Coercivity and remanence enhancement in hot-deformed Nd-Fe-B magnets by high-temperature short-term annealing process

It is challenging to simultaneously improve the coercivity, remanence and squareness factors of sintered and hot-deformed magnets, because of the complicated influences by various microstructure factors. In this work, we demonstrate that the coercivity, remanence and squareness factor of hot-deformed Nd-Fe-B magnet can be enhanced simultaneously by high-temperature short-term annealing process. The coercivity of the hot-deformed sample increased from 1.05 T to a maximum of 1.28 T after annealing at 800 °C, and decreased to 0.87 T at 900 °C. The remanence of the hot-deformed sample increased continuously from 1.41 to 1.49 T as the annealing temperature increased to 900 °C. The calculated squareness value firstly increased from 0.91 to 0.97 at 850 °C, then decreased to 0.74 at 900 °C. An block-shaped Nd-Fe-rich phase, Nd₆(Fe,Co)₁₃Ga, was identified in hot-deformed sample. The improvement of coercivity is due to the melting and infiltration of Nd₆(Fe,Co)₁₃Ga phase from the surface of powder flakes into the grain boundaries during annealing process. The slightly anisotropic growth of Nd₂Fe₁₄B grains results in an improved grain alignment thus the enhanced remanence. Furthermore, the melting point of Nd₆Fe₁₃Ga (810 ℃) and Nd₆(Fe,Co)₁₃Ga (760 ℃) phases were confirmed by DSC, indicates that adding a small amount of Co element in Nd₆Fe₁₃Ga compound can significantly reduce its melting point. Due to the micrometer scale infiltration distance from powder flake surface into the grain boundaries, a balance between infiltration within the powder flakes and grain growth can be achieved under the present processing conditions. This opens up a new route to improve the coercivity, remanence and squareness factor simultaneously for hot-deformed Nd-Fe-B sample.