Crystals are regularly ordered symmetrical arrays of atoms, but like anything else in nature they are not perfect and have many defects. Some defects are acquired during growth of the crystals when they first form, and others develop in the crystals during deformation. These defects determine the strength of crystals, minerals, and rocks. The motion of these defects accommodates the strain of crystals.
The motion of crystal defects is an important deformation mechanism, with different types of motion of different types of defects operating at different temperatures, pressures, and applied stresses. There are two main types of crystal defects: point defects and line defects. Point defects include vacancies, impurities, and interstitials, whereas line defects are known as dislocations.
Point defects are considered irregularities, or defects, that affect one point in a crystal lattice. several types include impurities, in which the wrong type of atom is present in the crystal lattice in the place of another; vacancies, in which an atom is missing from the atomic lattice; and interstitials, in which an atom occupies a site that is not normally occupied. Other types of point defects are more complex and involve more than one atom at a time.
In a regularly ordered crystal lattice most electric charges are satisfied by bonding and balancing positive and negative charges. Crystals having point or other defects have more internal energy because many bonds are broken or unsatisfied and the electric charges are not neutralized. Crystals are more apt
Cubic crystal of fluorite, about two inches (5 cm) on each face, on quartz with barite from Frazer's Hush Mine, Weardale, England (MarkA. Schneider/Photo Researchers, Inc.)
to react or deform when they have a higher internal energy. Temperature causes the number of vacancies to increase, whereas pressure causes the number of vacancies to decrease.
When a crystal is stressed, vacancies tend to move, or diffuse, in an orderly manner related to the stress field, migrating toward the crystal face with the highest stress, whereas atoms tend to migrate in the opposite direction toward the crystal faces with the lowest stress. This process of migration of vacancies is called Nabarro-Herring creep. This kind of diffusion can accommodate a general shape change of a crystal, such as occurs during regional deformation in many mountain belts worldwide.
Twinning, the misorientation of a crystallo-graphic plane across a plane in the crystal lattice, can be caused by mismatched growth or by deformation. Mechanical or stress-induced twinning differs from growth twinning in that shear across a crystal plane changes the lattice orientation. The shear occurs across a crystal plane, which must be a symmetry plane of the crystal. The amount of strain is limited by the crystallographic relationship for each type of crystal, though it typically falls in the range of 20°-45°.
Translation gliding is a deformation mechanism in crystals whereby the crystal lattice structure slips along some internal crystallographic plane, after some critical value of shear stress is reached. slip, or translation gliding, usually occurs in very specific crystallographic directions in crystals called slip systems, favoring directions that
• have short distances between equivalent atoms
• occur in directions that do not juxtapose ions of like charge.
slip on these crystallographic directions begins when the critical resolved shear stress for that crystallo-graphic direction is surpassed.
Most crystals have many slip systems activated at different critical resolved shear stresses, so slip begins on the planes with the lowest critical resolved shear stress. soon different slip systems start to interfere with each other and deformation will either stop or the stress will rise to enable continued deformation. This increase in stress is known as work hardening.
Crystals need to have five independent slip systems to accommodate any general homogenous strain. If the crystal has fewer than five, it will eventually crack or fracture. Different slip systems may be activated at different temperatures, or strain rates, so the relationships of which slip systems are active under different deformation conditions are complex.
Dislocations are best thought of as an extra plane of atoms that terminates somewhere in the crystal lattice. They are often referred to as an extra half-plane because of this geometry. The presence of dislocations weakens the crystal structure. Their motion accounts for much of the strain in crystals. Dislocations move through a crystal lattice much like a ridge can be moved across a carpet, slowly moving the carpet across a floor without moving the whole carpet at one time. The stress required to break one bond at a time to move a dislocation from one place to another in a crystal lattice is much less than breaking all the bonds along a specific crystallographic direction simultaneously.
There are two basic types of dislocations: edge dislocations and screw dislocations. Edge dislocations are simpler, and are basically an extra halfplane of atoms that extends partway across the crystal lattice structure. screw dislocations, however, are more complex, and have a twisted shape that resembles a parking garage, where one layer of the lattice is offset by a twisted, coil-like motion about an axis perpendicular to the planes. Most dislocations in crystals have both edge and screw components and form complex geometrical shapes.
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