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BDS2 - DMS > Metals and Alloys 2 > Flashcards

Metals and Alloys 2 Flashcards

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1
Q

What is an alloy?

A

A combination (or mixture) of two or more metals OR metals with a mettalloid

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2
Q

What is a metalloid?

A

Something that a metal can be replaced with e.g. Si, C

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3
Q

What are 3 advantages of alloys over metals?

A

They have improved properties:

  • Mechanical (EL, UTS, hardness)
  • Corrosion resistance

AND

  • Lower melting point than an individual metal
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4
Q

What is STEEL used for in dentistry? (2 points)

A

Burs, instruments

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5
Q

What is AMALGAM used for in dentistry?

A

Filling material

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6
Q

What is GOLD ALLOY used for in dentistry? (5 points)

A
  • Inlays
  • Crowns
  • Bridges
  • Partial dentures
  • Wires
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7
Q

What is NICKEL CHROMIUM used for in dentistry? ( 3 points)

A
  • Crowns
  • Bridges
  • Wires
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8
Q

What is the definition for ‘phase’?

A
  • Physically distinct homogeneous structure (can have more than one component)
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9
Q

What is the definition for a solution?

A
  • Homogeneous mixture at an atomic scale (lattice arrangement with 2 or more metals that coexist in the same lattice structure)
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10
Q

How many phases are there for grains composed of one metal only?

A

1 phase

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11
Q

How many phases are these for grains composed of individual grains of metal A and B situated in a lattice network?

A
  • Two phases as the 2 metals don’t coexist in the same grain
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12
Q

How many phases are there for grains composed of Metal A and B in a homogeneous mixture?

A
  • One phase (homogeneous as the 2 metals are coexisting in the same lattice structure)
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13
Q

What is a solid solution?

A

2 or more types of atoms or molecules that share a crystal lattice

  • The grains are of varying size and shape
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14
Q

Are metals usually soluble or insoluble when molten?

A

Usually soluble

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15
Q

On crystallisation of alloys, what are the 3 possible options that can occur?

A
  1. Metals are insoluble with each other so there is no common lattice - 2 phases
  2. The metals form an intermetallic compound with specific chemical formulation
  3. The metals are soluble in each other and form a solid solution i.e. form a common lattice (there are 3 types of solid solution)
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16
Q

What is a ‘substitutional’ solid solution?

A
  • Atoms of one metal replace the other metal in the crystal lattice/grain
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17
Q

What is a ‘random’ substitutional solid solution?

A
  • Metals are similar in: size, valency, crystal structure

- They are randomly positioned

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18
Q

What is an ‘ordered’ substitutional solid solution?

A
  • Metal atoms in regular lattice arrangement

- Forms an ordered solid solution where you can predict the position of the different types of metals

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19
Q

What is an ‘interstitial’ solid solution?

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A
  • Atoms markedly different in size

- Smaller atoms are located in spaces in lattice/grain structure of larger atoms

20
Q

What does the plateau on the cooling curve if a pure metal indicate?

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A
  • Where the crystallisation begins and the metal is becoming a solid (happens at one temperature)
21
Q

What is the cooling curve like for an alloy?

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A
  • Temp TL is the temp at which crystallisation of the alloy begins
  • Crystallisation continues and the temp drops
  • TS is when the crystallisation process is complete
  • This is different from the cooling curve of a pure metal as the process of crystallisation doesn’t happen over a single temp
22
Q

For an alloy, are the cooling curves the same or different for alloys with different compositions of metals?

23
Q

What is the lower line of a phase diagram called?

24
Q

What is the liquidous on a phase diagram?

Study These Flashcards
A

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

25
What is the solidus in a phase diagram?
The line representing the temperatures which different alloy compositions have completely crystallised  
26
What does slow cooling of molten alloys allow? (3 point s)
- Allows metal atoms to diffuse through the lattice - ENSURES grain composition is homogenous BUT - This results in large grains (which is not desirable as it causes poor mechanical properties)
27
What is the tie line on a phase diagram?
Horizontal line from liquidous to solidus - This is roughly when the first grain is going to form which is going to be a composition of 85% A and 15% B
28
What does rapid cooling of alloys produce?
Coring 
29
Rapid cooling of a molten alloy causes CORING. What is this?
The composition of metals varies throughout the grain 
30
What are the conditions required for coring? (2 points)
- Fast cooling of liquid state | - Liquidous and solidus must be separated and determines extent of the coring
31
Is a phase diagram has a liquidous and a solidus that are quite far apart will you get a lot of or little coring?
A lot of coring 
32
What may coring reduce of the solid form of an alloy?
May reduce the corrosion resistance
33
What does fast cooling of alloys cause? (2 points)
- Generates many small grains which impede dislocation movement, improving its MECHANICAL PROPERTIES - BUT causes coring, which is undesirable
34
What is homogenising annealing of alloys?
- Once the solid, cored alloy is formed: - REHEAT to allow atoms to diffuse and so cause grain composition to become homogenous - NOTE: keep below recrystallisation temperature, otherwise grains are altered
35
What is solution hardening? (2 points)
- 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
36
What is the dislocation movement like in a metal lattice and how much energy is needed for this? (2 points)
- Defects 'roll' over the atoms in the lattice plane | - Little energy/force is needed for defect to move along the slip plane
37
What is the dislocation movement like for a solid solution and how much energy is required for this? (3 points)
- Defect does not 'roll' over the lattice plane. Instead it falls into the larger space existing between the large and small atom - More energy/force is needed for the defect to overcome the different sized atoms, and move along lattice to the grain boundary - Hence, it requires greater stress to move dislocations in a solid solution - making alloys inherently more fracture resistant (i.e. stronger) than metals
38
What is order hardening?
- Alloys forming an ORDERED SOLID SOLUTION (atoms distributed at specific lattice sites) have a DISTORTED grain structure - This IMPEDES dislocation movement and so improves mechanical properties
39
What is an Eutectic alloy?
2 metals that are not soluble in one another 
40
What are the properties of Eutectic alloys? (4 points)
- Metals are soluble in liquid state - Metals INSOLUBLE in solid state (2 phases) - i.e. each metal forms physically distinct grains - CONTRAST this with Au-Pt alloy
41
What is the melting point of a eutectic alloy like?
- Les than the MP  of metal A&B that are combined to make the alloy 
42
What is eutectic composition?
- Where liquidous and solidus coincide (i.e. where crystallisation process occurs at a single temperature) - Where grains of individual metals are formed simultaneously
43
what are 3 properties of eutectic alloys?
- Lowest melting point - Hard but brittle - Poor corrosion resistance
44
What happens with alloys that are non-eutectic composition ? (3 points)
- Excess metal crystallises first - Then liquid reaches eutectic composition - And BOTH metals crystallise (forming separate grains)
45
What does the solubility limit line in a partially soluble alloy phase diagram indicate?
- Indicates that a range of compositions of Ag and Cu (corresponding to the horizontal section of the solidus) ARE NOT POSSIBLE - Hence molten alloy composition z: - DOES NOT cool rapidly to produce a 50:50 grain comprising Ag and Cu - Instead grains of alpha and beta are formed
46
In partially soluble alloys on annealing, a supersaturated alloy will undergo what?
- Precipitation hardening 
47
Alloys have better mechanical properties than metals, give 3 examples of these and say what it is due to?
- Fracture strength, rigidity, elastic limit surface hardness - This is due to solution, order and precipitation hardening BUT - CORED structure must be removed by annealing

BDS2 - DMS (14 decks)

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