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# Coble creep

Coble Creep is named after Robert L. Coble, who first reported his theory of how materials creep over time in 1962 in the Journal of Applied Physics[1] . 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:

$\frac{d\epsilon}{dt}= \frac{\sigma}{d^3} D_{gb} e^{-Q_{Coble}/RT}$

where

• 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 Kelvin

Note that in Coble Creep, the strain rate $\frac{d\epsilon}{dt}$ 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 $\frac{d\epsilon}{dt}~\alpha ~d^n$ and conclude whether Coble or Nabarro-Herring Creep is dominant[1].

## References

(1) Meyers and Chawla (1999): "Mechanical Behavior of Materials," 555-557.