KEY2CHEM

Properties of Metal Alloys

Solid-solid mixtures of metals or other elements to create a mixture with metallic character are called metal alloys. The size of the metals used in the alloys determines whether the alloy is interstitial (atoms of one element fill in spaces between the other element due to size difference) or substitutional (atoms of one element replace atoms of the other element because they are similar in size). The incorporation of another type of atom does not allow the metal cores to slide as easily past one another. As a result, the alloy is generally less malleable and less ductile that the individual metal components. This often means the alloy has improved mechanical properties, including hardness, relative to the individual metals.

Example 1.

What type of metal alloy is shown?

A. pure metal

B. interstitial alloy

C. substitutional alloy

Solution

B. interstitial alloy

Interstitial alloys form between atoms of different radius, where the smaller atoms fill the interstitial spaces between the larger atoms. Substitutional alloys form between atoms of comparable radius, where one atom substitutes for the other in the lattice.

Example 2.

Brass is an example of an alloy between copper and zinc. The atomic radius of copper is $$128\text{ pm}$$. The atomic radius of zinc is $$134\text{ pm}$$. What type of alloy would a mixture of copper and zinc formis brass?

A. pure metal

B. interstitial alloy

C. substitutional alloy

Solution

C. substitutional alloy

Because the sizes of copper and zinc atoms are similar, their mixture (brass) would be a substitutional alloy.

Example 3.

Steel is an example of an alloy of iron and other elements. Steel is used in many applications, including building materials. Why isn’t pure iron used in the same applications?

A. Pure iron is softer than steel.

B. Pure iron is less malleable than steel.

C. Pure iron is less ductile than steel.

Solution

A. Pure iron is softer than steel.

Pure iron is soft and corrodes easily. Alloying it with another element, often carbon, makes the resulting steel alloy more rigid, meaning it is less malleable and less ductile.