Optimization of TbAl grain boundary diffusion sources for sintered Nd–Fe–B magnets by low cost Nd substitution for Tb

Abstract

Tb–Al alloys have been frequently employed as grain boundary diffusion (GBD) sources to enhance the coercivity of Nd–Fe–B magnets. With the aim of further optimizing the diffusion efficiency of Tb and reducing the material cost of the Tb–Al alloys, Nd_xTb_80−xAl₂₀ alloys with various Nd substitutions for Tb have been developed as diffusion sources for a commercial sintered Nd–Fe–B magnet (N50) in this work. It is found that the binary Tb₈₀Al₂₀ alloy diffusion enhanced the coercivity of the magnet from 924 kA m⁻¹ to 1718 kA m⁻¹. Although 50 at% Tb was replaced by Nd in the diffusion alloy of Nd₄₀Tb₄₀Al₂₀, the coercivity of the magnet was significantly enhanced from 924 kA m⁻¹ to 1836 kA m⁻¹, along with greatly enhanced thermal stability. The diffusion temperature has also been optimized to 900 °C for the highest coercivity enhancement. The microstructure characterization indicated that the dramatic performance improvement of the Nd₄₀Tb₄₀Al₂₀ diffused magnet is mainly attributed to the effective construction of diffusion channels by Nd, which enables more Tb atoms to enter into the interior of the magnet along the Nd-rich grain boundary phase and, thus, a magnetically hardened shell structure can be formed deep inside the magnets. This work hence presents a potential approach to enhance the utilization efficiency of Tb in the GBD process for Nd–Fe–B magnets.