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As we have already seen,
dislocation
climb couples point defects and
dislocations in a very direct way. This has the immediate consequence that
climb processes will depend on temperature,
because: |
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The types and concentration of equilibrium point
defects are temperature dependent. |
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The supersaturation, which is the driving force for point
defect reactions including climb, is temperature
dependent. |
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The mobility of point defects, i.e. their diffusion
coefficient, is temperature dependent. |
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Unfortunately for most applications,
climb makes immobile dislocations mobile again (albeit they may move v e
r y s l o w l y ). |
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Coupled to the slow dislocation movement by climb is a slow
plastic deformation with a strong temperature dependence, which would not occur
without point defects - we have an
ageing mechanism. If
screws lose their tension, cables start to bow, and metals suddenly fracture
after years of dutiful service, you are probably looking at the results of
climb processes. |
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The major mechanism by which climb processes enable
dislocations to move, is the circumvention of otherwise insurmountable
obstacles, as shown below. |
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Screw dislocations can climb, too,
turning into a helix shape. The mechanism is illustrated in the advanced
section; examples of
climbed screw dislocations are provided in chapter 6 |
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© H. Föll
