Magnetic properties optimization of nanocomposite Nd₉Fe₈₅B₆ magnets by controlling microstructure of as-quenched ribbons

To optimize the magnetic properties of nanocomposite Nd₉Fe₈₅B₆ magnets, the as-quenched ribbons with different microstructures were prepared at six wheel velocities from 10 to 30 m·s⁻¹ through rapid quenching, followed by a series of annealing treatments at 550–800 °C for 5–10 min. It is found that both the large initial grains at low cooling rate and high content of amorphous phase at high cooling rate cause α-Fe grains coarsening, which leads to a decline in the strength of exchange coupling interaction and the deterioration of magnetic properties. In order to optimize the magnetic properties, the as-quenched ribbons should be chosen with relatively small initial grains as well as a small amount of amorphous phase. For nanocomposite Nd₉Fe₈₅B₆ materials, the optimized magnetic properties of H _cj = 446 kA·m⁻¹, B _r = 0.86 T, (BH) _max = 80 kJ·m⁻³ are obtained for ribbons prepared at 18 m·s⁻¹ after annealing at 620 °C for 5 min.