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BDS2 - DMS > Metals and Alloys 1 > Flashcards

Metals and Alloys 1 Flashcards

1
Q

Metals and alloys are widely used in dentistry. Give examples of this? (5 points)

A
  • Partial denture framework (CoCr)
  • Crowns (stainless steel)
  • Denture base (Stainless steel)
  • Orthodontic appliance (NiTi)
  • Restorations (amalgam)
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2
Q

Why are metals used in dentistry?

A
  • They have superior mechanical properties e.g. strength, rigidity etc
  • BUT poor aesthetics
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3
Q

What is the definition of a metal?

A

Aggregate of atoms in crystalline structure

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4
Q

What is the definition of an alloy?

A

Combination of metal atoms in a crystalline structure

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5
Q

What are the building blocks of alloys?

A

Metals

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6
Q

What is meant by the term malleability?

A

Able to be hammered or pressed into shape without breaking or cracking

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7
Q

What is meant by the term ‘ductility’?

A

The ability of a metal to be easily bent or stretched

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8
Q

What is the elastic limit?

A

The maximum stress without plastic deformation

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9
Q

What is plastic limit?

A

A process in which enough stress is placed on metal or plastic to cause the object to change its size or shape in a way that is not reversible

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10
Q

What is the definition of ductility in relation to plastic deformation?

A
  • Amount of plastic deformation prior to fracture

e. g. measure of the extent that a material can be shaped/manipulated

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11
Q

What does the crystalline structure of metals depend on?

A
  • History (method of production)

- Shaping (crucial for dental applications (cold working, swaging)

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12
Q

What are the 3 factors that affect the mechanical propertied of metals?

A
  • Crystalline structure
  • Grain size (single crystal)
  • Grain imperfections
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13
Q

What are 3 examples of some simple CRYSTAL or LATTICE structures?

A
  • Cubic
  • Face-centred cubic
  • Body centred cubic
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14
Q

What is the process of crystal growth? (3 points)

A
  • Atoms act as nuclei of crystallisation
  • Crystals grow to form dendrites
  • Crystals (or grains) grow until they impinge on other crystals
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15
Q

What is the region where grains make contact with each other called?

A

The GRAIN BOUNDARY

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16
Q

If crystal growth is of equal dimension in each direction, what do we call the grains?

A

Equi-axed grains

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17
Q

How is a radical grain structure produced?

A

Molten metalal is cooled quickly in cylindrical mould

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18
Q

How is a fibrous grain produced?

A

Wire pulled through die (cold worked metal/alloy)

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19
Q

What type of crystals does fast cooling (quenching) cause? (2 points)

A
  • more nuclei

- small fine grains

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20
Q

What type of crystals does slow cooling produce? (2 points)

A
  • Few nuclei

- Large coarse grains

21
Q

What are nucleating agents?

A

Impurities or additives that act as foci for crystal growth

22
Q

What is a grain?

A

Each grain is a single crystal (lattice) with atoms orientated in given directions (dendrites)

23
Q

What is a grain boundary?

A
  • Change in orientation of the crystal planes

- (impurities concentrate here)

24
Q

What are the advantages of small, fine grains? (3 points)

A
  • High elastic limit

- Increased UTS (fracture stress), hardness

25
Q

What is a disadvantage of small, fine grains?

A

Decreased ductility

26
Q

What are factors for rapid cooling (quenching)? (4 points)

A
  • Small bulk
  • Heat metal/alloy just about Tm
  • Mould - high thermal conduction
  • Quench (take bucket of molten metal and put it in a contained of cold water)
27
Q

What does a defect in the lattice represent?

A

A misalignment of the atoms in the lattice network

28
Q

What is another name for defect?

A

Dislocation

29
Q

If you apply a force to a defect what happens?

A

You have pushed the defect to the grain boundary so they grain is no longer there and you have removed the defect

30
Q

What are dislocations?

A

Imperfections/defects in the crystal lattice

31
Q

What is slip due to in relation to dislocations?

A
  • Due to propagation of dislocations and involves rupture of only a few bonds at a time
32
Q

What does impede movement of dislocations increase? (3 points)

A
  • Elastic limit
  • UTS
  • Hardness
33
Q

What dies impede movements of dislocations decrease? (2 points)

A
  • Ductility

- Impact resistance (so if drop it would be more likely to break)

34
Q

What are 3 factors that can impede dislocation movement?

A
  • Grain boundaries
  • Alloys: different atom sizes
  • Cold working - dislocations build up at grain boundaries
35
Q

What is cold working?

A
  • Dislocations build up at grain boundaries

- Pushing all of the defects to the grain boundary - take a metal and hammer it

36
Q

What is cold working?

A
  • Work done on metal/alloy
  • Done at low temperatures (below recrystallisation temperature)
  • Causes slip - so dislocations collect at grain boundaries
  • Hence stronger, harder material
37
Q

What is another term for cold work?

A

Work or strain hardening

38
Q

Cold work modifies grain structure. What does it increase?

A
  • Elastic limit
  • UTS (ultimate tensile strength)
  • Hardness
39
Q

Cold work modifies grain structure. What does it decrease? (3 points)

A
  • Ductility
  • Impact strength
  • Lower corrosion resistance
40
Q

Residual stress causes instability in the lattice which results in distortion over time. This is undesirable, what can it be relived by ?

A

Can be relieved by the annealing process

41
Q

What is annealing?

A

Heating a metal or alloy so that greater thermal vibrations allow migration of atoms (i.e. re-arrangement of atoms)

42
Q

What can cold work lead to in the metal?

A

Can result in internal stresses - may lead to distortion of the appliance over time

43
Q

What does stress relief annealing do?

A
  • Eliminates stresses by allowing atoms to re-arrange within grains
  • Grain structure and mechanical properties unchanged
  • Some further cold work possible (final shaping)
44
Q

Recrystallisation occurs when a metal/alloy is heated. What can this cause? (3 points)

A
  • New smaller equiaxed grains
  • Lower EL, UTS, hardness
  • Increased ductility
45
Q

What does recrystallisation do in relation to cold work? (3 points)

A
  • Spoils benefits of cold work
  • Allows further cold work
  • Cold work/recrystallisation repeated until correct shape obtained
46
Q

What does recrystallisation temperature depend on?

A
  • Depends on about of cold work

- Greater the amount of cold work the lower the recrystallisation temperature

47
Q

What does excessive temperature rise cause in relation to grain growth?

A
  • Large grains to replace smaller coarse grains yielding poorer mechanical properties -> careful when annealing
48
Q

What is an alloy?

A

A co,mbination (or mixture) of two or more metals OR metal(s) with a metalloid (Fe, C)

BDS2 - DMS (14 decks)

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