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 lattice structure. The alpha iron (α) is a body-centered cubic (BCC) and the gamma iron (γ) is a face-centered cubic (FCC).

Iron represents perhaps the best-known example for allotropy in a metal. At atmospheric pressure, there are three allotropic forms of iron: alpha iron (α) a.k.a. ferrite, gamma iron (γ) a.k.a. austenite, and delta iron (δ). At very high pressure, a fourth form exists, called epsilon iron (ε) hexaferrum. Some controversial experimental evidence exists for another 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[edit]

Delta iron (δ-Fe)[edit]

As molten iron cools down, it solidifies at 1,538 °C (2,800 °F) into its δ allotrope, which has a body-centered cubic (BCC) crystal structure.[5] δ-iron can dissolve as much as 0.09% of carbon by mass at 1,493 °C.

Gamma iron / Austenite(γ-Fe)[edit]

As the iron cools further to 1,394 °C 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.

Beta iron (β-Fe)[edit]

Beta ferrite (β-Fe) and beta iron (β-iron) are obsolete terms for the paramagnetic form of ferrite (α-Fe).[6][7] The primary phase of low-carbon or mild steel and most cast irons at room temperature is ferromagnetic ferrite (α-Fe). As iron or ferritic steel is heated above the critical temperature A2 or Curie temperature of 771 °C (1044K or 1420 °F),[8] the random thermal agitation of the atoms exceeds the oriented magnetic moment of the unpaired electron spins in the 3d shell.[9] The A2 forms the low-temperature boundary of the beta iron field in the phase diagram in Figure 1. Beta ferrite is crystallographically identical to alpha ferrite, except for magnetic domains and the expanded body-centered cubic lattice parameter as a function of temperature, and is therefore of only minor importance in steel heat treating. For this reason, the beta "phase" is not usually considered a distinct phase but merely the high-temperature end of the alpha phase field.

Alpha iron / Ferrite (α-Fe)[edit]

At 912 °C (1,674 °F) the crystal structure again becomes BCC as α-iron is formed. The substance assumes a paramagnetic property. α-iron can dissolve only a small concentration of carbon (no more than 0.021% by mass at 723 °C).

At 770 °C (1,418 °F), the Curie point (TC), the iron is a fairly soft metal and becomes ferromagnetic. As the iron passes through the Curie temperature there is no change in crystalline structure, but there is a change in the magnetic properties as the magnetic domains become aligned. This is the stable form of iron at room temperature.

High pressure allotropes[edit]

Epsilon iron / Hexaferrum (ε-Fe)[edit]

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;[10] 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.[11]

Experimental high temperature and pressure[edit]

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.

See also[edit]

References[edit]

  1. ^ a b Boehler, Reinhard (2000). "High-pressure experiments and the phase diagram of lower mantle and core materials". Review of Geophysics. American Geophysical Union. 38 (2): 221–245. Bibcode:2000RvGeo..38..221B. doi:10.1029/1998RG000053. 
  2. ^ Cohen, Ronald; Stixrude, Lars. "Crystal at the Center of the Earth". Archived from the original on 5 February 2007. Retrieved 2007-02-05. 
  3. ^ Lars Stixrude and R. E. Cohen, "High-Pressure Elasticity of Iron and Anisotropy of Earth's Inner Core", Science 31 March 1995: Vol. 267. no. 5206, pp. 1972–1975 doi:10.1126/science.267.5206.1972
  4. ^ BBC News, "What is at the centre of the Earth?
  5. ^ Metals Handbook, Vol. 8 Metallography, Structures and Phase Diagrams (8th ed.). Metals Park, Ohio: ASM International. 1973. 
  6. ^ D. K. Bullens et al., Steel and Its Heat Treatment, Vol. I, Fourth Ed., J. Wiley & Sons Inc., 1938, p. 86.
  7. ^ S. H. Avner, Introduction to Physical Metallurgy, 2nd Ed., McGraw-Hill, 1974, p. 225.
  8. ^ ASM Handbook, Vol. 3: Alloy Phase Diagrams, ASM International, 1992, p. 2.210 and 4.9, ISBN 0-87170-381-5.
  9. ^ B. D. Cullity & C. D. Graham, Introduction to Magnetic Materials, Second Ed., IEEE Inc., 2009, p. 91, ISBN 978-0-471-47741-9.
  10. ^ Mathon O; Baudelet F; Itié JP; Polian A; d'Astuto M; Chervin JC; Pascarelli S. (December 14, 2004). "Dynamics of the magnetic and structural alpha-epsilon phase transition in iron". Physical Review Letters. 93 (25): 255503. PMID 15697906. doi:10.1103/PhysRevLett.93.255503. 
  11. ^ G. C. Fletcher; R. P. Addis. "The magnetic state of the phase of iron" (PDF). Journal of Physics F: Metal Physics. 4 (11 (November 1974)). p. 1954. doi:10.1088/0305-4608/4/11/020. Retrieved December 30, 2011.