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.

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.

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