Effect of nano-Cu addition on the diffusion behaviors of Dy in hot-deformed Nd-Fe-B magnets

It is a major challenge for Nd-Fe-B based permanent magnets to improve the utilization of heavy rare-earth resources. For enhancing the diffusion efficiency of heavy rare-earth elements in the hot-deformed Nd-Fe-B magnets, an intergranular diffusion of DyF3 aided by a trace of nano-Cu was implemented by dual alloy process. Through the trace addition (0.2 wt%) of nano-Cu, the coercivity increment of the Cu-containing magnet is higher than that of the Cu-free magnet after the diffusion of DyF3, and the remanence was maintained at a high level. SEM results revealed that a considerable amount of bulky elongated Dy-rich phase was blocked by (Nd,Pr)-rich phase and hardly infiltrated into powder flakes in the Cu-free magnet. But most of the bulky Dy-rich phase dissolved and infiltrated into adjacent powder flakes with the aid of nano-Cu. TEM results revealed that the Dy concentrations of main phase and grain boundary phase in the Cu-containing magnet are both higher than those in the Cu-free magnet. SEM and TEM results confirmed that the trace addition of nano-Cu contributes to the diffusion of Dy into the main phase and grain boundary phase at the surface of powder flakes, which is the main reason for the further enhancement of coercivity. Further discussions on the enhancement mechanism of coercivity were made by using micromagnetic simulations.