Microstructural modification of grain boundary area in WS₂/Al co-doped Nd-Fe-B sintered magnet

We investigated the effect of microstructural modification of grain boundary area and WS₂ powder size on the magnetic properties of WS₂/Al-doped Nd-Fe-B sintered magnet. Grain growth inhibition (7.6 → 6.4 μm) in optimally doped magnet with Dy-rich core-shell microstructure was effectively improved the magnetic properties compared with only doped magnet. In the case of only doped magnet, the lattice misfit between the (002) WFeB precipitates and (110) Nd₂Fe₁₄B interfaces was about 9.9%, which caused the strong strain fields at the grain boundaries. However, the doped magnet with Dy-rich core-shell exhibited a decreased lattice misfit (9.9 → 3.4%) owing to the formation of the high anisotropic Dy-rich (Nd,Dy)₂Fe₁₄B phase around the GB area, thus suppressing reverse-domain nucleation at the grain boundaries. As a result, the coercivity increments in the optimally doped magnets with Dy-rich core-shell (6.5%) was larger than that of only doped magnets (3.9%) compared with each corresponding undoped magnet. When the particle size of WS₂-doped powders was reduced (∼2.0 → ∼0.6 μm), the coercivity further increased owing to the improved inhibition of grain growth although the doping content of WS₂ was reduced from 0.6 to 0.4 wt%.