Diffusion creep

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Diffusion creep is a mechanism of deformation in crystalline solids at high homologous temperatures (i.e. within about a tenth of its absolute melting temperature). It is caused by the migration of crystalline defects through the lattice of a crystal such that when a crystal is subjected to a greater degree of compression in one direction relative to another, defects migrate to the crystal faces along the direction of compression, causing a net mass transfer that shortens the crystal in the direction of maximum compression.

The migration of defects is in part due to vacancies, whose migration is compensated by a net mass transport in the opposite direction such that the most compressed direction yields a net shortening of the crystal. Vacancies always exist in a crystalline solid, and the balance of factors is the increase in their intrinsic mixing entropy which competes with the strain induced in the crystal around a site missing an ion. Vacancies can actually be thought of as chemical species themselves (or part of a compound species/component) that may then be treated using classical heterogeneous phase equilibria. The number of vacancies may also be influenced by the number of chemical impurities in the crystal lattice, if such impurities require the formation of vacancies to exist in the lattice. Highly mobile chemical components substituting for other species in the lattice can also cause a net differential mass transfer (i.e. segregation) of chemical species inside the crystal itself, often promoting shortening of the rheologically more difficult substance and enhancing deformation.

All these phenomena that make diffusion creep work require the migration of defects or vacancies toward the faces of a crystal having the greatest compression. Current theory holds that the elastic strain in the neighborhood of a defect is smaller toward the axis of greatest differential compression, creating a defect chemical potential gradient (depending upon lattice strain) within the crystal that leads to net accumulation of defects at the faces of maximum compression by diffusion. Hence the name "diffusion creep"