A cost-effective dual-layer grain boundary diffusion strategy to significantly enhance coercivity of sintered Nd-Ce-Fe-B magnets

In an attempt to obtain cost-effective sintered Nd-Ce-Fe-B magnet and overcome the coercivity-remanence trade-off challenge caused by heavy rare earth (HRE) diffusion, a novel dual-layer coating method has been proposed for the grain boundary diffusion process (GBDP). A high coercivity increment of 8.56 kOe is achieved by dual-layer coated DyH _x/Pr ₈₀Al ₂₀ (DH/PA) diffusion with 0.8 wt% DyH _x and a slight remanence reduction from 12.85 to 12.71 kG. Although the DyH _x diffused magnet has a coercivity increment of 9.24 kOe, it consumes twice as much DyH _x as the DH/PA diffused magnet at the great sacrifice of remanence. This can be attributed to the formation of continuous grain boundary phase and thin Dy-rich shell via microstructural observations. Further TEM characterization indicates that the amorphous triple junction grain boundary phase (TJP) gradually transforms into the crystalline Nd ₂O ₃ structure after GBDP. The coexistence of cubic-Nd ₂O ₃ phase and hexagonal-Nd ₂O ₃ phase found in the dual-layer coated Pr ₈₀Al ₂₀/DyH _x (PA/DH) diffused magnet creates a more intricate grain boundary phase structure, hindering the diffusion of Dy. Comparably, the formation of single cubic-Nd ₂O ₃ ( Ia 3 ̅ structure) grain boundary phase in the DH/PA and DyH _x diffused magnets is an important reason for the significant increase in coercivity. Based on the evaluation of magnetic performance and cost, the DH/PA diffused magnet achieves a higher price-performance ratio (PPR). This cost-effective bilayer grain boundary diffusion strategy provides a valuable reference for high PPR Nd-Ce-Fe-B magnet fabrication.