Grain boundary diffusion effects of heavy-rare-earth-free alloys with multi-elements in sintered Nd-Fe-B magnets

Abstract

Heavy-rare-earth-free Pr₆₀Fe₁₀Cu₅Al₁₀Ga₁₅ (at.%) alloy powders with certain ferromagnetism were designed and prepared as the diffusion sources to conduct grain boundary diffusion (GBD) on commercial N55 Nd-Fe-B sintered magnets, in which the ferromagnetic alloy powders had completed the attachment on the surface of the magnets by a magnetic attraction deposition method before heat-treatments. The coercivity of the magnet increased significantly from the original 14.78 kOe to 22.80 kOe after completing the GBD process, accompanied by the noticeable declines both in remanence and magnetic energy product. Microstructure and muti-element distribution analyses showed that the significant improvement in coercivity was mainly attribute to the excellent diffusion effects of Pr-Fe-Cu-Al-Ga alloys in GBD, and the apparent networks of RE-rich grain boundaries had formed and spanned across considerable areas both below the pole and side surface of the magnet. Specially, some typical continuous and penetrating grain boundaries mainly vertical to the c-axis had been formed, that could be closely related to both the intrinsic anisotropy of Nd-Fe-B magnet and the generation of 6:14 type phases during annealing process, based on the detailed microstructural and line-scan analyses. In addition, dynamic domain evolution analyses indicating the positive effect of Pr-Fe-Cu-Al-Ga diffusion on anti-demagnetization. This work provided a foundation for the development of heavy-rare-earth-free diffusion sources also with their corresponding GBD technology in Nd-Fe-B sintered magnets.