Metals

Question 1

metal definition

Answer 1

aggregate of atoms in crystalline structure

Question 2

what are metals used for in dentistry

Answer 2

RPD [CoCr, gold], crowns, denture base, restorations

Question 3

factors affecting mechanical properties

Answer 3

crystalline structure, grain size, grai n imperfections

Question 4

crystal growth

Answer 4

crystals grow to form dendrites, crystals for grains until they impinge on other crystals, region where grains make contact is grain boundary

Question 5

types of metallic grains

Answer 5

equi-axed = growth is of equal dimension in each direction
radial = molten metal cooled quickly in cylindrical mould
fibrous = wire pulled through die [cold worked]

Question 6

you can alter crystals by;

Answer 6

quenching - fast cooling, more nuclie, small fine grains
slow cooling - few nuclei, large coarse grains
nucleating agents - impurities or additives act as foci

Question 7

grains in metals

Answer 7

each grain is a single crystal lattice with atoms orientated in given directions [dendrites]

Question 8

grain boundary

Answer 8

change in orientation of crystal planes, impurities concentrate here
small fine grains advantageous as high eleastic limit, increased hardness, increased fracture strength and tensile

Question 9

dislocation

Answer 9

imperfections in crystal lattice, force is applied and this defect moves along by propagation
when defect reaches boundary, lattic changes to new shape free of the defect SLIP

Question 10

adv and disadv of dislocation

Answer 10

increases - elastic limit, fracture stress, hardness
decreases - ductility, impact resistance
factors impeding - grain boundaries, alloys, cold working

Question 11

cold working

Answer 11

done to metal/alloy [bending/rolling]
done at a low temperature, causes SLIP so dislocations collect at grain boundary, creating stronger and harder material

Question 12

adv and disadv of cold working

Answer 12

adv - higher elastic limit, fracture stress, hardness
disadv - lower ductility, impact strength, corrosion resistance
results in internal stress

Question 13

residual stress

Answer 13

instability in lattic, distortion over time, undesirable effect
relieved by annealing process

Question 14

annealing process

Answer 14

heating metal/alloy so that greater thermal vibrations allows migration/rearrgamgement of atoms
eliminates stresses, allows atoms to rearrgange with grains
grain structure and mechanical properties unchanged
care taken - if too high temo, grains will swell = worse mechanical properties

Question 15

recrystallisation

Answer 15

spoils benefit of cold work, but allows further cold work
repeated until correct shape is obtained
greater amount of cold work - the lower the recrystallisation temp