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.

Aluminium, copper and gold are useful metals. The table shows common alloys that contain these metals.

AlloyComposition
MagnaliumAluminium and magnesium
BrassCopper and zinc
Jewellery goldGold and copper

The uses of metals are related to their properties.

Aluminium and magnalium

Aluminium does not react with water. Its surface is protected by a natural layer of aluminium oxide that allows the metal to resist corrosion. Aluminium foil is used in the home for wrapping and storing food because it does not react to substances in food. It is malleable, so it is easily folded into shape around the food.

Aluminium has a low density, so pieces of aluminium are relatively lightweight. Magnalium is stronger than aluminium alone but still has a low density. It is used to make aircraft parts.

Copper and brass

Copper and brass resist corrosion and are good electrical conductors. Copper is a better conductor than brass, and it is used in electrical wiring. Brass is stronger than copper, so it is used for the pins in electrical plugs.

Two electrical cables that have had the outer insulating layer removed, showing the internal cables
Electrical wires are protected by a tough layer of an insulating polymer

Gold

Gold is a very soft and malleable metal. It is also very unreactive, so it resists corrosion and stays shiny. The gold used for jewellery is gold alloyed with other metals, often copper. This makes the jewellery much stronger while keeping its ability to stay shiny.

Alloy steels

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

AlloyOther 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.

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:

MetalRelative tensile strength
Copper3
Zinc 2
Brass 5

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 have 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 atoms of one metal (shown in black) have a lattice structure that is distorted by atoms of another metal (shown in red)