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BDS2 Dental materials science > Metal and alloys 1 and 2 > Flashcards

Metal and alloys 1 and 2 Flashcards

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1
Q

What metals and alloys are widely used in dentistry

A
  • partial denture framework (CoCr, Type 4 gold)
  • crowns (stainless steel)
  • denture base (stainless steel)
  • orthodontic appliance (NiTi)
  • restorations (amalgam-unique)
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2
Q

What properties are we interested in looking at regarding metals and alloys

A
  • strength
  • ductility
  • rigidity
  • hardness
  • elastic limit
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3
Q

What can affect properties of metals

A

defects (dislocations) on properties

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4
Q

What can affect structure

A

processing (e.g. quenching, cold working, annealing)

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5
Q

Disadvantage of metals

A

poor aesthetics

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6
Q

Define metal

A

aggregate of atoms in crystalline structure

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7
Q

Define alloy

A

combination of metal atoms in a crystalline structure

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8
Q

What information can we gain from a stress-strain curve (know what this looks like)

A
  • Fracture strength
  • Elastic limit
  • Ultimate tensile strength
  • Ductility
  • malleability
  • ductility
  • rigidity
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9
Q

What is the elastic limit

A

maximum stress without plastic deformation

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10
Q

What is ductility

A

amount of plastic deformation prior to fracture

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11
Q

What is on the y and x axis of a stress strain curve

A 
x = strain %
y= stress (MPa)
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12
Q

What do mechanical properties depend on

A
  1. Choice of metal

2. crystalline structure

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13
Q

what does crystalline structure depend on

A
  • history (method of production)

- shaping - crucial for dental applications e.g. cold working, swaging

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14
Q

What are the factors which affect mechanical properties

A
  • crystalline structure

- grain size and grain imperfections

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15
Q

What are some crystal or lattice structures

A
  • cubic
  • face-centred cubic
  • body centred cubic
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16
Q

What does the cooling curve of pure metal look like

A
  • gradual decrease in temp
  • plateau
  • gradual decrease in temp
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17
Q

what is the plateu phase of the cooling curve of pure metal

A

melting point

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18
Q

what triggers the end of the plateu phase on the cooling curve of pure metal

A

retains temp until all metal changes from liquid to solid, then it cools down further

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19
Q

How does crystal growth occur

A
  • 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
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20
Q

what is the grain boundary

A

where grains (crystals) make contact

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21
Q

what is the grain structure where crystal growth of equal dimension in each direction

A

equi-axed grains

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22
Q

what is the grain structure where crystal growth is molten metal cooled quickly in cylindrical mould

A

radial

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23
Q

what is the grain structure where wire is pulled through die

A

fibrous

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24
Q

what are the different names for the different grain structures of crystal growth

A
  • equi-axed
  • radial
  • fibrous
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25
Q

What happens to crystal growth when you cool it quickly

A
  • more nuclei

- small fine grains

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26
Q

what happens to crystal growth when you cool it slowly

A
  • few nuclei

- large coarse grains

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27
Q

what size of grains do we want and why

A

we want lots of small fine grains as large grains are weak

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28
Q

what are nucleating agents

A

impurities or additives act as foci for crystal growth

- they help crystallising process

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29
Q

what are grains

A

each grain is a single crystal (lattice) with atoms orientated in given directions (dendrites)

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30
Q

what is a grain boundary

A

change in orientation of the crystal planes (impurities concentrate here)

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31
Q

why are small fine grains advantageous

A
  • high elastic limit

- increased ultimate tensile strength (UTS), hardness

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32
Q

disadvantage to small fine grains?

A

decreased ductility (less easily stretched)

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33
Q

What are the factors for rapid cooling (for small fine grains)

A
  • small bulk
  • heat metal/ alloy just above Tm
  • mould- high thermal conduction
  • quench
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34
Q

What happens when you apply a force to a crystal lattice which has a defect (most do)

A

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

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35
Q

What are dislocations

A

dislocations are imperfections/defects in the crystal lattice

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36
Q

What is slip and what is it due to

A

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)

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37
Q

What would be the effects of impeding the movement of dislocations

A

Increases

  • elastic limit
  • UTS
  • hardness

Decreases

  • ductility
  • impact resistance
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38
Q

What are factors which impede dislocation movement

A
  • grain boundaries
  • alloys (different atom sizes)
  • cold working (dislocations build up at grain boundaries)
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39
Q

How is cold working done and what does it cause

A
  • bending, rolling, swaging
  • done at low temperature
  • causes SLIP (so dislocations collect at grain boundaries)
  • hence, stronger, harder material
40
Q

How does cold working modify the grain structure

A

Higher

  • elastic limit
  • UTS
  • hardness

Lower

  • ductility
  • impact strength
  • lower corrosion resistance (negative)
41
Q

What is the effect of cold working on strength, residual stress and ductility

A

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

42
Q

Why is increased residual stress bad

A
  • causes instability in lattice

- results in distortion over time (undesirable)

43
Q

how is increased residual stress relieved

A

by annealing process

44
Q

what is annealing

A

heating metal (or alloy) so that greater thermal vibrations allows migration of atoms (i.e. re-arrangement of atoms)

45
Q

Why is stress relief annealing better(?) than cold working

A

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

46
Q

when does recrystallisation occur and what does it result in

A
  1. occurs when metal/alloy heated causing
    - new smaller equiaxed grains
    - lower EL, UTS, hardness
    - increased ductility
  2. spoils benefits of cold work
  3. allows further cold work
  4. cold work/ recrystallisation repeated until correct shape obtained
47
Q

What should the recrystallisation temperature be

A
  • depends on amount of cold work

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

48
Q

What would an excessive temperature rise do to grain growth

A

large grains to replace smaller coarse grains yielding poorer mechanical properties –> careful when annealing

49
Q

what are the dental appliance manipulation processes

A
  • cold working
  • stress relief annealing
  • recrystallisation
50
Q

what determines the properties of metals

A
  • grain size
  • whether there are dislocations
  • how you shape it
51
Q

what is an alloy

A

a combination (or mixture) or 2 or more metals, or metal(s) with a metalloid (Fe, C)

52
Q

what is a solid solution

A

Two metals that form a lattice structure as they are soluble in one another (coexist in a common lattice)

53
Q

Advantages to alloys

A

improved:

  • mechanical properties (EL, UTS, hardness)
  • corrosion resistance
  • lower melting point that individual metal
54
Q

dental uses of alloys

A
  • steel - burs, instruments
  • amalgam
  • gold alloy - inlays, crowns, bridges, partial dentures, wires
  • nickel chromium - crowns bridges, wires

etc

55
Q

Define phase

A

physically distinct homogeneous structure (can have more than one component)

56
Q

Define solution

A

homogenous mixture at an atomic scale

57
Q

How many phases would grains composed of metal A only have

A

1 phase

58
Q

How many phases would individual grains composed of metal A and B have

A

2 phases

59
Q

How many phases would grains in a homogenous mixture have composed of metal A and B

A

1 phase (solid solution)

n.b. grains can be of varying size and shape

60
Q

Are metals soluble or insoluble when molten

A

soluble

61
Q

What forms can metal take on crystallisation

A
  1. insoluble (no common lattice, 2 phases)
  2. form intermetallic compound with specific chemical formulation (e.g. Ag3Sn)
  3. be soluble and form a solid solution i.e. form common lattice… 3 types of solid solution
62
Q

What are the 3 types of solid solution

A
  1. substitutional (random and ordered)

2. interstitial

63
Q

what is a substiutional solid solution

A

atoms of one metal replace the other metal in the crystal lattice/ grain

64
Q

what metals form a random substitutional solid solution

A

metal atoms similar in size, valency, crystal structure e.g. AuCu

65
Q

what is an ordered substitutional solid solution

A

metal atoms in regular lattice arrangement

66
Q

what metals form an ordered substitutional solid solution

A

metal atoms similar in size, valency, crystal structure

67
Q

what is an interstitial solid solution

A

smaller atoms located in spaces in lattice/grain structure of larger atom

68
Q

what metals form an interstitial solid solution

A

atoms markedly different in size e.g. Fe-C

69
Q

How does the cooling curve differ between pure metal and an alloy

A

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
70
Q

What does TL and TS stand for

A

TL - liquidous

TS - solidous

71
Q

Know what a phase diagram looks like, what can you plot on it?

A

temp (y axis) vs alloy composition (x axis)

plot TL and TS for both

72
Q

what does a phase diagram tell you

A

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

73
Q

What is the liquidous (TL) line on a phase diagram

A

line representing the temperatures which different alloy compositions begin to crystallise

74
Q

what is the solidus (TS) line on a phase diagram

A

line representing the temperatures which different alloy compositions have completely crystallised

75
Q

what happens when you cool an alloy slowly

A

allows metal atoms to diffuse through lattice

  • positive: ensures grain composition is homogenous
  • negative: results in large grains
76
Q

what happens when you cool an alloy rapidly

A

coring

77
Q

what is coring

A

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

78
Q

how does coring produce positive and negative results

A

positive: small grains
negative: different percentages within the grains that you form, not desirable as more likely to corrode

79
Q

how do you get rid of the disadvantage of coring

A

annealing as eliminates core structure

80
Q

what does rapid cooling of molten alloy cause

A

prevents atoms diffusing through lattice, causes coring as composition varies throughout grain

81
Q

what are the conditions for coring

A
  • fast cooling of liquid state

- liquidous and solidus must be separated and determines extent of coring e.g. AuPt

82
Q

is coring desirable

A

no

83
Q

how do we get small grains but avoid coring

A

homogenising anneal

84
Q

how does a homogenising anneal work

A

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

85
Q

how do alloys improve mechanical properties

A

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)

86
Q

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

A

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

87
Q

what is order hardening

A

Alloys forming an ordered solid solution it will also impede dislocations and improve mechanical properties as it has a distorted grain structure

88
Q

What is an eutectic alloy

A

two metals exist in separate grains

89
Q

properties of eutectic alloys

A
  • 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
90
Q

what determines the eutectic composition

A

where liquidus and solidus coincide i.e. where crystalliastion process occurs at a single temp, where grains of individual metals formed simulataneously

91
Q

what is a non-eutectic composition

A
  • excess metal crystallises first
  • then liquid reaches eutectic composition
  • and both metals crystallise (forming separate grains)
92
Q

what is a partially soluble alloy

A

somewhere between solid solutions and eutectic (complicated phase diagram)

93
Q

key things about partially soluble alloy’s phase diagram

A
  • 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
94
Q

why are silver and copper important

A

Silver and copper are important cause one of the RPD alloys = type 4 gold. Have silver and copper present

95
Q

what do you benefit from after annealing a partially soluble alloy

A

precipitation hardening

96
Q

what is precipitation hardening

A

One of the metals is pushed to the grain boundary and properties are inhanced

97
Q

generally, how do alloys compare to metals in terms of mechanical properties

A

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

BDS2 Dental materials science (10 decks)

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