Metals & Alloys Flashcards
Why are metals used in denstistry
Superior mechanical properties eg - strength
-rigidity
-hardness
-elastic limit
-ductility
What is a metal
aggregation of atoms in crystalline structure
What is an alloy
combination of metal atoms in a crystalline structure
What are some simple crystal or lattice structures
Simple cubic
Face-centred cubic
Body centric cubic
What is the cooling curve of pure metal
Decreases gradually then stops for a while and then decreases once more
What is a grain boundary
The region where grains make contact
What are the types of grain structures
Equi-axed
-Crystal growth is of equal dimension in each direction
Radical
-molten metal cooled quickly in cylindrical mould
Fibrous
-wire pulled through die
What impact does cooling have on crystal growth
Quenching
-more nuclei
-small fine grains
Slow cooling
-few nuclei
-large coarse grains
Where do impurities concentrate
At the grain boundary
What are the advantages of small fine grains
High elastic limit
increased FS and UTS, hardness
decreased ductillity
What qualifies a metal for quenching
small bulk
heat metal/alloy just above Tm
mould - high thermal conduction
quench
What are dislocations
imperfections/defects in the crystal lattice
What is SLIP
SLIP is due to Propagation of Dislocations and involves rupture of only a few bonds at a time
What does stopping movement of dislocations do
INCREASES
elastic limit
fracture stress /UTS
hardness
DECREASES
ductility
impact resistance
What natural factors reduce dislocation movement
Grain boundaries (hence fine grains)
Alloys: different atom sizes
Cold working - dislocations stopped at grain boundaries
What are the effects of cold working
Causes SLIP
increase strength and residual stress
decreases ductillity
How is residual stress relieved
Annealing process
What are the effects of residual stress
Causes instability in lattice
Results in distortion over time
Undesirable
What is annealing
heating metal (or alloy) so that greater thermal vibrations allows migration of atoms (ie re-arrangement of atoms)
What does recrystallisation cause and when does it occur
occurs when metal/alloy heated causing :-
- new smaller equiaxed grains
- lower EL, UTS, hardness
- increased ductility
Why would recrytallisation be used
spoils benefits of cold work
allows further cold work
cold work/recrystallisation repeated until correct shape obtained
What is an alloy
a combination (or mixture) of:
two or more metals
OR
a metal(s) with a metalloid (Si, C)
What is phase
physically distinct homogeneous structure
Which are preferable metals or alloys
Alloys
What are the advantages to alloys
Harder
More resistant to corrosion
Lower melting point than metal
What is a solution
Homogeneous mixture at an atomic scale
What can happen to an alloy of two metals on crystallisation
Forms one of three solid solutions
a) be insoluble, ie no common lattice – exist as 2 phases
OR
b) form an intermetallic compound with a specific chemical formulation (eg Ag3Sn)
OR
c) be SOLUBLE and form a SOLID SOLUTION, ie form a common lattice…
NOTE: there are 3 types of solid solution.
Types of solid solutions
Substitutional
-ordered
-random
Interstitial
-bigger and smaller atoms together
What is happening in the flat section of a cooling curve of a pure metal
Crystallisation
How do the cooling trends of a metal and alloy differ
Metal crystallises at one temp (flat section)
Alloy crystallises over temp range
What is the liquidus line
line representing the temperatures which different alloy compositions begin to crystallise
What does a solidus line show
line representing the temperatures which different alloy compositions have completely crystallised
What does slow cooling of a molten alloy do
Allows metal atoms to diffuse through lattice
ENSURES grain composition is homogeneous
BUT this results in LARGE GRAINS
What does rapid cooling of a molten alloy do
Causes coring
prevents atoms diffusing through lattice as composition varies throughout grain.
What conditions are required for coring to occur
fast cooling of liquid state;
LIQUIDUS and SOLIDUS must be separated (see Phase Diagram) and determines extent of coring
What is the disadvantage of coring
Can reduce the corrosion reistance of the solid form alloy
What type of cooling is preferred
Fast cooling - as it generates many small grains which impede dislocation movement and improve mechanical properties
HOWEVEr causes coring which can be resolved by homogenising anneal
How is coring resolved
Homogenising anneal
How is homogenising anneal carried out
once solid cored alloy formed
REHEAT to allow atoms to diffuse and so cause grain composition to become homogeneous
If the temp used to homogenise anneal is above the recrystallisation temp what happens
Grains altered
How does a distorted grain structure improve mechanical properties
Impedes dislocation movement
What substances would have a distorted grain structure
Alloys forming an ordered solid solution (atoms at specific lattice sites)
Alloys forming a solid solution and consisting of metals of different atomic size
What is the dislocation movement within a metal lattice
Defect “rolls” over the atoms in the lattice plane.
Little energy/force is needed for defect to move along slip plane.
What is the dislocation movement for a solid solution (different sized grains)
Defect does not “roll” over the lattice plane.
Instead it falls into the spaces between the large & small atom.
More energy/force is needed for the defect to overcome the different-sized atoms, and move along lattice to the grain boundary.
Hence, it requires greater stress to move any dislocations present in a solid solution
What are eutectic alloys
Liquidus and solidus coincide (crystallisation process occurs at a single temp)
metals are soluble in liquid state
metals INSOLUBLE in solid state (so 2 PHASES)
ie each metal forms physically distinct grains
What does binary eutectic indicate
Complete solid insolubility
What are the mechanical properties of eutectic alloys
lowest melting point - at eutectic composition: used for solder
hard but brittle
poor corrosion resistance
What is the summary of alloys
HAVE BETTER MECHANICAL PROPERTIES THAN METALS
(eg FRACTURE STRENGTH, RIGIDITY, ELASTIC LIMIT, SURFACE HARDNESS) DUE TO :- SOLUTION, ORDER & PRECIPITATION HARDENING
BUT
CORED ALLOY STRUCTURE FORMED FOLLOWING QUENCHING MUST BE REMOVED BY AN ANNEALING PROCESS
(OTHERWISE ALLOY IS MORE LIKELY TO CORRODE)
Are H1 to H2 compositions possible in partially solluble alloys
No - this is the solubility limit line the alloy formed here forms grains of both alpha or beta NOT both in a 50/50 ratio
When does precipitation hardening occur
In partially soluble alloys:
on annealing, a supersaturated alloy will undergo PRECIPITATION HARDENING
