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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

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Plastic deformation l.jpg

PLASTIC DEFORMATION

Dislocations and their role in plastic deformation


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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


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Types of dislocations

Screw

Edge


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Dislocation motionplastic deformation

Note: Dislocations normally move under a shear stress


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How does a dislocation move?


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Stress field of a dislocation


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Analog to an electric charge


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Modes of deformation

  • Slip

  • Twinning

  • Shear band formation


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Slip

  • 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


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Slip…..

  • 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>


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Slip systems


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Dislocation interaction

Positive

Positive

Repulsion

Positive

Negative

Attraction

&

Annihilation

Note: More positive-positive interactions in reality


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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


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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


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Compare slip and twinning


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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


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Plastic deformation movement of dislocations

Strengthening methods


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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


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Solid solution strengthening

  • Interaction between stress fields of alloy atoms and dislocations

  • This is the purpose of alloying


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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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Mechanism

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


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Precipitation hardening

  • Precipitates are second-phase particles

  • Hard precipitates act as barriers to dislocation motion

  • Applicable only to some alloy systems


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