Steel is iron strengthened by small carbon atoms sitting in the spaces between iron atoms in the crystal lattice. Steel is therefore classified as which type of alloy?

Core Concept

Interstitial alloys form when solute atoms small enough to fit into the gaps between host atoms occupy those spaces without replacing host atoms. Carbon's tiny radius lets it slip into holes among much larger iron atoms, distorting the lattice and hindering layer sliding, which adds hardness and strength to the metal.