metals and alloys Flashcards
what are metals and alloys used for
- partial denture framework = CoCr, type IV gold
- crowns = stainless steel
- denture base =stainless steel
- orthodontic appliance = NiTi
- restorations = amalgam
why is amalgam different to all the other metals and alloys
- as it undergoes a setting reaction and the rest don’t
what does stainless steel being a denture base show of alloys
- shows alloys are versatile
- able to take on complex shapes
what are the properties of interest of metals and alloys
- strength
- rigidity
- elastic limit
- ductility
- hardness
what is the downside of alloys
- their aesthetics
what is the definition of a metal
- aggregate of atoms in crystalline structure
- building blocks of alloys
what is the definition of alloys
- combination of metal atoms in a crystalline structure
what is the stress/strain curve
- shows that when you increases stress you increase strain = change shape
- shows the fracture stress, elastic limit and UTS of the material
- also shows the malleability and ductility of material
what is the UTS
- ultimate tensile strength
what is ductility
- amount of plastic deformation prior to fracture
- measure of extent material can be shaped
what is malleability caused by
- if apply compressive stress
what does the crystalline structure depend on
- history = method of production, structure depends on how you produce it
- shaping = crucial for dental application (cold working, swaging)
what factors can affect mechanical properties
- crystalline structure = lattice of atoms
- grain size and grain imperfections = grain is a single crystal
what is a grain
- a single crystal with atoms orientated in given directions (dendrites)
what are the 3 crystal structures
- cubic = simple cubic and relate structures
- face-centered cubic = cubic close packed and related structure
- body-centred cubic = body centred cubic and relate structures
- metals have atoms positioned at regularly arranged sites
how do metals cool
- alloys start molten in a container and are then allowed to cool down
- reaches a plateau where it goes from liquid to solid state and maintains temperature until all molten metal is now solid then it cools down again
how do crystals grow
- atoms at these sites act as nuclei of crystallisation
- crystals then grow to form dendrites = atoms cool around the nuclei
- crystals (or grains) grow until they impinge on other crystals and then stop growing
- dendrites grow until they impact on another
what is the area called where dendrites meet
- grain boundary
what do the size and shape of grain boundaries depend on
- how it is processed
- can be in all different sizes = sizes can also affect properties
what are the grains called if crystal growth is equal in all dimensions
- equi-axed grains
what are some grain structures other then equi-axed
- radial = molten metal cooled quickly in cylindrical mould
- fibrous = wire pulled through die = cold worked metal/alloy
what is quenching
- fast cooling
- more nuclei of crystallisation and lots of grains are produced
- mechanical properties are enhanced = best you can get
what is produced from slow cooling
- few nuclei
- large coarse grains = weaker
what do nucleating agents do
- help crystallisation process
- impurities or additives act as foci for crystal growth
- using these is another way in controlling grain size
what is the grain boundary
- change in orientation of crystal planes
- impurity’s concentrate here
why are small fine grains advantageous
- high elastic limit and high fracture stress
- increased UTS, hardness
what is not so good about small fine grains
- they have decreased ductility = less easily stretched while avoiding fracture
what are factors for rapid cooling (quenching)
- small bulk = take small amount and cool rapidly
- heat metal/alloy just above melting point
- mould = high thermal conduction
- quench = take molten metal bucket and put in container of cold water
what is dislocation
- a misalignment of atoms in lattice network
- there are 2 planes and a slight mismatch between them
- a defect
how do defects affect crystals/ grains
- make them weaker
what happens if you apply a force to the defect
- it will move along the singular plane
- slide along lattice plane
- defect moves along till grain looks a different shape
- push defect to grain boundary and defect is gone by changing shape of lattice structures
- change shape and characteristics of metal
what is the name of the way the defect is moved
- ‘slip’
- slip is due to propagation of dislocations and involves rupture of only a few bonds at a time
- only need to break one bond at a time over period of time to allow defect to ripple through structure
what does impeding dislocations do
- increases = elastic limit, UTS, hardness
- decreases = ductility and impact resistance
- means metal more likely to break
what factors can impeded dislocation movement
- have lots of grain boundaries = stops defect moving to another grain once reach boundary
- can use alloy instead of metal = different atom sizes
- cold working = defects build up at grain boundaries
what is cold working also called
- work/strain hardening
what happens in cold working
- push all defects to boundaries = take metal and hammer it
- improves mechanical properties
- work is done on metal/alloy = swaging, rolling, bending
what temperature is cold working done at
- low temperatures
- below melting point
- below recrystallisation temperature
what is swaging
- press sheets together
what does cold working cause
- slip
- so dislocations collect at grain boundaries
- causes a stronger, harder material
- modifies grain structure
what happens to the metal properties after cold working
- higher = elastic limit, UTS, hardness
- lower = ductility, impact strength, lower corrosion resistance
what is good about pushing defects to grain boundaries
- increased elastic limit and fracture stress
what is not good about pushing defects to grain boundaries
- lower corrosion resistance which is not good for something that is going in patient’s mouth
what are the effects of cold working
- increases strength but reduces ductility and increases residual stress which is not good
- more cold working = more defect to boundaries = stronger
- increases residual stress = increases instability of lattice
what is residual stress
- causes instability in lattice
- results in distortion over time = undesirable
- a reconfiguring of position of metal atoms leading to distortion later on
what is residual stress relieved by
- annealing process
what is annealing
- heating metal so that greater thermal vibrations allows migration of atoms = rearrangement of atoms
- rearrange so don’t have residual stress
what else can annealing be called
- homogenisation annealing
what does stress relief annealing do
- eliminates stresses by allowing atoms to re-arrange within grains
- grain structure and mechanical properties unchanged
- can do further cold work after for final shaping
why must annealing be done in a controlled and specific way
- so the grain structure is unaffected
- need to push atoms around and rearrange but don’t change the grain structure
what does recrystallisation do
- causes new smaller, equipment-axed grains
- lower EL, UTS and hardness
- increased ductility
- occurs when metal/alloy is heated
when do you recrystallised
- when cold working and annealing haven’t worked
- take metal and recycle = start process again
- spoils work of cold working and allows for more
- cold work and recrystallisation continues until you get correct shape
what temperature is recrystallisation at
- depends on amount of cold work
- greater the amount of cold work = the lower the recrystallisation temperature
what does excessive temperature rise cause to grains
- causes large grains to replace smaller coarse grains yielding poorer mechanical properties
- need to be careful to avoid during annealing
what is an alloy
- a combination or mixture of 2 or more metals
- OR a metal with a metalloid = such as iron or calcium
what is a solid solution
- two metals that form a common lattice structure and so are soluble in each other
- because both exist in one structure then they are soluble within each other = they co-exist in a common lattice structure
- means a lattice structure with 2 or more types of metal atom present
