- A solid solution is formed when two metals are completely soluble in liquid state and also completely soluble in solid state.
- In other words, when homogeneous mixtures of two or more kinds of atoms (of metals) occur in the solid state, they are known as solid solutions.
- The more abundant atomic form is referred as solvent and the less abundant atomic form is referred as solute.
- Solid solutions are of two type: i) Substitution solid solution; ii) Interstitial solid solution
- By studying a number of alloy systems, Hume Rothery formulated certain rules which govern the formation of substitutional solid solutions. These are:
Crystal Structure Factor- For complete solid solubility, the two elements should have the same type of crystal structure i.e., both elements should have FCC, BCC or HCP structure.
Relative Size Factor- As the size difference that is atomic radii difference between two elements increases, the solid solubility becomes more restricted. For extensive solid solubility the difference in atomic radii of two elements should be less than about 15 percent. If the relative size factor is more than 15 percent, solid solubility is limited.
Chemical Affinity Factor- Solid solubility is favoured when the two metals have lesser chemical affinity. If the chemical affinity of the two metals is greater, then greater is the tendency towards compound formation. Generally, if the two metals are separated in the periodic table widely then they possess greater chemical affinity and are very likely to form some type of compound instead of solid solution
Relative Valency Factor - It is found that a metal of lower valence tends to dissolve more of a metal of higher valence than vice versa.