Metals and Alloys 2 Flashcards
What advantages does an alloy have over a metal
Superior mechanical properties - EL stress, FS, rigidity and hardness
Higher corrosion resistance
Lower melting point
What are some common alloy uses in dentistry
Steel - burs and instruments
Gold alloy - inlays, crowns, bridges, partial dentures
CoCr - partial dentures
Amalgam - restorative material
What is a phase
A physically distinct homogenous structure (can have more than one component)
What is a solution
A homogenous mixture at an atomic scale
What is a single phase
A phase with grains consisting of atoms of one metal
Describe 2 phases with 2 metals
2 metals in a lattice - each grain is a separate entity so there are 2 phases
What is a solid solution
2 different metals co-existing in the same grain
What are the different possible outcomes of crystallisation of an alloy
Be insoluble - will exist as 2 phases
Form an intermetallic compound with a specific chemical formula
Be soluble and form a solid solution
How may metal atoms be substituted in a solid solution and what conditions are necessary for this to take place
A random way
An ordered way
Atoms musty be similar in size, valency and crystal structure
What are the different types of solid solution
Substitutional
Interstitial
Describe an interstitial solid solution
Atoms are markedly different in size so smaller atoms will be located in spaced in the grain structure of larger atoms
Describe their cooling curve of a pure metal
Plateau when the metal changes state
Crystallisation of the metal happens without a change in temperature
Only when all the metal has crystallised will its temperature gradually decrease
Describe the cooling curve of an alloy
Crystallisation begins when temp reaches TL
First few metal atoms are cooled sufficiently to form individual nuclei of crystallisation
On further cooling, other metal atoms will have different mp and crystallise when these are reached
As crystallisation takes more time, there is a slight drop in temperature from TL to Ts
What does a phase diagram show
If you change the ratio of each metal in an alloy, it will produce different cooling curves with different TL and Ts values
This gathers data for the cooling combinations of the different metals
Describe the liquidus and solidus in a phase diagram
Above the liquidus, the alloy is in a liquid state
Below the solidus, the alloy is in a solid state
Between the two curves, the allow is partly liquid and partly solid
From this you can determine the TL and Ts values for any alloy ratio
What is the liquidus
The line representing the temperatures which different alloy compositions begin to crystallise
What is the solidus
The line representing the temperatures which different alloy compositions have completely crystallised
What happens if alloys are cooled slowly
Allows metal atoms to diffuse through the lattice and ensures grain composition is homogenous
BUT - this results in large grains which is undesirable
What happens if alloys are cooled rapidly
Prevents atoms diffusing through the lattice causing coring so the composition of each metal will vary throughout the grain creating a concentration gradient
What are the necessary conditions for coring and how does this affect it
Phase diagram for the alloy must have the liquidus and solidus separated
The greater the separation, the more coring and the larger the concentration gradient
What is the disadvantage of coring
May reduce corrosion resistance of the alloy
How is coring of an alloy resolved
Homogenising annealing
What is homogenising annealing
Once a solid cored alloy is formed, it is reheated to allow atoms to diffuse and cause the grain composition to become homogenous
Must keep temperature below recrystallisation temperature
What is solution hardening
Alloys in a solid solution consisting of metals of different atomic size have a distorted grain structure
This impedes dislocation movement and so improves mechanical properties (EL, UTS, hardness)
Describe defect movement in a metal lattice
Defect rolls over the atoms in the lattice plane
Little energy is needed for the defect to move along the slip plane
Describe defect movement in a solid solution and how this enhances the alloy
Defect falls into the larger space existing between the large and small atom
More energy is needed for the defect to overcome the different sized atoms and move along the lattice to the grain boundary
So it requires more stress to move dislocations - making alloys more fracture resistant than metals
What is order hardening
Alloys forming an ordered solid solution with atoms distributed at specific lattice sites have a distorted grain structure which impedes dislocation movement and so improves mechanical properties
What is a eutectic alloy
An alloy in which the metals are soluble in a liquid state but insoluble in a solid state so 2 phases are formed
What is the eutectic composition
The point at which the liquidus and solidus collide and the crystallisation process occurs at a single temperature
What are the disadvantages of eutectic alloys
They are brittle
Poor corrosion resistance
What is the dental application for eutectic alloys
Soldering alloy parts together
Describe the phase diagram for a partially soluble alloy
The liquidus and solidus are separate
There are dashed lines emanating from points H1 and H2 - solubility limit lines
The alloy cannot form grains with a composition between H1 and H2
Instead the alloy forms grains either less than H1% - alpha grains or grains more than H2% - beta grains
Give an example of a partially soluble alloy
Ag-Cu
Describe alpha and beta grains of AgCu
Alpha grains contain both Ag and Cu but are Ag rich
Beta grains contain both Ag and Cu but are Cu rich
What are solubility limit lines
Found in partially soluble alloy phase diagrams
Indicates that a range of compositions of the metals are not possible and instead, alpha and beta grains are formed
What is precipitation hardening
When annealing a partially soluble alloy, its temperature is raised causing one of the metals to be pushed to the grain boundary
This makes the alloy stronger and increases surface hardness
