The structure of iron

Abstract

Iron has two different crystal structures at atmospheric pressure: the body centered cubic (bcc) and the face centered cubic (fcc). In the ground state the bcc α-phase is stable, and at the temperature T=1184 K (A₃ point), α-Fe transforms into fcc α-Fe, which is stable up to 1665 K(A₄ point). Above this temperature, iron transforms back into the bcc phase (σ-Fe), which remains stable up to the melting temperature T_m =1809 K. Since α and σ-Fe are isomorphic, the two are usually not distinguished when referred to bcc Fe. Steels with bcc and fcc structures are usually referred to as “ferrite” and “austenite” respectively. The boiling point of iron is about 3300K. A third form of iron, which has the hexagonal close packed (hcp) structure, can be stabilized under high pressures.