Metals and alloys

Question 1

metals and alloys used in dentistry - give examples

Answer 1

partial denture framework
- CoCr, type IV gold
crowns
- stainless steel
denture base
- stainless steel
orthodontic appliances
- NiTi
restorations
- amalgam

Question 2

metal - definition

Answer 2

aggregate of atoms in crystalline structure

Question 3

Elastic limit - define

Answer 3

maximum stress without plastic/permanent deformation

Question 4

ductility - define

Answer 4

amount of plastic deformation prior to fracture
- the extent that a material can be shaped/manipulated

Question 5

factors affecting mechanical properties of an alloy

Answer 5

choice of metal
- each has its own inherent characteristics
crystalline structure - depends on
- history - method of production
shaping - e.g. cold working, swaging
grain size
grain imperfections

Question 6

what is a grain boundary?

Answer 6

a region where grains (or crystals) make contact
- impurities concentrate here

Question 7

Fast cooling/quenching - effects

Answer 7

more nuclei
small fine grains

Question 8

slow cooling effects

Answer 8

few nuclei
large coarse grains

Question 9

pros and cons of small fine grains

Answer 9

high elastic limit
increased fracture strength, hardens and ultimate tensile strength
BUT
decreased ductility

Question 10

Factors for rapid cooling/quenching

Answer 10

small bulk
heat metal/alloy just above melting point
mould - high thermal conduction
quench

Question 11

What are dislocations?

Answer 11

imperfections/defects in the crystal lattice

Question 12

What is slip?

Answer 12

due to propagation of dislocations and involves rupture of only a few bonds at a time

Question 13

impeding movement of dislocations in metals/alloys - consequences

Answer 13

increased
- elastic limit
- fracture strength
- ultimate tensile strength
- hardness
decreased
- ductility
- impact resistance

Question 14

factors impeding dislocation movement

Answer 14

grain boundaries
- hence fine grains
alloys
- different atom sizes
cold working
- dislocations stopped at grain boundaries

Question 15

What is cold work?

Answer 15

work done on a metal or alloy
- e.g bending, rolling, swaging
- at low temperature - below recrystallisation temperature
causes slip
- dislocations collect at grain boundaries
leads to stronger, harder metal
- more prone to work or strain hardening

Question 16

cold working - effect on the metal

Answer 16

modifies grain structure
increases
- elastic limit
- fracture strength/UTS
- hardness
decreases
- ductility
- impact strength
- corrosion resistance

Question 17

Residual stress consequences and how to remedy

Answer 17

causes instability in lattice
results in distortion over time
relieved by annealing process

Question 18

What is annealing?

Answer 18

heating a metal or alloy so that greater thermal vibrations allow migration/rearrangement of atoms

Question 19

cold work - negative consequences

Answer 19

results in internal stresses
- may lead to distortion of appliance over time

Question 20

stress relief annealing effects

Answer 20

eliminates stresses by allowing atoms to rearrange within grains
- grain structure and mechanical properties unchanged
- some further cold work possible

Question 21

recrystallisation - causes and effects

Answer 21

occurs when metal/alloy heated causing
- new smaller equiaxed grains (crystal growth is of equal dimension in each direction)
- lower elastic limit, UTS and hardness
- increased ductility

Question 22

recrystallisation consequences

Answer 22

spoils benefits of cold work
allows further cold work
cold/work/recrystalisation repeated until correct shape obtained

Question 23

recrystallisation temperature depends on…

Answer 23

the amount of cold work
greater the amount of cold work the lower the recrystallisation temperature

Question 24

excessive temperature rise causes…(how to avoid)

Answer 24

large grains to replace smaller coarse grains yielding poorer mechanical properties
- careful when annealing