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
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