Strengthening of Materials Flashcards
What is cold working?
Strengthening of a material through plastic deformation.
Occurs due to dislocation movement and dislocation generation within the crystal structure. Happens at a temp where dislocations are created faster than they are annihilated.
Ductile material becomes harder and stronger
Increase dislocation density
What is the dislocation density?
Total number of dislocation lengths per unit volume.
What is the reason for strain hardening?
The increase of dislocation density between with plastic deformation
What is restoration?
The process of returning the material to the state before cold work by heat treatment
What are the two processes for restoration
Recovery and recrystallisation
Explain recovery
Used to increase ductility
Decrease strength
Recovers electrical and thermal conductivity
No change in micro structure
Heated which increases diffusion which decreases dislocation density
What is recrystallisation?
Where the dislocation density decreases, no stored energy, becomes weak and ductile.
Strained grains replaced with strain free grains with low density
What is the recrystallisation temperature?
The temp at which recrystallisation reaches completion in one hour
What happens following recrystallisation?
Grain growth
What is grain growth?
Grains continue to grow following recrystallisation. Energy reduced as grains grow, small grains shrink.
Involves short range diffusion
Why are high angle grain boundaries used?
They block slip and increase strength
How do you control the grain sizes?
By the rate of solidification, by plastic deformation and heat treatment
What is solid solution strengthening?
The strengthening of material due to impurities and interstitials
Where do impurities diffuse during solid solution strengthening?
The segregate around dislocations comes to find atomic sites that suit their radius. This reduces the overall strain energy and anchors the dislocation
What forces are exerted on small and large impurity atoms?
Small - tensile
Large - compressive
