Metal & Alloys

Question 1

give examples of uses of metal in dentistry

Answer 1

cocr - partial denture
stainless steel - crown or denture base
titanium - orthodontic wire

Question 2

what are the mechanical properties of metal

Answer 2

high elastic limit - very rigid, high UTS - resist tension, high fracture stress - a lot for it to fracture

Question 3

what is ductility/malleability

Answer 3

the difference between elastic limit and fracture stress, the stress it can undergo causing permanent deformation but not causing fracture. ductility - tensile stress, malleability - compressive stress

Question 4

what affects the mechanical properties and what is the knock on affect from this

Answer 4

grain size, impurities and lattice structure. these are affected by the processing of the metal, therefore, processing affects the mechanical properties

Question 5

what is the difference between a metal and an alloy

Answer 5

metal - atoms arranged in lattice structure

alloy - atoms of 2 or more metals arranged in lattice structure

Question 6

describe the over all process of producing metal

Answer 6

molten metal allowed to cool, reaches a temperature where solid formation begins, stays at this temperature until all metal is transformed to solid, then allowed to cool further

Question 7

describe crystal growth

Answer 7

nuclei of crystallisation - atom from which other atoms crystallise. crystals grow from here to form dendrites - 3d lattice structure, growing in an orientation. as grains impinge on other crystals - grain boundary - change of orientation

Question 8

what are the 2 types of grain growth

Answer 8

radial growth - small crystals, many of them, good mechanical properties
fibrous growth - large crystals, few of them, poor mechanical properties

Question 9

what affects grain growth

Answer 9

the way in which the metal is cooled. fast cooling - small grains formed, slow cooling - large grains formed

Question 10

how can a metal be cooled quickly

Answer 10

quenching - putting it into cold water

Question 11

what is the problem with defects in lattice structure

Answer 11

misalign the structure. if nothing done about it, the defect will move along the lattice - slip - propagation of dislocation. in doing so, weakens the material

Question 12

how can defects be reduced

Answer 12

cold work - applying a force in cold temperature eg hammering. moves the defect along the lattice structure until it gets to a grain boundary where it is stopped. this prevents the defect from moving and improves the UTS, EL and FS

Question 13

what is the disadvantage of cold work

Answer 13

results in an increase in internal residual stress

Question 14

how can the residual stress be dealt with

Answer 14

stress relief annealing - heating up the metal after cold work to realign the lattice. doesnt affect grain size or shape or mechanical properties but reduces the stress

Question 15

what can be done if a poor result has come from cold work and stress relief annealing

Answer 15

recrystalisation - heating the metal back up to molten state, then allow for cooling. un-does all cold work

Question 16

what is a phase

Answer 16

distinct homogenous structure

Question 17

if two metals in an alloy form two separate lattice structures, how many phases are there

Answer 17

2

Question 18

if two metals in an alloy form one lattice structure with 2 types of grains, how many phases are there

Answer 18

1

Question 19

if a molten alloy is decreased down to crystallisation temperature, what are the possible results

Answer 19

two metals are insoluble in one another - 2 separate phases are formed
intermetallic
two metals are soluble in one another - 1 phase formed with 2 types of grains

Question 20

what is a solid solution

Answer 20

solid metal lattice with 2 grain types - 1 phase

Question 21

how can 2 grains in 1 phase be arranged

Answer 21

substitution - one atom replaces another, either random or ordered
interstitial - smaller atoms fill in the gaps between larger atoms

Question 22

how does cooling of an alloy for crystallisation differ to that of a metal

Answer 22

metal - happens at one temperature

alloy - happens at a range of temperatures, continues to cool while crystallisation is happening

Question 23

how does the composition of metal in an alloy effect the crystallisation temperature

Answer 23

different compositions results in a different range of temperatures for crystallisation

Question 24

what is shown in a phase diagram

Answer 24

temperature of cyrstallisation against composition of alloy. liquidus - above this line, the alloy is molten, below, it begins crystallisation. solidus - below this line, the alloy is fully solid, crystallisation has finished

Question 25

how does cooling of the alloy affect the structure

Answer 25

slow cooling - homogenous structure, however, large particles reducing the mechanical properties
fast cooling - results in coring, but small fine particles with good mechanical properties

Question 26

what is coring

Answer 26

when crystals are produced at different temperatures, resulting in different compositions of metal in crystals in the one alloy. results in a concentration gradient across the alloy, reduces corrosion resistance, not a homogenous structure

Question 27

how can the affect of coring be reduced

Answer 27

homogenous annealing, increasing the temperature of the metal to allow the crystals to all have the same composition.

Question 28

what is solution hardening

Answer 28

dislocation movement can reduce mechanical properties. this can be impeded by smaller atoms between larger atoms. means the defect has to have energy to be able to move over the top of the larger atoms. instead, it just stays in the space - no movement, no reduction of mechanical properties. different atomic size - distorted grain structure

Question 29

what is eutectic alloy

Answer 29

metals that are insoluble in one another, crystallise at different temperatures and form 2 separate phases

Question 30

what is a partially soluble alloy

Answer 30

when only certain compositions of metals in the alloy are soluble in one another. below the solid solubility lines - possible compositions to form alloy

Question 31

what type of gold is important in partial denture

Answer 31

type 4

Question 32

how is type 4 gold produced for partial denture

Answer 32

quench cooled, rapid cooling, results in coring, homogenous annealing, can then be cold worked, followed by stress relief annealing, then order hardening

Question 33

what is type 4 gold composed of

Answer 33

gold, silver, copper, paldium, platinum

Question 34

what does chromium produce in CoCr

Answer 34

passive layer that is corrosion resistant

Question 35

how is cocr produced

Answer 35

different to gold
heated up in investment material, too hot to be gypsum, has to be silicone or phosphate, heated up, casting and finishing

Question 36

what are ideal properties of a partial denture alloy

Answer 36

high hardness, high UTS, high EL and FS, low density for patient comfort

Question 37

compare the properties of CoCr and Type IV gold

Answer 37

CoCr - low density more comfortable, has high hardness and very rigid - high YM but low ductility, very little adjustments can be made
Type IV gold - high density, not very comfortable, high ductility and higher fracture strength
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