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DS1102 > Week 3 - Alloys and Metals > Flashcards

Week 3 - Alloys and Metals Flashcards

1
Q

How do metals bond

A

Via metallic bonding
In metal bonding they form an electron sea where the meta valent electrons are delocalized around a regular array of metal ions.

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2
Q

What are properties of metals

A
  • solids
  • malleable
  • ductile
  • good electrical and thermal conductors
  • moderate melting and high boiling points
  • crystallize in 1 of 3 structures
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3
Q

What is a unit cell

A

smallest, regularly repeating parallelepiped from which a crystal lattice is formed

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4
Q

What are the 3 types of metal structures

A
  • Face centered cubic (FCC)
  • Body Centered Cubic (BCC)
  • Hexagonal Close Packed (HCP)
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5
Q

What is an example of a Face centered cubic FCC metal

A

Au - gold

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6
Q

How many near neighbors do face centered cubic metals have

A

12

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7
Q

What is an example of a body centered cubic (BCC) metal

A

Fe - Iron

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8
Q

How many near neighbors do body centered cubic (BCC) metals have

A

8

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9
Q

What is an example of a hexagonal close packed (HCP) metal

A

Ti -Titanium

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10
Q

How many near neighbors do hexagonal close packed (HCP) metals have

A

12

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11
Q

What is a polycrystalline

A

metal made of crystals of different size (grains), orientation, shape.

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12
Q

How do metals form

A
  1. At the freezing point groups of atoms from small crystals that increase in size by the addition of atoms
  2. many crystals fuse together to form grains
  3. Grains grow until they hit adjacent growing grains. The interface forced between grains is a grain boundary
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13
Q

What is a grain boundary

A

where different grains meet

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14
Q

What are the 2 types of crystal defects

A
  • point defects
  • dislocation
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15
Q

What are point defects

A

vacancies where an atom is missing or atom is substituted with something else (e.g. alloys)

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16
Q

what are dislocations as a crystal defect

A

lines of defective bonding

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17
Q

Why are polycrystalline stronger than pure metals

A

As polycrystals have grain boundaries which stop dislocations (cracks) from propagating
As under stress the dislocations propagate to edge of the grain but it is hard to move across grain boundaries

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18
Q

What is an alloy

A

a material with metallic properties resulting from the melting together of 2 or more elements, where 1 element will be a metal.

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19
Q

What are solution alloys

A

homogenous mixtures, where components are dispersed randomly and uniformly.

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20
Q

What are the 2 types of solution alloys

A

-Substitutional Alloy
- Interstitial Alloy

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21
Q

What is a substitutional alloy

A

Where atoms of the minor component substitutes for atoms in the main structure. The atoms which are being substituted need to be similar size and have similar chemical bonding tendencies.

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22
Q

What is a interstitial Alloy

A

Occurs when one atomy type is much smaller (mainly the main group) and fits into the spaces interstice) of the bulk structure

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23
Q

In a solution alloy what is the solvent

A

The meal that is present in the greater amount

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24
Q

In a solution alloy what is the solute

A

the metal that is present in a lesser amount

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25
Q

What are noble metals

A

metals which are relatively inert (non reactive) and resistant to oxidation e.g. gold, platinum, palladium (Pd), silver

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26
Q

What are the 3 noble metals

A

Au - gold
Pt - platinum
Pd - Palladium
(silver Ag)

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27
Q

What are base metals

A

Metals which are non-noble and are more easily oxidised

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28
Q

What are the different classes of noble dental alloys

A
  • High Noble (HN)
  • Noble (N)
  • Predominantly Base (PB)
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29
Q

What is a highly noble alloy

A

alloys containing at least 60% noble metals, with at least 40% being gold
they are commonly used in crowns, bridges

30
Q

What are noble alloys

A

contains at least 25% noble metals, with no specific requirements for the amount of gold

31
Q

What are base metal alloys

A

contain less than 25% noble metals, typically including metals like nickel, chromium or cobalt

32
Q

What are the main metals in dentistry

A
  • steel
  • titanium
33
Q

What is steel

A
  • Alloy of iron + carbon + other stuff
34
Q

What is the composition of stainless steel

A

steel with 18% Cr, 0.1% C, 8% Ni

35
Q

What is a heterogenous alloy of iron

A

pearlite steel

36
Q

How is steel used in dentistry

A

used to make orthodontic wires ( austenitic stainless steel)

37
Q

What reaction does titanium undergo

A

Ti reacts with oxygen to form a thin oxide TIO2 layer on the surface - which makes it corrosive resistance

38
Q

What are the alloy advantages of titanium

A
  • The most biocompatible metal
  • same strength of steel but it is lighter
  • Easily modified by alloying
  • Corrosion resistant on account of the surface TiO2 film
39
Q

What is the structure of titanium

A

Ti has Hexagonal Closest Pack HCP structural form - with it having traces of oxygen in interstitial sites (space inbetween Ti atoms)

40
Q

What metal is used in dental implants

A

Ti-6Al-4V

41
Q

How does fast cooling affect alloy formation

A

causes small grains, more grain boundaries = less corrosion resistant, but higher stress resistant

42
Q

How does slow cooling affect alloy formation

A

causes fewer grain boundaries = more corrosion resistant (but less stress resistant)

43
Q

What do the boundary lines on a phase diagram indicate

A

where equilibria exists between 2 states

44
Q

What crystal structure is a-form titanium

A

hexagonal close pack arrangement i

45
Q

What crystal structure is b-form titanium

A

body centered cubic arrangement

46
Q

What is the composition of Ti-6Al-4V Alloy

A

89.3 Ti
6.00 Al
4.00 V
it is a combination of both a and b phase titanium
- also an substitutional alloy

47
Q

Why is Al added in Ti-6Al-4V

A
  • Al promotes the a phase
  • decreases density and increases strength
48
Q

Why is V added to Ti-6Al-4V

A

to promote b phase Ti

49
Q

what is the point of having both alpha and beta phase present in Ti-6Al-4V

A

The combination of these two phases allows Ti-6Al-4V to achieve a balance between strength and ductility, which is crucial for applications requiring high performance under varying loads and conditions.

alpha = provides high strength and good creep resistance at elevated temperatures
beta = increased ductility and toughness

50
Q

What is the phase diagram of water

A
51
Q

What are the different types of alloys

A
  • solution alloys (substitution, interstitial)
  • Heterogenous alloy
  • Intermetallic compounds
52
Q

What are heterogenous alloys

A

alloys where the components are not dispersed uniformly
e.g. pearlite steel

53
Q

What is an intermetallic compound

A

An intermetallic alloy is a type of compound composed of two or more metallic elements, in which the elements are in a discrete composition (no range)
e.g. Fe3C - cementite in pearlite steel
e.g. Ag3Sn - y phase dental amalgam

54
Q

what is an example of a solution alloy

A

austenitic stainless steel - for orthodontic wires

55
Q

What type of alloy is austenitic stainless steel

A

Interstitial solid solution alloy

56
Q

How does freeze drying work

A
  • first cool a liquid so that it forms into a solid state
  • decrease pressure so that when increasing the temperature a little bit will cause the substance to instantly turn into a gas
  • (this enables you to remove liquid without high temperatures - decreasing damage to the sample)
57
Q
A

temperature (oC)

58
Q
A

liquidus line

59
Q
A

solidus line

60
Q
A

liquid

61
Q
A

Solid

62
Q

Draw the titanium phase diagram

A
63
Q

Why is the atomic packing factor for body centered cubic BCC metals lower than for face centered cubic FCC metals

A

Fewer nearer neighbors in BCC compared to FCC so the space is more occupied in FCC increasing APF

64
Q

What type of alloy is brass and what is the solute and solvent

A

solution substitutional alloy
solute = zinc
solvent = copper

65
Q

What is the composition of brass

A

67% Cu 33% Zn

66
Q

What is an example of an intermetallic alloy of steel and is this a solution alloy

A

cementite - not a solution alloy, fixed ratio of elements
Fe3C is a carbide

67
Q

What is an intermetallic compound

A

Cementite Fe3C
or
Ag3Sn - y phase dental amalgam

68
Q

What metals are added to iron to provide corrosion resistance in steel alloys

A

Chromium and Nickle
Cr, Ni : 18/8 is the most corrosion resistant alloy used in dentistry as chromium reacts with the oxygen in the air and forms a protective chromium oxide layer
Nickel in corrosion resistance of stainless steels is often quite subtle, as it affects the passive oxide layer and the micro structure by reducing the formation of detrimental phases

69
Q

what is quenching

A

fast cooling = smaller grains which will promote corrosion resistance

70
Q

Why do small grains of a steel alloy increase corrosion

A

Small grain sizes promotes corrosion where there is a concentration cell as the grain boundaries behave as anodes, grains at cathodes

71
Q

What are 3 factors that promote stress corosion in an alloy

A
  1. susceptible material - quality of alloy - defects in structure anodic, grain size anodic boundaries
  2. Corrosive environment - caries active
  3. tensile stress
72
Q

What is the temperature and pressure at the triple point for water phase diagram

A

0.01C, 0.006atm

DS1102 (30 decks)

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