Hydrogen-based functional recycling of Nd-Fe-B sintered magnets: influence on GBDP microstructure evolution and possibilities to improve the resulting properties

◼ Hydrogen decrepitation (HD) of over 100 commercial GBDP magnets (0.5 kg) ◼ Composition: Nd 23.0 Pr 6.15 Tb 1.76 Dy 0.24 Fe bal B 0.93 Co 0.89 Ga 0.21 Al 0.13 Cu 0.08 (wt.%) ◼ Jet-milling to particle size D 50 of 5.4 μm, aligning, pressing, and sintering at 1100 °C for 4 h followed by annealing at 700 °C for 1 h and 500 °C for 1 h ◼ Investigations on the changes of the GBDP microstructure (core-shell) and magnetic properties ◼ GBDP with 1.5 wt.% pure Tb-foil at 900 °C for 9 h with subsequent lowtemperature annealing to restore the magnetic properties SEM-BSE images, EDS mappings, and EDS line scans through the functional recycling process. While at the GBD scrap magnet a narrow Tb-core-shell structure with 0.5 μm can be observed, the other samples show less clear coreshell formation but broader Tb enrichments at the outer regions of the grains. APT investigation of GBD scrap magnet (left) and recycled magnet (right) reveals beside a more homogenous distribution of Tb and disappearance of the coreshell microstructure also changes in the grain boundary elemental distribution through functional recycling.