Enhancing the grain boundary diffusion efficiency of Tb for Nd-Fe-B magnets using dual-alloy diffusion source

Grain boundary diffusion is an effective approach to enhance coercivity for Nd-Fe-B magnets with reduced consumption of heavy rare earths (HRE). However, during diffusion treatment, the HRE tends to accumulate at the magnet surface, leading to an insufficient coercivity improvement and a decreased magnetic hysteresis loop squareness. Here, a Pr-Al-Cu/Tb-Al-Cu dual-alloy diffusion source is proposed to improve the diffusion of Tb element. During the GBD process, the low melting point Pr-Al-Cu alloy has higher diffusion rate than the Tb-Al-Cu alloy. The formation of continuous grain boundary phase by Pr-Al-Cu diffusion is beneficial to constructing effective channels for the subsequent diffusion of Tb. As a result, the dual-alloy diffusion enhances the coercivity of an initial magnet from 1040 to 1911 kA/m and a high loop squareness of 91.31% is also obtained. In comparison, the coercivity of the magnet treated by the Pr-Tb-Al-Cu single-alloy source was only enhanced to 1832 kA/m with loop squareness of 87.44%. The microstructure characterizations revealed that, by dual-alloy diffusion, both Pr and Tb can infiltrate deeply from the diffusion surface to the interior of the magnet. The formation of (Nd,Tb)2Fe14B phase with high anisotropy field and continuous grain boundary phase contribute to the coercivity enhancement. Using the dual-alloy diffusion source, the HRE resources can be efficiently utilized for increased infiltration depth by avoiding the formation of over-thick Tb-rich grain shells. This facile and flexible process is also attractive for industrial application.