Improvement of microstructure and coercivity for Nd-Fe-B sintered magnets by boundary introducing low melting point alloys

Abstract

Different from the grain boundary diffusion process (GBDP), which is suitable for modifying thin magnet, a green-pressing agents permeation process (GAPP) that uses low melting point alloys was applied to the Nd-Fe-B green compact with a thickness over 15 mm to reconstruct the boundary microstructure of a sintered Nd-Fe-B magnet. The coercivity increases from 12.3 kOe for the sample free of Pr80Al20 to 16.8 kOe for the sample with 2 wt% Pr80Al20. By further increasing the Pr80Al20 content to 3 wt%, the coercivity increases slightly, but the remanence and Hk/Hcj deteriorate obviously. The optimal comprehensive properties of Hcj = 16.8 kOe, Br = 13.4 kG and Hk/Hcj = 0.975 are obtained at 2 wt% Pr80Al20, since matrix phase grains are separated by relatively continuous thin grain boundary layers, which weaken the magnetic coupling between adjacent grains. The coercivities of the samples from the GAPP that use 2 wt% Pr80Al20, Pr70Cu30 and Pr60Tb20Al20 alloys, respectively, can be enhanced to a large extent. However, the coercivity of the magnet reconstructed with Pr80Al20 is lower than that of the sample with Pr60Tb20Al20 but is higher than that of the sample reconstructed with Pr70Cu30 alloy. Moreover, the coercivity of the sample from the GAPP using 2 wt% Pr80Al20 is much higher than that of the sample from the GBDP, which is due to a nearly uniform boundary microstructure from the surface to the interior of the thick magnet from the GAPP, thus providing new insights into the fabrication of thick and bulky permanent magnets with high coercivity.