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BDS2 Dental Materials Sciences > metals and alloys 1 > Flashcards

metals and alloys 1 Flashcards

(54 cards)

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1
Q

5 examples of metal uses in dentistry

A

Partial denture framework (CoCr, Type IV gold)

  • 3D object has to fit a number of surfaces neatly
  • Fitted in readily without causing pain for pt on regular basis

Crowns (stainless steel)

Denture base (stainless steel)

Orthodontic appliance (NiTi and others)

  • Lots of wires, Some with twists
  • Brackets
  • Forces are exerted to re-position individual teeth

Restorations (amalgam)

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2
Q

what is unique about amalgam

A

Undergoes a setting reaction

Most alloys do not undergo a setting reaction

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3
Q

metal

A

aggregate of atoms in a crystalline structure

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4
Q

alloy

A

combination of metal atoms in a crystalline structure

- 2 or more metals (sometimes up to 5)

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5
Q

what are the building blocks of alloys

A

metals

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6
Q

increase stress leads to

A

increased strain

then change in shape (deformation)

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7
Q

stress-strain curve

A

unique to material

show the about of stress needed before a permanent deformation (fracture) in incurred

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8
Q

malleability

A

Ability to deform under pressure

- Compressive strength

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9
Q

elastic limit EL

A

maximum stress without plastic deformation

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10
Q

ductility

A

Amount of plastic deformation prior to fracture (hammered, stretched before fracture)

i.e. measure of the extent that a material can be shaped/manipulated
= (y-x)%

Tensile strength

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11
Q

what are 2 factors for crystalline structure

A

history - method of production
dictates crystalline structure – alike gypsum

shaping - crucial for dental applications
e.g. cold working, swaging

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12
Q

grain

A

single crystal

lattice arrangement of one (metal) or more (alloy) types of atom

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13
Q

what are the 3 factors affecting mechanical properties of a metal

A

crystalline structure

grain size

grain imperfections

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14
Q

3 simple crystal/lattice structures of metals

A

cubic

face centred cubic

body centred cubic

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15
Q

what is true about all metals in a lattice

A

metals have atoms positioned at regularly arranged sites, whether in a cubic, BCC or FCC or other configuration

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16
Q

stages of crystal growth

A

first atoms cooling to form solid

  • Atoms at these sits act as nuclei of crystallisation
  • Other atoms cool around these nuclei

Crystals grow to form dendrites
- 3D branched lattice network

Crystals (or grains) grow until they impinge on other crystals
- stop the other one from growing any further in that direction/area/volume

Region where grains make contact is called Grain Boundary

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17
Q

describe the cooling curve for a pure metal

A

molten Metal atom in container
- Allow to cool

Gradually cooling down to plateau

Liquid to solid
- Takes time

Maintains till all from liquid to solid
- Tails off at the end

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18
Q

how can grains of metal differ in shape and size

A

can vary in shape and size depending on processing and cooling
- effect structure and properties of alloys made

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19
Q

3 types of crystal growth

A

equi-axed

radial

fibrous

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20
Q

equi-axed crystal growth

A

crystal growth of equal dimension in each direction

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21
Q

radial crystal growth

A

molten metal cooled quickly in cylindrical mould

- burst out from centre to diameter of cylider

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22
Q

fibrous crystal growth

Study These Flashcards
A

wire pulled through die (narrow circular aperture)

- (cold worked metal/alloy)

23
Q

what is the term for fast cooling a metal

24
Q

benefits of fast cooling a metal

Study These Flashcards
A

more nuclei of crystallisation
- crucial

small fine grains
- mechanical properties enhanced

25
disadvantages of slow cooling of metal
Few nuclei Large coarse grains - Weak
26
what are nucleating agents
Impurities or additives act as foci for crystal growth So help the crystallisation process - control grain size E.g. steel industry
27
what is a structure of grain
single crystal (lattice) with atoms orientated in given directions (dendrites)
28
what is the grain boundary
where individual grains make contact change in orientation of the crystal planes - (impurities concentrate here)
29
3 advantages of small fine grains
High elastic limit Increased UTS (fracture stress) Increase surface hardness all desirable for dental prostheses - small grains are best
30
disadvantage of small grains
decreased ductility | - less easily stretched while avoiding fracture
31
4 factors for rapid cooling (quenching)
small bulk heat metal/alloy just above Tm (melting point) mould - high thermal conduction - pull heat away from metal quickly quench - put in container of cold water - quickest way to cool down
32
what are dislocation/defects in crystalline structures
Representing misalignment of atoms in that lattice network - 3 rows of 6 To 3 rows of 5 Two planes have slight mismatch - Weaken grain and overall metal structure
33
how to correct a dislocation/defect in crystalline structure
Force to individual crystal - Defect moves/slides along the singular plane - Until grain changes shape Step change between top 3 rows and next 3 Push to grain boundary then Defect removed - Changes shape of the lattice structure
34
what is metal slip
is due to Propagation of Dislocations and involves rupture of only a few bonds at a time Forced from left to right side - Only need to break one bond at a time - Don’t need supreme large force to do Low level over long time to ripple through structure
35
what does impeding movement of dislocations in metal atoms cause an increase in
elastic limit UTS suface hardness
36
what does impeding movement of dislocations in metal atoms cause a decrease in
ductility impact resistance (drop more likely to break)
37
what is an advantage of many grain boundaries (small fine grains)
many places to slip defect out
38
how do alloys impede dislocation movement
alloy with different metal atoms , of different sizes (obviously) HAS an INHERENT resistance to the movement of dislocations within a grain.
39
how does cold working impede dislocation movement
dislocations build up at grain boundaries Equivalent of pushing all defects to grain boundary - Improve mechanical properties of the object
40
what is cold work/work hardening/ strain hardening
work done on metal/alloy (e.g. bending, rolling (steel), swaging (car), hammering (blacksmith with horseshoe)) done at LOW TEMPERATURE (i.e. below recrystallisation/ melting temperature) causes SLIP – so dislocations collect at grain boundaries - hence stronger, harder material
41
2 other names for cold work
work hardening strain hardening
42
what does cold working lead to increase of (3)
elastic limit UTS hardness
43
what does cold working lead to decrease of (4)
Ductility Impact strength Lower corrosion resistance - Not ideal for dental prostheses Increased residual strength - Causes instability in lattice reconfiguration of metal ions ------Results in distortion over time – undesirable Relieved by annealing process
44
purpose of annealing
heating metal (or alloy) so that greater thermal vibrations allows migration of atoms (i.e. re-arrangement of atoms) push atoms around, no more residual stress
45
what does cold work result in the metal structure
internal stresses may lead to distortion of appliance over time
46
what is stress relief annealing
eliminates stresses by allowing atoms to re-arrange within grains grain structure and mechanical properties unchanged - got to be carried out in a controlled and specific way so the grain is unaffected some further cold work possible (final shaping) - effects the properties of the metal or alloy
47
2 other names for stress relief annealing
annealing homogenisation
48
why would you recrystallise a metal
occurs when metal/alloy heated causing - new smaller equiaxed grains - lower EL, UTS, hardness - increased ductility spoils benefits of cold work (recycle if cold work hasn't had desired effect) allows further cold work - cold work/recrystallisation can be repeated until correct shape obtained - lose all grains structure and mechanical properties
49
what happens to the recrystallisation temperature
decreases with the greater amount of cold work | - melting point decreases
50
what can cause grain growth
excessive temperature rise causes large grains to replace smaller coarse grains - yielding poorer mechanical properties CAUTION WHEN ANNEALING
51
what 2 things influence and affect grain stucture
influenced by mechanical properties affected by dislocations (SLIP)
52
what are 3 ways dentists can manipulate metal properties
cold working stress relief annealing recrystallisation
53
definition of an alloy
``` a combination (or mixture) of two or more metalsor metal(s) with metalloid - Fe (iron), C (carbon ```
54
what is the term used to describe an alloy structure
2 metals form a common lattice structure - soluble in one another - form a solid solution

BDS2 Dental Materials Sciences (13 decks)

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