Diffusion behavior controlled performance enhancement of Nd-Fe-B magnets diffused by Tb-Ni alloy at various temperatures

Abstract

High coercivity and anti-corrosion properties are required for sintered Nd-Fe-B magnets used in new energy and off-shore wind power industries. Here, we found that both magnetic properties and corrosion resistance of Nd-Fe-B magnets can be significantly improved by grain boundary diffusion (GBD) of Tb₆₉Ni₃₁ alloy. The diffusion temperature is a key factor in determining both the magnetic properties and corrosion resistance. The wettability of Tb₆₉Ni₃₁ alloy with the magnet and the fluidity of Tb₆₉Ni₃₁ alloy gradually increase with increasing temperature above 750 ℃, which facilitates the elements diffusion. Tb and Ni show different diffusion behaviors at different temperatures. The magnet diffused at a relatively high temperature exhibits high coercivity due to the formation of a thick Tb-rich shell in the grain with high magnetocrystalline anisotropy field. However, excessive grain growth leads to a reduction in coercivity as the diffusion temperature further increases. The highest corrosion resistance is achieved by Tb₆₉Ni₃₁ alloy diffusion at a relatively lower temperature. By analyzing the Volta potential distribution using scanning Kelvin probe force microscopy, the Ni-rich phase with high potential formed in grain boundaries is mainly responsible for the improved corrosion resistance. The current results suggested that the performance of Tb-Ni diffused magnets can be controlled by the diffusion behaviors of Tb and Ni.