An alloy is a substance with metallic properties that is composed of a mixture of two or more elements. Alloys can be classified as interstitial or substitutional. In an interstitial alloy, smaller elements fill holes that are in the main metallic structure. The smaller element may be a nonmetallic element, such as boron, carbon, nitrogen, or silicon. For example, steel is an interstitial alloy in which carbon atoms fill the holes between the crystal structure of iron. In substitutional alloys, some of the atoms of the main metal are substituted with atoms of another metal. If the two metal atoms are about the same size and have the same crystallographic structure, then the two metals may form a solid solution. Hume-Rothery rules predict which metals will form solid-solutions based on the relative sizes and electronic properties of the metal atoms. Brass, an alloy composed of copper and zinc...