Deformation mechanism

In structural geology, metallurgy and materials science, deformation mechanisms refer to the various mechanisms at the grain scale that are responsible for accommodating large plastic strains in rocks, metals and other materials.

Mechanisms

The active deformation mechanism in a material depends on the homologous temperature, confining pressure, strain rate, stress, grain size, presence or absence of a pore fluid and its composition, presence or absence of impurities in the material, mineralogy, and presence or absence of a lattice-preferred orientation. Note these variables are not fully independent e.g. for a pure material of a fixed grain size, at a given pressure, temperature and stress, the strain-rate is given by the flow-law associated with the particular mechanism(s). More than one mechanism may be active under a given set of conditions and some mechanisms cannot operate independently but must act in conjunction with another in order that significant permanent strain can develop. In a single deformation episode, the dominant mechanism may change with time e.g. recrystallization to a fine grain size at an early stage may allow diffusive mass transfer processes to become dominant.

The recognition of the active mechanism(s) in a material almost always requires the use of microscopic techniques, in most cases using a combination of optical microscopy, SEM and TEM.

Using a combination of experimental deformation to find the flow-laws under particular conditions and from microscopic examination of the samples afterwards it has been possible to represent the conditions under which individual deformation mechanisms dominate for some materials in the form of deformation mechanism maps.

Five main mechanisms are recognized; cataclastic flow, dislocation creep, recrystallization, diffusive mass transfer and grain-boundary sliding.

Cataclastic flow

This is a mechanism that operates under low to moderate homologous temperatures, low confining pressure and relatively high strain rates and involves fracturing, sliding and rolling of fragments, and further fragmentation of these into smaller particles. During cataclastic flow, a rock will deform without any obvious strain localization at the mesoscopic scale, yet the process of deformation is microfracturing and frictional sliding where tiny fractures, called microcracks, and the associated rock fragments move past each other. Frictional sliding is strongly pressure dependent, where with increasing pressure the ability of sliding is reduced. The microfractures can be intergranular (along grain boundaries) or intragranular (within individual grains), where the process occurs by breaking many atomic bonds at the same time; however the crystal structure away from the fracture is unaffected. Cataclastic flow can occur by grain-boundary sliding with limited continuous fracturing of grains, or continued fracturing and other deformation processes can limit the rate of cataclastic flow.

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, as high fluid pressure will promote cataclastic flow in any metamorphic environment. Cataclastic flow is generally instable and will terminate by the localization of deformation into slip on fault planes, where fault propagation can allow cataclasis to migrate into nearby areas of the rock volume.

Dislocation creep

Main article: Dislocation creep

Dislocation creep, or grain-size insensitive creep, occurs at intermediate stress and temperatures, and is accommodated by dislocation climb and glide of lattice defects, the rate of which is controlled by the rate at which the dislocations can climb out of the lattice. Dislocation creep is often accommodated by dynamic recrystallization and associated with the generation of lattice-preferred orientations (LPOs).

Dislocation glide is the main process but 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. Dislocations can move through a crystal due to the energy introduced to the system by deformation and temperature. However, the dislocations cannot move in any direction through the crystal. In dislocation glide and at low temperatures, the dislocations are restricted to glide planes, or crystallographic planes across which the bonds are relatively weak. The glide plane of a dislocation is the plane that contains the Burgers vector and the dislocation line.

Some form of recovery process, such as dislocation climb or grain-boundary migration must also be active.

Dynamic recrystallization

Main article: Dynamic recrystallization

Dynamic recrystallization is the reorganization of a material with a change in grain size, shape, and orientation within the same mineral, and is the process of removing the internal strain that remains in grains after recovery. In isotropic stress conditions or when differential stress is removed, recrystallization is called static recrystallization or annealing. In static recrystallization, the internal strain energy is reduced by the formation of relatively large, strain-free grains that will grow to decrease the total free energy in the material.

Recrystallization in an anisotropic stress field is called dynamic recrystallization, and results in grain-size reduction. 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).

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.

Migration recrystallization (grain-boundary migration) is the processes by which a grain grows at the expense of the neighboring grain(s). 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.

Diffusive mass transfer

In this group of mechanisms, strain is accommodated by a change in shape involving the transfer of mass by diffusion. Diffusion creep 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 compressive stress 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, 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. Coble creep has a stronger grain-size dependence than Nabarro-Herring creep, and occurs at lower temperatures while remaining temperature dependent.
• Pressure solution operates at moderate homologous temperatures and relatively low strain-rates and requires the presence of a pore fluid. The process of pressure solution is similar to that of Coble creep (grain-boundary diffusion), but involves the presence of a fluid film along the grain boundaries. Pressure solution is localized along a grain where the stress in the grain is high, often where the grains are in contact along surfaces that are at high angle to the instantaneous shortening direction. The solubility of a mineral in an aqueous fluid is higher where the crystal lattice is under high stress than where the stress is lower, and a locally higher density of crystal defects near high-stress sites may also enhance the solubility. The material at the high-stress sites is dissolved and will be redeposited at sites of low differential stress, changing the shape of the grains without internal deformation. The dissolved material can travel down a stress-induced chemical gradient to nearby sites of low solubility, called solution transfer, where the redeposition of the material can occur along free grain boundaries that are in contact with the fluid; newly precipitated material may be of a different mineral composition or phase than the dissolved material, known as incongruent pressure solution. Dissolved material may also flow over a large distance and deposit in sites such as veins or strain shadows, or migrate out of the deforming rock volume.
• Pressure solution is dominant at diagenetic to low-grade metamorphic conditions, where there are abundant fluids and the high-temperature deformation mechanisms are hindered.

Grain-boundary sliding

Main article: Grain-boundary sliding

This mechanism is grain-size sensitive and works to change the shapes of the grains so that they can slide past each other without friction and without creating significant voids. This mechanism, acting with diffusive mass transfer has been linked with the development of superplasticity.

Grain-boundary sliding occurs at the highest temperature conditions and strain is produced by neighbor switching. This can result in very large strains without any appreciable internal deformation of the grains, except at the grain boundaries to accommodate the grain sliding; this processes is called superplastic deformation.

Grain-boundary sliding is grain-size dependent and favors small grain sizes, since diffusion pathways are relatively short, and secondary mineral phases may enhance the process since they hamper grain growth.

References

• C.W. Passchier & R.A.J. Trouw. Microtectonics. Berlin: Springer. ISBN 3-540-58713-6.
• Drury, M.R. and Urai, J.L., 1990, Deformation-related recrystallization processes: Tectonophysics, v. 172, p. 235-253.
• Passchier, C.W. and Trouw, R.A.J., 2005, Microtectonics: Berline, Springer, 366 pp. ISBN 3-540-58713-6.
• Urai, J.L., Means, W.D., and Lister, G.S., 1986, Dynamic Recrystallization of Minerals in Hobbs, B.E. and Heard, H.C., eds., Mineral and Rock Deformation: Laboratory Studies – The Patterson Volume: Geophysical Monograph 36, p. 161-199.
• Van der Pluijm, B.A. and Marshak, S., 2004, Earth Structure: An Introduction to Structural Geology and Tectonics: W.W. Norton & Company, Inc., 656 pp. . ISBN 0-393-92467-X.