Comparative Study of Bond Orientational Order and Pair Analysisin Describing Local Symmetry in Liquid Metals

The similarity and difference between the bond orientational order and pair analysis methods have been first identified with the help of molecular dynamics simulation and the steepest descent minimization of potentials techniques. The binary liquid K-Rb alloys are chosen as an example.Results show that the bond orientational order parameters may be a relevant direct measurement on the perfection of an icosahedron. The PA formula (1551 bonded pairs) is, however, responsible for the total number of various icosahedra. In addition, it is found that pair analysis approach is more sensible to local structure than that for bond orientational order.     

Liquid immiscibility in ternary metallic alloys

Abstract

Two models have been proposed which can be used to predict the occurrence of liquid immiscibility in ternary metallic alloys. One model, derived from an earlier proposal by Axon for predicting liquid immiscibility in binary alloys, was based on differences in the atomic sizes and melting points of the metals. The second model, based on a proposal made by Mott (from the binary metallic liquid immiscibility rule of Hildebrand and Scott), utilises solubility parameters and electronegativities as the determinants. The models were tested against the known behaviour of 27 metallic ternary systems which show liquid immiscibility and 36 systems which show either complete liquid miscibility or only a small region of liquid immiscibility confined to a region adjacent to an immiscible binary system. Both models showed good predictive ability when applied to the first group of 27 systems, but the first model proved to be superior when applied to the 36 miscible liquid systems. The behaviour of the liquid phase was always predicted correctly by one or other of the models in all the ternary systems examined.

MST/1412     

金属间化合物与大块玻璃合金的形成

综述了大块玻璃合金液态结构,结晶动力学行为,玻璃态形成机制以及其与金属间化合的关系,应用二元合金液态结构的理论模型,分析与推测,分析与推测大块玻璃合金的液相结构,解释了大块玻璃合金的特殊热力学性质和大块玻璃态形成机制,介绍了Ｚｒ基大块玻璃合金的一些研究结果。     

Validation of an effective oxidation pressure model for liquid binary alloys

Dynamic surface-tension measurements using the sessile drop method and acquisition times of a few seconds make it possible to study the evolution of the surface of molten metals and alloys and so reliably validate the predictive models of the interactions between pure liquid metals and an oxidizing atmosphere, in both an inert gas carrier and in a vacuum. The presence of active oxidation contributes to maintaining surface cleanness and then strongly affects the shape of the boundary separating oxidation and de-oxidation regimes. Recently the general physical–mathematical analysis we developed for pure liquid metals has been extended to liquid binary alloys and their oxides. In this work we present the experimental results of tests on some binary alloys chosen as test systems to try to obtain a preliminary validation of the extended model. The theoretical results obtained, indicating that the behaviour of the alloy towards oxidation tends to be similar to that of the less oxidizable component, have thus been confirmed experimentally.     

Two-Particle Separation in the Point Vortex Gas Model of Superfluid Turbulence

Within the framework of the self-consistent scheme proposed by Singwi, Tosi, Land and Sjölander (STLS) for an interacting system we study the properties of superfluid liquid ⁴He. By employing the Aziz potential (HFD-B) as the interaction potential between helium atoms, we have calculated the static structure factor, the pair-correlation function, the elementary excitation spectrum and the effective two-body interaction as a function of wave-vector, for different densities. Our results show considerable improvement over the Ng–Singwi’s model potential of a hard core plus an attractive tail and are comparable with experimental data. We have compared our results with experimental data and with the results of some theoretical models. Agreement between our results and the experimental data for the static structure factor for the small k values is fairly good.     

The effect of added oversized elements on the microstructure of binary alloy nanoparticles

Molecular dynamics simulations are used to investigate the microstructures of Cu-Ni nanoparticles with different concentrations of oversized atoms added to them. A many body second moment tight binding approximation potential is adopted to model the interatomic interactions. The Honeycutt-Anderson (HA) pair analysis technique is adopted to analyse in detail the transformation between local structures at different temperatures. From the simulation results, at temperatures higher than the melting point, the nanoparticles are in a liquid state and an icosahedral local structure is most frequently found inside the nanoparticles. At temperatures beneath the melting point, the fraction of FCC local structure increases with decreasing concentrations of the larger size atoms, whereas a larger fraction of amorphous structure still remains in the solid state for higher concentrations of oversized atoms. This is because the effects of distortion and misfit are more significant for a nanoparticle having a higher concentration of oversized atoms.     

Viscosity of Al–Cu liquid alloys: measurement and thermodynamic description

In the present work a high temperature oscillating cup viscometer has been used to measure the viscosities of liquid binary Al–Cu alloys. The dependence of viscosity on temperature is well described by the Arrhenius law. For constant temperature, the viscosity as a function of copper concentration exhibits a maximum at a mole fraction x _Cu = 0.7. This might be due to a pronounced chemical short range order in the liquid phase at this composition. As the comparison of existing phenomenological models describing viscosity as a function of composition to the experimental data is unsatisfactory, a new model for the viscosity has been developed within this work based only on a few assumptions and using the enthalpy of mixing as input parameter which is easily accessible. The agreement between model calculation and experimental data is excellent.     

Short-range order in some alloys of the Ge-As-Te semiconducting glassy system by X-ray diffraction

A study of short-range order in amorphous semiconducting alloys of the Ge-As-Te system has been carried out by X-ray diffraction. The different hypotheses on germanium coordination have been taken into account, following the different criteria cited in the literature for binary Ge-Te alloys. The result of this study shows the possibility of structural units based on tetra-or tricoordinated germanium atoms, which bonded together give way to short-range order in these alloys. !t has also been found that, for these alloys, germanium dicoordination is not compatible with the data obtained experimentally.     

Microstructural studies of NiAl-based model alloys and commercial coatings after isothermal oxidation

Abstract

Four commercial aluminide diffusion coatings (one Pt-free) and four polycrystalline NiAl model materials, isothermally oxidised in laboratory air at 1050°C for 1 hour, have been investigated. The coatings were deposited on a single crystal Ni-based superalloy, CMSX-4. Two of the model materials are binary alloys containing different Al amounts. The two other have the same Ni/Al ratio as the binary alloys but were alloyed with 4 at% Pt. In order to control the surface roughness, all materials were polished prior to oxidation. Microstructural studies have been conducted using X-ray diffraction and scanning electron microscopy. To get a deeper insight on the platinum influence on the oxide structure, two commercial coatings (one Pt free and the other modified with Pt) were also investigated using transmission electron microscopy. The results obtained are discussed according to the influence of Al and Pt contents in the materials.     

Thermodynamic–kinetic model for the simulation of solidification in binary copper alloys and calculation of thermophysical properties

An earlier developed thermodynamic–kinetic solidification model for binary copper alloys is extended to take into account the formation of the bcc phase via the peritectic transformation and the formation of binary compounds from the fcc phase. Also the eutectic and eutectoid transformations are simulated but only approximately, by modeling the movement of the fcc/eutectic and fcc/eutectoid interfaces due to the diffusion kinetics of the fcc phase only. The new model can handle binary copper alloys containing solutes Ag, Al, Cr, Fe, Mg, Mn, Ni, P, Si, Sn, Te, Ti, Zn and Zr. Depending on the alloy composition, cooling rate and dendrite arm spacing, the model determines the fractions and compositions of the phases (liquid, fcc, bcc, compounds) and calculates thermophysical material properties (enthalpy, specific heat, thermal conductivity, density and liquid viscosity), needed in heat transfer models, from the liquid state down to room temperature. The model is applied to Cu–Sn and Cu–Zn alloys but also to some other binary alloys to show the effect of cooling on the phases formed. Depending on the alloy system, the solidification structures obtained after real cooling processes are shown to be quite different from those estimated from phase diagrams.     

Overview of metallic materials for heat exchangers for ocean thermal energy conversion systems

Candidate materials for use in ocean thermal energy conversion systems, OTECS, heat exchangers include aluminium, Cu-Ni, stainless steel and titanium alloys. These are considered in this review, and their advantages and disadvantages are highlighted and discussed. Aluminium alloys have shortcomings for the anticipated long life span of OTEC heat exchangers; however they may still offer an economic alternative in the form of short-life, disposable units of low initial capital cost. The long-term effects of exposure to ammonia and to erosion pose questions about the suitability of Cu-Ni alloys. Although stainless steel alloys provide a strong challenge for OTECS use, titanium exhibits better seawater performance, has good fabricability, and has an excellent service history in marine environments. These factors, together with current and projected developments promise major savings in materials usage, reduced OTECS structure sizes and increased efficiencies consequent on the use of titanium and its alloys.     

Bound-state pairing singularities in the 3He Galitskii-Feynman T-matrix: Temperature dependence

The temperature-dependent Galitskii-Feynman T-matrix, which sums the two-body scattering series, allowing any number of pairs of either particles outside or holes inside the Fermi sea in intermediate states, for a two-body potential appropriate to liquid ³He, is shown to exhibit a bound-state singularity. The two-body binding energy within the ³He background is shown to be highly sensitive to both the temperature and the density of the system. A critical density below which the binding disappears is observed to be a function of the temperature and of the total momentum of the interacting pair. Detailed numerical computations of this structure are presented using the modified Frost-Musulin potential. To our knowledge, these computations represent the first attempt at a temperature-dependent many-body calculation based on a realistic ³He potential. Possible consequences are drawn for more detailed and realistic calculations of the properties of liquid ³He.     

Rapid solidification of zirconium-based alloys

Zr based metal-metal binary and ternary alloys can be obtained in the amorphous state in very wide composition ranges. Several eutectic reactions and intermetallic compounds are present in these alloy systems which provide opportunities for examining the validity of different theories on glass formation. The amorphous phases in these alloys decompose by a variety of crystallization mechanisms. Instances of polymorphic, primary and eutectic crystallization have been encountered in these glasses. Zr-based metallic glasses possess excellent corrosion resistance and mechanical properties. In several studies their properties have been compared with that of their crystalline counterparts and interesting differences have emerged. In the solute lean Zr-based alloys very large freezing ranges are available for studying the liquid to solid transformation. It has been possible to study the formation of some of the low temperature phases directly from the liquid. This paper describes some of the aforementationed studies carried out on Zr-based amorphous and crystalline alloys.     

Ab Initio Molecular-Dynamics Simulation Liquid and Amorphous Al_94-xNi₆La_x (x=3-9) Alloys

Ab initio molecular-dynamics simulations have been used to investigate the liquid and amorphous Al94-xNi6Lax (x=3-9) alloys. Through calculating the pair distribution functions and partial coordination numbers, the structure and properties of these alloys are researched, which will help the design bulk metallic glass. The concentration of La atoms can affect the short-range order of Al94-xNi6Lax alloys, which is also studied in this calculation result.     

Density of Liquid Ni-Mo Alloys Measured by a Modified Sessile Drop Method

The density of liquid binary Ni-Mo alloys with molybdenum concentration from 0 to 20% (mass fraction) wasmeasured by a modified sessile drop method. It has been found that the density of the liquid Ni-Mo alloys decreaseswith increasing temperature, but increases with the increase of molybdenum concentration in the alloys. The molarvolume of liquid Ni-Mo binary alloys increases with the increase of temperature and molybdenum concentration. Thepartial molar volume of molybdenum in Ni-Mo binary alloy has been approximately calculated as [13.18-2.65×10~(-3)T+(-47.94+3.10×10~(-2)T)×10~(-2)X_(Mo)]×10~(-6)m~3·mol~(-1). The molar volume of Ni-Mo alloy determined inthe present work shows a negative deviation from the ideal linear mixing molar volume.     

Melting Enthalpy and Entropy of Binary Systems of Alloys of 3d Transition VII and VIII Metals and Carbon: Thermodynamic Calculation and Experimental Study

Thermodynamic calculations have been performed of the melting enthalpy and entropy of the Co–C, Ni–C, and Fe–C binary system alloys having eutectic compositions and of alloys whose compositions correspond to the minimum temperatures of the Mn–C, Fe–Co, Fe–Ni, and Mn–Ni melting diagrams. Heat capacities of one- and two-phase alloys of investigated systems have also been calculated as a function of temperature. In order to express a dependence of the Gibbs energy (G) of the liquid phase on the composition, the model of regular solutions has been used. The relation between the G of interstitial solid solutions and concentration was described by the two-sublattice model. The values have been experimentally verified using a high temperature calorimeter. These results have shown that the discrepancy between experimental and calculated results is within the calorimetric measurement uncertainty and does not exceed 10%.     

Effect of Microstructure and Sulfide on Corrosion of Cu-Ni Alloys in Seawater

1. IntroductionThe Cu-Ni alloys become an important category ofmaterials in marine engineering since they are highlyresistant to corrosion caused by seawater and biofouling. These alloys have been widely applied in marineengineering, such ajs condensing tubes in various ships,heat exchangers for electric plants near seashore, andtubes in chlorinating systems. However, early failureand even leakage can occasionally occur during theirservice though Cu-Ni alloys come to practical use fordecades…