Materials Science and Metallurgy: Crystal Structure, Phase Diagrams and Heat Treatment
Structure and defects
Metals crystallise as BCC, FCC or HCP. FCC (Cu, Al, γ-iron) has 4 atoms per unit cell and APF 0.74; BCC (α-iron, W) has 2 and 0.68; HCP has 6 and 0.74. Point defects are vacancies and interstitials; line defects are edge and screw dislocations, which explain slip and work hardening.
Phase diagrams and heat treatment
- Iron–carbon diagram: eutectoid at 0.76% C and 727 °C gives pearlite; eutectic at 4.3% C and 1147 °C gives ledeburite.
- Annealing softens and relieves stress; normalising refines grain; quenching forms martensite (hard, brittle, body-centred tetragonal); tempering restores toughness.
- Hardenability is measured by the Jominy end-quench test and is not the same as hardness.
- TTT diagrams describe isothermal transformation; CCT diagrams describe continuous cooling.
Worked example
Density of BCC iron with a = 0.286 nm, M = 55.85 g/mol, N_A = 6.022 × 10²³. ρ = nM/(N_A a³) = 2 × 55.85/(6.022 × 10²³ × (0.286 × 10⁻⁹)³). a³ = 2.34 × 10⁻²⁹ m³, so N_A a³ = 1.409 × 10⁻⁵. ρ = 111.7/1.409 × 10⁻⁵ = 7.93 × 10⁶ g/m³ = 7.93 g/cm³, matching the measured value.
Traps
- Calling martensite an equilibrium phase; it is a diffusionless transformation product.
- Confusing hardness with hardenability.
- Mixing up annealing and normalising cooling rates.
- Quoting the eutectoid carbon content as 2% instead of 0.76%.