Materials: structure, properties, selection and environmental impact
Materials questions in the PAU exam test whether you can link atomic structure to measurable properties, justify a selection with numbers, and argue about end-of-life impact. Expect the iron–carbon diagram, tensile-test calculations and an eco-design comparison.
Structure and what follows from it
- Metals crystallise in BCC (ferrite, Fe-α), FCC (austenite, Al, Cu) or HCP (Mg, Zn) cells. FCC metals are ductile because they offer 12 slip systems; HCP metals are brittle at room temperature.
- Carbon dissolved in the octahedral interstices of iron distorts the lattice and raises strength. Up to 2.11 % C we speak of steel; above that it is cast iron (eutectic at 4.3 % C).
- Eutectoid point: 0.77 % C at 727 °C, where austenite transforms into pearlite (alternating ferrite and cementite). Hypoeutectoid steels carry proeutectoid ferrite; hypereutectoid steels form a cementite network, hence their brittleness.
Properties and tests
- Design properties: elastic modulus E, yield strength Re (0.2 % proof stress for non-ferrous metals), tensile strength Rm, elongation A %, hardness (Brinell HB, Rockwell HRC, Vickers HV), toughness (Charpy V-notch, J), fatigue limit.
- Technological properties concern the workshop: machinability, weldability, formability, castability, hardenability.
- Heat treatments: annealing (soften, relieve stresses), normalising, quenching (martensite: hard but brittle), tempering (restores toughness), case-hardening (hard skin, tough core).
Worked example
A 12 mm diameter steel specimen with 50 mm gauge length yields at 22 kN and breaks at 34 kN with a final length of 62 mm. A0 = π·6² = 113.1 mm². Re = 22 000/113.1 = 194.5 MPa; Rm = 34 000/113.1 = 300.6 MPa; A = (62−50)/50 × 100 = 24 %. Conclusion: a ductile structural steel (Re ≈ 195 MPa, Rm ≈ 300 MPa, 24 % elongation) valid for profiles, unsuitable for cutting tools.
Selection and environment
Rank candidates with a performance index for the function (strength/density for a tie rod), then filter by process compatibility and cost. Life-cycle thinking matters: primary aluminium costs roughly 11–14 kg CO2-eq per kg against about 2 for steel, but its low density and low-energy recycling (around 5 % of the primary energy) can repay that in transport applications.
Traps that cost marks
- 1 MPa = 1 N/mm²; converting the area to m² gives absurd results.
- Hardness is not a design strength - use Re.
- Quenching raises hardness but destroys toughness; a temper is always implied.
- Above 2.11 % C the alloy is cast iron, not steel.