[en] The effect of solute hydrogen on the stability of vacancy clusters in hexagonal closed packed zirconium is investigated with an ab initioapproach, including contributions of H vibrations. Atomistic simulations within the density functional theory evidence a strong binding of H to small vacancy clusters. The hydrogen effect on large vacancy loops is modeled through its interaction with the stacking faults. A thermodynamic modeling of H segregation on the various faults, relying on ab initiobinding energies, shows that these faults are enriched in H, leading to a decrease of the stacking fault energies. This is consistent with the trapping of H by vacancy loops observed experimentally. The stronger trapping, and thus the stronger stabilization, is obtained for vacancy loops lying in the basal planes, i.e.the loops responsible for the breakaway growth observed under high irradiation dose.

[en] Dislocation motion through a random alloy is impeded by its interactions with the compositional fluctuations intrinsic to the alloy, leading to strengthening. A recent theory predicts the strengthening as a function of the solute-dislocation interaction energies and composition. First-principles calculations of solute/dislocation interaction energies are computationally expensive, motivating simplified models. An elasticity model for the interaction reduces to the pressure field of the dislocation multiplied by the solute misfit volume. Here, the elasticity model is formulated and evaluated for cubic anisotropy in fcc metals, and compared to a previous isotropic model. The prediction using the isotropic model with Voigt-averaged elastic constants is shown to represent the full anisotropic results within a few percent, and so is the recommended approach for studying anisotropic alloys. Application of the elasticity model using accessible experimentally-measured properties and/or first-principles-computed properties is then discussed so as to guide use of the model for estimating strengths of existing and newly proposed alloys. (paper)

[en] The ab initio calculation of charged super-cells within density-functional theory is a necessary step to access several important properties of matter. The relaxation volume of charged point defects or the partial molar volume of ions in solution are two such examples. However, the total energy and therefore the pressure of charged systems is not uniquely defined when periodic boundary conditions are employed. This problem is tightly related to the origin of the electrostatic potential in periodic systems. This effect can be easily observed by modifying the electrostatic convention or modifying the local ionic potential details.We propose an approach to uniquely define the pressures in charged super-cells with the use of the absolute deformation potentials. Only with such a definition could the ab initio calculations provide meaningful values for the relaxation volumes and for the elastic interactions for charged defects in semiconductors or ions in solution. The proposed scheme allows one to calculate sensible data even when charge neutrality is not enforced, thus going beyond the classical force-field-based approaches. (authors)

[en] Different descriptions used to model a point-defect in an elastic continuum are reviewed. The emphasis is put on the elastic dipole approximation, which is shown to be equivalent to the infinitesimal Eshelby inclusion and to the infinitesimal dislocation loop. Knowing this elastic dipole, a second rank tensor fully characterizing the point-defect, one can directly obtain the long-range elastic field induced by the point-defect and its interaction with other elastic fields. The polarizability of the point-defect, resulting from the elastic dipole dependence with the applied strain, is also introduced. Parameterization of such an elastic model, either from experiments or from atomic simulations, is discussed. Different examples, like elasto diffusion and bias calculations, are finally considered to illustrate the usefulness of such an elastic model to describe the evolution of a point-defect in a external elastic field. (authors)

[en] The interaction of point defects with an external stress field or with other structural defects is usually well described within continuum elasticity by the elastic dipole approximation. Extraction of the elastic dipoles from atomistic simulations is therefore a fundamental step to connect an atomistic description of the defect with continuum models. This can be done either by a fitting of the point-defect displacement field, by a summation of the Kanzaki forces, or by a linking equation to the residual stress. We perform here a detailed comparison of these different available methods to extract elastic dipoles, and show that they all lead to the same values when the supercell of the atomistic simulations is large enough and when the anharmonic region around the point defect is correctly handled. But, for small simulation cells compatible with ab initio calculations, only the definition through the residual stress appears tractable. The approach is illustrated by considering various point defects (vacancy, self-interstitial, and hydrogen solute atom) in zirconium, using both empirical potentials and ab initio calculations. (authors)

[en] We present a review of solid solution strengthening models for random concentrated solid solutions of which high entropy alloys are an interesting subset. High entropy alloys (HEAs) usually refer to a class of multicomponent alloys in equal or near equal concentrations. These complex compositions break the conventional notion of solutes and solvents. Few attempts have been made to extend the conventional solute strengthening models to HEAs. Among these, the model based on an average effective medium, does not include any adjustable parameter, allows all model inputs to be computed by atomistic simulations, and has predicted the strength of fcc HEAs in good agreement with experiments. The basic concepts of this theory is explained and its capabilities are compared with few other existing models for solute strengthening of HEAs.

[en] We combine atomistic calculations and continuum laws to model irradiation-induced vacancy and interstitial dislocation loops in $\alpha$-zirconium. A comprehensive set of Burgers vectors/stacking sequences in the prismatic and basal planes, of sizes accessible to experiments, are studied by Molecular Statics (MS) simulations. Their formation energies and structural details are determined using two different interatomic potentials for $\alpha$-Zr, considering dislocation loops in hexagonal and circular shapes. Molecular Dynamics annealing of dislocation loops then validates the envisioned potential energy landscape. Finally, the continuum modelling hybridly calibrated on MS results and ab initio data indicate that the coexistence of vacancy and interstitial $〈a〉$ loops is supported by stability arguments. We also establish the limitations of such an approach for quantitative predictions.