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

Abstract 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 (Hci) of A0 magnet is significantly enhanced from the original 11.4 kOe to 19.5 kOe with remanence (Br) 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 Hci and Br 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 Br due to the increased fraction of Nd-rich phase and too much substitution of Al for Fe in Nd2Fe14B phase. Therefore, the Hci (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.