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Dental Materials Science BDS2 > Metals > Flashcards

Flashcards in Metals Deck (39)
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1
Q

what is a metal?

A

an aggregate of atoms in a crystalline structure

2
Q

what is an alloy?

A

contribution of two or more types of metal atoms in a crystalline structure

3
Q

what does it mean if the stress-strain curve has a large gradient?

A

it is rigid

4
Q

what does large stress give?

A

small strain

5
Q

what is ductility?

A

amount of plastic deformation prior to fracture

6
Q

what does the crystalline structure depend on?

A

method of production and shaping

7
Q

what factors affect mechanical properties?

A

crystalline structure, grain size and grain imperfections

8
Q

what are the different lattice structures?

A

cubic, face-centred, body centred

9
Q

describe the cooling curve of pure metal

A

molten form flows readily, as temperature drops some metal atoms change in state to solid, temperature then remains constant with more atoms crystallising. When all atoms have crystallised the temperature drops to room temperature

10
Q

what are nuclei of crystallisation?

A

crystals which grow as dendrites and continue to grow until they come into contact with each other

11
Q

what does it mean if grains are equi-axed?

A

crystal growth is of equal dimension in each direction

12
Q

apart from equi-axed grains, what other grain structures are available

A

radial, fibrous

13
Q

if a material is fast cooled (quenched) what are the grains like?

A

more nuclei, small fine grains

14
Q

if a material is slow cooled what are the grains like?

A

few nuclei, large coarse grains

15
Q

what are nucleating agents?

A

impurities or additives which ac as foci for crystal growth

16
Q

where do grain boundaries occur?

A

where there is a change in orientation of the crystal planes

17
Q

what are the advantages of small fine grains?

A

high elastic limit and increased FS, UTS, hardness

18
Q

what is the disadvantage of small fine grains?

A

decreased ductility

19
Q

what are factors needed for rapid cooling

A

small bulk, heat metal just above Tm, mould - high thermal conduction, quench

20
Q

what happens if there is a dislocation within the lattice?

A

it will fracture unless the dislocation is pushed along to the end of the lattice

21
Q

what are dislocations?

A

imperfections/defects in the crystal lattice

22
Q

what is SLIP due to?

A

propagation of dislocations and involves rupture of only a few bonds at a time

23
Q

how can you impede the movement of dislocations in metals and alloys?

A

have grain boundaries to prevent dislocation moving from one grain to another, have smaller finer grains

24
Q

what is the advantage of impeding dislocations?

A

metal/alloy has increased Fs, El and Sh

25
Q

what is cold working?

A

work done on a metal/alloy at a low temperature (below recrystallisation temperature) causing SLIP and making the material stronger and harder

26
Q

what does cold working increase?

A

El, Fs and Sh

27
Q

what does cold working decrease?

A

ductility, impact strength and corrosion resistance

28
Q

what is residual stress

A

stresses developed in the metal itself where atoms become oriented out of their equilibrial position in the grain/lattice

29
Q

what does residual stress cause?

A

instability in metal lattice, distortion of metal overtime

30
Q

what is residual stress relieved by?

A

subjecting metal or alloy to annealing

31
Q

what is annealing?

A

heating metal so that greater thermal vibrations allows migration of atoms

32
Q

what does annealing do?

A

eliminates stresses by allowing atoms to re-arrange within grains

33
Q

when does recrystallisation happen?

A

when metal/alloy is heated

34
Q

what does recrystallisation do?

A

allow new smaller equiaxed grains, lower El, UTS, hardness, increased ductility

35
Q

what is the relationship between recrystallisation and cold working?

A

recrystallisation spoils the benefits of cold work but allows further cold work

36
Q

what is the recrystallisation temperature dependent on?

A

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

37
Q

what does excessive temperature rise cause?

A

large grains to replace smaller coarse grains which yields poorer mechanical properties

38
Q

what are the metal/alloy manipulation processes

A

quenching, cold working, stress relief annealing, recrystallisation

39
Q

what is grain structure affected by?

A

dislocations, processing methods