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DMS :( > metals and alloys :(( > Flashcards

metals and alloys :(( Flashcards

1
Q

uses

A 
RPD framework (CoCr, t4 gold)
crowns (SS)
denture base (SS)
ortho (NiTi)
Rxs (amalgam)
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2
Q

factors affecting mechanical properties

A

crystalline structure
grain size
grain dislocations

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3
Q

metal

A

aggregate of atoms in crystalline structure

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4
Q

alloy

A

combination of 2 or more metal atoms in a crystalline structure
(or metal(s) with a metalloid)

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5
Q

ductility

A

amount of plastic deformation prior to fracture

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6
Q

EL

A

max stress without plastic deformation

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7
Q

ductility is between

A

El and FS

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8
Q

UTS on graph

A

at top

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9
Q

crystalline structures

A

cubic
face-centred cubic
body-centred cubic

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10
Q

cooling curve - metal

A

molten
plateau
metal

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11
Q

crystal growth

A

first atoms cooling - nuclei of crystallisation
dendrites
impinge - grain boundary
(can get nucleating agents)

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12
Q

types of crystal growth

A

equiaxed
radial
fibrous

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13
Q

equiaxed crystal growth

A

equal

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14
Q

radial crystal growth

A

cooled quickly in a cylindrical mould

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15
Q

fibrous crystal growth

A

wire pulled through die

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16
Q

fast cooling - quenching

A

more nuclei
small fine grains
good mechanical properties

+ high EL, UTS, hardness
- reduced ductility

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17
Q

slow cooling

A

few nuclei

large coarse grains

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18
Q

grains

A

each grain is a single crystal (lattice)

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19
Q

grain boundary

A

change in orientation of the crystal planes

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20
Q

ways to achieve quenching

A

small bulk
heat metal/alloy just above Tm
mould - high thermal conduction
quench

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21
Q

dislocations

A

imperfections/defects in the crystal lattice

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22
Q

SLIP

A

due to the propagation of dislocations, involves rupture of only a few bonds at a time
process by which defects move through grain

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23
Q

impacts of impeding dislocation movement

A

increased EL, UTS, hardness

reduced ductility and impact resistance

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24
Q

ways of impeding dislocation movement

A

grain boundaries
alloys - different atom sizes - inherent resistance to movement of dislocations
cold working - dislocations build up at grain boundaries

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25
Q

cold work (strain hardening)

A

work done to change shape
low temp
causes SLIP

increases EL, UTS, hardness
reduces ductility, impact strength, corrosion resistance

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26
Q

residual stress

A

instability in lattice
results in distortion over time
relieved by annealing process

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27
Q

annealing

A

relieve residual stress
heat - thermal vibrations allow migration of atoms
grain structure and mechanical properties unchanged

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28
Q

alloys structure

A

2 metals form a common lattice structure

solid solution

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29
Q

advantages of alloys

A

mechanical properties
corrosion resistance
reduced mp
more versatile

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30
Q

phase

A

physically distinct homogeneous structure

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31
Q

solution

A

homogeneous mixture at an atomic scale

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32
Q

recrystallisation

A

re-melt - lose all grain structure - start again
new smaller equiaxed grains
reduced EL, UTS, hardness
increased ductility

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33
Q

cold work and recrystallisation temp

A

greater the amount of cold work done the lower the recrystallisation temp

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34
Q

grain growth

A

annealing has the potential for this

excessive temp rise causes large grains to replace smaller ones - poorer mechanical properties

35
Q

crystallisation

A 
metals normally soluble when molten
upon crystallisation:
1 - insoluble - 2 phases
2 - intermetallic compound with specific chemical formulation (Ag3Sn)
3 - soluble - solid solution
36
Q

substitutional solid solution

A

atoms of one metal replace the other metal in the crystal lattice/grain
metal atoms similar in size, valency, crystal structure
random
ordered

37
Q

interstitial solid solution

A

atoms v different in size

smaller atoms located in spaces in lattice/grain structure of larger atom e.g. Fe-C

38
Q

alloy cooling curve

A

crystallises over a temp range

39
Q

slow cooling of alloys

A

allows metal atoms to diffuse through lattice
ensures grain composition homogeneous
BUT - large grains

40
Q

what does rapid cooling of alloys - coring - prevent?

A

atoms diffusing through lattice - grains of different composition being formed

41
Q

coring conditions

A

fast cooling of liquid state

solidus and liquidus must be separated and determines extent of coring

42
Q

fast cooling of allows - coring - effects

A

small grains - impede dislocation movement so better mechanical properties
coring - reduced corrosion resistance

43
Q

dislocation movement metals

A

metal lattice - defect ‘rolls’ over the atoms in the lattice plane
little energy/force/stress needed for defect to move along slip plane

44
Q

dislocation movement alloys

A

defect falls into larger space between large and small atom
more energy/force/stress needed for the defect to overcome the different sized atoms therefore requires greater stress to move dislocations in a solid solution

makes alloys inherently more fracture resistant i.e. stronger than metals

45
Q

partially soluble alloy

A

solubility limit lines - only soluble in certain proportions

46
Q

homogenising anneal

A

once solid cored alloy formed reheat to allow atoms to diffuse
keep below recrystallisation temp

47
Q

solution hardening

A

alloys with metals of different atomic size have a distorted grain structure - impedes dislocation movement and so improves mechanical properties (EL, UTS, hardness)

48
Q

order hardening

A

alloys forming an ordered solid solution (atoms distributed at specific lattice sites) have a distorted grain structure - impedes dislocation movement

49
Q

eutectic alloys

A

metals soluble in liquid state, insoluble in solid state

where liquidus and solidus coincide - crystallisation process occurs at a single temp
grains of individual metals formed simultaneously

lowest mp
hard but brittle
poor corrosion resistance

50
Q

non-eutectic composition

A

excess metal crystallises first

then liquid reaches eutectic composition and both metals crystallise

51
Q

precipitation hardening

A

applies to T4 gold
in partially soluble alloys
on annealing, some of the atoms get pushed to the grain boundary
harder for dislocations to move - better mechanical properties

52
Q

T4 gold composition

A 
Au 65%
Cu 14%
Ag 14%
Zn 2%
Pd 3%
Pt 2%
53
Q

T4 gold - effect of Cu

A 
solid solution in all proportions
solution hardening
order hardening
reduced mp
v little coring
red colour (try to avoid)
reduces density
base metal - can corrode if too much
54
Q

T4 gold - effect of Ag

A 
solid solution in all proportions
solution hardening
precipitation hardening with copper and heat tx
can allow tarnishing
molten Ag absorbs gas
whitens alloy - compensates for Cu
55
Q

T4 gold alloys - heat tx

A

quench after casting
homogenising anneal (700 degrees, 10mins)
if cold worked - stress relief anneal
heat harden

heat treated

  • properties more suitable for clasp
  • need thickness for base (expense)
56
Q

partial denture alloys ideal properties

A 
rigid (YM)
strong (UTS, EL)
hard
ductile
precise casting (shrinkage)
melting point
low density
57
Q

types of partial denture alloy

A

ADA T4 gold
white gold (AgPd)
CoCr
Ti

58
Q

one piece casting: RPD

A

base - high YM, high EL
clasp - lower YM, high EL
compromise - thick section - rigid base
thin section - flexible clasp

59
Q

partial solid solubility example

A

AgCu (in T4 gold)

60
Q

platinum in T4 gold

A

solid solution in all proportions
solution hardening
fine grain structure
coring

61
Q

palladium in T4 gold

A

less coring than Pt
coarser grains than Pt
absorbs gases when molten - porous casting

62
Q

Zn in T4 gold

A

scavenger

63
Q

Ni in T4 gold

A

increases hardness and strength

wrought alloys

64
Q

Indium

A

fine grain structure

65
Q

CoCr uses

A

wires
implants
RPDs
connectors

66
Q

CoCr composition

A 
Co 54%
Cr 25%
Ni 15%
Mo 5%
C 0.4%
67
Q

effects of Co

A

forms solid solution with Cr
increased strength, hardness, rigidity
coring possible

68
Q

effects of Cr

A

forms solid solution with Co
increased strength, hardness, rigidity
coring possible
forms passive oxide layer on surface - corrosion resistance

69
Q

effects of Ni in CoCr

A

replaces some Co
improves ductility
slight decrease in strength
sensitivity

70
Q

effects of C in CoCr

A

undesirable
excess causes carbide at grain boundaries
hard and brittle - reduces ductility

71
Q

effect of Mo in CoCr

A

reduces grain size so increases strength

72
Q

effect of W in CoCr

A

increases strength

73
Q

other molecules in CoCr

A

scavengers

74
Q

CoCr investment material

A

high tem 1200-1400 degrees - can’t use standard gypsum therefore silica or phosphate bonded

75
Q

CoCr melting

A

electric induction preferred

- oxyacetylene - carbon pickup

76
Q

CoCr casting

A

centrifugal force required
avoid overheating - coarse grains
cooling too fast/slow - carbides brittle

77
Q

CoCr finishing

A

takes time to finish and polish
but less likely to experience wear
HARD
sandblast, electroplate, abrasive wheel, polishing buff

78
Q

CoCr ductility

A

low
work hardens rapidly - amount of cold work you can do ti it is low
need precision casting
adjustment difficult

79
Q

CoCr rigidity

A

high rigidity

80
Q

CoCr density

A

low

81
Q

CoCr shrinkage

A

high

82
Q

uses of titanium

A

implants, RPDs, crown and bridge, MF skull plates

83
Q

titanium properties

A

good biocompatibility
good corrosion resistance (passive oxide layer)
absorbs gases so need specialised investment and casting

84
Q

T4 gold properties

A 
ductile
high density
low rigidity
low hardness
low shrinkage

DMS :( (18 decks)

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