Alloys

An alloy is a mixture of two or more elements, where at least one element is a metal. Many alloys are mixtures of two or more metals.

Alloy strength

Converting pure metals into alloys often increases the strength of the product. For example, brass is an alloy of copper and zinc. It is stronger than copper or zinc alone.

MetalTensile strength (MPa)
Copper220
Zinc139
Brass350

Explaining alloy strength

Solid metals have a regular lattice structure. When a force is applied to a metal, layers of atoms can move past each other. The more difficult it is for the layers to move, the more force is needed and the stronger the metal.

Copper and zinc atoms are different sizes. This distorts the regular lattice structure in brass, so layers of atoms cannot slide over each other so easily. This makes brass stronger than copper or zinc alone.

Atoms of two different sizes packed together to form an irregular arrangement.The metal lattice structure is distorted in alloys

Alloy steels

Iron is alloyed with other metals to produce a range of alloy steels. Different steels have different properties, depending on their composition. This table shows three common examples:

Alloy steelAdditional elementsProperties
Mild steelCarbonMalleable, ductile
Tool steelTungstenHard, resistant to high temperatures
Stainless steelChromiumHard, resistant to rusting

Mild steel is useful for making car body parts because it is easily pressed into shape. Although mild steel rusts, it can be protected by galvanising and painting. Tool steel is useful for making drill bits. These do not easily become damaged by the heating caused by friction during drilling.

Question

Explain why stainless steel is used to make washing machines and dishwashers.

Washing machines and dishwashers contain water. This would cause other steels to rust, but stainless steel does not rust.