Alloys are homogeneous mixtures that contain uniformly distributed components. Interstitial alloys form when the main alloying agents mix with much smaller agent atoms. The smaller atoms slip into the gaps, or interstices, between the main alloying agents.
Steel is a common interstitial alloy. It contains a lattice of large iron atoms with a few smaller carbon atoms in the interstices. In contrast, in substitution alloys, the alloying agents replace some of the atoms of the metal. Brass is a substitution alloy in which zinc atoms replace a considerable amount of the metal's copper atoms. Since zinc and copper are similar in size, they form a substitution alloy instead of an interstitial alloy.