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

The oxidation behaviour of the Nd2Fe14B phase in Nd-Fe-B sintered magnets containing additional elements (Al, Co, V, Nb, Mo) was investigated by Mossbauer 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 (X = Nb, V, Mo) precipitates was evidenced by high-resolution scanning electron microscopy, x-ray diffraction and Mossbauer spectrometry. The presence of Al and Co substituted to Fe in the Nd2Fe14B phase was evidenced, and quantified using Mossbauer spectrometry and Curie temperature measurements. Powdered magnets sieved to a particle size less than 20 mu m were oxidized in an ambient air furnace in the 200-300 degrees C temperature range. These conditions are known to allow the oxidation process of the hard Nd2Fe14B matrix (namely the intragranular diffusion process) to be followed accurately. The experimental oxidation kinetics were determined using Mossbauer 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 mol(-1)), the pre-exponential factor of the diffusivity has a greater influence on the dissociation rate (0.4 mm(2) s(-1) for the magnets containing V or Mo and 0.9 mm(2) s(-1) for the magnet containing Nb, compared with 2.4 mm(2) s(-1) for a magnet without additives). The slowing down of the dissociation rate is attributed to the presence of corrosion-resistant X-Fe-B (X = Nb, V, Mo) intragranular precipitates in the oxidized layer and appears to be strongly dependent on the density of the intragranular precipitates.