metals and alloys :(( Flashcards
1
Q
uses
A
RPD framework (CoCr, t4 gold) crowns (SS) denture base (SS) ortho (NiTi) Rxs (amalgam)
2
Q
factors affecting mechanical properties
A
crystalline structure
grain size
grain dislocations
3
Q
metal
A
aggregate of atoms in crystalline structure
4
Q
alloy
A
combination of 2 or more metal atoms in a crystalline structure
(or metal(s) with a metalloid)
5
Q
ductility
A
amount of plastic deformation prior to fracture
6
Q
EL
A
max stress without plastic deformation
7
Q
ductility is between
A
El and FS
8
Q
UTS on graph
A
at top
9
Q
crystalline structures
A
cubic
face-centred cubic
body-centred cubic
10
Q
cooling curve - metal
A
molten
plateau
metal
11
Q
crystal growth
A
first atoms cooling - nuclei of crystallisation
dendrites
impinge - grain boundary
(can get nucleating agents)
12
Q
types of crystal growth
A
equiaxed
radial
fibrous
13
Q
equiaxed crystal growth
A
equal
14
Q
radial crystal growth
A
cooled quickly in a cylindrical mould
15
Q
fibrous crystal growth
A
wire pulled through die
16
Q
fast cooling - quenching
A
more nuclei
small fine grains
good mechanical properties
+ high EL, UTS, hardness
- reduced ductility
17
Q
slow cooling
A
few nuclei
large coarse grains
18
Q
grains
A
each grain is a single crystal (lattice)
19
Q
grain boundary
A
change in orientation of the crystal planes
20
Q
ways to achieve quenching
A
small bulk
heat metal/alloy just above Tm
mould - high thermal conduction
quench
21
Q
dislocations
A
imperfections/defects in the crystal lattice
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
23
Q
impacts of impeding dislocation movement
A
increased EL, UTS, hardness
reduced ductility and impact resistance
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
25
cold work (strain hardening)
work done to change shape
low temp
causes SLIP
increases EL, UTS, hardness
reduces ductility, impact strength, corrosion resistance
26
residual stress
instability in lattice
results in distortion over time
relieved by annealing process
27
annealing
relieve residual stress
heat - thermal vibrations allow migration of atoms
grain structure and mechanical properties unchanged
28
alloys structure
2 metals form a common lattice structure
| solid solution
29
advantages of alloys
mechanical properties
corrosion resistance
reduced mp
more versatile
30
phase
physically distinct homogeneous structure
31
solution
homogeneous mixture at an atomic scale
32
recrystallisation
re-melt - lose all grain structure - start again
new smaller equiaxed grains
reduced EL, UTS, hardness
increased ductility
33
cold work and recrystallisation temp
greater the amount of cold work done the lower the recrystallisation temp
34
grain growth
annealing has the potential for this
| excessive temp rise causes large grains to replace smaller ones - poorer mechanical properties
35
crystallisation
```
metals normally soluble when molten
upon crystallisation:
1 - insoluble - 2 phases
2 - intermetallic compound with specific chemical formulation (Ag3Sn)
3 - soluble - solid solution
```
36
substitutional solid solution
atoms of one metal replace the other metal in the crystal lattice/grain
metal atoms similar in size, valency, crystal structure
random
ordered
37
interstitial solid solution
atoms v different in size
| smaller atoms located in spaces in lattice/grain structure of larger atom e.g. Fe-C
38
alloy cooling curve
crystallises over a temp range
39
slow cooling of alloys
allows metal atoms to diffuse through lattice
ensures grain composition homogeneous
BUT - large grains
40
what does rapid cooling of alloys - coring - prevent?
atoms diffusing through lattice - grains of different composition being formed
41
coring conditions
fast cooling of liquid state
| solidus and liquidus must be separated and determines extent of coring
42
fast cooling of allows - coring - effects
small grains - impede dislocation movement so better mechanical properties
coring - reduced corrosion resistance
43
dislocation movement metals
metal lattice - defect 'rolls' over the atoms in the lattice plane
little energy/force/stress needed for defect to move along slip plane
44
dislocation movement alloys
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
partially soluble alloy
solubility limit lines - only soluble in certain proportions
46
homogenising anneal
once solid cored alloy formed reheat to allow atoms to diffuse
keep below recrystallisation temp
47
solution hardening
alloys with metals of different atomic size have a distorted grain structure - impedes dislocation movement and so improves mechanical properties (EL, UTS, hardness)
48
order hardening
alloys forming an ordered solid solution (atoms distributed at specific lattice sites) have a distorted grain structure - impedes dislocation movement
49
eutectic alloys
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
non-eutectic composition
excess metal crystallises first
| then liquid reaches eutectic composition and both metals crystallise
51
precipitation hardening
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
T4 gold composition
```
Au 65%
Cu 14%
Ag 14%
Zn 2%
Pd 3%
Pt 2%
```
53
T4 gold - effect of Cu
```
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
T4 gold - effect of Ag
```
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
T4 gold alloys - heat tx
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
partial denture alloys ideal properties
```
rigid (YM)
strong (UTS, EL)
hard
ductile
precise casting (shrinkage)
melting point
low density
```
57
types of partial denture alloy
ADA T4 gold
white gold (AgPd)
CoCr
Ti
58
one piece casting: RPD
base - high YM, high EL
clasp - lower YM, high EL
compromise - thick section - rigid base
thin section - flexible clasp
59
partial solid solubility example
AgCu (in T4 gold)
60
platinum in T4 gold
solid solution in all proportions
solution hardening
fine grain structure
coring
61
palladium in T4 gold
less coring than Pt
coarser grains than Pt
absorbs gases when molten - porous casting
62
Zn in T4 gold
scavenger
63
Ni in T4 gold
increases hardness and strength
| wrought alloys
64
Indium
fine grain structure
65
CoCr uses
wires
implants
RPDs
connectors
66
CoCr composition
```
Co 54%
Cr 25%
Ni 15%
Mo 5%
C 0.4%
```
67
effects of Co
forms solid solution with Cr
increased strength, hardness, rigidity
coring possible
68
effects of Cr
forms solid solution with Co
increased strength, hardness, rigidity
coring possible
forms passive oxide layer on surface - corrosion resistance
69
effects of Ni in CoCr
replaces some Co
improves ductility
slight decrease in strength
sensitivity
70
effects of C in CoCr
undesirable
excess causes carbide at grain boundaries
hard and brittle - reduces ductility
71
effect of Mo in CoCr
reduces grain size so increases strength
72
effect of W in CoCr
increases strength
73
other molecules in CoCr
scavengers
74
CoCr investment material
high tem 1200-1400 degrees - can't use standard gypsum therefore silica or phosphate bonded
75
CoCr melting
electric induction preferred
| - oxyacetylene - carbon pickup
76
CoCr casting
centrifugal force required
avoid overheating - coarse grains
cooling too fast/slow - carbides brittle
77
CoCr finishing
takes time to finish and polish
but less likely to experience wear
HARD
sandblast, electroplate, abrasive wheel, polishing buff
78
CoCr ductility
low
work hardens rapidly - amount of cold work you can do ti it is low
need precision casting
adjustment difficult
79
CoCr rigidity
high rigidity
80
CoCr density
low
81
CoCr shrinkage
high
82
uses of titanium
implants, RPDs, crown and bridge, MF skull plates
83
titanium properties
good biocompatibility
good corrosion resistance (passive oxide layer)
absorbs gases so need specialised investment and casting
84
T4 gold properties
```
ductile
high density
low rigidity
low hardness
low shrinkage
```
