metals and alloys

Question 1

metal vs alloys

Answer 1

metal - single species of atoms in a crystalline structure
alloys - combination of metal atoms in a crystalline structure
metals are the building blocks of alloys

Question 2

when would a material need to display ductility

Answer 2

when subject to tensile strength (pulling)

ductility - ability to deform without fracture

Question 3

when would a material need to display malleability

Answer 3

when subject to compressive strength

malleability = ability to be pressed/hammered etc

Question 4

are mechanical properties identical in every metal

Answer 4

no

Question 5

aside from choice of metal what else can influence a metals mechanical properties

Answer 5

crystalline structure

this structure depends on method of production and how it was shaped e.g cold working, swaging, annealing

Question 6

describe the temperature changes/metal composition as a metal cools from a molten state to a solid

Answer 6

in molten form atoms are arranged randomly and flow readily
as it drops to melting point atoms start to crystallise as they go from liquid to solid
temperature is constant at this stage
only when crystallisation has complete will the temperature begin to cool again

Question 7

what term is used to describe what crystals grow as to form a 3D lattice network

Answer 7

dendrites

Question 8

what are crystals also known as

Answer 8

grains

Question 9

grain boundaries

Answer 9

form where a series of individual grains make contact with one another - stopping each other from further growth

Question 10

equi axed grains

Answer 10

form when atom crystals grow equally in each direction

Question 11

how is radial grain structure achieved

Answer 11

by cooling molten metal quickly in a cylindrical mould

Question 12

quenching

Answer 12

fast cooling

Question 13

what effect does quenching have on crystalline structure

Answer 13

causes more nuclei (of crystallisation) to form therefore forms more grains , these are small in size

Question 14

what effect does slow cooling have on crystalline structure

Answer 14

less nuclei (of crystallisation) form therefore less grains, these grains are large in size

Question 15

what may be added to molten metals to act as additional nuclei of crystalisation and therefore produce more small grains

Answer 15

nucleating agents such as impurities or addidives

Question 16

are small or large grains advantageous and why

Answer 16

small grains - they have a higher EL, FS, UTS and hardness however decreased ductility

Question 17

grain

Answer 17

single crystal with atoms in given directions

Question 18

dislocation

Answer 18

‘imperfection’ in crystal lattice structure causing a discontinuity , this area is most likely to fracture

Question 19

slip

Answer 19

propagation of dislocations , towards a grain boundary

dislocations move one lattice bond at a time until they reach a grain boundary

Question 20

what makes small grains have better mechanical properties

Answer 20

many small grains = many grain boundaries , this impedes the movement of dislocations making it stronger

Question 21

cold working

Answer 21

work such as bending, rolling or swaging done at a low temp - one below the metals recrystalisation temperature

Question 22

benefits of cold working

Answer 22

the applied force/ pressure from the work causes slip and produces a harder and stronger material

Question 23

disadvantages of cold working

Answer 23

lots of cold working can increase residual stress (internal stresses)
this causes instability of the lattice and may result in distortion of the metal over time

Question 24

how are residual stresses relieved

Answer 24

annealing - heating of the metal/alloy which causes increased thermal vibrations allowing the metal atoms to migrate and re arrange so as to eliminate instability

Question 25

stress relieving annealing

Answer 25

when annealing is done to cold worked materials

doesn’t alter grain structure or mechanical properties and allows further cold working to be carried out

Question 26

recrystallisation

Answer 26

can be carried out if cold working doesnt work out so that the metal can be recycled
however this changes grain structure therefore changes mechanical properties - decreased EL, FS, hardness but increased ductility

Question 27

metalloid

Answer 27

element with properties in between a metal and a non metal e.g silicon , chloride

Question 28

what advantages do alloys have in comparison to metals

Answer 28

increased mechanical properties, increased corrosion resistance

Question 29

solution vs phase

Answer 29

solution - homogenous mixture at an atomic scale

phase - physically distinct homogenous structure - can have more than one component

Question 30

solid solution

Answer 30

on crystallisation 2 metals come together to form a common lattice i.e one phase

Question 31

substitutional solid solution

Answer 31

atoms of one metal replace the other in the crystal lattice
2 types
- random - randomly replaced, atoms of similar size
- ordered - orderly replaced in lattice arrangement, atoms of similar size

Question 32

interstitial solid solution

Answer 32

atoms markedly different in size, smaller atoms don’t have a specific location and are located within the larger atoms lattice structure

Question 33

metal vs alloy cooling curve

Answer 33

metal - crystallisation of metals occurs at a constant temp before cooling resumes
alloy - crystallisation of alloy occurs over a temperature range as the different metals will have different melting points

Question 34

liquidus

Answer 34

line representing temp at which crystallisation begins - above this ally is in a liquid state

Question 35

solidus

Answer 35

line representing temp at which crystallisation ends - below this alloy is in a solid state

Question 36

slow cooling of alloys

Answer 36

ensures grain composition is homogenous however forms large grains which have decreased mechanical properties

Question 37

fast cooling of alloys

Answer 37

produces small grains and many grain boundaries which can impede dislocations therefore better mechanical properties
however grains formed are not homogenous they experience coring

Question 38

coring of alloy grains

Answer 38

happens when alloys are quenched
occurs as the metals within the alloy have different melting points
happens as atoms are prevented from freely moving through the lattice so grains have a varying composition
extend of coring dependent on separation between liquidus and solidus

Question 39

downfall of coring

Answer 39

may decrease corrosion resistance

Question 40

how can you overcome the downfall of decreased corrosion resistance caused by coring

Answer 40

homogenising annealing

reheat the solid core to allow atoms to diffuse and grain composition to become homogenous

Question 41

why do alloys have improved mechanical properties in comparison to metals

Answer 41

alloys resist movements of dislocations more

differing atom sizes means more force is needed to move the dislocations

Question 42

eutectic alloys

Answer 42

metals are soluble when molten but insoluble when solid so form 2 phases
brittle and poor corrosion resistance

Question 43

partially soluble alloys

Answer 43

a range of compositions are not possible , do not cool to form 50:50 grains
instead forms alpha and beta grains , with them being made up majority of the respective metal
can’t form grains between the solubility limit lines

Question 44

when annealed what do partially soluble alloys undergo

Answer 44

precipitation hardening which makes them stronger and harder