How is an interstitial solid solution formed?

An interstitial solid solution is formed when smaller atoms fit into the empty spaces, or interstices, between larger metal atoms in a crystal lattice. This occurs because the smaller atoms can occupy the voids without significantly disrupting the overall structure of the material. The incorporation of these smaller atoms enhances certain properties of the metal, such as strength and hardness, because they create distortions in the lattice that impede the movement of dislocations.

The other options do not describe the correct mechanism for forming an interstitial solid solution. For instance, replacing parent metal atoms with larger atoms would not yield an interstitial solution; it involves substitutional solid solutions instead, where the larger atoms replace some of the metallic lattice sites. Introducing larger atoms that occupy voids in the lattice is not feasible since larger atoms typically cannot fit into the interstitial spaces created by smaller atoms. Lastly, mixing metals in equal proportions does not specifically relate to the formation of an interstitial solid solution, as that scenario likely describes a simple mixture or possibly a different type of solid solution. Thus, option C accurately represents the characteristic process of forming an interstitial solid solution.