Toshiyuki Koyama

Size dependence of ordering in FePt nanoparticles

We have investigated the size effect of A1→L10 ordering of FePt nanoparticles in FePt–Al2O3 granular and FePt/SiO2 particulate films by transmission electron microscopy (TEM). The TEM results have shown convincingly that ordering does not progress when the particle size has a diameter of less than 4 nm. Calculation of the order parameter profile from the surface to the volume of the FePt nanoparticles based on diffuse-interface theory justified the experimentally observed size dependence of the ordering. The transition length from disorder to order depends on the interfacial energy, hence the critical particle size of ordering should vary depending on the type of matrix and substrate.

Crystallization of amorphous germanium in an Al/a-Ge bilayer film deposited on a SiO2 substrate

The crystallization of amorphous Ge(a-Ge) in an Al (134 nm) and a-Ge (108 nm) thin-film bilayer deposited on a SiO2 substrate has been examined by a cross section transmission electron microscope technique. When crystallization of a-Ge begins at 125 °C, amorphous AlGe (a-AlGe) alloy is formed in the Ge layer. Then, the a-AlGe alloy layer also appeared at the surface of the bilayer. After complete crystallization, those amorphous layers disappeared and the bilayer film has been converted to a polycrystalline film. We discussed the crystallization of a-Ge and proposed the mechanism of the diffusion of Ge atoms from the inner a-Ge layer through the outer Al layer to the topmost surface that involves the formation of the metastable a-AlGe alloy in the Ge layer, followed by the crystallization of this alloy by the pseudo-eutectic reaction, leading to the decomposition into an equilibrium Al and Ge crystal mixture and a-Ge. Then, Ge atoms is released to the Al layer for the compensation of the Al diffusion down...

Phase-field modeling of microstructure evolutions in magnetic materials

Abstract Recently, the phase-field method has been extended and utilized across many fields of materials science. Since this method can incorporate, systematically, the effect of the coherency induced by lattice mismatch and the applied stress as well as the external electrical and magnetic fields, it has been applied to many material processes including solidification, solid-state phase transformations and various types of complex microstructure changes. In this paper, we focus on the recent phase-field simulations of real magnetic materials, and the simulation method for magnetic materials is explained comprehensively. Several applications of the phase-field method to clarifying the microstructure changes in magnetic materials, such as Ni2MnGa ferromagnetic shape memory alloy, FePt nanogranular thin film, Co–Sm–Cu rare-earth magnet, Fe–Cr–Co spinodal magnet, and Fe–C steel with external magnetic field, are demonstrated. Furthermore, the general concept of the effective strategy for controlling microstructure in magnetic materials is proposed.

A new characterization method of the microstructure using the macroscopic composition gradient in alloys

A new experimental method to determine the phase boundary and phase equilibria is accomplished by using the transmission electron microscopic observation of alloys having the macroscopic composition gradient. The various phase boundaries,i.e., the coherent binodal and spinodal lines and incoherent binodal line, are distinctly determined for the Cu-Ti alloy system. Furthermore, the equilibrium compositions at the interface of precipitate/matrix are experimentally obtained for various particle sizes, and therefore, the Gibbs-Thomson relation is verified. It is expected that the composition gradient method proposed in the present study will become an important experimental method for microstructural characterization.

Computer simulations of phase decomposition in real alloy systems based on the modified Khachaturyan diffusion equation

Recently, Khachaturyan’s group proposed a new calculation method for phase decomposition on the basis of the Onsager equation. In the present study, we modified the Khachaturyan diffusion equation to allow simulation of the phase decomposition in actual alloy systems. Two-dimensional (2-D) computer calculations are performed for the phase decompositions of Al-Zn, Cu-Co, and Fe-Mo binary systems by using the thermodynamic data related to the equilibrium phase diagrams. The calculated microstructures are very similar to the actual micrographs experimentally obtained for these alloys.

Computer simulations of the phase transformation in real alloy systems based on the phase field method

Abstract The kinetic simulation based on the non-linear diffusion equation becomes a very powerful method in fundamental understanding of the dynamics of phase transformation with recent remarkable development of the computer. In the present study, we calculate the dynamics of microstructure changes in real alloy systems, i.e. Fe–Mo, Al–Zn and Fe–Al–Co based on the phase field method. The composition dependencies of atomic interchange energy are taken into account so as to be applicable for the phase diagram of the real alloy systems. The elasticity and mobility of atoms are assumed to depend on the local order parameters such as the composition, the degree of order, etc. Time dependent morphological changes of the microstructure such as formation of modulated structure by spinodal decomposition, strain induced morphological changes of precipitates, the order–disorder phase transition with phase decomposition will be demonstrated. The results simulated are quantitatively in good agreement with the experimental results in the real alloy systems.

Formation mechanism of the hierarchic structure in the lath martensite phase in steels

Martensitic transformation is the phase transformation accompanying orderly shear deformation without atomic diffusion. The structures made by martensitic transformation are classified as thin plate, lens or lath in steels. The mechanism by which the hierarchic microstructure in the lath martensite phase forms has heretofore not been understood. We have made clear the mechanism by considering, independently, two plastic deformations using the slip deformation model proposed by Khachaturyan, and present herein a deformation matrix for each of the six crystallographic variants in a packet of the hierarchic structure. Our results are quantitatively consistent with experimental results for the Kurdjumov–Sachs (K-S) crystal orientation relationship and habit plane. Furthermore, the important points of our study are as follows: the origin of the sub-block structure and the specific combination of the sub-block structure are clarified; the laths existing in a block can be explained; and deviations between the directional parallel and plane parallel are obtained quantitatively, without any adjustable parameters.

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.

Effect of coherent strain energy on γ/γ′ phase equilibria in NiAlTi alloys

The characteristic influences of elastic strain energy on the phase equilibria of the [gamma] + [gamma][prime] two phases in Ni-Al-Ti alloys have experimentally been investigated by means of analytical transmission electron microscopy. The results obtained are as follows; the tie-line ends of phase decomposition do not always coincide with the equilibrium phase boundaries and move inside the miscibility gap when the precipitates are coherent with the matrix, but move both the right hand for the incoherent precipitates and are finally fixed for the large incoherent precipitates on the equilibrium compositions of the phase diagram. These experimental results are theoretically rationalized on the basis of the free energy of the microstructure proposed by the authors.

Morphological changes of the Ti3Al5 phase formed by phase-decomposition of TiAl intermetallics

Abstract When TiAl intermetallics containing Al of 56, 58 and 60 at.% are aged at 973 K, the TiAl phase decomposes into the two-phase state of Ti 3 Al 5 and TiAl. Transmission electron microscopy observations showed the following microstructure changes because of ageing: (1) in Ti-56Al, first Ti 3 Al 5 precipitate particles are thin plates having (001) surfaces and then they coalesce with each other to form large plates whose surfaces are not flat but undulating and consist of {113} planes; and (2) in Ti–58Al and Ti–60Al the tweed structure first appears, then Ti 3 Al 5 particles grow and, finally, the coalescence takes place to form large rafts like folded strata. The computer simulations based on the Cahn–Hilliard non-linear diffusion equation can successfully reproduce such microstructure changes.