Materials Chemistry (Aerospace)

Q1. (4 marks)

• Precipitation hardening: a heat treatment in which a supersaturated solid solution is aged so that fine, coherent second-phase precipitates form, obstructing dislocation motion and increasing strength/hardness. (2)
• Three stages: (i) solution treatment (heat to dissolve solute into single-phase solid solution); (ii) quenching (rapid cool to retain supersaturated solution); (iii) ageing (natural or artificial — reheat to precipitate fine particles). (1½, ½ each)
• Example: Al 7075 (or 2024, or Inconel). (½)

Q2. (4 marks — 1 mark each: correct material + valid property) (a) Reinforced Carbon–Carbon (RCC) — retains strength at very high temperature / low thermal expansion. (b) Refractory metal e.g. tungsten (W) or rhenium/tantalum — very high melting point ($\sim3400\,^\circ\text{C}$ for W). (c) Al alloy (2024/7075) or CFRP — high specific strength (strength-to-weight ratio). (d) Ni-based superalloy (Inconel) — high creep/oxidation resistance at elevated temperature.

Q3. (4 marks)

• Annealing: slow cooling (usually furnace cool). Produces soft, ductile, stress-relieved microstructure; coarse equilibrium grains. (2)
• Quenching: rapid cooling (water/oil). Produces hard, strong but brittle non-equilibrium structure (e.g. martensite in steel). (2)

Q4. (4 marks)

• SCC: cracking from the combined action of tensile stress and a corrosive environment; neither alone would cause failure. Promoted by e.g. chloride environment + sustained tensile stress. (2)
• Hydrogen embrittlement: loss of ductility/cracking caused by atomic hydrogen diffusing into the metal lattice. Promoted by e.g. exposure to hydrogen (electroplating, acid pickling, cathodic reactions). (2)

Q5. (4 marks) Rule of mixtures (isostrain): $E_1 = E_f V_f + E_m V_m$, with $V_m = 1 - V_f = 0.40$. (1) $E_1 = 230(0.60) + 3.5(0.40)$ (1) $E_1 = 138 + 1.4$ (1) $E_1 = 139.4\ \text{GPa}$ (1)

Q6. (4 marks)

• Ablative material: a sacrificial thermal-protection material that removes heat by pyrolysis/charring and mass loss (sublimation/vaporisation), carrying heat away from the structure. (2)
• Mechanism: heat causes the material to decompose/char; endothermic reactions and outgassing (transpiration) absorb energy and the boundary layer is cooled/blocked, protecting the substrate. (1)
• Example: PICA, AVCOAT, or SLA. (1)

Q7. (4 marks) (a) 6 wt% aluminium and 4 wt% vanadium (balance Ti). (2) (b) Any two: high specific strength; excellent corrosion resistance; good high-temperature performance; low density (~$4.4\ \text{g cm}^{-3}$). (2, 1 each)

Q8. (4 marks)

• Alumina ($\text{Al}_2\text{O}_3$): hard, electrically insulating, chemically inert, good oxidation resistance (but lower thermal-shock resistance). (1½)
• Silicon carbide (SiC): high hardness, high thermal conductivity, good thermal-shock resistance, strength retention at high T. (1½)
• UHTC example: $\text{ZrB}_2$ or $\text{HfB}_2$. (1)

Q9. (4 marks) $\rho = \rho_f V_f + \rho_m V_m$, $V_m = 0.50$. (1) $\rho = 2.55(0.50) + 1.20(0.50)$ (1) $\rho = 1.275 + 0.600$ (1) $\rho = 1.875\ \text{g cm}^{-3}$ (1)

Q10. (4 marks)

• Anodising: an electrolytic surface treatment that grows a controlled, thick oxide layer (e.g. $\text{Al}_2\text{O}_3$) on the metal surface by making it the anode in an acid electrolyte. (2)
• Purposes (any two): corrosion protection; improved wear/hardness; better paint/adhesive adhesion; electrical insulation; decorative finish. (2, 1 each)

[
  {"claim":"Q5 CFRP longitudinal modulus E1 = 139.4 GPa","code":"Ef=230; Em=3.5; Vf=0.60; Vm=1-Vf; E1=Ef*Vf+Em*Vm; result=(abs(E1-139.4)<1e-9)"},
  {"claim":"Q9 GFRP density = 1.875 g/cm3","code":"rf=2.55; rm=1.20; Vf=0.50; Vm=1-Vf; rho=rf*Vf+rm*Vm; result=(abs(rho-1.875)<1e-9)"},
  {"claim":"Q5 matrix volume fraction is 0.40","code":"Vf=0.60; Vm=1-Vf; result=(abs(Vm-0.40)<1e-9)"}
]