Iron (Fe)

From isarn, the old Saxon word for iron.

The atoms in metals are arranged in a regular three-dimensional pattern called a crystal structure. In the case of iron it can be visualised as a series of cubes stacked side by side, and one on top of the other. The corners of the cube are atoms and each corner is shared by eight adjoining cubes or cells. As well as corner atoms each unit cell contains additional atoms, with one atom at the centre of the cell, it is termed a body-centred cubic structure (bcc), with atoms at the centre of each face of the cell it is termed a face centred cubic structure (fcc).

Pure iron is capable of existing in three forms, all of which are stable within different temperature ranges. Between room temperature and 911°C iron has a body-centred cubic, bcc crystal structure and is termed ά (Alpha) iron, (commonly known as ferrite). At 91I°C a crystalline transformation occurs and the bcc structure changes to face- centred cubic, fcc. This form is termed γ (Gamma) iron (austenite) and exists up to 1392°C, at which temperature the structure again changes to bcc, the high temperature, δ (Delta-ferrite) form.

Other metallic elements, when added to iron, have their atoms interspersed in the gaps between the iron atoms and in this way alloys are formed. The addition of carbon to iron, as in the case of steel, causes alterations to the crystal structure by the imposition of carbon atoms into the gaps between iron atoms; e.g. in gamma-iron, austenite. Rapid cooling of steel by quenching from the austenitic temperature range produces crystallographic transformation to the meta-stable hard phase, martensite.

See also ferrous.