[en] The energetics of defects in B_4+xC boron carbide and β-boron are studied through first-principles calculations, the supercell phonon approach and the Debye–Grüneisen model. It is found that suitable sublattice models for β-boron and B_4+xC are B₁₀₁(B,C)₄ and B₁₁(B,C) (B,C,Va) (B,Va) (B,C,Va), respectively. The thermodynamic properties of B_4+xC, β-boron, liquid and graphite are modeled using the CALPHAD approach based on the thermochemical data from first-principles calculations and experimental phase equilibrium data in the literature. The concentrations of various defects are then predicted as a function of carbon composition and temperature.

[en] The thermodynamic properties and phase equilibria of the Cu–Hf binary system with five intermetallic compounds were studied by experiments, first-principles calculations and CALPHAD modeling. The experimental investigations included differential thermal analysis, scanning electron microscopy, energy dispersive X-ray microanalysis and micro-X-ray diffraction focusing on the 30–60 at.% Hf composition range to determine the invariant reaction temperatures. Cu₁₀Hf₇ was confirmed to melt incongruently. The enthalpies of formation of all five binary Cu–Hf compounds were predicted through first-principles calculations. The atomic configuration of one of the compounds, Cu₅₁Hf₁₄, was postulated through systematic first-principles calculations with 65 atoms instead of 68 atoms, denoted by hp68 in the literature. The thermodynamic description of the Cu–Hf binary system was then obtained from the new experimental data and first-principles calculations.

[en] Fe-rich Fe–Si alloys show peculiar bulk-modulus changes depending on the Si concentration in the range of 0–15 at.%Si. In order to clarify the origin of this phenomenon, we have performed density-functional theory calculations of supercells of Fe–Si alloy models with various Si concentrations. We have applied our recent techniques of ab initio local energy and local stress, by which we can obtain a local bulk modulus of each atom or atomic group as a local constituent of the cell-averaged bulk modulus. A2-phase alloy models are constructed by introducing Si substitution into bcc Fe as uniformly as possible so as to prevent mutual neighboring, while higher Si concentrations over 6.25 at.%Si lead to contacts between SiFe₈ cubic clusters via sharing corner Fe atoms. For 12.5 at.%Si, in addition to an A2 model, we deal with partial D03 models containing local D03-like layers consisting of edge-shared SiFe₈ cubic clusters. For the cell-averaged bulk modulus, we have successfully reproduced the Si-concentration dependence as a monotonic decrease until 11.11 at.%Si and a recovery at 12.5 at.%Si. The analysis of local bulk moduli of SiFe₈ cubic clusters and Fe regions is effective to understand the variations of the cell-averaged bulk modulus. The local bulk moduli of Fe regions become lower for increasing Si concentration, due to the suppression of bulk-like d–d bonding states in narrow Fe regions. For higher Si concentrations till 11.11 at.%Si, corner-shared contacts or 1D chains of SiFe₈ clusters lead to remarkable reduction of local bulk moduli of the clusters. At 12 at.%Si, on the other hand, two- or three-dimensional arrangements of corner- or edge-shared SiFe₈ cubic clusters show greatly enhanced local bulk moduli, due to quite different bonding nature with much stronger p-d hybridization. The relation among the local bulk moduli, local electronic and magnetic structures, and local configurations such as connectivity of SiFe₈ clusters and Fe-region sizes has been analyzed. The ab initio local stress has opened the way for obtaining accurate local elastic properties reflecting local valence-electron behaviors. (paper)