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Dental material science > Metals & Alloys > Flashcards

Metals & Alloys Flashcards

1
Q

Why are metals used in denstistry

A

Superior mechanical properties eg - strength
-rigidity
-hardness
-elastic limit
-ductility

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2
Q

What is a metal

A

aggregation of atoms in crystalline structure

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3
Q

What is an alloy

A

combination of metal atoms in a crystalline structure​

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4
Q

What are some simple crystal or lattice structures

A

Simple cubic
Face-centred cubic
Body centric cubic

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5
Q

What is the cooling curve of pure metal

A

Decreases gradually then stops for a while and then decreases once more

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6
Q

What is a grain boundary

A

The region where grains make contact

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7
Q

What are the types of grain structures

A

Equi-axed
-Crystal growth is of equal dimension in each direction

Radical
-molten metal cooled quickly in cylindrical mould

Fibrous
-wire pulled through die

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8
Q

What impact does cooling have on crystal growth

A

Quenching
-more nuclei
-small fine grains

Slow cooling
-few nuclei
-large coarse grains

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9
Q

Where do impurities concentrate

A

At the grain boundary

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10
Q

What are the advantages of small fine grains

A

High elastic limit
increased FS and UTS, hardness

decreased ductillity

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11
Q

What qualifies a metal for quenching

A

small bulk​

heat metal/alloy just above Tm​

mould - high thermal conduction​

quench

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12
Q

What are dislocations

A

imperfections/defects in the crystal lattice

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13
Q

What is SLIP

A

SLIP is due to Propagation of Dislocations and involves rupture of only a few bonds at a time

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14
Q

What does stopping movement of dislocations do

A

INCREASES​

elastic limit ​

fracture stress /UTS​

hardness​

​

DECREASES​

ductility​

impact resistance

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15
Q

What natural factors reduce dislocation movement

A

Grain boundaries (hence fine grains)​

Alloys: different atom sizes​

Cold working - dislocations stopped at grain boundaries

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16
Q

What are the effects of cold working

A

Causes SLIP
increase strength and residual stress
decreases ductillity

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17
Q

How is residual stress relieved

A

Annealing process

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18
Q

What are the effects of residual stress

A

Causes instability in lattice
Results in distortion over time​

Undesirable

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19
Q

What is annealing

A

heating metal (or alloy) so that greater thermal vibrations allows migration of atoms (ie re-arrangement of atoms)

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20
Q

What does recrystallisation cause and when does it occur

A

occurs when metal/alloy heated causing :-​
- new smaller equiaxed grains​
- lower EL, UTS, hardness​
- increased ductility

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21
Q

Why would recrytallisation be used

A

spoils benefits of cold work​

allows further cold work​

cold work/recrystallisation repeated until correct shape obtained

22
Q

What is an alloy

A

a combination (or mixture) of:
two or more metals ​

OR​

a metal(s) with a metalloid (Si, C)

23
Q

What is phase

A

physically distinct homogeneous structure

24
Q

Which are preferable metals or alloys

25
What are the advantages to alloys
Harder More resistant to corrosion Lower melting point than metal
26
What is a solution
Homogeneous mixture at an atomic scale
27
What can happen to an alloy of two metals on crystallisation
Forms one of three solid solutions a) be insoluble, ie no common lattice – exist as 2 phases OR b) form an intermetallic compound with a specific chemical formulation (eg Ag3Sn) OR c) be SOLUBLE and form a SOLID SOLUTION, ​ie form a common lattice… ​ NOTE: there are 3 types of solid solution.​
28
Types of solid solutions
Substitutional -ordered -random Interstitial -bigger and smaller atoms together
29
What is happening in the flat section of a cooling curve of a pure metal
Crystallisation
30
How do the cooling trends of a metal and alloy differ
Metal crystallises at one temp (flat section) Alloy crystallises over temp range
31
What is the liquidus line
line representing the temperatures which different alloy compositions begin to crystallise​
32
What does a solidus line show
line representing the temperatures which different alloy compositions have completely crystallised
33
What does slow cooling of a molten alloy do
Allows metal atoms to diffuse through lattice ​ ENSURES grain composition is homogeneous​ BUT this results in LARGE GRAINS​
34
What does rapid cooling of a molten alloy do
Causes coring prevents atoms diffusing through lattice as composition varies throughout grain.​
35
What conditions are required for coring to occur
fast cooling of liquid state;​ LIQUIDUS and SOLIDUS must be separated (see Phase Diagram) and determines extent of coring
36
What is the disadvantage of coring
Can reduce the corrosion reistance of the solid form alloy
37
What type of cooling is preferred
Fast cooling - as it generates many small grains which impede dislocation movement and improve mechanical properties HOWEVEr causes coring which can be resolved by homogenising anneal
38
How is coring resolved
Homogenising anneal
39
How is homogenising anneal carried out
once solid cored alloy formed​ REHEAT to allow atoms to diffuse and so cause grain composition to become homogeneous
40
If the temp used to homogenise anneal is above the recrystallisation temp what happens
Grains altered
41
How does a distorted grain structure improve mechanical properties
Impedes dislocation movement
42
What substances would have a distorted grain structure
Alloys forming an ordered solid solution (atoms at specific lattice sites) Alloys forming a solid solution and consisting of metals of different atomic size
43
What is the dislocation movement within a metal lattice
Defect “rolls” over the atoms in the lattice plane.​ Little energy/force is needed for defect to move along slip plane.​
44
What is the dislocation movement for a solid solution (different sized grains)
Defect does not “roll” over the lattice plane. ​ Instead it falls into the spaces between the large & small atom. ​ More energy/force is needed for the defect to overcome the different-sized atoms, and move along lattice to the grain boundary.​ Hence, it requires greater stress to move any dislocations present in a solid solution
45
What are eutectic alloys
Liquidus and solidus coincide (crystallisation process occurs at a single temp) metals are soluble in liquid state​ metals INSOLUBLE in solid state (so 2 PHASES)​ ie each metal forms physically distinct grains
46
What does binary eutectic indicate
Complete solid insolubility
47
What are the mechanical properties of eutectic alloys
lowest melting point - at eutectic composition: used for solder​ hard but brittle​ poor corrosion resistance
48
What is the summary of alloys
HAVE BETTER MECHANICAL PROPERTIES THAN METALS ​ (eg FRACTURE STRENGTH, RIGIDITY, ELASTIC LIMIT, SURFACE HARDNESS) DUE TO :- SOLUTION, ORDER & PRECIPITATION HARDENING​ BUT ​ CORED ALLOY STRUCTURE FORMED FOLLOWING QUENCHING MUST BE REMOVED BY AN ANNEALING PROCESS​ (OTHERWISE ALLOY IS MORE LIKELY TO CORRODE)
49
Are H1 to H2 compositions possible in partially solluble alloys
No - this is the solubility limit line the alloy formed here forms grains of both alpha or beta NOT both in a 50/50 ratio
50
When does precipitation hardening occur
In partially soluble alloys: on annealing, a supersaturated alloy will undergo PRECIPITATION HARDENING​

Dental material science (13 decks)

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