Microstructure and magnetic properties of Fe_81.3Si₄B₁₃Cu_1.7 nanocrystalline alloys with minor Nb addition

The amorphous matrix containing dispersive high number density (N_d) α-Fe nuclei with average grain sizes (D) of 4.3–6.2 nm was formed in the melt-spun Fe_81.3–xSi₄B₁₃Cu_1.7Nb_x (x = 0–2) alloys, and the N_d and D values reduce with increase in the Nb content. The fine nanocrystalline structure with α-Fe grains of 14.0–21.6 nm in size was obtained for the annealed alloys, which showed high saturation magnetic flux density of 1.60–1.77 T and low coercivity (H_c) of 7.1–17.0 A/m. Addition of minor Nb significantly expands the optimum annealing temperature range for obtaining good soft magnetic properties, while coarsens the α-Fe grains, leading to a slight increase in the H_c. The mechanism of the effect of Cu and Nb elements on the structure and magnetic properties was discussed in terms of the formation and growth of the α-Fe nuclei of the alloys.