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Allotropes of iron
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Iron represents perhaps the best-known example for allotropy in a metal. There are three allotropic forms of iron, known as alpha, gamma, and delta.
As molten iron cools down it crystallises at 1538°C into its delta allotrope, which has a body-centred cubic (BCC) crystal structure. As it cools further its crystal structure changes to face centred cubic (FCC) at 1394°C, when it is known as gamma-iron, or austenite.
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Iron represents perhaps the best-known example for allotropy in a metal. There are three allotropic forms of iron, known as alpha, gamma, and delta.
As molten iron cools down it crystallises at 1538°C into its delta allotrope, which has a body-centred cubic (BCC) crystal structure. As it cools further its crystal structure changes to face centred cubic (FCC) at 1394°C, when it is known as gamma-iron, or austenite. At 912°C the crystal structure again becomes BCC as alpha-iron also known as ferrite, is formed, and at 770°C (the Curie point, Tc ) the iron becomes magnetic as alpha-iron, which is also BCC, is formed. As the iron passes through the Curie temperature there is no change in crystalline structure, but there is a change in the magnetic response. In unmagnetised iron, all the electronic spins of the atoms within one domain are in the same direction. However, in neighbouring domains they point in various directions and thus cancel out. In magnetised iron, the electronic spins of all the domains are all aligned, so that the magnetic effects of neighbouring domains reinforce each other. Although each domain contains billions of atoms, they are very small, about one thousandth of a centimetre across.
Iron, of course, is of most importance when mixed with certain other metals and with carbon to form steels. There are many types of steels, all with different properties; and an understanding of the properties of the allotropes of iron is key to the manufacture of good quality steels.
Alpha iron, also known as ferrite, is the stable form of iron at normal temperatures. It is a fairly soft metal that can dissolve only a small concentration of carbon (no more than 0.021% by mass at 910 °C).
Above 912°C and up to 1394°C alpha iron undergoes a phase transition from body-centred cubic to the face-centred cubic configuration of gamma iron, also called austenite. This is similarly soft and ductile but can dissolve considerably more carbon (as much as 2.04% by mass at 1146°C). This gamma form of iron is exhibited by the most commonly used type of stainless steel for making and hospital and food-service equipment.
At one time, the beta phase designation was give to the non-magnetic form of iron that is stable between 770 and 912°C. However, the magnetic transition at 770°C is not now considered a phase transition and the beta phase designation has been dropped.
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