metals and alloys 1 Flashcards
5 examples of metal uses in dentistry
Partial denture framework (CoCr, Type IV gold)
- 3D object has to fit a number of surfaces neatly
- Fitted in readily without causing pain for pt on regular basis
Crowns (stainless steel)
Denture base (stainless steel)
Orthodontic appliance (NiTi and others)
- Lots of wires, Some with twists
- Brackets
- Forces are exerted to re-position individual teeth
Restorations (amalgam)
what is unique about amalgam
Undergoes a setting reaction
Most alloys do not undergo a setting reaction
metal
aggregate of atoms in a crystalline structure
alloy
combination of metal atoms in a crystalline structure
- 2 or more metals (sometimes up to 5)
what are the building blocks of alloys
metals
increase stress leads to
increased strain
then change in shape (deformation)
stress-strain curve
unique to material
show the about of stress needed before a permanent deformation (fracture) in incurred
malleability
Ability to deform under pressure
- Compressive strength
elastic limit EL
maximum stress without plastic deformation
ductility
Amount of plastic deformation prior to fracture (hammered, stretched before fracture)
i.e. measure of the extent that a material can be shaped/manipulated
= (y-x)%
Tensile strength
what are 2 factors for crystalline structure
history - method of production
dictates crystalline structure – alike gypsum
shaping - crucial for dental applications
e.g. cold working, swaging
grain
single crystal
lattice arrangement of one (metal) or more (alloy) types of atom
what are the 3 factors affecting mechanical properties of a metal
crystalline structure
grain size
grain imperfections
3 simple crystal/lattice structures of metals
cubic
face centred cubic
body centred cubic
what is true about all metals in a lattice
metals have atoms positioned at regularly arranged sites, whether in a cubic, BCC or FCC or other configuration
stages of crystal growth
first atoms cooling to form solid
- Atoms at these sits act as nuclei of crystallisation
- Other atoms cool around these nuclei
Crystals grow to form dendrites
- 3D branched lattice network
Crystals (or grains) grow until they impinge on other crystals
- stop the other one from growing any further in that direction/area/volume
Region where grains make contact is called Grain Boundary
describe the cooling curve for a pure metal
molten Metal atom in container
- Allow to cool
Gradually cooling down to plateau
Liquid to solid
- Takes time
Maintains till all from liquid to solid
- Tails off at the end
how can grains of metal differ in shape and size
can vary in shape and size depending on processing and cooling
- effect structure and properties of alloys made
3 types of crystal growth
equi-axed
radial
fibrous
equi-axed crystal growth
crystal growth of equal dimension in each direction
radial crystal growth
molten metal cooled quickly in cylindrical mould
- burst out from centre to diameter of cylider
fibrous crystal growth
wire pulled through die (narrow circular aperture)
- (cold worked metal/alloy)
what is the term for fast cooling a metal
quenching
benefits of fast cooling a metal
more nuclei of crystallisation
- crucial
small fine grains
- mechanical properties enhanced
disadvantages of slow cooling of metal
Few nuclei
Large coarse grains
- Weak
what are nucleating agents
Impurities or additives act as foci for crystal growth
So help the crystallisation process - control grain size
E.g. steel industry
what is a structure of grain
single crystal (lattice) with atoms orientated in given directions (dendrites)
what is the grain boundary
where individual grains make contact
change in orientation of the crystal planes
- (impurities concentrate here)
3 advantages of small fine grains
High elastic limit
Increased UTS (fracture stress)
Increase surface hardness
all desirable for dental prostheses
- small grains are best
disadvantage of small grains
decreased ductility
- less easily stretched while avoiding fracture
4 factors for rapid cooling (quenching)
small bulk
heat metal/alloy just above Tm (melting point)
mould - high thermal conduction
- pull heat away from metal quickly
quench
- put in container of cold water
- quickest way to cool down
what are dislocation/defects in crystalline structures
Representing misalignment of atoms in that lattice network
- 3 rows of 6 To 3 rows of 5
Two planes have slight mismatch
- Weaken grain and overall metal structure
how to correct a dislocation/defect in crystalline structure
Force to individual crystal
- Defect moves/slides along the singular plane
- Until grain changes shape
Step change between top 3 rows and next 3
Push to grain boundary then Defect removed
- Changes shape of the lattice structure
what is metal slip
is due to Propagation of Dislocations and involves rupture of only a few bonds at a time
Forced from left to right side
- Only need to break one bond at a time
- Don’t need supreme large force to do
Low level over long time to ripple through structure
what does impeding movement of dislocations in metal atoms cause an increase in
elastic limit
UTS
suface hardness
what does impeding movement of dislocations in metal atoms cause a decrease in
ductility
impact resistance (drop more likely to break)
what is an advantage of many grain boundaries (small fine grains)
many places to slip defect out
how do alloys impede dislocation movement
alloy with different metal atoms , of different sizes (obviously) HAS an INHERENT resistance to the movement of dislocations within a grain.
how does cold working impede dislocation movement
dislocations build up at grain boundaries
Equivalent of pushing all defects to grain boundary
- Improve mechanical properties of the object
what is cold work/work hardening/ strain hardening
work done on metal/alloy
(e.g. bending, rolling (steel), swaging (car), hammering (blacksmith with horseshoe))
done at LOW TEMPERATURE
(i.e. below recrystallisation/ melting temperature)
causes SLIP – so dislocations
collect at grain boundaries
- hence stronger, harder material
2 other names for cold work
work hardening
strain hardening
what does cold working lead to increase of (3)
elastic limit
UTS
hardness
what does cold working lead to decrease of (4)
Ductility
Impact strength
Lower corrosion resistance
- Not ideal for dental prostheses
Increased residual strength
- Causes instability in lattice reconfiguration of metal ions
——Results in distortion over time – undesirable
Relieved by annealing process
purpose of annealing
heating metal (or alloy) so that greater thermal vibrations allows migration of atoms (i.e. re-arrangement of atoms)
push atoms around, no more residual stress
what does cold work result in the metal structure
internal stresses
may lead to distortion of appliance over time
what is stress relief annealing
eliminates stresses by allowing atoms to re-arrange within grains
grain structure and mechanical properties unchanged
- got to be carried out in a controlled and specific way so the grain is unaffected
some further cold work possible (final shaping)
- effects the properties of the metal or alloy
2 other names for stress relief annealing
annealing
homogenisation
why would you recrystallise a metal
occurs when metal/alloy heated causing
- new smaller equiaxed grains
- lower EL, UTS, hardness
- increased ductility
spoils benefits of cold work (recycle if cold work hasn’t had desired effect)
allows further cold work
- cold work/recrystallisation can be repeated until correct shape obtained
- lose all grains structure and mechanical properties
what happens to the recrystallisation temperature
decreases with the greater amount of cold work
- melting point decreases
what can cause grain growth
excessive temperature rise causes
large grains to replace smaller coarse grains
- yielding poorer mechanical properties
CAUTION WHEN ANNEALING
what 2 things influence and affect grain stucture
influenced by mechanical properties
affected by dislocations (SLIP)
what are 3 ways dentists can manipulate metal properties
cold working
stress relief annealing
recrystallisation
definition of an alloy
a combination (or mixture) of two or more metalsor metal(s) with metalloid - Fe (iron), C (carbon
what is the term used to describe an alloy structure
2 metals form a common lattice structure
- soluble in one another
- form a solid solution
