rpd alloys

Question 1

4 types of hardening

Answer 1

• Cold working
• Solution hardening (for complete solubility metals)
• Order hardening (to make the solution ordered -> improve mechanical properties)
• Precipitation hardening (for partially soluble metals)

Question 2

function of hardening

Answer 2

to improve the mechanical properties, move the dislocations to grain boundaries

Question 3

2 types of annealing

Answer 3

• Stress relief (for internal stresses created by quenching)
• Homogenisation (for coring)

Question 4

Ideal properties of partial denture alloys

Answer 4

• 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

Question 5

should a denture base have high or low YM/EL

Answer 5

High YM = rigid
High EL = to maintain shape and avoid plastic deformation

Question 6

should clasps have high/low EL/YM

Answer 6

low YM = flexible
high EL

Maintains elasticity over wide range of movement

Question 7

2 main materials used for RPD

Answer 7

• Type 4 gold
• Co-Cr

Question 8

gold copper solubility?

Answer 8

complete

forms solid solution

benefits from solution and order hardening

Question 9

effect of copper in type 4

Answer 9

• little coring
• reduced melting point
• red colour from copper
• reduces density
• base metal , adding copper can cause corrosion

Question 10

gold-silver solubility

Answer 10

complete solubility

forms solid solution

benefits from solution hardening

Question 11

silver-copper solubility

Answer 11

partial solubility

benefits from precipitation hardening

Question 12

effects of silver in type 4 gold

Answer 12

• tarnishing
• molten silver absorbs gas CO2
• whitens alloys, cancels out the redness of copper

Question 13

gold and platinum Pt solubility

Answer 13

complete solubility
solid solution formed
solution hardening

Question 14

effects of Pt

Answer 14

• fine grain structure
• coring can occur since there is a wide solidus-liquidus gap

Question 15

Effect of Pd

Answer 15

• less coring than Pt (good)
• coarser grains than Pt (bad)
• absorbs gases when molten -> porous casting

Question 16

what are the steps to heat treat type 4 gold

Answer 16

1. quench causes coring
2. homogenizing anneal eliminates coring then
3. further cold working but causes residual stress
4. stress relief annealing eliminates the stress
5. extra heat treatment for order and precipitation hardening

Question 17

soft vs hard type 4 gold

Answer 17

soft = normal
hard = heat treated ie there was order and precipitation hardening

Question 18

are cobalt and chromium soluble?

Answer 18

completely soluble

forms solid solution that is hard, strong and rigid

coring possible

Question 19

what are the benefits of chromium

Answer 19

forms passive layer which is ideal for corrosion resistance

Question 20

what are the benefits and disadvantages of nickel in co cr?

Answer 20

benefits => improves ductility

disad => nickel sensitivity in some + slight reduction in strength

Question 21

effects of carbon in co cr

Answer 21

• too much carbon causes carbides to form at grain boundaries making the alloy brittle

Question 22

Effects of Molybdenum in cocr

Answer 22

reduces grain size to increase strength

Question 23

How to make Co-Cr denture?

Answer 23

investment material

melt using electric induction

cast

finishing using sandblasting or electroplate

Question 24

what type of investment material is needed for cocr

Answer 24

silica or phosphate bonded

Question 25

what happens if you over heat during casting or cool too fasr

Answer 25

 overheating causes coarse grains

 cooling too fast or slow causes carbides which is brittle

Question 26

is gold or cocr harder

Answer 26

co cr (BHN 370)

Question 27

properties of cocr

Answer 27

hard
good wear resistance ie
abrades less easily

low ductility
elongation 4%
difficult to polish
difficult to adjust

Question 28

why do we need precision casting for cocr??

Answer 28

o work hardens rapidly,
o cannot keep cold working and annealing , making adjustments difficult

Question 29

gold or cocr is more ductile?

Answer 29

gold

Question 30

gold or cocr is more dense?

Answer 30

gold

Question 31

gold or cocr has higher UTS? ie higher FS

Answer 31

gold

Question 32

gold or cocr is more rigid?

Answer 32

cocr by alot

Question 33

gold or cocr is more hard?

Answer 33

cocr

Question 34

gold or cocr more shrinkage?

Answer 34

cocr

Question 35

cocr is more ___ than gold

Answer 35

rigid
hard
more shrinkage

Question 36

gold is more __ than cocr

Answer 36

ductile
dense
higher FS ie higher UTS