Enhancing the grain boundary diffusion efficiency of Tb for Nd-Fe-B magnets using multi-element composite diffusion source

Grain boundary diffusion of the heavy rare-earth element Tb, facilitated by multielement alloys, was investigated in Nd-Fe-B sintered magnets. This study systematically examined magnets treated with various diffusion sources: conventional TbHx powders, a mixture of TbHx and Pr45.0Cu18.3Al27.5Ga9.2 (at%) alloys, and integrated Tb10.0Pr35.0Cu18.3Al27.5Ga9.2 (at%) alloys. Results indicated that, although the total Tb content was considerably reduced from 2.80 to 0.49 wt%, the coercivities of the magnets employing multielement composite diffusion sources were still enhanced to a high level, comparable to those treated with TbHx alone. Notably, the coercivity of the magnets diffused with Tb10.0Pr35.0Cu18.3Al27.5Ga9.2 substantially increased to 24.14 kOe from the original value of 14.19 kOe, and the Tb utilization rate reached 73.47%. In addition, the coercivity increment per unit mass of Tb was 20.31 kOe/wt%. Microstructural observations and analyses of the diffusion process revealed that the assisted diffusion by the multielement composite effectively prevented superficial Tb accumulation, thereby enhancing diffusion efficiency. This process resulted in the formation of typical continuous grain boundaries and a core-shell structure. Furthermore, it was observed that low-melting-point metal elements, particularly Al, preferentially infiltrated the magnet interior and wetted the grain boundary channels during the early diffusion stages, facilitating efficient Tb diffusion. This study lays the groundwork for developing highly efficient magnets with reduced heavy rare-earth content.