Electrical resistivity of liquid binary and ternary alloys

New method of calculation of the electrical resistivity of liquid and amorphous alloys is presented. The method is based on the Morgan–Howson–S̆aub (MHS̆) model but the pseudopotentials are replaced by the scattering matrix operators. The Fermi energy is properly determined by the accurate values of the phase shifts. The model depends on a very small number of universal parameters and gives stable results. The calculated values of the resistivity agree well with available experimental data for a substantial number of binary alloys. Moreover, the results for some ternary alloys were also obtained.     

Investigation of valence band structures in Cu-Pd alloys

With the use of a HP 5950 A spectrometer X-ray photoelectron spectra (XPS) of valence band were obtained for Cu-Pd alloys and the density of electron states was calculated by the coherent potential method. It is found that d-resonance states of Pd are present in the alloys containing a high percentage of copper but there are no apparant d-resonance states of Cu in the alloys containing a high percentage of palladium.     

The pseudopotential method and the thermodynamics of alloys

The authors review their work on the use of the pseudopotential method in the thermodynamics of alloys. Treatment of the theory of phase equilibria in alloys on the basis of the pseudopotential method makes it possible to use first principles in order to set up a thermodynamics of alloys in which the energy parameters are not presumed to be phenomenological but are calculated from the electronic characteristics of the pure components. The approach developed allows a priori calculation of the phase diagrams of alloys, the effect of cubic compression on phase diagrams, and the various thermodynamic functions of alloys. A classification of phases in alloys on the basis of analysis of their thermodynamic characteristics is proposed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 22–39, August, 1976.     

Vibrational properties of crystalline and amorphous Ge1−xSixalloys

A five-atom cluster Bethe lattice method treating the short-range order with proper statistical effects has been proposed for understanding the crystalline and amorphous alloys. Numerical results for Ge_1?xSi_x alloys are in excellent agreement with the experimental data. In the system studied the network is random but the nearest-neighbour coordination should be treated exactly. The long-range coordination is seen to be concentration-dependent in the crystalline alloys but concentration-independent in the amorphous alloys.     

Splitting of acceptor ground state in zincblende semiconductor alloys

We have evaluated the splitting of the acceptor ground state induced by compositional disorder in zincblende semiconductor alloys, by a perturbation method. We show that the splitting is proportional to the quadrupolar moment of the impurity distribution around the acceptor, weighted by the hole probability distribution. We discuss applications of this calculation to exciton bound to N in III–V alloys, and possible consequences for magnetic polarons bound to acceptors in II–VI semimagnetic alloys.     

The magnetic entropy change in La0.8Ce0.2Fe11.4Si1.6Bx compounds prepared by copper-mold casting

The magnetocaloric effect (MCE) of La_0.8Ce_0.2Fe_11.4Si_1.6B_x (x=0.0-0.5) compounds, prepared by a copper-mold casting (CMC) method, has been investigated. Comparing with the conventional arc-melting (CAM) method, the relatively homogenous composition and microstructure were achieved in the precursor alloys prepared by the CMC method. As a result, the annealing time is dramatically shortened from several weeks for CAM alloys to 2 h for CMC alloys, suggesting that CMC method is a time-saving and energy-saving method for fabrication of MCE alloys. On the other hand, it is revealed that B addition gives rise to an enhancement of Curie temperature (T_C), a reduction of thermal lag and magnetic hysteresis and a broadening of working temperature span as well. Although the peak value of magnetic entropy change decreases with B content, various B-contained compounds hold close refrigerant capacities. Comprehensively considering magnetocaloric properties of the B-contained La_0.8Ce_0.2Fe_11.4Si_1.6B_x compounds, it can be concluded that the B-contained compounds prepared by CMC method are promising candidates of magnetocaloric materials in practical application.     

Occupation of energy bands by electrons in transition metals and alloys

A method is proposed for calculating the degree of occupation of energy bands by electrons with variously directed spins in transition metals and their alloys in a ferromagnetic state. The method is used to calculate the degree of occupation of bands in metals of the first transition period and in several binary and ternary alloys based on these metals.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 64–71, September, 1977.     

Theoretical study of the structural,electronic and magnetic properties of equiatomic quaternary CoTcCrZ (Z = Si,Ge, P) Heusler alloys

In this study, full potential linearized augmented plane wave (FP-LAPW) method has been used to calculate structural, electronic and magnetic characteristics of CoTcCrZ (Z = Si, Ge, P) Equiatomic Quaternary Heusler alloys (EQHAs). Furthermore, Generalized Gradient Approximation (GGA) and Hubbard potential (GGA+U) are adopted in the parametrization of Perdew-Burke-Ernzerhof (PBE). Structural stability calculations confirmed that the Type II is the most stable structure of all considered alloys. Further investigations were carried out for the most stable Type (i.e. Type II). Comparison of electronic structure calculations performed by GGA and GGA+U methods concluded that considered alloys show half-metallic nature for GGA+U method. Magnetic moments for all these alloys are determined which are found in accordance to the Slater-Pauling rule. Half-metallicity has been verified in all these considered Heusler alloys (HAs) from the calculations of spin-polarization at the Fermi level (E_F). Moreover, the Curie temperature (Tc) is estimated by employing mean field approximation (MFA).     

Atomic approximation for alloys and their surfaces

We report a simple, ab initio method for calculating the electronic structure of compositionally disordered alloys. Results are shown for Cu/Ni and Ag/Pd bulk systems, and the first calculations are reported for the surface electronic structure of random alloys, exemplified by {111} surface states of Cu/Ni and Cu/Al alloys.     

Studies of concentration and temperature dependences of precipitation kinetics in iron-copper alloys using kinetic Monte Carlo and stochastic statistical simulations

The previously developed ab initio model and the kinetic Monte Carlo method (KMCM) are used to simulate precipitation in a number of iron-copper alloys with different copper concentrations x and temperatures T. The same simulations are also made using an improved version of the previously suggested stochastic statistical method (SSM). The results obtained enable us to make a number of general conclusions about the dependences of the decomposition kinetics in Fe-Cu alloys on x and T. We also show that the SSM usually describes the precipitation kinetics in good agreement with the KMCM, and using the SSM in conjunction with the KMCM allows extending the KMC simulations to the longer evolution times. The results of simulations seem to agree with available experimental data for Fe-Cu alloys within statistical errors of simulations and the scatter of experimental results. Comparison of simulation results with experiments for some multicomponent Fe-Cu-based alloys allows making certain conclusions about the influence of alloying elements in these alloys on the precipitation kinetics at different stages of evolution.     

Voigt-based methods for arbitrary shape quadrupole splitting distributions (QSD's) applied to quasi-crystals

It is now well established that, as with amorphous alloys, the Mössbauer spectra of icosahedral alloys exhibit QSD's. This is evidence for intrinsic local disorder present in icosahedral structures. The shapes of the QSD's have not been unambiguously established and, as a result, it has been difficult to link these shapes with other physical properties of these novel and complex alloys. We apply a recently developed arbitrary shape static hyperfine parameter distribution method to the case of a new icosahedral alloy series: Al₆₅Cu₂₀Cr_15-xFe_x. The method proves to be a powerful and useful tool in this application. It enables us to evaluate several robust features and to identify other features that are subject to large uncertainties due to extreme sensitivity to details.     

Anisotropic lattice distortions in random alloys from first-principles theory

Within the framework of the exact muffin-tin orbitals (EMTO) theory we have developed a new method to calculate the total energy for random substitutional alloys. The problem of disorder is treated within the coherent potential approximation (CPA), and the total energy is obtained using the full charge density (FCD) technique. The FCD-EMTO-CPA method is suitable for determination of energy changes due to anisotropic lattice distortions in random alloys. In particular, we calculate the elastic constants of the Cu-rich face centered cubic Cu-Zn alloys ( alpha-brass) and optimize the c/a ratio for the hexagonal Zn-rich alloys for both the epsilon and eta phases.     

First-principles calculations of structural and electronic properties of Tl_xGa_(1-x)As alloys

The zincblende ternary alloys Tl_xGa_(1-x) As(0 x 1) are studied by numerical analysis based on the plane wave pseudopotential method within the density functional theory and the local density approximation. To model the alloys,16-atom supercells with the 2 × 2 × 2 dimensions are used and the dependency of the lattice parameter, bulk modulus,electronic structure, energy band gap, and optical bowing on the concentration x are analyzed. The results indicate that the ternary Tl_xGa_(1-x) As alloys have an average band gap bowing parameter of 4.48 eV for semiconductor alloys and 2.412 eV for semimetals. It is found that the band gap bowing strongly depends on composition and alloying a small Tl content with GaAs produces important modifications in the band structures of the alloys.     

GexSi1-x合金中的键长及其对电子能带结构的影响

采用经验紧束缚理论，以类似闪锌矿结构的晶体模型模拟Ge_xSi_1-x合金，根据总能最小原则计算了Ge_xSi_1-x合金中的键长及点阵常数．同时以紧束缚方法计算了原子位置发生弛豫前后的电子能带结构，并与虚晶近似下的计算结果进行了比较．计算结果表明，Ge_xSi_1-x合金中的键长基本上与合金组分无关，各自接近于Ｇｅ，Ｓｉ晶体中的键长，与广延ｘ射线吸收精细结构（ＥＸＡＦＳ）测量结果符合得 关键词：     

On accuracy of different cluster models used in describing ordering phase transitions in fcc alloys

A general formulation of cluster methods applied to calculations of thermodynamic quantities of alloys in terms of renormalizing fields describing interaction between a cluster and its environment is given. We have shown that the well-known cluster variation method and the cluster field method, which was suggested earlier, are special cases of our approach. These methods have been used in calculations of phase diagrams of fcc alloys with L1₂ and L1₀ ordering transitions with several realistic interaction models. It turns out that, for all these models, the simple tetrahedron version of the cluster field method suggested in this paper describes the phase diagrams almost as accurately as more complicated cluster variation techniques. Possible applications of the tetrahedron version of the cluster field method to inhomogeneous states and kinetics of phase transitions in fcc alloys are discussed. Zh. éksp. Teor. Fiz. 115, 158–179 (January 1999)     

Concentration dependence of Fe segregation near the surface in V-Fe alloys after V+ ion irradiation

The effect of Fe segregation near the free surface of V model alloys containing 2, 5, or 7 at % Fe is investigated by x-ray photoelectron spectroscopy. Segregation is induced by 50-keV V⁺ ion irradiation at a temperature of 30–40°C with fluences ranging from 10¹⁹ to 10²¹. Young’s moduli in these alloys are measured by the torsion pendulum method. The degree of Fe segregation is estimated, and its dependence on the irradiation dose and iron concentration in these alloys is analyzed. Correlation is found between the behavior of Young’s moduli and the degree of iron surface segregation as functions of the Fe concentration in the alloys.     

Scaling analysis of the magnetocaloric effect in Gd5Si2Ge1.9X0.1 (X=Al, Cu, Ga, Mn, Fe, Co)

The field dependence of the magnetic entropy change has been studied for a series of doped Gd₅Si₂Ge₂ alloys, which possess a magnetic phase transition that is either entirely second order or a combination of primarily second-order mixed to a very minor degree with a first-order transition arising from a magneto-structural phase change. By analyzing the field scaling of the refrigerant capacity as well as of the reference temperatures used for constructing a universal scaling curve, a procedure for estimating the values of the critical exponents for the alloys was developed. For the cases where the transition is entirely second order, the results obtained from this procedure are comparable to the values obtained from the Kouvel–Fisher method. For the case of Fe-doped alloys which partially possess a first-order phase change, the Kouvel–Fisher method is inapplicable. However, their critical exponents determined by our developed procedure can be used to estimate the Curie temperature of the orthorhombic majority phase.     

Nucleation of deformation twins in nanocrystalline fcc alloys

An earlier dislocation model for predicting the grain size effect on deformation twinning in nanocrystalline (nc) face-centred-cubic (fcc) metals has been found valid for pure metals but problematic for alloys. The problem arises from the assumption that the stacking-fault energy (γ_SF) is twice the coherent twin-boundary energy (γ_fcc), which is approximately correct for pure fcc metals, but not for alloys. Here we developed a modified dislocation model to explain the deformation twinning nucleation in fcc alloy systems, where γ_SF ≠ 2γ_twin. This model can explain the differences in the formations of deformation twins in pure metals and alloys, which is significant in low stacking-fault energy alloys. We also describe the procedure to calculate the optimum grain size for twinning in alloy systems and present a method to estimate γ_twin.     

The origins of phase stability in ordered and disordered Ni-Pt alloys

Using the atomic-sphere approximation formulation of the Korringa-Kohn-Rostoker coherent potential approximation (KKR CPA) method, we have studied the effects of relativity on the electronic structure of ordered and substitutionally disordered Ni-Pt alloys. The inclusion of mass-velocity and Darwin terms are found to be essential for describing the experimentally observed ground-state properties. For the stability of disordered Ni-Pt alloys we find that, in addition to relativity, the minimization of charge-transfer effects are important. We also find that the treatment of ordering tendencies based on the band energy term alone is not sufficiently accurate for alloys with charge-transfer effects. Further analysis, in terms of basis functions, densities of states and non-spherically averaged charge densities, indicate the importance ofs- andd-electrons of Pt for the stability of both ordered and disordered Ni-Pt alloys.     

Property-composition correlations and short-range order in bcc and fcc binary solid solutions

The properties of an ordering solid solution are substantially dependent on the long-range and short-range order parameters; a method is given for representing a binary solid solution as a set of cluster components whose short-range order parameters are incorporated out to the second coordination sphere. The scheme is illustrated via the lattice parameter and Debye temperature for Ti-Mo alloys (bcc lattice) and via the magnetic moment of Fe-Pd alloys (fcc lattice). The method allows one to describe nonmonotonic composition-property relationships and to define the values of properties for ordered structures.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 18–24, February 1978.