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PLASTIC DEFORMATION

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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  1. PLASTIC DEFORMATION Dislocations and their role in plastic deformation

  2. 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

  3. Types of dislocations Screw Edge

  4. Dislocation motionplastic deformation Note: Dislocations normally move under a shear stress

  5. How does a dislocation move?

  6. Stress field of a dislocation

  7. Analog to an electric charge

  8. Modes of deformation • Slip • Twinning • Shear band formation

  9. 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

  10. 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>

  11. Slip systems

  12. Dislocation interaction Positive Positive Repulsion   Positive Negative Attraction & Annihilation  Note: More positive-positive interactions in reality

  13. 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

  14. 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

  15. Compare slip and twinning

  16. 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

  17. Plastic deformation movement of dislocations Strengthening methods

  18. 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

  19. Solid solution strengthening • Interaction between stress fields of alloy atoms and dislocations • This is the purpose of alloying

  20. 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

  21. 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

  22. 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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