1. Last class
More mechanical testing
Fracture toughness - pre-cracked specimen
Creep - constant load, elevated temperature
Fatigue - constant load, S/N curves, endurance limit
Fracture surfaces
Brittle vs. ductile
Crack origins, chevron markings
Beachmarks

2. Today
Atom arrangements in metals
Dislocations

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

12. Dislocations
These things are real!
We can see them! (in an electron microscope)
Dislocation line
0.00001 cm

13. Dislocations
Move under a shear stress

14. Dislocations
Move under a shear stress

15. Dislocations
Move under a shear stress

16. Dislocations
Move under a shear stress

17. Dislocations
Move under a shear stress

18. Dislocations
Move under a shear stress

19. Dislocations
Permanent change in shape results
Dislocation has disappeared
Dislocations move along a slip plane

20. Dislocations
Watch a real one move!

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