Temperature-dependent grain boundary diffusion behavior of Nd-Dy-Al alloy in sintered Nd-Fe-B magnet

The diffusion temperature has shown significant effects on the microstructure and magnetic properties of the grain boundary diffusion (GBD) treated Nd-Fe-B magnets, but the underlying mechanism is still not quite clear. Here, the sintered Nd-Fe-B magnets were processed by GBD with Nd₂₀Dy₆₀Al₂₀ alloy as the diffusion source. After diffusion at 800 °C, 900 °C, and 950 °C, the magnets exhibit increased intrinsic coercivity from 924 kA/m to 1231 kA/m, 1346 kA/m, and 1163 kA/m, respectively. The microstructure analysis reveals two types of diffusion behavior for Dy element in the magnet during GBD, i.e. lattice diffusion and inter-grain diffusion. Lattice diffusion helps to form core-shell structured hard magnetic grains while inter-grain diffusion promotes the diffusion depth. These two types of diffusion can be modified by the diffusion temperature. At the diffusion temperature of 800 °C, Dy element tends to enter into the interior of the magnet along the grain boundary by inter-grain diffusion, but at 950 °C, it prefers to substitute Nd to enter into the grain by lattice diffusion. These two diffusion behaviors achieve a delicate balance at 900 °C, and both the effective diffusion depth and the desired thickness of the magnetically hardened shell can be realized, which results in the highest coercivity enhancement.