In substitution alloys, the atoms of the components are approximately the same size and the various atoms are simply substituted for one another in the crystal structure.
An example of a (binary) substitution alloy is brass, made up of copper and zinc.
This structure is the reason, that substitution alloys are stable over wide range of compositions.
Interstitial alloys occur when the atoms of one component are substantially smaller than the other and the smaller atoms fit into the spaces (interstices) between the larger atoms.
An example for an interstitial alloy is steel, made up iron and carbon. In contradiction to substitution alloys the solubility is restricted. For steel it is max 1.7% carbon