On the anisotropic grain boundary diffusion of sintered Nd-Fe-B magnets

Abstract

• Anisotropic diffusion of heavy rare earth can be avoided for sintered Nd-Fe-B magnets. • The alloying of diffusion source distinctly enhance diffusion efficiency of Tb along a -axis. • The alloying of diffusion source inhibits the excessive lattice diffusion of Tb. Grain boundary diffusion (GBD) efficiency of heavy rare earths (HREs) for sintered Nd-Fe-B magnets along the easy magnetization direction ( c -axis) was reported to be higher than that along the difficult one ( a -axis). However, this conclusion is challenged by the present results, which indicate that the anisotropic diffusion does not exist, at least for some low-melting HRE-based diffusion sources. The alloying of Tb by Pr, Al and Cu can distinctly enhance the diffusion efficiency of Tb along the a -axis, resulting in quite similar coercivity enhancement to that diffused along the c -axis. The diffusion kinetic analysis and microstructure characterizations demonstrate that the alloying lowers the melting point of diffusion source and wets the intergranular phase as effective diffusion paths for Tb. This suppresses the excessive lattice diffusion of Tb and the formation of over-thick Tb-rich shells, which is beneficial to enhancing the Tb diffusion efficiency along the a -axis and reducing the total Tb consumption.