Clarifying the dominated coercivity enhancement mechanism of grain boundary diffused Nd-Fe-B magnets by Pr-based alloys

Low-melting point Pr-based alloys have been confirmed to be cost-effective grain boundary diffusion sources for enhancing the coercivity of Nd-Fe-B magnets. Various coercivity enhancement mechanisms have been reported for these alloys so far, but the dominated one has not been clarified. Here, the sintered Nd-Fe-B magnet was treated by Pr75Al25 alloy grain boundary diffusion at different temperatures. A detailed investigation on the diffusion kinetics of the elements and the microstructure evaluation have been carried out. After 800 °C diffusion, the intrinsic coercivity of the magnet increased from 1070 kA/m to 1348 kA/m without significant reduction of remanence. The increased rare earth-rich phase continuously distributed along the grain boundaries has great contribution to the magnetic decoupling, leading to high coercivity. In comparison, after 900 °C diffusion, although Pr and Al diffused more sufficiently in the magnet, a drastic lattice diffusion occurred and less grain boundary layer was formed, resulting in insufficient coercivity enhancement. The results thus indicate that, for Pr-Al diffusion, the formation of RE-rich layer is more important than that of (Nd,Pr)2Fe14B shell in order to enhance the coercivity. Therefore, different from that for heavy rare earth diffusion, the diffusion treatment for Pr-based sources should be carefully optimized to form continuous grain boundary phase and suppress the excessive lattice diffusion.