- Coble creep
Coble creep, a form of diffusion creep, is a mechanism for deformation of crystalline solids. Coble creep occurs through the diffusion of atoms in a material along the grain boundaries, which produces a net flow of material and a sliding of the grain boundaries.
The strain rate in a material experiencing Coble creep is given by:
- σ is the applied stress
- d is the average grain boundary diameter
- Dgb is the diffusion coefficient in the grain boundary
- − QCoble is the activation energy for Coble creep
- R is the molar gas constant
- T is the temperature in kelvins
Note that in Coble creep, the strain rate is proportional to the applied stress σ; the same relationship is found for Nabarro-Herring creep. However, the two mechanisms differ in their relationship between the strain rate and grain size d. In Coble creep, the strain rate is proportional to d − 3, whereas the strain rate in Nabarro-Herring creep is proportional to d − 2. Researchers commonly use these relationships to determine which mechanism is dominant in a material; by varying the grain size and measuring how the strain rate is affected, they can determine the value of n in and conclude whether Coble or Nabarro-Herring creep is dominant.
(1) Meyers and Chawla (1999): "Mechanical Behavior of Materials," 555-557.
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