Key Parts

Question 1

Explain the process of work hardening

Answer 1

1. Upon solidification, atoms arrange themselves to form crystals
2. Atom arrays with different orientations then form grains (with grain boundaries in-between them)
3. As dislocation movement deforms the crystal structure, the grain boundary impedes the dislocation movement.
4. The longer and the more grain boundaries, the more difficult for the dislocation to move.

• By deforming the metal (working), the metal grains get deformed and grain boundaries are longer and closer together, increasing dislocation density (more difficult for dislocations to move).

Question 2

Explain the process of Solid solution hardening

Answer 2

1. Dissolve another metal into the metal to create an alloy.
2. Solid solution has defects (alloying solute) making it harder for dislocations to move under stress.

• “Large/small substitutional solid solution strengthening” occurs when the solute atom is large enough that it can replace solvent atoms in their lattice positions (large solute atoms stretch out nearby bonds, small solute atoms pull in nearby bonds)
• “interstitial solid solutions” occur when the solute atom is small enough to fit at interstitial sites between the solvent atoms

Question 3

Explain the process of precipitation hardening

Answer 3

• Form fine particles of a second phase within the metal matrix to increase strength of metal alloy.

1. Heat the metal to dissolve the alloying elements (solutionising) then cool rapidly (quenching) to create a supersaturated solution.
2. Metal is aged at a lower temperature, causing alloying elements to precipitate out of the solution (coarsening) and form tiny particles that hinder movement of dislocations.

Question 4

What is the manufacturing process for glass fibres?

Answer 4

1. Liquid glass formed from blending quarry products (sand, limestone)
2. Heat the mixture in a furnace to 1200-1600°C
3. Liquid glass passed through a platinum bushing with very fine holes.
4. Fibres are cooled using a water spray to produce glass filaments 10-20 microns in diameter.

Question 5

What is the manufacturing process for aramid fibres?

Answer 5

1. “Wet spinning” - polymer powder is dissolved in a solvent and the polymer solution is extruded through spinneret into a mixture (coagulant)
2. Raw materials of aramid react and is isolated by precipitation with water
3. After washing, fibres are dried and stretched at 500°C to improve molecular alignment
4. Chain molecules are highly orientated along the fibre axis, so the strength of the chemical bond can be exploited.

Question 6

What is the manufacturing process for carbon fibres?

Answer 6

• Controlled oxidation, carbonisation and graphitisation of carbon-rich organic precursors (chemical) which are already in fibre form.

1. PAN fibres stretched by 500-1300% to improve molecular alignment.
2. Stabilised (oxidised) in air at 300°C
3. They are then carbonised at 1500°C to improve crystallinity. Nitrogen released, fibres 90% carbon (high strength and chemical inertness)
4. Graphitised by heating and stretching at temperatures up to 3000°C