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BDS2 - dental material science > metals and alloys > Flashcards

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
Q

what is the grain boundary

A
  • change in orientation of crystal planes

- impurity’s concentrate here

26
Q

why are small fine grains advantageous

A
  • high elastic limit and high fracture stress

- increased UTS, hardness

27
Q

what is not so good about small fine grains

A
  • they have decreased ductility = less easily stretched while avoiding fracture
28
Q

what are factors for rapid cooling (quenching)

A
  • 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
Q

what is dislocation

A
  • a misalignment of atoms in lattice network
  • there are 2 planes and a slight mismatch between them
  • a defect
30
Q

how do defects affect crystals/ grains

A
  • make them weaker
31
Q

what happens if you apply a force to the defect

A
  • 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
Q

what is the name of the way the defect is moved

A
  • ‘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
Q

what does impeding dislocations do

A
  • increases = elastic limit, UTS, hardness
  • decreases = ductility and impact resistance
  • means metal more likely to break
34
Q

what factors can impeded dislocation movement

A
  • 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
Q

what is cold working also called

A
  • work/strain hardening
36
Q

what happens in cold working

A
  • push all defects to boundaries = take metal and hammer it
  • improves mechanical properties
  • work is done on metal/alloy = swaging, rolling, bending
37
Q

what temperature is cold working done at

A
  • low temperatures
  • below melting point
  • below recrystallisation temperature
38
Q

what is swaging

A
  • press sheets together
39
Q

what does cold working cause

A
  • slip
  • so dislocations collect at grain boundaries
  • causes a stronger, harder material
  • modifies grain structure
40
Q

what happens to the metal properties after cold working

A
  • higher = elastic limit, UTS, hardness

- lower = ductility, impact strength, lower corrosion resistance

41
Q

what is good about pushing defects to grain boundaries

A
  • increased elastic limit and fracture stress
42
Q

what is not good about pushing defects to grain boundaries

A
  • lower corrosion resistance which is not good for something that is going in patient’s mouth
43
Q

what are the effects of cold working

A
  • 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
Q

what is residual stress

A
  • causes instability in lattice
  • results in distortion over time = undesirable
  • a reconfiguring of position of metal atoms leading to distortion later on
45
Q

what is residual stress relieved by

A
  • annealing process
46
Q

what is annealing

A
  • heating metal so that greater thermal vibrations allows migration of atoms = rearrangement of atoms
  • rearrange so don’t have residual stress
47
Q

what else can annealing be called

A
  • homogenisation annealing
48
Q

what does stress relief annealing do

A
  • 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
Q

why must annealing be done in a controlled and specific way

A
  • so the grain structure is unaffected

- need to push atoms around and rearrange but don’t change the grain structure

50
Q

what does recrystallisation do

A
  • causes new smaller, equipment-axed grains
  • lower EL, UTS and hardness
  • increased ductility
  • occurs when metal/alloy is heated
51
Q

when do you recrystallised

A
  • 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
Q

what temperature is recrystallisation at

A
  • depends on amount of cold work

- greater the amount of cold work = the lower the recrystallisation temperature

53
Q

what does excessive temperature rise cause to grains

A
  • causes large grains to replace smaller coarse grains yielding poorer mechanical properties
  • need to be careful to avoid during annealing
54
Q

what is an alloy

A
  • a combination or mixture of 2 or more metals

- OR a metal with a metalloid = such as iron or calcium

55
Q

what is a solid solution

A
  • 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

BDS2 - dental material science (13 decks)

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