Effect of Al–Zn alloy wafer grain boundary diffusion on the magnetism and microstructure of sintered NdFeB magnets

Abstract

This study investigates Al–Zn grain boundary diffusion (GBD) treatment on sintered Nd-Fe-B magnets using Al ₈₀ Zn ₂₀ alloy sheets as the diffusion source. The alloy sheets were placed at both ends of cylindrical samples and diffusion-annealed at 900 °C and 700 °C for 7 h under vacuum (

 Pa). Magnetic measurements show that coercivity increases from 951.5 kA/m in the untreated sample to 1,158.2 kA/m at 900 °C (a gain of 206.7 kA/m, 21.7 %) and to 1,039.6 kA/m at 700 °C (a gain of 88.1 kA/m, 9.3 %), while remanence declines modestly from 1282 mT to 1256 mT after the high-temperature treatment. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffractometer (XRD) analyses reveal that the 900 °C treatment produces a thinner, more continuous grain boundary phase and a distinct core–shell structure around the main-phase grains. EDS mapping shows that Al preferentially enriches the shell region of the Nd ₂ Fe ₁₄ B grains, while Zn predominantly resides in the grain boundary phase, where it lowers the melting point of the intergranular phase and improves its fluidity. XRD shows no detectable secondary phases, while the combined XRD, EDS results, and lattice expansion are consistent with possible limited Al incorporation into the main-phase lattice. Verified by computational analysis, the coercivity enhancement is attributed to three synergistic factors: improved grain boundary decoupling, the formation of a high-anisotropy shell layer that strengthens domain-wall pinning, and the smoothing of grain edges to suppress reverse-domain nucleation. Overall, the 900 °C treatment proves considerably more effective than 700 °C, providing a non-heavy-rare-earth pathway for enhancing coercivity in sintered Nd-Fe-B magnets for high-temperature applications.