Get your paper clip for experiment on the way into class!!

Dislocations And now the rest of the story! Dislocations And now the rest of the story! 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 Dislocations Situation is this: strength of a material w/ no dislocations is times greater than ordinary materials Think of edge dislocation as extra plane of atoms partially inserted into crystal

Dislocations These things are real! We can see them! (in an electron microscope) Dislocations These things are real! We can see them! (in an electron microscope) Dislocation line 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 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 Dislocations Permanent change in shape results Dislocation has disappeared Dislocations move along a slip plane

Dislocations Watch a real one move! 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 If hundreds of thousands of dislocations move through material, microscopic steps produced in the surface as below

Dislocations The world famous paper clip experiment!! Dislocations The world famous paper clip experiment!!

Instructions Roughly straighten a portion of the clip Use thumbnail to judge the smoothness of the wire Now bend & straighten wire sharply at the same point about 4 times without fracturing it Check roughness at bend point; compare Instructions Roughly straighten a portion of the clip Use thumbnail to judge the smoothness of the wire Now bend & straighten wire sharply at the same point about 4 times without fracturing it Check roughness at bend point; compare

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

Assignments for next class: 1. review today's classnotes a. fatigue & beachmarks b. absence of dislocs means? c. presence of dislocs means? d. disloc slip 2. Read text: sections 8.9, Assignments for next class: 1. review today's classnotes a. fatigue & beachmarks b. absence of dislocs means? c. presence of dislocs means? d. disloc slip 2. Read text: sections 8.9,