Steels + aluminium

What are steels

alloys that contain iron and carbon

2 distinct crystallographic forms of polymorphs that iron exists in

how can carbon content be determined

by the maximum amount that can be dissolved into solid iron

what are compositions

alloys that contain 0.01 - 2 wt% C

graph of volume change against temperature showing different structures

what is equilibrium

a state in which no further change is perceptible, no matter how long one waits

• the state has the lowest free energy

2 words to describe a phase

1. homogenous

2. mechanically separable

how does cementite form

once C is added beyond the solubility limit, the metastable iron carbide Fe3C (cementite) forms

what are intersitials

• small atoms which can sit between larger atoms

• e.g. carbon in steel

• interstiitial locking occurs when dislocations are trying to move

microstructure at 0.8 wt% C

microstructure at hypoeutectoid compositions < 0.8 wt% C

microstructure at hypereutectoid compositions > 0.8 wt% C

how is C rejected from a solid solution and diffuses towards plates of Fe₃C

Via diffusion (thermally activated)

• so if the cooling rate increases, shorter time for diffusion

  • as a result, the lamellae of pearlite will become finer

final composition of hyoeutectoid vs hypereutectoid

Hypoeutectoid

• proeutectoid ferrite + pearlite

Hypereutectoid

• Fe₃C + pearlite

when is the time temperature transformation curve used

used for ISOTHERMAL heat treatments

• only austenite can transform into something

what are cast irons

alloys of iron and carbon containing varying amounts of manganese, silicon, sulphur and phosporus

describe the 3 types of cast iron

WHITE CAST IRON

• consists of Fe₃C and pearlite

• fast cooling rate

• white irons have no free carbon

• very hard, difficult to machine + good wear resistance

GREY CAST IRON

• consists of graphite (flakes) and pearlite

• have free graphite present

• more readily machinable than white irons

• poor tensile properties

DUCTILE CAST IRON

• consists of graphite + pearlite / graphite + ferrite

• pearlitic (moderate cooling) or ferritic (slow cooling)

• additions of magnesium or cerium change the graphite form to spheroidal

5 properties of aluminium + examples of uses

1) Low density (1/3 of weight steel)

2) Highly resistant to most forms of corrosions

• protective coating Al oxide barrier to air, temperature, moisture and chemical attack

3) Super conductor of electricity (replaces Cu where weight is also a consideration)

4) Non-magnetic and non-combusible

• advanced industries such as electronics or in off-shore structures

5) Non-toxic and impervious

• food packaging

Describe the 2 types of alloys

WORK-HARDENING ALLOYS

Hardened by deformation (an increase in dislocation density)

Designated

• F = as fabricated

• H = strain hardened

• O = annealed state

HEAT-TREATABLE ALLOYS

Properties controlled by heat treatment through the dissolution and controlled precipitation of second phase particles (solution treatment + aging)

• Designated T (with a number to indicate the treatment)

3 diagram heat treatment of alloys and designation

Hardness/Strength vs log aging time graph

why is the assumption that there are no further changes in the structure below the eutectic temperature incorrect for 2.5% Cu alloy

at a temperature below the eutectic temperature, θ would precipitate

for a 2.5% Cu alloy design a heat treatment that would give you refinement of the θ particles with the smallest carbon footprint

1. Heat to the maximum solubility of Cu in Al (547 degrees)

2. and QUENCH

3. leave to naturally age

would this work for 10% and 40% Cu

it would not work as at this temperature, liquid would start to form