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Dental Materials Science BDS2 > Alloys > Flashcards

Alloys Flashcards

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1
Q

what is an alloy?

A

a combination of two or more metals or a metal with a metalloid

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2
Q

what are the advantages of an alloy?

A

improved properties (mechanical, corrosion resistance, lower melting point)

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3
Q

what are the uses of alloys?

A

burs, instruments, inlays, crowns, partial dentures, wires, bridges, restorative materials

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4
Q

what does phase mean?

A

physically distinct homogenous structure

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5
Q

what is solution

A

homogenous mixture at an atomic scale

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6
Q

what does it mean if a metal has one phase?

A

it is consisted of only one type of metal

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7
Q

what does it mean if an alloy has two phases?

A

individual grains of metal A and B situated in lattice network

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8
Q

what does it mean if an alloy has one phase?

A

metals A and B are in a homogenous mixture

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9
Q

what happens to metals when they are crystallising?

A

they can be insoluble and exit as 2 phases, or they can form an intermetallic compound with a specific chemical formulation or be soluble and form a solid solution

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10
Q

how many types of solid solution are there?

A

3

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11
Q

what are the 3 types of solid solution

A

random substitutional, ordered substiutional and interstitial

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12
Q

what do metals do in a substitutional solution?

A

atoms of one metal replace the other metal in the crystal lattice/grain

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13
Q

what are metals in a random substituional solid solution like?

A

they are similar in size, valency, crystal structure

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14
Q

what is the lattice like with ordered substiutional solutions?

A

it is regular lattice arrangement with one row of metal A and another row of metal B

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15
Q

what are the atoms like in an interstitial lattice?

A

atoms are markedly different in size, smaller atoms are located in spaces of the lattice/grain structure of larger atoms

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16
Q

what is the difference in the cooling curve between alloys and metals?

A

with metals, there is no change in temperature during the crystallisation period, with alloys there is a change in temperature over the crystallisation period

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17
Q

if a metal is soluble what type of solution is formed?

A

solid solution (homogenous mixture of metals in each grain)

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18
Q

if a metal is insoluble what is formed?

A

grains of individual metals formed

19
Q

in the phase diagram what is the liquidus?

A

line representing the temperature which different alloy compositions begin to crystallise

20
Q

in the phase diagram what is the solidus?

A

line representing the temperature at which different alloy compositions have completely crystallised

21
Q

what does slow cooling of molten alloys do?

A

allows metal atoms to diffuse through lattice and ensures grain composition is homogenous but has larger grains

22
Q

what is coring

A

when the alloy is cooled in layers and it is rapid

23
Q

what does rapid cooling of molten alloys do?

A

prevent atoms diffusing through lattice, cause coring, composition of metals varies throughout grain

24
Q

what are the necessary coring conditions?

A

fast cooling of liquid state, liquidus and solidus must be separated and determines extent of coring

25
Q

what can coring do to corrosion resistance?

A

reduce it

26
Q

what does fast cooling do?

A

generates many small grains which impedes dislocation movement and improves mechanical properties, causes coring which is undesirable

27
Q

what is homogenous annealing

A

reheating an alloy to allow atoms to diffuse and so cause grain composition to become homogenous

28
Q

what sort of grain structure do alloys forming a solid solution and consisting of different atomic size have?

A

distorted which impedes dislocation movement and so improves mechanical properties

29
Q

what happens in a metal lattice with a defect?

A

the defect goes up and around the other metal atoms and slides along until it reaches the grain boundary, little energy is needed for it to move along the plane

30
Q

what happens in an alloy lattice with a defect?

A

the defect goes up and over each atom but this is repeated for each atom there is instead of the defect rolling over al of the atoms so more energy is needed and hence greater stress to move dislocations

31
Q

what happens once an alloy is cooled to room temperature

A

it needs to be annealed to remove coring

32
Q

with eutectic alloys what are the grains like?

A

physically distinct

33
Q

what is the eutectic composition?

A

where liquidus and solidus coincide (where crystallisation process occurs at a single temperature) and where grains of individual metals are formed simultaneously

34
Q

what are the advantages of eutectic alloys?

A

lowest melting point, hard, poor corrosion resistance

35
Q

what occurs during crystallisation with non-eutectic composition

A

excess metal crystallises first, then liquid reaches eutectic composition and both metals crystallise

36
Q

give an example of a solid solution

A

gold platinum

37
Q

give an example of a partially solid solubility

A

silver copper

38
Q

what is the phase diagram like for partially solid solubility?

A

there are dashed lines of H1 and H2 (solubility limit lines)

39
Q

what do solubility limit lines mean?

A

that alloys of silver copper cannot form grains with composition between H1 and H2

40
Q

what are the two solutions formed on a partially solid solubility phase diagram for AgCu

A

one solid solution which is mostly Ag and another solid solution which is mostly Cu

41
Q

what does a solubility limit line indicate?

A

that a range of compositions of Ag and Cu are not possible

42
Q

what happens to partially soluble alloys during annealing?

A

precipitation hardening

43
Q

what does precipitation hardening do?

A

makes the alloy stronger and surface harder

44
Q

why do alloys have better mechanical properties than metals?

A

due to solution, order and precipitation hardening

Dental Materials Science BDS2 (15 decks)

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