Intergranular Phase Engineering in Nd–Fe–B Magnets: Achieving Deep Dy Diffusion through Cu-Modified Grain Boundaries

Grain boundary diffusion (GBD) is an effective technique to enhance the coercivity of Nd-Fe-B magnets, but achieving sufficient diffusion depth to maintain high squareness in thick magnets remains challenging. Here, we investigate the effect of different Cu addition methods on the microstructure and magnetic properties of GBD processed Nd-Fe-B magnets. Three distinct Cu incorporation strategies are explored: surface diffused, grain boundary-doped, and melt-added. Results indicate that the addition of Cu significantly optimizes the grain boundary structure. The melt-added method produces the most uniform and continuous grain boundary network, widening the grain boundary width from nearly 0 to 2.35 nm and effectively enhancing magnetic isolation between grains. Using DyH₃ nanopowder (1.0 wt %) as the diffusion source, these optimized grain boundaries expand diffusion channels and increase diffusion depth (from 401 to 593 μm). Enhanced Dy GBD forms more core(Nd₂Fe₁₄B)-shell[(Nd,Dy)₂Fe₁₄B] structures, where Dy-substitution significantly enhances the magnetocrystalline anisotropy field at the grain surface region, suppressing reverse domain nucleation at the Nd₂Fe₁₄B grain surface region, thus increasing coercivity. The coercivity increment of melt-added Cu magnets after Dy GBD reaches 11.76 kOe, significantly higher than the 5.73 kOe of Cu-free magnets and exceeding previous related reports for magnets of similar thickness. Notably, this strategy is successfully applied to 10 mm thick bulk magnets, improving coercivity while maintaining excellent squareness of 92.9%. These results offer the possibility of applying GBD technology to applications requiring thick magnets, such as high-temperature wind turbines.