Minoru Ichimura

Monte Carlo simulation of grain boundary cross effect on hydrogen diffusivity in aluminum

Assuming the jumping of interstitial impurities to neighboring sites, the diffusivity is simulated by the Monte Carlo method based on the random walk theory. The jumping probability along a grain boundary is greater than that in the lattice of an inside grain. The crossing point of grain boundaries has a smaller jummping probability to neighboring sites on the grain boundary than that in the lattice. The results of simulation support the grain boundary cross effect, proposed experimentally by us, on the grain size dependence of hydrogen diffusivity in an aluminum polycrystal. The diffusivity shows a peak at some critical grain size. The peak includes two conflicting effects of grain boundaries: enhancement of diffusion along grain boundaries and suppression of diffusion by trapping at grain boundary junctions.

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.

Computer Simulation of the Film Growth Process on the Two-Dimensional Penrose Pattern

Abstract We performed computer simulation of the film growth process on the two-dimensional (2D) Penrose pattern, which is considered a typical structural model of quasicrystal. The atomistic structure of the deposited atoms was obtained as a function of time under various conditions of atomic binding energy, temperature and deposition rate. The Monte Carlo method based upon the solid-on-solid model was utilized for the present calculation. We found a geometrical restriction on the growth front of grains in the 2D Penrose pattern, which is also expected for the growth of an actual quasicrystal.

Computer Simulation of Film Growth Process on the Two-Dimensional Penrose Pattern

We performed computer simulation of the film growth process on the two-dimensional (2D) Penrose pattern, which is considered a typical structural model of quasicrystal. The atomistic structure of the deposited atoms was calculated as a function of time under various conditions of atomic binding energy, temperature and deposition rate. The Monte Carlo method based upon the solid-on-solid model was utilized for the present calculation. We found a geometrical restriction on the growth front of grains in the 2D Penrose pattern, which is also expected for the growth of an actual quasicrystal.

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.

Monte Carlo Simulation of Growth Process of Two-Dimensional Quasicrystal.

Simulations of the growth process of quasicrystal and square crystal lattice have been performed by the canonical Monte Carlo method. By varying the deposition rate and dimensionless temperature, the conditions for obtaining defect-free single crystal were investigated. The two-dimensional Penrose pattern was adopted as a model of the quasicrystal lattice where the positions of particles are restricted to the lattice sites and the interaction range between particles was assumed to be only within the nearest-neighbor distance. It was found that the diffusivity of quasicrystal was higher than that of square crystal lattice if attractive interaction was assumed. This tendency is opposite to that in the case of free particles. The conditions for obtaining a defect-free single crystal were that the deposition rate was low, under 6.57×10-6 atom/(site MCS) and the dimensionless temperature was as low as possible but higher than that for polynuclear growth.

Molecular Dynamics Simulation of Nucleation and Growth of a Binary Quasicrystal

Abstract The quasicrystal structure is considered to be a new type of ordered phase because its Fourier transform has Laue spots with icosahedral symmetry, which is inconsistent with crystal structure. Computer simulation of the formation process of a quasicrystal was performed by the molecular dynamics method. On the basis of the Strandburg type of quasicrystal model, we developed an algorithm of the formation process of binary quasicrystal reflecting the procedure as realistically as possible. The Fourier transform of some of the obtained structures has shown decagonal symmetry although the spots are rather diffused. It has been shown that the potential parameter and experimental condition should be limited to produce a perfect quasicrystal 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.