Improvement of coercivity and its thermal stability of hot-deformed Nd-Fe-B magnets processed by Tb70Cu30 doping and subsequent Nd85Cu15 diffusion

Abstract

A new processing route comprising of Tb-Cu doping and subsequent Nd-Cu diffusion was applied to fabricate hot-deformed Nd-Fe-B magnets with high coercivity and high thermal stability. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and corresponding energy-dispersive X-ray spectroscopy (EDS) analysis revealed that the thin Fe-rich amorphous intergranular phase was formed in hot-deformed samples with a small amount of Tb70Cu30 doping (2 and 5 wt.%). The core-shell structure was dependent on the amount of Tb70Cu30 and obvious Tb-rich shell with the thickness about 20 nm was observed in the hot-deformed sample with 5 wt.% Tb70Cu30 addition. Ordered atomic arrangement of the core-shell interface was observed by the atomic-scale HAADF-STEM image viewed along [100] zone axis. After Nd85Cu15 diffusion a large amount of Nd3Co and NdCu phases and obvious Tb-rich shell were observed in the hot-deformed sample doped with 2 wt.% Tb70Cu30, which results in a substantial enhancement of coercivity from 1.77 to 2.68 T and improved temperature coefficient of coercivity from - 0.452 to - 0.358%/°C. For the hot-deformed sample doped with 5 wt% Tb70Cu30, the high coercivity of 3.07 T and low temperature coefficient of - 0.348%/°C were achieved after Nd85Cu15 diffusion. The evolution mechanism of the core-shell microstructure and phase composition were analyzed.