Metals and Alloys 1 Flashcards Preview

BDS2 Dental Material Science > Metals and Alloys 1 > Flashcards

Flashcards in Metals and Alloys 1 Deck (38)
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1
Q

what are metals (& alloys) used for in dentistry?

A
  • partial denture framework
  • crowns
  • denture base
  • orthodontic appliance
  • restorations
2
Q

why are metals used?

A

they have superior mechanical properties:

  • strength
  • rigidity
  • elastic limit
  • ductility
  • hardness
3
Q

what is meant by good ductility?

A

shape is able to be adjusted chairside

4
Q

what is a negative of metals?

A

poor aesthetics

5
Q

what is the definition of a metal?

A

aggregate of atoms in a crystalline structure

6
Q

what is meant by an alloy?

A

combination of metal atoms in a crystalline structure

7
Q

what metal factors affect its mechanical properties?

A
  • crystalline structure
  • grain size
  • grain imperfections
8
Q

what type of structures can metals take?

A
  • crystal

- lattice

9
Q

when a PURE METAL cools, what occurs (in terms of the cooling curve)?

A
  • molten
  • temperature drops to melting point
  • atoms begin to crystallise (straight line on graph)
  • all atoms are crystallised
  • solid metal cools to room temp
10
Q

explain the growth of metal crystals?

A
  • atoms act as nuclei of crystallisation
  • crystals grow to form dendrites
  • crystals (or grains) grow until they impinge on other crystals
  • region where grains make contact is called GRAIN BOUNDARY
11
Q

what is meant by equi-axed grains?

A

occurs when crystal growth is of equal dimension in each direct

12
Q

when would a radical grain structure occur?

A

molten metal cooled quickly in cylindrical mould

13
Q

when would a fibrous grain structure occur?

A

wire pulled through die

14
Q

in terms of crystal growth, what happens when the metal undergoes fast cooling (quenching)?

A
  • more nuclei

- small fine grains

15
Q

in terms of crystal growth, what occurs when a metal undergoes slow cooling?

A
  • few nuclei

- large coarse grains

16
Q

what are some nucleating agents?

A

impurities or additives act as foci for crystal growth

more impurities = more small grains

17
Q

what is meant by a ‘grain’ ?

A

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

18
Q

what is meant by a ‘grain boundary’?

A
  • where grains collect with one another

- impurities concentrate here

19
Q

what are some advantages of small grains?

A
  • high elastic limit (deformation requires a large stress)

- increased FS & hardness

20
Q

what is a disadvantage of small fine grains?

A

decreased ductility

21
Q

what is meant by the dislocation area of a lattice?

A

where the lattice structure is most likely to fracture

22
Q

what occurs if a force is applied to the dislocation area?

A

the forces moves the dislocation along the lattice until it reaches the grain boundary

23
Q

what is the propagation of dislocations through a lattice known as?

A

slip

24
Q

what are dislocations?

A

imperfections/defects in the crystal lattice

25
Q

what role do grain boundaries play in the movement of dislocations? why is this important?

A
  • grain boundaries stop the propagation of defects spreading to another grain
  • therefore in metals/alloys made up of many fine grains… the movement of dislocations are impeded leading to SPECIFIC PROPERTIES
26
Q

what specific properties does impeding movement of dislocations in metals/alloys cause?

A

INCREASES:

  • elastic limit
  • fracture stress
  • hardness

DECREASES:

  • ductility
  • impact resistance
27
Q

what is meant by cold work?

A
  • applying forces to metals/alloys alongside a LOW TEMP
28
Q

what is the purpose of cold work?

A
  • causes SLIP
  • therefor dislocations collect at grain boundaries
  • hence stronger, harder material
29
Q

what is a negative aspect of cold work?

A
  • lower ductility
  • lower impact strength
  • lower corrosion resistance**
30
Q

what is a residual stress?

A

stresses that form within the metal/alloy itself (UNDESIRABLE)

31
Q

what does residual stress lead to?

A
  • instability in lattice

- distortion over time

32
Q

how do residual stresses occur?

A

residual stresses increase as cold work increases

33
Q

how can residual stresses be relieved?

A

ANNEALING PROCESS

34
Q

what is meant by annealing?

A

heating metal (or alloy) so that greater thermal vibrations allow migration of atoms (any instability in the lattice is reversed)

35
Q

does annealing alter the grain structure and mechanical properties?

A

NO

36
Q

what occurs when metal/alloy is heated?

A

recrystallisation of new smaller equiaxed grains

37
Q

the greater the amount of cold work….

A

the lower the recrystallisation temperature

38
Q

what would occur if there was an excessive temperature rise of a metal?

A

large grains replace the smaller coarse grains yielding poorer mechanical properties