Liquid alloy

Using the symbols introduced in Section 1.1, an expression for rrLV can be derived from the regular solution model (Guggenheim 1945)  

A negative value of adsorption energy leads to a high value of the enrichment factor Yb/Xb and to a sharp decrease of v with the first additions of B in A. Equation (4.8) shows that factors favoring adsorption of B are (i) a low value of ofv compared to and (ii) weak A-B interactions i.e., a tendency of A-B alloy to form a miscibility gap (A 0). 

It should be noted that the regular solution model has been extended to ternary alloys and applied successfully for systems for which interactions cause positive deviations from ideality (Joud et al. 1974). 

These treatments suppose the surface molar areas of A and B atoms to be equal. However, for alloys with A 0, the low rLv component, B, can have a molar volume much greater than that of A (up to 100%). Typical examples are the Cu-Pb, Cu-Bi and Al-Pb systems, for which the experimental decreases in rxLV caused by adding small quantities of B are significantly more than those predicted by the regular solution model. These differences have been attributed to the fact that not all the sites of the quasicrystalline lattice on which A and B atoms are distributed are equivalent (Goumiri et al. 1979). 

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Binary Alloys. Aluminum-rich binary phase diagrams show tliree types of reaction between liquid alloy, aluminum solid solution, and otlier phases eutectic, peritectic, and monotectic. Table 16 gives representative data for reactions in tlie systems Al—Al. Diagrams are shown in Figures 10—19. Compilations of phase diagrams may be found in reference 41. 

Fig. 6. Section of the Fe—Cr—Ni diagram at 8% nickel (5) where a, y, and represent phases and L = liquid alloy.

Figure 6.3 The iron-carbon phase diagram showing the alternative production of iron and cementite from the liquid alloy, which occurs in practice, to the equilibrium production of graphite...

Here we present first results of ab-initio LDA-MD studies of crystalline and liquid K-Sb alloys[51]. The liquid alloys are modelled by 64-atom ensembles of appropriate... 

AB INITIO MOLECULAR DYNAMICS SIMULATIONS OF LIQUID ALLOYS ... 

Many liquid alloys, in particular, the alkali-group IV alloys, exhibit (Zintl) anion clustering and show strong effects of compound formation. A typical example of such Zintl systems are sodium-tin alloys. In the solid NaSn crystal the Zintl anions Sn appear [1]. An interesting question is the stability of these anions in the liquid. Furthermore, the electrical conductivity of these alloys shows a strong dependence on composition [2] For the limiting (sodium-rich or tin-rich) cases a metallic (small) conductivity appears, but for the nearly equimolar compositions a semi-metallic behavior - with a considerably smaller conductivity - is observed. 

The experiments [2] yielded also a considerable temperature dependence of the conductivities - including solid phases too [3]. Hence, we are going to consider now the T dependence of the conductivity for the liquid alloys. 

If the major constituents of a solid alloy in contact with a liquid alloy are highly soluble in the latter without formation of compounds, progressive attack by solution is to be expected. If, on the other hand, a stable inter-metallic compound is formed, having a melting point above the temperature of reaction, a layer of this compound will form at the interface and reduce the rate of attack to a level controlled by diffusion processes in the solid state. By far the most serious attack, however, occurs in the presence of stresses, since in this case the liquid alloy, or a product of its reaction with the solid alloy, may penetrate along the grain boundaries, with resultant embrittlement and serious loss of strength. 

The double-layer structure of Ga and its liquid alloys was discussed by Trasatti in a chapter in this series in 19807 and by Bagotskaya in 1986.120 Other discussions can be found in books of the NATO series.25,26... 

Solid Sn + Pb alloys have been studied by Shuganova et al.s25 by impedance. As found by Kukk and PUttsepp,808 was independent of the Pb content in a wide region of alloy composition, i.e., S2dPI> 0. and only at Sn content 95% is a marked shift of observed. A comparison of solid and liquid alloys indicates that Pb is the surface-active... 

Liquid alloys of Hg with a variety of metals (amalgams) constitute particularly complex systems in view of the potential dependence of surface composition. A detailed study of In and T1 amalgams, with... 

Results in other solvents are scanty for metals other than Hg. Liquid Ga and its T1 and In liquid alloys have been studied in DMSO, DMF, NMF, AN,343,894 MeOH360 andEtOH.361 Among solid metals, only Bi,28,152 Au,25,26,109 Al,750,751 and Fe729 have been investigated in a number of nonaqueous solvents. Pt and Pd have been studied in DMSO and... 

Serious problems may arise if gallium or its liquid alloys contact aluminium alloy structural components in aircraft, when rapid amalgamation and weakening occurs. 

The liquid alloy gives mixtures with halocarbons even more shock-sensitive than those with potassium. Highly chlorinated methane derivatives are more reactive than those of ethane, often exploding spontaneously after a delay [1]. Contact of... 

The liquid alloy was used in an unlabelled Dreschel bottle to dry inert gas. A violent explosion occurred when the bottle was being cleaned, owing to contact of the alloy with water. Other desiccants are much safer (and would avoid the possibility of mistaking the alloy for mercury). 

In substitutional metallic solid solutions and in liquid alloys the experimental data have been described by Epstein and Paskin (1967) in terms of a predominant frictional force which leads to the accumulation of one species towards the anode. The relative movement of metallic ion cores in an alloy phase is related to the scattering cross-section for the conduction electrons, which in turn can be correlated with the relative resistance of the pure metals. Thus iron, which has a higher specific resistance than copper, will accumulate towards the anode in a Cu-Fe alloy. Similarly in a germanium-lithium alloy, the solute lithium atoms accumulate towards the cathode. In liquid alloys the same qualitative effect is observed, thus magnesium accumulates near the cathode in solution in bismuth, while uranium, which is in a higher Group of the Periodic Table than bismuth, accumulated near the anode in the same solvent. 

The situation in the solid state is generally more complex. Several examples of binary systems were seen in which, in the solid state, a number of phases (intermediate and terminal) are formed. See for instance Figs 2.18-2.21. Both stoichiometric phases (compounds) and variable composition phases (solid solutions) may be considered and, as for their structures, both fully ordered or more or less completely disordered phases. This variety of types is characteristic for the solid alloys. After a few comments on liquid alloys, particular attention will therefore be dedicated in the following paragraphs to the description and classification of solid intermetallic phases. 

Figure 3.1. Li-Pb system averaged values (in the temperature range between 400°C and 800°C) of the resistivity of the liquid alloys (Van der Lugt 1996).

Figure 3.2. Li-Pb system trend of the mixing enthalpy, AH, of the liquid alloys (after Predel and Oehme 1979) and dAmixH/dx vs. composition.

In a special issue (4/2006) of the International Journal of Materials Research (formerly Zeitschrift fur Metallkunde) a number of papers have been collected which have been dedicated to liquid alloys and their properties. A short summary of these, even without any claim to be systematic, highlights some research problems into liquid metals and alloys. 

Surface tension and density of liquid alloys have been studied by Moser et al. (2006). Measurements by maximum bubble pressure and dilatometric techniques were carried out in an extensive range of temperatures on liquid alloys close to the ternary eutectic Sn3 3Ag0 76Cu with different Sb additions, which decrease surface tension and density. The experimental data were discussed in comparison also with values calculated on the basis of different models. 

By using a thermodynamic model based on the formation of self-associates, as proposed by Singh and Sommer (1992), Akinlade and Awe (2006) studied the composition dependence of the bulk and surface properties of some liquid alloys (Tl-Ga at 700°C, Cd-Zn at 627°C). Positive deviations of the mixing properties from ideal solution behaviour were explained and the degree of phase separation was computed both for bulk alloys and for the surface. 

In a review, Van der Lugt (1996) observed that a number of liquid alloys (K-Pb, Rb-Pb, Cs-Pb, K-Sn, Rb-Sn, Cs-Sn, K-Te) behave as if all the anions were in the form of perfect simple Zintl anions like tetrahedra and dumbbells this being suggested by the behaviour of many physical properties (resistivity, thermodynamic properties, etc.) as a composition function. In this context a simple Zintl ion was defined as a polyanion that assumes the same configuration as a neutral isoelec-tronic element. A similar definition was reported by Van der Lugt and Verkerk... 

A clear indication of the various intermediate phase stability may be obtained from the values of their Afi (AfG). The enthalpies of formation, in the liquid and in the solid state, of divalent metal alloys with Pb, as a typical element in the p-block, have been measured, and their trend discussed, by Sommer et al. (2006). The most exothermic values observed, for instance, in the Ca-Pb and Ba-Pb systems, correspond (for the solid compounds, in kJ/mol of atoms, at 300 K) to — 62 2 (for Ca2Pb) and —13 2 (for BaPb). A relevant compound forming tendency was observed also for the liquid alloys, for which the association model (see 3.2) was successfully applied confirming the existence of strong chemical short-range order. 

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