When you ask a question do you really

When you ask a question, do you really want to know the answer, or are you merely flaunting your power? -Dmitri Harkonnen

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

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

Dislocation entanglements

Dislocation entanglements

Work hardening

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

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

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

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

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

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

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

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

Assignments for next class: 1. review today's classnotes a. work hardening b. dislocation tangles c. precipitation hardening d. solution hardening e. grain boundary pileups f. slip systems and critical resolved shear stress 2. Exam comments