Metals and Alloys 1 Flashcards
When are metals and alloys used in dentistry
Partial denture framework - CoCr Crowns - Stainless steel Denture bases - stainless steel Orthodontic appliance - NiTi Restorations - amalgam
What mechanical properties of metals make them superior
Strength, rigidity and surface hardness
Can experience large stresses and abrasive forces without being distorted or fractured
What is a metal
Aggregate of atoms in a crystalline structure
What is an alloy
A combination of two or more types of metal atoms in a crystalline structure
What is ductility
The amount a material will deform when it is subjected to tensile stress
What is malleability
The amount a material is deformed when it is subjected to compressive strength
What factors affect the mechanical properties of metals and alloys
Crystalline structure
Grain size
Grain imperfections
What crystalline structures can metals form
Simple cubic
Face centred cubic
Body centred cubic
Describe the cooling curve of a metal
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
Describe crystal growth as a metal cools
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
What is a dendrite
A 3D branched lattice network
What are grain boundaries
The regions where grains (crystals) make contact
How can crystal growth be influenced
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
What is a grain
A single crystal with atoms orientated in given directions (dendrites)
Where are impurities and defects found
At grain boundaries because they can’t jump from one grain to another
What are the properties of small fine grains
High elastic limit
Increased fracture strength and hardness
Decreased ductility
What is required for rapid cooling (quenching)
Using a small quantity of metal
Heating it just above mp before cooling
Storing the metal in a mould that readily conducts heat
What is the consequence of having a defect or dislocation in a grain
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
What is the propagation of a defect to the grain boundary known as
Slip
How are defects controlled and what does this cause
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
What is cold working
Work done on a metal/alloy at low temperature that causes slip, creating a stronger, harder material
Give examples of cold working
Bending
Rolling
Swaging
What is the disadvantage of cold working
Increases the residual stress within a material - stresses that develop within the material itself causing instability in the lattice and resulting in distortion
How is residual stress within an metal/alloy relieved
Stress relief annealing
