1. Dislocations - Initial overview
Material w/ NO dislocations is very strong
But it cannot be deformed plastically
Dislocations weaken a material
But dislocations make plastic deformation possible

2. Dislocations - need to control slip
Ease of disloc movement along slip planes
is key to ductility
Easier movement along slip planes means
greater ductility
Well-defined slip planes are best for easy motion
flat!!

3. Brief description of metal crystal structures
Most metals exhibit one of three very simple types of atom arrangements (crystal structures):
cubic close-packed (ccp)
hexagonal close-packed (hcp)
close-packed structures built up of atom layers like this
Stacking of layers different for cubic or hexagonal

4. Brief description of metal crystal structures
Most metals exhibit one of three very simple types of atom arrangements (crystal structures):
cubic close-packed (ccp)
hexagonal close-packed (hcp)
"body centered cubic metal structure" ("bcc")
No close-packed layers in "bcc"

5. Brief description of metal crystal structures
Just for fun……here are several views of the structure of a sodium silicate zeolite – a somewhat complicated crystal structure

6. Brief description of metal crystal structures
Where are mostly densely packed planes
in these three crystal structures?
cubic close-packed (ccp)
hexagonal close-packed (hcp)
"body centered cubic metal structure" ("bcc")
four

7. Brief description of metal crystal structures
Where are mostly densely packed planes
in these three crystal structures?
cubic close-packed (ccp)
hexagonal close-packed (hcp)
"body centered cubic metal structure" ("bcc")
four
one

8. Brief description of metal crystal structures
Where are mostly densely packed planes
in these three crystal structures?
cubic close-packed (ccp)
hexagonal close-packed (hcp)
"body centered cubic metal structure" ("bcc")
four
one
six

9. Dislocations
Slip planes are most densely packed planes
in crystal structure
Disloc movement easier in simple crystal structures cuz
slip planes well-defined

10. Dislocations
Metals are so useful in our society because
they have simple crystal structures
Metals are so useful in our society because
they can be plastically deformed
Can be plastically deformed because
dislocations can move easily
Can move easily because
the slip planes are well-defined
Slip planes well-defined because
metals have simple crystal structures

11. Dislocations
Problem: as material deforms, dislocs disappear - at surface, at grain boundaries, elsewhere
If dislocs disappear completely, mat'l becomes extremely strong & no longer ductile
Never happens……
Instead, more dislocs produced during deformation!!
Highly deformed mat'l contains BILLIONS of dislocs
Undeformed – 106 lines/cm2
Deformed – 1012 lines/cm2

12. Dislocation generators
Dislocation generators at work!
Internal generators act to constantly generate new dislocs during deformation