Question

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

Short answer

An alloy is a mixture of two or more elements, typically a metal and another element or multiple elements, created to obtain desired properties. In substitutional alloys, atoms of added elements replace some parent metal atoms in the crystal lattice, while in interstitial alloys, they occupy interstitial positions, distorting the lattice structure. Substitutional alloys involve elements with similar atomic radii, while interstitial alloys involve elements with considerably smaller atomic radii. Brass (copper and zinc) is an example of a substitutional alloy, and steel (iron and carbon) is an example of an interstitial alloy.

Step-by-step solution

Definition of an alloy

An alloy is a mixture of two or more elements, typically a metal and another element or multiple elements. Alloys are created to obtain desired properties not found in individual elements, such as increased strength, resistance to corrosion, and reduced brittleness. Alloys have a metallic structure and, compared to pure metals, display an enhanced range of mechanical and physical properties.

Substitutional alloys

In substitutional alloys, the atoms of the added elements replace some of the atoms of the parent metal within the crystal lattice. Substitutional alloys are typically formed when the added elements have similar atomic radii and are chemically compatible with the parent metal. In this type of alloy, the overall crystal structure is maintained, and the elements are randomly distributed within the lattice.

Interstitial alloys

Interstitial alloys are characterized by the atoms of the added elements occupying the interstitial positions (empty spaces) within the crystal lattice of the parent metal. Interstitial alloys are formed when the added elements have considerably smaller atomic radii compared to the parent metal, enabling them to fit into the interstitial gaps. The interstitial impurities tend to distort the lattice structure, resulting in varied properties compared to the parent metal.

Comparison between substitutional and interstitial alloys

The main differences between substitutional and interstitial alloys are related to their atomic arrangement and composition. In substitutional alloys, the added elements replace parent metal atoms, while in interstitial alloys, they occupy the interstitial positions. Moreover, substitutional alloys generally involve elements with similar atomic radii, while interstitial alloys involve elements with considerably smaller atomic radii than the parent metal.

Example of a substitutional alloy

Brass is an example of a substitutional alloy, composed primarily of copper (Cu) and zinc (Zn). In brass, zinc atoms replace some copper atoms in the crystal lattice, resulting in a solid solution with enhanced properties like strength, corrosion resistance, and machinability compared to pure copper.

Example of an interstitial alloy

Steel is an example of an interstitial alloy, which is predominantly composed of iron (Fe) and a small percentage of carbon (C). In steel, the carbon atoms occupy the interstitial positions within the iron lattice, leading to increased strength, hardness, and resistance to wear compared to pure iron.