Metals, ceramics and polymers

Overview

Recall questions on the structure of metals, dislocations and slip, ductility, hardening a metal with foreign atoms, work hardening and annealing, the structure of ceramics and why they are brittle and stronger in compression, polymer chains and their large extensions, stiffening rubber, cross-links, brittle cold polymers, and metal grains.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Describe the structure of a metal.

   Model answer

   Positive ions sit in a regular crystalline lattice held together by a sea of delocalised electrons, usually arranged in many small grains of differing orientation.

2. 2

   What is a dislocation?

   Model answer

   A dislocation is a line defect in a crystal, where a plane of atoms comes to an end part way through the lattice.

3. 3

   Explain how the movement of dislocations allows slip in a metal.

   Model answer

   A dislocation moves by breaking and remaking one row of bonds at a time, so planes of atoms slide over each other at a far smaller stress than would be needed to break every bond at once.

4. 4

   Explain why metals are ductile.

   Model answer

   They contain mobile dislocations that move easily under stress, which allows large plastic deformation without fracture.

5. 5

   Explain how adding atoms of a different size makes a metal harder.

   Model answer

   The foreign atoms distort the lattice and pin dislocations in place, so a greater stress is needed to make them move.

6. 6

   Explain why work hardening makes a metal harder but more brittle.

   Model answer

   Deformation multiplies and tangles the dislocations so that they obstruct one another, which resists further slip but leaves the metal little capacity for plastic deformation.

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