1. Structure and bonding
page 6
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Dislocations in the crystal structure
Metals are malleable and ductile; much more so than can be explained using the simple model of layers slipping past one another (see page 2). There is another feature which also helps metals change shape without breaking. The malleability and ductility are helped by the presence of dislocations in the crystal structure.
Interactive graphic showing defects and movement
Picture 1.11 The lattice will have a number of defects called dislocations. These can move through the lattice.

Click to step through the movement.

Note: this simple cubic lattice does not exist in nature - however, it is simple to see.

A dislocation is simply a defect in the lattice structure in which a few ions in a layer are missing. This causes the neighbouring layers to be displaced slightly to minimise the strain from the defect. You can also think of it as being an extra row of ions.
Moving dislocations
There can be millions of dislocations in each cubic millimetre of a metal. When a force is applied to the metal, the dislocations move through the lattice structure (see picture 1.11). This only needs to be a small force (smaller than that needed to produce slip) because very few bonds are being broken at any one time.

This movement does not affect the vast majority of ions and does not require large scale movement of the layers in the structure. It is the ease of movement of dislocations that helps explain why metals are so malleable and ductile. As with slip, moving dislocations causes plastic deformation of the metal.

Po=hoto of steel structure
Picture 1.12 Photomicrograph of a steel. You can see the grains in which the crystals have grown in different directions.
Crystal grains
Most metals are manufactured by casting molten liquid. As the liquid begins to solidify, a small number of solid nuclei appear.
A single crystal grows around each nucleus. These individual crystals are called grains. The grains grow in all directions until they meet other grains. The edges of the grains are called grain boundaries.
Interactive graphic of grains
Picture 1.13 Diagram of grains and grain boundaries.
How grains affect dislocations
The movement of dislocations is hindered by grain boundaries. The more grain boundaries there are the more difficult it is for the dislocations to move and for the metal to change shape. The result is that the metal is stiffer and harder. It is also stronger.

A fine grained metal contains a large number of grains. A coarse grained metal contains fewer grains. A fine grained metal is therefore stronger than a coarse grained metal.

Sometimes, grain refining agents are added to the molten metal before it is cast to provide nuclei around which grains can grow. This will tend to produce a finer grained material which is stronger than it would have been with fewer grains.

Question 1-5.
a) Explain why dislocations move with a smaller force than is needed to cause slip.

b) What are the crystals called that grow around nuclei as a molten metal cools?


Summary                   Close
  • metals have moveable dislocations in their lattice structure
  • these make them so malleable and ductile
  • metal crystals grow in grains; the grain boundaries hinder the movement of dislocations
  • fine grained metals are stronger than coarse grained metals