The extent to which the components of an alloy are miscible depends on the interaction between the atoms:
- If the species do not tend to bond to each other, then separate phases will form with limited miscibility
- If strong mutual attraction occurs, a single crystal of a different structure can form, such as in intermetallic compounds
- If there is little difference between like and unlike bonds, then a solid solution can occur, over a wide range of chemical compositions
In these solid solutions, different types of atoms or molecules exist in the same crystal lattice. A good example of a solid solution is the Cu-Ni system, for which the phase diagram is shown below.
Both metals are completely soluble in each other. The α phase is a substitutional solid solution. This occurs because both the Cu and Ni form face centred cubic structures, and have similar atomic radii, electronegativities and valences. Copper and nickel show very different physical properties in their pure states, and the α phase provides a continuous change between the extremes.
Other systems can exist as multiple phases, and revert to a single solid solution at high temperatures. The thermodynamic factors influencing such transitions are discussed in this Teaching and Learning Package.
With a liquid, it is easy to visualise the atoms or molecules moving past one another. With solids, the crystalline lattice is not completely static: atoms can move through the lattice by solid-state diffusion. This involves swapping of atoms; interstitial atoms; and motion of vacancies. These mechanisms are thermally activated.