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PLASTIC DEFORMATION PowerPoint PPT Presentation


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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

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

  • The dislocation density in annealed metals is normally r = 106/cm2


Types of dislocations

Screw

Edge


Dislocation motionplastic deformation

Note: Dislocations normally move under a shear stress


How does a dislocation move?


Stress field of a dislocation


Analog to an electric charge


Modes of deformation

  • Slip

  • Twinning

  • Shear band formation


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


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>


Slip systems


Dislocation interaction

Positive

Positive

Repulsion

Positive

Negative

Attraction

&

Annihilation

Note: More positive-positive interactions in reality


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


Plastic deformation movement of dislocations

Strengthening methods


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

  • Interaction between stress fields of alloy atoms and dislocations

  • This is the purpose of alloying


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


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


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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