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Dental Materials > Metals and Alloys 2 > Flashcards

Metals and Alloys 2 Flashcards

1
Q

What advantages does an alloy have over a metal

A

Superior mechanical properties - EL stress, FS, rigidity and hardness
Higher corrosion resistance
Lower melting point

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2
Q

What are some common alloy uses in dentistry

A

Steel - burs and instruments
Gold alloy - inlays, crowns, bridges, partial dentures
CoCr - partial dentures
Amalgam - restorative material

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3
Q

What is a phase

A

A physically distinct homogenous structure (can have more than one component)

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4
Q

What is a solution

A

A homogenous mixture at an atomic scale

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5
Q

What is a single phase

A

A phase with grains consisting of atoms of one metal

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6
Q

Describe 2 phases with 2 metals

A

2 metals in a lattice - each grain is a separate entity so there are 2 phases

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7
Q

What is a solid solution

A

2 different metals co-existing in the same grain

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8
Q

What are the different possible outcomes of crystallisation of an alloy

A

Be insoluble - will exist as 2 phases
Form an intermetallic compound with a specific chemical formula
Be soluble and form a solid solution

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9
Q

How may metal atoms be substituted in a solid solution and what conditions are necessary for this to take place

A

A random way
An ordered way
Atoms musty be similar in size, valency and crystal structure

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10
Q

What are the different types of solid solution

A

Substitutional

Interstitial

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11
Q

Describe an interstitial solid solution

A

Atoms are markedly different in size so smaller atoms will be located in spaced in the grain structure of larger atoms

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12
Q

Describe their cooling curve of a pure metal

A

Plateau when the metal changes state
Crystallisation of the metal happens without a change in temperature
Only when all the metal has crystallised will its temperature gradually decrease

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13
Q

Describe the cooling curve of an alloy

A

Crystallisation begins when temp reaches TL
First few metal atoms are cooled sufficiently to form individual nuclei of crystallisation
On further cooling, other metal atoms will have different mp and crystallise when these are reached
As crystallisation takes more time, there is a slight drop in temperature from TL to Ts

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14
Q

What does a phase diagram show

A

If you change the ratio of each metal in an alloy, it will produce different cooling curves with different TL and Ts values
This gathers data for the cooling combinations of the different metals

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15
Q

Describe the liquidus and solidus in a phase diagram

A

Above the liquidus, the alloy is in a liquid state
Below the solidus, the alloy is in a solid state
Between the two curves, the allow is partly liquid and partly solid
From this you can determine the TL and Ts values for any alloy ratio

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16
Q

What is the liquidus

A

The line representing the temperatures which different alloy compositions begin to crystallise

17
Q

What is the solidus

A

The line representing the temperatures which different alloy compositions have completely crystallised

18
Q

What happens if alloys are cooled slowly

A

Allows metal atoms to diffuse through the lattice and ensures grain composition is homogenous
BUT - this results in large grains which is undesirable

19
Q

What happens if alloys are cooled rapidly

A

Prevents atoms diffusing through the lattice causing coring so the composition of each metal will vary throughout the grain creating a concentration gradient

20
Q

What are the necessary conditions for coring and how does this affect it

A

Phase diagram for the alloy must have the liquidus and solidus separated
The greater the separation, the more coring and the larger the concentration gradient

21
Q

What is the disadvantage of coring

A

May reduce corrosion resistance of the alloy

22
Q

How is coring of an alloy resolved

A

Homogenising annealing

23
Q

What is homogenising annealing

A

Once a solid cored alloy is formed, it is reheated to allow atoms to diffuse and cause the grain composition to become homogenous
Must keep temperature below recrystallisation temperature

24
Q

What is solution hardening

A

Alloys in a solid solution consisting of metals of different atomic size have a distorted grain structure
This impedes dislocation movement and so improves mechanical properties (EL, UTS, hardness)

25
Q

Describe defect movement in a metal lattice

A

Defect rolls over the atoms in the lattice plane

Little energy is needed for the defect to move along the slip plane

26
Q

Describe defect movement in a solid solution and how this enhances the alloy

A

Defect falls into the larger space existing between the large and small atom
More energy is needed for the defect to overcome the different sized atoms and move along the lattice to the grain boundary
So it requires more stress to move dislocations - making alloys more fracture resistant than metals

27
Q

What is order hardening

A

Alloys forming an ordered solid solution with atoms distributed at specific lattice sites have a distorted grain structure which impedes dislocation movement and so improves mechanical properties

28
Q

What is a eutectic alloy

A

An alloy in which the metals are soluble in a liquid state but insoluble in a solid state so 2 phases are formed

29
Q

What is the eutectic composition

A

The point at which the liquidus and solidus collide and the crystallisation process occurs at a single temperature

30
Q

What are the disadvantages of eutectic alloys

A

They are brittle

Poor corrosion resistance

31
Q

What is the dental application for eutectic alloys

A

Soldering alloy parts together

32
Q

Describe the phase diagram for a partially soluble alloy

A

The liquidus and solidus are separate
There are dashed lines emanating from points H1 and H2 - solubility limit lines
The alloy cannot form grains with a composition between H1 and H2
Instead the alloy forms grains either less than H1% - alpha grains or grains more than H2% - beta grains

33
Q

Give an example of a partially soluble alloy

A

Ag-Cu

34
Q

Describe alpha and beta grains of AgCu

A

Alpha grains contain both Ag and Cu but are Ag rich

Beta grains contain both Ag and Cu but are Cu rich

35
Q

What are solubility limit lines

A

Found in partially soluble alloy phase diagrams

Indicates that a range of compositions of the metals are not possible and instead, alpha and beta grains are formed

36
Q

What is precipitation hardening

A

When annealing a partially soluble alloy, its temperature is raised causing one of the metals to be pushed to the grain boundary
This makes the alloy stronger and increases surface hardness

Dental Materials (19 decks)

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