Earth Materials. John O'Brien. Читать онлайн. Newlib. NEWLIB.NET

Автор: John O'Brien
Издательство: John Wiley & Sons Limited
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Жанр произведения: География
Год издания: 0
isbn: 9781119512219
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illustration of two-dimensional depiction of how an edge dislocation created by slip due to shear can migrate through a crystal by breaking one bond at a time, so that no fractures develop as the crystal changes shape during deformation (steps 1–6)."/>

      Source: Adapted from Hobbs et al. (1976). © John Wiley & Sons.

Schematic illustration of three types of planar defect (shown in two dimensions): (a) intergranular grain boundary between two different crystals; (b) intragranular mechanical twin boundary resulting from mechanical slip; (c) intragranular subgrain boundary within a crystal, separated by a wall of dislocations.

      4.8.3 Planar defects

      Plastic deformation at high temperatures and low strain rates largely results from two significant types of diffusion creep (Figure B4.2a) that are dependent on the existence of omission defects in minerals: (1) Coble (grain boundary diffusion) creep, and (2) Herring–Nabarro (volume diffusion) creep. Elevated temperatures are associated with elevated molecular vibration in an expanded crystal lattice. Such vibrations lower bond strength and increase the number of omission defects (also called holes or vacancies) in the crystal structure. As holes are created, adjacent atoms can migrate into the vacancy by breaking only one weak bond a time. The movement of the ions in one direction causes the holes or vacancies to migrate in the opposite direction (Figure B4.2b).

      Under conditions of differential stress, ions tend to be forced toward the direction of least compressive stress (σ3), which tends to lengthen the crystal in that direction. Simultaneously, holes tend to migrate toward the direction of maximum compressive stress (σ1) until they reach the surface of the crystal where they disappear, causing the crystal to shorten in this direction (Figure B4.2b). In Coble creep, the vacancies and ions migrate near grain boundaries to achieve the strain, whereas in Herring–Nabarro creep, the vacancies and ions migrate through the interior of the crystals. Since thousands of omission defects are created over long periods of time, even in small crystals, the long‐term summative effects of plastic strain as each crystal changes shape by diffusion creep can be very large indeed.

      At higher strain rates related to higher differential stresses, dislocation creep processes become dominant (Figure B4.1). In these environments edge dislocations and screw dislocations migrate through the crystal structure, once again breaking only one bond at a time, while producing plastic changes in shape. Because such dislocations result from strain, large numbers are produced in response to stress, and their migration accommodates large amounts of plastic strain. Imagining the summative plastic changes in shape that can be accomplished by the migration of thousands of diffusing vacancies and/or migrating dislocations in a small crystal or l020 dislocations migrating through the many crystals in a large mass of rock offers insight into the power of crystal defects to accommodate plastic deformation on scales that range from microcrystals to regionally metamorphosed mountain ranges.

Image described by caption.

      Source: Modified from Davis et al. (1996). © John Wiley & Sons.

      (b) Diagrams showing how the existence of omission defects permits adjacent ions to move into their former locations, effectively causing the omission or hole to migrate in one direction as the ions migrate in the other. The flux of atoms (blue arrows) toward regions of least compressive stress (σ3) and of vacancies (black arrows) toward areas of maximum compressive stress (σ1) cause crystals to change shape.

      4.9.1 Polymorphs