Dislocations, ductile and brittle fracture

Overview

Recall questions on Dislocations, ductile and brittle fracture.

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

   What is a dislocation in a crystalline metal?

   Model answer

   A dislocation is a line defect in the lattice, most simply an extra half-plane of atoms ending part way through the crystal.

2. 2

   Explain why the presence of dislocations makes a metal much easier to deform plastically.

   Model answer

   A dislocation lets one plane of atoms slip over another a single row of bonds at a time rather than all at once. The stress needed is therefore far smaller than for a perfect lattice.

3. 3

   Define plastic strain.

   Model answer

   Plastic strain is deformation that remains after the deforming load has been removed.

4. 4

   State three ways of strengthening a metal by hindering the movement of dislocations.

   Model answer

   Foreign atoms can be introduced by alloying, the dislocation density can be raised by work hardening so dislocations obstruct one another, and the grain size can be reduced so there are more grain boundaries.

5. 5

   Why does a grain boundary obstruct a moving dislocation?

   Model answer

   The lattice orientation changes abruptly across the boundary, so the slip plane the dislocation was travelling along does not continue into the next grain.

6. 6

   What is meant by necking in a ductile metal?

   Model answer

   Necking is a local narrowing of the specimen at one point. The cross-sectional area there is reduced, so the stress at the neck rises and further deformation concentrates in that region.

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