Effect of Al/Cu on the magnetic properties and microstructure of Nd-Fe-B sintered magnet by diffusing Pr-Tb-(Cu, Al) alloys

Grain boundary diffusion process (GBDP) using Pr60Tb10Cu30-xAlx (x = 0, 5, 15, 25, 30, atomic fraction) alloy ribbons, named as A0-A30 alloys, was applied to high maximum energy product (BH)(max) Nd-Fe-B sintered magnet. After diffusion, coercivity (H-ci) of AO magnet is significantly enhanced from the original 11.4 kOe to 19.5 kOe with remanence (B-r) decreasing from 14.65 kGs to14.32 kGs, which is mainly attributed to magnetic hardening effect by Tb-rich shells. With substituting 5 at% Al for Cu, the H-ci, and B-r of A5 magnet are simultaneously enhanced by the homogenized distribution of Tb-rich shells and the reduced interface energy of Nd-rich phase and Nd2Fe14B phase. The highest coercivity of 22.4 kOe is obtained for A25 magnet by further strengthening magnetic isolation of Nd2Fe14B phase, however, which results in a drastic decrease in B-r due to the increased fraction of Nd-rich phase and too much substitution of Al for Fe in Nd2Fe14B phase. Therefore, the H-ci (kOe) + (BH)(max) (MGOe) reaches to the peak value for A5 magnet. Also, it is proven that Cu/Al co-addition can effectively improve Tb diffusion efficiency by modifying intergranular Nd-rich phase.