Coercivity Enhancement of Dy-Free Sintered Nd–Fe–B Magnets by Grain Refinement and Induction Heat Treatment

Using conventional N
₂
gas jet milling, a coercivity of 19.01 kOe was obtained in Dy-free sintered Nd-Fe-B magnets with the grain size refined to 2.6 μm. Through the high-frequency induction heat treatment (HIHT) at 600°, the coercivity was enhanced to 20.56 kOe, while the remanence remained unchanged. The transmission electron microscope observation showed that the grain boundaries were broadened through the HIHT process. The increasement of coercivity may be related to the strong stirring effect of induction vortex, which facilitates the continuous distribution of the Nd-rich grain boundary phase and thus enhances the magnetic insulation between the matrix grains.