Futoshi Shimizu

Crack-tip dislocation nanostructures in dynamical fracture of fcc metals: A molecular dynamics study

AbstractDynamic behavior of dislocations near a crack tip in an fcc lattice, studied through parallel molecular dynamics (MD) simulation with visualization facilitated by newly developed software, reveals three-dimensional features of dislocation nucleation and subsequent entanglement. Results obtained for copper and aluminum show multiple emissions of dislocation loops from the crack tip and incipient evolution of plastic deformation during crack extension. 〈100〉 dislocations are found to be emitted in aluminum at zero temperature, which however are unstable and subsequently disassociate into bundles of

Chain structure of liquid and amorphous selenium: tight-binding molecular-dynamics simulation

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Molecular Dynamics Analysis of Negative Poisson Ratios over the α-β Transition in Cristobalite, SiO2

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Theory of Shear Banding in Metallic Glasses and Molecular Dynamics Calculations

Prototype of the middleware for Grid project promoted by national institutes in Japan has been developed. Key technologies that are indispensable for construction of virtual organization were already implemented onto the prototype of middleware and examined in practical computer/network system from a view point of availability. In addition several kinds of scientific applications are being executed on the prototype system. It seems that successful result in the implementation of those technologies such as security infrastructure, component programming and collaborative visualization in practical computer/network systems means significant progress in Science Grid in Japan.

Yield point of metallic glass

Tight-binding molecular-dynamics (TB-MD) simulation has been performed to study the properties on the microscopic scale of liquid and amorphous selenium at 375 and 760 K. Since a diagonalization of the tight-binding (TB) Hamiltonian matrix is time consuming calculation, we have made parallel computations in the simulation program to realize a long time simulation of 512 atoms up to the 105 time steps corresponding to the order of 10 ps. The result shows that the radial distribution function, which becomes zero at around 0.275 nm, agrees with experiments and the dihedral angle distribution function has a maximum around 0°, a minimum at 30° and a smaller maximum around 100°. This tendency differs from a previous study either by the tight-binding Monte Carlo (TB-MC) simulation or by the first principle molecular-dynamics (MD) simulation.

Atomistic simulation of shear localization in Cu–Zr bulk metallic glass

Macroscopic elastic properties of materials depend on the underlying microscopic structures. We have investigated the topological structure of three-dimensional network glass, such as vitreous SiO2, and its effect on the rigidity, using a parallel molecular-dynamics (MD) approach. The topological analysis based on the graph theory is employed to characterize disordered networks in the computer generated model of vitreous SiO2. The nature of connectivity of the elementary units beyond the nearest-neighbor, which is related to the medium-range order structure of amorphous state, is described in terms of the ring distribution by the shortest-path analysis. In large-scale MD simulations, the task of detecting these rings from a large amount of data is computationally demanding. Elastic moduli of vitreous SiO2 are calculated with the fluctuation formula for internal stress. The quantitative relation between the statistics of rings for vitreous SiO2 and the elastic moduli are discussed.

Molecular dynamics study on the formation of stacking fault tetrahedra and unfaulting of Frank loops in fcc metals

Cristobalite, a low-density polymorph of silicon dioxide (SiO2), shows an unusual elastic behavior of negative Poisson ratio, contracting laterally when compressed and expanding laterally when stretched. We have successfully evaluated the adiabatic second-order elastic stiffness constants in the temperature range of 300-1,800K, including a displacive phase transition region, with the equilibrium molecular dynamics method using the statistical fluctuation formula. We have found that the cristobalite exhibits a negative Poisson ratio over this temperature range, however, with different mechanisms between low-temperature (α) and high-temperature (β) phases.