Dislocations - Initial overview

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

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

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

4. Brief description of metal crystal structures No close-packed layers in "bcc" 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")

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 four 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")

7. Brief description of metal crystal structures four one 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")

8. Brief description of metal crystal structures four one six 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")

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 Undeformed – 106 lines/cm2 Deformed – 1012 lines/cm2 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

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