Metals under stress: dislocations and ductile fracture

Overview

Elastic strain and plastic strain, happening to the atoms during plastic deformation of a metal and dislocation in a metal crystal.

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 shape of the force–extension graph for a ductile metal such as copper.

   Model answer

   It rises as a straight line to the limit of proportionality, then curves over as the metal yields, continues to extend for very little extra force, and finally falls slightly before fracture.

2. 2

   Distinguish between elastic strain and plastic strain.

   Model answer

   Elastic strain disappears completely when the load is removed, while plastic strain is a permanent change of shape that remains.

3. 3

   What is happening to the atoms during plastic deformation of a metal?

   Model answer

   Whole planes of atoms slide over one another, so the atoms take up new permanent positions relative to their neighbours.

4. 4

   What is a dislocation in a metal crystal?

   Model answer

   A dislocation is a line defect in the crystal, such as the edge of an extra half-plane of atoms inserted into the lattice.

5. 5

   Explain how a dislocation allows a metal to deform plastically at a low stress.

   Model answer

   A dislocation lets slip happen by breaking and remaking one line of bonds at a time as the defect moves along, instead of requiring a whole plane of bonds to break at once.

6. 6

   Name three ways of introducing barriers to dislocation movement in a metal.

   Model answer

   Dislocation movement is impeded by foreign atoms, by other dislocations and by additional grain boundaries.

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