94. Metals & Alloys

Question 1

Provide examples of metals used in dentistry.

Answer 1

• PD framework (CoCr, gold);
• Denture bases (stainless steel);
• Crowns (stainless steel);
• Orthodontic appliances (NiTi);
• Restorations (amalgam).

Question 2

Why is amalgam different to all other metals used in dentistry?

Answer 2

There is a distinct difference that amalgam requires a setting reaction during its use in clinic

Question 3

What are the properties of interest with regards to choosing a metal?

Answer 3

• Strength;
• Rigidity;
• Elastic Limit;
• Ductility;
• Hardness.

Question 4

What are the superior properties of interest with regards to choosing a metal?

Answer 4

Strength and rigidity

Question 5

What is the major disadvantage of using metals in dental appliances?

Answer 5

Poor aesthetics

Question 6

What is the difference between a metal and an alloy with regards to their chemical structure?

Answer 6

A metal is an aggregation of a single type of atom in a crystalline structure whereas in an alloy, the crystalline structure combines different metal atoms.

Question 7

What is elasticity with regards to metals?

Answer 7

The maximum stress that can be applied without plastic deformation/ fracturing

Question 8

What is ductility with regards to metals?

Answer 8

The amount of plastic deformation prior to fracture i.e. a measure of the extent a metal can be shaped/ manipulated

Question 9

What is malleability with regards to metals?

Answer 9

The amount of compressive stress/ force a metal can withstand

Question 10

What determines the crystalline structure of a metal?

Answer 10

• Method of production;

- Shaping e.g. cold working or swaging.

Question 11

What is a grain?

Answer 11

A single atom within the lattice

Question 12

What is a crystal?

Answer 12

Lattice arrangement of atoms

Question 13

Describe the cooling curve of a pure metal.

Answer 13

The cooling curve illustrates the change in temperature, over time as a metal cools from molten to a solid.

Question 14

What does the plateau on a cooling curve indicate?

Answer 14

This shows where all molten metal atoms have changed to a solid. From this point, the temperature of the solid metal gradually decreases.

Question 15

Describe the crystal growth of a metal.

Answer 15

• A metal forms dendrites around a site of nuclei of crystallisation;
• Crystals (or grains) grown until the impinge on other crystals;
• This region is known as a grain boundary.

Question 16

What are equi-axed grain structure and how do you achieve it during crystal formation?

Answer 16

Crystal growth of equal dimension in each direction

Question 17

What is radial grain structure and how do you achieve it during crystal formation?

Answer 17

Molten metal cooled quickly in a cylindrical mould

Question 18

What are fibrous grain structure and how do you achieve it during crystal formation?

Answer 18

Molten metal pulled through a narrow aperture leading to a fibrous grain, e.g. wire

Question 19

What are the major structural differences between quick and slow cooled metals?

Answer 19

• Quick cooling (quenching), results in more nuclei and small fine grains;
• Slow cooling results in fewer nuclei and large, coarse grains.

Question 20

What are the major mechanical differences between quick and slow cooled metals?

Answer 20

Quick cooled metals have enhanced mechanical properties, which is why manufacturers often add nucleating agents to encourage nucleation of molten metal

Question 21

What mechanical properties are enhanced with quick cooled metals?

Answer 21

• High elastic limit;
• Increased UTS (tensile strength);
• Increased hardness.

Question 22

What is a mechanical disadvantage of quenched metals?

Answer 22

Decreased ductility

Question 23

What are the factors needed for quenching?

Answer 23

• Small bulk;
• Heat metal/alloy just above Tm;
• Mould has to have high thermal conduction e.g. metal bucket;
• Quickly quenched (bucket into a bucket of cold water).

Question 24

What is a defect or dislocation in a metallic crystal lattice?

Answer 24

A misalignment of atoms in the network leading to 2 or more planes of crystallography

Question 25

What happens if a force is applied to the defect?

Answer 25

• Atoms move along the dislocation which results in a different shape of metal;
• E.g. ‘step change’ where the metal no longer has a defect but is now a different shape.

Question 26

What is metal slip?

Answer 26

• Propagation of Dislocations;

- Like a domino effect through the atoms when a force is applied to the defect/ dislocation.

Question 27

How do the presence of defects/ dislocations effect the mechanical properties of metals?

Answer 27

• Increase in elasticity;
• Increase in UTS;
• Increase in hardness;
• Decrease in ductility;
• Decrease in impact resistance.

Question 28

What factors impede dislocation movement?

Answer 28

• Grain boundaries (the more boundaries, the less a defect can move to other grains);
• Alloys (different atom sizes);
• Cold working (dislocations build up at grain boundaries).

Question 29

How does cold working effect the distribution of dislocations in a lattice?

Answer 29

This leads to slip so that dislocations collect at grain boundaries

Question 30

How does the effects of cold working on dislocations effect the metal’s mechanical properties?

Answer 30

• Dislocations at grain boundaries;
• Stronger, harder (at surface) metal;
• Higher elasticity;
• Higher UTS;
• Lower ductility;
• Lower impact strength;
• Lower corrosion resistance.

Question 31

What is residual stress?

Answer 31

Stresses that remain in a solid material after the original cause of the stresses has been removed

Question 32

What is the relationship between residual stress and cold working?

Answer 32

The more cold work done to a metal, the more residual stress there is - undesirable

Question 33

Why is residual stress undesirable and how is this overcome in manufacturing of metals?

Answer 33

Residual stress causes instability in the lattice which can lead to distortion over time. This is overcome by annealing.

Question 34

What is the relationship between recrystallisation temperature and cold working?

Answer 34

The greater the amount of cold work done to the metal, the lower the recrystallisation temperature

Question 35

Why should excessive temperature rises be avoided?

Answer 35

Large grains replace smaller, coarse grains leading to poorer mechanical properties