1. PLASTIC DEFORMATION Dislocations and their role in plastic deformation

2. What are dislocations? n Dislocations are line defects that exist in metals n There are two types of dislocations: edge and screw n The symbol for a dislocation is The dislocation density in annealed metals is normally  = 10 6 /cm 2

3. Types of dislocations Edge Screw

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 n Slip n Twinning n Shear band formation

9. Slip n Dislocations move on a certain crystallographic plane: slip plane n Dislocations move in a certain crystallographic direction: slip direction n The combination of slip direction and slip plane is called a slip system

10. Slip….. n Slip planes are normally close-packed planes n Slip directions are normally close-packed directions Recall for fcc close-packed planes are {111} Close-packed directions are

11. Slip systems

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

13. Positive-positive dislocation interaction n Results in more stress to move dislocations (or cause plastic deformation):called work hardening n This type of interaction also leads to dislocation multiplication which leads to more interactions and more work hardening

14. Twinning n Common in hcp and bcc structures n Limited deformation but help in plastic deformation in hcp and bcc crystals n Occurs on specific twinning planes and twinning directions

15. Compare slip and twinning

16. Shear band formation n Limited non-homogeneous deformation Very large localized strain  ~1 or 100% n Occurs especially under high strain rates n Mechanism of deformation still unclear

17. Plastic deformation  movement of dislocations Strengthening methods

18. Cold working n Deformation at temperatures below 0.4 T m n Dislocation density increases from 10 6 /cm 2 to 10 10-12 /cm 2 n High dislocation density results in a large number of dislocation interactions which results in high strength and hardness

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

20. Grain size refinement n Small grains result in higher strength n Small grains is equivalent to a large number of grain boundaries in the same volume n Grain boundaries act as barriers to dislocation motion

21. Mechanism Strength is inversely proportional to grain size  =  0 + k y d -1/2 Hall-Petch equation Smaller grains have more boundary area and hence more barriers to dislocation motion

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