Mechanisms Deformation mechanism

1 mechanisms

1.1 cataclastic flow
1.2 dislocation creep
1.3 dynamic recrystallization
1.4 diffusive mass transfer
1.5 grain-boundary sliding

mechanisms

the active deformation mechanism in material depends on homologous temperature, confining pressure, strain rate, stress, grain size, presence or absence of pore fluid , composition, presence or absence of impurities in material, mineralogy, , presence or absence of lattice-preferred orientation. note these variables not independent e.g. pure material of fixed grain size, @ given pressure, temperature , stress, strain-rate given flow-law associated particular mechanism(s). more 1 mechanism may active under given set of conditions , mechanisms cannot operate independently must act in conjunction in order significant permanent strain can develop. in single deformation episode, dominant mechanism may change time e.g. recrystallization fine grain size @ stage may allow diffusive mass transfer processes become dominant.

the recognition of active mechanism(s) in material requires use of microscopic techniques, in cases using combination of optical microscopy, sem , tem.

using combination of experimental deformation find flow-laws under particular conditions , microscopic examination of samples afterwards has been possible represent conditions under individual deformation mechanisms dominate materials in form of deformation mechanism maps.

five main mechanisms recognized; cataclastic flow, dislocation creep, recrystallization, diffusive mass transfer , grain-boundary sliding.

cataclastic flow

this mechanism operates under low moderate homologous temperatures, low confining pressure , relatively high strain rates , involves fracturing, sliding , rolling of fragments, , further fragmentation of these smaller particles. during cataclastic flow, rock deform without obvious strain localization @ mesoscopic scale, yet process of deformation microfracturing , frictional sliding tiny fractures, called microcracks, , associated rock fragments move past each other. frictional sliding pressure dependent, increasing pressure ability of sliding reduced. microfractures can intergranular (along grain boundaries) or intragranular (within individual grains), process occurs breaking many atomic bonds @ same time; crystal structure away fracture unaffected. cataclastic flow can occur grain-boundary sliding limited continuous fracturing of grains, or continued fracturing , other deformation processes can limit rate of cataclastic flow.

cataclastic flow occurs @ diagenetic low-grade metamorphic conditions, depends on mineralogy of material , extent of pore fluid pressure, high fluid pressure promote cataclastic flow in metamorphic environment. cataclastic flow instable , terminate localization of deformation slip on fault planes, fault propagation can allow cataclasis migrate nearby areas of rock volume.

dislocation creep

dislocation creep, or grain-size insensitive creep, occurs @ intermediate stress , temperatures, , accommodated dislocation climb , glide of lattice defects, rate of controlled rate @ dislocations can climb out of lattice. dislocation creep accommodated dynamic recrystallization , associated generation of lattice-preferred orientations (lpos).

dislocation glide main process cannot act on own produce large strains due effects of strain-hardening, dislocation ‘tangle’ can inhibit movement of other dislocations, pile behind blocked ones causing crystal become difficult deform. dislocations can move through crystal due energy introduced system deformation , temperature. however, dislocations cannot move in direction through crystal. in dislocation glide , @ low temperatures, dislocations restricted glide planes, or crystallographic planes across bonds relatively weak. glide plane of dislocation plane contains burgers vector , dislocation line.

some form of recovery process, such dislocation climb or grain-boundary migration must active.

dynamic recrystallization

dynamic recrystallization reorganization of material change in grain size, shape, , orientation within same mineral, , process of removing internal strain remains in grains after recovery. in isotropic stress conditions or when differential stress removed, recrystallization called static recrystallization or annealing. in static recrystallization, internal strain energy reduced formation of relatively large, strain-free grains grow decrease total free energy in material.

recrystallization in anisotropic stress field called dynamic recrystallization, , results in grain-size reduction. dynamic recrystallization can occur under wide range of metamorphic conditions, , can influence mechanical properties of deforming material. dynamic recrystallization result of 2 end-member processes: (1) formation , rotation of subgrains (rotation recrystallization) , (2) grain-boundary migration (migration recrystallization).

rotation recrystallization (subgrain rotation) progressive misorientation of subgrain more dislocations move dislocation wall (a zone of dislocations resulting climb, cross-slip, , glide), increases crystallographic mismatch across boundary. eventually, misorientation across boundary sufficiently large enough recognize individual grains (usually 10-15° misorientation). grains tend elongate or ribbon-shape, many subgrains, characteristic gradual transition low-angle subgrains high-angle boundaries.

migration recrystallization (grain-boundary migration) processes grain grows @ expense of neighboring grain(s). @ low temperatures, mobility of grain boundary may local, , grain boundary may bulge neighboring grain high dislocation density , form new, smaller, independent crystals process called low-temperature grain boundary migration, or bulging recrystallization. bulges produced can separate original grain form new grains formation of subgrain (low-angle) boundaries, can evolve grain boundaries, or migration of grain boundary. bulging recrystallization occurs along boundaries of old grains @ triple junctions. @ high temperatures, growing grain has lower dislocation density grain(s) consumed, , grain boundary sweeps through neighboring grains remove dislocations high-temperature grain-boundary migration crystallization. grain boundaries lobate variable grain size, new grains larger existing subgrains. @ high temperatures, grains highly lobate or ameboid, can strain-free.

diffusive mass transfer

in group of mechanisms, strain accommodated change in shape involving transfer of mass diffusion. diffusion creep grain-size sensitive , occurs @ low strain rates or high temperatures, , accommodated migration of lattice defects areas of low compressive stress of high compressive stress. main mechanisms of diffusive mass transfer nabarro-herring creep, coble creep, , pressure solution.

nabarro-herring creep, or volume diffusion, acts @ high homologous temperatures , grain size dependent strain-rate inversely proportional square of grain size (creep rate decreases grain size increases). during nabarro-herring creep, diffusion of vacancies occurs through crystal lattice, causes grains elongate along stress axis. nabarro-herring creep has weak stress dependence.
coble-creep, or grain-boundary diffusion, diffusion of vacancies occurs along grain-boundaries elongate grains along stress axis. coble creep has stronger grain-size dependence nabarro-herring creep, , occurs @ lower temperatures while remaining temperature dependent.
pressure solution operates @ moderate homologous temperatures , relatively low strain-rates , requires presence of pore fluid. process of pressure solution similar of coble creep (grain-boundary diffusion), involves presence of fluid film along grain boundaries. pressure solution localized along grain stress in grain high, grains in contact along surfaces @ high angle instantaneous shortening direction. solubility of mineral in aqueous fluid higher crystal lattice under high stress stress lower, , locally higher density of crystal defects near high-stress sites may enhance solubility. material @ high-stress sites dissolved , redeposited @ sites of low differential stress, changing shape of grains without internal deformation. dissolved material can travel down stress-induced chemical gradient nearby sites of low solubility, called solution transfer, redeposition of material can occur along free grain boundaries in contact fluid; newly precipitated material may of different mineral composition or phase dissolved material, known incongruent pressure solution. dissolved material may flow on large distance , deposit in sites such veins or strain shadows, or migrate out of deforming rock volume.
pressure solution dominant @ diagenetic low-grade metamorphic conditions, there abundant fluids , high-temperature deformation mechanisms hindered.

grain-boundary sliding

this mechanism grain-size sensitive , works change shapes of grains can slide past each other without friction , without creating significant voids. mechanism, acting diffusive mass transfer has been linked development of superplasticity.

grain-boundary sliding occurs @ highest temperature conditions , strain produced neighbor switching. can result in large strains without appreciable internal deformation of grains, except @ grain boundaries accommodate grain sliding; processes called superplastic deformation.

grain-boundary sliding grain-size dependent , favors small grain sizes, since diffusion pathways relatively short, , secondary mineral phases may enhance process since hamper grain growth.