Dislocations and Work Hardening Flashcards
Review of what dislocations are and their implications, and how Copper undergoes work hardening
When was work hardening first used?
~9000BC in Middle East, of Cu
How are slip steps formed in crystals?
Atom planes move against one another when force applied to both ends; form slip steps in mirror finish/single crystal
What occurs when slip happens in polycrystals?
Slip occurs in each grain differently on slip system with highest resolved shear stress; overall extension if sum of the slips
Why is the theoretical strength of materials different to the actual strength?
Theoretically, two planes of atoms slide bodily over one another and would have to break bonds between each plane simultaneously; concept based on a perfect lattice; in reality, have imperfections in crystal lattice which allow slip to occur gradually
What are dislocations?
Lines of discontinuity in the crystal lattice that are created by the application of force
What are edge dislocations?
Row of atoms missing during solidification
What are screw dislocations?
Dislocations which occur @90 degrees to force application
What are the implications of the dislocations?
One set of bonds break and reform at a time
What are compressions in a lattice?
Where extra half plane is present; causes bonds to be compressed
What are tensions in a lattice?
Where planes of atoms fall into void of half plane; causes bonds to be stretched
What happens when edge dislocations of same sign interact?
Hard to push them against each other; work hardening
What happens when edge dislocations of opposite sign interact?
Cancel each other out; annihilation
How do you shape something which is being work hardened?
Heat and anneal; causes increased diffusion, which causes increased dislocation climb (movement of dislocations), causing more dislocation sinking and annihilation; dislocation density is decreased; material becomes softer
