Grain boundary microstructure in DyF3-diffusion processed Nd–Fe–B sintered magnets

The effect of post-sinter tempering on the DyF3-diffusion processed Nd–Fe–B was investigated using two kinds of starting magnets. The increase of coercivity after diffusion process using as-sintered magnet was higher than that using two-stage tempered magnet. The grain boundary phase of the tempered magnet became discontinuous upon further annealing at the temperature of diffusion process. This clearly indicates that a continuous grain boundary phase is helpful to the DyF3-diffusion process. When sufficiently diffused, there is no enrichment of Dy in the grain boundary phase. The excess Nd as a result of Dy substitution in the Nd2Fe14B matrix phase forms Nd–O phase at grain boundary and on the surface of the magnet. The increase of coercivity can be related to the (Nd,Dy)2Fe14B grains as well as to the improved decoupling by the grain boundary phase.