Edge Dislocation: Definition & Motion

Instructor: Betsy Chesnutt

Betsy teaches college physics, biology, and engineering and has a Ph.D. in Biomedical Engineering

Some pieces of metal are able to deform more than others, even if the chemical makeup of the metal is exactly the same! What is going on? In this lesson, learn about edge dislocations and how they affect a metal's ability to stretch and change its shape.

Can This Metal Stretch?

Imagine that you pick up a steel rod and decide to pull on both ends. What do you think would happen? Would the rod stretch out like a rubber band? Of course not! Metals like steel, copper, and aluminum seem like they are very hard and stiff, right? It's hard to imagine that anything could make a steel rod get longer.

However, if you apply a big enough force to the rod, that's exactly what will happen! This is known as deformation, and it can happen in all metals when they are subjected to large forces.

In fact, it's been known since ancient times that repeatedly applying large forces to shape a piece of metal, a process known as forming, could have a big effect on its ability to deform. This happens even though the forming does not change the chemical structure of the metal at all! For many years, metalworkers knew that forming was a good way to change the properties of a piece of metal, but they did not really know why this happened.

Shaping a piece of metal by forming can have a big effect on its deformability even though the chemical composition does not change.
girl working with metal

Dislocations and Crystal Structure

If you could make yourself really small and travel inside a piece of metal, you would find that the atoms are arranged in a regular pattern. This is called the crystal structure of the material. Not all materials have atoms that are arranged in this way, but most metals do.

In 1934, a group of scientists theorized that defects in this regular crystal structure were usually responsible for allowing deformation in metals, but they didn't have any way to prove their theory. Finally, in the 1950's, transmission electron microscopes that could actually SEE the arrangement of atoms inside a material were used to show that this was really true. Defects in the crystal structure were present in almost all metals and these defects were responsible for most deformations.

Edge Dislocations

One of the most common crystal structure defects is known as an edge dislocation. This occurs when there are extra atoms inserted into a plane in the crystal lattice.

Edge dislocations make it easier for atoms to slip past one another, making it easier for the metal to deform.
Edge dislocation

When there are lot of edge dislocations in a metal, it will deform much more easily than a similar metal containing fewer edge dislocations. Edge dislocations make it easier for a material to deform because they stretch out bonds between adjacent atoms and make it easier for the atoms to slide past one another.

Edge dislocations pass through a material in a way that looks a big like an inchworm crawling across the ground. The dislocation slips from one atom to the next, creating a ''bump'' in the lattice and eventually causing the whole plane of atoms to move forward.

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