Hexaferrum
Encyclopedia
Hexaferrum and epsilon iron (ε-Fe) are synonym
s for the hexagonal close-packed (HCP) phase of iron
that is stable only at extremely high pressure. Takahashi and Bassett at the University of Rochester
mixed 99.8% pure alpha iron powder with sodium chloride
, and pressed a 0.5-mm diameter pellet between the flat faces of two diamond anvils. The deformation of the NaCl lattice, as measured by x-ray diffraction (XRD), served as a pressure indicator. At a pressure of 130 kbar (13 GPa) and room temperature, the body-centered cubic ferrite
powder transformed to the HCP phase in Figure 1. When the pressure was lowered, ε-Fe transformed back to ferrite (α-Fe) rapidly. A specific volume
change of -0.20 cm3/mole ± 0.03 was measured. Hexaferrum, much like austenite
, is more dense than ferrite at the phase boundary. A shock wave
experiment confirmed the diamond anvil results.
The triple point between the alpha, gamma and epsilon phases in the unary phase diagram of iron has been calculated as T = 770K and P = 11 GPa, although it was determined at a lower temperature of T = 750K (477°C) in Figure 1. The Pearson symbol
for hexaferrum is hP2 and its space group
is P63/mmc.
Giles et al. studied the ferrite-hexaferrum transformation metallographically
and noted that it is a martensitic rather than equilibrium
transformation.
While hexaferrum is purely academic in metallurgical engineering, it may have significance in geology
. The pressure and temperature of Earth's iron core
are on the order of 1500-3500 kbar and 3000 ± 1000°C. An extrapolation of the austenite-hexaferrum phase boundary in Figure 1 suggests hexaferrum might be stable or metastable in Earth’s core.
Synonym
Synonyms are different words with almost identical or similar meanings. Words that are synonyms are said to be synonymous, and the state of being a synonym is called synonymy. The word comes from Ancient Greek syn and onoma . The words car and automobile are synonyms...
s for the hexagonal close-packed (HCP) phase of iron
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...
that is stable only at extremely high pressure. Takahashi and Bassett at the University of Rochester
University of Rochester
The University of Rochester is a private, nonsectarian, research university in Rochester, New York, United States. The university grants undergraduate and graduate degrees, including doctoral and professional degrees. The university has six schools and various interdisciplinary programs.The...
mixed 99.8% pure alpha iron powder with sodium chloride
Sodium chloride
Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...
, and pressed a 0.5-mm diameter pellet between the flat faces of two diamond anvils. The deformation of the NaCl lattice, as measured by x-ray diffraction (XRD), served as a pressure indicator. At a pressure of 130 kbar (13 GPa) and room temperature, the body-centered cubic ferrite
Ferrite (iron)
Ferrite or alpha iron is a materials science term for iron, or a solid solution with iron as the main constituent, with a body centred cubic crystal structure. It is the component which gives steel and cast iron their magnetic properties, and is the classic example of a ferromagnetic material...
powder transformed to the HCP phase in Figure 1. When the pressure was lowered, ε-Fe transformed back to ferrite (α-Fe) rapidly. A specific volume
Specific volume
In thermodynamics, the specific volume of a substance is the ratio of the substance's volume to its mass. It is the reciprocal of density:In thermodynamics, the specific volume of a substance is the ratio of the substance's volume to its mass...
change of -0.20 cm3/mole ± 0.03 was measured. Hexaferrum, much like 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...
, is more dense than ferrite at the phase boundary. A shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...
experiment confirmed the diamond anvil results.
The triple point between the alpha, gamma and epsilon phases in the unary phase diagram of iron has been calculated as T = 770K and P = 11 GPa, although it was determined at a lower temperature of T = 750K (477°C) in Figure 1. The Pearson symbol
Pearson symbol
The Pearson symbol, or Pearson notation, is used in crystallography as a means of describing a crystal structure, and was originated by W.B. Pearson. The symbol is made up of two letters followed by a number. For example:* Diamond structure, cF8...
for hexaferrum is hP2 and its space group
Space group
In mathematics and geometry, a space group is a symmetry group, usually for three dimensions, that divides space into discrete repeatable domains.In three dimensions, there are 219 unique types, or counted as 230 if chiral copies are considered distinct...
is P63/mmc.
Giles et al. studied the ferrite-hexaferrum transformation metallographically
Metallography
Metallography is the study of the physical structure and components of metals, typically using microscopy.Ceramic and polymeric materials may also be prepared using metallographic techniques, hence the terms ceramography, plastography and, collectively, materialography.-Preparing metallographic...
and noted that it is a martensitic rather than equilibrium
Thermodynamic equilibrium
In thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium, mechanical equilibrium, radiative equilibrium, and chemical equilibrium. The word equilibrium means a state of balance...
transformation.
While hexaferrum is purely academic in metallurgical engineering, it may have significance in geology
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
. The pressure and temperature of Earth's iron 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...
are on the order of 1500-3500 kbar and 3000 ± 1000°C. An extrapolation of the austenite-hexaferrum phase boundary in Figure 1 suggests hexaferrum might be stable or metastable in Earth’s core.