Metals and alloys Flashcards
metals and alloys used in dentistry - give examples
partial denture framework
- CoCr, type IV gold
crowns
- stainless steel
denture base
- stainless steel
orthodontic appliances
- NiTi
restorations
- amalgam
metal - definition
aggregate of atoms in crystalline structure
Elastic limit - define
maximum stress without plastic/permanent deformation
ductility - define
amount of plastic deformation prior to fracture
- the extent that a material can be shaped/manipulated
factors affecting mechanical properties of an alloy
choice of metal
- each has its own inherent characteristics
crystalline structure - depends on
- history - method of production
shaping - e.g. cold working, swaging
grain size
grain imperfections
what is a grain boundary?
a region where grains (or crystals) make contact
- impurities concentrate here
Fast cooling/quenching - effects
more nuclei
small fine grains
slow cooling effects
few nuclei
large coarse grains
pros and cons of small fine grains
high elastic limit
increased fracture strength, hardens and ultimate tensile strength
BUT
decreased ductility
Factors for rapid cooling/quenching
small bulk
heat metal/alloy just above melting point
mould - high thermal conduction
quench
What are dislocations?
imperfections/defects in the crystal lattice
What is slip?
due to propagation of dislocations and involves rupture of only a few bonds at a time
impeding movement of dislocations in metals/alloys - consequences
increased
- elastic limit
- fracture strength
- ultimate tensile strength
- hardness
decreased
- ductility
- impact resistance
factors impeding dislocation movement
grain boundaries
- hence fine grains
alloys
- different atom sizes
cold working
- dislocations stopped at grain boundaries
What is cold work?
work done on a metal or alloy
- e.g bending, rolling, swaging
- at low temperature - below recrystallisation temperature
causes slip
- dislocations collect at grain boundaries
leads to stronger, harder metal
- more prone to work or strain hardening
cold working - effect on the metal
modifies grain structure
increases
- elastic limit
- fracture strength/UTS
- hardness
decreases
- ductility
- impact strength
- corrosion resistance
Residual stress consequences and how to remedy
causes instability in lattice
results in distortion over time
relieved by annealing process
What is annealing?
heating a metal or alloy so that greater thermal vibrations allow migration/rearrangement of atoms
cold work - negative consequences
results in internal stresses
- may lead to distortion of appliance over time
stress relief annealing effects
eliminates stresses by allowing atoms to rearrange within grains
- grain structure and mechanical properties unchanged
- some further cold work possible
recrystallisation - causes and effects
occurs when metal/alloy heated causing
- new smaller equiaxed grains (crystal growth is of equal dimension in each direction)
- lower elastic limit, UTS and hardness
- increased ductility
recrystallisation consequences
spoils benefits of cold work
allows further cold work
cold/work/recrystalisation repeated until correct shape obtained
recrystallisation temperature depends on…
the amount of cold work
greater the amount of cold work the lower the recrystallisation temperature
excessive temperature rise causes…(how to avoid)
large grains to replace smaller coarse grains yielding poorer mechanical properties
- careful when annealing
