Types of Solid Solution

There are two types of solid solution in metals: Substitutional and Interstitial

Substitutional – In this form of alloying, solute atoms replace or substitute the host atoms e.g. Au-Ag. The following parameters are important for substitutional solutions:

Difference between atomic radii of the solute and the solvent atoms should be within ± 15 %. The other important issues are -

• Electronegativity
• Relative valencies of participating atoms
• Crystal structures of metals of both atom types should be the same.

Interstitial – When the difference in radii between the participating atoms is too high, the alloying could occur in the form of interstitial filling. Impurity atoms fill voids or interstices among host atoms e.g. Fe-C. The figure below shows substitutional and interstitial alloying schematically.