Physical Metallurgy and Phase Transformations
This domain tests your ability to read phase diagrams quantitatively, to link thermal history to microstructure, and to explain how microstructure controls properties.
Core rules
- Nucleation: ΔG* = 16πγ³/(3ΔGv²); heterogeneous nucleation on grain boundaries, inclusions or mould walls dominates in real casts because the barrier is lower.
- Growth and overall transformation follow Avrami-type kinetics; the fraction transformed depends on nucleation rate and growth rate, both strongly temperature dependent, so the TTT curve is C-shaped.
- The Fe–Fe3C diagram: eutectoid at 0.76 wt% C and 727 °C; maximum ferrite solubility 0.022 wt% C; austenite holds up to 2.14 wt% C at 1147 °C.
- Martensite forms by diffusionless, displacive transformation; carbon is trapped, giving a body-centred tetragonal lattice, high hardness and low toughness. Ms and Mf fall as carbon rises.
- TTT diagrams describe isothermal holding; CCT diagrams describe continuous cooling and are displaced to longer times and lower temperatures.
- Recrystallisation of a cold-worked metal typically begins near 0.4 Tm; below that, only recovery occurs.
Worked example
A 0.40 wt% C steel is cooled just below 727 °C. Proeutectoid ferrite fraction = (0.76 − 0.40)/(0.76 − 0.022) = 0.36/0.738 = 0.488. Pearlite = 0.512. Ferrite inside the pearlite = 0.512 × (6.70 − 0.76)/(6.70 − 0.022) = 0.455, so total ferrite is about 94% and cementite about 6%.
Traps that cost marks
Reading a continuous-cooling treatment off a TTT diagram, or vice versa. Treating martensite as an equilibrium phase, or claiming it forms by diffusion. Applying the lever rule outside a two-phase field or with wrong tie-line endpoints. Forgetting that the eutectoid composition is 0.76%, not 0.8% or the steel's own carbon content.