Influence of particle size of iron powder on the microstructure of Nd-Fe-B alloy powder prepared by reduction-diffusion

Neodymium-iron-boron (Nd-Fe-B) alloy powder is technologically important for the manufacture of high energy product permanent magnets. Calciothermic reduction of neodymium salt in the presence of iron and ferroboron is reported to be one of the cost-effective processes of synthesizing Nd-Fe-B alloy powders. The dependence of magnetic properties such as coercivity, remanence and energy product on grain size has already been established. Small grains in the size range of 1-5 μm have been found to give high coercivity magnets. It is reported that diffusion reaction in the R-D process is a one-way reaction, i.e., diffusion of neodymium into the iron metal. In view of this finer size of the iron powder is favoured as it allows faster rate of formation of the desired intermetallic compound (Nd2Fe14B). The present paper examines the effect of iron powder size for obtaining Nd-Fe-B alloy powders of desired characteristics. Atomised iron powders of three different size ranges, i.e., <10, 20-40 and >53 μm were used for calciothermic reduction of Nd2O3 in the presence of ferroboron. The Nd-Fe-B alloy powder obtained has been studied with respect to grain size, shape, phases present and ease of washing. Finally, it is concluded that use of very fine iron powders (<10 μm) does not give a distinct advantage for the manufacture of Nd-Fe-B alloy powder