Using a combination of statistical mechanics and finite-element interpolation, we develop a coarsegrained (CG) alternative to molecular dynamics (MD) for crystalline solids at constant temperature. The new approach is significantly more efficient than MD and generalizes earlier work on the quasicontinuum method. The method is validated by recovering equilibrium properties of single crystal Ni as a function of temperature. CG dynamical simulations of nanoindentation reveal a strong dependence on temperature of the critical stress to nucleate dislocations under the indenter. DOI: 10.1103/PhysRevLett.95.060202 PACS numbers: 02.70.Ns, 02.70.Dh, 62.25.+g, 68.60.Dv Many processes involving the physics and chemistry of materials result from microscopic interactions between the constituent atoms. As a result, molecular dynamics (MD) simulations pervade the literature of a variety of materialsrelated disciplines. However, large-scale atomistic simulations remain computationally demanding, resulting in the continued effort to seek alternatives which permit the examination of larger spatial domains or longer time scales.An important step in this direction is a variety of multiscale methods which combine atomistic simulation with coarse-graining schemes (see [1] for a recent review). These methods exploit the fact that in many cases the critical dynamics may involve a relatively small subset of the entire set of atoms with the remainder of the atoms serving primarily to guarantee appropriate boundary conditions for the region of interest. One example is the quasicontinuum (QC) method, a zero-temperature energy minimization technique, which significantly reduces the total number of degrees of freedom that must be considered when simulating the deformation of crystalline solids [2,3]. In this method an approximation to the total potential energy is obtained by making use of finite-element constraints to remove atoms where the deformation field varies slowly on the scale of the lattice parameter. An attractive feature of this approach is its ''seamlessness'' in that the same underlying atomistic model is used in the energy calculations in both the coarse-grained (CG) and fully atomistic regions.The aim of this Letter is to extend the QC method to treat the dynamics of systems at constant temperature. Our procedure is based on the concept of a potential of mean force (PMF), which was first introduced by Kirkwood in 1935 [4]. In principle, this approach enables the calculation of a variety of equilibrium and nonequilibrium properties of large systems using only a limited number of degrees of freedom. In practice, however, calculating the PMF directly from a molecular dynamics simulation is often computationally demanding. This drawback can either make the PMF approach less efficient than the full atomistic calculation it is seeking to replace, or limits its use to linear coupling terms [5]. In this Letter we propose a method to substantially expedite the calculation of the PMF by making use of finite-element interpolation a...
Understanding the dissolution of silicate glasses and minerals from atomic to macroscopic levels is a challenge with major implications in geoscience and industry. One of the main uncertainties limiting the development of predictive models lies in the formation of an amorphous surface layer––called gel––that can in some circumstances control the reactivity of the buried interface. Here, we report experimental and simulation results deciphering the mechanisms by which the gel becomes passivating. The study conducted on a six-oxide borosilicate glass shows that gel reorganization involving high exchange rate of oxygen and low exchange rate of silicon is the key mechanism accounting for extremely low apparent water diffusivity (∼10−21 m2 s−1), which could be rate-limiting for the overall reaction. These findings could be used to improve kinetic models, and inspire the development of new molecular sieve materials with tailored properties as well as highly durable glass for application in extreme environments.
International audienceA generalization of the quasi-continuum (QC) method to finite temperature is presented. The resulting "hot-QC" formulation is a partitioned domain multiscale method in which atomistic regions modeled via molecular dynamics coexist with surrounding continuum regions. Hot-QC can be used to study equilibrium properties of systems under constant or quasistatic loading conditions. Two variants of the method are presented which differ in how continuum regions are evolved. In "hot-QC-static" the free energy of the continuum is minimized at each step as the atomistic region evolves dynamically. In "hot-QC-dynamic" both the atomistic and continuum regions evolve dynamically in tandem. The latter approach is computationally more efficient, but introduces an anomalous "mesh entropy" which must be corrected. Following a brief review of related finite-temperature methods, this review article provides the theoretical background for hot-QC (including new results), discusses the implementational details, and demonstrates the utility of the method via example test cases including nanoindentation at finite temperature
A multiscale characterization of the microstructural evolutions taking place in 9 to 12 pct Cr martensitic steels subjected to fatigue and creep-fatigue (CF) loadings is presented. Specimens of a P91 steel subjected to high-temperature cyclic loadings are examined using several experimental techniques. Bright-field transmission electron microscopy (TEM), electron backscattered diffraction (EBSD), and TEM orientation mapping are used to characterize and quantify the microstructural evolutions. A recovery phenomenon consisting of the coarsening of the subgrains and a decrease of the dislocation density is observed. This coarsening is heterogeneous and depends on the strain amplitude and on the applied hold time. The size distribution of subgrains and the dislocation density are measured from bright-field TEM observations. Orientation mapping on scanning electron microscopy (SEM) and TEM show that, even though a correlation between the crystallographic orientation and the recovery phenomenon is highlighted, a complex dependency related to the orientation of neighboring blocks exists. These microstructural observations are consistent with the very fast deterioration of creep properties due to cyclic loadings (reported in the first part of this study).
International audienceDislocation climb mobilities, assuming vacancy bulk diffusion, are derived and implemented in dislocation dynamics simulations to study the coarsening of vacancy prismatic loops in fcc metals. When loops cannot glide, the comparison of the simulations with a coarsening model based on the line tension approximation shows a good agreement. Dislocation dynamics simulations with both glide and climb are then performed. Allowing for glide of the loops along their prismatic cylinders leads to faster coarsening kinetics, as direct coalescence of the loops is now possible
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