94. Metals & Alloys Flashcards
Provide examples of metals used in dentistry.
- PD framework (CoCr, gold);
- Denture bases (stainless steel);
- Crowns (stainless steel);
- Orthodontic appliances (NiTi);
- Restorations (amalgam).
Why is amalgam different to all other metals used in dentistry?
There is a distinct difference that amalgam requires a setting reaction during its use in clinic
What are the properties of interest with regards to choosing a metal?
- Strength;
- Rigidity;
- Elastic Limit;
- Ductility;
- Hardness.
What are the superior properties of interest with regards to choosing a metal?
Strength and rigidity
What is the major disadvantage of using metals in dental appliances?
Poor aesthetics
What is the difference between a metal and an alloy with regards to their chemical structure?
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.
What is elasticity with regards to metals?
The maximum stress that can be applied without plastic deformation/ fracturing
What is ductility with regards to metals?
The amount of plastic deformation prior to fracture i.e. a measure of the extent a metal can be shaped/ manipulated
What is malleability with regards to metals?
The amount of compressive stress/ force a metal can withstand
What determines the crystalline structure of a metal?
- Method of production;
- Shaping e.g. cold working or swaging.
What is a grain?
A single atom within the lattice
What is a crystal?
Lattice arrangement of atoms
Describe the cooling curve of a pure metal.
The cooling curve illustrates the change in temperature, over time as a metal cools from molten to a solid.
What does the plateau on a cooling curve indicate?
This shows where all molten metal atoms have changed to a solid. From this point, the temperature of the solid metal gradually decreases.
Describe the crystal growth of a metal.
- 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.
What are equi-axed grain structure and how do you achieve it during crystal formation?
Crystal growth of equal dimension in each direction
What is radial grain structure and how do you achieve it during crystal formation?
Molten metal cooled quickly in a cylindrical mould
What are fibrous grain structure and how do you achieve it during crystal formation?
Molten metal pulled through a narrow aperture leading to a fibrous grain, e.g. wire
What are the major structural differences between quick and slow cooled metals?
- Quick cooling (quenching), results in more nuclei and small fine grains;
- Slow cooling results in fewer nuclei and large, coarse grains.
What are the major mechanical differences between quick and slow cooled metals?
Quick cooled metals have enhanced mechanical properties, which is why manufacturers often add nucleating agents to encourage nucleation of molten metal
What mechanical properties are enhanced with quick cooled metals?
- High elastic limit;
- Increased UTS (tensile strength);
- Increased hardness.
What is a mechanical disadvantage of quenched metals?
Decreased ductility
What are the factors needed for quenching?
- 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).
What is a defect or dislocation in a metallic crystal lattice?
A misalignment of atoms in the network leading to 2 or more planes of crystallography
