Influence of annealing on microstructural and magnetic properties of Nd-Fe-B magnets by grain boundary diffusion with Pr-Cu and Dy-Cu alloys

Highlights: • Diffusion process with (Pr,Dy)-Cu alloy was applied to the Nd-Fe-B sintered magnet. • Coercivity of the sample with Dy-Cu without annealing is largely enhanced. • Coercivity of the sample with Pr-Cu is raised only after annealing treatment. • Coercivity increment in the diffused sample with Dy-Cu is due to magnetic hardening. • Coercivity increment in the annealed sample with Pr-Cu stems from magnetic isolating. - Abstract: The paper presented the effect of post-diffusion annealing on the magnetic properties and microstructure of Nd-Fe-B sintered magnets by grain boundary diffusion with Pr₆₈Cu₃₂, Dy₇₀Cu₃₀ and Pr₃₅Dy₃₅Cu₃₀ alloys. The coercivities of the samples changed slightly by diffusing Pr₆₈Cu₃₂ alloy then increased sharply by subsequent post-diffusion annealing. Differently, the coercivities of the samples showed large rise by diffusing Dy₇₀Cu₃₀ and Pr₃₅Dy₃₅Cu₃₀ alloys then slight rise by subsequent annealing. Investigation on microstructure revealed that it was mainly due to different mechanism of grain boundary modification by Pr-Cu and Dy-Cu alloys. The coercivity enhancement of the diffused magnets by Dy₇₀Cu₃₀ and Pr₃₅Dy₃₅Cu₃₀ alloys was mainly attributed to continuous Dy-rich shells in extensive layers of Nd₂Fe₁₄B grains. Nevertheless, the coercivity enhancement of the diffused magnets by Pr₆₈Cu₃₂ alloy was mainly resulted from relatively continuous and thin intergranular layers isolating Nd₂Fe₁₄B grains, which were achieved only by post-diffusion annealing.