Metal and Alloys Flashcards
What is cold working?
aka work hardening
Strengthening of a metal by plastic deformation which occurs because of dislocation movement.
What are the effects of cold working?
It modifies the grain structure
This leads to higher elastic limit, higher UTS, higher hardness
BUT also causes lower ductility, lower impact strength, lower corrosion resistance
ALSO causes residual stress
Why is residual stress a disadvantage? How is it caused? How is it relieved?
Residual stress causes instability in lattice and results in distortion over time
Caused by cold working
Relieved by annealing
What factors impede dislocation movement?
grain boundaries, alloys (different atom sizes), cold working
What are the results of slow/fast cooling of metals?
fast cooling -> more nuclei, small fine grains
slow cooling -> few nuclei, large coarse grains
What are the properties of small fine grains?
high elastic limit, increased UTS, increased hardness
decreased ductility
What are the effects of rapid cooling a molten alloy?
prevents atoms diffusing through lattice
generates many small grains which impede dislocation
causes coring, as composition varies throughout grain
alloy is not as resistant to corrosion if coring occurs (the further aprt the liquidus and solidus, the more coring occurs)
How do we resolve coring?
homogenising anneal - reheat alloy to allow atoms to diffuse and so cause grain composition to become homogenous
NB: keep below recrystallisation temperature, otherwise grains altered
