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BDS2 CLINICAL Dental Materials > Metals and Alloys > Flashcards

Metals and Alloys Flashcards

1
Q

What are metals and alloys used for in dentistry

A 
RPD framework 
crowns
denture base
orthodontic appliance
restorations
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2
Q

What metal/alloy is used in partial denture framework

A

cobalt chrome

type IV gold

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3
Q

What metal/alloy is used in crowns

A

stainless steel

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4
Q

What metal/alloy is used in denture base

A

stainless steel

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5
Q

What metal/alloy is used in orthodontic appliance

A

NiTi

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6
Q

What metal/alloy is used in restorations

A

amalgam

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7
Q

Why do we use metals

A

They have superior mechanical properties

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8
Q

What are the superior mechanical properties of metals

A 
strength
rigidity 
elastic limit 
ductility
hardness
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9
Q

What are the disadvantages of metals

A

poor aesthetics

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10
Q

What is the definition of a metal

A

aggregate of atoms in a crystalline structure

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11
Q

What is an alloy

A

combination of metal atoms in a crystalline structure

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12
Q

What are the building blocks of alloys

A

metals

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13
Q

What is ductility

A

Ductility is a measure of a metal’s ability to withstand tensile stress—any force that pulls the two ends of an object away from each other

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14
Q

What is malleability

A

Malleability is a substance’s ability to deform under compressive stress

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15
Q

What is the elastic limit

A

maximum stress without plastic deformation

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16
Q

What is the UTS

A

ultimate tensile strength

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17
Q

What is ductility defined as

A

amount of PLASTIC DEFORMATION prior to fracture

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18
Q

What do mechanical properties depend on

A

choice of metal

crystalline structure

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19
Q

What does the crystalline structure depend on

A

history - method of production

shaping - crucial for dental applications

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20
Q

What are the factors that affect mechanical properties

A

crystalline structure
grain size
grain imperfections

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21
Q

What is a grain

A

a single crystal

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22
Q

What is a crystal

A

a lattice arrangement

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23
Q

What are different crystal/lattice structures

A

cubic
face centre cubic
body centre cubic

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24
Q

In the cooling curve of a pure metal what is the straight line

A

plateau phase

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25
Q

What does the plateau phase mean in a pure metal

A

the melting point

changes from liquid to solid

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26
Q

Describe the process crystal growth

A
  1. atoms act as nuclei of crystallization
  2. crystals grow to form dendrites
  3. crystals (or GRAINS) grow until they impinge on other crystals
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27
Q

What is the nuclei of crystallization

A

the centre of around which other atoms will crystallite

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28
Q

What are dendrites

A

3D branched lattice network

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29
Q

What is the region where grains make contact called

A

the GRAIN BOUNDARY

30
Q

What parameters can effect the size and shape of grains

A

how you process it

which container you use to cool it

31
Q

If you change the size and shape what else do you effect

A

the properties

32
Q

What are EQUI-AXED grains

A

crystal growth of equal direction

33
Q

What is a radial grain structure

A

molten metal cooled quickly in cylindrical mould

34
Q

What is a fibrous grain structure

A

wire pulled through die (cold worked metal/alloy)

35
Q

What are the different types of cooling

A 
Fast cooling (quenching)
slow cooling
36
Q

How does fast cooling effect the crystal growth

A

more nuclei
small fine grains
MECHANICAL PROPERTIES ENHANCED

37
Q

How does slow cooling effect the crystal growth

A

few nuclei
large coarse grains
WEAK

38
Q

What are nucleating agents

A

impurities or additives act as foci for crystal growth

helps the crystallization process and improves the product

39
Q

What is each grain

A

a single crystal (lattice) with atoms orientation in given directions (dendrites)

40
Q

What is the grain boundary a result of

A

change in orientation of the crystal planes - impurities concentrate here

41
Q

why are small fine grains advantageous

A

high elastic limit

increased UTS, hardness

42
Q

What are the disadvantages of small fine grains

A

have a decreased ductility

43
Q

What are the factors for rapid cooling

A

small bulk required
heat metal/alloy just above its melting point
pull the heat away from the metal quickly and quench

44
Q

What does a defect in the crystalline structure represent

A

a misalignment of atoms in that lattice network

45
Q

What is dislocation

A

imperfections/defects in crystal lattice

46
Q

What happens to a defect when force is applied

A

it moves which in the end results in a lattice that has no defect but is a different shape

47
Q

What is slip due to

A

propagation of dislocations and involves rupture of only a few bonds at a time
this is crucial as it is a cascade effect and doesn’t require supreme large forces to do it

48
Q

If you impede the movement of dislocations what does this increase

A

elastic limit
UTS
hardness

49
Q

If you impede the movement of dislocations what does this decrease

A

impact resistance

50
Q

What are the factors that impede dislocation movement

A

grain boundaries (hence fine grains)
alloys: different atom sizes
cold working

51
Q

In cold working where do the dislocations build up at

A

grain boundary

52
Q

How do grain boundaries impede dislocation movement

A

they stop the defect moving to another grain

53
Q

How is cold working achieved

A

bending, rolling, swaging

push the effect within the metal alloy to the grain boundary and improve the mechanical properties of that object

54
Q

What temperature is cold working done

A

at a low one (below recrystallization temperature)

55
Q

What does cold working cause

A

slip - hence stronger harder material

56
Q

What properties are higher after cold work

A

elastic limit
UTS
hardness

57
Q

What properties are lower after cold working

A

ductility
impact strength
low corrosion resistance

58
Q

What is the effect of cold working on residual stress

A

increases it

59
Q

What does residual stress cause

A

instability in lattice resulting in distortion over time

60
Q

What is residual stress relieved by

A

annealing process

61
Q

What is annealing

A

heating the metal/alloy so that greater thermal vibrations allow migrations of atoms

62
Q

What does stress relief annealing do

A

ELIMINATE stresses by allowing atoms to REARRANGE within grains

63
Q

How are grain structure and mechanical properties changed in annealing

A

they are not

64
Q

When does recrustallisation happen

A

when meta alloy is heated causing new smaller equiaxed grains

65
Q

What is the effect of recrystallisaton on the properties

A

lower EL, UTS, hardness

increased ductility

66
Q

What does recrystallization spoil

A

spoils benefit of cold work
allows further cold work
cold work/recrystallisation repeated until correct shape obtained

67
Q

What does the recrystallization depend on

A

amount of cold work

greater the cold work, lower the recrystallization temperature

68
Q

What does excessive temperature rise cause

A

large grains to replace smaller coarse grains yielding poor mechanical properties so have to be careful when annealing

69
Q

What is the definition of an alloy

A

two metals that form a COMMON LATTICE structure, are SOLUBLE in one another and form a SOLID SOLUTION

70
Q

What does metal alloys forming a solid solution mean

A

it means two metals co-existing in a common lattice structure
NOTHING TO DO WITH TEMP

BDS2 CLINICAL Dental Materials (15 decks)

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