Dislocations

1. Dislocations Today's objective – how to make these guys work harder

2. Work hardening So what happened (coat hanger)? Dislocations moved when coat hanger bent Dislocation generators produced large amounts of new dislocations New dislocations so numerous that they entangled & had difficulty moving

3. Dislocation entanglements

4. Dislocation entanglements

5. Work hardening

6. Work hardening plate sheet extremely hard - need backup rolls to keep work rolls from deforming So….cheap, convenient way to strengthen mat'ls On the other hand, work hardening can be problem Example: cold rolling plate to sheet harder!! too hard to continue rolling harder fairly soft

7. Dislocation interactions With each other: annihilate entangle form generators With grain boundaries: disappear pileups Thus, small grained mat'ls stronger cuz more grain bdy area, ––> more pileups

8. Dislocation interactions With tiny precipitate particles Why don't dislocs move thru precipitates? a. complex structure - poorly defined slip planes and/or b. slip planes wrongly oriented

9. Solid solution strengthening Solute atoms different size than solvent atoms Crystal structure distorted – slip planes less well-defined Disloc movement restricted Cu solid solutions

10. Strengthening methods - summary Work hardening Solution hardening Precipitation hardening Grain size reduction

11. Dislocations Oh, yeah….one more thing about dislocs Ease of disloc motion depends on slip plane orientation

12. Dislocations Slip occurs only as a result of shear & only along slip plane in a slip direction The greater the no. of slip systems, the greater the probability some slip plane & slip direction correctly oriented

13. Dislocations "bcc" - 12 slip systems Slip system = slip plane + slip direction ccp - 12 slip systems hcp - 3 slip systems