Mechanisms
Basic Concept of dislocation:
Dislocations can be edge dislocations, screw dislocations and exist in combination of the two.
Their motion (slip) occurs by sequential bond breaking and bond reforming . The number of
dislocations per unit volume is the dislocation density, in a plane they are measured per unit area.
Characteristics of Dislocations.
There is strain around a dislocation which influences how they interact with other dislocations,
impurities, etc. There is compression near the extra plane (higher atomic density) and tension
following the dislocation line. Dislocations interact among themselves. When they are in the
same plane, they repel if they have the same sign and annihilate if they have opposite signs
(leaving behind a perfect crystal). In general, when dislocations are close and their strain fields
add to a larger value, they repel, because being close increases the potential energy (it takes
energy to strain a region of the material). The number of dislocations increases dramatically
during plastic deformation. Dislocations spawn from existing dislocations, and from defects,
grain boundaries and surface irregularities.
Plastic Deformation:
Slip directions vary from crystal to crystal. When plastic deformation occurs in a grain, it will be
constrained by its neighbors, which may be less favorably oriented. As a result, polycrystalline
metals are stronger than single crystals (the exception is the perfect single crystal, as in
whiskers.)
