Abstract
Fully dense spark plasma sintered recycled and fresh HDDR Nd-Fe-B nanocrystalline bulk magnets were processed by surface grain boundary diffusion (GBD) treatment to further augment the coercivity and investigate the underlying diffusion mechanism. The fully dense SPS processed HDDR based magnets were placed in a crucible with varying the eutectic alloys Pr68Cu32 and Dy70Cu30 at 2–20 wt. % as direct diffusion source above the ternary transition temperature for GBD processing followed by secondary annealing. The changes in mass gain was analyzed and weighted against the magnetic properties. For the recycled magnet, the coercivity (HCi) values obtained after optimal GBDP yielded ~60% higher than the starting recycled HDDR powder and 17.5% higher than the SPS-ed processed magnets. The fresh MF-15P HDDR Nd-Fe-B based magnets gained 25–36% higher coercivities with Pr-Cu GBDP. The FEG-SEM investigation provided insight on the diffusion depth and EDXS analysis indicated the changes in matrix and intergranular phase composition within the diffusion zone. The mechanism of surface to grain boundary diffusion and the limitations to thorough grain boundary diffusion in the HDDR Nd-Fe-B based bulk magnets were detailed in this study.
Highlights
The Nd-Fe-B based rare-earth permanent magnets (REPMs) possess great significance for microelectronics, data storage, electric motors and medical devices [1]
The results indicate that, the grain boundary diffusion processing (GBDP) of the eutectic alloy ribbons is limited in mass gain and diffusion depth to near surface regions (~600 μm for Pr-Cu alloy) in the HDDR system, the Dy-Cu alloys are comparatively less effective in improving the coercivity as compared to Pr-Cu alloys, besides a more significant impact on the magnetization reduction
The results indicate that the GBDP of the eutectic alloy ribbons is limited in mass gain and diffusion depth to near surface regions (~600 μm for Pr-Cu alloy) in the HDDR
Summary
The Nd-Fe-B based rare-earth permanent magnets (REPMs) possess great significance for microelectronics, data storage, electric motors and medical devices [1]. The grain size refinement has been theorized to improve the coercivity (HC ), i.e. resistance to demagnetization in REPMs [2]. The hydrogenation-disproportionation-desorption-recombination (HDDR) is a well-established and greener route for developing anisotropic ultrafine grains (~400 nm) with preferential easy-axis orientation [3]. The overall surge in production volume and application demand has necessitated the utilization of recycled REPMs into the industrial feedstocks [4,5]. The effectiveness of hydrogen gas towards decrepitating (HD) and disproportionation (HDDR) of the end-of-life rare-earth (RE) scrap has convincing been proven in previous studies [6,7,8,9,10,11,12].
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