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© Infobase Publishing
© Infobase Publishing
Types of point and line defects in crystals, including vacancies, interstitials, edge dislocation, and screw dislocation
Dislocations form loops within crystals, marking boundaries between slipped and unslipped portions. This type of motion and slipping in crystals is exactly analogous to motions on faults.
During deformation there are so many dislocations moving around in a crystal that they are bound to interact in several ways. Dislocation annihilation occurs when two dislocations of opposite signs move toward each other on the same slip plane and their extra half planes meet and form a complete crystal lattice, effectively canceling each other out of existence. Each dislocation in a crystal induces a stress field known as the self-stress field, since they disrupt the normal crystallographic structure. These stress fields interact for large distances within the crystals, causing either repulsions or attractions between dislocations, much as magnets repel or attract each other depending on charge. In the case of dislocations, however, attractions are deadly because if two dislocations are on the same slip plane and are attracted, they come together and annihilate each other. If they repel each other, the number of dislocations increases and higher and higher stresses are needed to make the dislocations move as the number of repelled dislocations increases during deformation.
Dislocations often encounter immobile, tightly bounded impurities, such as interstitials, which pin dislocations behind them. As more dislocations move toward the region with the impurities, they too become stuck and are repelled by each other's stress fields. This causes dislocation pile-ups. Dislocations can also interact with other dislocations, also causing pile-ups.
When the temperature of the deforming crystal is high, vacancies can diffuse toward the obstacle, or atoms can move away from the half-plane, enabling dislocations to climb over obstacles. Thus there are two main mechanisms by which dislocations move through crystals: gliding and climbing.
When dislocations of different slips systems move through each other, they offset the other slip plane, forming a dislocation jog, which is basically a step in the slip system that one of the dislocations moved along. once there is a jog in the slip plane, it becomes more difficult for dislocations to move along that slip plane, and they must climb over the jog to progress. Dislocation jogs can be made to disappear, or evaporate, by diffusion of vacancies toward them or movement of atoms away from the jog.
Work hardening is any process that makes increased deformation harder, requiring more stress to do the same amount of deformation. Work hardening can occur by
• formation of dislocation jogs
• dislocation pile-ups
• interaction of stress fields
• increase in dislocation density
Work hardening is more common at low temperatures, as high temperatures cause increased diffusion of vacancies and climb to occur.
Annealing is any process that tends to return a crystal lattice to a less deformed state, such as through a reduction in the number of dislocations. A lattice with fewer dislocations has lower energy and is more stable that one with a high dislocation density. There are several different common ways that a crystal anneals:
• group dislocations in a more stable configuration
• migration of dislocations to an edge of crystal
• recrystallization of grains
Diffusion helps all of these processes, so annealing is faster at high temperatures. Annealing mechanisms are also diverse and include the following:
• Dislocations of opposite signs can climb to the same slip plane and annihilate each other.
• Dislocations can glide and climb to grain boundaries.
• Formation of subgrain boundaries by dislocation motion concentrates dislocations into planes, or walls that bound domains of low-dislocation density.
• Recrystallization, or regrowth, of the entire crystal lattice, with new grains having low-dislocation density. often this process starts in regions of high-dislocation density or along grain boundaries. Prolonged heating leads to grain growth, and some grains grow at the expense of neighbors. Recrystalliza-tion forms grains that are equant, have 120° grain boundaries at triple junctions.
At temperatures lower than those where dislocation glide and climb operate, rocks flow by other mechanisms. Pressure solution, or grain boundary diffusion (also called Coble creep), is where crystals are flattened and dissolved along their edges. The extra dissolved material is either precipitated at the ends of the grains or moved away to be precipitated in veins or pores, or far away. This deformation mechanism can easily accommodate large bulk shortening and stretching. Pressure solution often produces seams, or stylolites, which are leftover concentrations of insoluble material from where the rock dissolved much of the other material. stylolites are interest ing because in three dimensions they form irregular surfaces with cones or teeth on them, pointed to the maximum compressive stress. Pressure solution works much like squeezing an ice cube. Grain-grain boundaries that are initially touching have the highest stresses on them and are the first to be dissolved.
Compaction is also an important deformation mechanism in sedimentary basins due to the weight of overlying rocks. Compaction often involves dewa-tering, or removal of fluids, from the pore spaces of a rock. The weight of newly deposited sediments and overlying rocks adds pressure and pushes the grain-to-grain contacts between crystals or grains closer together, expelling the fluids. Some muds begin with a porosity of 80 percent and end with 10 percent on burial. In these cases a large quantity of water is expelled from the system during compaction. sand stones have initial porosities of up to 45 percent, reduced to about 10-30 percent depending on the rock, pressure, and fluid. Porosity decreases with increasing burial depth.
See also deformation of rocks; mineral, mineralogy; structural geology.
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