subgrain rotation recrystallization
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In metallurgy, materials science and structural geology, subgrain rotation recrystallization is recognized as an important mechanism for
dynamic recrystallisation Dynamic recrystallization (DRX) is a type of recrystallization process, found within the fields of metallurgy and geology. In dynamic recrystallization, as opposed to static recrystallization, the nucleation and growth of new grains occurs during ...
. It involves the rotation of initially low-angle sub-grain boundaries until the mismatch between the crystal lattices across the boundary is sufficient for them to be regarded as
grain boundaries In materials science, a grain boundary is the interface between two grains, or crystallites, in a polycrystalline material. Grain boundaries are two-dimensional defects in the crystal structure, and tend to decrease the electrical and thermal ...
. This mechanism has been recognized in many minerals (including
quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...
, calcite,
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, pyroxenes, micas,
feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) felds ...
s,
halite Halite (), commonly known as rock salt, is a type of salt, the mineral (natural) form of sodium chloride ( Na Cl). Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, p ...
,
garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different s ...
s and
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s) and in metals (various
magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...
,
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
and
nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...
alloys An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility ...
).Microtectonics by C.W.Passchier and R.A.J.Trouw, 2nd rev. and enlarged ed., 2005, XVI, 366 p., 322 illus., with CDEarth Structure: an introduction to structural geology and tectonics, B.A Van Der Pluijm & S. Marshak, 2nd edition, 2004, 656 p.


Structure

In metals and minerals, grains are ordered structures in different crystal orientations. Subgrains are defined as grains that are oriented at a < 10–15 degree angle at the grain boundary, making it a low-angle grain boundary (LAGB). Due to the relationship between the energy versus the number of dislocations at the grain boundary, there is a driving force for fewer high-angle grain boundaries (HAGB) to form and grow instead of a higher number of LAGB. The energetics of the transformation depend on the interfacial energy at the boundaries, the lattice geometry (atomic and planar spacing, structure
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/Body-centered cubic">BCC/Hexagonal crystal family">HCP] of the material, and the degrees of freedom of the grains involved (misorientation, inclination). The recrystallized material has less total grain boundary area, which means that Failure (material), failure via
brittle fracture Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displa ...
along the grain boundary is less probable.


Mechanism

Subgrain rotation recrystallization is a type of continuous
dynamic recrystallization Dynamic recrystallization (DRX) is a type of recrystallization process, found within the fields of metallurgy and geology. In dynamic recrystallization, as opposed to static recrystallization, the nucleation and growth of new grains occurs during ...
. Continuous dynamic recrystallization involves the evolution of low-angle grains into high-angle grains, increasing their degree of misorientation. One mechanism could be the migration and agglomeration of like-sign
dislocation In materials science, a dislocation or Taylor's dislocation is a linear crystallographic defect or irregularity within a crystal structure that contains an abrupt change in the arrangement of atoms. The movement of dislocations allow atoms to s ...
s in the LAGB, followed by grain boundary shearing. The transformation occurs when the subgrain boundaries contain small precipitates, which pin them in place. As the subgrain boundaries absorb dislocations, the subgrains transform into grains by rotation, instead of growth. This process generally occurs at elevated temperatures, which allows dislocations to both glide and climb; at low temperatures, dislocation movement is more difficult and the grains are less mobile. By contrast, discontinuous dynamic recrystallization involves nucleation and growth of new grains, where due to increased temperature and/or pressure, new grains grow at high angles compared to the surrounding grains.


Mechanical properties

Grain strength generally follows the Hall–Petch relation, which states that material strength decreases with the square root of the grain size. A higher number of smaller subgrains leads to a higher
yield stress In materials science and engineering, the yield point is the point on a stress-strain curve that indicates the limit of elastic behavior and the beginning of plastic behavior. Below the yield point, a material will deform elastically and w ...
, and so some materials may be purposefully manufactured to have many subgrains, and in this case subgrain rotation recrystallization should be avoided. Precipitates may also form in grain boundaries. It has been observed that precipitates in subgrain boundaries grow in a more elongated shape parallel to the adjacent grains, whereas precipitates in HAGB are blockier. This difference in aspect ratio may provide different strengthening effects to the material; long plate-like precipitates in the LAGB may delaminate and cause brittle failure under stress. Subgrain rotation recrystallization reduces the number of LAGB, thus reducing the number of flat, long precipitates, and also reducing the number of available pathways for this brittle failure.


Experimental techniques

Different grains and their orientations can be observed using scanning electron microscope (SEM) techniques such as electron backscatter diffraction (EBSD) or polarized
optical microscopy Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultravio ...
(POM). Samples are initially cold- or hot-rolled to introduce a high degree of dislocation density, and then deformed at different strain rates so that dynamic recrystallization occurs. The deformation may be in the form of compression, tension, or torsion. The grains elongate in the direction of applied stress and the misorientation angle of subgrain boundaries increases.


References

{{Reflist Metallurgy Structural geology