Effects of Ribbon Thickness on Structure and Soft Magnetic Properties of a High-Cu-Content FeBCuNb Nanocrystalline Alloy

The effects of ribbon thickness (t) on the structure and magnetic properties of a Fe82.3B13Cu1.7Nb3 alloy in melt-spun and annealed states have been investigated. Increasing the t from 15 to 23 mu m changes the structure of the melt-spun ribbons from a single amorphous phase to a composite with dense alpha-Fe nanograins embedded in the amorphous matrix. The grain size (D alpha-Fe) of the alpha-Fe near the free surface of the ribbon is about 6.7 nm, and it gradually decreases along the cross section toward the wheel-contacted surface. Further increasing the t to 32 mu m coarsens the D alpha-Fe near the free surface to 15.2 nm and aggravates the D alpha-Fe ramp along the cross section. After annealing, the ribbon with t = 15 mu m has relatively large alpha-Fe grains with D alpha-Fe > 30 nm, while the thicker ribbons possessing the pre-existing nanograins form a finer nanostructure with D alpha-Fe < 16 nm. The structural uniformity of the ribbon with t = 23 mu m is better than that of the ribbon with t = 32 mu m. The annealed ribbons with t = 23 and 32 mu m possess superior soft magnetic properties to the ribbon with t = 15 mu m. The ribbon with t = 23 mu m exhibits a high saturation magnetic flux density of 1.68 T, low coercivity of 9.6 A/m, and high effective permeability at 1 kHz of 15,000. The ribbon with t = 32 mu m has a slightly larger coercivity due to the lower structural uniformity. The formation mechanism of the fine nanostructure for the ribbons with suitable t has been discussed in terms of the competitive growth effect among the pre-existing alpha-Fe nanograins