3. Crystal structures of materials -
where are the atoms?
Polymers - mostly not crystalline
structure extremely complicated
Ceramic materials - complicated
many different types of atom arrangements
Metals - really simple - most have one of three types
4. Crystal structures of pure metals
Most pure metals exhibit one of three types
1. cubic close packing (ccp or A1)
2. hexagonal close packing (hcp or A3)
3. A2 (almost universally referred to by
the confusing notation 'bcc')
5. Crystal structures of pure metals
cubic close-packed (ccp)
close-packed plane of atoms
6. Crystal structures of pure metals
cubic close-packed (ccp)
ABCABC layer sequence close-packed plane of atoms
A
B
C
A
A
B
C
7. Crystal structures of pure metals
hexagonal close-packed (hcp)
Zn, Cd, Co, Ti, Zr…… close-packed plane of atoms
hexagon
8. Crystal structures of pure metals
hexagonal close-packed (hcp)
ABAB layer sequence close-packed plane of atoms
A
B
A
B
A
B
9. Fe, V, Cr, Mo, W, Ta……
Crystal structures of pure metals
A2 structure - so-called "bcc" metal structure
almost close-packed atom planes
some empty space
10. Dislocations
And now…the rest of the story (on plastic deformation)!
Mechanism of plastic deformation connected
with existence of defects in atom arrangement
known as dislocations
Ideally, atom arrangement within a crystal repeats
perfectly
Mistakes (defects) in repetition occur in reality
11. Dislocations
Situation is this:
strength of a material w/ no dislocations is
20-100 times greater than ordinary materials
Think of edge
dislocation as
extra plane of
atoms partially
inserted into
crystal
21. Dislocations
If hundreds of thousands of dislocations move
through material, microscopic steps produced in
the surface as below
22. Dislocations - Initial overview
Material w/ NO dislocations is very strong
Dislocations weaken a material
But it cannot be deformed plastically
But dislocations make plastic deformation possible
