metals Flashcards
what is quenching
method for rapid cooling
use small bulk so easier to cool
heat metal just above melting temp
use a mould with high thermal conduction so the heat can escape more easily
quench using cooler rather than warmer water
Advantages of metals over cr
- strength
- rigid
- high elastic limit (ie wont fracture easily)
- ductility
- hardness
Disadvantages of metals
- poor aesthetics
metal definition
Aggregate of atoms in a crystalline structure
alloy defintion
Combination of metal atoms in a crystalline structure
EL defintion
Maximum stress without plastic deformation
UTS stand for
ultimate tensile strength
ductility definition
Amount of plastic deformation prior to fracture
Ie measure of the extent that a material can be shaped
what is the equation for ductility
(y-x)%
equi-axed grains def
crystal growth of equal dimensions in each direction
what are grains
each grain is a single crystal lattice with atoms orientated in given directons
what is a grain boundary
change in orientation of the crystal planes
ie where one grain comes into contact with another grain
where do impurities tend to concentrate at?
at grain boundaries
dislocations definition
imperfections/defects in the crystal lattice
slip defintion
propagation of dislocation
what factors affect the mechanical properties of metal?
grain size
grain defecs
crystalline structure
what r the three types of crystalline metal structures?
cubic
body centered cubic
face centered cubix
what are the 3 types of grains
equiaxed
radial
fibrous
how do metal crystals grow to form grains?
- some atoms act as nuclei of crystallisation
- crystals grow to form dendrites which are 3D branched lattice networks (atoms concrescence on the nuclei of crystallisation to form dendrites)
- crystals (aka grains) grow until they impinge on other crystals
- region where grains make contact with another is called grain boundary
- this can be done via fast cooling or slow cooling; sometimes with the addition of nucleating agents
what are nucleating agents
**nucleating agents are impurities that act as foci for crystal growth
they speed up growth
what is difference between fast and slow cooling
fast ->
more nuclei of crystallisation,
smaller grains and
better mechanical properties but
decreased ductility
slow ->
large coarse grains which is undesirable and weaker
what happens when the defect moves to the grain boundary
lattice has NO defects and is now a different shape
what is impeding dislocations desirable
increases
o EL
o FS
o UTS
o hardness
but
decreases
o ductility
o impact resistant
how to impede dislocation movement
- having smaller finer grains
- alloys
- cold working, pushing the defects to the boundaries before it sets
fast and slow cooling vs
annealing and cold working ??
fast (quenching) and slow cooling is for growing the crystals (affects grain size)
cold working and annealing is when you’re shaping the metals (processing it)
what is cold working
work done at low temperature causes slip. dislocations collect at grain boundaries resulting in a stronger harder materiald
does cold working modify the metal and its properties
yes
increases
o EL
o FS
o UTS
o hardness
o strength
o more residual stress which is bad
decreases
o ductility
o impact resistant
o less able to resist corrosion which is bad
what causes residual stress in metals
cold working
how do you overcome residual stress from cold wokring
annealing
what is annealing
- heating metal so that greater thermal vibrations allows migration of atoms and rearrangement of atoms within the grain
why is residual stress bad
causes instability in the lattice, results in distortion over time which is undesirable.
annealing vs recrystallization
annealing is done at not so highly raised temp, still below melting temp (in the hundreds of deg)
recrystallisation temp is much higher, above melting temp (in the thousands)
does annealing change the structure and properties of the grain?
no
why do we have to ensure that the annealing temp does not raise too much
- excessive annealing temperature can cause large grains to replace smaller coarse grains yielding poorer mechanical properties
more cold work = high or lower recrystallization temp??
lower
