Metal and alloys 1 and 2 Flashcards
What metals and alloys are widely used in dentistry
- partial denture framework (CoCr, Type 4 gold)
- crowns (stainless steel)
- denture base (stainless steel)
- orthodontic appliance (NiTi)
- restorations (amalgam-unique)
What properties are we interested in looking at regarding metals and alloys
- strength
- ductility
- rigidity
- hardness
- elastic limit
What can affect properties of metals
defects (dislocations) on properties
What can affect structure
processing (e.g. quenching, cold working, annealing)
Disadvantage of metals
poor aesthetics
Define metal
aggregate of atoms in crystalline structure
Define alloy
combination of metal atoms in a crystalline structure
What information can we gain from a stress-strain curve (know what this looks like)
- Fracture strength
- Elastic limit
- Ultimate tensile strength
- Ductility
- malleability
- ductility
- rigidity
What is the elastic limit
maximum stress without plastic deformation
What is ductility
amount of plastic deformation prior to fracture
What is on the y and x axis of a stress strain curve
x = strain % y= stress (MPa)
What do mechanical properties depend on
- Choice of metal
2. crystalline structure
what does crystalline structure depend on
- history (method of production)
- shaping - crucial for dental applications e.g. cold working, swaging
What are the factors which affect mechanical properties
- crystalline structure
- grain size and grain imperfections
What are some crystal or lattice structures
- cubic
- face-centred cubic
- body centred cubic
What does the cooling curve of pure metal look like
- gradual decrease in temp
- plateau
- gradual decrease in temp
what is the plateu phase of the cooling curve of pure metal
melting point
what triggers the end of the plateu phase on the cooling curve of pure metal
retains temp until all metal changes from liquid to solid, then it cools down further
How does crystal growth occur
- atoms act as nuclei of crystallisation
- crystals grow to form dendrites (3D branched lattice network)
- crystals (or grains) grow until they impinge on other crystals
- region where grains make contact is called grain boundary
what is the grain boundary
where grains (crystals) make contact
what is the grain structure where crystal growth of equal dimension in each direction
equi-axed grains
what is the grain structure where crystal growth is molten metal cooled quickly in cylindrical mould
radial
what is the grain structure where wire is pulled through die
fibrous
what are the different names for the different grain structures of crystal growth
- equi-axed
- radial
- fibrous
What happens to crystal growth when you cool it quickly
- more nuclei
- small fine grains
what happens to crystal growth when you cool it slowly
- few nuclei
- large coarse grains
what size of grains do we want and why
we want lots of small fine grains as large grains are weak
what are nucleating agents
impurities or additives act as foci for crystal growth
- they help crystallising process
what are grains
each grain is a single crystal (lattice) with atoms orientated in given directions (dendrites)
what is a grain boundary
change in orientation of the crystal planes (impurities concentrate here)
why are small fine grains advantageous
- high elastic limit
- increased ultimate tensile strength (UTS), hardness
disadvantage to small fine grains?
decreased ductility (less easily stretched)
What are the factors for rapid cooling (for small fine grains)
- small bulk
- heat metal/ alloy just above Tm
- mould- high thermal conduction
- quench
What happens when you apply a force to a crystal lattice which has a defect (most do)
If you apply a force to an individual crystal and the defect moves in a singular direction (slip) until you end up with grain looking a different shape
Defect goes to grain boundary, the only way to remove the defect is to change the shape of the lattice structure
What are dislocations
dislocations are imperfections/defects in the crystal lattice
What is slip and what is it due to
SLIP is when a dislocation/defect moves along the grain structure following a force. It’s due to the propagation of dislocations and involves rupture of only a few bonds at a time (doesn’t need to be a big force)
What would be the effects of impeding the movement of dislocations
Increases
- elastic limit
- UTS
- hardness
Decreases
- ductility
- impact resistance
What are factors which impede dislocation movement
- grain boundaries
- alloys (different atom sizes)
- cold working (dislocations build up at grain boundaries)
How is cold working done and what does it cause
- bending, rolling, swaging
- done at low temperature
- causes SLIP (so dislocations collect at grain boundaries)
- hence, stronger, harder material
How does cold working modify the grain structure
Higher
- elastic limit
- UTS
- hardness
Lower
- ductility
- impact strength
- lower corrosion resistance (negative)
What is the effect of cold working on strength, residual stress and ductility
The more cold working you do, the more you push dislocations to the grain boundary, the stronger but the more you increase the residual stress in the lattice and reduce ductility
Why is increased residual stress bad
- causes instability in lattice
- results in distortion over time (undesirable)
how is increased residual stress relieved
by annealing process
what is annealing
heating metal (or alloy) so that greater thermal vibrations allows migration of atoms (i.e. re-arrangement of atoms)
Why is stress relief annealing better(?) than cold working
Cold working results in internal stresses which may lead to distortion of appliance over time
Stress relief annealing eliminates stresses by allowing atoms to re-arrange within the grains. The grain structure and mechanical properties are unchanged. Final shaping by cold working possible
when does recrystallisation occur and what does it result in
- occurs when metal/alloy heated causing
- new smaller equiaxed grains
- lower EL, UTS, hardness
- increased ductility - spoils benefits of cold work
- allows further cold work
- cold work/ recrystallisation repeated until correct shape obtained
What should the recrystallisation temperature be
- depends on amount of cold work
- greater the amount of cold work the lower the recrystallisation temperature
What would an excessive temperature rise do to grain growth
large grains to replace smaller coarse grains yielding poorer mechanical properties –> careful when annealing
what are the dental appliance manipulation processes
- cold working
- stress relief annealing
- recrystallisation
what determines the properties of metals
- grain size
- whether there are dislocations
- how you shape it
what is an alloy
a combination (or mixture) or 2 or more metals, or metal(s) with a metalloid (Fe, C)
what is a solid solution
Two metals that form a lattice structure as they are soluble in one another (coexist in a common lattice)
Advantages to alloys
improved:
- mechanical properties (EL, UTS, hardness)
- corrosion resistance
- lower melting point that individual metal
dental uses of alloys
- steel - burs, instruments
- amalgam
- gold alloy - inlays, crowns, bridges, partial dentures, wires
- nickel chromium - crowns bridges, wires
etc
Define phase
physically distinct homogeneous structure (can have more than one component)
Define solution
homogenous mixture at an atomic scale
How many phases would grains composed of metal A only have
1 phase
How many phases would individual grains composed of metal A and B have
2 phases
How many phases would grains in a homogenous mixture have composed of metal A and B
1 phase (solid solution)
n.b. grains can be of varying size and shape
Are metals soluble or insoluble when molten
soluble
What forms can metal take on crystallisation
- insoluble (no common lattice, 2 phases)
- form intermetallic compound with specific chemical formulation (e.g. Ag3Sn)
- be soluble and form a solid solution i.e. form common lattice… 3 types of solid solution
What are the 3 types of solid solution
- substitutional (random and ordered)
2. interstitial
what is a substiutional solid solution
atoms of one metal replace the other metal in the crystal lattice/ grain
what metals form a random substitutional solid solution
metal atoms similar in size, valency, crystal structure e.g. AuCu
what is an ordered substitutional solid solution
metal atoms in regular lattice arrangement
what metals form an ordered substitutional solid solution
metal atoms similar in size, valency, crystal structure
what is an interstitial solid solution
smaller atoms located in spaces in lattice/grain structure of larger atom
what metals form an interstitial solid solution
atoms markedly different in size e.g. Fe-C
How does the cooling curve differ between pure metal and an alloy
Cooling curve of pure metal- crystallisation begins at start of plateau, once all has been crystallised temp drops
Of alloy:
- TL - crystallisation of alloy begins - Not all alloy can crystallise at one time - Crysatallisation continues over a drop in temp down to TS where crystallisation is complete (unlike pure metal) - Then whole alloy temp drops
I.e:
- metal crystallises at one temperature
- alloy crystallises over a temperature range
What does TL and TS stand for
TL - liquidous
TS - solidous
Know what a phase diagram looks like, what can you plot on it?
temp (y axis) vs alloy composition (x axis)
plot TL and TS for both
what does a phase diagram tell you
If go to temp far above TL you know that both metals are liquid
In the middle, part liquid part solid
Below TS, crystallisation complete for both metals
What is the liquidous (TL) line on a phase diagram
line representing the temperatures which different alloy compositions begin to crystallise
what is the solidus (TS) line on a phase diagram
line representing the temperatures which different alloy compositions have completely crystallised
what happens when you cool an alloy slowly
allows metal atoms to diffuse through lattice
- positive: ensures grain composition is homogenous
- negative: results in large grains
what happens when you cool an alloy rapidly
coring
what is coring
when you cool an alloy rapidly, the first grain to form will be about 80% A, as it cools, the next grains to form will be 75% A and so on
as you cool rapidly, you will have different compositions of A and B to give a concentration gradient
use tie line on phase diagram
how does coring produce positive and negative results
positive: small grains
negative: different percentages within the grains that you form, not desirable as more likely to corrode
how do you get rid of the disadvantage of coring
annealing as eliminates core structure
what does rapid cooling of molten alloy cause
prevents atoms diffusing through lattice, causes coring as composition varies throughout grain
what are the conditions for coring
- fast cooling of liquid state
- liquidous and solidus must be separated and determines extent of coring e.g. AuPt
is coring desirable
no
how do we get small grains but avoid coring
homogenising anneal
how does a homogenising anneal work
once solid cored alloy formed, reheat to allow atoms to diffuse and so cause grain composition to become homogenous
n.b. keep below recrystallisation temperature otherwise grains altered
how do alloys improve mechanical properties
Alloys forming a SOLID SOLUTION and consisting of metals of different atomic size have a distorted grain structure
which IMPEDES dislocation movement and so improves mechanical properties (EL, UTS, hardness)
what happens when you have a defect in an otherwise ‘perfect’ metal compared to in an alloy and apply a force to them? and what does this mean in terms of fracture resistance
Metal - It moves away easily (little stress involved) until it settles at the grain boundary.
Alloy - we have atoms of different sizes. So, if we apply a force, it takes more force for the defect to end up at the grain boundary
This is why alloys are more fracture resistant than metals
what is order hardening
Alloys forming an ordered solid solution it will also impede dislocations and improve mechanical properties as it has a distorted grain structure
What is an eutectic alloy
two metals exist in separate grains
properties of eutectic alloys
- metals are soluble in liquid state
- metals are insoluble in solid state (2 phases)
- each metal forms physically distinct grains
- lowest melting point at eutectic composition: used for solder
- hard but brittle
- poor corrosion resistance
what determines the eutectic composition
where liquidus and solidus coincide i.e. where crystalliastion process occurs at a single temp, where grains of individual metals formed simulataneously
what is a non-eutectic composition
- excess metal crystallises first
- then liquid reaches eutectic composition
- and both metals crystallise (forming separate grains)
what is a partially soluble alloy
somewhere between solid solutions and eutectic (complicated phase diagram)
key things about partially soluble alloy’s phase diagram
- solubility limit lines indicates that a range of compositions of Ag and Cu (corresponding to the horizontal section of the solidus (H1 to H2) ) ARE NOT POSSIBLE
- Hence molten alloy of composition Z DOES NOT cool rapidly to produce a 50:50 grain comprising Ag and Cu; instead grains of alpha and beta are formed
why are silver and copper important
Silver and copper are important cause one of the RPD alloys = type 4 gold. Have silver and copper present
what do you benefit from after annealing a partially soluble alloy
precipitation hardening
what is precipitation hardening
One of the metals is pushed to the grain boundary and properties are inhanced
generally, how do alloys compare to metals in terms of mechanical properties
Alloys have better mechanical properties than metals e.g. fracture strength, rigidity, elastic limit, surface hardness due to:
SOLUTION, ORDER & PRECIPITATION HARDENING
BUT
CORED STRUCTURE MUST BE REMOVED BY ANNEALING
