Materials Science and Engineering: Structure, Properties, Testing and Selection
In this domain the Andalusian PAU exam asks you to connect internal structure to measurable properties and then justify a selection with numbers. Questions typically give you a tensile or hardness result, a microstructure, or a component brief (low weight, corrosion resistance, electrical insulation) and expect a reasoned choice rather than a memorised name.
Structure and properties
- Crystal structures: BCC is ferrite (α-Fe); FCC covers austenite (γ-Fe), aluminium and copper; HCP covers magnesium, zinc and titanium. FCC metals are ductile because they present 12 slip systems; HCP metals, with fewer, tend to be brittle at room temperature.
- Carbon steel is ferrite plus pearlite, the eutectoid composition being 0.8 % C at 727 °C. Above 2.11 % C the alloy is cast iron, not steel.
- Read the tensile curve for elastic limit, yield strength σy (0.2 % offset), ultimate strength σUTS, elongation at break and toughness (area under the curve).
- Hooke's law σ = E·ε applies only in the elastic zone. Typical values: steel E ≈ 210 GPa, aluminium ≈ 70 GPa, copper ≈ 110 GPa.
- Heat treatments: annealing softens, quenching hardens and makes material brittle, tempering restores toughness.
Testing
- Brinell (HB): hardened 10 mm ball, F/D² = 30 for steels, reliable up to about 450 HB.
- Vickers (HV): diamond pyramid, valid for thin or very hard specimens.
- Rockwell C (HRC): 150 kgf diamond cone reading penetration depth; not exactly convertible to HB.
- Charpy impact testing measures toughness and reveals the ductile–brittle transition temperature.
Worked example
A 20 mm diameter, 200 mm long steel specimen breaks at 100 kN with a final length of 205 mm. Cross-section A = π·d²/4 = π·400/4 = 314.2 mm². Then σUTS = F/A = 100 000 N / 314.2 mm² = 318 MPa. Strain at break ε = ΔL/L₀ = 5/200 = 0.025, i.e. 2.5 %. If σy = 250 MPa, the elastic strain at yield is 250/210 000 ≈ 0.12 %, so nearly all the deformation was plastic and the material is ductile.
Traps that cost marks
- 1 MPa = 1 N/mm²; the commonest error is leaving loads in kN or areas in cm².
- Distinguish engineering stress (initial area) from true stress (instantaneous area).
- Never quote a hardness number without its scale, and never relabel HB as HRC.
- Choosing 'the strongest' material when the brief prioritises density, cost or corrosion resistance.