Allotropes of iron
Encyclopedia
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.
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. 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 atom
s 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 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.
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 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).
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.
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
represents perhaps the best-known example for allotropy
Allotropy
Allotropy or allotropism is the property of some chemical elements to exist in two or more different forms, known as allotropes of these elements...
in a metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
. At atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...
, 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.
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. As it cools further its crystal structure
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
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
Curie point
In physics and materials science, the Curie temperature , or Curie point, is the temperature at which a ferromagnetic or a ferrimagnetic material becomes paramagnetic on heating; the effect is reversible. A magnet will lose its magnetism if heated above the Curie temperature...
, 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 atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s 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
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
to form steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
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.
The solutions of carbon and α, γ and δ-iron are called respectively α-ferrite
Ferrite
Ferrite may refer to:* Ferrite , iron or iron alloys with a body centred cubic crystal structure.* Ferrite , ferrimagnetic ceramic materials used in magnetic applications....
, austenite
Austenite
Austenite, also known as gamma phase iron, is a metallic non-magnetic allotrope of iron or a solid solution of iron, with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of ; other alloys of steel have different eutectoid temperatures...
and δ-ferrite.
α-iron is the stable form of iron at normal temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
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).
Above 912 °C and up to 1,394 °C α-iron undergoes a phase transition
Phase transition
A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another.A phase of a thermodynamic system and the states of matter have uniform physical properties....
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
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
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
Inner core
The inner core of the Earth, its innermost hottest part as detected by seismological studies, is a primarily solid ball about in radius, or about 70% that of the Moon...
of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
is generally assumed to consist essentially of an iron-nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
alloy
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
with ε (or β) structure.