Alloys

Question 1

what is an alloy?

Answer 1

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

Question 2

what are the advantages of an alloy?

Answer 2

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

Question 3

what are the uses of alloys?

Answer 3

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

Question 4

what does phase mean?

Answer 4

physically distinct homogenous structure

Question 5

what is solution

Answer 5

homogenous mixture at an atomic scale

Question 6

what does it mean if a metal has one phase?

Answer 6

it is consisted of only one type of metal

Question 7

what does it mean if an alloy has two phases?

Answer 7

individual grains of metal A and B situated in lattice network

Question 8

what does it mean if an alloy has one phase?

Answer 8

metals A and B are in a homogenous mixture

Question 9

what happens to metals when they are crystallising?

Answer 9

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

Question 10

how many types of solid solution are there?

Answer 10

3

Question 11

what are the 3 types of solid solution

Answer 11

random substitutional, ordered substiutional and interstitial

Question 12

what do metals do in a substitutional solution?

Answer 12

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

Question 13

what are metals in a random substituional solid solution like?

Answer 13

they are similar in size, valency, crystal structure

Question 14

what is the lattice like with ordered substiutional solutions?

Answer 14

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

Question 15

what are the atoms like in an interstitial lattice?

Answer 15

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

Question 16

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

Answer 16

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

Question 17

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

Answer 17

solid solution (homogenous mixture of metals in each grain)

Question 18

if a metal is insoluble what is formed?

Answer 18

grains of individual metals formed

Question 19

in the phase diagram what is the liquidus?

Answer 19

line representing the temperature which different alloy compositions begin to crystallise

Question 20

in the phase diagram what is the solidus?

Answer 20

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

Question 21

what does slow cooling of molten alloys do?

Answer 21

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

Question 22

what is coring

Answer 22

when the alloy is cooled in layers and it is rapid

Question 23

what does rapid cooling of molten alloys do?

Answer 23

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

Question 24

what are the necessary coring conditions?

Answer 24

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

Question 25

what can coring do to corrosion resistance?

Answer 25

reduce it

Question 26

what does fast cooling do?

Answer 26

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

Question 27

what is homogenous annealing

Answer 27

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

Question 28

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

Answer 28

distorted which impedes dislocation movement and so improves mechanical properties

Question 29

what happens in a metal lattice with a defect?

Answer 29

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

Question 30

what happens in an alloy lattice with a defect?

Answer 30

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

Question 31

what happens once an alloy is cooled to room temperature

Answer 31

it needs to be annealed to remove coring

Question 32

with eutectic alloys what are the grains like?

Answer 32

physically distinct

Question 33

what is the eutectic composition?

Answer 33

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

Question 34

what are the advantages of eutectic alloys?

Answer 34

lowest melting point, hard, poor corrosion resistance

Question 35

what occurs during crystallisation with non-eutectic composition

Answer 35

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

Question 36

give an example of a solid solution

Answer 36

gold platinum

Question 37

give an example of a partially solid solubility

Answer 37

silver copper

Question 38

what is the phase diagram like for partially solid solubility?

Answer 38

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

Question 39

what do solubility limit lines mean?

Answer 39

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

Question 40

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

Answer 40

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

Question 41

what does a solubility limit line indicate?

Answer 41

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

Question 42

what happens to partially soluble alloys during annealing?

Answer 42

precipitation hardening

Question 43

what does precipitation hardening do?

Answer 43

makes the alloy stronger and surface harder

Question 44

why do alloys have better mechanical properties than metals?

Answer 44

due to solution, order and precipitation hardening