Influence of Al/Cu content on grain boundary diffusion in Nd-Fe-B magnet via in-situ observation

The influence of aluminum and copper content in the starting Nd-Fe-B magnet on grain boundary diffusion process (GBDP) was studied by observing the phase transformation behaviors of the magnets in-situ at high temperature. A higher coercivity increment is discovered in the sample with higher Al/Cu despite the fact that its Dy diffusion amount is the same as the other. DSC analysis shows an evident melting behavior in the higher Al/Cu sample. Laser scanning confocal microscopy (LSCM) in-situ characterization shows a large amount of melted intergranular phase spills out to the surface simultaneously at around 600 °C in the high Al/Cu sample, while the phase spills out gradually one after another in the range between 623 and 680 °C in the other sample, which indicates that the intergranular phase can be more easily melted in the sample containing more Al/Cu. The area fraction of matrix phase remarkably shrinks while that of intergranular phase enlarges after LSCM heating, which demonstrates the outer region of the Nd₂Fe₁₄B grains melt at the temperature of 900 °C. Electron probe microanalyzer result (EPMA) shows that the Nd and Dy concentrate in edge regions and subsequently mix into the intergranular phase with the melting of the grain edge, while a large amount of Al and Cu in the intergranular phase spill out. Nevertheless, the sample with higher starting Al/Cu still remains higher residual contents after LSCM experiments, and that could probably be the main reason why the high Al/Cu magnet shows smaller coercivity decrement after LSCM experiment. Overall, the increase of Al/Cu in the starting magnet optimizes the Dy distribution and the wettability of intergranular phase, enhancing coercivity increment effect further.