Optimizing nanostructure and magnetic softness of Fe-Si-B-Cu nanocrystalline alloys via tailoring melt-spun structure by Al microalloying

An Al microalloying strategy has been proposed to optimize the nanostructure and magnetic softness of Fe-based Fe-Si-B-Cu nanocrystalline alloys via tailoring their amorphous precursor structure. By alloying a minor amount of Al in a Fe_81.5Si₄B₁₃Cu_1.5 alloy, the melt-spun structure transforms from a completely amorphous phase to an amorphous matrix dispersed with appropriately high number-density (N_d) α-Fe nanograins with the average size (d_α-Fe) of less than 9 nm, while the alloy ribbons maintain ductile. The N_d and d_α-Fe in a Fe_81.5-xSi₄B₁₃Cu_1.5Al_x (x = 0.6) alloy ribbon are 2.9 × 10²¹ m⁻³ and 7.1 nm, respectively, and further raising x to 1.0 increases the d_α-Fe and N_d slightly. During melt-spinning, the attraction and aggregation of Al atoms with Cu atoms induce Cu clustering, which in turn promotes the precipitation of the high-N_d α-Fe nanograins by taking Cu-clusters as heterogeneous nucleation sites. The unique melt-spun structure of the Al-added alloys enables the achievement of a fine nanostructure and excellent magnetic softness under conventional annealing conditions. The average grain size of α-Fe (D_α-Fe), coercivity, and saturation magnetic flux density (B_s) of the nanocrystalline alloy with x = 0.6 are 21.0 nm, 13.2 A/m, and 1.77 T, respectively, demonstrating a significant improvement compared to the x = 0 alloy, whose values are 49.1 nm, 213.6 A/m and 1.78 T, respectively. The refined nanostructure is attributed to the competitive growth effect between the high-N_d pre-exiting and newly-formed α-Fe nanograins/nuclei promoted by the Al-induced Cu-clusters. The fine nanostructure facilitates the formation of regular magnetic domain and weakens pinning effects, which leads to improved magnetic softness. This work provides inspiration for the development of industrializable high-B_s nanocrystalline alloys. © 2025 Elsevier Ltd