Metals and Alloys

Question 1

give 4 properties of metals

Answer 1

1. hard
2. closely packed regular crystalline structures
3. opaque
4. conductors of heat and electricity

Question 2

which metal is an exception to these properties?

Answer 2

mercury

Question 3

what is a binary alloy?

Answer 3

an alloy composed of two metals

Question 4

what is a tertiary alloy?

Answer 4

an alloy composed of 3 metals

Question 5

define an alloy system

Answer 5

all possible combinations of an alloy

Question 6

what occurs in the metals in an alloy when in molten state?

Answer 6

they display mutual solubility

Question 7

what 2 things occur when cooling from a molten metal?

Answer 7

• impurities enable metal atoms to deposit upon them

- then nucleate crystal growth

Question 8

what are crystals in metal called?

Answer 8

grains

Question 9

when does grain growth stop?

Answer 9

when all the melt metal is consumed

Question 10

define a grain boundary

Answer 10

when the grains grow and push on neighbours creating a source of tension

Question 11

what are the two types of nucleation of metal crystals (grains)?

Answer 11

homogeneous and heterogeneous

Question 12

what is homogenous nucleation? what are the 2 requirements of it?

Answer 12

nucleation of metal grains at a single site.

requires

• 4 metal atoms to come together
• specialised equipment to control crystal orientation

Question 13

what is heterogeneous nucleation?

Answer 13

where grain nucleation occurs at many sites.

• majority of dental applications
• happens more randomly

Question 14

does grain size influence the physical properties?

Answer 14

yes

Question 15

how is grain size controlled?

Answer 15

by the manufacturer

Question 16

what are the 3 ways that the manufacturers can influence grain size?

Answer 16

• rapid solidification
• provision of ‘extra’ nucleation sites
• both of the above (promotes small grain size)

Question 17

what other 2 factors affect grain size?

Answer 17

1. the temperature of the mould

2. the shape of the mould

Question 18

if the mould is similar temperature to the melt then… 2 things

Answer 18

• slow cooling

- large grain size

Question 19

if the mould is a different temperature to the melt then… 2 things

Answer 19

• fast cooling

- small grain size

Question 20

how are grain boundaries visualised?

Answer 20

a light reflecting microscope

Question 21

how can the quality of grain boundaries be assured?

Answer 21

by using the correct etching chemical that shows up areas under high stress

Question 22

4 ways that metals and alloys can be shaped?

Answer 22

• hammering
• rolling
• drawing through a wire
• pressing

Question 23

define ductility

Answer 23

the maximum degree of EXTENSION in response to an applied TENSILE force

Question 24

define malleable

Answer 24

the maximum degree of COMPRESSION in response to an applied COMPRESSION force

Question 25

what is casting?

Answer 25

molten to investment mould

Question 26

what is amalgamation?

Answer 26

alloy and mercury mixing in a hardening reaction that brings about crystallisation

Question 27

why are elemental metals really used practically?

Answer 27

as there are severe limitations of their properties that are inferior to that of an alloy

Question 28

what are the 4 possible solubilities of molten metals that occurs on cooling?

Answer 28

1. remain soluble in each other (SOLID SOLUTION)
2. are completely insoluble in each other
3. are partially soluble in each other
4. from inter metallic compounds if the metals have affinity for each other (e.g Ag3Sn)

Question 29

what are the 3 possible solid solutions?

Answer 29

1. ordered solid solution
2. random solid solution
3. interstitial solid solution

Question 30

whats the benefit of solid solutions?

Answer 30

• the hardening effect due to atoms of differing radii form a mechanical resistance to dislocation

Question 31

when 2 metals are totally insoluble in each other way can occur and is it common?

Answer 31

it is rare.

risk of electrolytic corrosion due to 2 dissimilar metals

Question 32

what are the 4 main properties of an alloy where inter metallic compounds have been formed on cooling?

Answer 32

• hard
• brittle
• low ductility
• few crystal imperfections means less potential fro slip movement

Question 33

what are the two axis on a cooling curve diagram?

Answer 33

temperature vs time

Question 34

what does Tm mean?

Answer 34

melting point

Question 35

when does crystallisation occur?

Answer 35

at Tm

Question 36

what does a plateau in the cooling curve indicate?

Answer 36

a constant temperature during crystallisation

Question 37

what is a phase diagram?

Answer 37

a series of cooling curves for alloys of different compositions

Question 38

what does a large separation between liquidous or solidous indicate..

Answer 38

a greater coring. (more of a cored structure)

Question 39

what does an eutectic alloy behave like?

Answer 39

a pure metal

Question 40

give a fact about the melting point of an eutectic alloy

Answer 40

it is lower than that of the component metals

Question 41

how are eutectic alloys applied?

Answer 41

by a solder