Defects Work Like Waves

In crystals one of the more common defects is that of the dislocation. This is where there is an irregularity in the crystal structure such as the two demonstrated here.

A screw dislocation (thank you to Wikityke on Wikipedia for this image)
An edge dislocation (also by Wikityke)

Both of these would be defined as line defects as whole rows of atoms have been put out of place in these structures. The line of dislocation, more clearly existing and drawn in blue for the edge dislocation, is where the displacement is most severe. These defects can act to either strengthen or weaken the crystal in which they exist.

The two main aspects of studying dislocations are that of production and transport. Production, at least the most common kind, is relatively simple. Stress applied laterally to a row of crystals can cause them to dislocate. The transport is a lot more varied with the dislocations moving through the crystal by a number of mechanisms. The production and distribution of these defects can produce many different structural patterns but the most common is the persistent slip band, a block of rows of atoms that are very willing to slip over each other. In order to predict the behaviour of the slip bands two main theories called the low energy dislocation structure model and the self organized dislocation structure model are used. These models are considered quite primitive and are still ineffective when it comes to predicting more complex systems beyond a single dislocation. Massive dislocations and their distribution require some more work.

The realisation made in this paper is that the dislocation motion can be perceived as the movement of an elastic wave. The edge dislocations act like longitudinal waves and the screw dislocations like transverse waves. Not only this, but it is believed they act together like a standing wave to collect the persistent slip bands at intervals equivalent to the nodes of the wave.

Paper links: Standing wave effect and fractal structure in dislocation evolution



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