Deformation mechanism
Encyclopedia
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.
, confining pressure, strain rate, stress
, grain size, presence or absence of a pore fluid, presence or absence of impurities in the material. 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.
s, low confining pressure and relatively high strain rates and involves fracturing, sliding and rolling of fragments.
process, such as dislocation climb or grain-boundary migration must also be active.
and grain boundary migration, and only the former involves actual deformation. Grain boundary migration involves no strain in itself, but is one of the recovery mechanisms that can allow dislocation processes to proceed to large strains.
) and via a pore fluid (Pressure solution
).
.
Creep (deformation)
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 information about the history of deformation in the rocks, and...
, metallurgy
Metallurgy
Metallurgy is a domain of materials science that studies the physical and chemical behavior of metallic elements, their intermetallic compounds, and their mixtures, which are called alloys. It is also the technology of metals: the way in which science is applied to their practical use...
and materials science
Materials science
Materials science is an interdisciplinary field applying the properties of matter to various areas of science and engineering. This scientific field investigates the relationship between the structure of materials at atomic or molecular scales and their macroscopic properties. It incorporates...
, deformation mechanisms refer to the various mechanisms at the grain scale that are responsible for accommodating large plastic
Plasticity
Plasticity may refer to:Science* Plasticity , in physics and engineering, plasticity is the propensity of a material to undergo permanent deformation under load...
strains in rocks, metals and other materials.
Mechanisms
The active deformation mechanism in a material depends on the homologous temperatureHomologous temperature
Homologous temperature expresses the temperature of a material as a fraction of its melting point temperature using the Kelvin scale. For example, the homologous temperature of lead at room temperature is approximately 0.50 ....
, confining pressure, strain rate, stress
Stress (physics)
In continuum mechanics, stress is a measure of the internal forces acting within a deformable body. Quantitatively, it is a measure of the average force per unit area of a surface within the body on which internal forces act. These internal forces are a reaction to external forces applied on the body...
, grain size, presence or absence of a pore fluid, presence or absence of impurities in the material. 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
Transmission electron microscopy
Transmission electron microscopy is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through...
.
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
Deformation mechanism maps
A deformation mechanism map is a way of representing the dominant deformation mechanism in a material loaded under a given set of conditions and thereby its likely failure mode. Deformation mechanism maps consist of some kind of stress plotted against some kind of temperature axis, typically stress...
.
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 temperatureHomologous temperature
Homologous temperature expresses the temperature of a material as a fraction of its melting point temperature using the Kelvin scale. For example, the homologous temperature of lead at room temperature is approximately 0.50 ....
s, low confining pressure and relatively high strain rates and involves fracturing, sliding and rolling of fragments.
Dislocation creep
Dislocation glide is the main process but cannot act on its own to produce large strains due to the effects of strain-hardening. Some form of recoveryRecovery (metallurgy)
Recovery is a process by which deformed grains can reduce their stored energy by the removal or rearrangement of defects in their crystal structure. These defects, primarily dislocations, are introduced by plastic deformation of the material and act to increase the yield strength of a material...
process, such as dislocation climb or grain-boundary migration must also be active.
Dynamic recrystallization
Two main mechanisms of recrystallization are known, sub-grain rotationContinuous sub-grain rotation type dynamic recrystallization
In Metallurgy, Materials Science and Structural geology continuous sub-grain rotation type dynamic 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...
and grain boundary migration, and only the former involves actual deformation. Grain boundary migration involves no strain in itself, but is one of the recovery mechanisms that can allow dislocation processes to proceed to large strains.
Diffusive mass transfer
In this group of mechanisms, strain is accommodated by a change in shape involving the transfer of mass by diffusion; through the lattice (Nabarro-Herring creep), the grain boundaries (Coble creepCoble creep
Coble creep, a form of diffusion creep, is a mechanism for deformation of crystalline solids. Coble creep occurs through the diffusion of atoms in a material along the grain boundaries, which produces a net flow of material and a sliding of the grain boundaries.Coble creep is named after Robert L...
) and via a pore fluid (Pressure solution
Pressure solution
In structural geology and diagenesis, pressure solution or pressure dissolution is a deformation mechanism that involves the dissolution of minerals at grain-to-grain contacts into an aqueous pore fluid in areas of relatively high stress and either deposition in regions of relatively low stress...
).
- Nabarro-herring creep acts at high homologous temperatures and is grain size dependent with the strain-rate inversely proportional to the square of the grain size.
- Coble-creep acts at high homologous temperatures and is strongly grain-size dependent, with a flow-law where strain-rate is inversely proportional to the cube of the grain size .
- Pressure solution operates at moderate homologous temperatures and relatively low strain-rates and requires the presence of a pore fluid.
Grain-boundary sliding
This mechanism must act with another to change the shapes of the grains so that they can slide past each other without creating significant voids. This mechanism, acting with diffusive mass transfer has been linked with the development of superplasticitySuperplasticity
In materials science, superplasticity is a state in which solid crystalline material is deformed well beyond its usual breaking point, usually over about 200% during tensile deformation. Such a state is usually achieved at high homologous temperature, typically half the absolute melting point. ...
.
See also
Deformation mechanism mapsDeformation mechanism maps
A deformation mechanism map is a way of representing the dominant deformation mechanism in a material loaded under a given set of conditions and thereby its likely failure mode. Deformation mechanism maps consist of some kind of stress plotted against some kind of temperature axis, typically stress...
Creep (deformation)
Creep (deformation)
In materials science, creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material....