Metals and Alloys Flashcards
What are metals and alloys used for in dentistry
RPD framework crowns denture base orthodontic appliance restorations
What metal/alloy is used in partial denture framework
cobalt chrome
type IV gold
What metal/alloy is used in crowns
stainless steel
What metal/alloy is used in denture base
stainless steel
What metal/alloy is used in orthodontic appliance
NiTi
What metal/alloy is used in restorations
amalgam
Why do we use metals
They have superior mechanical properties
What are the superior mechanical properties of metals
strength rigidity elastic limit ductility hardness
What are the disadvantages of metals
poor aesthetics
What is the definition of a metal
aggregate of atoms in a crystalline structure
What is an alloy
combination of metal atoms in a crystalline structure
What are the building blocks of alloys
metals
What is ductility
Ductility is a measure of a metal’s ability to withstand tensile stress—any force that pulls the two ends of an object away from each other
What is malleability
Malleability is a substance’s ability to deform under compressive stress
What is the elastic limit
maximum stress without plastic deformation
What is the UTS
ultimate tensile strength
What is ductility defined as
amount of PLASTIC DEFORMATION prior to fracture
What do mechanical properties depend on
choice of metal
crystalline structure
What does the crystalline structure depend on
history - method of production
shaping - crucial for dental applications
What are the factors that affect mechanical properties
crystalline structure
grain size
grain imperfections
What is a grain
a single crystal
What is a crystal
a lattice arrangement
What are different crystal/lattice structures
cubic
face centre cubic
body centre cubic
In the cooling curve of a pure metal what is the straight line
plateau phase
What does the plateau phase mean in a pure metal
the melting point
changes from liquid to solid
Describe the process crystal growth
- atoms act as nuclei of crystallization
- crystals grow to form dendrites
- crystals (or GRAINS) grow until they impinge on other crystals
What is the nuclei of crystallization
the centre of around which other atoms will crystallite
What are dendrites
3D branched lattice network
What is the region where grains make contact called
the GRAIN BOUNDARY
What parameters can effect the size and shape of grains
how you process it
which container you use to cool it
If you change the size and shape what else do you effect
the properties
What are EQUI-AXED grains
crystal growth of equal direction
What is a radial grain structure
molten metal cooled quickly in cylindrical mould
What is a fibrous grain structure
wire pulled through die (cold worked metal/alloy)
What are the different types of cooling
Fast cooling (quenching) slow cooling
How does fast cooling effect the crystal growth
more nuclei
small fine grains
MECHANICAL PROPERTIES ENHANCED
How does slow cooling effect the crystal growth
few nuclei
large coarse grains
WEAK
What are nucleating agents
impurities or additives act as foci for crystal growth
helps the crystallization process and improves the product
What is each grain
a single crystal (lattice) with atoms orientation in given directions (dendrites)
What is the grain boundary a result of
change in orientation of the crystal planes - impurities concentrate here
why are small fine grains advantageous
high elastic limit
increased UTS, hardness
What are the disadvantages of small fine grains
have a decreased ductility
What are the factors for rapid cooling
small bulk required
heat metal/alloy just above its melting point
pull the heat away from the metal quickly and quench
What does a defect in the crystalline structure represent
a misalignment of atoms in that lattice network
What is dislocation
imperfections/defects in crystal lattice
What happens to a defect when force is applied
it moves which in the end results in a lattice that has no defect but is a different shape
What is slip due to
propagation of dislocations and involves rupture of only a few bonds at a time
this is crucial as it is a cascade effect and doesn’t require supreme large forces to do it
If you impede the movement of dislocations what does this increase
elastic limit
UTS
hardness
If you impede the movement of dislocations what does this decrease
impact resistance
What are the factors that impede dislocation movement
grain boundaries (hence fine grains)
alloys: different atom sizes
cold working
In cold working where do the dislocations build up at
grain boundary
How do grain boundaries impede dislocation movement
they stop the defect moving to another grain
How is cold working achieved
bending, rolling, swaging
push the effect within the metal alloy to the grain boundary and improve the mechanical properties of that object
What temperature is cold working done
at a low one (below recrystallization temperature)
What does cold working cause
slip - hence stronger harder material
What properties are higher after cold work
elastic limit
UTS
hardness
What properties are lower after cold working
ductility
impact strength
low corrosion resistance
What is the effect of cold working on residual stress
increases it
What does residual stress cause
instability in lattice resulting in distortion over time
What is residual stress relieved by
annealing process
What is annealing
heating the metal/alloy so that greater thermal vibrations allow migrations of atoms
What does stress relief annealing do
ELIMINATE stresses by allowing atoms to REARRANGE within grains
How are grain structure and mechanical properties changed in annealing
they are not
When does recrustallisation happen
when meta alloy is heated causing new smaller equiaxed grains
What is the effect of recrystallisaton on the properties
lower EL, UTS, hardness
increased ductility
What does recrystallization spoil
spoils benefit of cold work
allows further cold work
cold work/recrystallisation repeated until correct shape obtained
What does the recrystallization depend on
amount of cold work
greater the cold work, lower the recrystallization temperature
What does excessive temperature rise cause
large grains to replace smaller coarse grains yielding poor mechanical properties so have to be careful when annealing
What is the definition of an alloy
two metals that form a COMMON LATTICE structure, are SOLUBLE in one another and form a SOLID SOLUTION
What does metal alloys forming a solid solution mean
it means two metals co-existing in a common lattice structure
NOTHING TO DO WITH TEMP
