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P8.26.1

Synthesis and Characterization of Fe-Co Nanocrystalline Alloys Mohammad Mujahid and Jie Zhu School of Materials Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798. ABSTRACT Nanostructured alloys have great potential as soft magnetic materials. In particular, nanocrystalline Fe-Co based alloys are believed to be good candidates for imparting improved magnetic behavior in terms of higher permeability, lower coercivity, reduced hysteresis losses and higher Curie temperatures. In the present work, Fe-50at.%Co alloy powders have been prepared using mechanical alloying (MA) in a planetary ball mill under controlled environment. The particle size and morphology of MA powders was investigated using scanning electron microscopy. The crystal size and internal strain was measured using X-ray diffraction. It has been shown that the crystal size could be reduced down to less than 15 nm in these alloys. Finally, the influence of grain size and internal strain on the magnetic properties has been discussed. INTRODUCTION Soft magnetic materials are usually characterized by low coercivity (< 1kAm-1) and high permeability. Therefore, a little change in the applied magnetic field can cause large change in the magnetization [1]. Since magnetization involves domain wall movement and domain rotation, grain boundaries are considered as impediments to these processes especially in conventional magnetic materials. It has been found that the magnetic hardness is approximately inversely proportional to the grain size for sizes exceeding 0.1-1µm [2]. However, recent developments in nanostructured soft magnetic materials, coercivity was found to follow a proportional relationship with the grain size for average grain diameters below the ferromagnetic exchange length (L). The underlying reason is related to the reduced effective average magnetocrystalline anisotropy due to many grains being exchange coupled within a volume of L3. Based on the random anisotropy model, Herzer predicted the well known D6 law (D, grain size) and extended it to explain lower exponent for the D-power dependence [3,4]. The term “Nanomaterials” refers to those nanostructured bulk materials, nanophase dispersive particles, or thin films, whose characteristic length scale of microstructure ranges from 1-100 nm [5]. Mechanical alloying (MA) has been extensively applied to produce nanostructured bulk materials with a broad range of chemical compositions and crystal structures [6]. In the MA process, continuous collisions between milling media and powders lead to the repeated plastic deformation, fragmentation and re-welding of powder particles [7,8]. The dynamics of milling process has been summarized in a recent report [9]. According to Fecht [10], plastic deformation during milling leads to successive accumulation of dislocations in the shear bands such that the crystals are subsequently disintegrated into subgrains delineated by low-angle grain boundaries.

P8.26.2

The size of subgrains can then be continuously reduced with further