Solid solution

A solid solution is formed when two metals which are mutually soluble in the liquid state remain dissolved in each other after crystallization. If the resulting structure is examined, even with the aid of a high power electron microscope, no trace can be seen of the parent metals as separate entities, and if the metals are completely soluble in each other there will be crystals of one type only. The term solvent refers to the more abundant atomic form, and solute to the less abundant. It will be clear that in order that one metal may dissolve in another to form a solid solution, its atoms must fit in some way into the crystal lattice of the other metal. This may be achieved by the formation of either a 'substitutional' or an 'interstitial' solid solution (Fig.1).

Fig.1 Ways in which solid solution can occur.

-Interstitial solid solutions

Interstitial solid solutions can be formed only when the atoms of the added element are very small compared with those of the parent metal, thus enabling them to fit into the interstices or spaces in the crystal lattice of the parent metal. Not only can this occur during solidification but, also in many cases, when the parent metal is already solid. Thus, carbon can form an interstitial solid solution with FCC iron during the solidification of steel, but it can also be absorbed by solid iron provided the latter is heated to a temperature at which the structure is face-centred cubic. This is the basis of carburising steel. Nitrogen is also able to dissolve interstitially in solid steel, making the nitriding process possible; whilst hydrogen, the atoms of which are very small indeed, is able to dissolve in this way in a number of solid metals, usually producing mechanical brittleness as a result. - substitutional solid solution

In a substitutional solid solution the atoms of the solvent or parent metal are replaced in the crystal lattice by atoms of the solute metal. This substitution can be either 'disordered' or 'ordered' (Fig.1). Solubility of one metal in another is governed in part by the relative sizes of their atoms. Thus disordered solid solutions are formed over a wide range of alloy compositions when the atoms are of similar size but when the atomic diameters differ by more than 14% of that of the atom of the solvent metal then solubility will be slight because the atomic size factor is unfavourable. The size factor effect arises from the strain which is induced in the crystal lattice by a badly misfitting solute atom. The atoms of the solvent metal are displaced either outwards or inwards according as the solute atom is either larger or smaller than the atom of the solvent metal. This distortion of the crystal lattice from the ideal shape increases the energy of the crystal and tends to restrict solubility in it. Electrochemical properties of the two metals also influence the extent to which they will form a solid solution. Thus if the two metals have very similar electrochemical properties, then they may form solid solutions over a wide range of compositions but if one metal is very strongly electropositive and the other only weakly electropositive, then they are very likely to form some type of compound. As might be expected, the lattice structures of the two metals will influence the extent to which they are likely to form substitutional solid solutions. If the structures are similar then it is reasonable to expect that one metal can replace the other satisfactorily in its lattice. Thus two FCC metals like copper and nickel will form a continuous series of solid solutions— assisted by the fact that atom sizes of these two metals are almost the same. To summarize, a high degree of substitutional solid solubility between two metals is likely to be obtained when: 1. The difference in atomic radii is less than 14%. 2. Electrochemical properties are similar. 3. The two metals have similar crystal structures.

Even though the size factor of the two metallic atoms may favour their forming a solid solution they are more likely to form compounds if one is much more strongly electropositive than the other.

- INTERMEDIATE PHASES

In many alloy systems, crystal structures or phases are found that are different from those of the elementary components (pure metals). If these structures occur over a range of compositions, they are solid solutions. However, when the new crystal structures occur with simple whole-number fixed ratios of the component atoms, they are intermetallic compounds with stoichiometric compositions. The difference between intermediate solid solutions and compounds can be more easily understood by actual examples. When copper and zinc are alloyed to form brass, a number of new structures are formed in different composition ranges. The crystal structure of this new phase is body-centered cubic, whereas that of copper is face- centered cubic, and zinc is close-packed hexagonal. Because this body-centered cubic structure can exist over a range of compositions (it is the only stable phase at room temperature between 47 and 50 weight percent of zinc), it is not a compound, but a solid solution. This is also sometimes called a nonstoichiometric compound or a nonstoichiometric intermetallic compound. On the other hand, when carbon is added to iron in an amount exceeding a small fraction of one-thousandth of a percent at ambient temperatures, a definite intermetallic compound is observed. This compound has a fixed composition (6.67 weight percent of carbon) and a complex crystal structure (orthorhombic, with 12 iron atoms and 4 carbon atoms per unit cell) which is quite different from that of either iron (body-centered cubic) or carbon (graphite).