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Allotropes of iron
Allotropes of iron
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.
Iron-carbon eutectic phase diagram, showing various forms of FexCy substances.
Iron allotropes, showing the differences in 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, depending on temperature: alpha iron (α-Fe, ferrite), gamma iron (γ-Fe, austenite), and delta iron (δ-Fe, similar to alpha iron). At very high pressure, a fourth form exists, epsilon iron (ε-Fe, hexaferrum). 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

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Alpha iron (α-Fe)

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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 line forms the boundary between the beta iron and alpha fields in the phase diagram in Figure 1.

Similarly, the A2 boundary 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 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 of carbon is about 0.02 wt% at 727 °C (1,341 °F) and 0.001% 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) consists mostly of α-Fe and increasing amounts of cementite (Fe3C, an iron carbide). The mixture adopts a lamellar 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

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Figure 1: The beta field and A2 critical temperature on the iron-rich side of the iron-carbon phase diagram.[5]

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 heating and ferromagnetic hysteresis losses. Above the A2 boundary, the hysteresis mechanism disappears and the required amount of energy per degree of temperature increase is thus 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)

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Main article: Austenite

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 austenitic stainless steel.

Delta iron (δ-Fe)

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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). δ-Fe cannot exist above 5.2 GPa, with austenite instead transitioning directly to a molten phase at these high pressures.[15]

High pressure allotropes

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Epsilon iron / Hexaferrum (ε-Fe)

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Main article: Hexaferrum

At pressures above approximately 10–13 GPa and temperatures up to around 700 K, α-iron changes into a hexagonal close-packed (hcp) structure, which is also known as ε-iron or hexaferrum;[16] the higher-temperature γ-phase also changes into ε-iron, but generally requires far higher pressures as temperature increases. The triple point of hexaferrum, ferrite, and austenite is 10.5 GPa at 750 K.[15] Antiferromagnetism in alloys of epsilon-Fe with Mn, Os and Ru has been observed.[17]

Experimental high temperature and pressure

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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

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Melting and boiling points

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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.[18]

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;[19] 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.[20]

Structural phase transitions

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"Gamma loop" redirects here; not to be confused with Gamma motor neuron.

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.[21]

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Allotropes of iron

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Allotropes of iron are the distinct crystalline phases of elemental iron, characterized by different atomic packing arrangements that emerge under varying temperature and pressure conditions. At , pure iron displays three primary allotropes: α-iron (ferrite), which adopts a body-centered cubic (BCC) and remains stable from room temperature up to 912 °C; γ-iron (), featuring a face-centered cubic (FCC) structure and stable from 912 °C to 1394 °C; and δ-iron, which reverts to a BCC structure and persists from 1394 °C until the at 1538 °C. Under extreme high pressures, such as those exceeding 10 GPa, a fourth allotrope, ε-iron (hexaferrum), forms with a hexagonal close-packed (HCP) structure, representing the densest phase of iron. These allotropic transformations play a pivotal role in and , particularly in the production and of steels, where the phase changes influence carbon , microstructure , and resulting mechanical properties like strength and . For instance, the FCC structure of γ-iron allows for higher interstitial carbon dissolution (up to 2.1 wt% at the eutectic temperature), enabling processes such as to form for hardened steels, while the BCC forms of α- and δ-iron exhibit lower carbon (maximum 0.02 wt%). The of α-iron, at approximately 770 °C, marks the transition from ferromagnetic to paramagnetic behavior, further impacting applications in magnetic materials. Beyond industrial uses, iron allotropes are relevant to , as ε-iron is believed to constitute part of under pressures of 300–360 GPa and temperatures around 5000–6000 K, influencing planetary and seismic properties. Recent studies continue to explore these phases at extreme conditions using advanced techniques like diamond anvil cells and shock compression, revealing potential for new high-pressure variants with unique electronic properties.

Ambient-Pressure Allotropes

Alpha iron (α-Fe)

Alpha iron, denoted as α-Fe, is the stable allotrope of pure iron at low temperatures and ambient pressure, exhibiting a body-centered cubic (BCC) crystal structure with a lattice parameter of approximately 2.87 Å at room temperature. This structure consists of iron atoms arranged at the corners and center of a cubic unit cell, resulting in two atoms per unit cell and contributing to its relatively high packing efficiency of about 68%. The BCC arrangement imparts alpha iron with notable ductility and malleability, making it a foundational phase in iron-based materials. It remains thermodynamically stable from absolute zero up to 1185 K (912°C) at ambient pressure, above which it transforms into the gamma phase. A key characteristic of alpha iron is its ferromagnetic behavior below the , known as the A2 point, at 1043 K (770°C), where it transitions from ferromagnetic to paramagnetic ordering. In the ferromagnetic state, the material organizes into magnetic domains—regions where atomic magnetic moments align spontaneously to minimize magnetostatic energy—leading to a net in the presence of an external field. The saturation , the maximum achievable magnetic induction, reaches approximately 2.15 T at low temperatures, reflecting the strong exchange interactions between unpaired electrons in the 3d orbitals of iron atoms. This magnetic property is crucial for applications in electromagnets and transformers. Physically, alpha iron has a of about 7.87 g/cm³ at , which arises from the compact yet open BCC lattice. It also displays a linear coefficient of approximately 12 × 10^{-6} K^{-1}, indicating moderate dimensional changes with variations. In metallurgical contexts, alpha iron forms the basis of ferrite, a phase that incorporates carbon atoms in the BCC lattice; however, its limit for carbon is very low, with a maximum of about 0.02 wt% near the eutectoid , dropping to around 0.005 wt% at due to the limited size of sites. This restricted prevents significant hardening via carbon dissolution, distinguishing ferrite from other phases. The recognition of alpha iron as ferrite in dates back to the late 19th century, when metallurgists like Henry Clifton Sorby used early microscopic techniques to observe its distinct microstructure in iron-carbon alloys, naming it after the Latin "ferrum" for iron to denote the pure alpha phase. This naming convention highlighted its role as the soft, magnetic constituent in steels, influencing the development of phase diagrams and processes.

Gamma iron (γ-Fe)

Gamma iron, denoted as γ-Fe or austenite, is the high-temperature allotrope of iron stable under ambient pressure conditions, exhibiting a face-centered cubic (FCC) crystal structure that enables greater interstitial site availability compared to lower-temperature phases. This structure features a lattice parameter of approximately 3.63 Å at 915°C, reflecting the close-packed arrangement of iron atoms in the cubic lattice. The FCC configuration results in an atomic packing factor of 0.74, higher than the 0.68 for the body-centered cubic (BCC) structure of alpha iron, which contributes to its distinct mechanical and thermodynamic behavior despite occurring at elevated temperatures. Throughout its stability range from 1185 K (912°C) to 1667 K (1394°C) at , gamma iron displays paramagnetic properties with no ferromagnetic ordering, distinguishing it from the ferromagnetic alpha phase below its . The of gamma iron is approximately 7.6 g/cm³ in this range, lower than that of alpha iron primarily due to effects, even though the denser FCC packing would otherwise suggest higher at equivalent temperatures. A key characteristic of gamma iron is its enhanced solubility for carbon, reaching up to 2.1 wt% at 1147°C, which forms the basis for in alloys and enables critical processes like to achieve desired microstructures. This allotrope forms upon heating alpha iron beyond 912°C and persists until transitioning to delta iron near the , providing a window for alloying and phase manipulation in metallurgical applications.

Delta iron (δ-Fe)

Delta iron (δ-Fe) is a high-temperature allotrope of iron characterized by a body-centered cubic (BCC) , identical to that of alpha iron (α-Fe). This phase forms upon cooling from the liquid state and is stable under in a narrow range from 1667 K (1394°C) to the at 1811 K (1538°C). Due to its position just below the , delta iron represents a transient solid phase before liquefaction. The lattice parameter of delta iron expands with increasing temperature owing to thermal effects, reaching approximately 2.93 Å in its high-temperature regime. Throughout its stability range, the phase exhibits , as the elevated temperatures exceed the point, preventing ferromagnetic ordering observed in lower-temperature phases. Density decreases progressively due to , approaching lower values near the compared to room-temperature iron. Carbon solubility in delta iron remains low, at less than 0.1 wt%, akin to alpha iron and restricting its utility in high-carbon alloying applications. The confined temperature interval poses significant challenges for direct observation and study, contributing to its initial identification via methods in early 20th-century metallurgical investigations.

High-Pressure Allotropes

Epsilon iron (ε-Fe)

Epsilon iron (ε-Fe), also known as hexaferrum, is the hexagonal close-packed (HCP) allotrope of iron that forms under high-pressure conditions, representing a denser phase compared to the ambient-pressure body-centered cubic (BCC) alpha iron. This phase was first discovered in 1956 through shock-wave experiments on iron, where compressive waves revealed a polymorphic transition from the BCC to HCP structure at pressures exceeding approximately 13 GPa. The HCP lattice features an ideal c/a ratio of about 1.63, with lattice parameters of a ≈ 2.50 and c ≈ 4.07 observed at pressures of 10-20 GPa, enabling efficient atomic packing. The of 0.74 is identical to that of the face-centered cubic (FCC) structure but differs in slip systems, primarily basal {0001}<11\overline{2}0> planes, which can limit relative to the more isotropic BCC form under ambient conditions. The ε-Fe phase exhibits non-magnetic behavior at and pressures within its stability range, though theoretical models predict antiferromagnetic ordering at low temperatures below approximately 55 near 20 GPa arising from layered spin arrangements, while experiments reveal no long-range order but persistent local magnetic moments in a -smectic-like state. Its density ranges from 8.5 to 9.0 g/cm³ in this pressure regime, significantly higher than the 7.87 g/cm³ of alpha iron due to compression-induced reduction, with a discontinuous reduction of approximately 0.2 cm³/mol at the transition. Stability occurs above ~10-13 GPa at and extends over a wide range of combined high-pressure and high-temperature conditions, including up to inner core pressures and temperatures, where it persists as the dominant solid phase before melting. Naturally, hexaferrum appears in meteorites as a high-pressure phase in iron-nickel alloys, often hosting platinum-group elements, providing direct evidence of HCP iron formation in extraterrestrial environments. In geophysical contexts, ε-Fe is highly relevant to the composition of , where pressures exceed 330 GPa and temperatures reach ~6000 ; seismic data and equation-of-state models indicate that the solid inner core consists primarily of this HCP iron phase, potentially alloyed with and lighter elements to match observed densities. This structure contributes to the core's elastic anisotropy and influences planetary generation through its thermodynamic properties.

Additional High-Pressure Phases

While early reports from experiments proposed orthorhombic distortions of the HCP structure ( Pbcm) at pressures above 50 GPa and , subsequent computational studies have shown these to be dynamically unstable and metastable relative to ε-Fe. Similarly, double-hexagonal close-packed (DHCP) phases with ABAC stacking have been suggested as metastable intermediates under extreme conditions exceeding 200 GPa and temperatures of 2000–4000 K, but not as thermodynamically stable for pure iron. Density functional theory (DFT) calculations have explored a body-centered tetragonal (BCT) phase at ultra-high pressures beyond 300 GPa, where the c/a ratio approaches 0.9, offering a distorted variant of the ambient BCC structure; however, these indicate it is metastable with dynamical instabilities at high temperatures, unlikely to persist in equilibrium at conditions (330–360 GPa). These proposed transitions often exhibit , with no recovery to ambient structures upon decompression in quenched samples. As of , advanced simulations including potentials confirm ε-Fe (HCP) as the stable phase dominating , potentially with minor FCC contributions at extreme temperatures, explaining seismic without requiring additional stable distorted phases. No direct natural samples of these high-pressure variants exist beyond low-pressure meteoritic hexaferrum. Such insights from and computational studies underscore the role of ε-Fe in dynamics.

Phase Transitions and Thermodynamic Properties

Structural Phase Transitions

The structural phase transitions in iron involve changes between its solid allotropes, driven by and pressure, and are characterized by distinct mechanisms involving , growth, and associated thermodynamic changes. At , the transition from alpha iron (body-centered cubic, BCC) to gamma iron (face-centered cubic, FCC) occurs at 1185 K (912°C), representing a reconstructive transformation where the FCC phase nucleates at grain boundaries and triple junctions of the BCC structure, leading to a volume contraction of approximately 1% due to the denser packing of the FCC lattice. This process is thermally activated and exhibits kinetics influenced by , with the transformation completing over a narrow range during heating. The reverse gamma-to-alpha transition upon cooling is exothermic and shows , typically completing 10-20 below the heating , due to the higher nucleation barrier for BCC formation. Further heating leads to the gamma-to-delta transition at 1665 (1392°C), where the FCC structure reverts to BCC (delta iron), accompanied by a volume expansion of about 1-2% and an endothermic change, with the process involving similar grain-boundary but faster kinetics owing to the proximity to . This transition also displays during cooling, with delta phase persistence down to around 1640 , reflecting the thermodynamic stability fields. The change for the alpha-to-gamma transition is approximately 900 J/mol, as measured by (DSC), highlighting the energetic cost of the structural reorganization. Under elevated pressures, the alpha-to-epsilon transition (BCC to hexagonal close-packed, HCP) initiates at 10-13 GPa and , proceeding via a diffusive mechanism with significant (up to 2-3 GPa between forward and reverse paths) due to the sluggish kinetics under hydrostatic conditions. The Clapeyron slope for this boundary is approximately 100 MPa/, indicating a positive dP/dT consistent with the volume decrease of about 3-4% during the transformation. The of iron up to 50 GPa reveals key triple points, including the alpha-gamma-epsilon point at around 8.2 GPa and 678 , and the gamma-epsilon-liquid point near 50 GPa and 2500 , where the epsilon phase dominates at higher pressures and lower temperatures relative to the gamma field. These transitions are modulated by external factors such as alloying elements, where carbon stabilizes the gamma phase by expanding its range (e.g., up to 1000 in low-carbon steels), while strain from deformation lowers activation barriers and accelerates through defect-assisted growth. Kinetics are governed by rates and interfacial mobility, with slower rates at lower favoring incomplete transformations and potential retention of metastable phases.

Melting and Boiling Points

Under ambient pressure, the highest-temperature solid allotrope of iron, delta iron (δ-Fe), melts into liquid iron at 1811 K (1538°C), absorbing a of fusion of approximately 13.8 kJ/mol. The liquid iron then boils at 3134 K (2861°C), requiring a of vaporization of about 340 kJ/mol to form monatomic iron vapor. These values represent the thermodynamic endpoints for iron's phase changes at standard conditions, with delta iron serving as the pre-melt solid phase. The melting temperature of iron exhibits a positive dependence, increasing at a rate of approximately 40–50 K/GPa in the low-pressure regime up to about 100 GPa. This behavior is critical for geophysical models of Earth's interior, where the extrapolated of iron at inner core boundary conditions (~360 GPa) reaches around 6000 K. Along the delta solid-to-liquid phase boundary, a density discontinuity arises, with liquid iron being less dense than the solid by roughly 3% near . Early determinations of iron's melting point relied on optical pyrometry techniques, which measured thermal radiation from heated samples to estimate temperatures in the 1500–1600°C range. Modern high-precision experiments, including those using gamma-ray attenuation and differential scanning calorimetry in the 2020s, have refined and confirmed the value at 1811 K under ambient pressure. In pure iron, kinetic effects such as supercooling (where the liquid persists below 1811 K) and superheating (where the solid endures above it) can occur, though these metastable states are limited by nucleation barriers and typically span tens of kelvin.

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