Allotropes of iron

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Low-pressure phase diagram of pure iron. BCC is body centered cubic and FCC is face-centered cubic. Pure iron phase diagram (EN).svg
Low-pressure phase diagram of pure iron. BCC is body centered cubic and FCC is face-centered cubic.
Iron-carbon eutectic phase diagram, showing various forms of FexCy substances. Iron carbon phase diagram.svg
Iron-carbon eutectic phase diagram, showing various forms of FexCy substances.
Iron allotropes, showing the differences in lattice structure. The alpha iron (a-Fe) is a body-centered cubic (BCC) and the gamma iron (g-Fe) is a face-centered cubic (FCC). IronAlfa&IronGamma.svg
Iron allotropes, showing the differences in lattice structure. The alpha iron (α-Fe) is a body-centered cubic (BCC) and the gamma iron (γ-Fe) is a face-centered cubic (FCC).

At atmospheric pressure, three allotropic forms of iron exist: alpha iron (α-Fe), gamma iron (γ-Fe), and delta iron (δ-Fe). At very high pressure, a fourth form exists, called epsilon iron (ε-Fe). Some controversial experimental evidence suggests the existence of a fifth high-pressure form that is stable at very high pressures and temperatures. [1]


The phases of iron at atmospheric pressure are important because of the differences in solubility of carbon, forming different types of steel. The high-pressure phases of iron are important as models for the solid parts of planetary cores. The inner core of the Earth is generally assumed to consist essentially of a crystalline iron-nickel alloy with ε structure. [2] [3] [4] The outer core surrounding the solid inner core is believed to be composed of liquid iron mixed with nickel and trace amounts of lighter elements.

Standard pressure allotropes

Alpha iron (α-Fe)

Below 912 °C (1,674 °F), iron has a body-centered cubic (bcc) crystal structure and is known as α-iron or ferrite. It is thermodynamically stable and a fairly soft metal. α-Fe can be subjected to pressures up to ca. 15 GPa before transforming into a high-pressure form termed ε-Fe discussed below.

Magnetically, α-iron is paramagnetic at high temperatures. However, below its Curie temperature (TC or A2) of 771 °C (1044K or 1420 °F), [5] , it becomes ferromagnetic. In the past, the paramagnetic form of α-iron was known as Beta iron (β-Fe). [6] [7] Even though the slight tetragonal distortion in the ferromagnetic state does constitute a true phase transition, the continuous nature of this transition results in only minor importance in steel heat treating. The A2 forms the boundary between the beta iron and alpha fields in the phase diagram in Figure 1.

Similarly, the A2 is of only minor importance compared to the A1 (eutectoid), A3 and Acm critical temperatures. The Acm, where austenite is in equilibrium with cementite + γ-Fe, is beyond the right edge in Fig. 1. The α + γ phase field is, technically, the β + γ field above the A2. The beta designation maintains continuity of the Greek-letter progression of phases in iron and steel: α-Fe, β-Fe, austenite (γ-Fe), high-temperature δ-Fe, and high-pressure hexaferrum (ε-Fe).

Molar volume vs. pressure for a-Fe at room temperature. Iron-alpha-pV.svg
Molar volume vs. pressure for α-Fe at room temperature.

The primary phase of low-carbon or mild steel and most cast irons at room temperature is ferromagnetic α-Fe. [8] [9] It has a hardness of approximately 80 Brinell. [10] [11] The maximum solubility is about 0.02 wt% at 727 °C (1,341 °F) and 0.001% carbon at 0 °C (32 °F). [12] When it dissolves in iron, carbon atoms occupy interstitial "holes". Being about twice the diameter of the tetrahedral hole, the carbon introduces a strong local strain field.

Mild steel (carbon steel with up to about 0.2 wt% C) consist mostly of α-Fe and increasing amounts of cementite (Fe3C, an iron carbide). The mixture adopts a laminar structure called pearlite. Since bainite and pearlite each contain α-Fe as a component, any iron-carbon alloy will contain some amount of α-Fe if it is allowed to reach equilibrium at room temperature. The amount of α-Fe depends on the cooling process.

A2 critical temperature and induction heating

Figure 1: The beta field and A2 critical temperature on the iron-rich side of the iron-carbon phase diagram. Beta iron carbon pd.TIF
Figure 1: The beta field and A2 critical temperature on the iron-rich side of the iron-carbon phase diagram.

β-Fe and the A2 critical temperature are important in induction heating of steel, such as for surface-hardening heat treatments. Steel is typically austenitized at 900–1000 °C before it is quenched and tempered. The high-frequency alternating magnetic field of induction heating heats the steel by two mechanisms below the Curie temperature: resistance or Joule (I2R) heating and ferromagnetic hysteresis losses. Above the A2, the hysteresis mechanism disappears and the required amount of energy per degree of temperature increase is substantially larger than below A2. Load-matching circuits may be needed to vary the impedance in the induction power source to compensate for the change. [13]

Gamma iron (γ-Fe)

When heating iron above 912 °C (1,674 °F), its crystal structure changes to a face-centered cubic (fcc) crystalline structure. In this form it is called gamma iron (γ-Fe) or Austenite. γ-iron can dissolve considerably more carbon (as much as 2.04% by mass at 1,146 °C). This γ form of carbon saturation is exhibited in stainless steel.

Delta iron (δ-Fe)

Peculiarly, above 1,394 °C (2,541 °F) iron changes back into the bcc structure, known as δ-Fe. [14] δ-iron can dissolve as much as 0.08% of carbon by mass at 1,475 °C. It is stable up to its melting point of 1,538 °C (2,800 °F).

High pressure allotropes

Epsilon iron / Hexaferrum (ε-Fe)

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 or hexaferrum; [15] the higher-temperature γ-phase also changes into ε-iron, but does so at a higher pressure. Antiferromagnetism in alloys of epsilon-Fe with Mn, Os and Ru has been observed. [16]

Experimental high temperature and pressure

An alternate stable form, if it exists, may 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. [1] as of December 2011, recent and ongoing experiments are being conducted on high-pressure and Superdense carbon allotropes.

Phase transitions

Melting and boiling points

The melting point of iron is experimentally well defined for pressures less than 50 GPa.

For greater pressures, published data (as of 2007) put the γ-ε-liquid triple point at pressures that differ by tens of gigapascals and 1000 K in the melting point. Generally speaking, molecular dynamics computer simulations of iron melting and shock wave experiments suggest higher melting points and a much steeper slope of the melting curve than static experiments carried out in diamond anvil cells. [17]

The melting and boiling points of iron, along with its enthalpy of atomization, are lower than those of the earlier group 3d elements from scandium to chromium, showing the lessened contribution of the 3d electrons to metallic bonding as they are attracted more and more into the inert core by the nucleus; [18] however, they are higher than the values for the previous element manganese because that element has a half-filled 3d subshell and consequently its d-electrons are not easily delocalized. This same trend appears for ruthenium but not osmium. [19]

Structural phase transitions

The exact temperatures at which iron will transition from one crystal structure to another depends on how much and what type of other elements are dissolved in the iron. The phase boundary between the different solid phases is drawn on a binary phase diagram, usually plotted as temperature versus percent iron. Adding some elements, such as Chromium, narrows the temperature range for the gamma phase, while others increase the temperature range of the gamma phase. In elements that reduce the gamma phase range, the alpha-gamma phase boundary connects with the gamma-delta phase boundary, forming what is usually called the Gamma loop. Adding Gamma loop additives keeps the iron in a body-centered cubic structure and prevents the steel from suffering phase transition to other solid states. [20]

See also

Related Research Articles

Iron Chemical element, symbol Fe and atomic number 26

Iron is a chemical element with symbol Fe and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in front of oxygen, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust.

Steel Metal alloy made by combining iron with other elements

Steel is an alloy made up of iron with typically a few tenths of a percent of carbon to improve its strength and fracture resistance compared to iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant need typically an additional 11% chromium. Because of its high tensile strength and low cost, steel is used in buildings, infrastructure, tools, ships, trains, cars, machines, electrical appliances, and weapons. Iron is the base metal of steel. Depending on the temperature, it can take two crystalline forms : body-centred cubic and face-centred cubic. The interaction of the allotropes of iron with the alloying elements, primarily carbon, gives steel and cast iron their range of unique properties.

Eutectic system Mixture with a lower melting point than its constituents

A eutectic system from the Greek "εύ" and "τήξις" is an homogeneous mixture of substances that melts or solidifies at a single temperature that is lower than the melting point of any of the constituents. This temperature is known as the eutectic temperature, and is the lowest possible melting temperature over all of the mixing ratios for the involved component species. On a phase diagram, the eutectic temperature is seen as the eutectic point.

Heat treating Process of heating something to alter it

Heat treating is a group of industrial, thermal and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve the desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing and quenching. Although the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.

Iron(III) oxide Chemical compound

Iron(III) oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron(II) oxide (FeO), which is rare; and iron(II,III) oxide (Fe3O4), which also occurs naturally as the mineral magnetite. As the mineral known as hematite, Fe2O3 is the main source of iron for the steel industry. Fe2O3 is readily attacked by acids. Iron(III) oxide is often called rust, and to some extent this label is useful, because rust shares several properties and has a similar composition; however, in chemistry, rust is considered an ill-defined material, described as Hydrous ferric oxide.


Martensite is a very hard form of steel crystalline structure. It is named after German metallurgist Adolf Martens. By analogy the term can also refer to any crystal structure that is formed by diffusionless transformation.


Cementite (or iron carbide) is a compound of iron and carbon, more precisely an intermediate transition metal carbide with the formula Fe3C. By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure. It is a hard, brittle material, normally classified as a ceramic in its pure form, and is a frequently found and important constituent in ferrous metallurgy. While cementite is present in most steels and cast irons, it is produced as a raw material in the iron carbide process, which belongs to the family of alternative ironmaking technologies. The name cementite originated from the research of Floris Osmond and J. Werth, where the structure of solidified steel consists of a kind of cellular tissue in theory, with ferrite as the nucleus and Fe3C the envelope of the cells. The carbide therefore cemented the iron.

Austenite Metallic, non-magnetic allotrope of iron or a solid solution of iron, with an alloying element

Austenite, also known as gamma-phase iron (γ-Fe), 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 1000 K (727 °C); other alloys of steel have different eutectoid temperatures. The austenite allotrope is named after Sir William Chandler Roberts-Austen (1843–1902); it exists at room temperature in some stainless steels due to the presence of nickel stabilizing the austenite at lower temperatures.


Bainite is a plate-like microstructure that forms in steels at temperatures of 125–550 °C. First described by E. S. Davenport and Edgar Bain, it is one of the products that may form when austenite is cooled past a temperature where it no longer is thermodynamically stable with respect to ferrite, cementite, or ferrite and cementite. Davenport and Bain originally described the microstructure as being similar in appearance to tempered martensite.


Pearlite is a two-phased, lamellar structure composed of alternating layers of ferrite and cementite that occurs in some steels and cast irons. During slow cooling of an iron-carbon alloy, pearlite forms by a eutectoid reaction as austenite cools below 723 °C (1,333 °F). Pearlite is a microstructure occurring in many common grades of steels.

Maraging steel

Maraging steels are steels that are known for possessing superior strength and toughness without losing ductility. Aging refers to the extended heat-treatment process. These steels are a special class of low-carbon ultra-high-strength steels that derive their strength not from carbon, but from precipitation of intermetallic compounds. The principal alloying element is 15 to 25 wt% nickel. Secondary alloying elements, which include cobalt, molybdenum and titanium, are added to produce intermetallic precipitates. Original development was carried out on 20 and 25 wt% Ni steels to which small additions of aluminium, titanium, and niobium were made; a rise in the price of cobalt in the late 1970s led to the development of cobalt-free maraging steels.

Solid oxygen forms at normal atmospheric pressure at a temperature below 54.36 K (−218.79 °C, −361.82 °F). Solid oxygen O2, like liquid oxygen, is a clear substance with a light sky-blue color caused by absorption in the red part of the visible light spectrum.

Titanium hydride Chemical compound

Titanium hydride normally refers to the inorganic compound TiH2 and related nonstoichiometric materials. It is commercially available as a stable grey/black powder, which is used as an additive in the production of Alnico sintered magnets, in the sintering of powdered metals, the production of metal foam, the production of powdered titanium metal and in pyrotechnics.

TRIP steel are a class of high-strength steel alloys typically used in naval and marine applications and in the automotive industry. TRIP stands for "Transformation induced plasticity," which implies a phase transformation in the material, typically when a stress is applied. These alloys are known to possess an outstanding combination of strength and ductility.


Hexaferrum and epsilon iron (ε-Fe) are synonyms for the hexagonal close-packed (HCP) phase of iron that is stable only at extremely high pressure.

Iron(II) selenide refers to a number of inorganic compounds of ferrous iron and selenide (Se2−). The phase diagram of the system Fe–Se reveals the existence of several non-stoichiometric phases between ~49 at. % Se and ~53 at. % Fe, and temperatures up to ~450 °C. The low temperature stable phases are the tetragonal PbO-structure (P4/nmm) β-Fe1−xSe and α-Fe7Se8. The high temperature phase is the hexagonal, NiAs structure (P63/mmc) δ-Fe1−xSe. Iron(II) selenide occurs naturally as the NiAs-structure mineral achavalite.

Iron–hydrogen alloy

Iron–hydrogen alloy, also known as iron hydride, is an alloy of iron and hydrogen and other elements. Because of its lability when removed from a hydrogen atmosphere, it has no uses as a structural material.

Solid nitrogen

Solid nitrogen is the solid form of the element nitrogen. It is an important component of the surfaces of Pluto and outer moons of the Solar System such as Neptune's Triton. Under low or moderate pressure solid nitrogen contains dinitrogen molecules held together by London dispersion forces. At standard atmospheric pressure for Earth, this solid melts at 63.23 K, but this is not true at other pressures. Non-molecular forms of solid nitrogen produced by extreme pressures have a higher energy density than any other non-nuclear material.

Ferritic stainless steel forms one of the four stainless steel families, the other three being austenitic, martensitic and duplex stainless steels

κ-Carbides are a special class of carbide structures. They are most known for appearing in steels containing manganese and aluminium where they have the molecular formula (Fe,Mn)


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