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BDS2 Dental Materials Sciences > metals and alloys 1 > Flashcards

metals and alloys 1 Flashcards

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

A

wire pulled through die (narrow circular aperture)

- (cold worked metal/alloy)

23
Q

what is the term for fast cooling a metal

A

quenching

24
Q

benefits of fast cooling a metal

A

more nuclei of crystallisation
- crucial

small fine grains
- mechanical properties enhanced

25
Q

disadvantages of slow cooling of metal

A

Few nuclei

Large coarse grains
- Weak

26
Q

what are nucleating agents

A

Impurities or additives act as foci for crystal growth

So help the crystallisation process - control grain size

E.g. steel industry

27
Q

what is a structure of grain

A

single crystal (lattice) with atoms orientated in given directions (dendrites)

28
Q

what is the grain boundary

A

where individual grains make contact

change in orientation of the crystal planes
- (impurities concentrate here)

29
Q

3 advantages of small fine grains

A

High elastic limit

Increased UTS (fracture stress)

Increase surface hardness

all desirable for dental prostheses
- small grains are best

30
Q

disadvantage of small grains

A

decreased ductility

- less easily stretched while avoiding fracture

31
Q

4 factors for rapid cooling (quenching)

A

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
Q

what are dislocation/defects in crystalline structures

A

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
Q

how to correct a dislocation/defect in crystalline structure

A

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
Q

what is metal slip

A

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
Q

what does impeding movement of dislocations in metal atoms cause an increase in

A

elastic limit

UTS

suface hardness

36
Q

what does impeding movement of dislocations in metal atoms cause a decrease in

A

ductility

impact resistance (drop more likely to break)

37
Q

what is an advantage of many grain boundaries (small fine grains)

A

many places to slip defect out

38
Q

how do alloys impede dislocation movement

A

alloy with different metal atoms , of different sizes (obviously) HAS an INHERENT resistance to the movement of dislocations within a grain.

39
Q

how does cold working impede dislocation movement

A

dislocations build up at grain boundaries

Equivalent of pushing all defects to grain boundary
- Improve mechanical properties of the object

40
Q

what is cold work/work hardening/ strain hardening

A

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
Q

2 other names for cold work

A

work hardening

strain hardening

42
Q

what does cold working lead to increase of (3)

A

elastic limit

UTS

hardness

43
Q

what does cold working lead to decrease of (4)

A

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
Q

purpose of annealing

A

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
Q

what does cold work result in the metal structure

A

internal stresses

may lead to distortion of appliance over time

46
Q

what is stress relief annealing

A

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
Q

2 other names for stress relief annealing

A

annealing

homogenisation

48
Q

why would you recrystallise a metal

A

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
Q

what happens to the recrystallisation temperature

A

decreases with the greater amount of cold work

- melting point decreases

50
Q

what can cause grain growth

A

excessive temperature rise causes

large grains to replace smaller coarse grains
- yielding poorer mechanical properties

CAUTION WHEN ANNEALING

51
Q

what 2 things influence and affect grain stucture

A

influenced by mechanical properties

affected by dislocations (SLIP)

52
Q

what are 3 ways dentists can manipulate metal properties

A

cold working

stress relief annealing

recrystallisation

53
Q

definition of an alloy

A 
a combination (or mixture) of two or more metalsor metal(s) with metalloid 
- Fe (iron), C (carbon
54
Q

what is the term used to describe an alloy structure

A

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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