metals and alloys 2

Question 1

what are the advantages of alloys over metals?

Answer 1

improved mechanical properties
-EL, FS, hardness
corrosion resistance
lower melting point

Question 2

what are dental uses of alloys?

Answer 2

steel- burs, instruments
gold- inlays, crowns, bridges, RPDs, wires
CoCr- partial dentures
nickel-titanium- wires
nickel chromium- crowns, bridges, wires
amalgam -restorative

Question 3

what is a phase?

Answer 3

physically distinct homogeneous structure

Question 4

what is a solution?

Answer 4

homogenous mixture at an atomic scale

Question 5

describe the solubility of molten metals?

Answer 5

any 2 metals are soluble when molten

Question 6

what state can 2 metals be on crystallisation?

Answer 6

insoluble- no common lattice- 2 phases
intermetallic compound with specific chemical formulation
soluble= form solid solution, common lattice

Question 7

how many phases if grains of metal A only?

Answer 7

one phase

Question 8

how many phases if individual grains of metal A & metal B in lattice?

Answer 8

2 phases

as grains are insoluble

Question 9

how many phases if metals A & B in homogeneous mix?

Answer 9

one phase

co-existing in same lattice

Question 10

what are the 2 types of solid solution?

Answer 10

substitutional

interstitial

Question 11

what is a substitutional solution?

Answer 11

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

Question 12

what are the 2 types of substitutional solution?

Answer 12

random

ordered

Question 13

describe a random substitutional solution

Answer 13

metal atoms similar in size, valency, crystal structure
can substitute randomly
in lattice arrangement with no order- random

Question 14

describe an ordered substitutional solution

Answer 14

metal atoms in regular lattice arrangement
similar valency, crystal structure, size
ordered- predict next atom in lattice

Question 15

what is an interstitial solution?

Answer 15

atoms markedly different in size
smaller atoms located in spaces in lattice/grain structure of larger atom
-small atoms have no specific location
-squeezed into arrangement of ordered larger atoms

Question 16

what happens at the plateau of cooling metal?

Answer 16

crystallisation happens

when crystallisation ends temp will resume cooling

Question 17

how does crystallisation happen when cooling an alloy?

Answer 17

temp reaches set temp
continues cooling at slower pace
first few atoms cooled sufficiently to form individual nuclei of crystallisation-grows
-2 metals w/ dif melting points so temp range not point

Question 18

what does the liquidus line on a phase diagram represent?

Answer 18

temp where different alloy compositions begin to crystallise- above line = liquid

Question 19

what does the solidus line on a phase diagram represent?

Answer 19

temp at which dif alloy compositions have completely crystallised

Question 20

what does slow cooling of molten alloy achieve?

Answer 20

metal atoms diffuse through lattice
ensure grain composition is homogeneous
results in large grains
-only small grains can be used in cold working

Question 21

what is coring?

Answer 21

rapid cooling
ensure small grains
results in 4 layers of different concentration

Question 22

what are the conditions for coring?

Answer 22

fast cooling of liquid state

liquidus and solidus must be separated and determines extend of coring

Question 23

what is a negative consequence of coring?

Answer 23

may reduce corrosion resistance of the solid form of alloy

Question 24

what is homogenising anneal?

Answer 24

once solid cored alloy formed
reheat to allow atoms to diffuse and so cause grain composition to become homogeneous
-has to stay below recrystallisation temp otherwise grains altered

Question 25

what is solution hardening?

Answer 25

alloys forming a solid solution and consisting of metals of dif atomic size have a distorted grain structure
impedes dislocation movement so improved mechanical properties
-EL, UTS, hardness

Question 26

describe slip in an alloy with metal atoms samesize

Answer 26

stress/force from LHS applied
defect undergoes slip and moves along lattice plane
needs to get around adjacent metal atom
defect slides along along lattice plane until reaches grain boundary
little force needed- rolls over atoms

Question 27

describe slip in an alloy with a defect

Answer 27

metals dif size
distorts regular lattice array
for defect to undergo slip, needs to get round adjacent atom-climbs
settles between small & large atoms
more energy needed to overcome next atom etc. until reaches grain boundary

Question 28

why is requiring more stress to move dislocations an advantage?

Answer 28

makes alloy more fracture resistant (strong) than metals

Question 29

what is order hardening?

Answer 29

alloys forming ordered solid solution have distorted grain structure
impedes dislocation movement so improved mechanical properties

Question 30

what are the eutectic alloys?

Answer 30

metals soluble in liquid state
metals insoluble in solid state
each metal forms physically distinct grains

Question 31

what are the properties of eutectic alloys?

Answer 31

lower melting point
hard but brittle
poor corrosion resistance

Question 32

what is eutectic composition?

Answer 32

where liquidus and solidus coincide
-crystallisation process occurs at single temp
where grains of individual metals formed simultaneously

Question 33

what is non-eutectic composition?

Answer 33

excess metal crystallises first
then liquid reaches eutectic composition
both metals crystallise forming separate grains

Question 34

when does precipitation hardening occur?

Answer 34

in partially soluble alloys on annealing, a supersaturated alloy undergoes precipitation hardening

Question 35

what is the result of precipitation hardening?

Answer 35

stronger & surface harder