Optimizing Tb diffusion behavior in Tb-Cu-Al grain boundary diffused Nd-Fe-B magnets by Y substitution

Abstract

Grain boundary diffusion (GBD) is a well-established technique for improving the coercivity of Nd-Fe-B permanent magnets while saving heavy rare earths (HRE) resource. However, a common challenge for employing HRE-based diffusion sources is the pronounced HRE accumulation at the near-surface of the magnets, often leading to the formation of excessively thick HRE-rich shell on the grain or even undesired anti-core-shell grain structure due to the aggressive lattice diffusion. This study presents a strategy to suppress excessive Tb lattice diffusion by incorporating Y element with high diffusivity into Tb-based diffusion source. Even with 50 at.% Tb was replaced in Tb80Cu10Al10 diffusion source by Y, the Tb40Y40Cu10Al10 alloy diffusion increased the coercivity of magnet from 12.28 kOe to 21.21 kOe, with a coercivity increment (ΔHcj) of 8.93 kOe, only slightly lower than 10.32 kOe achieved by Tb80Cu10Al10 diffusion. The optimized thermal stability was also obtained. The microstructure analysis revealed that the introduction of Y effectively reduced the accumulation of Tb in the magnet surface and eliminated the anti-shell-core structure. The preferable core-shell structured grains with Tb-rich shell were formed. Further investigation on the element distribution showed that Y is depleted quickly within the near-surface region, whereas Tb preferentially diffuses toward the interior. The optimized core-shell structure is mainly attributed to the lower substitution energy of Y for Nd than Tb. Y exhibits stronger ability to diffuse into the 2:14:1 grain, which has been further demonstrated by the first-principles calculations. The synergistic effect of Tb and Y contributes to the improved efficiency of Tb diffusion on the coercivity. This study thus demonstrated an effective strategy for efficiently utilizing Tb in grain boundary diffusion of Nd-Fe-B magnets.