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Deformation mechanism

Deformation mechanisms refer to the various processes occurring at micro-scale that are responsible for changes in a materials internal structure, shape and volume. The process involves planar discontinuity and/or displacement of atoms from their original position within the crystal lattice system . These small changes are preserved in various microstructures of materials such as rocks, metals and plastics, and can be studied in depth using optical or digital microscopy.

Deformation mechanism processes

Summary of various mechanisms process that occurs within brittle and ductile conditions. These mechanisms can overlap in the brittle-ductile settings.

Deformation mechanisms is commonly characterized as brittle, ductile and brittle-ductile. The driving mechanism responsible is a subject of interplay between internal (e.g composition, grain size and lattice-preferred orientation) and external (e.g temperature and fluid pressure) factors. These mechanisms produce a range of micro-structures studied in rocks to constrain the conditions, rheology, dynamics and motions of tectonic events. More than one mechanism may be active under a given set of conditions and some mechanisms can develop independently. Detailed microstructures analysis can be used to define the conditions and timing under which individual deformation mechanisms dominate for some materials. Common deformation mechanisms processes subdivisions are:

• Fracturing

• Cataclastic flow

• Diffusive mass transfer

• Grain-boundary sliding

• Dislocation creep

• Dynamic recrystallization (recovery)

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Fracturing

Cross polarized image of a high concentration of variably oriented joints within a granitic rock from the San Andreas Fault, CA. No obvious displacement along fractures.

Fracturing is a brittle deformation process that creates permanent linear breaks, that are not accompanied by displacement within materials. These linear breaks or openings can be independent or interconnected. For fracturing to occur, the ultimate strength of the materials need to be exceeded to a point where the material rupture. Rupturing is aided by the accumulations of high differential stress (the difference between the maximum and minimum stress acting on the object). Most fracture grow into faults. However, the term fault is only used when the fracture plane accommodate some degree of movement. Fracturing can happen across all scales, from microfractures to macroscopic fractures and joints in the rocks.

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Catalcasitic flow

Rounded to sub-rounded grains within very fine grained matrix. Fracture processes "grind"/"roll"/"slide" grains past each other creating the rounded appearance of the individual grains.

Cataclasis is a non-elastic brittle mechanism that operates under low to moderate homologous temperatures, low confining pressure and relatively high strain rates. It occurs only above a certain differential stress level, which is dependent on fluid pressure and temperature. Cataclasis accommodates the fracture and crushing of grains, causing grain size reduction, along with frictional sliding on grain boundaries and rigid body grain rotation. Intense cataclasis occurs in thin zones along slip or fault surfaces where extreme grain size reduction occurs. In rocks, cataclasis forms a cohesive and fine-grained fault rock called cataclasite. Cataclasitic flow occurs during shearing when a rock deform by microfracturing and frictional sliding where tiny fractures (microcracks), and associated rock fragments move past each other. Cataclastic flow usually occurs at diagenetic to low-grade metamorphic conditions. However, this depends on the mineralogy of the material and the extent of pore fluid pressure. Cataclastic flow is generally unstable and will terminate by the localization of deformation into slip on fault planes.

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Grain boundary sliding

Grain boundary sliding is a plastic deformation mechanism where crystals can slide past each other without friction and without creating significant voids as a result of diffusion. The deformation process associated with this mechanism is referred to as granular flow .The absence of voids results from solid-state diffusive mass transfer, locally enhanced crystal plastic deformation, or solution and precipitation of a grain boundary fluid. This mechanism operates at a low strain rate produced by neighbor switching. Grain boundary sliding is grain size and temperature-dependent. It is favored by high temperatures and the presence of very fine-grained aggregates where diffusion paths are relatively short. Large strains operating in this mechanism don’t result in the development of a lattice preferred orientation or any appreciable internal deformation of the grains, except at the grain boundary to accommodate the grain sliding; this process is called superplastic deformation.

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Diffusive mass transfer

In this group of mechanisms, the strain is accommodated by migration of vacancies in crystallographic lattice . This results in a change in crystal shape involving the transfer of mass by diffusion. These migrations are oriented towards sites of maximum stress and are limited by the grain boundaries; which conditions a crystallographic shape fabric or strain. The result is a more perfect crystal. This process is grain-size sensitive and occurs at low strain rates or very high temperatures, and is accommodated by migration of lattice defects from areas of low to those of high compressive stress. The main mechanisms of diffusive mass transfer are Nabarro-Herring creep, Coble creep, and pressure solution.

• Nabarro-herring creep, or volume diffusion, acts at high homologous temperatures and is grain size dependent with the strain-rate inversely proportional to the square of the grain size (creep rate decreases as the grain size increases). During Nabarro-Herring creep, the diffusion of vacancies occurs through the crystal lattice [microtectonics], which causes grains to elongate along the stress axis. Nabarro-Herring creep has a weak stress dependence.

• Coble-creep, or grain-boundary diffusion, is the diffusion of vacancies occurs along grain-boundaries to elongate the grains along the stress axis [microtectonics]. Coble creep has a stronger grain-size dependence than Nabarro-Herring creep, and occurs at lower temperatures while remaining temperature dependent. It play a more important role than Nabarro–Herring creep and is more important in the deformation of the plastic crust. In this group of mechanisms, the strain is accommodated by migration of vacancies in crystallographic lattice . This results in a change in crystal shape involving the transfer of mass by diffusion. These migrations are oriented towards sites of maximum stress and are limited by the grain boundaries; which conditions a crystallographic shape fabric or strain. The result is a more perfect crystal.

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Dislocation creep

Dislocation creep is a non-linear (plastic) deformation mechanism in which vacancies in the crystal glide and climb past obstruction sites within the crystal lattice. These migrations within the crystal lattice can occur in one or more directions and are triggered by the effects of increased differential stress. It occurs at lower temperatures relative to diffusion creep.The mechanical process presented in dislocation creep is called slip. The principal direction in which dislocation takes place are defined by a combination of slip planes and weak crystallographic orientations resulting from vacancies and imperfections in the atomic structure. Each dislocation causes a part of the crystal to shift by one lattice point along the slip plane, relative to the rest of the crystal. Each crystalline material has different distances between atoms or ions in the crystal lattice, resulting in different lengths of displacement. The vector that characterizes the length and orientation of the displacement is called the Burgers vector. The development of strong lattice preferred orientation can be interpreted as evidence for dislocation creep as dislocations move only in specific lattice planes.

Dislocation glide cannot act on its own to produce large strains due to the effects of strain-hardening, where a dislocation ‘tangle’ can inhibit the movement of other dislocations, which then pile up behind the blocked ones causing the crystal to become difficult to deform. Diffusion and dislocation creep can occur simultaneously. The effective viscosity of a stressed material under given conditions of temperature, pressure, and strain rate will be determined by the mechanism that delivers the smallest viscosity. Some form of recovery process, such as dislocation climb or grain-boundary migration must also be active. Slipping of the dislocation results in a more stable state for the crystal as the pre-existing imperfection is removed. It requires much lower differential stress than that required for brittle fracturing. This mechanism does not damage the mineral or reduce the internal strength of crystals.

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Dynamic recystallization

Dynamic recrystallization is the process of removing the internal strain that remains in grains during deformation. This happens by the reorganization of a material with a change in grain size, shape, and orientation within the same mineral. When recrystallization occurs after deformation has come to an end and particularly at high temperatures, the process is called static recystallization or annealing. Dynamic recystallization results in grain size-reduction and static recystallization results in the formation of larger equant grains.

Dynamic recrystallization can occur under a wide range of metamorphic conditions, and can strongly influence the mechanical properties of the deforming material. Dynamic recrystallization is the result of two end-member processes: (1) The formation and rotation of subgrains (rotation recrystallization) and (2) grain-boundary migration (migration recrystallization).[1

1. Rotation recrystallization (subgrain rotation) is the progressive misorientation of a subgrain as more dislocations move into the dislocation wall (a zone of dislocations resulting from climb, cross-slip, and glide), which increases the crystallographic mismatch across the boundary. Eventually, the misorientation across the boundary is sufficiently large enough to recognize individual grains (usually 10-15° misorientation). Grains tend to be elongate or ribbon-shape, with many subgrains, with a characteristic gradual transition from low-angle subgrains to high-angle boundaries.

2. Migration recrystallization (grain-boundary migration) is the processes by which a grain grows at the expense of the neighboring grains. At low temperatures, the mobility of the grain boundary may be local, and the grain boundary may bulge into a neighboring grain with a high dislocation density and form new, smaller, independent crystals by a process called low-temperature grain boundary migration, or bulging recrystallization. The bulges produced can separate from the original grain to form new grains by the formation of subgrain (low-angle) boundaries, which can evolve into grain boundaries, or by migration of the grain boundary. Bulging recrystallization often occurs along boundaries of old grains at triple junctions. At high temperatures, the growing grain has a lower dislocation density than the grain(s) consumed, and the grain boundary sweeps through the neighboring grains to remove dislocations by high-temperature grain-boundary migration crystallization. Grain boundaries are lobate with a variable grain size, with new grains generally larger than existing subgrains. At very high temperatures, grains are highly lobate or ameboid, but can be nearly strain-free.

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