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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:

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

where

$σ$ is the applied stress
$d$ is the average grain boundary diameter
$D g b$ is the diffusion coefficient in the grain boundary
$- Q C o b l e$ 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].