Materials science: what are dislocations?

Dislocations occur primarily in metals, where the atoms are arranged in a regular pattern to form crystalline structures. But what would happen if a metal such as iron had a completely perfect internal structure, with no crystal defects at all?

The perhaps surprising answer is that such a metal would be of little practical use in engineering.

Paradoxical as it may sound, metals work precisely because of their crystal defects. The most important of these defects is the dislocation. It enables plastic deformation in metals and has a decisive influence on properties such as strength and toughness.

An illustrative model: a wrinkle in a carpet

The physical description of dislocations is complex. However, a familiar everyday example helps explain the principle: imagine the material as a large carpet, with a dislocation represented by a wrinkle in the carpet. To move the carpet, we create a wrinkle and move it across the surface. If this does not move the carpet far enough, we create another wrinkle.

By moving and orienting many such wrinkles, the carpet can be shifted in any direction. In the material, this means that a dislocation moves through the crystal lattice, causing a very small displacement of the atomic planes.

Dislocations and plastic deformation

Plastic deformation of metals, for example during rolling, takes place through the movement of dislocations through the crystal lattice until the component reaches the required shape. Compared with the wrinkle in the carpet, however, a dislocation is extremely small and only ‘shifts’ the material by around one ten-millionth of a millimetre.

During bulk forming of a material, for example forging, an unimaginably large number of dislocations therefore act together simultaneously. It is this collective movement that makes large-scale changes in shape technically possible.

Why blacksmiths work at high temperatures

For a material to be easily formable (moving the carpet), its dislocations (the wrinkles in the carpet) must be able to move as freely as possible. Obstacles in the material (on the carpet), such as precipitates or foreign atoms (objects on the carpet), impede this movement.

For this reason, a blacksmith often heats steel to temperatures well above 1000 °C before forming it. At these temperatures, many precipitates dissolve and the crystal structure of the steel changes in a way that allows dislocations to move more easily.

The result: the material becomes considerably easier to form.

Dislocation movement and mechanical properties

The mobility of dislocations has a major influence on the mechanical properties of a metal. If dislocations can move easily, the material typically has low strength but high toughness and good formability.

If dislocations are strongly impeded, however, the strength of the material increases. At the same time, ductility usually decreases and the material becomes more brittle. Materials engineering therefore often involves deliberately controlling this relationship.

Strength through deliberately introduced obstacles

If a component is to retain its shape over time, the movement of dislocations must be restricted. In steels, this is often achieved through heat treatment, for example quenching and tempering. During this heat treatment, fine precipitates, such as iron carbides, are deliberately formed. These act as obstacles to dislocations in the crystal lattice, much like objects on the carpet that make it harder to move a wrinkle.

The more obstacles there are, and the more favourably they are distributed, the more strongly dislocation movement is blocked and the higher the strength of the material.

Blacksmiths and heat treatment specialists – two perspectives on the same mechanism

Blacksmiths and heat treatment specialists have been working with dislocations for thousands of years, without knowing it in the past. The difference is that blacksmiths want dislocations to move unimpeded through the crystal lattice of metals. Heat treatment specialists generally want exactly the opposite: to impede dislocations wherever they can.

Interestingly, blacksmiths and heat treatment specialists work with the same physical mechanism, but pursue opposite goals.

Blacksmiths want dislocations to move as freely as possible through the crystal lattice so that the material can be formed easily. Heat treatment specialists, on the other hand, usually aim for exactly the opposite: they deliberately introduce obstacles that impede the movement of dislocations and thus increase the strength of the material.

Both processes – forming and heat treatment – ultimately rely on controlling dislocation movement within the crystal lattice.

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