[en] A three-phase miscibility gap including the austenite and two carbonitrides is presented to describe the phase equilibria between the austenite matrix and carbonitride precipitates. It is demonstrated that the three-phase equilibrium has a lower Gibbs energy than the conventional two-phase equilibrium of austenite and one carbonitride, and is more consistent with experimental data in the literature

[en] Constitutional liquation means the local eutectic melting of second-phase particles in a matrix at temperatures above the eutectic temperature and below the solidus of the alloy, which may occur in the heat-affected zone (HAZ) during welding. In the present paper, the constitutional liquation in the Al-Cu system was computationally investigated using the DICTRA program coupled with critically assessed thermodynamic and kinetic databases. Computer simulated results are in quantitative agreement with existing experimental data. The computational procedures for obtaining the critical heating rate to avoid constitutional liquation are demonstrated. The critical heating rate was found to be inversely proportional to the square of the precipitate size. The present computational procedures can be readily extended to predict the susceptibility of multicomponent commercial alloys to constitutional liquation during welding with available thermodynamic and kinetic databases

[en] In an attempt to obtain reliable first-principles phonon dispersions of random alloys, we have developed a method to calculate the dynamical matrix, with respect to the wavevector space of the ideal lattice, by averaging over the force constants of a special quasi-random structure. Without additional approximations beyond standard density functional theory, the present scheme takes into account the local atomic position relaxations, the composition disorder, and the force constant disorder in a random alloy. Numerical results are presented for disordered Cu₃Au, FePd, and NiPd and good agreement between the calculations and the inelastic neutron scattering data is observed. (paper)

[en] The structural and elastic properties of BiMnO₃ with monoclinic (C 2/c) and orthorhombic (Pnma) ferromagnetic (FM) structures have been studied by first-principles calculations within LDA + U and GGA + U approaches. The equilibrium volumes and bulk moduli of BiMnO₃ phases are evaluated by equation of state (EOS) fittings, and the bulk properties predicted by LDA + U calculations are in better agreement with experiment. The orthorhombic phase is found to be more stable than the monoclinic phase at ambient pressure. A monoclinic to monoclinic phase transition is predicted to occur at a pressure of about 10 GPa, which is ascribed to magnetism versus volume instability of monoclinic BiMnO₃. The single-crystal elastic stiffness constants c_ijs of the monoclinic and orthorhombic phases are investigated using the stress-strain method. The c₄₆ of the monoclinic phase is predicted to be negative. In addition, the polycrystalline elastic properties including bulk modulus, shear modulus, Young's modulus, bulk modulus-shear modulus ratio, Poisson's ratio, and elastic anisotropy ratio are determined based on the calculated elastic constants. The presently predicted phase transition and elastic properties open new directions for investigation of the phase transitions in BiMnO₃, and provide helpful guidance for the future elastic constant measurements.

[en] The 0 K pressure-induced magnetic phase transformations of face-centered cubic (FCC) and hexagonal close packed (HCP) Co have been examined using first-principles calculations. Issues of fitting an equation of state to the first-principles energy versus volume data points containing a magnetic transformation and comparing to experimental phase equilibria are discussed. It is found that a fitting scheme employing only data where the magnetic moment decreases linearly with volume offers a physically meaningful behavior for the equation of state at metastable volumes. From this fitting, the ferromagnetic to nonmagnetic transformations with increasing pressure at 0 K are at 77 GPa and 123 GPa for FCC and HCP, respectively, and are first order and second order, respectively, on the basis of an unambiguous measure proposed in the paper. In addition to the HCP/FCC structure transformation at 99 GPa, another transformation at negative pressures is predicted, at - 31 GPa. These results are shown to be consistent with the extrapolations of the experimental pressure-temperature phase diagram to 0 K.

[en] Temperature-dependent elastic stiffness constants (c_ijs), including both the isothermal and isoentropic ones, have been predicted for rhombohedral α-Al₂O₃ and monoclinic θ-Al₂O₃ in terms of a quasistatic approach, i.e., a combination of volume-dependent c_ijs determined by a first-principles strain versus stress method and direction-dependent thermal expansions obtained by first-principles phonon calculations. A good agreement is observed between the predictions and the available experiments for α-Al₂O₃, especially for the off-diagonal elastic constants. In addition, the temperature-dependent c_ijs predicted herein, in particular the ones for metastable θ-Al₂O₃, enable the stress analysis at elevated temperatures in thermally grown oxides containing α- and θ-Al₂O₃, which are crucial to understand the failure of thermal barrier coatings in gas-turbine engines.

[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 effects of alloying elements (Al, Cr, Hf, Pt, Y, and Zr) on the coefficients of thermal expansion (CTE) of face-centered-cubic Ni (γ) and L1₂-Ni₃Al (γ′) phases have been investigated in terms of the first-principles quasi-harmonic approach by considering both the vibrational and thermal electronic contributions. By combining the present CTE data and the compositions and phase fractions of γ and γ′, the CTE for some Ni–Al-base alloys are predicted and compared with available experimental measurements in the literature, showing good agreement. It is observed that the addition of Pt is effective in reducing the CTE of both γ and γ′ phases, resulting in lower expansion mismatch between the bond coat and the thermally grown oxide alumina layer, as shown by the predicted CTE of Ni–22 Al–x Pt (at.%) alloys (x = 5, 10, 15, 20, 25, 30).

[en] In the present work, ternary special quasirandom structures (SQSs) for a fcc solid solution phase are generated at different compositions, x_A=x_B=x_C=(1/3) and x_A=(1/2), x_B=x_C=(1/4), whose correlation functions are satisfactorily close to those of a random fcc solution. The generated SQSs are used to calculate the mixing enthalpy of the fcc phase in the Ca-Sr-Yb system. It is observed that first-principles calculations of all the binary and ternary SQSs in the Ca-Sr-Yb system exhibit very small local relaxation. It is concluded that the fcc ternary SQSs can provide valuable information about the mixing behavior of the fcc ternary solid solution phase. The SQSs presented in this work can be widely used to study the behavior of ternary fcc solid solutions

[en] Phase stabilities of Hf-Si-O and Zr-Si-O have been studied with first-principles and thermodynamic modeling. From the obtained thermodynamic descriptions, phase diagrams pertinent to thin film processing were calculated. We found that the relative stability of the metal silicates with respect to their binary oxides plays a critical role in silicide formation. It was observed that both the HfO₂/Si and ZrO₂/Si interfaces are stable in a wide temperature range and silicide may form at low temperatures, partially at the HfO₂/Si interface