Magnetic performances and diffusion shell-structures of Nd-Fe-B sintered magnets treated with grain boundary diffusion by low Dy-content multi-component alloys

Commercial N55 sintered Nd-Fe-B magnets were treated by grain boundary diffusion (GBD) technology with a series of designed low Dy-content alloys, whose chemical composition could be expressed as Pr_(70-x)Dy_xM₃₀ (at%, M = Fe, Al, Ga). Results showed that the coercivities (H_cj) of the diffused magnets were gradually enhanced with the Dy atomic ratios (in the diffusion-source alloys) increased from 0 % to 20 %, and especially, the coercivity was greatly improved to 22.82 kOe from the initial 14.38 kOe when adopting Pr₅₀Dy₂₀M₃₀ alloys in GBD, and the coercivity increment (∆H_cj) reached 8.44 kOe with the unit Dy strengthening effect (∆H_cj/wt%Dy) of 16.55 kOe/wt%, according to the actual ICP component analyses, which were even better than that when adopting conventional DyH_x diffusion sources with the ∆H_cj of 7.74 kOe and the ∆H_cj/wt%Dy of 4.05 kOe/wt%. Note that the comprehensive diffusion effects were starting to deteriorate when the atomic ratio of Dy further increased to 30 % in the diffusion alloys. Systemic microstructure and composition analyses indicated that the excellent diffusion effects of Pr₅₀Dy₂₀M₃₀ alloys were mainly attributed to the deeper Dy-rich shells along diffusion direction simultaneously with more continuous Pr-rich phases in grain boundaries compared with other diffusion sources in this work. Meanwhile, magnetic domain evolution observation by Kerr microscopy confirmed that the diffused magnets using low Dy-content diffusion sources could also achieve a similar anti-demagnetization ability to that using the conventional DyH_x sources in the surface layers of the magnets. Besides, the characterization and analyses of the evolutions of typical core-shell structures during diffusion processes were also performed systematically.