A phase-field model for phase transformations in glass-forming alloys

A phase-field model is proposed for phase transformations in glass-forming alloys. The glass transition is introduced as a structural relaxation, and the competition between the glass and crystalline phases is investigated. The simulations are performed for Cu-Zr alloys, employing thermodynamic and kinetic parameters derived from reported thermodynamic modeling and molecular dynamics simulation results,[1–3] respectively. Four distinct phase fields are treated with a multi-phase-field approach, representing the liquid/glass, Cu10Zr7, CuZr, and CuZr2 phases. In addition, a continuum-field method is applied to the liquid to accommodate the liquid–glass transformation. The combined phase-field approach is used to investigate the glass formation tendency, and critical cooling rates are estimated and compared with the reported experimental values

Search for Tensor, Vector, and Scalar Polarizations in the Stochastic Gravitational-Wave Background

The detection of gravitational waves with Advanced LIGO and Advanced Virgo has enabled novel tests of general relativity, including direct study of the polarization of gravitational waves. While general relativity allows for only two tensor gravitational-wave polarizations, general metric theories can additionally predict two vector and two scalar polarizations. The polarization of gravitational waves is encoded in the spectral shape of the stochastic gravitational-wave background, formed by the superposition of cosmological and individually unresolved astrophysical sources. Using data recorded by Advanced LIGO during its first observing run, we search for a stochastic background of generically polarized gravitational waves. We find no evidence for a background of any polarization, and place the first direct bounds on the contributions of vector and scalar polarizations to the stochastic background. Under log-uniform priors for the energy in each polarization, we limit the energy densities of tensor, vector, and scalar modes at 95% credibility to Ω0T<5.58×10-8, Ω0V<6.35×10-8, and Ω0S<1.08×10-7 at a reference frequency f0=25 Hz. © 2018 American Physical Society

On the progenitor of binary neutron star merger GW170817

On 2017 August 17 the merger of two compact objects with masses consistent with two neutron stars was discovered through gravitational-wave (GW170817), gamma-ray (GRB 170817A), and optical (SSS17a/AT 2017gfo) observations. The optical source was associated with the early-type galaxy NGC 4993 at a distance of just ∼40 Mpc, consistent with the gravitational-wave measurement, and the merger was localized to be at a projected distance of ∼2 kpc away from the galaxy's center. We use this minimal set of facts and the mass posteriors of the two neutron stars to derive the first constraints on the progenitor of GW170817 at the time of the second supernova (SN). We generate simulated progenitor populations and follow the three-dimensional kinematic evolution from binary neutron star (BNS) birth to the merger time, accounting for pre-SN galactic motion, for considerably different input distributions of the progenitor mass, pre-SN semimajor axis, and SN-kick velocity. Though not considerably tight, we find these constraints to be comparable to those for Galactic BNS progenitors. The derived constraints are very strongly influenced by the requirement of keeping the binary bound after the second SN and having the merger occur relatively close to the center of the galaxy. These constraints are insensitive to the galaxy's star formation history, provided the stellar populations are older than 1 Gyr

Electronic contribution for materials of Hume-Rothery type: a first-principles enlightment to !xalphad modelling

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First-principles approach to phase stability for a ternary sigma phase: Application to Cr-Ni-Re

International audienceFirst-principles calculations of formation energies for 243 different configurations of the Cr-Ni-Re sigma phase were used to calculate a ternary phase diagram in the Bragg-Williams-Gorsky approximation (BWG) and to model finite-temperature thermodynamic properties. The binary and ternary phase diagrams were then calculated at different temperatures. Correct topology of the experimental ternary isothermal section of the phase diagram has been obtained with a relatively small difference in temperature between calculations and experiments

First-principles approach to phase stability for a ternary sigma phase: Application to Cr-Ni-Re

International audienceFirst-principles calculations of formation energies for 243 different configurations of the Cr-Ni-Re sigma phase were used to calculate a ternary phase diagram in the Bragg-Williams-Gorsky approximation (BWG) and to model finite-temperature thermodynamic properties. The binary and ternary phase diagrams were then calculated at different temperatures. Correct topology of the experimental ternary isothermal section of the phase diagram has been obtained with a relatively small difference in temperature between calculations and experiments

Anharmonicity, mechanical instability, and thermodynamic properties of the Cr-Re sigma-phase.

International audienceUsing density-functional theory in combination with the direct force method and molecular dynamics we investigate the vibrational properties of a binary Cr-Re sigma-phase. In the harmonic approximation, we have computed phonon dispersion curves and density of states, evidencing structural and chemical effects. We found that the s-phase is mechanically unstable in some configurations, for example, when all crystallographic sites are occupied by Re atoms. By using a molecular-dynamics-based method, we have analysed the anharmonicity in the system and found negligible effects (similar to 0.5 kJ/mol) on the Helmholtz energy of the binary Cr-Re sigma-phase up to 2000 K (similar to 0.8T(m)). Finally, we show that the vibrational contribution has significant consequences on the disordering of the sigma-phase at high temperature. (C) 2014 AIP Publishing LLC

Development of contacts for Mg2(Si,Sn)-based thermoelectric devices within a CALPHAD-based ICME framework

Mg (Si,Sn)-based/electrode contacts are crucial in the manufacturing of thermoelectric devices. The contact of dissimilar materials, of semiconducting and metallic type, is typically done using several contacting layers, e. g. solder fillers, diffusion barriers, which produce bonding layers by diffusionreaction in the interconnection zone (IZ). The multiphase/multilayered bonded region has intrinsic dissimilar physico-chemical properties; therefore, the IZ is susceptible to degradation and/or failure during long-term operation under a thermal gradient. A CALPHAD-based ICME approach using the ThermoCalc-Python framework is being used to simulate the diffusion-reaction phenomena at the IZ to identify expected bonding layers, thickness and morphology. Thermodynamic and kinetic calculations proved valuable for selection of optimal process conditions and potential contacting layers. Process conditions can be optimized to avoid undesired phases or to fine tune layer thicknesses and microstructure of desired phases. The interaction between the model parameters and the optimization as a whole largely profits from the ICME approach