DislocationSSWeb Class_13a_final

DislocationSSWeb Class_13a_final - Twinning Dislocation...

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Twinning Dislocation Reactions
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Deformation by Twinning The second important mechanism by which metals deform is the process known as twinning. Twinning results when a portion of the crystal takes up an orientation that is related to the orientation of the rest of the untwinned lattice in a definite, symmetrical way. The twinned portion of the crystal is a mirror image of the parent crystal. The plane of symmetry between the two portions is called the twinning plane.
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Note that the twin is visible on the polished surface because of the change in elevation produced by the deformation and because of the difference in crystallographic orientation between the deformed and unreformed regions. It should be noted that twinning differs from slip in several specific respects. In slip, the orientation of the crystal above and below the slip plane is the same after deformation as before, while twinning results in an orientation difference across the twin plane.
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Slip is usually considered to occur in discrete multiples of the atomic spacing, while in twinning the atom movements are much less than an atomic distance. Slip occurs on relatively widely spread planes, but in the twinned region of a crystal every atomic plane is involved in the deformation.
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Figure 12-2. Classic picture of twinning.
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In general, a dislocation line cannot end inside of a crystal. The exception is at a node, where three or four dislocation lines meet. At a node two dislocations with Burgers vectors b 1 and b 2 combine to produce a resultant dislocation b 3 . The vector b 3 is given by the vector sum of b 1 and b 2 . Dissociation or Combination of Dislocations
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A dislocation with a Burgers vector equal to one lattice spacing is said to be a dislocation of unit strength . Because of energy considerations, dislocations with strengths larger than unity are generally unstable and dissociate into two or more dislocations of lower strength.
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The criterion for deciding whether or not the dissociation will occur is based on two
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DislocationSSWeb Class_13a_final - Twinning Dislocation...

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