metals

Question 1

what is quenching

Answer 1

method for rapid cooling

 use small bulk so easier to cool

 heat metal just above melting temp

 use a mould with high thermal conduction so the heat can escape more easily

 quench using cooler rather than warmer water

Question 2

Advantages of metals over cr

Answer 2

• strength
• rigid
• high elastic limit (ie wont fracture easily)
• ductility
• hardness

Question 3

Disadvantages of metals

Answer 3

• poor aesthetics

Question 4

metal definition

Answer 4

Aggregate of atoms in a crystalline structure

Question 5

alloy defintion

Answer 5

Combination of metal atoms in a crystalline structure

Question 6

EL defintion

Answer 6

Maximum stress without plastic deformation

Question 7

UTS stand for

Answer 7

ultimate tensile strength

Question 8

ductility definition

Answer 8

Amount of plastic deformation prior to fracture

Ie measure of the extent that a material can be shaped

Question 9

what is the equation for ductility

Answer 9

(y-x)%

Question 10

equi-axed grains def

Answer 10

crystal growth of equal dimensions in each direction

Question 11

what are grains

Answer 11

each grain is a single crystal lattice with atoms orientated in given directons

Question 12

what is a grain boundary

Answer 12

change in orientation of the crystal planes

ie where one grain comes into contact with another grain

Question 13

where do impurities tend to concentrate at?

Answer 13

at grain boundaries

Question 14

dislocations definition

Answer 14

imperfections/defects in the crystal lattice

Question 15

slip defintion

Answer 15

propagation of dislocation

Question 16

what factors affect the mechanical properties of metal?

Answer 16

grain size
grain defecs
crystalline structure

Question 17

what r the three types of crystalline metal structures?

Answer 17

cubic
body centered cubic
face centered cubix

Question 18

what are the 3 types of grains

Answer 18

equiaxed
radial
fibrous

Question 19

how do metal crystals grow to form grains?

Answer 19

1. some atoms act as nuclei of crystallisation
2. crystals grow to form dendrites which are 3D branched lattice networks (atoms concrescence on the nuclei of crystallisation to form dendrites)
3. crystals (aka grains) grow until they impinge on other crystals
4. region where grains make contact with another is called grain boundary
5. this can be done via fast cooling or slow cooling; sometimes with the addition of nucleating agents

Question 20

what are nucleating agents

Answer 20

**nucleating agents are impurities that act as foci for crystal growth

they speed up growth

Question 21

what is difference between fast and slow cooling

Answer 21

fast ->
more nuclei of crystallisation,
smaller grains and
better mechanical properties but
decreased ductility

slow ->
large coarse grains which is undesirable and weaker

Question 22

what happens when the defect moves to the grain boundary

Answer 22

lattice has NO defects and is now a different shape

Question 23

what is impeding dislocations desirable

Answer 23

increases
o EL
o FS
o UTS
o hardness

but
decreases
o ductility
o impact resistant

Question 24

how to impede dislocation movement

Answer 24

1. having smaller finer grains
2. alloys
3. cold working, pushing the defects to the boundaries before it sets

Question 25

fast and slow cooling vs

annealing and cold working ??

Answer 25

fast (quenching) and slow cooling is for growing the crystals (affects grain size)

cold working and annealing is when you’re shaping the metals (processing it)

Question 26

what is cold working

Answer 26

work done at low temperature causes slip. dislocations collect at grain boundaries resulting in a stronger harder materiald

Question 27

does cold working modify the metal and its properties

Answer 27

yes

increases
o EL
o FS
o UTS
o hardness
o strength
o more residual stress which is bad

decreases
o ductility
o impact resistant
o less able to resist corrosion which is bad

Question 28

what causes residual stress in metals

Answer 28

cold working

Question 29

how do you overcome residual stress from cold wokring

Answer 29

annealing

Question 30

what is annealing

Answer 30

• heating metal so that greater thermal vibrations allows migration of atoms and rearrangement of atoms within the grain

Question 31

why is residual stress bad

Answer 31

causes instability in the lattice, results in distortion over time which is undesirable.

Question 32

annealing vs recrystallization

Answer 32

annealing is done at not so highly raised temp, still below melting temp (in the hundreds of deg)

recrystallisation temp is much higher, above melting temp (in the thousands)

Question 33

does annealing change the structure and properties of the grain?

Answer 33

no

Question 34

why do we have to ensure that the annealing temp does not raise too much

Answer 34

• excessive annealing temperature can cause large grains to replace smaller coarse grains yielding poorer mechanical properties

Question 35

more cold work = high or lower recrystallization temp??

Answer 35

lower