- Ferrite (iron)
Ferrite or alpha iron (α-Fe) is a
materials scienceterm for iron, or a solid solutionwith iron as the main constituent, with a body centred cubic crystal structure. It is the component which gives steeland cast irontheir magneticproperties, and is the classic example of a ferromagnetic material.
It can be considered as pure
ironpractically (strength = 280N/mm2). Ferrite can be strictly defined as a solid solution of iron in body-centered cubic ( BCC) containing a maximum of 0.03% carbon at 723oC and 0.006% carbon at room temperature.
steels(plain carbon steels with up to about 0.2 wt% C) consist mostly of ferrite, with increasing amounts of pearlite(a fine lamellar structure of ferrite and cementite) as the carbon content is increased. Since bainite(shown as ledeburite on the diagram) and pearlite each have ferrite as a component, any iron-carbon alloy will contain some amount of ferrite if it is allowed to reach equilibrium at room temperature.
In pure iron, ferrite is stable below 910°C. Above this temperature the
face-centered cubicform of iron, austenite(gamma-iron) is stable. Above 1390°C, up to the melting pointat 1539°C, the body-centred cubic crystal structure is again the more stable form of delta-ferrite (δ-Fe).
Only a very small amount of
carboncan be dissolved in ferrite; the maximum solubilityis about 0.02wt% at 723°C. This is because carbon dissolves in iron interstitially, with the carbon atoms being about twice the diameter of the interstitial "holes", so that each carbon atom is surrounded by a strong local strain field. Hence the enthalpyof mixing is positive (unfavourable), but the contribution of entropyto the free energy of solutionstabilises the structure for low carbon content. 723°C also is the minimum temperature at which iron-carbon austenite (0.8 wt% C) is stable; at this temperature there is a eutectoidreaction between ferrite, austenite and cementite.
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