Limitations in grain boundary processing of the recycled HDDR Nd-Fe-B system

Fully dense spark plasma sintered recycled and fresh HDDR Nd-Fe-B nanocrystalline 16 bulk magnets are processed by surface grain boundary diffusion (GBD) treatment to further 17 augment the coercivity and investigate the underlying diffusion mechanism. The fully dense SPS 18 processed HDDR based magnets were placed in a crucible with varying the eutectic alloys Pr68Cu32 19 and Dy70Cu30 from 2 – 20 wt.% as direct diffusion source above the ternary transition temperature 20 for GBD processing followed by secondary annealing. The changes in mass gain was analysed and 21 weighted against the magnetic properties. For the recycled magnet, the coercivity (HCi) values 22 obtained after optimal GBDP yielded ~ 60% higher than the starting recycled HDDR powder and 23 17.5% higher than the SPS-ed processed magnets. The fresh MF-15P HDDR Nd-Fe-B based magnets 24 gained 25 – 36% higher coercivities with Pr-Cu GBDP. The FEG-SEM investigation provided insight 25 on the diffusion depth and EDXS analysis indicated the changes in matrix and intergranular phase 26 composition within the diffusion zone. The mechanism of surface to grain boundary diffusion and 27 the limitations to thorough grain boundary diffusion in the HDDR Nd-Fe-B based bulk magnets are 28 detailed in this study.