What is an alloy? Explain the differences in structure between substitutional and interstitial alloys. Give an example of each type.

An alloy is a mixture of two or more elements, at least one of which is a metal, resulting in a material with enhanced properties. Alloys are created to improve the mechanical, electrical or chemical properties of materials, such as strength, corrosion resistance or conductivity.

There are two main types of alloys: substitutional and interstitial. The differences in structure between these types lie in the way the atoms of the elements are arranged in the alloy. In a substitutional alloy, atoms of the added elements replace or substitute some atoms of the base metal in the lattice, taking up the same positions in the crystal lattice of the host metal. This occurs when the radius of the added element is similar in size to that of the base metal, typically within 15% difference. Substitutional alloys typically involve metals with the same crystal structure and similar atomic radii. In an interstitial alloy, atoms of the added elements are smaller than the atoms of the base metal and fit into the interstices, or the spaces between the host atoms in the lattice. This arrangement usually occurs when there is a significant size difference between the atoms of the base metal and the added elements, typically when the atomic size of the added element is less than 59% of the base metal atom. Interstitial alloys exhibit higher strength and hardness compared to substitutional alloys, due to the added elements causing distortion in the lattice, leading to greater resistance to deformation.

Example of a substitutional alloy: Brass is a substitutional alloy composed of copper (Cu) and zinc (Zn). In brass, the zinc atoms replace some of the copper atoms in the lattice structure. Copper and zinc have similar atomic radii and share the same face-centered cubic crystal structure, which allows for this substitutional alloy. Example of an interstitial alloy: Steel is an interstitial alloy composed primarily of iron (Fe) and carbon (C). In steel, the smaller carbon atoms occupy the interstices between the larger iron atoms in the lattice structure. The atomic size of carbon is significantly smaller than that of iron, allowing it to fit within the interstices of the lattice structure without replacing iron atoms, thus resulting in an interstitial alloy.