156 research outputs found

    Large scale ab-initio simulations of dislocations

    Get PDF
    We present a novel methodology to compute relaxed dislocations core configurations, and their energies in crystalline metallic materials using large-scale ab-intio simulations. The approach is based on MacroDFT, a coarse-grained density functional theory method that accurately computes the electronic structure with sub-linear scaling resulting in a tremendous reduction in cost. Due to its implementation in real-space, MacroDFT has the ability to harness petascale resources to study materials and alloys through accurate ab-initio calculations. Thus, the proposed methodology can be used to investigate dislocation cores and other defects where long range elastic effects play an important role, such as in dislocation cores, grain boundaries and near precipitates in crystalline materials. We demonstrate the method by computing the relaxed dislocation cores in prismatic dislocation loops and dislocation segments in magnesium (Mg). We also study the interaction energy with a line of Aluminum (Al) solutes. Our simulations elucidate the essential coupling between the quantum mechanical aspects of the dislocation core and the long range elastic fields that they generate. In particular, our quantum mechanical simulations are able to describe the logarithmic divergence of the energy in the far field as is known from classical elastic theory. In order to reach such scaling, the number of atoms in the simulation cell has to be exceedingly large, and cannot be achieved with the state-of-the-art density functional theory implementations

    Nuevas perspectivas para la lengua española en Filipinas

    Get PDF
    El anuncio hecho por el Gobierno filipino sobre la reintroducción del español en la enseñanza secundaria ofrece unas nuevas perspectivas. La nueva situación merece una reflexión basada en los derechos individuales. El proyecto piloto del Departamento de Educación filipino significa que 17 escuelas públicas ofrecerán la asignatura optativa de español en secundaria. Esta oferta se suma a la de algunas escuelas privadas y universidades. España puede apoyar en formación de profesores, asesorías lingüísticas o aulas bilingües, como hace en otros países. Todo ello, sin olvidar la dimensión americana de la lengua y el derecho a elegir la enseñanza por parte de los interesados

    MXE: A package for simulating long-term diffusive mass transport phenomena in nanoscale systems

    Get PDF
    We present a package to simulate long-term diffusive mass transport in systems with atomic scale resolution. The implemented framework is based on a non-equilibrium statistical thermo-chemo-mechanical formulation of atomic systems where effective transport rates are computed using a kinematic diffusion law. Our implementation is built as an add-on to the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) code, it is compatible with other LAMMPS’ functionalities, and shows a good parallel scalability and efficiency. In applications involving diffusive mass transport, this framework is able to simulate problems of technological interest for exceedingly large time scales using an atomistic description, which are not reachable with the state-of-the-art molecular dynamics techniques. Several examples, involving complex diffusive behavior in materials, are investigated with the framework. We found good qualitative and quantitative comparison with known theories and models, with Monte Carlo methods, as well as with experimental results. Thus, our implementation can be used as a tool to understand diffusive behavior in materials where experimental characterization is difficult to perform

    Stress in a stimuli-responsive polymer brush

    Get PDF
    The application of a polymer brush in sensing, actuation, self-folding, among others acutely depends on the tuneable bending of a brush-grafted substrate caused by the stress in the brush. However, the stress in a stimuli-responsive brush has not been investigated. In this work, we study the stress in the stimuli-responsive planar polymer brushes of neutral water-soluble polymers with low to very high graft densities using strong stretching theory (SST). First, SST with the Langevin force-extension relation for a polymer chain is extended to the study of stimuli-responsive brushes. Stress profile and other properties of a Poly(N-isopropylacrylamide) (PNIPAm) brush are then obtained using the extended SST and an empirical Flory-Huggins parameter. The model predicts that the stress in a PNIPAm brush is inhomogeneous and compressive at all temperatures and graft densities. The resultant stress is predicted to increase in magnitude with increasing graft density. Moreover, it decreases in magnitude with an increase in temperature before plateauing in low graft density brushes. In contrast, its magnitude increases weakly with increasing temperature in high density brushes. This contrasting behavior is traced to the minimum in interaction free energy density \emph{vs} polymer volume fraction curve for PNIPAm solution at a large volume fraction, and stiffening of chains due to finite extensibility. Furthermore, our results indicate that the ability to tune the resultant stress by changing temperature diminishes with increasing graft density

    Influence of different interatomic potentials in void growth, dislocations emission and temperature evolution

    Get PDF
    We have studied the process of void growth by dislocation emission in Cu and Al under different loads using a nonequilibrium thermodynamic QC method. In addition, we have compared the results with different interatomic potentials to understand the influence of them in dislocation emission, void growth, and temperature evolution during the ductile failure of the material. We have observed that Mishin’s potential predicts the formation of shear and prismatic dislocations loops due to cross slip. On the contrary, Johnson’s potential predict a local change in the microstructure making a self-locking structure near the void that allows its grow only in the load direction. Because of the self-locking structure, a linear behavior of the material up to high levels of deformation is observed. Interestingly, because the different set the dislocations emitted from the void surface, the temperature evolution has also shown a different behavior for both potentials modifying the heat conduction process during the ductile failure. In fact, our simulations have shown that with the flux of material due to the dislocation emission there is a heat flux which is strongly connected to the plastic work generated in the material
    corecore