Preparation of flake-shaped Fe-based nanocrystalline soft magnetic alloy particles subjected to plastic deformation

ABSTRACT Fe-based nanocrystalline alloy powder prepared by ball-milling is a potential candidate as a soft magnetic composite (SMC). Since the magnetic properties of particles having a random geometry arising from brittle fracture deteriorate by the presence of a demagnetising field, plastically deformed flake-shaped powders, exhibiting better magnetic properties on account of the suppression of any demagnetising field, are desirable. Microstructure such as grain size, lattice distortion and the distribution of dislocations, are affected by ball-milling treatment which changes the magnetic properties. In this study, Fe-based nanocrystalline alloy sheets are ball-milled with lubricant oil as a process control agent (PCA) and the microstructure of the particles investigated. The PCA effectively suppresses the brittle fracture of the alloy sheet during the ball-milling treatment and plastically deformed flake-shaped particles are then successfully obtained. Transmission electron microscopy reveals that there were few lattice defects in the α-Fe grain of the alloy, which indicated that almost only grain-boundary-mediated processes such as GB diffusion/sliding/migration and grain rotation dominate the deformation mechanism. However, Williamson–Hall analysis based on synchrotron radiation exhibits a slope indicating micro-strain in the α-Fe grains. It is found that the plastic deformation induced by the ball-milling treatment forms a microstructure having lattice distortion but containing few lattice defects. It is considered that a slight growth of the existing grains, which can be induced by thermal treatment, can achieve a strain- and dislocation-free microstructure, which is desirable for soft magnetic alloys.