Optimizing structure and magnetic softness of low-Nb-content Fe-Si-B-Nb-Cu nanocrystalline alloys by regulating Si/B ratio and Cu content

The influences of Si/B ratio and Cu content on the melt-spun and crystallized structures and magnetic properties of low-Nb-content Fe-Si-B-Nb-Cu alloys have been studied. The results show that increasing the Si/B ratio in Fe78SixB19.5-xNb1.5Cu1 (Cu1Six) alloys deteriorates amorphous-forming ability, and gradually transforms the melt-spun structure from an amorphous phase (x = 9.5–10.5) to amorphous phase plus α-Fe texture (x = 11.5–13.5). An appropriate rise in Cu content eliminates the α-Fe texture in melt-spun structure and Fe77.5SixB19.5-xNb1.5Cu1.5 (Cu1.5Six) alloys all appear amorphous within x = 9.5–13.5. After annealing, the average α-Fe grain size (Dα-Fe) and coercivity (Hc) of the Cu1Six nanocrystalline alloys initially decrease and then increase with rising the Si/B ratio, reaching their minimums at x = 11.5. For the Cu1.5Six nanocrystalline alloys, both the Dα-Fe and Hc consistently decrease with increasing the Si/B ratio, and remain significantly lower than those of the Cu1Six alloys. The increased Si/B ratio also steadily reduces the saturation magnetic flux density (Bs) of both alloy series. The Cu1.5Six nanocrystalline alloys with x = 12.5–13.5 exhibit the optimized nanostructure and soft magnetic properties featuring the Dα-Fe, Hc, Bs, and effective permeability at 100 kHz of 24.8–25.2 nm, 10.5–11.0 A/m, 1.50–1.52 T, and 7300–7500, respectively. The increase in the Si/B ratio facilitates α-Fe nucleation whereas hinders grain growth, and a higher content of Cu promotes high-number-density α-Fe nucleation and strengthens the competitive growth among the α-Fe grains, further inhibiting their overgrowth. The synergistic effect leads to a refined nanostructure and improved magnetic softness.