Metals and Alloys 1

Question 1

When are metals and alloys used in dentistry

Answer 1

Partial denture framework - CoCr
Crowns - Stainless steel
Denture bases - stainless steel
Orthodontic appliance - NiTi
Restorations - amalgam

Question 2

What mechanical properties of metals make them superior

Answer 2

Strength, rigidity and surface hardness

Can experience large stresses and abrasive forces without being distorted or fractured

Question 3

What is a metal

Answer 3

Aggregate of atoms in a crystalline structure

Question 4

What is an alloy

Answer 4

A combination of two or more types of metal atoms in a crystalline structure

Question 5

What is ductility

Answer 5

The amount a material will deform when it is subjected to tensile stress

Question 6

What is malleability

Answer 6

The amount a material is deformed when it is subjected to compressive strength

Question 7

What factors affect the mechanical properties of metals and alloys

Answer 7

Crystalline structure
Grain size
Grain imperfections

Question 8

What crystalline structures can metals form

Answer 8

Simple cubic
Face centred cubic
Body centred cubic

Question 9

Describe the cooling curve of a metal

Answer 9

When a molten metal cools, it changes state
As the temp drops to its mp, some of the atoms begin to undergo a change in state, crystallising from liquid to solid
The temperature plateaus as more atoms begin to crystallise
Only when crystallisation is complete will the temperature drop gradually to room temp

Question 10

Describe crystal growth as a metal cools

Answer 10

The first atoms crystallise to form nuclei of crystallisation
Other atoms cool around these nuclei
Crystals grow to form dendrites
Crystals (GRAINS) grow until they impinge on other crystals

Question 11

What is a dendrite

Answer 11

A 3D branched lattice network

Question 12

What are grain boundaries

Answer 12

The regions where grains (crystals) make contact

Question 13

How can crystal growth be influenced

Answer 13

Fast cooling (quenching) - causes more nuclei and small fine grains
Slow cooling - causes few nuclei and large course grains
Adding nucleating agents to act as nuclei of crystallisation

Question 14

What is a grain

Answer 14

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

Question 15

Where are impurities and defects found

Answer 15

At grain boundaries because they can’t jump from one grain to another

Question 16

What are the properties of small fine grains

Answer 16

High elastic limit
Increased fracture strength and hardness
Decreased ductility

Question 17

What is required for rapid cooling (quenching)

Answer 17

Using a small quantity of metal
Heating it just above mp before cooling
Storing the metal in a mould that readily conducts heat

Question 18

What is the consequence of having a defect or dislocation in a grain

Answer 18

If a force is exerted on the lattice, the defect or dislocation is the most likely to be fractured
Applying force causes the defect to propagate along the lattice plane until it reaches the grain boundary
Now there is no defect and no lattice zone with weak bonding between atoms

Question 19

What is the propagation of a defect to the grain boundary known as

Answer 19

Slip

Question 20

How are defects controlled and what does this cause

Answer 20

The grain boundary prevents them from progressing to another grain
Increases EL, FS and hardness
Decreases ductility and impact resistance
Carried out by cold working

Question 21

What is cold working

Answer 21

Work done on a metal/alloy at low temperature that causes slip, creating a stronger, harder material

Question 22

Give examples of cold working

Answer 22

Bending
Rolling
Swaging

Question 23

What is the disadvantage of cold working

Answer 23

Increases the residual stress within a material - stresses that develop within the material itself causing instability in the lattice and resulting in distortion

Question 24

How is residual stress within an metal/alloy relieved

Answer 24

Stress relief annealing

Question 25

What is annealing

Answer 25

Heating a metal or alloy so that greater thermal vibrations alloy the rearrangement of atoms allowing any instability or distortion to be eliminated

Question 26

What are the advantages of annealing

Answer 26

Doesn’t alter grain structure
Doesn’t affect the mechanical properties
Allows further cold work to be carried out

Question 27

What is recrystallisation

Answer 27

Heating a metal or alloy to give a second attempt at using it

Question 28

How does recrystallisation affect metals and alloys

Answer 28

Will change the grain structure and the mechanical properties, spoiling the benefit of any cold work
Allows further cold work to be carried out

Question 29

How does cold work affect recrystallisation temperature

Answer 29

The greater amount of cold working done on the metal/alloy, the lower the recrystallisation temperature value

Question 30

Why must temperature be monitored during annealing

Answer 30

If the temperature is excessive, there will be grain growth with large grains replacing small ones
This results in poorer mechanical properties