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, 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.

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.

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

Dynamic recrystallization

Two main mechanisms of recrystallization are known, sub-grain rotation 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 creep) and via a pore fluid (Pressure solution).

• 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 superplasticity.

References

• C.W. Passchier & R.A.J. Trouw. Microtectonics. Berlin: Springer. ISBN 3-540-58713-6. 

See also

Deformation mechanism maps

Creep (deformation)