metals and alloys :(( Flashcards
uses
RPD framework (CoCr, t4 gold) crowns (SS) denture base (SS) ortho (NiTi) Rxs (amalgam)
factors affecting mechanical properties
crystalline structure
grain size
grain dislocations
metal
aggregate of atoms in crystalline structure
alloy
combination of 2 or more metal atoms in a crystalline structure
(or metal(s) with a metalloid)
ductility
amount of plastic deformation prior to fracture
EL
max stress without plastic deformation
ductility is between
El and FS
UTS on graph
at top
crystalline structures
cubic
face-centred cubic
body-centred cubic
cooling curve - metal
molten
plateau
metal
crystal growth
first atoms cooling - nuclei of crystallisation
dendrites
impinge - grain boundary
(can get nucleating agents)
types of crystal growth
equiaxed
radial
fibrous
equiaxed crystal growth
equal
radial crystal growth
cooled quickly in a cylindrical mould
fibrous crystal growth
wire pulled through die
fast cooling - quenching
more nuclei
small fine grains
good mechanical properties
+ high EL, UTS, hardness
- reduced ductility
slow cooling
few nuclei
large coarse grains
grains
each grain is a single crystal (lattice)
grain boundary
change in orientation of the crystal planes
ways to achieve quenching
small bulk
heat metal/alloy just above Tm
mould - high thermal conduction
quench
dislocations
imperfections/defects in the crystal lattice
SLIP
due to the propagation of dislocations, involves rupture of only a few bonds at a time
process by which defects move through grain
impacts of impeding dislocation movement
increased EL, UTS, hardness
reduced ductility and impact resistance
ways of impeding dislocation movement
grain boundaries
alloys - different atom sizes - inherent resistance to movement of dislocations
cold working - dislocations build up at grain boundaries
cold work (strain hardening)
work done to change shape
low temp
causes SLIP
increases EL, UTS, hardness
reduces ductility, impact strength, corrosion resistance
residual stress
instability in lattice
results in distortion over time
relieved by annealing process
annealing
relieve residual stress
heat - thermal vibrations allow migration of atoms
grain structure and mechanical properties unchanged
alloys structure
2 metals form a common lattice structure
solid solution
advantages of alloys
mechanical properties
corrosion resistance
reduced mp
more versatile
phase
physically distinct homogeneous structure
solution
homogeneous mixture at an atomic scale
recrystallisation
re-melt - lose all grain structure - start again
new smaller equiaxed grains
reduced EL, UTS, hardness
increased ductility
cold work and recrystallisation temp
greater the amount of cold work done the lower the recrystallisation temp
grain growth
annealing has the potential for this
excessive temp rise causes large grains to replace smaller ones - poorer mechanical properties
crystallisation
metals normally soluble when molten upon crystallisation: 1 - insoluble - 2 phases 2 - intermetallic compound with specific chemical formulation (Ag3Sn) 3 - soluble - solid solution
substitutional solid solution
atoms of one metal replace the other metal in the crystal lattice/grain
metal atoms similar in size, valency, crystal structure
random
ordered
interstitial solid solution
atoms v different in size
smaller atoms located in spaces in lattice/grain structure of larger atom e.g. Fe-C
alloy cooling curve
crystallises over a temp range
slow cooling of alloys
allows metal atoms to diffuse through lattice
ensures grain composition homogeneous
BUT - large grains
what does rapid cooling of alloys - coring - prevent?
atoms diffusing through lattice - grains of different composition being formed
coring conditions
fast cooling of liquid state
solidus and liquidus must be separated and determines extent of coring
fast cooling of allows - coring - effects
small grains - impede dislocation movement so better mechanical properties
coring - reduced corrosion resistance
dislocation movement metals
metal lattice - defect ‘rolls’ over the atoms in the lattice plane
little energy/force/stress needed for defect to move along slip plane
dislocation movement alloys
defect falls into larger space between large and small atom
more energy/force/stress needed for the defect to overcome the different sized atoms therefore requires greater stress to move dislocations in a solid solution
makes alloys inherently more fracture resistant i.e. stronger than metals
partially soluble alloy
solubility limit lines - only soluble in certain proportions
homogenising anneal
once solid cored alloy formed reheat to allow atoms to diffuse
keep below recrystallisation temp
solution hardening
alloys with metals of different atomic size have a distorted grain structure - impedes dislocation movement and so improves mechanical properties (EL, UTS, hardness)
order hardening
alloys forming an ordered solid solution (atoms distributed at specific lattice sites) have a distorted grain structure - impedes dislocation movement
eutectic alloys
metals soluble in liquid state, insoluble in solid state
where liquidus and solidus coincide - crystallisation process occurs at a single temp
grains of individual metals formed simultaneously
lowest mp
hard but brittle
poor corrosion resistance
non-eutectic composition
excess metal crystallises first
then liquid reaches eutectic composition and both metals crystallise
precipitation hardening
applies to T4 gold
in partially soluble alloys
on annealing, some of the atoms get pushed to the grain boundary
harder for dislocations to move - better mechanical properties
T4 gold composition
Au 65% Cu 14% Ag 14% Zn 2% Pd 3% Pt 2%
T4 gold - effect of Cu
solid solution in all proportions solution hardening order hardening reduced mp v little coring red colour (try to avoid) reduces density base metal - can corrode if too much
T4 gold - effect of Ag
solid solution in all proportions solution hardening precipitation hardening with copper and heat tx can allow tarnishing molten Ag absorbs gas whitens alloy - compensates for Cu
T4 gold alloys - heat tx
quench after casting
homogenising anneal (700 degrees, 10mins)
if cold worked - stress relief anneal
heat harden
heat treated
- properties more suitable for clasp
- need thickness for base (expense)
partial denture alloys ideal properties
rigid (YM) strong (UTS, EL) hard ductile precise casting (shrinkage) melting point low density
types of partial denture alloy
ADA T4 gold
white gold (AgPd)
CoCr
Ti
one piece casting: RPD
base - high YM, high EL
clasp - lower YM, high EL
compromise - thick section - rigid base
thin section - flexible clasp
partial solid solubility example
AgCu (in T4 gold)
platinum in T4 gold
solid solution in all proportions
solution hardening
fine grain structure
coring
palladium in T4 gold
less coring than Pt
coarser grains than Pt
absorbs gases when molten - porous casting
Zn in T4 gold
scavenger
Ni in T4 gold
increases hardness and strength
wrought alloys
Indium
fine grain structure
CoCr uses
wires
implants
RPDs
connectors
CoCr composition
Co 54% Cr 25% Ni 15% Mo 5% C 0.4%
effects of Co
forms solid solution with Cr
increased strength, hardness, rigidity
coring possible
effects of Cr
forms solid solution with Co
increased strength, hardness, rigidity
coring possible
forms passive oxide layer on surface - corrosion resistance
effects of Ni in CoCr
replaces some Co
improves ductility
slight decrease in strength
sensitivity
effects of C in CoCr
undesirable
excess causes carbide at grain boundaries
hard and brittle - reduces ductility
effect of Mo in CoCr
reduces grain size so increases strength
effect of W in CoCr
increases strength
other molecules in CoCr
scavengers
CoCr investment material
high tem 1200-1400 degrees - can’t use standard gypsum therefore silica or phosphate bonded
CoCr melting
electric induction preferred
- oxyacetylene - carbon pickup
CoCr casting
centrifugal force required
avoid overheating - coarse grains
cooling too fast/slow - carbides brittle
CoCr finishing
takes time to finish and polish
but less likely to experience wear
HARD
sandblast, electroplate, abrasive wheel, polishing buff
CoCr ductility
low
work hardens rapidly - amount of cold work you can do ti it is low
need precision casting
adjustment difficult
CoCr rigidity
high rigidity
CoCr density
low
CoCr shrinkage
high
uses of titanium
implants, RPDs, crown and bridge, MF skull plates
titanium properties
good biocompatibility
good corrosion resistance (passive oxide layer)
absorbs gases so need specialised investment and casting
T4 gold properties
ductile high density low rigidity low hardness low shrinkage
