Metals

Question 1

what is a metal?

Answer 1

an aggregate of atoms in a crystalline structure

Question 2

what is an alloy?

Answer 2

contribution of two or more types of metal atoms in a crystalline structure

Question 3

what does it mean if the stress-strain curve has a large gradient?

Answer 3

it is rigid

Question 4

what does large stress give?

Answer 4

small strain

Question 5

what is ductility?

Answer 5

amount of plastic deformation prior to fracture

Question 6

what does the crystalline structure depend on?

Answer 6

method of production and shaping

Question 7

what factors affect mechanical properties?

Answer 7

crystalline structure, grain size and grain imperfections

Question 8

what are the different lattice structures?

Answer 8

cubic, face-centred, body centred

Question 9

describe the cooling curve of pure metal

Answer 9

molten form flows readily, as temperature drops some metal atoms change in state to solid, temperature then remains constant with more atoms crystallising. When all atoms have crystallised the temperature drops to room temperature

Question 10

what are nuclei of crystallisation?

Answer 10

crystals which grow as dendrites and continue to grow until they come into contact with each other

Question 11

what does it mean if grains are equi-axed?

Answer 11

crystal growth is of equal dimension in each direction

Question 12

apart from equi-axed grains, what other grain structures are available

Answer 12

radial, fibrous

Question 13

if a material is fast cooled (quenched) what are the grains like?

Answer 13

more nuclei, small fine grains

Question 14

if a material is slow cooled what are the grains like?

Answer 14

few nuclei, large coarse grains

Question 15

what are nucleating agents?

Answer 15

impurities or additives which ac as foci for crystal growth

Question 16

where do grain boundaries occur?

Answer 16

where there is a change in orientation of the crystal planes

Question 17

what are the advantages of small fine grains?

Answer 17

high elastic limit and increased FS, UTS, hardness

Question 18

what is the disadvantage of small fine grains?

Answer 18

decreased ductility

Question 19

what are factors needed for rapid cooling

Answer 19

small bulk, heat metal just above Tm, mould - high thermal conduction, quench

Question 20

what happens if there is a dislocation within the lattice?

Answer 20

it will fracture unless the dislocation is pushed along to the end of the lattice

Question 21

what are dislocations?

Answer 21

imperfections/defects in the crystal lattice

Question 22

what is SLIP due to?

Answer 22

propagation of dislocations and involves rupture of only a few bonds at a time

Question 23

how can you impede the movement of dislocations in metals and alloys?

Answer 23

have grain boundaries to prevent dislocation moving from one grain to another, have smaller finer grains

Question 24

what is the advantage of impeding dislocations?

Answer 24

metal/alloy has increased Fs, El and Sh

Question 25

what is cold working?

Answer 25

work done on a metal/alloy at a low temperature (below recrystallisation temperature) causing SLIP and making the material stronger and harder

Question 26

what does cold working increase?

Answer 26

El, Fs and Sh

Question 27

what does cold working decrease?

Answer 27

ductility, impact strength and corrosion resistance

Question 28

what is residual stress

Answer 28

stresses developed in the metal itself where atoms become oriented out of their equilibrial position in the grain/lattice

Question 29

what does residual stress cause?

Answer 29

instability in metal lattice, distortion of metal overtime

Question 30

what is residual stress relieved by?

Answer 30

subjecting metal or alloy to annealing

Question 31

what is annealing?

Answer 31

heating metal so that greater thermal vibrations allows migration of atoms

Question 32

what does annealing do?

Answer 32

eliminates stresses by allowing atoms to re-arrange within grains

Question 33

when does recrystallisation happen?

Answer 33

when metal/alloy is heated

Question 34

what does recrystallisation do?

Answer 34

allow new smaller equiaxed grains, lower El, UTS, hardness, increased ductility

Question 35

what is the relationship between recrystallisation and cold working?

Answer 35

recrystallisation spoils the benefits of cold work but allows further cold work

Question 36

what is the recrystallisation temperature dependent on?

Answer 36

amount of cold work - the more cold work the lower the recrystallisation temperature

Question 37

what does excessive temperature rise cause?

Answer 37

large grains to replace smaller coarse grains which yields poorer mechanical properties

Question 38

what are the metal/alloy manipulation processes

Answer 38

quenching, cold working, stress relief annealing, recrystallisation

Question 39

what is grain structure affected by?

Answer 39

dislocations, processing methods