Hideo Kaburaki

Empirical bond-order potential description of thermodynamic properties of crystalline silicon

Thermodynamic properties of silicon (diamond cubic phase) are calculated using an empirical many-body potential developed by Tersoff [Phys. Rev. Lett. 56, 632 (1986)] based on the concept of bond order. It is shown that this model gives predictions in good agreement with experiment for those properties governed by energetics (free energy, entropy, and heat capacity). The thermal expansion coefficient is less well described, which is traced to the fact that the model potential, in its present version, is overly stiff and therefore unable to account properly for the volume dependence of the transverse acoustic modes. Furthermore, sensitivity of the potential to whether each atom remains bonded to only four neighbors indicates that the short-range nature of the potential may necessitate model improvement before it is suitable for studies of thermomechanical properties at elevated temperatures or large deformations.

Energetics of segregation and embrittling potency for non-transition elements in the Ni Σ5 (012) symmetrical tilt grain boundary: a first-principles study

A series of non-transition elements bound to the Ni Σ5 (012) symmetrical tilt grain boundary (GB) and the (012) free surface (FS) systems has been studied by first-principles calculation using WIEN2k code, which is based on the full-potential linearized augmented plane wave method with the generalized gradient approximation. The multilayer relaxations in the presence and absence of solutes are determined by the force minimization procedure. The binding energies at some GB/FS/bulk sites including both interstitial and substitutional sites are calculated for all the non-transition elements between H and Rn (from the first-row to the sixth-row elements). The GB/FS segregation energy is obtained by calculating the binding energy difference between the GB/FS site and the inner bulk site. The embrittling potency energy is obtained by calculating the difference between the GB and FS segregation energies on the basis of the Rice–Wang model. The calculated results show that most of the non-transition elements have negative GB/FS segregation energies. In our definition, this means that there exists a segregation site in the GB/FS that is more stable for the solute atom than in the bulk. The embrittling potency energies are positive for most of the solutes. However, some exceptions such as Be, B, C, and Si having negative and large embrittling potency can enhance the GB cohesion. The calculated results are found to be consistent with the various experimental findings within the discussion based on the simple site competition model neglecting the interactions between different solutes.

Thermal Conductivity of Solid Argon by Classical Molecular Dynamics

Following the Green-Kubo formalism in linear response theory, the lattice thermal conductivity of solid argon is determined by using classical molecular dynamics simulation to calculate the heat current correlation function. Comparing the absolute conductivities obtained using the Lennard-Jones potential with experiments, the authors find the predicted results to uniformly underestimate the measurements in magnitude, whereas the calculated temperature dependence corresponds well with the data. The temporal behavior of the heat current autocorrelation function shows that while a single exponential decay description is appropriate at elevated temperatures, below the half of the Debye temperature, the heat current relaxation clearly consists of two stages, an initial rapid decay associated with local dynamics followed by a slower component associated with the dynamics of lattice vibrations (phonons).

Solution softening in magnesium alloys: the effect of solid solutions on the dislocation core structure and nonbasal slip

There is a pressing need to improve the ductility of magnesium alloys so that they can be applied as lightweight structural materials. In this study, a mechanism for enhancing the ductility of magnesium alloys has been pursued using the atomistic method. The generalized stacking fault (GSF) energies for basal and prismatic planes in magnesium were calculated by using density functional theory, and the effect of the GSF energy on the dislocation core structures was examined using a semidiscrete variational Peierls-Nabarro model. Yttrium was found to have an anomalous influence on the solution softening owing to a reduction in the GSF energy gradient.

Dynamical thermal conductivity of argon crystal

The thermal conductivity of a rare-gas crystal (Ar) is computed using equilibrium molecular dynamics in conjunction with the Green-Kubo linear response formalism, and the Lennard-Jones potential with an appropriately long cutoff (4σ). Besides predicting absolute values of the conductivity from low temperature up to the liquid, the approach allows heat conduction to be understood as a dynamical process through the temporal behavior of the heat current correlation function. At low temperatures the correlation function shows a characteristic two-stage decay, a short-time relaxation which we attribute to single-particle motions in a local environment, and a more extended component corresponding to collective atomic motions (phonons). As temperature increases the second correlation component diminishes much faster than the first component, indicating a transition from mainly phase-coherent phonon transport to mainly phase-incoherent interatomic energy transfer in solids.

Thermal conductivity in one-dimensional lattices of Fermi-Pasta-Ulam type

Abstract Computer experiments have been done on the steady state thermal conduction of one-dimensional Fermi-Pasta-Ulam (FPU) type lattices. The thermal conductivity was determined for a lattice size of up to 5000. It is found that the nonlinearity of the lattice has a large on the formation of the internal temperature gradient.

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 n

Novel Cross-Slip Mechanism of Pyramidal Screw Dislocations in Magnesium.

frac{1}{2}