First-principles phase diagram calculations for the HfC–TiC, ZrC–TiC, and HfC–ZrC solid solutions

We report first-principles phase diagram calculations for the binary systems HfC–TiC, TiC–ZrC, and HfC–ZrC. Formation energies for superstructures of various bulk compositions were computed with a plane-wave pseudopotential method. They in turn were used as a basis for fitting cluster expansion Hamiltonians, both with and without approximations for excess vibrational free energies. Significant miscibility gaps are predicted for the systems TiC–ZrC and HfC–TiC, with consolute temperatures in excess of 2000 K. The HfC–ZrC system is predicted to be completely miscibile down to 185 K. Reductions in consolute temperature due to excess vibrational free energy are estimated to be ~7%, ~20%, and ~0%, for HfC–TiC, TiC–ZrC, and HfC–ZrC, respectively. Predicted miscibility gaps are symmetric for HfC–ZrC, almost symmetric for HfC–TiC and asymmetric for TiC–ZrC

First principles phase diagram calculations for the CdSe-CdS wurtzite, zincblende and rock-salt structures

The phase diagrams of CdSe1-xSx alloys were calculated for three different crystal structure types: wurtzite (B4); zinc-blende (B3); and rocksalt (B1). Ab initio calculations of supercell formation energies were fit to cluster expansion Hamiltonians, and Monte Carlo simulations were used to calculate finite temperature phase relations. The calculated phase diagrams have symmetric miscibility gaps for B3 and B4 structure types and a slightly asymmetric diagram for B1 structure. Excess vibrational contributions to the free energy were included, and with these, calculated consolute temperatures are: 270 K for B4; 300 K for B3; and 270 K for B1. Calculated consolute temperatures for all structures are in good quantitative agreement with experimental data

Nanostructured Metallic Glasses: Tailoring the Mechanical Properties of Amorphous Metals

The mechanical properties of metallic glasses cannot only be influenced by their chemical composition, but also by their nanostructure: Secondary phases in the form of precipitates, as well as a nanocrystalline-like structure in the glass are viable options to increase the plasticity of the material. We performed molecular dynamics simulations on Cu-Zr based metallic glass systems to investigate the influence of these nanostructures on the mechanical deformation

Pressure induced phase transitions and elastic properties of CaCO3 polymorphs: a density functional theory study

First-principles calculations, based on density functional theory, were carried out to investigate phase transitions, structural and elastic properties of three polymorphs of calcium carbonate, in the pressure range up to 40 GPa. Our calculations led to the following stability sequence: calcite-IIIb → calcite-III → calcite-IIIb → calcite-VI, where phase transitions of the first order occur at 4.3 GPa, 14.9 GPa and 18.2 GPa, respectively. From 4.3 to 40 GPa, the elastic properties, the acoustic wave velocities and the Debye temperature of each polymorph exhibit a linear dependence over pressure. A nonlinear behavior is observed from 2.0 to 4.3 GPa for the properties of calcite-IIIb. As there are no available experimental data on the elastic properties of calcite-III, calcite-IIIb, calcite-VI and their pressure dependence, our present findings can serve to a better understanding of the behavior of calcium carbonate in the Earth's mantle

Disclinations provide the missing mechanism for deforming olivine-rich rocks in the mantle

International audienceMantle flow involves large strains of polymineral aggregates. The strongly anisotropic plastic response of each individual grain in the aggregate results from the interactions between neighbouring grains and the continuity of material displacement across the grain boundaries. Orthorhombic olivine, which is the dominant mineral phase of the Earth's upper mantle, does not exhibit enough slip systems to accommodate a general deformation state by intracrystalline slip without inducing damage. Here we show that a more general description of the deformation process that includes the motion of rotational defects referred to as disclinations can solve the olivine deformation paradox. We use high-resolution electron backscattering diffraction (EBSD) maps of deformed olivine aggregates to resolve the disclinations. The disclinations are found to decorate grain boundaries in olivine samples deformed experimentally and in nature. We present a disclination-based model of a high-angle tilt boundary in olivine, which demonstrates that an applied shear induces grain-boundary migration through disclination motion. This new approach clarifies grain-boundary-mediated plasticity in polycrystalline aggregates. By providing the missing mechanism for describing plastic flow in olivine, this work will permit multiscale modelling of the rheology of the upper mantle, from the atomic scale to the scale of the flow