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PLASTIC DEFORMATION. Dislocations and their role in plastic deformation. What are dislocations?. Dislocations are line defects that exist in metals There are two types of dislocations: edge and screw The symbol for a dislocation is

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plastic deformation


Dislocations and their role in plastic deformation

what are dislocations
What are dislocations?
  • 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

Dislocation motionplastic deformation

Note: Dislocations normally move under a shear stress

modes of deformation
Modes of deformation
  • Slip
  • Twinning
  • Shear band formation
  • Dislocations move on a certain crystallographic plane: slip plane
  • Dislocations move in a certain crystallographic direction: slip direction
  • The combination of slip direction and slip plane is called a slip system
  • Slip planes are normally close-packed planes
  • Slip directions are normally close-packed directions

Recall for fcc close-packed planes are {111}

Close-packed directions are <110>

dislocation interaction
Dislocation interaction









Note: More positive-positive interactions in reality

positive positive dislocation interaction
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
  • 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
shear band formation
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
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
solid solution strengthening
Solid solution strengthening
  • Interaction between stress fields of alloy atoms and dislocations
  • This is the purpose of alloying
grain size refinement
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

Strength is inversely proportional to grain size

s = s0 + kyd-1/2

Hall-Petch equation

Smaller grains have more boundary area and hence more

barriers to dislocation motion

precipitation hardening
Precipitation hardening
  • Precipitates are second-phase particles
  • Hard precipitates act as barriers to dislocation motion
  • Applicable only to some alloy systems