Effect of milling on the soft magnetic behavior of nanocrystalline alloy cores

The variation of soft magnetic properties of nanocrystalline Fe73Si16B7Nb3Cu1 (numbers indicate at.%) alloy powder cores as a function of milling time has been investigated. Amorphous ribbons were pulverized using a hammer mill after crystallization heat treatment at 550 °C for 1 h. Ball milling was carried out for the pulverized flake powder at room temperature for up to 36 h. The powders were classified into 53–75 μm and under 45 μm particles and mixed with a binder for consolidation into a toroidal shape. The consolidated powder was annealed at 450 °C for 1 h for stress relief. Results show that the consolidation density of the flake-shaped nanocrystalline alloy powder increased by 2.5–5% in theoretical density (0.2–0.4 g/cm3) by the ball milling process. The density increase is attributed mainly to the morphological change due to the milling, which yields smoothened particle edges. Excess milling induced fracture of particles, resulting in reduced density due to the reappearance of sharp edges. In coarse flake powder the permeability is mainly controlled by the density of the consolidates whereas it is dominated by the size distribution of particles in finer powder. The intrinsic coercivity of the powder was significantly influenced by the residual stress and demagnetization effect arising from the change in morphology.