Deformation mechanism

Deformation mechanisms are commonly characterized as brittle, ductile, and brittle-ductile. 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. More than one mechanism may be active under a given set of conditions and some mechanisms can develop independently. 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: (recovery) Fracturing Fracturing is a brittle deformation process that creates permanent linear breaks, that are not accompanied by displacement within materials. and temperature. Cataclasis accommodates the fracture and crushing of grains, causing grain size reduction, along with frictional sliding on grain boundaries and rigid body grain rotation. Intense cataclasis occurs in thin zones along slip or fault surfaces where extreme grain size reduction occurs. 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. Some form of recovery process, such as dislocation climb or grain-boundary migration must also be active. Slipping of the dislocation results in a more stable state for the crystal as the pre-existing imperfection is removed. It requires much lower differential stress than that required for brittle fracturing. This mechanism does not damage the mineral or reduce the internal strength of crystals. Dynamic recrystallization Dynamic recrystallization is the process of removing the internal strain that remains in grains during deformation. This happens by the reorganization of a material with a change in grain size, shape, and orientation within the same mineral. When recrystallization occurs after deformation has come to an end and particularly at high temperatures, the process is called static recrystallization or annealing. Dynamic recrystallization results in grain size-reduction and static recrystallization results in the formation of larger equant grains. 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 grains. 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. ==Deformation mechanism map==

A deformation mechanism map is a way of representing the dominant deformation mechanism in a material loaded under a given set of conditions. The technique is applicable to all crystalline materials, metallurgical as well as geological. Additionally, work has been conducted regarding the use of deformation maps to nanostructured or very fine grain materials. Deformation mechanism maps usually consist of some kind of stress plotted against some kind of temperature axis, typically stress normalized using the shear modulus versus homologous temperature with contours of strain rate. The normalized shear stress is plotted on a log scale. While plots of normalized shear stress vs. homologous temperature are most common, other forms of deformation mechanism maps include shear strain rate vs. normalized shear stress and shear strain rate vs. homologous temperature. Thus deformation maps can be constructed using any two of stress (normalized), temperature (normalized), and strain rate, with contours of the third variable. A stress/strain rate plot is useful because power-law mechanisms then have contours of temperature which are straight lines. For a given set of operating conditions, calculations are conducted and experiments performed to determine the predominant mechanism operative for a given material. Constitutive equations for the type of mechanism have been developed for each deformation mechanism and are used in the construction of the maps. 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. Constant strain rate contours can be constructed on the maps using the constitutive equations of the deformation mechanisms which makes the maps extremely useful. Process maps The same technique has been used to construct process maps for sintering, diffusion bonding, hot isostatic pressing, and indentation. Construction Repeated experiments are performed to characterize the mechanism by which the material deforms. The dominant mechanism is the one which dominates the continuous deformation rate (strain rate), however at any given level of stress and temperature, more than one of the creep and plasticity mechanisms may be active. The boundaries between the fields are determined from the constitutive equations of the deformation mechanisms by solving for stress as a function of temperature. and an archive of its development is online. \dot{\gamma}\propto (\frac{\sigma_s}{\mu})^2 \exp[-\frac{\Delta E}{kT}(1-\frac{\sigma_s}{\widehat{\tau}})] Power Law creep region In this region, the dominant deformation mechanism is power law creep, such that the strain rate goes as the stress raised to a stress exponent n. This region is dominated by dislocation creep. The value of this stress exponent is dependent upon the material and the microstructure. If deformation is occurring by slip, n=1-8, and for grain boundary sliding n=2 or 4. The general equation for power law creep is as follows, The effective diffusion coefficient in the strain rate equation depends on whether or not the system is dominated by core diffusion or lattice diffusion and can be generalized as follows having only been reported in a select few materials at low stresses including aluminium, lead, and tin. The equation for Nabarro-Herring creep is dominated by vacancy diffusion within the lattice, whereas Coble creep is dominated by vacancy diffusion within the grain boundaries. The equation for these mechanisms is shown below where \sigma_s 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 D_{eff} is the effective diffusion coefficient. In the low temperature regime of a polymer melt (T < Tg), crazing or shear banding can occur. The former mechanism resembles crack formation, but this deformation mechanism actually involves the formation of fibrils separated by porous domains or voids. The latter mechanism (shear banding) involves the formation of localized regions of plastic deformation, which typically arise near the position of the maximal shear point in a polymer melt. It is important to note that crazing and shear banding are deformation mechanisms observed in glassy polymers. For crystalline polymers, the deformation mechanism is best described by a stress-strain curve for a crystalline polymer, such as nylon. The stress-strain behavior exhibits four characteristic regions. The first region is the linear-elastic regime, where the stress-strain behavior is elastic with no plastic deformation. The characteristic deformation mechanism in the second region is yielding, where plastic deformation can occur in the form phenomena such as twinning. The third region shows the formation of a neck, and the fourth region is characterized as a steep increase in stress due to viscous flow. Additionally, region four corresponds to alignment and elongation of the polymer backbone from its coiled or folded state—eventually leading to fracture. == External links ==