Metals solid solubility

As you can see from the tables in Chapter 1, few metals are used in their pure state -they nearly always have other elements added to them which turn them into alloys and give them better mechanical properties. The alloying elements will always dissolve in the basic metal to form solid solutions, although the solubility can vary between <0.01% and 100% depending on the combinations of elements we choose. As examples, the iron in a carbon steel can only dissolve 0.007% carbon at room temperature the copper in brass can dissolve more than 30% zinc and the copper-nickel system - the basis of the monels and the cupronickels - has complete solid solubility. 

The catalysts used are themselves complexes produced by interaction of alkyls of metals in Groups l-IIl of the Periodic Table with halides and other derivatives of Groups IV-VIII metals. Although soluble co-ordination catalysts are known, those used for the manufacture of stereoregular polymers are usually solid or adsorbed on solid particles. 

In some metal components it is possible to form oxides and carbides, and in others, especially those with a relatively wide solid solubility range, to partition the impurity between the solid and the liquid metal to provide an equilibrium distribution of impurities around the circuit. Typical examples of how thermodynamic affinities affect corrosion processes are seen in the way oxygen affects the corrosion behaviour of stainless steels in sodium and lithium environments. In sodium systems oxygen has a pronounced effect on corrosion behaviour whereas in liquid lithium it appears to have less of an effect compared with other impurities such as C and Nj. According to Casteels Li can also penetrate the surface of steels, react with interstitials to form low density compounds which then deform the surface by bulging. For further details see non-metal transfer. 

Just as the saturated solubility of sugar in water is limited, so the solid solubility of element B in metal A may also be limited, or may even be so low as to be negligible, as for example with lead in iron or carbon in aluminium. There is extensive interstitial solid solubility only when the solvent metal is a transition element and when the diameter of the solute atoms is < 0 6 of the diameter of the solvent atom. The Hume-Rothery rules state that there is extensive substitutional solid solubility of B in >1 only if ... 

However, just as two liquids may be completely miscible and form a complete range of solutions from one pure liquid to the other, so certain metals, for example copper and nickel, exhibit complete solid solubility over the whole range of compositions from pure copper to pure nickel. Clearly for two metals to be soluble in each other over the whole compositional range, they must have the same crystal structure, i.e. they must be isomorphous. 

Oxygen and carbon have substantial solid solubilities in niobium at the temperatures normally required for reduction. As the activity coefficients of both carbon and oxygen in niobium are low, their retention in the niobium metal produced by the carbothermic reduction of niobium oxide is expected. It is, however, possible (as explained later) to remove these residual impurities by extending the pyrovacuum treatment to still higher temperatures and lower pressures. 

For ionic solutions the strain energy seem to be relatively more important than for the metallic alloy systems [38-40] and the size difference between the two components being mixed dominates the energetics, although other factors are also of importance. In cases where the the covalency or ionicity of the components being mixed are largely different a limited solid solubility also must be expected, even... 

Mutual solid solubility of the component metals in alloy systems... 

Figure 2.1. Examples of melting phase diagrams of binary systems showing complete mutual solubility in the solid and in the liquid states (L liquid field, S solid field). The melting behaviour of the Mo-V, Cs-Rb and Ca-Sr alloys is presented. Notice the different ranges of temperature involved. The melting points of the pure metal components are shown on the corresponding vertical axes. The Cs-Rb is an example of a system showing a minimum in the melting temperature. In the Sr-Ca system complete mutual solid solubility is shown in both the allotropic forms a and (3 of the two metals.

For a review of the application of the Darken and Gurry method to predict solid solubility see Gschneidner Jr. (1980). An improvement of the method by means of simultaneous use of rules based on the electronic and crystal structures of the metals involved is also presented. Notice that, in a way, the Darken and Gurry diagram corresponds to a row, or to a column, of the map reported in Fig. 2.8. 

In the general case a complex behaviour may be expected for the extension of the terminal solid solutions which, for a pair of metals Mb M2, also depends on the stoichiometry and stability of the M (or, respectively, M2) richest phase. However a certain regularity of the dependence of the mutual solid solubility on the position of the metals involved in the Periodic Table may be observed. This can be related to the so-called Hume-Rothery rules (1931) ... 

Figure 2.15. Relative extent of the mutual solid solubility in binary alloys of transition metals, ordered according to their group number in the Periodic Table. The group number is reported on the left and on the top of the figure.

Besides solid transition metals, certain soluble transition-metal complexes are active hydrogenation catalysts.4. The most commonly used example is tris(triphenylphosphine)-chlororhodium, which is known as Wilkinson s catalyst.5 This and related homogeneous catalysts usually minimize exchange and isomerization processes. Hydrogenation by homogeneous catalysts is believed to take place by initial formation of a rc-complex, followed by transfer of hydrogen from rhodium to carbon. 

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