The soft magnetic properties of Fe-based nanocrystalline alloys are determined by their grain size. In the present article, the (Fe0.4Co0.6)79Nb₃B18 nanocrystalline alloys have been successfully prepared by isothermal annealing. The variation of soft magnetic properties as a function of annealing temperature and incubation time is investigated in detail. Two distinct crystallization behaviors were found for the (Fe0.4Co0.6)79Nb₃B18 alloys. The initial nanocrystallization products comprise a mixture of α-Fe(Co), Fe₂B, and Fe23B₆-type crystalline metastable phases, and the final crystallization products are composed of α-Fe(Co), Fe₂B, and Fe₃B crystalline phases. The grain size decreases first and then increases with the increasing annealing temperature in the range of 764⁻1151 K, and a fine grain size with mean grain size of 12.7 nm can be achieved for alloys annealed at 880 K. As the annealing temperature increases from 764 K to 1151 K, the saturation magnetization increases first and then decreases without a significant increase of the coercivity. The alloys annealed at 880 K exhibit the optimized soft magnetic properties with high Ms of 145 emu g-1 and low Hc of 0.04 Oe.
Keywords: Fe-based nanocrystalline alloys; grain size; isothermal annealing; soft magnetic properties.