metallurgy Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys. Metallurgy encompasses both the ...

materials science Materials science is an interdisciplinary field of researching and discovering materials. Materials engineering is an engineering field of finding uses for materials in other fields and industries. The intellectual origins of materials sci ...

and

structural geology Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover informati ...

, subgrain rotation recrystallization is recognized as an important mechanism for dynamic recrystallisation. It involves the rotation of initially low-angle sub-grain boundaries until the mismatch between the

crystal lattice In crystallography, crystal structure is a description of ordered arrangement of atoms, ions, or molecules in a crystal, crystalline material. Ordered structures occur from intrinsic nature of constituent particles to form symmetric patterns that ...

s 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 crystallographic defect, defects in the crystal structure, and tend to decrease the ...

. This mechanism has been recognized in many minerals (including

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...

pyroxene The pyroxenes (commonly abbreviated Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents ions of calcium (Ca), sodium (Na), iron ( ...

mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into fragile elastic plates. This characteristic is described as ''perfect basal cleavage''. Mica is co ...

feldspar Feldspar ( ; sometimes spelled felspar) is 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 ''plagiocl ...

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, pi ...

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. Garnet minerals, while sharing similar physical and crystallographic properties, exhibit a wide range of chemical compositions, de ...

s and

zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of th ...

s) and in metals (various

magnesium Magnesium is a chemical element; it has Symbol (chemistry), 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 ...

aluminium Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...

and

nickel Nickel is a chemical element; it has 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 slo ...

alloys An alloy is a mixture of chemical elements of which in most cases at least one is a metallic element, although it is also sometimes used for mixtures of elements; herein only metallic alloys are described. Metallic alloys often have properties ...

).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 FCC/Body-centered cubic">BCC/Hexagonal crystal family">HCP">html" ;"title="Face-centered_cubic.html" ;"title=".e. Face-centered cubic">FCC/Body-centered cubic">BCC/Hexagonal crystal family">HCPof 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 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 sli ...

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 In thermodynamics, nucleation is the first step in the formation of either a new Phase (matter), thermodynamic phase or Crystal structure, structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically def ...

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 elasticity (physics), elastic behavior and the beginning of plasticity (physics), plastic behavior. Below the yield point ...

, 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 A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that ...

(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