Regulation of the grain boundary CeFe₂ phase via Cu addition for anisotropic Ce-Fe-B magnet fabrication

A low melting point rare earth (RE) rich phase is essential for fabricating anisotropic nanocrystalline RE-Fe-B magnets. Ce-Fe-B alloys exhibit distinctive metallurgical behavior due to the precipitation of CeFe₂ phase, which reduces the RE-rich phase content. To date, developing anisotropic Ce-Fe-B magnets by eliminating CeFe₂ phase has not been successful. Here, we propose an alternative strategy to take advantage of CeFe₂ phase by modifying its physical properties via Cu doping. The segregation of Cu at grain boundaries changes the phase transition behavior of CeFe₂ phase. By preparing annealed CeFe_2-xCu_x alloys, it was found that the melting temperature of the CeFe₂ phase decreased from 1198 K for x = 0 to 973 K for x = 0.2. For the nanocrystalline Ce₁₆Fe_78-xCu_xB₆ (x = 0-2.0 at%) alloys, Cu doping transforms the morphology of Ce(Fe, Cu)₂ phase from bulk aggregate to elongated shape, indicating increased flowability and wettability. The improved wettability of Ce(Fe, Cu)₂ phase with low melting-point in the grain boundary facilitates the texture development during hot-deformation. As a result, the hot-deformed Ce₁₆Fe₇₆Cu₂B₆ magnet exhibits pronounced magnetic anisotropy with maximum energy product (BH)_max of 22.5 kA m^-3, coercivity H_cj of 109 kA m^-1, and remanence J_r of 0.54 T. On the contrary, the hot-deformed Ce₁₆Fe₇₈B₆ magnet shows isotropic behavior with (BH)_max of 0.9 kA m^-3, H_cj of 27 kA m^-1, and J_r of 0.15 T. This work provides a practical process for fabricating anisotropic Ce-Fe-B permanent magnets by tuning the properties of CeFe₂ phase.