Modifying Tb-Cu grain boundary diffusion behavior in Nd-Fe-B magnets by Gd or other rare earth elements

The grain boundary diffusion using mixed rare earths (REs) as the diffusion source is very effective in enhancing the coercivity of Nd-Fe-B magnets. Here, we firstly investigated the Gd added Tb-Cu diffusion alloys and found that the coercivity of Nd-Fe-B magnets can be improved from 1094 kA/m to 1704, 1677 and 1593 kA/m by Tb70Cu30, Tb60Gd10Cu30 and Tb50Gd20Cu30 diffusion, respectively. The material cost of diffusion source in grain boundary diffusion process can be reduced from 1091 $/kg of Tb70Cu30 to 788 $/kg of Tb50Gd20Cu30. Binary GdCu alloy source has no positive effect on the coercivity enhancement. The enhanced thermal stability with increased temperature coefficient of coercivity from -0.62 %/degrees C for the initial magnet to -0.55 %/degrees C and -0.56 %/degrees C is also obtained by Tb70Cu30 and Tb50Gd20Cu30 diffusion, respectively. Tb exhibits a higher diffusion coefficient than Gd, but the introduction of Gd decreases the melting point of Tb-Cu alloy and promotes the infiltration of Tb. The typically core-shell structure with Tb-rich shell is observed after diffusion, which enhances the coercivity. The effects of RE elements in the Tb-RE-Cu diffusion source on the diffusion behavior of Tb has also been discussed. Y and Gd elements tend to enter into RE2Fe14B grain, resulting in the reduction of thickness for the Tb-rich shell, but Pr, La and Ce elements prefer to stay in the grain boundaries, which provides the diffusion channel for Tb, leading to the increased diffusion depth. This work provides a design idea for the RE -based diffusion sources of Nd-Fe-B magnets.