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| [[Image:CobleCreepgraindiagram.svg|frame|A diagram showing how atoms and vacancies move through a grain as the mechanism of Coble creep]]
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| '''Coble creep''', a form of [[diffusion creep]], is a mechanism for [[Deformation (engineering)|deformation]] of [[crystalline]] [[solid]]s. 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. | |
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| Coble creep is named after [[Robert L. Coble]], who first reported his theory of how materials [[Creep (deformation)|creep]] over time in 1962 in the Journal of Applied Physics [http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JAPIAU000034000006001679000001&idtype=cvips&gifs=yes].
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| The strain rate in a material experiencing Coble creep is given by:
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| :<math> \frac{d\epsilon}{dt}= \frac{\sigma}{d^3} D_{gb} e^{-Q_{Coble}/RT}</math>
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| where
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| *<math>\sigma</math> is the applied stress
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| *<math>d</math> is the average grain boundary diameter
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| *<math>D_{gb}</math> is the diffusion coefficient in the grain boundary
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| *<math>-Q_{Coble}</math> is the activation energy for Coble creep
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| *<math>R</math> is the molar gas constant
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| *<math>T</math> is the temperature in kelvins
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| Note that in Coble creep, the strain rate <math>\frac{d\epsilon}{dt}</math> is proportional to the applied stress <math>\sigma</math>; the same relationship is found for [[Nabarro-Herring creep]]. However, the two mechanisms differ in their relationship between the strain rate and grain size <math>d</math>. In Coble creep, the strain rate is proportional to <math>d^{-3}</math>, whereas the strain rate in Nabarro-Herring creep is proportional to <math>d^{-2}</math>. 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 <math>n</math> in <math>\frac{d\epsilon}{dt}~\alpha ~d^n</math> and conclude whether Coble or Nabarro-Herring creep is dominant[1].
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| ==References==
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| (1) Meyers and Chawla (1999): "Mechanical Behavior of Materials," 555-557.
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| [[Category:Materials degradation]]
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