Phosphorus addition-tuned melt-spun structure enables refined nanostructure and enhanced magnetic softness in Fe-Si-B-Cu nanocrystalline alloys

Stringent annealing requirements, such as high heating rates, hinder the industrial production of Fe-based Fe-(B, P, Si)-Cu nanocrystalline soft magnetic alloys with high saturation magnetic flux density (B_s). In this study, substitution of B with an appropriate amount of P in Fe_81.5Si₄B_13-xCu_1.5P_x (x = 0–8) alloys moderately reduces the amorphous-forming ability and promotes the formation of local crystal-like ordering (CLO) structures in the melt-spun state, arising from reduced atomic packing density, weakened interatomic interactions, and enhanced atomic diffusivity during quenching. Furthermore, the mutual affinity and aggregation of P atoms with Cu atoms facilitates the formation of abundant Cu(P) clusters in the amorphous matrix. These P-regulated CLO structures in the amorphous precursor enable high number-density α-Fe precipitation during conventional annealing, accompanied by sufficient Cu(P) clusters serving as heterogeneous nucleation sites for α-Fe, which induces a strong competitive growth effect among the α-Fe grains during annealing, contributing to significantly refined nanostructure, suppressed magneto-crystalline anisotropy, and thus enhanced magnetic softness of the nanocrystalline alloys. The nanocrystalline alloy with x = 6 exhibits an average α-Fe grain size, coercivity, effective permeability at 1 kHz, and B_s of 19.2 nm, 6.2 A/m, 11000, and 1.73 T, respectively, substantially improved compared to those of 47.6 nm, 213.6 A/m, 550, and 1.78 T for the x = 0 alloy. These findings provide key insights into the role of melt-spun precursor structure in regulating the nanocrystalline structure and magnetic properties, and offer a promising route for developing high-B_s nanocrystalline alloys suitable for industrial applications. © 2026 Elsevier B.V.