Symplectite

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Scanning Electron Microscope interface image of a fayalite-pyroxene symplectite (at right) in a Martian meteorite Symplectite.JPG
Scanning Electron Microscope interface image of a fayalite-pyroxene symplectite (at right) in a Martian meteorite

A symplectite (or symplektite) is a material texture: a micrometre-scale or submicrometre-scale intergrowth of two or more crystals. Symplectites form from the breakdown of unstable phases, and may be composed of minerals, ceramics, or metals. Fundamentally, their formation is the result of slow grain-boundary diffusion relative to interface propagation rate. [1] [2] [3]

If a material undergoes a change in temperature, pressure or other physical conditions (e.g., fluid composition or activity), one or more phases may be rendered unstable and recrystallize to more stable constituents. If the recrystallized minerals are fine grained and intergrown, this may be termed a symplectite. A cellular precipitation reaction, in which a reactant phase decomposes to a product phase with the same structure as the parent phase and a second phase with a different structure, can form a symplectite. [4] Eutectoid reactions, involving the breakdown of a single phase to two or more phases, neither of which is structurally or compositionally identical to the parent phase, can also form symplectites. [5]

Symplectites may be formed by reaction between adjacent phases or to decomposition of a single phase. The intergrown phases may be planar or rodlike, depending on the volume proportions of the phases, their interfacial free energies, the rate of reaction, the Gibbs free energy change, and the degree of recrystallization. Lamellar symplectites are common in retrogressed eclogite. Kelyphite is a symplectite formed from the decomposition of garnet. [6] Myrmekite is a globular or bulbous symplectite of quartz in plagioclase. [6]

Chemical reaction process that results in the interconversion of chemical species

A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking of chemical bonds between atoms, with no change to the nuclei, and can often be described by a chemical equation. Nuclear chemistry is a sub-discipline of chemistry that involves the chemical reactions of unstable and radioactive elements where both electronic and nuclear changes can occur.

Eclogite A dense, mafic metamorphic rock

Eclogite is a mafic metamorphic rock. Eclogite forms at pressures greater than those typical of the crust of the Earth. An unusually dense rock, eclogite can play an important role in driving convection within the solid Earth.

Garnet mineral, semi-precious stone

Garnets are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.

Examples of symplectites formed in Earth materials include dolomite + calcite, [7] aragonite + calcite, [8] and magnetite + clinopyroxene. [9] Symplectite formation is important in metallurgy: bainite or pearlite formation from the decomposition of austenite, for example. [3]

Bainite

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 lamellar structure of ferrite and cementite

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 727 °C (1,341 °F). Pearlite is a microstructure occurring in many common grades of steels.

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 stainless steel.

See also

Related Research Articles

Calcite carbonate mineral

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). The Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 3 as "calcite".

Eutectic system

A eutectic system from the Greek "εύ" and "τήξις" is a homogeneous mixture of substances that melts or solidifies at a single temperature that is lower than the melting point of either of the constituents.

Heat treating process of heating something to alter it

Heat treating is a group of industrial 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 a desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing and quenching. It is noteworthy that while 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.

Aragonite carbonate mineral

Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, CaCO3 (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including precipitation from marine and freshwater environments.

Martensite most commonly refers to a very hard form of steel crystalline structure, but it can also refer to any crystal structure that is formed by diffusionless transformation.

Martensite is named after the German metallurgist Adolf Martens (1850–1914). The term most commonly refers to a very hard form of steel crystalline structure, but can also refer to any crystal structure that is formed by diffusionless transformation. Martensite includes a class of hard minerals that occur as lath- or plate-shaped crystal grains.

Cementite iron and carbon compound

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.

Carbon steel steel in which the main interstitial alloying constituent is carbon

Carbon steel is a steel with carbon content up to 2.1% by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states:

Quenching rapid cooling of a workpiece to obtain certain material properties

In materials science, quenching is the rapid cooling of a workpiece in water, oil or air to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, such as phase transformations, from occurring. It does this by reducing the window of time during which these undesired reactions are both thermodynamically favorable, and kinetically accessible; for instance, quenching can reduce the crystal grain size of both metallic and plastic materials, increasing their hardness.

Tempering (metallurgy) metallurgy

Tempering is a process of heat treating, which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product. For instance, very hard tools are often tempered at low temperatures, while springs are tempered to much higher temperatures.

Lamellar structure

Lamellar structures or microstructures are composed of fine, alternating layers of different materials in the form of lamellae. They are often observed in cases where a phase transformation front moves quickly, leaving behind two solid products, as in rapid cooling of eutectic or eutectoid systems.

Annealing, in metallurgy and materials science, is a heat treatment that alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable. It involves heating a material above its recrystallization temperature, maintaining a suitable temperature for a suitable amount of time, and then cooling.

In petrology, micrographic texture is a fine-grained intergrowth of quartz and alkali feldspar, interpreted as the last product of crystallization in some igneous rocks which contain high or moderately high percentages of silica. Micropegmatite is an outmoded terminology for micrographic texture.

Acicular ferrite is a microstructure of ferrite in steel that is characterised by needle-shaped crystallites or grains when viewed in two dimensions. The grains, actually three-dimensional in shape, have a thin lenticular shape. This microstructure is advantageous over other microstructures because of its chaotic ordering, which increases toughness.

Isothermal transformation diagram

Isothermal transformation diagrams are plots of temperature versus time. They are generated from percentage transformation-vs time measurements, and are useful for understanding the transformations of an alloy steel at elevated temperatures.

Neomorphism refers to the wet metamorphic process in which diagenetic alterations systematically transform minerals into either polymorphs or crystalline structures that are structurally identical to the rock(s) from which they developed.

Dual-phase steel

Dual-phase steel (DP steel) is a high-strength steel that has a ferritic–martensitic microstructure. DP steels are produced from low or medium carbon steels that are quenched from a temperature above A1 but below A3 determined from continuous cooling transformation diagram. This results in a microstructure consisting of a soft ferrite matrix containing islands of martensite as the secondary phase (martensite increases the tensile strength). Therefore, the overall behaviour of DP steels is governed by the volume fraction, morphology (size, aspect ratio, interconnectivity, etc.), the grain size and the carbon content. For achieving these microstructures, DP steels typically contain 0.06–0.15 wt.% C and 1.5-3% Mn (the former strengthens the martensite, and the latter causes solid solution strengthening in ferrite, while both stabilize the austenite), Cr & Mo (to retard pearlite or bainite formation), Si (to promote ferrite transformation), V and Nb (for precipitation strengthening and microstructure refinement). The desire to produce high strength steels with formability greater than microalloyed steel led the development of DP steels in the 1970s.

Austempering

Austempering is heat treatment that is applied to ferrous metals, most notably steel and ductile iron. In steel it produces a bainite microstructure whereas in cast irons it produces a structure of acicular ferrite and high carbon, stabilized austenite known as ausferrite. It is primarily used to improve mechanical properties or reduce / eliminate distortion. Austempering is defined by both the process and the resultant microstructure. Typical austempering process parameters applied to an unsuitable material will not result in the formation of bainite or ausferrite and thus the final product will not be called austempered. Both microstructures may also be produced via other methods. For example, they may be produced as-cast or air cooled with the proper alloy content. These materials are also not referred to as austempered.

References

  1. ↑ Cahn, J. W. (1959), The kinetics of cellular segregation reactions, Acta Metall., 7, 18– 28.
  2. ↑ Elliott, R. (1983), Eutectic Solidification Processing, 370 pp., Butterworths, London.
  3. 1 2 Lee, H. J., G. Spanos, G. J. Shiflet, and H. I. Aaronson (1988), Mechanisms of the bainite (non-lamellar eutectoid) reaction and a fundamental distinction between the bainite and pearlite (lamellar eutectoid) reactions, Acta Metall., 36, 1129– 1140.
  4. ↑ Sundquist, B. E. (1973), Cellular precipitation, Metall. Trans., 4, 1919– 1934.
  5. ↑ Spencer, C. W., and D. J. Mack (1962), Eutectoid transformations in nonferrous and ferrous alloy systems, in Decomposition of Austenite by Diffusional Processes, edited by V. F. Zackay and H. I. Aaronson, pp. 549– 606, John Wiley, New York.
  6. 1 2 Passchier, Cees W. and Rudolph A. J. Trouw, Microtectonics, Springer, 2nd ed. 205 p. 231 ISBN   978-3-540-64003-5
  7. ↑ Ogasawara, Y., R. Y. Zhang, and J. G. Liou (1998), Petrogenesis of dolomitic marbles from Rongcheng in the Su-Lu ultrahigh-pressure metamorphic terrane, eastern China, Island Arc, 7, 82– 97.
  8. ↑ Hacker, B.R., S.R. Bohlen, S.H. Kirby, and D.C. Rubie, Calcite --> aragonite transformation in marble: textures and reaction mechanisms of an archetypal polymorphic phase transformation, Journal of Geophysical Research, 110, doi:10.1029/2004JB003302, 2005.
  9. ↑ Ashworth, J. R., and A. D. Chambers (2000), Symplectic reaction in olivine and the controls of intergrowth spacing in symplectites, J. Petrol., 41, 285–304.
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