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Abstract
Intergranular adding Dy-containing powders is effective to enhance the coercivity of Nd2Fe14B-based sintered magnets with slight remanence loss due to the formation of (Nd, Dy)2Fe14B hardening shell surrounding the 2:14:1 grains. Here, the influences of post-sinter annealing (PSA) time on microstructure and coercivity of Dy69Ni31-added Nd-Fe-B magnets have been investigated. With prolonging the annealing time at 580 °C, the coercivity increases rapidly from 10.1 kOe to 16.2 kOe when the annealing time is less than 6 h and keeps nearly unchanged after longer time annealing. Longer time annealing promotes the formation of thicker Dy-enriched 2:14:1 hardening shells as well as optimized and continuous grain boundary (GB) layers, which plays positive contributions to enhance coercivity. However, the transmission electron microscope (TEM) investigations reveal that crystal structure of RE-oxides at the triple junctions changes with prolonging PSA time, transforming from double hexagonal close-packed (d-hcp) to a mixture of face-centered cubic (fcc) and ordered body-centered cubic (bcc) and to hexagonal close-packed (hcp). The latter transformation may play a negative role on coercivity enhancement, offsetting the positive contributions. These findings may provide a useful guide for engineering the microstructure of Nd-Fe-B sintered magnets.
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