MICROSTRUCTURE AND CORROSION-RESISTANCE OF ND-FE-B MAGNETS CONTAININGADDITIVES

The oxidation behaviour of the Nd2Fe14B phase in Nd-Fe-B sintered magn ets containing additional elements (Al, Co, V, Nb, Mo) was investigate d by Mossbauer spectrometry. The microstructure of the different sampl es was first characterized. Added elements were detected in both inter granular and intragranular precipitates. The presence of X-Fe-B (X = N b, V, Mo) precipitates was evidenced by high-resolution scanning elect ron microscopy, x-ray diffraction and Mossbauer spectrometry. The pres ence of Al and Co substituted to Fe in the Nd2Fe14B phase was evidence d, and quantified using Mossbauer spectrometry and Curie temperature m easurements. Powdered magnets sieved to a particle size less than 20 m u m were oxidized in an ambient air furnace in the 200-300 degrees C t emperature range. These conditions are known to allow the oxidation pr ocess of the hard Nd2Fe14B matrix (namely the intragranular diffusion process) to be followed accurately. The experimental oxidation kinetic s 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 f or 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 t he 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 lay er and appears to be strongly dependent on the density of the intragra nular precipitates.