Metal and alloys 1 and 2 Flashcards
(97 cards)
1
Q
What metals and alloys are widely used in dentistry
A
- partial denture framework (CoCr, Type 4 gold)
- crowns (stainless steel)
- denture base (stainless steel)
- orthodontic appliance (NiTi)
- restorations (amalgam-unique)
2
Q
What properties are we interested in looking at regarding metals and alloys
A
- strength
- ductility
- rigidity
- hardness
- elastic limit
3
Q
What can affect properties of metals
A
defects (dislocations) on properties
4
Q
What can affect structure
A
processing (e.g. quenching, cold working, annealing)
5
Q
Disadvantage of metals
A
poor aesthetics
6
Q
Define metal
A
aggregate of atoms in crystalline structure
7
Q
Define alloy
A
combination of metal atoms in a crystalline structure
8
Q
What information can we gain from a stress-strain curve (know what this looks like)
A
- Fracture strength
- Elastic limit
- Ultimate tensile strength
- Ductility
- malleability
- ductility
- rigidity
9
Q
What is the elastic limit
A
maximum stress without plastic deformation
10
Q
What is ductility
A
amount of plastic deformation prior to fracture
11
Q
What is on the y and x axis of a stress strain curve
A
x = strain % y= stress (MPa)
12
Q
What do mechanical properties depend on
A
- Choice of metal
2. crystalline structure
13
Q
what does crystalline structure depend on
A
- history (method of production)
- shaping - crucial for dental applications e.g. cold working, swaging
14
Q
What are the factors which affect mechanical properties
A
- crystalline structure
- grain size and grain imperfections
15
Q
What are some crystal or lattice structures
A
- cubic
- face-centred cubic
- body centred cubic
16
Q
What does the cooling curve of pure metal look like
A
- gradual decrease in temp
- plateau
- gradual decrease in temp
17
Q
what is the plateu phase of the cooling curve of pure metal
A
melting point
18
Q
what triggers the end of the plateu phase on the cooling curve of pure metal
A
retains temp until all metal changes from liquid to solid, then it cools down further
19
Q
How does crystal growth occur
A
- atoms act as nuclei of crystallisation
- crystals grow to form dendrites (3D branched lattice network)
- crystals (or grains) grow until they impinge on other crystals
- region where grains make contact is called grain boundary
20
Q
what is the grain boundary
A
where grains (crystals) make contact
21
Q
what is the grain structure where crystal growth of equal dimension in each direction
A
equi-axed grains
22
Q
what is the grain structure where crystal growth is molten metal cooled quickly in cylindrical mould
A
radial
23
Q
what is the grain structure where wire is pulled through die
A
fibrous
24
Q
what are the different names for the different grain structures of crystal growth
A
- equi-axed
- radial
- fibrous
25
What happens to crystal growth when you cool it quickly
- more nuclei
| - small fine grains
26
what happens to crystal growth when you cool it slowly
- few nuclei
| - large coarse grains
27
what size of grains do we want and why
we want lots of small fine grains as large grains are weak
28
what are nucleating agents
impurities or additives act as foci for crystal growth
| - they help crystallising process
29
what are grains
each grain is a single crystal (lattice) with atoms orientated in given directions (dendrites)
30
what is a grain boundary
change in orientation of the crystal planes (impurities concentrate here)
31
why are small fine grains advantageous
- high elastic limit
| - increased ultimate tensile strength (UTS), hardness
32
disadvantage to small fine grains?
decreased ductility (less easily stretched)
33
What are the factors for rapid cooling (for small fine grains)
- small bulk
- heat metal/ alloy just above Tm
- mould- high thermal conduction
- quench
34
What happens when you apply a force to a crystal lattice which has a defect (most do)
If you apply a force to an individual crystal and the defect moves in a singular direction (slip) until you end up with grain looking a different shape
Defect goes to grain boundary, the only way to remove the defect is to change the shape of the lattice structure
35
What are dislocations
dislocations are imperfections/defects in the crystal lattice
36
What is slip and what is it due to
SLIP is when a dislocation/defect moves along the grain structure following a force. It's due to the propagation of dislocations and involves rupture of only a few bonds at a time (doesn't need to be a big force)
37
What would be the effects of impeding the movement of dislocations
Increases
- elastic limit
- UTS
- hardness
Decreases
- ductility
- impact resistance
38
What are factors which impede dislocation movement
- grain boundaries
- alloys (different atom sizes)
- cold working (dislocations build up at grain boundaries)
39
How is cold working done and what does it cause
- bending, rolling, swaging
- done at low temperature
- causes SLIP (so dislocations collect at grain boundaries)
- hence, stronger, harder material
40
How does cold working modify the grain structure
Higher
- elastic limit
- UTS
- hardness
Lower
- ductility
- impact strength
- lower corrosion resistance (negative)
41
What is the effect of cold working on strength, residual stress and ductility
The more cold working you do, the more you push dislocations to the grain boundary, the stronger but the more you increase the residual stress in the lattice and reduce ductility
42
Why is increased residual stress bad
- causes instability in lattice
| - results in distortion over time (undesirable)
43
how is increased residual stress relieved
by annealing process
44
what is annealing
heating metal (or alloy) so that greater thermal vibrations allows migration of atoms (i.e. re-arrangement of atoms)
45
Why is stress relief annealing better(?) than cold working
Cold working results in internal stresses which may lead to distortion of appliance over time
Stress relief annealing eliminates stresses by allowing atoms to re-arrange within the grains. The grain structure and mechanical properties are unchanged. Final shaping by cold working possible
46
when does recrystallisation occur and what does it result in
1. occurs when metal/alloy heated causing
- new smaller equiaxed grains
- lower EL, UTS, hardness
- increased ductility
2. spoils benefits of cold work
3. allows further cold work
4. cold work/ recrystallisation repeated until correct shape obtained
47
What should the recrystallisation temperature be
- depends on amount of cold work
| - greater the amount of cold work the lower the recrystallisation temperature
48
What would an excessive temperature rise do to grain growth
large grains to replace smaller coarse grains yielding poorer mechanical properties --> careful when annealing
49
what are the dental appliance manipulation processes
- cold working
- stress relief annealing
- recrystallisation
50
what determines the properties of metals
- grain size
- whether there are dislocations
- how you shape it
51
what is an alloy
a combination (or mixture) or 2 or more metals, or metal(s) with a metalloid (Fe, C)
52
what is a solid solution
Two metals that form a lattice structure as they are soluble in one another (coexist in a common lattice)
53
Advantages to alloys
improved:
- mechanical properties (EL, UTS, hardness)
- corrosion resistance
- lower melting point that individual metal
54
dental uses of alloys
- steel - burs, instruments
- amalgam
- gold alloy - inlays, crowns, bridges, partial dentures, wires
- nickel chromium - crowns bridges, wires
etc
55
Define phase
physically distinct homogeneous structure (can have more than one component)
56
Define solution
homogenous mixture at an atomic scale
57
How many phases would grains composed of metal A only have
1 phase
58
How many phases would individual grains composed of metal A and B have
2 phases
59
How many phases would grains in a homogenous mixture have composed of metal A and B
1 phase (solid solution)
n.b. grains can be of varying size and shape
60
Are metals soluble or insoluble when molten
soluble
61
What forms can metal take on crystallisation
1. insoluble (no common lattice, 2 phases)
2. form intermetallic compound with specific chemical formulation (e.g. Ag3Sn)
3. be soluble and form a solid solution i.e. form common lattice... 3 types of solid solution
62
What are the 3 types of solid solution
1. substitutional (random and ordered)
| 2. interstitial
63
what is a substiutional solid solution
atoms of one metal replace the other metal in the crystal lattice/ grain
64
what metals form a random substitutional solid solution
metal atoms similar in size, valency, crystal structure e.g. AuCu
65
what is an ordered substitutional solid solution
metal atoms in regular lattice arrangement
66
what metals form an ordered substitutional solid solution
metal atoms similar in size, valency, crystal structure
67
what is an interstitial solid solution
smaller atoms located in spaces in lattice/grain structure of larger atom
68
what metals form an interstitial solid solution
atoms markedly different in size e.g. Fe-C
69
How does the cooling curve differ between pure metal and an alloy
Cooling curve of pure metal- crystallisation begins at start of plateau, once all has been crystallised temp drops
Of alloy:
- TL - crystallisation of alloy begins
- Not all alloy can crystallise at one time
- Crysatallisation continues over a drop in temp down to TS where crystallisation is complete (unlike pure metal)
- Then whole alloy temp drops
I.e:
- metal crystallises at one temperature
- alloy crystallises over a temperature range
70
What does TL and TS stand for
TL - liquidous
| TS - solidous
71
Know what a phase diagram looks like, what can you plot on it?
temp (y axis) vs alloy composition (x axis)
plot TL and TS for both
72
what does a phase diagram tell you
If go to temp far above TL you know that both metals are liquid
In the middle, part liquid part solid
Below TS, crystallisation complete for both metals
73
What is the liquidous (TL) line on a phase diagram
line representing the temperatures which different alloy compositions begin to crystallise
74
what is the solidus (TS) line on a phase diagram
line representing the temperatures which different alloy compositions have completely crystallised
75
what happens when you cool an alloy slowly
allows metal atoms to diffuse through lattice
- positive: ensures grain composition is homogenous
- negative: results in large grains
76
what happens when you cool an alloy rapidly
coring
77
what is coring
when you cool an alloy rapidly, the first grain to form will be about 80% A, as it cools, the next grains to form will be 75% A and so on
as you cool rapidly, you will have different compositions of A and B to give a concentration gradient
use tie line on phase diagram
78
how does coring produce positive and negative results
positive: small grains
negative: different percentages within the grains that you form, not desirable as more likely to corrode
79
how do you get rid of the disadvantage of coring
annealing as eliminates core structure
80
what does rapid cooling of molten alloy cause
prevents atoms diffusing through lattice, causes coring as composition varies throughout grain
81
what are the conditions for coring
- fast cooling of liquid state
| - liquidous and solidus must be separated and determines extent of coring e.g. AuPt
82
is coring desirable
no
83
how do we get small grains but avoid coring
homogenising anneal
84
how does a homogenising anneal work
once solid cored alloy formed, reheat to allow atoms to diffuse and so cause grain composition to become homogenous
n.b. keep below recrystallisation temperature otherwise grains altered
85
how do alloys improve mechanical properties
Alloys forming a SOLID SOLUTION and consisting of metals of different atomic size have a distorted grain structure
which IMPEDES dislocation movement and so improves mechanical properties (EL, UTS, hardness)
86
what happens when you have a defect in an otherwise 'perfect' metal compared to in an alloy and apply a force to them? and what does this mean in terms of fracture resistance
Metal - It moves away easily (little stress involved) until it settles at the grain boundary.
Alloy - we have atoms of different sizes. So, if we apply a force, it takes more force for the defect to end up at the grain boundary
This is why alloys are more fracture resistant than metals
87
what is order hardening
Alloys forming an ordered solid solution it will also impede dislocations and improve mechanical properties as it has a distorted grain structure
88
What is an eutectic alloy
two metals exist in separate grains
89
properties of eutectic alloys
- metals are soluble in liquid state
- metals are insoluble in solid state (2 phases)
- each metal forms physically distinct grains
- lowest melting point at eutectic composition: used for solder
- hard but brittle
- poor corrosion resistance
90
what determines the eutectic composition
where liquidus and solidus coincide i.e. where crystalliastion process occurs at a single temp, where grains of individual metals formed simulataneously
91
what is a non-eutectic composition
- excess metal crystallises first
- then liquid reaches eutectic composition
- and both metals crystallise (forming separate grains)
92
what is a partially soluble alloy
somewhere between solid solutions and eutectic (complicated phase diagram)
93
key things about partially soluble alloy's phase diagram
- solubility limit lines indicates that a range of compositions of Ag and Cu (corresponding to the horizontal section of the solidus (H1 to H2) ) ARE NOT POSSIBLE
- Hence molten alloy of composition Z DOES NOT cool rapidly to produce a 50:50 grain comprising Ag and Cu; instead grains of alpha and beta are formed
94
why are silver and copper important
Silver and copper are important cause one of the RPD alloys = type 4 gold. Have silver and copper present
95
what do you benefit from after annealing a partially soluble alloy
precipitation hardening
96
what is precipitation hardening
One of the metals is pushed to the grain boundary and properties are inhanced
97
generally, how do alloys compare to metals in terms of mechanical properties
Alloys have better mechanical properties than metals e.g. fracture strength, rigidity, elastic limit, surface hardness due to:
SOLUTION, ORDER & PRECIPITATION HARDENING
BUT
CORED STRUCTURE MUST BE REMOVED BY ANNEALING
