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Dislocation creep
Dislocation creep is the non-linear (plastic) deformation mechanism in which vacancies in the crystal glide and climb past obstruction sites in the crystal lattice. These migrations within the crystal lattice can occur in one or more directions and are triggered by the effects of increased differential stress. It occurs at lower temperatures relative to diffusion creep.

The mechanical process present in dislocation creep is called slip. The direction(s) in which dislocation takes place are defined by slip planes conditioned by weak crystallographic orientations resulting from vacancies and imperfections in the atomic structure. Each dislocation causes a part of the crystal to shift by one lattice point along the slip plane, relative to the rest of the crystal. Each crystalline material has different distances between atoms or ions in the crystal lattice, resulting in different lengths of displacement. The vector that characterizes the length and orientation of the displacement is called the Burgers vector. The development of strong lattice preferred orientation can be interpreted as evidence for dislocation creep as dislocations move only in specific lattice planes.

Dislocation glide cannot act on its own to produce large strains due to the effects of strain-hardening, where a dislocation ‘tangle’ can inhibit the movement of other dislocations, which then pile up behind the blocked ones causing the crystal to become difficult to deform. Diffusion and dislocation creep can occur simultaneously. The effective viscosity of a stressed material under given conditions of temperature, pressure, and strain rate will be determined by the mechanism that delivers the smallest viscosity. Some form of recovery process, such as dislocation climb [needs link] or grain-boundary migration must also be active.

Slipping of the dislocation results in a more stable state for the crystal as the pre-existing imperfection is removed. It requires much lower differential stress than that required for brittle fracturing. This mechanism does not damage the mineral or reduce the internal strength of crystals.

Grain boundary sliding
Grain boundary sliding is a plastic deformation mechanism where crystals can slide past each other without friction and without creating significant voids as a result of diffusion. The deformation process associated with this mechanism is referred to as granular flow (Boullier and Gueguen 1975) .The absence of voids results from solid state diffusive mass transfer, locally enhanced crystal plastic deformation, or solution and precipitation of a grain boundary fluid. This mechanism operates at a low strain rate produced by neighbor switching.

Grain boundary sliding is grain size and temperature dependent. It is favored by high temperatures and the presence of very fine-grained aggregates where diffusion paths are relatively short. Large strains operating in this mechanism don’t result in the development of a lattice preferred orientation or any appreciable internal deformation of the grains, except at the grain boundaries to accommodate the grain sliding; this process is called superplastic deformation.