Austenite → Martensite: diffusionless shear transformation
Martensitic transformation is a first-order diffusionless (military) phase transition. FCC austenite transforms to BCT martensite via a collective shear of atoms — no diffusion needed. The Bain correspondence maps {001}_γ → {001}_α. Carbon atoms become trapped in interstitial sites of BCT, causing ~4% volume expansion and the characteristic surface relief/tilt. Martensite fraction: Koistinen-Marburger: f = 1 − exp(−0.011(Ms−T)).