Significantly enhancing the coercivity of NdFeB magnets by ternary Pr-Al-Cu alloys diffusion and understanding the elements diffusion behavior

Low melting point Pr-Al-Cu alloys were employed in grain boundary diffusion for sintered NdFeB magnets. The coercivity of the 7 × 7 × 2 mm magnet was enhanced from 1000 kA/m to 1360, 1615, and 1714 kA/m by diffusing Pr70Al10Cu20, Pr70Al15Cu15 and Pr70Al20Cu10 alloys, respectively, at 800 °C for 2 h followed by 500 °C for 3 h. The optimized microstructure with strip-like grain boundary phase was evident in diffused magnets, which helps to reduce the magnetic exchange interaction between the hard magnetic grains and improve the coercivity. Under the same diffusion process, the coercivity of the diffused magnets decreased with increasing sample thickness, but 27% coercivity enhancement can still be obtained in the magnet with 6 mm in height. A Φ10×10 mm block magnet was diffused by Pr70Al20Cu10 alloy at 800 °C for 2-10 h. Up to 49% enhancement in coercivity was achieved, which confirms the diffusion ability of Pr-Al-Cu alloy. The diffusion behavior of Pr, Al and Cu elements has been analyzed by Fick's law. Based on the diffusion coefficients obtained at 800 and 900 °C, Pr diffuses faster than Cu and Al along the grain boundary. The current results demonstrated that Pr-Al-Cu alloys can effectively optimize the grain boundary structure and improve the coercivity of NdFeB magnets.