Sulfur doping effect on microstructure and magnetic properties of Nd-Fe-B sintered magnets

Highlights: • The melting point of the Nd-rich eutectic phases was lowered by S addition. • The grain growth of the S-doped Nd-Fe-B sintered magnets was effectively inhibited. • Coercivity was maximum with S content of 0.2 wt%, 7.3% higher than the S-free magnets. • The mechanism of the coercivity enhancement with proper S doping is outlined. In this paper, the effects of sulfur (S) doping on microstructure and magnetic properties of Nd-Fe-B sintered magnets were studied. With 0.2 wt% S doping, the melting point of the Nd-rich eutectic phases decreased from 1038 K to 1021 K. Clear and continuous grain boundary phases were also formed with smaller grain size. The average grain size was 7.83 μm, which was approximately 1.3 μm smaller than that of the undoped magnets. The coercivity enhancement was attributed to boundary microstructure modification and grain size optimization. The coercivity of the 0.2 wt% S-doped magnets increased from 15.54 kOe to 16.67 kOe, with slight changes of the remanence and the maximum magnetic energy production. The magnetic properties of the overdoped magnets deteriorated, due to the reduction in density and decrease of the volume fraction of the main phase. Globular S precipitates in the Nd-rich triple junctions were hexagonal Nd₂O₂S phase and tetragonal NdS₂ phase. S addition allows reducing Dy usage in magnets with comparable magnetic properties.