Allotropes of iron
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Iron represents perhaps the best-known example for allotropy in a metal. At atmospheric pressure, there are three allotropic forms of iron, known as α, γ and δ. At high pressure, a fourth form exists, called ε. Some controversial experimental evidence exists for a phase β stable at very high pressures and temperatures.[1]
As molten iron cools down it crystallizes at 1,538 °C (2,800 °F) into its δ allotrope, which has a body-centered cubic (BCC) crystal structure.[2] As it cools further its crystal structure changes to face centered cubic (FCC) at 1,394 °C, when it is known as γ-iron. At 912 °C (1,674 °F) the crystal structure again becomes BCC as α-iron is formed, and at 770 °C (1,418 °F) (the Curie point, TC) the iron becomes magnetic as α-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 properties. In unmagnetized iron, all the electron spins of the atoms within one domain are in the same direction. However, in neighboring domains they point in various directions and thus cancel out. In magnetized iron, the electronic spins of all the domains are all aligned, so that the magnetic effects of neighboring domains reinforce each other. Although each domain contains billions of atoms, they are very small; each domain is about one thousandth of a centimeter across. At pressures above approximately 10 GPa and temperatures of a few hundred kelvin or less, α-iron changes into a hexagonal close-packed (hcp) structure, which is also known as ε-iron; the higher-temperature γ-phase also changes into ε-iron, but does so at higher pressure. The β-phase, if it exists, would appear at pressures of at least 50 GPa and temperatures of at least 1,500 K; it has been thought to have an orthorhombic or a double hcp structure.
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.
The solutions of carbon and α, γ and δ-iron are called respectively α-ferrite, austenite and δ-ferrite.
α-iron 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 1,394 °C α-iron undergoes a phase transition from body-centered cubic to the face-centered cubic configuration of γ-iron. This is similarly soft and ductile but can dissolve considerably more carbon (as much as 2.04% by mass at 1,146°C). This γ form of iron is exhibited by the most commonly used type of stainless steel for making hospital and food-service equipment.
At one time, the β phase designation was given to the non-magnetic form of iron that is stable between 770 °C and 912°C. However, the magnetic transition at 770°C is nowadays not considered a phase transition and the β-phase designation has been dropped for this phase. Today it is assigned to a still controversial allotrope thought to exist at very high pressures and temperatures.
The high-pressure phases of iron are important as endmember models for the solid parts of planetary cores. The inner core of the Earth is generally assumed to consist essentially of an iron-nickel alloy with ε (or β) structure.
See also
References
- ^ Boehler, Reinhard (2000). "High-pressure experiments and the phase diagram of lower mantle and core materials". Review of Geophysics. 38. American Geophysical Union: 221–245. Bibcode:2000RvGeo..38..221B. doi:10.1029/1998RG000053.
- ^ Metals Handbook, Vol. 8 Metallography, Structures and Phase Diagrams (8th ed.). Metals Park, Ohio: ASM International. 1973.