L6 - Strengthning Mechanisms And Annealing Flashcards
1
Q
What is work hardening?
A
- dislocations and solute atoms create local stress fields
- as material is worked, more dislocations are formed
- these can have stress fields in opposite direction to those of other dislocations
- results in dislocation annihilation
- means dislocations become harder to move
- more stress needed for further strain
2
Q
What happens to the material properties as a result of work hardening?
A
- yield stress increases
- percentage elongation for a given stress decreases
3
Q
What is grain boundary strengthening?
A
- dislocations accumulate at grain boundary
- creates local stress concentration
- initiates dislocation movement in next grain
4
Q
How can grain size affect the yield stress?
A
- larger grain means dislocation accumulation is larger
- thus higher stress concentration
- results in easier dislocation initiation in neighbouring grains
- larger grain means—> lower yield stress
5
Q
How does precipitation hardening work?
A
- precipitates are formed within the metal matrix
- are hard to shear, oppose dislocation motion
- takes large amount of stress to either bow around the precipitates or shear through
6
Q
What is annealing?
A
- process to reduce the effects of work hardening
- resets the microstructure of a metal
- is a diffusion based process
7
Q
What are the conditions for annealing?
A
- carried out at 0.5 * melting point
8
Q
What are the stages of annealing?
A
- recovery
- recrystallisation
- further grain growth
9
Q
What happens in the recovery stage?
A
- dislocations line up with each other
- creates dislocation free areas for recrystallisation
10
Q
What happens in the recrystallisation phase?
A
- atoms diffuse into dislocation free zones
- create new crystals
- is a diffusion process
- heat dependent
11
Q
How does work hardening affect annealing grain size?
A
- more work hardening results in dislocations being propagated and formed
- creates more initiation sites for new crystals
- results in finer grain structure
12
Q
How does temperature affect annealing grain size?
A
- annealing is a diffusion controlled process
- grains will grow quicker at higher temperatures
- results in larger grains size
13
Q
What drives grain growth?
A
- grain boundaries have higher energy than inside the grains
- atoms want to get to a lower energy state
- amalgamation of smaller grains allows for this
14
Q
What are coherent precipitates?
A
- have a structure aligned with the metal matrix around it
15
Q
What are incoherent precipitates?
A
- have a different structure to the matrix around them
- are created at higher temperatures
