Phenomenology of reaction-diffusion binary-state cellular automata

We study a binary-cell-state eight-cell neighborhood two-dimensional cellular automaton model of a quasi-chemical system with a substrate and a reagent. Reactions are represented by semitotalistic transitions rules: every cell switches from state 0 to state 1 depending on if the sum of neighbors in state 1 belongs to some specified interval, cell remains in state 1 if the sum of neighbors in state 1 belong to another specified interval. We investigate space-time dynamics of 1296 automata, establish morphology-bases classification of the rules, explore precipitating and excitatory cases and scrutinize collisions between mobile and stationary localizations (gliders, cycle life and still-life compact patterns). We explore reaction-diffusion like patterns produced as a result of collisions between localizations. Also, we propose a set of rules with complex behavior called Life 2c22. © World Scientific Publishing Company

Displacive Phase Transformations and Generalized Stacking Faults

The displacive phase transformations can be considered as composed of two processes, namely, pure displacements, shuffling or shearing of atomic planes, and supplementary homogeneous lattice deformation changing also the dimensions of the moving planes. Such deformation causes shape memory effect when the structural transformation is reversed. General displacements of atomic planes will be examined, i.e. γ-surface type calculations will be reported for single plane shuffling, alternate shuffling of every other bcc atomic plane and successive displacements of parallel atomic planes producing in combination with homogeneous deformation the close packed structures. The results of calculations using the many-body potentials of the Finnis-Sinclair type will be compared with ab initio calculations that indicate in which way the phase transformation can be initiated

ACTA PHYSICA POLONICA A Displacive Phase Transformations and Generalized Stacking Faults

The displacive phase transformations can be considered as composed of two processes, namely, pure displacements, shuing or shearing of atomic planes, and supplementary homogeneous lattice deformation changing also the dimensions of the moving planes. Such deformation causes shape memory eect when the structural transformation is reversed. General displacements of atomic planes will be examined, i.e. γ-surface type calculations will be reported for single plane shuing, alternate shuing of every other bcc atomic plane and successive displacements of parallel atomic planes producing in combination with homogeneous deformation the close packed structures. The results of calculations using the many-body potentials of the FinnisSinclair type will be compared with ab initio calculations that indicate in which way the phase transformation can be initiated

Comparison of the intergranular segregation for eight dilute binary metallic systems in the Σ 11′ {332} tilt grain boundary

International audienceIntergranular segregation is studied in the limit of infinitely diluted solution for eight dilute metallic systems made of four face centred cubic metals, one transition metal, nickel, and three noble metals, copper, silver and gold. The grain boundary (GB) chosen is the symmetrical tilt Σ = 11′ {332} 〈110〉 GB with its characteristic “zigzag” structural pattern as numerically calculated and experimentally observed by high resolution transmission electronic microscopy in nickel. The metallic interactions are modelled with Finnis-Sinclair like potentials. The atomic sites are characterised by a geometrical parameter defined with their exact Voronoï' volumes and the tensor of the stresses locally exerted. The {332} GB presents the most diversity of sites in these respects. The segregation energies are computed and analysed versus the only two ‘driving forces' which can play a role in metallic intergranular segregation, viz. the elastic size effect and the excess cohesion energy effect. The elastic size effect calculated by the method of virtual impurity represents the main segregation driving force in most cases of the considered systems. It is worth noting however that the excess cohesion energy effect is important for non hydrostatic or compressive sites. It can even be predominant, as in the case of Ni(Cu)

Order and disorder in solid oxygen: Neutron scattering investigations

Solid oxygen exists at zero pressure in three crystallographic phases, denoted α, β, γ. Various results have been obtained by neutron scattering in the two disordered β and у phases. Plastic γ-O₂ has a A15 structure with 8 molecules per unit cell. The molecules, located at 2α and 6c sites, exhibit two different types of disorder of molecular axes: a spherical-like disorder for the 6c molecules, and a disc-like disorder for the 6c molecules. A precise diffraction analysis on large single crystals shows that the disorder of molecular axis of the molecules can be interpreted in terms of localized forbidden orientations, or orientational holes, with a wide disorder among the other orientations. An important contribution of the translation-rotation coupling must be taken into account for the 6c molecules. Analysis of diffuse scattering by both an approximate analytical model and molecular dynamics simulation shows an important contribution to the orientational correlations between 6c-6c and 2a-6c molecules. Magnetic disorder can be investigated by neutron polarization analysis of the paramagnetic scattering. The 3D long range antiferromagnetic order of the α phase becomes a 2D short range helicoidal order in the β phase, with a very short 0.5 nm correlation range. In the γ phase, one observes, as in the liquid, a strong antiferromagnetic correlation, with a suggestion of a ID order along the linear chains of 6c molecules. In all the measured phases, the variation of the scattering intensity with scattering vector q cannot be reproduced at high q by the known magnetic form factor. This suggests the need for a better description of the л orbital and the theoretical magnetic form factor of the molecule

Detailed analysis of the segregation driving forces for Ni(Ag) and Ag(Ni) in the sigma = 11{113} and sigma = 11{332} grain boundaries

International audienceThe behaviour with respect to segregation is studied for two qualitatively different symmetrical tilt grain boundaries (GBs): a low energy boundary with mirror-symmetric structure on the {1 1 3} plane and the high energy {3 3 2} GB with a "zigzag" structural pattern. These two structures have been confirmed by high resolution transmission electronic microscopy observations in nickel. Intergranular segregation is studied in the limit of infinitely diluted solution for Ni(Ag) and Ag(Ni) systems modelled with Finnis–Sinclair like potentials. The atomic sites are characterized by a geometrical parameter defined by their exact Voronoy volumes and the pressure locally exerted. The segregation energies and driving forces are compared with these parameters in order to characterize a segregation "susceptibility" of intergranular atomic sites. The size effect calculated by the method of virtual impurity represents the main segregation driving force for the considered system

Sagittal elastic waves in infinite and semi-infinite superlattices

International audienceElastic waves, polarized in the sagittal plane, are investigated in infinite and semi-infinite superlattices made from alternate layers of two isotropic media. By the use of a transfer-matrix method, we obtain explicit equations for the dispersion of the bulk modes as well as of the surface modes of a semi-infinite superlattice. We present a few illustrations of the theory for Al-W superlattices in which the relative thicknesses of the two media or the thickness of the layer at the surface are varied

Theoretical Study of Superficial Segregation of Infinitely Dilute Binary Alloys made of Nickel, Copper, Silver and Gold.

International audienceThe behavior of the surface segregation in infinitely diluted bimetallic alloys is addressed by calculating the segregation energy, local pressure and relative changes in the volume of atomic sites. The atomic interactions are described by a Finnis-Sinclair semi-empirical N-body potential derived from the second moment approximation of the tight binding theory. The segregation energies by plane parallel to the surface are calculated by numerical simulation soaked in molecular dynamics. It models the surface segregation at the atomic scale in order to identify the physical phenomena that govern and so-called traditional driving forces. These different driving forces at the surface have been identified in the metals as a size effect, an effect of excess cohesion and an alloy effect. Surface segregation is studied in the limit of infinitely dilute solution for Ni(Ag), Cu(Ag), Ni(Cu), Au(Ag), as well as their opposite, Ag(Ni), Ag(Cu), Cu(Ni), Ag(Au). The study's main objective is the verification of the model of the driving forces involving the contributions of the size effect, the effect of excess cohesion and the alloy effect. The (110) surface is the surface which has the highest segregation in all systems among the three simple surfaces. Surface segregation is mainly localized on the surface. It is favoured for direct systems and is no longer for Inverse Systems. The results for the surface show that the energy of segregation is well reproduced by the sum the three effects: size, excess cohesion and alloy

Analysis of dissociated dislocations in a deformed bicrystal close to the rhombohedral twin orientation in -alumina

International audienceA bicrystal close to a rhombohedral twin orientation in -alumina has been deformed by compression at high temperature. After deformation the twin contained a periodic array of intrinsic dislocations. This configuration is interpreted by the decomposition of incoming basal lattice dislocations into interfacial disconnections and further interactions between sessile and glissile products. The dislocations are widely dissociated and the separation width between partials being close to 30 angstrom. The translation state of the interfacial structure between partials is different from those of the perfect twin. Possible structural models are envisaged and simulated using ab initio calculations and the less efficient Streitz and Mintmire modelling. One model in particular fits the experimental image rather well. The interfacial energies are higher than those estimated using elasticity theory