Crystal structure

Most materials form crystals when they cool down from the molten state. In metals, this crystal structure can normally only be seen clearly using a high-power microscope, the individual crystals then being called grains.

Crystals are most-commonly found when a hot, concentrated, liquid solution of a suitable crystalline chemical (e.g. sugar) cools down slowly. However, some minerals occur naturally in the form of large crystals.

Some metals can have more than one crystal structure and this is what enables iron to be heat treated. At room temperature pure iron crystals are body-centred cubic (bcc) and are called ferrite. Above 911ºC they are face-centred cubic (fcc) and are called austenite.

Components which are cast in such a way that they consist of a single crystal only are extremely strong and are used for arduous duties such as high temperature turbine blades.

See also grain.