High coercivity of 2.8 T in heavy-rare-earth-free anisotropic magnets by microstructure engineering

We demonstrate that a high coercivity of 2.8 T can be achieved in a 5-mm-thick anisotropic Nd-Fe-B magnet by applying a Pr–Cu grain boundary diffusion process to a hot-deformed Nd-Fe-B-based magnet. Microstructural analysis reveals two key changes; the formation of an Fe-lean intergranular phase, which weakens magnetic exchange coupling, and the development of a Pr-rich shell around the matrix grains, which enhances the local anisotropy field after the diffusion process. These modifications shift the angular dependence of coercivity from the Kondorsky type to the Stoner–Wohlfarth type, and persist even at elevated temperatures. This explains the observed high coercivity and its improved thermal stability. Benchmarking the performance against 8–9 wt.% Dy-containing sintered magnets confirms that these Dy-free magnets exhibit comparable high-temperature performance due to improved thermal stability. Our findings highlight that ultrafine-grained, Dy-free, hot-deformed magnets are a promising alternative to commercial Dy-containing magnets for high-performance applications.