Substitutional solid solutions are generally formed if three criteria are fulfilled:
1. The atomic radii of the elements are within about 15 per cent of each other.
2. The crystal structures of the two pure metals are the same; this similarity indicates that
the directional forces between the two types of atom are compatible with each other.
3. The electropositive characters of the two components are similar; otherwise compound
formation, where electrons are transferred between species, would be more likely.