Strengthening magnetic and corrosion performances of NdFeB magnets via grain boundary diffusion with Tb element

The structure and phase modification of sintered Nd-Fe-B magnets through Tb diffusion from two different sources, nano TbH_x and TbF₃ particles, and their effects on magnetic and corrosion-resistant properties, were investigated. The results demonstrate that Tb diffusion from TbH_x leads to an optimized microstructure with fewer anti-core-shell grains, a longer diffusion distance, and better magnetic isolation than TbF₃. In contrast, TbF₃ facilitates Tb diffusion into the main phase and also blocks its diffusion channels in the grain boundaries, resulting in accumulation at the surface and significantly reducing the diffusion efficiency of the Tb element. Additionally, the study reveals a key role of Tb in influencing the electrode potential of different phases and, consequently, the corrosion behavior of Nd-Fe-B magnets. Tb diffusion slightly increases the electrode potential of the shell of main phase grains while significantly increasing that of the NdO phase. It thus reduces the interphase potential difference thereby enhancing the corrosion resistance of the magnets. However, when TbF₃ is used for diffusion, the formation of a new (Nd, Tb)-O-F phase, which exhibits significantly lower potential than the (Nd, Tb)-O phase, leads to a deterioration in corrosion resistance. These findings provide valuable insights into the magnetic and corrosion behaviors of grain boundary diffusion magnets and highlight key factors for developing advanced permanent magnets with enhanced overall performance.