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BDS2: Dental Material Science > Metals & Alloys > Flashcards

Metals & Alloys Flashcards

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0
Q

What properties does recrystallisation change?

A

Lower EL, UTS and hardness
Increased ductility
New smaller equiaxed grains

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1
Q

What is recrystallisation?

A

Deformed grains are replaced by a new set of undeformed grains that nucleate and grow until the original grains have been entirely consumed.
Occurs when the metal/alloy is heated.

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2
Q

What is a disadvantage of recrystallisation?

A

Spoils the benefits of cold working

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3
Q

What temperature is required for recrystallisation?

A

Depends on the amount of cold work

The greater the amount of cold work, the lower the recrystallisation temperature

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4
Q

What can happen if the recrystallisation temperature is too high?

A

Causes grain growth

Large grains replace smaller grains during the process creating poorer mechanical properties

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5
Q

What is annealing?

A

Heating the metal/alloy so that greater thermal vibrations allows migration of atoms (i.e. rearrangement of atoms)

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6
Q

What properties are improved in alloys compared to metals?

A

Improved mechanical properties - EL, UTS, hardness
Corrosion resistance
Lower melting point than the individual metal

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7
Q

What uses are there for alloys in dentistry?

A

Steel - burs, instruments
Amalgam - restorative material
Gold alloy - inlays, crowns, bridges, partial dentures, wires
Nickel chromium - crowns, bridges, wires

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8
Q

How many phases does a:

a) soluble metal have?
b) insoluble metal have?

A

a) one phase - solid solution

b) two phases - no common lattice

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9
Q

What are the two different types of solid solution?

A

Substitutional (random or ordered)

Interstitial

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10
Q

What is a substitutional solid solution?

A

Atoms of one metal replace the other metal in the crystal lattice/grain
Can be either random (size, valency, crystal structure etc) or ordered (regular lattice arrangement)

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11
Q

What is an interstitial solid solution?

A

Atoms markedly different in size

Smaller atoms located in spaces in lattice/grain structure of a larger atom

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12
Q

Alloy crystallises over ____________

Metal crystallises at ______________

A

Temperature range

One temperature

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13
Q

In a phase diagram, what is do the liquidus & solidus lines represent?

A

Liquidus - line representing the temperatures which different alloy compositions begin to crystallise
Solidus - line representing the temperatures which different alloy compositions have completely crystallised

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14
Q

What are the pros & cons of rapidly cooling a molten alloy?

A

+ generates many small grains which prevent dislocation movement, improving its mechanical properties
- Causes coring - every layer has a different composition (ratio of material A:material B)

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15
Q

What are the pros and cons of slow cooling a molten alloy?

A

+ allows metal atoms to diffuse through lattice
+ ensures grain composition is homogenous
- this results in large grains

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16
Q

What are the two types of annealing?

A 
Stress relief annealing (used to combat cons of cold working)
Homogenising annealing (used to combat cons of coring)
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17
Q

What is dislocation movement like in a:

a) Metal lattice
b) Solid solution

A

a) Rolling movement along slip plane, little energy required to reach the grain boundary.
b) Defect falls into the larger space in between the large and small atoms, more energy/force is required for the defect to overcome the different sized atoms, and ‘climb’ out of the space, to move along the lattice to the grain boundary.

18
Q

What is the significance of the difference in dislocation movement between metals and alloys?

A

Alloys (solid solutions) require more energy to move defects along the lattice and therefore are more fracture resistant than metals.

20
Q

What is a metal?

A

An aggregate of atoms in a crystalline structure

21
Q

What factors affect the mechanical properties of a metal?

A

Crystalline structure
Grain size
Grain imperfections

22
Q

What are the properties of small fine grains?

A

High elastic limit
Increased UTS & hardness
Overall stronger material
Decreased ductility

23
Q

Why are metals used in dentistry?

A

Superior mechanical properties when compared to other materials. However they have poorer aesthetics.

24
Q

What is cold working/work hardening?

A

The strengthening of a metal by plastic deformation. Processes include rolling, drawing, pressing, spinning, extruding and heading.
Done on a metal at low temperature (below recrystallisation temp)
Causes slip.
Increases hardness and tensile strength.
Decreases ductility.

25
Q

What is the grain boundary?

A

The region where one crystal comes into contact with the adjacent crystal.
Impurities concentrate here.
Crystals are a.k.a. grains.

26
Q

What is a grain?

A

A single crystal (lattice) with atoms orientated in given directions (dendrites)

27
Q

What is slip?

A

A force is applied creating a defect in the lattice.
Continual application of the force causes the defect to move along the lattice plane.
The lattice has no defects and now is a different shape (plastic deformation).

28
Q

What is a dislocation?

A

Imperfection/defect in the crystal lattice

29
Q

What are the factors preventing dislocation movement?

A

Grain boundaries - hence fine grains
Alloys - different atom sizes
Cold working - dislocations build up at grain boundaries

30
Q

What does cold working do?

A

Modifies the grain structure:
Higher - EL, UTS, hardness, residual stress
Lower - ductility, impact strength, low corrosion resistance

31
Q

What are the problems with residual stress?

A

Causes instability in the lattice
Results in distortion over time
Undesirable
*however can be relieved by annealing process

32
Q

What is annealing?

A

Heating metal (or alloy) so that greater thermal vibrations allows migration of atoms (i.e. rearrangement of atoms)

33
Q

What are the features of eutectic alloys?

A

Where the crystallisation process of an alloy occurs at one temperature.
Lowest melting point.
Hard but brittle.
Poor corrosion resistance.

34
Q

What are the different types of alloys?

A

Solid solution forming alloys
Eutectic alloys
Partially soluble alloys

35
Q

What is precipitation hardening?

A

Enhancing the strength and hardness of alloys by the formation of extremely small, uniformly dispersed particles of a second phase within the original phase matrix. This is accomplished by appropriate heat treatments.

36
Q

What effect does copper have when added to gold alloys?

A

Increased strength and hardness

Decreased ductility

37
Q

What effect does silver have when added to gold alloys?

A

Increased hardness and strength

Increased tarnishing and porosity

38
Q

What effect does adding platinum/palladium have on gold alloys?

A

Increased melting point (advantage for soldering)

Can only be added to type III/IV gold alloys

39
Q

What are the uses for the different types of gold alloys?

A

Type I and II - low load bearing situations such as inlays
Type III - where demands are higher such as full cuspal coverage onlays
Type IV - where rigidity is required for RPD bases or clasps

40
Q

What is the difference between wrought and cast alloys?

A

Wrought alloy - worked by being hammered, rolled, drawn or bent into the desired shape when they are solid
Cast alloy - molten alloy is poured into a mould to give it it’s shape, has very little strength

41
Q

What are examples of wrought alloys?

A

Stainless steel, cobalt chromium, titanium alloys, gold

42
Q

What are examples of casting alloys?

A

Base metal casting alloys - Nickel chromium, cobalt chromium
Titanium
Gold alloys

43
Q

What are the uses of cast alloys in dentistry?

A

Cast CoCr is used for RPD connectors because it has a high proof stress & modulus of elasticity and thin castings are strong, rigid and lightweight
May also be used for RPD clasps as it allows you to cast connector and clasps in one

44
Q

What are the uses for wrought alloys in dentistry?

A

Wrought gold alloys are used for clasps

BDS2: Dental Material Science (9 decks)

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