Microstructure and magnetic properties of Nd-Fe-B sintered magnet by diffusing Pr-Cu-Al and Pr-Tb-Cu-Al alloys

•Coercivities of PCA, PTCA-5, PTCA-20 magnets are 19.6 kOe, 24.1 kOe and 24.3 kOe.•Coercivity enhancement of PCA magnet is owing to magnetic isolation by Nd-rich GBP.•Coercivity enhancement of PTCA-20 magnet results from magnetic hardening by TRS.•Magnetic isolation and hardening result in coercivity enhancement of PTCA-5 magnet. Grain boundary diffusion process (GBDP) with low melting-point alloy ribbons, namely Pr70Cu15Al15 (PCA), Pr65Tb5Cu15Al15 (PTCA-5) and Pr50Tb20Cu15Al15 (PTCA-20, at%), was applied to Nd-Fe-B sintered magnet (52 M) as thick as 6 mm. The coercivities of diffused magnets by PCA, PTCA-5 and PTCA-20 increase significantly to 19.6 kOe, 24.1 kOe and 24.3 kOe, respectively, from original 14.7 kOe. Microstructural investigations indicate that the coercivity enhancement of PCA magnet is mainly attributed to magnetic isolation by continuous Nd-rich grain boundary phases (GBP). However, the coercivity enhancement of PTCA-20 magnet is mainly owing to magnetic hardening by Tb-rich shells (TRS), and that of PTCA-5 magnet results from a combined strengthening of magnetic isolation by GBP and magnetic hardening by TRS. Meanwhile, the coercivity of PTCA-5 magnet is as high as that of PTCA-20 magnet, but the former consumes Tb only 0.60 wt%, lower than 1.23 wt% for the latter.