Metals And Alloys 2 Flashcards
What are advantages of alloys over metals?
Improved properties:
- Mechanical
- EL, FS, hardness
- corrosion resistance
- lower melting pointe
List some dental uses of alloys
- Steel - burs, instruments
- Gold alloy - inlays, crowns, bridges, PDs, wires
- Cobalt Chromium - partial dentures
- Amalgam - restorative
What is meant by the term ‘phase’ and ‘solution’ in relation to alloys?
- Phase - physically distinct homogenous structure (can have more than one component
- Solution - homogenous mixture at an atomic scale
What are grains composed of one metal called?
One phase
Describe a two phase structure
- Individual grains of metal A and B situated in lattice network
Are grains composed two different metals in a homogenous mixture considered as one phase?
Yes
What events can occur to two metals when crystallisation occurs?
On crystallisation, two metals may:
- Be insoluble, ie no common lattice - exist as two phases
- Form an Inter metallic compound with a specific chemical formulation (eg silver and tin)
- Be soluble and form a solid solution ie form a common lattice
What are the 3 categories of solid solution?
- Substitutional
- Interstitial
3.
Describe a substitutional solid solution
- Atoms of one metal replace the other metal in the crystal lattice/grain
May be:
A) random: metal atoms similar in
- size valency, crystal structure (eg gold and copper, silver and copper)
B) ordered: metal atoms in regular lattice arrangement, conditions as above
Describe an interstitial solid solution
- Atoms markedly different in size
2. Smaller atoms located in spaces in lattice/grain structure of larger atom eg (iron and copper)
What event occurs at the plateau of a cooling curve graph of a metal?
Crystallisation
Why is there no plateau on an alloy cooling curve?
Two metals have two different melting points so will crystallise at different rates as temperature decreases
Summarise the difference between cooling curves of alloys and metals
- Metal - crystallises at one temperature
2. Alloys - crystallises over temperature range
Describe what occurs when two metals are soluble vs insoluble
- Soluble - solid solution formed (homogenous mixture of metals in each grain)
- Insoluble - grains of individual metals formed
What does the liquidus line represent on a phase diagram?
Line representing the temperatures at which different alloy compositions begin to crystallise
What does the solidus line represent on a phase diagram?
Line representing the temperatures which at different alloy compositions have completely crystallised
How does slow cooling effect a molten alloy?
- Allows metal atoms to diffuse through lattice
- Ensures grain composition is homogenous
- this results in LARGE grains
How does rapid cooling effect molten alloys?
- Prevents atoms diffusing through the lattice
- Causes coring
- as composition varies throughout grain - Generates many small grains which impede dislocation movement, improving its mechanical properties
What are Conditions required for coring?
- Fast cooling of liquid state
2. Liquidus and solidus must be separated and this determines the extent of coring e.g gold and platinum
What is a negative consequence of coring?
- May reduce corrosion resistance of the solid form of alloy
What process helps a rapidly cooled alloy resist corrosion?
Homogenising anneal
Describe the process of homogenising anneal
• once solid core alloy Is formed, reheat to allow atoms to diffuse and so cause grain composition to become homogenous
- make sure to keep BELOW recrystallisation temperature, otherwise grains are altered
Describe what causes a distorted grain structure and the advantages of this
- Alloys forming a solid solution and consisting of metals of different atomic size have a distorted grain structure
- This impedes dislocation movement and so improves mechanical properties (EL, UTS, hardness)
Where does the defect in a distorted grain structure of a metal end up when force is applied and why?
- Defect “rolls” over the atoms in the lattice plane
- Little energy/force is needed for defect to move along slip plane
- Defect settles at the grain boundary
Why are alloys inherently more fracture resistant (stronger) than metals?
- More energy/force is needed for the defect to overcome the different sized atoms and move along lattice to grain boundary
- Therefore greater stress is required to move dislocations in a solid solution
Describe order hardening in metal alloys
- Alloys form an ordered solid solution ( atoms distributed specific lattice sites) have a distorted grain structure
- which impedes dislocation movement and so improves mechanical properties (EL, FS, hardness)
Describe the properties of a eutectic alloy
- Metals are soluble I’m a liquid state
- Metals are insoluble in a solid state
- ie each metal forms physically distinct grains
What is the eutectic composition on a graph?
Where liquidus and solidus coincide
- ie where crystallisation process occurs at a single temperature
- grains of individual metals formed simultaneously
What are eutectic alloys used for?
- Soldering
Describe properties of a eutectic alloy
Hard but brittle
Poor corrosion resistance
What is a typical partially soluble alloy?
Silver copper
What becomes impossible for an alloy between the solid solubility limit lines represented by h1 and h2?
The alloy cannot form grains between h1 and h2%
Results in alpha and beta grains
What does the solubility limit line represent?
Indicates that a range of compositions of Ag and Cu are not possible
In partially soluble alloys:
What happens to a supersaturated alloy on annealing?
Precipitation hardening
Describe how precipitation hardening improves the copper silver alloy
When annealing silver copper (raising the temperature by a few hundred degrees Celsius), either copper or silver atoms will be pushed to the grain boundary making the alloy stronger and harder
Why do alloys have better mechanical properties than metals?
This is due to:
Solution, order and precipitation hardening
What must be done to an alloy to reduce the risk of corrosion?
Cored structure must be removed by annealing
