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

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

Dislocation entanglements

Work hardening

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

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

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

Solute atoms different size than solvent atoms Solid solution strengthening Crystal structure distorted – slip planes less well-defined Disloc movement restricted 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 Work hardening Solution hardening Precipitation hardening Grain size reduction

Oh, yeah….one more thing about dislocs Dislocations Ease of disloc motion depends on slip plane orientation 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 hcp - 3 slip systems "bcc" - 12 slip systems