Hidehiro Onodera

Cu clustering and Si partitioning in the early crystallization stage of an Fe73.5Si13.5B9Nb3Cu1 amorphous alloy

Solute clustering and partitioning behavior in the early crystallization stage of an Fe{sub 73.5}Si{sub 13.5}B{sub 9}Nb{sub 3}Cu{sub 1} amorphous alloy have been studied by employing a three-dimensional atom probe (3DAP) and a high resolution electron microscope (HREM). Results from the 3DAP have clearly shown that Cu atom clusters are present in the amorphous state after annealing below the crystallization temperature. The density of these clusters is in the order of 10{sup 24}/m{sup 3}, which is comparable to that of the {alpha}-Fe grains in the optimum nanocrystalline microstructure. In the early stage of primary crystallization, Cu clusters are in direct contact with the {alpha}-Fe nanocrystals, suggesting that the {alpha}-Fe primary particles are heterogeneously nucleated at the site of Cu clusters. In the early stage of crystallization, the concentration of Si is lower in the primary crystal than in the amorphous matrix phase, unlike in the late stage of the primary crystallization, where Si partitions into the {alpha}-Fe phase with a composition of approximately 20 at.%.

Microstructure of Co–Al–O granular thin films

We have investigated the microstructures of Co–Al–O granular thin films, which were prepared by the sputter-deposition technique with various oxygen partial pressures. The constituent phases, grain sizes of granular particles, and width of insulating channels have been evaluated quantitatively. The specimen with the optimum giant magnetoresistance (GMR) is composed of nanoscale Co particles, and these are completely isolated by amorphous aluminum oxide channels. The GMR behavior observed in the Co–Al–O films has a close correlation with the width of the insulating channel and the grain size of the magnetic particles, which is consistent with the spin-dependent tunneling conduction mechanism of GMR

Small-angle neutron scattering and differential scanning calorimetry studies on the copper clustering stage of Fe–Si–B–Nb–Cu nanocrystalline alloys

Abstract The kinetics of copper clustering and primary crystallization of FINEMET type alloys with the compositions Fe 74.5− x Si 13.5 B 9 Nb 3 Cu x and Fe 77 Si 11 B 9 Nb 3− x Cu x have been studied by small-angle neutron scattering (SANS) and high-sensitivity differential scanning calorimetry (DSC) in order to explain the different optimized Cu contents, x , for obtaining the highest permeability in these two alloys. SANS results have shown that the alloys with the optimized Cu contents have the finest nanocrystalline microstructures. Kinetic analyses of Cu clustering prior to primary crystallization have shown that the number density of Cu clusters becomes highest at the crystallization stage of α-Fe primary crystals in the alloy containing an optimized amount of Cu.

Microstructures and magnetic properties of Co–Al–O granular thin films

The microstructures of Co–Al–O thin films of wide varieties of compositions are studied by transmission electron microscopy and small angle x-ray scattering (SAXS). In the superparamagnetic specimens, high resolution electron microscope images reveal that isolated spherical Co particles are surrounded by an amorphous aluminum oxide matrix. However, in the soft ferromagnetic films, the shape of the Co particles is prolate ellipsoidal. SAXS intensities from the soft magnetic specimens decrease inversely with the wave vector, q, in a low wave-vector region, while an interparticle interference peak is observed for the superparamagnetic specimens. The scattering profiles of the soft magnetic films imply that the Co particles have a cylindrical shape and are randomly oriented. The correlation between the magnetic properties and the microstructures is discussed.

Optimization of the microstructure and properties of Co-substituted Fe–Si–B–Nb–Cu nanocrystalline soft magnetic alloys

The effect of Co replacement for Fe on the microstructure and soft magnetic properties of Fe78.8−xCoxNb2.6Si9B9Cu0.6 (x=5–60) nanocrystalline alloys has been studied for improving the soft magnetic properties of Fe–Si–B–Nb–Cu type alloys at a high frequency range. The magnetic anisotropy constant increases with x, but the coercivity increases when x exceeds 20, indicating that magnetic softness is degraded by replacing Fe with Co. Three-dimensional atom-probe observations have revealed that the number density of Cu-enriched clusters decreases with x, thereby decreasing the number density of the heterogeneous nucleation sites for bcc-Fe primary crystals. In addition, differential scanning calorimetry measurements show that the Cu clustering temperature shifts to a higher temperature with increasing x, suggesting that the kinetics for the Cu clustering decreases as Co content. These experimental results are discussed from the thermodynamical point of view, and the optimized Cu composition to achieve a low c...

Thermodynamic assessment of the Cu−Pt system

A CALPHAD assessment of the Cu−Pt system has been carried out. Two and four sublattice models were applied to describe the Gibbs free energies of ordered phases where the contribution of SRO is taken explicity into account through the reciprocal parameters. The disordered fcc A1 and liquid phases were treated as substitutional solutions. A consistent set of parameters for the phases in the Cu−Pt system as obtained, and those parameters can satisfactorily reproduce the experimental phase equilibria and thermodynamic properties, such as enthalpies, activity of Cu, and long-range order parameters.

Molecular dynamics study on formation and crystallization of Ti-Al amorphous alloys

Abstract Formation of the amorphous phase and its relaxation and crystallization processes in the Ti–Al binary system are investigated by using molecular dynamics techniques. For various Al concentrations ranging from 0.0 to 1.0, amorphous alloys are prepared by quenching from liquid state. By varying the cooling rates, we can obtain the critical cooling rate to get the amorphous state in each composition. The geometrical analysis shows that a structural inhomogeneity on nanometer scale exists in the amorphous state, which mainly consists of two regions. One is dominated by the icosahedral clusters and the other is filled with approximate crystalline packing. The excess of low frequency vibration modes called the boson peak is observed in the amorphous states, which is getting weaker during the annealing process and completely vanishes after crystallization.

Modeling of microstructure changes in Fe-Cr-Co magnetic alloy using the phase-field method

During the last decade, the phase-field method has emerged in many fields if materials science as a powerful tool to simulate and predict complex microstructure development. In this study, the microstructure changes during thermomagnetic treatment and step aging of the Fe−Cr−Co magnetic alloy are modeled using phase-field methods. Using phase-field simulation, the model reasonably represented microstructural changes in the Fe−Cr−Co system quantitatively. In particular, it is shown that gradual step aging is important to obtain the lamellar microstructure that provides excellent permanent magnetic properties. Modeling of microstructure development with the framework of the phase-field method is shown to be a very effective strategy to predict and analyze complex microstructure formation.

Icosahedral Order in Supercooled Liquids and Glassy Alloys

Formation of the icosahedral order in supercooled liquids and glassy phases is investigated for a model alloy system by using molecular dynamics simulations. The simulation results show that the short-range icosahedral order grows in the supercooled liquids as well as in the glassy phases. Structural analyses reveal that the icosahedral clusters form a network in which the clusters are connected via the pentagonal-bicap sharing. Geometrical property of the network formed by the icosahedral clusters is an origin of medium-range order in the glassy phases

Phase-Field simulation of phase decomposition in Fe−Cr−Co alloy under an external magnetic field

Phase decomposition during isothermal aging of a Fe−Cr−Co ternary alloy under an external magnetic field is simulated based on the phase-field method. In this simulation, since the Gibbs energy available from the thermodynamic CALPHAD database of the equilibrium phase diagram is employed as a chemical free energy, the present calculation provides the quantitative microstructure changes directly linked to the phase diagram. The simulated microstructure evolution demonstrates that the lamella like microstructure elongated along the external magnetic field is evolved with the progress of aging. The morphological and temporal developments of the simulated microstructures are in good agreement with experimental results that have been obtained for this alloy system.