Microstructure evolution of Dy₆₉Ni₃₁-added Nd-Fe-B sintered magnets during annealing

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.