metals and alloys Flashcards

1
Q

what are metals and alloys used for

A
  • partial denture framework = CoCr, type IV gold
  • crowns = stainless steel
  • denture base =stainless steel
  • orthodontic appliance = NiTi
  • restorations = amalgam
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2
Q

why is amalgam different to all the other metals and alloys

A
  • as it undergoes a setting reaction and the rest don’t
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3
Q

what does stainless steel being a denture base show of alloys

A
  • shows alloys are versatile

- able to take on complex shapes

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4
Q

what are the properties of interest of metals and alloys

A
  • strength
  • rigidity
  • elastic limit
  • ductility
  • hardness
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5
Q

what is the downside of alloys

A
  • their aesthetics
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6
Q

what is the definition of a metal

A
  • aggregate of atoms in crystalline structure

- building blocks of alloys

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7
Q

what is the definition of alloys

A
  • combination of metal atoms in a crystalline structure
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8
Q

what is the stress/strain curve

A
  • 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
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9
Q

what is the UTS

A
  • ultimate tensile strength
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10
Q

what is ductility

A
  • amount of plastic deformation prior to fracture

- measure of extent material can be shaped

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11
Q

what is malleability caused by

A
  • if apply compressive stress
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12
Q

what does the crystalline structure depend on

A
  • history = method of production, structure depends on how you produce it
  • shaping = crucial for dental application (cold working, swaging)
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13
Q

what factors can affect mechanical properties

A
  • crystalline structure = lattice of atoms

- grain size and grain imperfections = grain is a single crystal

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14
Q

what is a grain

A
  • a single crystal with atoms orientated in given directions (dendrites)
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15
Q

what are the 3 crystal structures

A
  • 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
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16
Q

how do metals cool

A
  • 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
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17
Q

how do crystals grow

A
  • 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
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18
Q

what is the area called where dendrites meet

A
  • grain boundary
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19
Q

what do the size and shape of grain boundaries depend on

A
  • how it is processed

- can be in all different sizes = sizes can also affect properties

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20
Q

what are the grains called if crystal growth is equal in all dimensions

A
  • equi-axed grains
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21
Q

what are some grain structures other then equi-axed

A
  • radial = molten metal cooled quickly in cylindrical mould

- fibrous = wire pulled through die = cold worked metal/alloy

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22
Q

what is quenching

A
  • fast cooling
  • more nuclei of crystallisation and lots of grains are produced
  • mechanical properties are enhanced = best you can get
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23
Q

what is produced from slow cooling

A
  • few nuclei

- large coarse grains = weaker

24
Q

what do nucleating agents do

A
  • help crystallisation process
  • impurities or additives act as foci for crystal growth
  • using these is another way in controlling grain size
25
what is the grain boundary
- change in orientation of crystal planes | - impurity's concentrate here
26
why are small fine grains advantageous
- high elastic limit and high fracture stress | - increased UTS, hardness
27
what is not so good about small fine grains
- they have decreased ductility = less easily stretched while avoiding fracture
28
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
29
what is dislocation
- a misalignment of atoms in lattice network - there are 2 planes and a slight mismatch between them - a defect
30
how do defects affect crystals/ grains
- make them weaker
31
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
32
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
33
what does impeding dislocations do
- increases = elastic limit, UTS, hardness - decreases = ductility and impact resistance - means metal more likely to break
34
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
35
what is cold working also called
- work/strain hardening
36
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
37
what temperature is cold working done at
- low temperatures - below melting point - below recrystallisation temperature
38
what is swaging
- press sheets together
39
what does cold working cause
- slip - so dislocations collect at grain boundaries - causes a stronger, harder material - modifies grain structure
40
what happens to the metal properties after cold working
- higher = elastic limit, UTS, hardness | - lower = ductility, impact strength, lower corrosion resistance
41
what is good about pushing defects to grain boundaries
- increased elastic limit and fracture stress
42
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
43
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
44
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
45
what is residual stress relieved by
- annealing process
46
what is annealing
- heating metal so that greater thermal vibrations allows migration of atoms = rearrangement of atoms - rearrange so don't have residual stress
47
what else can annealing be called
- homogenisation annealing
48
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
49
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
50
what does recrystallisation do
- causes new smaller, equipment-axed grains - lower EL, UTS and hardness - increased ductility - occurs when metal/alloy is heated
51
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
52
what temperature is recrystallisation at
- depends on amount of cold work | - greater the amount of cold work = the lower the recrystallisation temperature
53
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
54
what is an alloy
- a combination or mixture of 2 or more metals | - OR a metal with a metalloid = such as iron or calcium
55
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