Enlarge this picture Figure 1 (left). Body-centered cubic crystal structure; Figure 1(right). Body-centered cubic crystal structure

Iron is “polymorphic.” That is, it has the ability as a solid material to exist in more than one crystallographic form or structure. This is one of the “secrets” to the success of iron to combine with other elements to form alloys (such as steel) to dramatically extend its range as an engineering material.

At room temperature, the most stable form of iron is the body-centered cubic (bcc) structure (Fig. 1) known as ferrite or α-Fe (alpha iron). In this state, iron is relatively soft and can dissolve only a small amount of carbon – no more than 0.021 wt% at 910°C (1670°F).

Enlarge this picture Figure 2(left). Face-centered cubic crystal structure; Figure 2(right). Face-centered cubic crystal structure

Above 910°C (1670°F), ferrite undergoes a phase transition from body-centered cubic to a face-centered cubic (fcc) structure (Fig. 2), called austenite or γ-Fe (gamma iron). Austenite is also relatively soft but can dissolve considerably more carbon - as much as 2.03 wt% carbon at 1154°C (2110°F).

Enlarge this picture Figure 3(left). Body-centered tetragonal crystal structure; Figure 3(right). Body-centered tetragonal crystal structure

Finally, above 1400°C (2550°F) and up until the melting point at about 1535°C (2795°F), austenite undergoes another phase transformation from a face-centered cubic structure back to a body-centered cubic (bcc) structure.

Perhaps the most important polymorphic form of iron occurs when, in the form of steel, it is converted from austenite to martensite, a metastable structure with about four to five times the strength of ferrite. In fact, martensite is often referred to as supersaturated ferrite.

A minimum of 0.4 wt% of carbon is needed to form martensite. When austenite is rapidly quenched to form martensite, the carbon is "frozen" in place as the crystal structure attempts to change from fcc to bcc. The carbon atoms being too large to fit in the interstitial vacancies of the crystal lattice distort the structure into a body-centered tetragonal (bct) structure (Fig. 3). Martensite and austenite have an identical chemical composition. As such, martensite requires extremely little thermal activation energy to form. Austenite is transformed to martensite on quenching at approximately the speed of sound - too fast for the carbon atoms to come out of solution. The resulting transformation of the unit cell results in numerous lattice dislocations in each crystal, which consists of millions of unit cells. These dislocations make the crystal structure extremely resistant to shear stress and extremely strong.