Fe-B sintered magnets containing additional elements (Al, Co, V, Nb, Mo) was investigated by Mössbauer spectrometry. The microstructure of the different samples was first characterized. Added elements were detected in both intergranular and intragranular precipitates. The presence of X-Fe-B (, V, Mo) precipitates was evidenced by high-resolution scanning electron microscopy, x-ray diffraction and Mössbauer spectrometry. The presence of Al and Co substituted to Fe in the phase was evidenced, and quantified using Mössbauer spectrometry and Curie temperature measurements. Powdered magnets sieved to a particle size less than m were oxidized in an ambient air furnace in the 200-C temperature range. These conditions are known to allow the oxidation process of the hard matrix (namely the intragranular diffusion process) to be followed accurately. The experimental oxidation kinetics were determined using Mössbauer spectrometry and fitted according to a single-particle analysis model. The results show a decrease of the dissociation rate of the hard matrix compared with the rate obtained for a sintered magnet containing no additives. As the activation energy was found to be comparable for each sample (106-112 kJ ), the pre-exponential factor of the diffusivity has a greater influence on the dissociation rate ( for the magnets containing V or Mo and for the magnet containing Nb, compared with for a magnet without additives). The slowing down of the dissociation rate is attributed to the presence of corrosion-resistant X-Fe-B (, V, Mo) intragranular precipitates in the oxidized layer and appears to be strongly dependent on the density of the intragranular precipitates.

A Mössbauer investigation of the dissociation of the Nd2Fe14B phase

“…However, for the DCVA, CVA and CMA samples, the α-Fe contribution is characterized by a higher mean hyperfine field (34.6-34.7 T) than that obtained for the Nd 16 Fe 76 B 8 and DNA samples (33.3-33.4 T). This is attributed to the presence of Co in the α-Fe phase, in agreement with the fact that Co atoms substitute for Fe atoms in the Nd 2 Fe 14 B unit cell [27]. By interpolating the results of a study of the mean hyperfine field in Fe 1−x Co x compounds (with 0 x 0.5) [28], the Co content in α-Fe was estimated to be 8 at.%.…”

“…The nominal atomic compositions of the samples containing additives (DCVA, CVA, CMA and DNA) are reported in table 1. The microstructural investigation of these samples is reported elsewhere [27]. This study revealed the presence of both intergranular and intragranular Layer composed of: Layer composed of: X-Fe-B precipitates (with X = V, Nb, Mo).…”

“…The fact that the Fe 3 B and para4 contributions are not observed in the spectrum of oxidized Nd 16 Fe 76 B 8 or DNA powders is attributed to the presence of Co in the Nd 2 Fe 14 B phase, which leads to the formation of other dissociation products (in weak proportions) during the dissociation process. The formation of some Fe 3 B during the process could be related to the fact that the Co content in the α-Fe phase (8 at.%) is higher than in the Nd 2 Fe 14 B phase (5 at.% [27]). The para4 contribution, in very weak amount, remains unidentified.…”

Effects of Dy and Nb on the magnetic properties and corrosion resistance of sintered NdFeB

“…Therefore, considerable effort has been devoted to enhancing the corrosion resistance of NdFeB magnets using surface coating such as electroplating of Ni, electroplating of Zn, and spraying of epoxy resin [1][2][3][4]. In parallel, the attempts have been made to improve the corrosion resistance NdFeB alloys by addition of other alloying elements, including Pb, Ti, Zr, Sn and Cr [5][6][7][8]. The addition of these elements, however, is likely associated with the degradation of magnetic properties.…”

Thin films of NdFeB deposited by PLD technique

“…First studies revealed that nanocrystalline magnets have a higher environmental susceptibility than sintered microcrystalline magnets and that their corrosion behaviour depends strongly on the powder processing route generating different grain sizes. Other rare earth elements (Dy, Pr), transitional elements (Co, Ni) or metals (Ga and/or Al) are suitable additives for improving the magnetic properties and corrosion resistance, as reported in literature [18][19][20].…”