The ability to improve the coercivity of sintered Nd-Fe-B diffused by Dy-Al-Cu alloy derived from electrolysis
in a fluoride salt-oxide system was evaluated. The results show that with the increase in heat treatment time,
the coercivity of the magnet firstly increased and then decreased. Holding at 900 ℃ for 4 h, tempering at low
temperature for 3 h at 550 ℃, the coercivity of GBDPed magnet increased by 44.34 %, the remanence
decreased by 1.26 %, the Dy-rich shell phase was recognizable, and the Nd-rich phase evenly distributed. Electron
microscope analysis showed that when the GBDP time was longer than 4h, the diffusion of Dy from the
shell phase to the matrix phase dominated, decreasing the coercivity with the increase in the diffusion time. The
increase of Fe content in the grain boundary phase enhanced the exchange coupling between grains, which also
reduced the coercivity of the GBDPed magnet. The infiltration of the matrix phase by excessive Dy and deterioration
of (00L) texture of Nd-Fe-B resulted in the reduction of the remanence and the maximum energy product.