The Structure of Metals

Metals are polycrystalline. They are made up of many small crystalline regions which we call grains.

Within each grain there is a very regular 3 dimensional arrangement of the metal atoms.

They are held together by metallic bonding. The atoms are actually positive ions in a sea of free electrons.

The crystal structure of copper	Grains in a metal.	A photo of a metal treated with acid to show grains

The grains can vary greatly in size. Their size and what may be found in the grain boundaries can have a big effect on the properties of the metal.

Dislocations

Imagine a brick wall where one of the bricks is slightly bigger than the others or if a brick was missing. Imagine how weak the wall would be. If put under stress this layer is certainly where the wall would fail. We call this irregularity a dislocation.

In a metal this could correspond to a missing atom (perhaps the crystal formed to quickly and there are imperfections) or the presence of an impurity atom. The effect is the same though. The plane where the dislocation exists is weak and when put under stress this is where plastic deformation occurs. The planes on either side slide against each other and the dislocation moves through the material.

So dislocations make a material weak? Not necessarily. A few dislocations make a material weaker but lots of dislocations actually make it stronger.