rpd alloys Flashcards
4 types of hardening
- Cold working
- Solution hardening (for complete solubility metals)
- Order hardening (to make the solution ordered -> improve mechanical properties)
- Precipitation hardening (for partially soluble metals)
function of hardening
to improve the mechanical properties, move the dislocations to grain boundaries
2 types of annealing
- Stress relief (for internal stresses created by quenching)
- Homogenisation (for coring)
Ideal properties of partial denture alloys
- Rigid
- Strong
- High EL and FS
- Hard
- Ductile
- Precise casting
- Less shrinkage
- Melting point (investment material needs to have a higher melting point than the RPD melting point)
- Density
should a denture base have high or low YM/EL
High YM = rigid
High EL = to maintain shape and avoid plastic deformation
should clasps have high/low EL/YM
low YM = flexible
high EL
Maintains elasticity over wide range of movement
2 main materials used for RPD
- Type 4 gold
- Co-Cr
gold copper solubility?
complete
forms solid solution
benefits from solution and order hardening
effect of copper in type 4
- little coring
- reduced melting point
- red colour from copper
- reduces density
- base metal , adding copper can cause corrosion
gold-silver solubility
complete solubility
forms solid solution
benefits from solution hardening
silver-copper solubility
partial solubility
benefits from precipitation hardening
effects of silver in type 4 gold
- tarnishing
- molten silver absorbs gas CO2
- whitens alloys, cancels out the redness of copper
gold and platinum Pt solubility
complete solubility
solid solution formed
solution hardening
effects of Pt
- fine grain structure
- coring can occur since there is a wide solidus-liquidus gap
Effect of Pd
- less coring than Pt (good)
- coarser grains than Pt (bad)
- absorbs gases when molten -> porous casting
what are the steps to heat treat type 4 gold
- quench causes coring
- homogenizing anneal eliminates coring then
- further cold working but causes residual stress
- stress relief annealing eliminates the stress
- extra heat treatment for order and precipitation hardening
soft vs hard type 4 gold
soft = normal
hard = heat treated ie there was order and precipitation hardening
are cobalt and chromium soluble?
completely soluble
forms solid solution that is hard, strong and rigid
coring possible
what are the benefits of chromium
forms passive layer which is ideal for corrosion resistance
what are the benefits and disadvantages of nickel in co cr?
benefits => improves ductility
disad => nickel sensitivity in some + slight reduction in strength
effects of carbon in co cr
- too much carbon causes carbides to form at grain boundaries making the alloy brittle
Effects of Molybdenum in cocr
reduces grain size to increase strength
How to make Co-Cr denture?
investment material
melt using electric induction
cast
finishing using sandblasting or electroplate
what type of investment material is needed for cocr
silica or phosphate bonded
what happens if you over heat during casting or cool too fasr
overheating causes coarse grains
cooling too fast or slow causes carbides which is brittle
is gold or cocr harder
co cr (BHN 370)
properties of cocr
hard
good wear resistance ie
abrades less easily
low ductility
elongation 4%
difficult to polish
difficult to adjust
why do we need precision casting for cocr??
o work hardens rapidly,
o cannot keep cold working and annealing , making adjustments difficult
gold or cocr is more ductile?
gold
gold or cocr is more dense?
gold
gold or cocr has higher UTS? ie higher FS
gold
gold or cocr is more rigid?
cocr by alot
gold or cocr is more hard?
cocr
gold or cocr more shrinkage?
cocr
cocr is more ___ than gold
rigid
hard
more shrinkage
gold is more __ than cocr
ductile
dense
higher FS ie higher UTS
