Shell formation and coercivity change in Pr-free and Pr-alloyed Nd-Fe-B sintered magnets via grain-boundary-diffusion-process using low-melting Pr-Cu-Al-Ga alloy

• Unlike the Pr-GBDP Pr-alloyed magnets, the Pr-GBDP Pr-free magnets formed unique shells divided into low-Pr and high-Pr zones. • The low-Pr zone in the Pr-GBDP Pr-free magnets reduced the consumption of GBD-Pr in the shell formation. • The high-Pr zone in the Pr-GBDP Pr-free magnets enhanced the anisotropy field of the grain boundary region. • Therefore, to obtain high-coercivity by Pr-GBDP, it is advantageous to design the Pr-content of base magnets to be lower. • This can be a guide to develop high-performance permanent magnets without heavy-rare earths. Change in Pr-distribution of Pr-free and Pr-alloyed Nd-Fe-B sintered magnets during heavy rare-earth free Pr-Cu-Al-Ga grain-boundary-diffusion-process (GBDP) were comparatively investigated to reveal influence of pre-alloyed Pr on the formation behavior of Pr-rich shell that is a critical determinant for coercivity of magnets. The thicknesses of Pr-rich shell, which is formed by chemically-induced-liquid-film-migration, were similar between the GBDP-Pr-free and −Pr-alloyed magnets. Interestingly, unlike the GBDP-Pr-alloyed magnets, two distinct zones with different Pr-concentrations [Zone Ⅰ. Low-Pr-concentration zone formed near core/shell interface, Zone Ⅱ. High-Pr-concentration zone formed near GB] were observed within the shell in GBDP-Pr-free magnets. The average Pr-concentration of shell for the GBDP-Pr-free magnets was lower than that for the GBDP-Pr-alloyed one, but those near GB between the samples became comparable due to the high-Pr zone in GBDP-Pr-free magnets. Notably, GBD-depth of Pr for the GBDP-Pr-free magnets was deeper than that for the GBDP-Pr-alloyed magnets. As a result, coercivity of the GBDP-Pr-free magnets was much higher than that of the GBDP-Pr-alloyed magnets. Our findings indicate that the Pr-concentration near GB has a greater influence on the coercivity than the average Pr-concentration of shell, and the GBD-depth of Pr is the most critical factor for achieving higher coercivity in magnets with Pr-GBDP.