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Creep mechanisms

Diffusion Creep

Diffusion creep occurs by transport of material via diffusion of atoms within a grain. Like all diffusional processes, it is driven by a gradient of free energy (chemical potential), created in this case by the applied stress. For example, an applied tensile stress creates regions of high hydrostatic tension at the extremities of each grain, along the loading direction. In what might be termed the “equatorial” regions of the grain, the hydrostatic stress is lower. Since atoms have a lower free energy in these “polar” regions of high hydrostatic stress (ie “low pressure” regions), they will tend to diffuse towards such regions and this motion will lead to elongation of the grain along the loading direction. Since this occurs on the scale of the individual grains, diffusion distances are shorter in fine-grained materials, which thus tend to be more susceptible to creep.

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There are two types of diffusion creep, depending on whether the diffusion paths are predominantly through the grain boundaries, termed Coble creep (favoured at lower temperatures) or through the grains themselves, termed Nabarro-Herring creep (favoured at higher temperatures).

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Dislocation Creep

Dislocation creep is a mechanism involving motion of dislocations. This mechanism of creep tends to dominate at high stresses and relatively low temperatures.

Dislocations can move by gliding in a slip plane, a process requiring little thermal activation.

This is discussed in the Introduction to Dislocations TLP.

However, the rate-determining step for their motion is often a climb process, which requires diffusion and is thus time-dependent and favoured by higher temperatures. Obstacles in the slip plane, such as other dislocations, precipitates or grain boundaries, can lead to such situations.

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