Alloys as useful materials: High carbon steel is strong but brittle.

The idea

High carbon steel contains a relatively large proportion of carbon within its iron alloy. The carbon atoms disrupt the regular layers of iron atoms, so the layers cannot slide over one another easily. This makes the steel strong and hard, able to resist forces without deforming. However, the same disruption means the material cannot absorb much energy by bending, so it tends to crack or shatter rather than deform when struck or overloaded; this is brittleness. High carbon steel is therefore chosen where hardness and strength matter more than flexibility, such as cutting tools, blades and some springs.

What earns the marks

• State that high carbon steel contains a relatively high proportion of carbon in the iron alloy.
• Explain that carbon atoms disrupt the layers of iron atoms, preventing them from sliding easily.
• Link the disrupted structure to strength and hardness, so the steel resists deformation.
• Explain that the same structure makes the steel brittle, so it cracks or shatters instead of bending.
• Apply the properties to a suitable use, such as cutting tools or blades, where hardness is important.

Common mistakes and how to put them right

• Using brittle to mean weak.

  Put it right by explaining that brittle means it cracks or shatters rather than deforming, even though it may be strong.

• Saying high carbon steel bends easily.

  Put it right by stating that its layers cannot slide, so it resists bending and may fracture instead.

• Confusing high carbon steel with low carbon steel.

  Put it right by comparing their carbon content and their resulting hardness and flexibility.

Exam tips

• Use the word brittle precisely, linking it to cracking or shattering rather than to low strength.
• Explain the property in terms of layers of atoms not sliding, which shows understanding of structure.
• Match the property to a use, such as tools that need a hard, sharp edge.