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

•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. 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.