Thermal stability improvement and microstructure optimization of high cobalt content Nd-Fe-B magnets via terbium grain boundary diffusion

The substitution of Fe by Co in the 2:14:1 phase is an effective method to increase the Curie temperature and enhance the thermal stability of the Nd-Fe-B magnets. However, the accumulation of Co element at the grain boundaries (GBs) changes the GBs from nonmagnetic to ferromagnetic and causes the thin-layer GBs to become rare. In this paper, the method of diffusing Tb element was chosen to improve the microstructure and temperature stability of high-Co magnets. Three original sintered Nd28.5Dy3CoxFebalM0.6B1 (x = 0, 6 wt%, 12 wt%; M = Cu, Al, Zr) magnets with different Co contents were diffused with Tb by grain boundary diffusion (GBD). After GBD, high-Co magnets exhibit more continuously distributed thin-layer GBs, and their thermal stability is significantly improved. In high-Co magnets (x = 6 wt%), the absolute value of the temperature coefficient of coercivity decreases from 0.603 %/K to 0.508 %/K in the temperature range of 293–413 K, that of remanence decreases from 0.099 %/K to 0.091 %/K, and the coercivity increases from 18.44 to 25.04 kOe. Transmission electron microscopy (TEM) characterization reveals that there are both the 1:2 phase and the amorphous phase in the high-Co magnet before and after GBD. EDS elemental analysis shows that Tb element is more likely to preferentially replace the rare earth elements in the 2:14:1 main phase than in the 1:2 phase and the amorphous phase. The concentration of Tb at the edge of the main phase is much higher than that in the 1:2 phase and amorphous phase, which is beneficial to the improvement of the microstructure. The preferential replacement of Tb elements at the edge of the 2:14:1 phase and thin-layer GBs with a more continuous distribution are synergistically responsible for improving the thermal stability of high-Co magnets. The study indicates that GBD is an effective method to improve the microstructure and thermal stability of high-Co magnets.