Distinct roles of metalloid elements (Si, P, C) in tailoring as-quenched nanostructure and magnetic properties of Fe–B–Cu nanocrystalline alloys

Novel high-Cu-content nanocrystalline alloys exhibit high saturation magnetic flux density (B_s) and favorable processing characteristics (e.g., tolerance to prolonged annealing), which are intrinsically linked to their as-quenched structure. In this work, the effects of metalloid elements (Si, P, and C) on the as-quenched nanostructure, crystallization behavior, and soft magnetic properties of Fe₈₂B_16.5-xCu_1.5M_x (M = Si, P, C; x = 2, 4, 6) nanocrystalline alloys are systematically investigated, leading to the establishment of distinct composition-structure-property relationships and a new compositional design strategy. It is found that Si and P refine the α-Fe nanocrystals and reduce their number density in the as-quenched state, while C promotes the precipitation of the γ-Fe phase alongside α-Fe, resulting in grain coarsening. A higher nanocrystal number density in the as-quenched state effectively suppresses abnormal grain growth during subsequent annealing, leading to a finer final grain size. However, the γ-Fe phase significantly deteriorates magnetic softness by disrupting intergranular exchange coupling, and consequently leads to a high coercivity (H_c). After optimal annealing, the alloy with 2 at% Si exhibits the best combination of soft magnetic properties, achieving a high B_s of 1.81 T and a low H_c of 4.9 A/m. Guided by these principles, a multi-metalloid synergistic design approach is proposed and explored. The understanding of these mechanisms indicates that the targeted regulation of alloy properties can be achieved through tailored multi‑metalloid design, offering a promising pathway for developing next‑generation soft magnetic materials with customized performance profiles.