Thermo-physical properties of ternary Al–Cu–Fe alloy in liquid state

ABSTRACT

The thermodynamic and surface properties of the ternary Al–Cu–Fe alloy in the liquid state have been computed using different models. The thermodynamic properties, such as activity and excess free energy of mixing and the surface properties, such as surface tension have been calculated. The temperature dependence of activity and surface concentration of the components of the ternary Al–Cu–Fe alloy in fixed proportion of any two components have also been calculated. The surface tension of the alloy with respect to the change in temperature in the range 1823–2073?K has also been studied.     

Surface and transport properties of Au-In liquid alloys

Thermodynamic quantities on Au-In liquid alloys have been used as the input data for the interaction parameter calculations in the framework of the complex formation model (CFM). Once the interaction energies are computed the surface (surface tension and surface composition) and transport properties (chemical diffusion and viscosity) as well as the microscopic functions (concentration fluctuations in the long-wavelength limit and chemical short-range order parameter) have been calculated. The concentration and temperature dependent surface tension values have been compared with our new set of experimental data, obtained by the large drop method in the temperature range of T = 1273-1493 K. The anomalous change of surface tension for some alloy compositions may be attributed to a retention of order in the Au-In melts which is similar to the atomic arrangement in solid Au-In.     

Determination of the surface composition of palladium-nickel alloy film catalysts using Auger electron spectroscopy

Palladium-nickel films evaporated in UHV on room temperature substrates form alloys of good bulk homogeneity as indicated by X-ray diffraction. The average composition of the outermost 3 to 5 atom layers has been determined from the intensities of the high energy 848 eV nickel and 330 eV palladium Auger electrons. This average composition is in close agreement with the bulk composition determined by X-ray diffraction, X-ray fluorescence and atomic absorption spectrophotometry. If the nickel concentration is determined from the intensity of the low energy 61 eV nickel Auger electrons, when the analysis refers more critically to the first 1 to 3 atom layers, then a surface enrichment of palladium is indicated for all alloy compositions. From the decrease in the relative intensities of the low energy and high energy nickel Auger electrons with increasing palladium concentration it may be deduced that the enrichment of palladium in the first atom layer is higher than in the second and third layers and that a complete monolayer of palladium is formed for bulk concentrations of 65 at% or more. The experimental observations are in qualitative agreement with theoretical predictions of surface composition from bulk thermodynamic data. The palladium-nickel alloys form a range of surface compositions which can be controlled by changing the bulk composition and which are useful for studying catalytic activity as a function of composition. The alloy films are stable under electron irradiation in the AES analysis in UHV but air exposed films analysed in a residual pressure of 1.3 microPa water vapour show a decrease in palladium surface concentration on irradiation indicating a diffusion of nickel to the surface to form an overlayer of nickel oxide.     

Diagrammatic method for the calculation of short-range order effects in an ordering binary alloy

A statistical thermodynamic theory of short-range order effects in an ordering binary alloy with an arbitrary composition and an arbitrary radius of the interatomic interaction has been constructed in the approximation of ring diagrams for the free energy. For concreteness, a solid solution, which orders according to the Cu₃Au type, has been considered. A formula has been obtained for the Fourier components of the correlation parameters, which is valid for a wide range of variation of concentration and temperature of the alloy. Correlation parameters of the system Cu₃Au with a stoichiometric composition have been calculated according to this formula for given values of the displacement energy for various coordination spheres at several values of temperature and the interaction radius.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 62–67, August, 1988.The authors thank D. M. Sedrakyan and É. M. Kazaryan for a discussion of the results.     

Structure,microhardness, and strength of a directionally crystallized Al-Ge alloy

The structure, microhardness, and strength of binary directionally crystallized aluminum alloys with 35, 43, 53, 57, and 64 wt % germanium have been investigated. It has been shown that the eutectic microhardness is constant in the composition region under study. The microstrength of primary crystals of the solid solution of germanium in aluminum with the dendrite structure increases with increasing germanium concentration. However, the difference in the microhardnesses of the eutectic and dendrites, which was determined for each of compositions on the same specimen, does not exceed the measurement error. It has been assumed that the change in the strength of the alloy having the composition in the hypoeutectic region is determined by the redistribution of the volume fractions of the eutectic (α-Al and eutectic germanium) and the domains of primary crystals of the solid solution. This dependence can be described by the mixture rule. Above the eutectic composition, the alloy decomposes in a brittle manner; its strength is likely dependent not only on the content of the components, but also on the form and orientation of primary germanium crystals.     

Phase segregation and exchange biasing in TbFeCo films with perpendicular anisotropy

A TbFeCo film was deposited by DC magnetron sputtering and studied by transmission electron microscopy, polar and longitudinal magneto-optical Kerr effect, and magnetometry measurements. Transmission electron microscopy has shown the existence of lateral compositional inhomogeneity. Magneto-optical measurements have shown that the initial layer at the bottom consists of only magnetic perpendicular component and the top surface layer has a compositional inhomogeneity and consists of in-plane components and perpendicular one. The perpendicular components in the bottom and the surface layers have identical composition. Two in-plane components have been shown by magnetometry measurements. It is shown that phase segregation exists in the TbFeCo film and possible form of compositional inhomogeneity has been discussed. The two in-plane components are exchange coupled with a magnetization off-alignment of 35°. For the soft in-plane component, the in-plane and out-of-plane angular dependence of the exchange biasing is similar to those of the conventional one. Within temperatures from 100 to 300 K, the exchange field and the coercivity are both linear functions of temperature.     

Orbital free ab initio simulation of surface freezing in a dilute Ga-Tl alloy

We have used the orbital-free ab initio molecular dynamics method to study the phenomenon of surface freezing in a liquid Ga-Tl alloy (nominal Tl concentration of 0.14). Several thermodynamic states are considered from a high temperature of 675 K to a low temperature of 350 K. At the higher temperature the Tl atoms segregate to the surface and form a liquid layer. Upon cooling the layer eventually crystallizes into a close-packed hexagonal solid layer. These results agree with experiments for a dilute liquid alloy of Tl in Ga, except for some overestimation of the surface freezing temperature. X-ray reflectivity data are also well reproduced. The wavelength of the oscillatory density profile coincides with that deduced from experiments and with that obtained in earlier computer simulation results. We also analyze the consequences of surface freezing on the structure of the liquid in contact with the solid layer, finding no influence whatsoever, as the structure basically coincides with the bulk liquid one.     

Surface segregation in a Ni-1 at% Pd alloy

The surface composition of a Ni-1 at% Pd alloy has been determined by Auger electron spectroscopy after equilibration of the alloy at several temperatures in the range 550 to 800°C. Moderate segregation of Pd to the surface of the alloy has been observed, with concentrations of up to ~～30 at% being detected at the surface after equilibration at 550°C. The heat of segregation (or adsorption) of Pd has been found to agree reasonably well with the predictions of a previously developed unified model of surface segregation in alloys. The kinetics of Pd segregation to the surface have also been measured between 500 and 700°C. Analysis of the kinetic data yields apparent diffusivities of Pd through the alloy that are suggestive of a dominant grain boundary transport mechanism.     

The theory of equilibrium antiphase boundaries in ordered solid solutions of the AuCu3 type

The Gorsky-Bragg-Williams approximation gives expressions which determine the equilibrium values of the long-range-order parameter and the concentrations of components in the vicinity of the antiphase boundary 1/2 (110) {111} in an L1₂ super-structure of stoichiometric composition AB₃. On the assumption that the changes in the alloy due to the presence of an antiphase boundary are distributed over a great number of planes on both sides of the boundary, the long-range-order parameter and the concentration of components in these planes have been calculated. It is found that the long-range-order parameter at the antiphase boundary is considerably lower than it is in the matrix over a wide temperature range. The concentration of the components at the antiphase boundary under conditions of thermodynamic equilibrium is somewhat lower than the mean concentration in the alloy.Estimates are made of the critical stress for the start of superdislocations with equilibrium antiphase boundaries, the equilibrium width of the superdislocations, and the defect in the elastic modulus due to the reversible movement of the superparticle dislocations.     

NMR study of pure and doped β-LiAl

The NMR spin-lattice relaxation time, T_I, has been measured as a function of temperature for both ⁷Li and ²⁷Al in pure and doped β-LiAl alloys. Compositions with ⁷Li concentration in the range 48.3–54.5% and doping in the form Li₅₀Al_50?xM_x, where M = Ag or In, were studied. The relaxation rates T₁^?1 for the ²⁷Li and the ²⁷Al resonances were found to be peaked functions of temperature with the maxima for ⁷Li appearing at composition dependent temperatures. The ²⁷Al maxima always appeared at a lower temperature, independent of composition, and the ²⁷Al maximum relaxation rate was a strong function of composition in contrast with ⁷Li where the maximum rate was only weakly dependent on composition. The principle relaxation mechanisms are identified as dipole-dipole coupling in the ⁷Li and coupling of the ²⁷Al quadrupole moment to electric field gradients. The temperature dependence of these rates is attributed to the thermally activated diffusion of vacancies of a non-thermal origin in the Li sub-lattice. These vacancies are also responsible for the fluctuating electric field gradients. The results have been analyzed to give the Li diffusion coefficients with associated activation energies and estimates of the vacancy concentration as functions of alloy composition.     

Local order in the bulk and surface of liquid K-Pb alloy

Ordering in the bulk and surface of liquid K-Pb alloy has been studied throughout the bulk concentration range. At low K concentrations, the liquid alloy is strongly ordered both in the bulk and on the surface. The existence of the zintl structure in the liquid alloy is predicted to be predominant below 0.6 atomic fraction of K. While above this concentration a complex of the form K₃Pb₂ is suggested to be present. PACS  28.52.Fa; 65.70.+y; 61.20.-p; 61.82.Bg     

Thermodynamic and surface properties of Sb-Sn and In-Sn liquid alloys

The thermodynamic properties of Sb-Sn and In-Sn liquid alloys have been studied using the quasi-chemical model for compound forming binary alloys and that for simple regular alloys. The concentration fluctuation S _cc(0) and the Warren-Cowley short-range order parameter (α ₁) were determined for the whole concentration range at a temperature of 770 K. The surface tensions of these liquid alloys were determined for the whole concentration range by using energetics determined from thermodynamic calculations. In all calculations, In-Sn manifested properties very close to alloys of ideal mixing, while Sb-Sn showed properties that are asymmetric about equiatomic composition. Our results suggest that a weak complex of the form SbSn₂ could be present in the Sb-Sn alloy at a temperature of about 770 K.     

Phase separation in Na–K liquid alloy

The quasi-chemical expression for weakly interacting binary alloy has been applied to obtain energy parameters and their temperature derivatives for Na–K liquid alloy at 384?K. These energy parameters have then been used to calculate thermodynamic functions, such as free energy of mixing, heat of mixing, entropy of mixing and microscopic functions, such as concentration fluctuation in long wavelength limit, Warren–Cowley short-range order parameter, ratio of mutual and self-diffusion coefficients. The analysis reveals that the energy parameters are temperature dependent and the Na–K liquid alloy at 384?K is a weakly interacting homocoordination system. The observed thermodynamic properties of Na–K alloy in molten state have successfully been explained by assuming Na₂K complex on the basis of the quasi-chemical formalism for a weakly interacting system.     

Theory of concentrational profile at the surface of a decomposing binary alloy

In the self-consistent-field approximation, a statistical theory is constructed for the atomic distribution in the near-surface region of a binary alloy which is able to disintegrate from the atomically smooth free surface of the crystal. The form of the surface-segregation functions of the components is found, in closed form, taking the relaxation of the coordination energies and the atomicinteraction energies at the surface into account. The possibility of the existence of metastable nonmonotonic concentrational profiles at the crystal surface is demonstrated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 51–56, December, 1981.     

Tests of surface segregation theory: I. The Ni-5Pt alloy

The surface composition of polycrystalline samples of the binary metallic alloy Ni-5Pt (at%) has been studied as a function of temperature. This alloy is of particular interest as it lies in a relatively unexplored quadrant of a plot of binary alloy surface tension ratios versus atomic size ratios. Such plots have been used to predict which component of a binary alloy will surface segregate and solute (Pt) segregation is anticipated in the present case. Mechanically polished specimens were purged of bulk impurities by prolonged heating and exposure to oxygen. Quantitative Auger data, collected from clean equilibrated surfaces after rapid cooling from elevated temperatures in the range 1050 to 1540 K, showed Pt surface enrichment compared to the bulk composition. The surface composition was temperature dependent with a heat of segregation in the range −10 to −30 kJ mol⁻¹. The expected heat of segregation for this alloy was calculated by assuming that segregation was driven solely by the lowering of surface free energy and the relief of bulk lattice strain. If the strain term is neglected, Ni surface enrichement is predicted but, when both terms are included, Pt enrichment with a heat of segregation of −14 kJ mol⁻¹ is predicted for the Ni-5Pt(100) surface. This value lies within the limits of the experimental data for polycrystalline Ni-5Pt surfaces and it is concluded that the simple theory adequately accounts for the main segregation mechanisms in this type of alloy.     

Observation of exciton localization in PbSrSe thin films grown by molecular beam epitaxy

Detailed temperature and excitation-intensity dependence of radiative recombination characteristics in PbSrSe thin films grown by molecular beam epitaxy has investigated. For the first time, localized excitonic structure due to the alloy disorder has been observed at low temperature, and is gradually delocalized as wave-vector-nonconserving band-to-band transitions at high temperature in PbSrSe with low Sr composition. Furthermore, we are able to observe the evolution of the exciton localization, due to the strong alloy fluctuations and lattice distortions, into either free or trapped (strongly localized) excitons at low temperature by studying a PbSrSe thin film with the Sr composition as high as 0.276. The lattice deformation has been shown to play a key role in the composition-dependent excitonic peak broadening in PbSrSe at low temperature. This gives clear evidence for the observation of excitonic effects in lead salts due to their low carrier concentration and enhanced exciton binding energy. The results are discussed in the framework of theories taking into account the carrier migration and exciton trapping due to the alloy fluctuations and lattice vibrations . PACS 78.55.Hx; 78.66.Li     

Surface Diffusion Induced Growth of Solid Solution over a Free Crystal Surface

It has been shown that interdiffusion along a free vicinal crystal surface can lead, at comparably low temperatures, to layered growth of solid solution over the surface without the involvement of bulk diffusion. The alloy concentration distribution along the surface, as well as the normal rate of solution growth has been calculated. The formation of a layer of the solid solution has been experimentally observed on a vicinal (111) surface of Ni single crystal as a result of surface interdiffusion between Ni and a thin film of Au deposited on a part of the Ni surface. The surface diffusion coefficients, as well as other parameters responsible for the exchange rate between the adatoms and kinks of elementary steps have been measured in the temperature range 550–700°C.     

Oscillations in the compositional depth profile of Cu/Ni alloys: A study by UPS

The composition in the surface region of a (110) oriented single crystal of Cu/Ni alloy was studied by ultraviolet photoemission spectroscopy (UPS) in the energy range hv = 32–240 eV. The data indicate a surface enrichment of Cu and that the composition, as a function of depth below the surface, does not approach the bulk value in a monotonic manner but shows at least one oscillation. The depth sampled by the UPS experiment was varied by changing the photon energy and thus the escape length of the photoemitted electrons. Two surfaces of a single bulk crystal (bulk atomic composition 90% Ni and 10% Cu) were studied. One was the high temperature equilibrium surface (AES measurements yielded a surface composition of ~65% Cu and 35% Ni); the second surface was higher in Ni concentration (AES measurements gave ~65% Ni and 35% Cu) and was obtained by annealing at a lower temperature. Theories based on the quasi-chemical and regular solution models predict a surface enrichment of Cu for the alloy, but do not predict any oscillation in composition below the surface.     

Spontaneous alloying of tin atoms into nanometer-sized gold clusters and phase stability in the resultant alloy clusters

Alloying behavior and phase stability has been studied in situ by transmission electron microscopy using clusters in the Au-Sn system. When tin atoms are vapor-deposited onto nm-sized gold clusters, rapid dissolution of tin atoms into gold clusters takes place and as a result Au-rich solid solution, amorphous-like Au-Sn alloy and AuSn compound clusters are formed depending upon the concentration of tin. The remarkable enhancement of solubility has been observed in Au-rich solid solution and AuSn compound. It becomes more difficult to form two phases in the interior of individual clusters even if the composition of alloy clusters falls in the two-phase region in the phase diagram for the bulk alloy and as a result amorphous-like phase is stabilized in nm-sized Au-Sn alloy clusters. Received 2 August 1999 and Received in final form 8 November 1999