Metals & Alloys Flashcards
What properties does recrystallisation change?
Lower EL, UTS and hardness
Increased ductility
New smaller equiaxed grains
What is recrystallisation?
Deformed grains are replaced by a new set of undeformed grains that nucleate and grow until the original grains have been entirely consumed.
Occurs when the metal/alloy is heated.
What is a disadvantage of recrystallisation?
Spoils the benefits of cold working
What temperature is required for recrystallisation?
Depends on the amount of cold work
The greater the amount of cold work, the lower the recrystallisation temperature
What can happen if the recrystallisation temperature is too high?
Causes grain growth
Large grains replace smaller grains during the process creating poorer mechanical properties
What is annealing?
Heating the metal/alloy so that greater thermal vibrations allows migration of atoms (i.e. rearrangement of atoms)
What properties are improved in alloys compared to metals?
Improved mechanical properties - EL, UTS, hardness
Corrosion resistance
Lower melting point than the individual metal
What uses are there for alloys in dentistry?
Steel - burs, instruments
Amalgam - restorative material
Gold alloy - inlays, crowns, bridges, partial dentures, wires
Nickel chromium - crowns, bridges, wires
How many phases does a:
a) soluble metal have?
b) insoluble metal have?
a) one phase - solid solution
b) two phases - no common lattice
What are the two different types of solid solution?
Substitutional (random or ordered)
Interstitial
What is a substitutional solid solution?
Atoms of one metal replace the other metal in the crystal lattice/grain
Can be either random (size, valency, crystal structure etc) or ordered (regular lattice arrangement)
What is an interstitial solid solution?
Atoms markedly different in size
Smaller atoms located in spaces in lattice/grain structure of a larger atom
Alloy crystallises over ____________
Metal crystallises at ______________
Temperature range
One temperature
In a phase diagram, what is do the liquidus & solidus lines represent?
Liquidus - line representing the temperatures which different alloy compositions begin to crystallise
Solidus - line representing the temperatures which different alloy compositions have completely crystallised
What are the pros & cons of rapidly cooling a molten alloy?
+ generates many small grains which prevent dislocation movement, improving its mechanical properties
- Causes coring - every layer has a different composition (ratio of material A:material B)
What are the pros and cons of slow cooling a molten alloy?
+ allows metal atoms to diffuse through lattice
+ ensures grain composition is homogenous
- this results in large grains
What are the two types of annealing?
Stress relief annealing (used to combat cons of cold working) Homogenising annealing (used to combat cons of coring)
What is dislocation movement like in a:
a) Metal lattice
b) Solid solution
a) Rolling movement along slip plane, little energy required to reach the grain boundary.
b) Defect falls into the larger space in between the large and small atoms, more energy/force is required for the defect to overcome the different sized atoms, and ‘climb’ out of the space, to move along the lattice to the grain boundary.
What is the significance of the difference in dislocation movement between metals and alloys?
Alloys (solid solutions) require more energy to move defects along the lattice and therefore are more fracture resistant than metals.
What is a metal?
An aggregate of atoms in a crystalline structure
What factors affect the mechanical properties of a metal?
Crystalline structure
Grain size
Grain imperfections
What are the properties of small fine grains?
High elastic limit
Increased UTS & hardness
Overall stronger material
Decreased ductility
Why are metals used in dentistry?
Superior mechanical properties when compared to other materials. However they have poorer aesthetics.
What is cold working/work hardening?
The strengthening of a metal by plastic deformation. Processes include rolling, drawing, pressing, spinning, extruding and heading.
Done on a metal at low temperature (below recrystallisation temp)
Causes slip.
Increases hardness and tensile strength.
Decreases ductility.
What is the grain boundary?
The region where one crystal comes into contact with the adjacent crystal.
Impurities concentrate here.
Crystals are a.k.a. grains.
What is a grain?
A single crystal (lattice) with atoms orientated in given directions (dendrites)
What is slip?
A force is applied creating a defect in the lattice.
Continual application of the force causes the defect to move along the lattice plane.
The lattice has no defects and now is a different shape (plastic deformation).
What is a dislocation?
Imperfection/defect in the crystal lattice
What are the factors preventing dislocation movement?
Grain boundaries - hence fine grains
Alloys - different atom sizes
Cold working - dislocations build up at grain boundaries
What does cold working do?
Modifies the grain structure:
Higher - EL, UTS, hardness, residual stress
Lower - ductility, impact strength, low corrosion resistance
What are the problems with residual stress?
Causes instability in the lattice
Results in distortion over time
Undesirable
*however can be relieved by annealing process
What is annealing?
Heating metal (or alloy) so that greater thermal vibrations allows migration of atoms (i.e. rearrangement of atoms)
What are the features of eutectic alloys?
Where the crystallisation process of an alloy occurs at one temperature.
Lowest melting point.
Hard but brittle.
Poor corrosion resistance.
What are the different types of alloys?
Solid solution forming alloys
Eutectic alloys
Partially soluble alloys
What is precipitation hardening?
Enhancing the strength and hardness of alloys by the formation of extremely small, uniformly dispersed particles of a second phase within the original phase matrix. This is accomplished by appropriate heat treatments.
What effect does copper have when added to gold alloys?
Increased strength and hardness
Decreased ductility
What effect does silver have when added to gold alloys?
Increased hardness and strength
Increased tarnishing and porosity
What effect does adding platinum/palladium have on gold alloys?
Increased melting point (advantage for soldering)
Can only be added to type III/IV gold alloys
What are the uses for the different types of gold alloys?
Type I and II - low load bearing situations such as inlays
Type III - where demands are higher such as full cuspal coverage onlays
Type IV - where rigidity is required for RPD bases or clasps
What is the difference between wrought and cast alloys?
Wrought alloy - worked by being hammered, rolled, drawn or bent into the desired shape when they are solid
Cast alloy - molten alloy is poured into a mould to give it it’s shape, has very little strength
What are examples of wrought alloys?
Stainless steel, cobalt chromium, titanium alloys, gold
What are examples of casting alloys?
Base metal casting alloys - Nickel chromium, cobalt chromium
Titanium
Gold alloys
What are the uses of cast alloys in dentistry?
Cast CoCr is used for RPD connectors because it has a high proof stress & modulus of elasticity and thin castings are strong, rigid and lightweight
May also be used for RPD clasps as it allows you to cast connector and clasps in one
What are the uses for wrought alloys in dentistry?
Wrought gold alloys are used for clasps
