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BDS2: Dental Material Science > Metals And Alloys 2 > Flashcards

Metals And Alloys 2 Flashcards

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1
Q

What are advantages of alloys over metals?

A

Improved properties:

  1. Mechanical
    - EL, FS, hardness
    - corrosion resistance
    - lower melting pointe
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2
Q

List some dental uses of alloys

A
  1. Steel - burs, instruments
  2. Gold alloy - inlays, crowns, bridges, PDs, wires
  3. Cobalt Chromium - partial dentures
  4. Amalgam - restorative
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3
Q

What is meant by the term ‘phase’ and ‘solution’ in relation to alloys?

A
  1. Phase - physically distinct homogenous structure (can have more than one component
  2. Solution - homogenous mixture at an atomic scale
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4
Q

What are grains composed of one metal called?

A

One phase

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5
Q

Describe a two phase structure

A
  1. Individual grains of metal A and B situated in lattice network
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6
Q

Are grains composed two different metals in a homogenous mixture considered as one phase?

A

Yes

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7
Q

What events can occur to two metals when crystallisation occurs?

A

On crystallisation, two metals may:

  1. Be insoluble, ie no common lattice - exist as two phases
  2. Form an Inter metallic compound with a specific chemical formulation (eg silver and tin)
  3. Be soluble and form a solid solution ie form a common lattice
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8
Q

What are the 3 categories of solid solution?

A
  1. Substitutional
  2. Interstitial
    3.
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9
Q

Describe a substitutional solid solution

A
  1. Atoms of one metal replace the other metal in the crystal lattice/grain

May be:

A) random: metal atoms similar in
- size valency, crystal structure (eg gold and copper, silver and copper)

B) ordered: metal atoms in regular lattice arrangement, conditions as above

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10
Q

Describe an interstitial solid solution

A
  1. Atoms markedly different in size

2. Smaller atoms located in spaces in lattice/grain structure of larger atom eg (iron and copper)

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11
Q

What event occurs at the plateau of a cooling curve graph of a metal?

A

Crystallisation

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12
Q

Why is there no plateau on an alloy cooling curve?

A

Two metals have two different melting points so will crystallise at different rates as temperature decreases

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13
Q

Summarise the difference between cooling curves of alloys and metals

A
  1. Metal - crystallises at one temperature

2. Alloys - crystallises over temperature range

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14
Q

Describe what occurs when two metals are soluble vs insoluble

A
  1. Soluble - solid solution formed (homogenous mixture of metals in each grain)
  2. Insoluble - grains of individual metals formed
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15
Q

What does the liquidus line represent on a phase diagram?

A

Line representing the temperatures at which different alloy compositions begin to crystallise

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16
Q

What does the solidus line represent on a phase diagram?

A

Line representing the temperatures which at different alloy compositions have completely crystallised

17
Q

How does slow cooling effect a molten alloy?

A
  1. Allows metal atoms to diffuse through lattice
  2. Ensures grain composition is homogenous
  • this results in LARGE grains
18
Q

How does rapid cooling effect molten alloys?

A
  1. Prevents atoms diffusing through the lattice
  2. Causes coring
    - as composition varies throughout grain
  3. Generates many small grains which impede dislocation movement, improving its mechanical properties
19
Q

What are Conditions required for coring?

A
  1. Fast cooling of liquid state

2. Liquidus and solidus must be separated and this determines the extent of coring e.g gold and platinum

20
Q

What is a negative consequence of coring?

A
  1. May reduce corrosion resistance of the solid form of alloy
21
Q

What process helps a rapidly cooled alloy resist corrosion?

A

Homogenising anneal

22
Q

Describe the process of homogenising anneal

A

• once solid core alloy Is formed, reheat to allow atoms to diffuse and so cause grain composition to become homogenous

  • make sure to keep BELOW recrystallisation temperature, otherwise grains are altered
23
Q

Describe what causes a distorted grain structure and the advantages of this

A
  1. Alloys forming a solid solution and consisting of metals of different atomic size have a distorted grain structure
  2. This impedes dislocation movement and so improves mechanical properties (EL, UTS, hardness)
24
Q

Where does the defect in a distorted grain structure of a metal end up when force is applied and why?

A
  1. Defect “rolls” over the atoms in the lattice plane
  2. Little energy/force is needed for defect to move along slip plane
  3. Defect settles at the grain boundary
25
Q

Why are alloys inherently more fracture resistant (stronger) than metals?

A
  1. More energy/force is needed for the defect to overcome the different sized atoms and move along lattice to grain boundary
  2. Therefore greater stress is required to move dislocations in a solid solution
26
Q

Describe order hardening in metal alloys

A
  1. Alloys form an ordered solid solution ( atoms distributed specific lattice sites) have a distorted grain structure
    - which impedes dislocation movement and so improves mechanical properties (EL, FS, hardness)
27
Q

Describe the properties of a eutectic alloy

A
  1. Metals are soluble I’m a liquid state
  2. Metals are insoluble in a solid state
  3. ie each metal forms physically distinct grains
28
Q

What is the eutectic composition on a graph?

A

Where liquidus and solidus coincide
- ie where crystallisation process occurs at a single temperature

  • grains of individual metals formed simultaneously
29
Q

What are eutectic alloys used for?

A
  1. Soldering
30
Q

Describe properties of a eutectic alloy

A

Hard but brittle

Poor corrosion resistance

31
Q

What is a typical partially soluble alloy?

A

Silver copper

32
Q

What becomes impossible for an alloy between the solid solubility limit lines represented by h1 and h2?

A

The alloy cannot form grains between h1 and h2%

Results in alpha and beta grains

33
Q

What does the solubility limit line represent?

A

Indicates that a range of compositions of Ag and Cu are not possible

34
Q

In partially soluble alloys:

What happens to a supersaturated alloy on annealing?

A

Precipitation hardening

35
Q

Describe how precipitation hardening improves the copper silver alloy

A

When annealing silver copper (raising the temperature by a few hundred degrees Celsius), either copper or silver atoms will be pushed to the grain boundary making the alloy stronger and harder

36
Q

Why do alloys have better mechanical properties than metals?

A

This is due to:

Solution, order and precipitation hardening

37
Q

What must be done to an alloy to reduce the risk of corrosion?

A

Cored structure must be removed by annealing

BDS2: Dental Material Science (8 decks)

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