Optimizing Ce-doped Nd-Fe-B magnets for high-efficient grain boundary diffusion of Tb-Fe alloy

The grain boundary diffusion (GBD) efficiencies of heavy rare earth (HRE) elements are strongly influenced by the composition of the initial Nd-Fe-B magnets. Here we reported the significant variation of terbium (Tb) diffusion efficiency in the Nd-Fe-B magnets with various cerium (Ce) contents. The magnets with 0 wt. %, 3.71 wt. %, and 8.45 wt. % Ce in the total rare earth content, denoted as Ce0, Ce4, and Ce8 magnets, were employed as the initial magnets. After diffusion by Tb₆₆Fe₃₄ alloy, the coercivity of the Ce0, Ce4, and Ce8 magnets increased from 1196 kA/m, 958 kA/m, and 986 kA/m to 1689 kA/m, 1305 kA/m, and 1711 kA/m, respectively. The Ce8 magnet demonstrated the highest diffusion efficiency of Tb with an increase of 1209 kA·m^-1 by 1 wt. % Tb. The diffused Ce8 magnet also exhibits excellent hysteresis loop squareness due to the superior diffusion behavior of Tb. In the Ce-free magnet, the presence of bulk rare earth (RE) oxides and a fully Tb₂Fe₁₄B structure leads to the wastage of Tb. Microstructure analysis revealed that the formation of a thick grain boundary (GB) layer and the cubic RE₂O₃ phase contribute to the coercivity enhancement of the Ce8 magnet. In contrast, the GB layer in Ce-free magnet is too thin to isolate the Nd₂Fe₁₄B hard magnetic grains effectively. Therefore, to achieve high GBD efficiency for Tb, the Ce8 initial magnet is effective. These findings suggest that the careful selection of initial magnet is crucial for maximizing property enhancement through GBD treatment and ensuring the efficient use of RE resources.