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

Today Atom arrangements in metals Dislocations

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

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')

Crystal structures of pure metals cubic close-packed (ccp) close-packed plane of atoms

Crystal structures of pure metals cubic close-packed (ccp) ABCABC layer sequence close-packed plane of atoms A B C C A A B

Crystal structures of pure metals hexagonal close-packed (hcp) Zn, Cd, Co, Ti, Zr…… hexagon close-packed plane of atoms

Crystal structures of pure metals hexagonal close-packed (hcp) ABAB layer sequence close-packed plane of atoms B A B B A A

Crystal structures of pure metals A2 structure - so-called "bcc" metal structure almost close-packed atom planes Fe, V, Cr, Mo, W, Ta…… some empty space

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

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

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

Dislocations Move under a shear stress

Dislocations Move under a shear stress

Dislocations Move under a shear stress

Dislocations Move under a shear stress

Dislocations Move under a shear stress

Dislocations Move under a shear stress

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

Dislocations Watch a real one move!

Dislocations If hundreds of thousands of dislocations move through material, microscopic steps produced in the surface as below

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