Allotropes of iron: Difference between revisions
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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. |
[[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. |
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As molten iron cools down it crystallises at |
As molten iron cools down, it crystallises at 1,538 °C (2800 °F) into its delta allotrope, which has a [[body-centred cubic]] (BCC) crystal structure.<ref>{{cite book |title= Metals Handbook, Vol. 8 Metallography, Structures and Phase Diagrams |origyear= 1973 |edition= 8th |publisher= ASM International |location= Metals Park, Ohio}}</ref> As it cools further, its [[crystal structure]] changes to [[face centred cubic]] (FCC) at 1,394 °C, when it is known as gamma-iron. At 912 °C (1674 °F) the crystal structure again becomes BCC as alpha-iron is formed and, at 770 °C (1,418 °F) (the [[Curie point]], T<sub>C</sub>), 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 properties. In unmagnetized iron, all the [[electron spin]]s of the [[atom]]s within one domain are in the same direction. However, in neighbouring 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 neighbouring domains reinforce each other. Although each domain contains billions of atoms, they are very small; each domain is about one thousandth of a centimetre across. |
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Iron, of course, is of most importance when mixed with certain other metals and with [[carbon]] to form [[steel]]s. 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. |
Iron, of course, is of most importance when mixed with certain other metals and with [[carbon]] to form [[steel]]s. 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. |
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Alpha iron is the stable form of iron at normal [[temperature]]s. 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). |
Alpha iron is the stable form of iron at normal [[temperature]]s. 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). |
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Above 912 °C and up to |
Above 912 °C and up to 1,394 °C, alpha iron undergoes a [[phase transition]] from body-centred cubic to the face-centred cubic configuration of gamma iron. 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 hospital and food-service equipment. |
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At one time, the beta 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 beta phase designation has been dropped. |
At one time, the beta 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 beta phase designation has been dropped. |
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Revision as of 22:34, 29 December 2010
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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 1,538 °C (2800 °F) into its delta allotrope, which has a body-centred cubic (BCC) crystal structure.[1] As it cools further, its crystal structure changes to face centred cubic (FCC) at 1,394 °C, when it is known as gamma-iron. At 912 °C (1674 °F) the crystal structure again becomes BCC as alpha-iron is formed and, at 770 °C (1,418 °F) (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 properties. In unmagnetized iron, all the electron 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 magnetized 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; each domain is 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.
The solutions of carbon and alpha, gamma and delta iron are called respectively alpha-ferrite, austenite and delta-ferrite.
Alpha 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, alpha iron undergoes a phase transition from body-centred cubic to the face-centred cubic configuration of gamma iron. 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 hospital and food-service equipment.
At one time, the beta 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 beta phase designation has been dropped.
See also
References
- ^ Metals Handbook, Vol. 8 Metallography, Structures and Phase Diagrams (8th ed.). Metals Park, Ohio: ASM International.