Microstructure and Magnetic Properties of High Coercivity Die-Upset Nd–Fe–B Magnets by Nd–Cu Alloy Addition

The coercivity of the die-upset Nd-Fe-B magnets was enhanced remarkably by the Nd-Cu eutectic alloy addition. The initial magnetization curves show that the magnetization reversal transforms from the nucleation fields for an Nd-Cu free magnet to the coexistence of the nucleation and domain-wall pinning fields for the magnet with Nd-Cu addition. The transmission electron microscopy images reveal that more triple-junction regions, which are enriched with Nd and Cu elements, distribute in the die-upset magnets with Nd-Cu addition. These regions become the domain-wall pinning center and enhance the coercivity of the magnets. Moreover, the thickness of the grain boundary phase increases with the Nd-Cu addition, which declines the coupling between the Nd-Fe-B grains and then further improves the coercivity.