metals and alloys

Question 1

give some dental uses of metals and alloys?

Answer 1

partial denture framework (CoCr)
crowns (stainless steel)
denture base (stainless steel)
orthodontist appliance (NiTi)
restorations (amalgam)

Question 2

an aggregate of atoms in a crystalline structure is a?

Answer 2

metal

Question 3

a combination of metal atoms in a crystalline structure is an?

Answer 3

alloy

Question 4

crystal growth of equal dimension in each direction is known as?

Answer 4

equi-axed grains

Question 5

regions of a crystal where grains make contact are known as?

Answer 5

grain boundaries

Question 6

a single crystal with atoms orientated in a given direction is a?

Answer 6

grain

Question 7

why are small fine grains advantageous ?

Answer 7

high elastic limit
increased UTS (ultimate tensile strength) and hardness

Question 8

why are small fine grains not advantageous?

Answer 8

decreased ductility

Question 9

does fast cooling give small fine grains or large course grains?

Answer 9

small fine grains

Question 10

does slow cooling give small fine grains or large course grains?

Answer 10

large course

Question 11

imperfections/defects in the crystal lattice are known as?

Answer 11

dislocations

Question 12

which technique allows dislocations to be collected at the grain boundary?

Answer 12

cold work

Question 13

how does cold work result in stronger/harder material?

Answer 13

results in dislocations being collected at the grain boundary

Question 14

what techniques does cold working involve?

Answer 14

benign and rolling a metal at low temperature

Question 15

what effect does cold working have on elastic limit (EL), ultimate tensile strength (UTS) and hardness?

Answer 15

increase all

Question 16

what effect does cold working have on the ductility of a metal?

Answer 16

reduced ductility

Question 17

cold working increases residual stress causing distortion of the metal over time, how is this relieved?

Answer 17

annealing process

Question 18

briefly describe what the annealing process involves?

Answer 18

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

Question 19

name the 3 stages of annealing?

Answer 19

recovery
recrystallisation
grain growth

Question 20

describe the purpose of the recovery stage of annealing?

Answer 20

eliminates residual stress caused from cold working allowing atoms to rearrange within grains

Question 21

what happens during the recrystallisation stage of the annealing process?

Answer 21

• cold work material is subject to a temp above recrystallisation temp
• this causes new smaller grains, lowers EL, UTS and hardness while increasing ductility
• spoils the benefits of cold work allowing more cold work

Question 22

what happens during the grain growth stage of the annealing process?

Answer 22

excessive temp rise causes large grains to replace smaller ones

Question 23

a combination of 2 or more metals or metals within a metalloid is known as?

Answer 23

an alloy

Question 24

list the advantages of an alloy compared to a metal?

Answer 24

improved mechanical properties (EL, UTS and hardness)
more corrosion resistant
lower melting pt than individual metal

Question 25

give some dental uses of alloys?

Answer 25

steel burs and instruments amalgam gold alloy crowns/bridges

Question 26

what is a phase in relation to an alloy?

Answer 26

a physically distinct homogenous structure

Question 27

a homogenous mixture at an atomic state is known as a ?

Answer 27

solution

Question 28

how many phases does a solid solution have?

Answer 28

1

Question 29

name the 3 types of solid solution?

Answer 29

random substitutional solid solution ordered substitutional solid solution interstitial solid solution

Question 30

smaller atoms located in lattice/grain stricture of larger atoms describes which type of solid solution?

Answer 30

interstitial

Question 31

which crystallises at a single temperature, an alloy or a pure metal?

Answer 31

pure metal

Question 32

which crystallises over a temperature range, an alloy or a pure metal?

Answer 32

alloy

Question 33

the line representing the temperature by which an alloys composition begins to crystallise is known as?

Answer 33

liquidus

Question 34

the line representing temp by which an alloys composition has completely crystallised is known as ?

Answer 34

solidus

Question 35

which type of alloy cooling, slow or fast, results in a desirable homogenous grain composition but undesirable large grains ?

Answer 35

slow cooling

Question 36

do we cool alloys slowly or rapidly?

Answer 36

rapidly

Question 37

does rapid cooling of an alloy result in a homogenous grain structure?

Answer 37

no

Question 38

a defect in an alloy caused by cooling too quickly is known as?

Answer 38

coring

Question 39

how can coring of an alloy be reversed?

Answer 39

homogenous annealing

Question 40

which requires little energy/force, dislocation of metal lattice or dislocation of alloy?

Answer 40

metal lattice

Question 41

describe dislocation of a defect in an alloy?

Answer 41

defect climbs over each atom and settles in-between metal atoms until it reaches grain boundary, this requires a lot of energy

Question 42

a mix of metals having melting pt lower than any of its components is known as?

Answer 42

eutectic alloy

Question 43

what are the ideal properties for a partial denture alloy?

Answer 43

rigid strong hard ductile

Question 44

for the denture base, do you want high or low youngs modulus and why?

Answer 44

high to maintain shape in use

Question 45

for the denture clasps, do you want high or low youngs modulus and why?

Answer 45

low to allow flexure over tooth