Dislocations and their role in plastic deformation
Dislocations are line defects that exist in metals
There are two types of dislocations: edge and screw
The symbol for a dislocation is
The dislocation density in annealed metals is normally r = 106/cm2
Positive-positive dislocation interaction
Results in more stress to move dislocations (or cause plastic deformation):called work hardening
This type of interaction also leads to dislocation multiplication which leads to more interactions and more work hardening.
Twinning
Common in hcp and bcc structures
Limited deformation but help in plastic deformation in hcp and bcc crystals
Occurs on specific twinning planes and twinning directions.
Compare slip and twinning
Shear band formation
Limited non-homogeneous deformation
Very large localized strain e~1 or 100%
Occurs especially under high strain rates
Mechanism of deformation still unclear
Cold working
Deformation at temperatures below 0.4 Tm
Dislocation density increases from 106/cm2 to 1010-12/cm2
High dislocation density results in a large number of dislocation interactions which results in high strength and hardness.
Grain size refinement
Small grains result in higher strength
Small grains is equivalent to a large number of grain boundaries in the same volume
Grain boundaries act as barriers to dislocation motion
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