Deformation mechanism
[1][2] The process involves planar discontinuity and/or displacement of atoms from their original position within a crystal lattice structure.
[1][3] 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.
The driving mechanism responsible is an interplay between internal (e.g. composition, grain size and lattice-preferred orientation) and external (e.g. temperature and fluid pressure) factors.
[1][2] These mechanisms produce a range of micro-structures studied in rocks to constrain the conditions, rheology, dynamics, and motions of tectonic events.
Detailed microstructure analysis can be used to define the conditions and timing under which individual deformation mechanisms dominate for some materials.
Common deformation mechanisms processes include: § Fracturing § Cataclastic flow § Grain boundary sliding § Diffusive mass transfer § Dislocation creep § Dynamic recrystallization (recovery) Fracturing is a brittle deformation process that creates permanent linear breaks, that are not accompanied by displacement within materials.
[2] Cataclastic flow is generally unstable and will terminate by the localization of deformation into slip on fault planes.
[8] 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.
It is favored by high temperatures and the presence of very fine-grained aggregates where diffusion paths are relatively short.
These migrations are oriented towards sites of maximum stress and are limited by the grain boundaries; which conditions a crystallographic shape fabric or strain.
During Nabarro-Herring creep, the diffusion of vacancies occurs through the crystal lattice (microtectonics), which causes grains to elongate along the stress axis.
[1] These migrations within the crystal lattice can occur in one or more directions and are triggered by the effects of increased differential stress.
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.
[2] Dynamic recrystallization is the process of removing the internal strain that remains in grains during deformation.
[2] Dynamic recrystallization can occur under a wide range of metamorphic conditions, and can strongly influence the mechanical properties of the deforming material.
Additionally, work has been conducted regarding the use of deformation maps to nanostructured or very fine grain materials.
The theoretical shear strength of the material is independent of temperature and located along the top of the map, with the regimes of plastic deformation mechanisms below it.
[14] The same technique has been used to construct process maps for sintering, diffusion bonding, hot isostatic pressing, and indentation.
The boundaries between the fields are determined from the constitutive equations of the deformation mechanisms by solving for stress as a function of temperature.
The main regions in a typical deformation mechanism map and their constitutive equations are shown in the following subsections.
The plasticity region is at the top of deformation map (at the highest normalized stresses), and is below the boundary set by the ideal strength.
Diffusional flow is a regime typically below dislocation creep and occurs at high temperatures due to the diffusion of point defects in the material.
is the applied shear stress, Ω is the atomic volume, k is the Boltzmann constant ,d is the grain size, T is the temperature, and
If the values place the point near the center of a field, it is likely that the primary mechanism by which the material will fail, i.e.: the type and rate of failure expected, grain boundary diffusion, plasticity, Nabarro–Herring creep, etc.
Deformation mechanism maps are only as accurate as the number of experiments and calculations undertaken in their creation.
For a given stress and temperature, the strain rate and deformation mechanism of a material is given by a point on the map.
Polymer melts exhibit different deformation mechanisms when subjected to shear or tensile stresses.
[23] In the low temperature regime of a polymer melt (T < Tg), crazing or shear banding can occur.
It is important to note that crazing and shear banding are deformation mechanisms observed in glassy polymers.
Additionally, region four corresponds to alignment and elongation of the polymer backbone from its coiled or folded state—eventually leading to fracture.
