Metal & Alloys Flashcards Preview

Dental Material Science > Metal & Alloys > Flashcards

Flashcards in Metal & Alloys Deck (37)
Loading flashcards...
1
Q

give examples of uses of metal in dentistry

A

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

2
Q

what are the mechanical properties of metal

A

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

3
Q

what is ductility/malleability

A

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

4
Q

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

A

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

5
Q

what is the difference between a metal and an alloy

A

metal - atoms arranged in lattice structure

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

6
Q

describe the over all process of producing metal

A

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

7
Q

describe crystal growth

A

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

8
Q

what are the 2 types of grain growth

A

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

9
Q

what affects grain growth

A

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

10
Q

how can a metal be cooled quickly

A

quenching - putting it into cold water

11
Q

what is the problem with defects in lattice structure

A

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

12
Q

how can defects be reduced

A

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

13
Q

what is the disadvantage of cold work

A

results in an increase in internal residual stress

14
Q

how can the residual stress be dealt with

A

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

15
Q

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

A

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

16
Q

what is a phase

A

distinct homogenous structure

17
Q

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

A

2

18
Q

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

A

1

19
Q

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

A

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

20
Q

what is a solid solution

A

solid metal lattice with 2 grain types - 1 phase

21
Q

how can 2 grains in 1 phase be arranged

A

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

22
Q

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

A

metal - happens at one temperature

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

23
Q

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

A

different compositions results in a different range of temperatures for crystallisation

24
Q

what is shown in a phase diagram

A

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

25
Q

how does cooling of the alloy affect the structure

A

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

26
Q

what is coring

A

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

27
Q

how can the affect of coring be reduced

A

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

28
Q

what is solution hardening

A

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

29
Q

what is eutectic alloy

A

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

30
Q

what is a partially soluble alloy

A

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

31
Q

what type of gold is important in partial denture

A

type 4

32
Q

how is type 4 gold produced for partial denture

A

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

33
Q

what is type 4 gold composed of

A

gold, silver, copper, paldium, platinum

34
Q

what does chromium produce in CoCr

A

passive layer that is corrosion resistant

35
Q

how is cocr produced

A

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

36
Q

what are ideal properties of a partial denture alloy

A

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

37
Q

compare the properties of CoCr and Type IV gold

A

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
have similar uts