V. I. Bogdanov

First-principles study of point defects in Ni3Al

The energetics and structural properties of native, substitutional and interstitial defects in NiAl have been investigated by first-principles methods. In particular, we have determined the formation energies of composition conserving defects and established that the so-called penta defect, which consists of four vacancies on Ni sublattice and Ni antisite on the Al sublattice, is the main source of vacancies in NiAl. We show that this is due to the strong Ni-site preference of vacancies in NiAl. We have also calculated the site substitution behaviour of Cu, Pd, Pt, Si, Ti, Cr, V, Nb, Ta and Mo and their effect on the concentration expansion coefficient. We show the latter information can used for an indirect estimate of the site substitution behaviour of the alloying elements. The solution energy of carbon and its effect on the lattice constant of NiAl have been obtained in the dilute limit in the first-principles calculations. We have also determined the chemical and strain-induced carbon–carbon interactions in the interstitial positions of NiAl. These interactions have been subsequently used in the statistical thermodynamic simulations of carbon ordering in NiAl.

Chemical and deformational interactions in solid solution of carbon in nickel

A first-principles stuy of ordering phenomena in hcp interstitial solid solutions of oxygen and nitrogen in Ti, Zr and Hf has demonstrated that the dominant contributions to the interaction energy of interstitial atoms are of chemical nature; thus, it is necessary to modify the previously established concepts about the priority role of deformational interactions in interstitial solutions. We have continued studies of the role of chemical and deformational interactions of interstitial atoms by the example of solid solutions of carbon in nickel. The results obtained also confirm a significant role of chemical interactions between carbon atoms in these solid solutions. The results were compared with the experimental data on the enthalpy of carbon dissolution in nickel and on the coefficient of solutal expansion of the lattice.

First-principles investigations of interatomic interactions in Ni3Al alloyed by interstitial and substitutional impurities

First-principles calculations of the total energy of interstitial and substitutional solid solutions in intermetallic compound Ni3Al were performed based on methods using Vienna ab-initio simulation package (VASP). The results of the calculations for interstitial solutions of carbon in Ni3Al confirmed the priority role of chemical interactions over deformational ones for the nearest neighbors. We attempted to use first-principles methods of calculation of the deformation interaction and continuum approaching in the theory of solutions to calculate coefficients of the concentration changes of the lattice spacing. Comparison of the calculation results with experimental data of substitutional impurities in Ni3Al has shown that the proposed method can aid in the study of the distribution of impurity atoms on the sublattices of the ordered phases, intermetallic compounds. We have proposed a method of calculating the partial molar volume of impurity in interstitial solid solutions.

Estimation of the thermodynamic parameters of second-order interactions in binary alloys on the basis of experimental thermodynamic data

The values of the thermodynamic parameters of second-order interaction in dilute binary substitutional solid solutions have been estimated for twenty seven systems on the basis of experimental thermodynamic data. The possibilities are outlined of using these data for checking the results of the first-principles calculations of the electronic theory of alloys. It has been established that for the majority of the systems examined the sign of the parameter of second-order interaction is opposite to that of the Wagner parameter of interaction. For the solid solutions of molybdenum in chromium at a temperature of 1471 K the values of the thermodynamic parameter of the second-order interaction and first-order enthalpy parameter are connected with the value of the Wagner parameter of interaction via formulas, which correspond to the model in which three-particle interaction is absent and the radius of pairwise interaction corresponds to the nearest coordination shell. The potential of pairwise interaction is temperature-independent. For the solid solutions of platinum in cobalt at a temperature of 1273 K, the value of the Wagner parameter of interaction calculated in the approximation of an Ising Hamiltonian with the use of effective pairwise interactions evaluated in the literature by the methods of the electronic theory of alloys agree in sign and in order of magnitude with the experimental value.

Interatomic interactions and thermodynamic parameters in dilute solid solutions of the Ag-Au system

The thermodynamic parameters of interaction and the enthalpy parameters are of fundamental importance in the theory of solutions, i.e., the coefficients of the expansion of partial excess thermodynamic functions into series in terms of the concentrations of the dissolved components. In the approximation of pairwise interactions between the impurity atoms in the solution, the above parameters can be computed using the methods of the density-functional theory in the electron theory of alloys. As an example, the substitutional solid solutions of Au in Ag have been chosen, which are formed by atoms of the components with close chemical properties, in which the deformation interactions should be small, and in which there is no need to take into account the complex magnetic contributions to the pair potentials. The total energy of the dilute solution of Au in Ag and the contributions from the chemical and strain-induced interactions to the potentials of pairwise interactions are calculated up to the seventh coordination shell. Quite satisfactory agreement with the thermodynamic parameters obtained from the experimental data has been obtained.

Mechanical alloying of Ni3Al with molybdenum and arrangement of Mo atoms in sublattices of the intermetallic

Mechanochemical synthesis (MS) of Ni70Al25Mo5 (composition 1) and Ni75Al20Mo5 (composition 2) mixtures, in which 5 at % Mo substitutes for the equal amount of Ni or Al, leads to the formation of Ni-based nanocrystalline (coherent domains are ∼7–12 nm in size) solid solutions; in this case, some amount of molybdenum remains free. A comparison of the lattice parameters of solid solutions, which were determined experimentally, with the magnitudes determined theoretically using Vegard law and Bozollo-Ferrante simulation, which takes into account volume modules of elasticity of elements, showed an increase in interactions between atoms composed the solid solution and the formation of regions characterized by short-range order. The heating of mechanically synthesized three-component Ni(Al, Mo) solid solutions to 720°C in a calorimeter chamber forms the ordered γ′ phase (L12) at T ∼ 450°C. An analysis of the ratio of relative intensities of superlattice and fundamental reflections showed that, whatever the composition of initial mixture, Mo atoms always occupy positions in the Al sublattice. This arrangement of Mo atoms was confirmed by calculations of coefficients of concentrational variations of the lattice parameters. When molybdenum is added to Ni3 Al, Mo atoms, rather than Ni atoms, complete the Al sublattice. In this case, vacancies compensate for the lack of atoms in the Ni sublattice.

Method for calculating coefficients of concentrational variations in lattice constants and the distribution of impurity atoms between sublattices in intermetallic compounds

A method is proposed for calculating coefficients of concentrational variations in lattice constants in solid solutions, based on first-principles determination of the total energy of solid solutions and the continual approximation in the solutions theory, allowing for the deformation interaction of impurity atoms due to distortions of the solvent crystal lattice.

Thermodynamic analysis of the data on the solubility of nitrogen in gamma iron at a high pressure

An equation is proposed for an isotherm of the solubility of gaseous nitrogen in γ iron at a high gas pressure. This equation takes into account the partial volume of nitrogen in austenite. This equation is used to analyze the experimental data on the solubility of nitrogen in γ iron at a nitrogen pressure of up to 348 MPa and to estimate the experimental value of the Wagner interaction parameter in austenite at a temperature of 1273 K (ɛNN = 6.3 ± 1.0). This value is compared to the theoretical value (ɛNN = 9.0) that corresponds to the interaction potential between nitrogen atoms in austenite that was determined earlier using Mössbauer spectroscopy. The possible causes of the discrepancy between the experimental and calculated values of parameter ɛNN are discussed.

Statistical thermodynamics of low-concentration gold melts in copper

The Wagner interaction parameter ɛAuAu and first-order enthalpy parameter ηAuAu in Cu-Au melts at 1550 K were calculated in terms of the lattice solution model and statistical theory of low-concentration melts. The hi potential of the approach of two gold atoms in the face-centered cubic lattice of copper was used. The theoretical parameter ɛAuAu = 3.2 was satisfactorily close to the experimental value ɛAuAu = 3.7, whereas the theoretical enthalpy parameter agreed with its experimental value only in sign and the order of magnitude.

Wagner's nitrogen-nitrogen interaction coefficient in iron-based liquid alloys

The Wagners nitrogen-nitrogen interaction coefficient NN in iron-base liquid alloys is considered. The statistical theory of low-concentrated alloys in the framework of lattice model of melts has been used for the calculation of this coefficient at a temperature of 1873 K. The theoretical value NN=7.3 has been obtained by means of nitrogen-nitrogen interatomic potential in austenite based on Mossbauer spectrometry data. The experimental value of this parameter obtained according to the data on nitrogen solubility in the melts of the iron-chromium system is 7.1. Both values are in agreement with each other.