Problem 21 - Chemistry

Question

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

Step-by-Step Solution

Answer

An alloy is a mixture of two or more elements, with at least one being a metal, created to obtain specific properties like increased strength or hardness. Substitutional alloys are formed by replacing atoms of the host metal with atoms of a similar size from a different element, such as brass, which consists of copper and zinc. In contrast, interstitial alloys have small atoms added to the host metal that occupy spaces between the bigger atoms in the crystal lattice, like steel, where small carbon atoms fit into the interstitial spaces in the iron lattice, increasing its strength and hardness.

Step 1: Introduction to Alloys

An alloy is a mixture of two or more elements, where at least one of them is a metal. Alloys are created to obtain specific properties like increased strength, hardness, or corrosion resistance, which are difficult to achieve with a single metallic element.

Step 2: Substitutional Alloys

Substitutional alloys are formed when one or more atoms in the host metal's crystal lattice are replaced by atoms of a different element that have a similar atomic size. The new atoms take up positions in the crystal lattice previously occupied by the host metal atoms. In this type of alloy, the solute atoms have a similar atomic radius to that of the host metal—usually within 15%.

Step 3: Example of Substitutional Alloy

A classic example of a substitutional alloy is brass, which is an alloy of copper and zinc. In brass, some of the copper atoms in the crystal lattice are replaced by zinc atoms. It comes in various compositions, having different ratios of copper and zinc, providing a range of mechanical properties.

Step 4: Interstitial Alloys

Interstitial alloys are formed when the added element's atoms are much smaller than the host metal's atoms. The smaller atoms occupy spaces (interstices) between the bigger atoms of the host metal in the crystal lattice. This type of alloying often leads to an increased hardness and strength compared to the original metal.

Step 5: Example of Interstitial Alloy

Steel is an example of an interstitial alloy, which is mainly composed of iron (Fe) with small amounts of carbon (C) atoms added. In this case, the carbon atoms, being smaller than the iron atoms, fit into the interstitial spaces in the iron crystal lattice. The interstitial alloy formed results in improved mechanical properties, such as strength and hardness compared to pure iron.