Grain Boundary Structure and Coercivity Enhancement of Nd90Al10 Alloy Modified NdFeB Permanent Magnets by GBD Process

The grain boundary diffusion (GBD) process is a remarkable achievement in sintered Nd-Fe-B permanent magnet manufacturing. Furthermore, the coercivity can be considerably improved by diffusing heavy rare earth (HRE) elements into the magnet along the grain boundary, and the reduced HRE consumption can also be realized. However, compared with parameters of GBD, previous research has focused less on improving the magnet. In this study, the magnet was prepared using low melting point alloy Nd90Al10 before GBD modification, after which the corresponding Tb-GBD was completed. The magnetic property results indicated that the coercivity increased to 1439 kA/m, which was 530 kA/m higher than the unmodified magnet. Thus, the effects of the grain boundary structure and composition on the coercivity were analyzed. The addition of Nd90Al10 did not affect the Curie temperature of the magnet, but it reduced the low-temperature phase transition temperature. The Tb replaced Nd at the margin of main phase, which moved the diffraction peak to the right in the XRD spectrum. Moreover, a clear Tb-rich shell surrounding the main phase formed in the diffused magnet modified by Nd90Al10 at the depth of 20 mu m, and the shell could still be clearly observed at 100 mu m. However, the main phase was surrounded by the continuous grain boundary when the depth increased to 500 mu m in Nd90Al10 modified magnet by GBD. The Tb-rich shell was observed by TEM and a noncrystalline Nd-rich phase was observed. The content peak of the Nd element appeared in the central region of the Nd-rich phase. This, the diffused depth and usage efficiency remarkably improved, because the Nd-rich phase acted as a channel for Tb diffusion, with the concentration of Tb being as high as 35%.