Dependence of grain size and aspect ratio on grain boundary additives in hot-deformed Nd-Fe-B magnets

Abstract Commercial Nd13.6Fe73.6Ga0.6Co6.6B5.6 (at.%) powders were mixed with 0-10 wt.% Nd80Cu15Al5 alloy powders to prepare hot-deformed Nd-Fe-B magnets. As the increasing Nd80Cu15Al5 content from 0 to 10 wt.%, the grain sizes along the c-plane (D||) decreased by more than 35% from 312 to 198 nm while the grain sizes along the ab-plane (D⊥) decreased by less than 7% from 58 to 54 nm, resulting in a decrease of more than 30% in the grain aspect ratio (D||/D⊥) from 5.4 to 3.7. Moreover, the temperature coefficient of coercivity increased significantly from -0.469 to -0.419%/°C by 8 wt.% Nd80Cu15Al5 addition. Based on microstructure observations, the improved thermal stability of the magnets without using heavy rare-earth elements is due to combined effect of the exchange decoupling between grains by the nonferromagnetic grain boundary phases, the refinement of grains and the decreased grain aspect ratio.