Mamoru Imabayashi

Molecular Dynamics Study of the Structural Changes of Ultra-fine Particles

The dynamics of ultra-fine particles (UFPs) have been studied by the molecular dynamics method. The Morse-type function has been assumed as the interaction potential between the atoms in UFPs. The numbers of atoms of the calculated systems have been 13 to 459. The particles with magic numbers, 13, 55 and 147, have shown the structural change from fcc to icosahedron, while the system of 459 atoms has not.

Monte Carlo simulation of diffusion process on quasi-lattice

The characteristics of the diffusion process of quasicrystals are studied for one-dimensional and two-dimensional models by the conventional Monte Carlo simulation. For the one-dimensional model, the tracer diffusion coefficients of an isolated particle are calculated for the three types of lattice models, i.e., single crystal, binary solid solution and quasicrystal. The characteristics of diffusivity of the one-dimensional quasicrystal model are derived from comparisons among the three types of models. For the two-dimensional model, the characteristics of diffusivity of a quasicrystal are discussed in comparison with those of the honeycomb lattice. In particular, the concentration dependence of diffusivity on the two-dimensional Penrose lattice is calculated and the effects of interatomic potential are discussed.

The stability of the 2D Penrose pattern: molecular dynamics study

The stability of the 2D Penrose pattern was studied by the molecular dynamics method. The potential used here is the Lennard-Jones type, the parameters of which were determined to stabilise the FCC(111) structure. All the systems studied are unstable and decompose into the FCC(111) structure; the system of 76 atoms changed into the single crystal, namely the FCC(111), and the other systems became polycrystals. The diffraction patterns were calculated at several stages of the relaxation process. At an early stage of relaxation the pattern shows fivefold symmetry and then the halo pattern, and finally shows sixfold symmetry. The crystallisation process, in which the metastable phase appeared, was clearly observed before reaching the final equilibrium structure.

The relaxation processes of the 2D Penrose pattern: lattice dynamics and electronic structures

The vibrational density of states and electronic spectra in the relaxation processes of the 2D Penrose pattern were calculated. The fractal-like spectra due to the self-similarity of the Penrose pattern drastically changed into the continuous spectra without gaps which reflects the FCC(111) crystal structure. At the critical stages defined as the intermediate states between the quasi-crystal and the crystal, the number of localised modes of phonons and electrons increased, indicating the structural transition.

Atomic size effects on the stability of the 2D Penrose pattern

The atomic size conditions for stabilising the 2D Penrose pattern were studied. The 2D Penrose patterns decorated by two kinds of Lennard-Jones particles according to Socolar and Steinhardt were relaxed by molecular dynamics. From structural analysis by Voronoi polygons and calculations of kinematical diffraction patterns, it was found that the atomic radius ratio (ra) had to be in the range 0.5177-0.7900 to keep the 2D Penrose pattern topologically stable. Interstitial and substitutional FCC(111) solid solutions were obtained as the stable atomic arrangements of the systems with ra=0.2071 and 1.000, respectively. The other systems with ra=0.3107-0.4660 and 0.8000-0.9000 showed the twelve-fold and ten-fold symmetry in the diffraction patterns, respectively, indicating the existence of the other kind of aperiodic structure and the multiply twinned structure respectively.

Removal of inclusions in aluminum melt by hydrogen fluxing.

The effectiveness of hydrogen fluxing on removal of inclusions in an aluminum melt has been studied. Based on the scheduled conditions, hydrogen gas was blown into the melt through an alumina tube whose tip had been filled up with porous alumina. The effect of the luxing treatment was evaluated by optical microscope observation for the electropolished surface of the sample which represents the melt.Oxide particles in the melt are removed effectively by hydrogen fluxing. While significant increase in hydrogen content of the melt is accompanied by hydrogen fluxing. Directional solidification of the melt treated with hydrogen attains both the removal of oxide and the low level of hydrogen content.