the martensitic transformation
/ mar-TEN-sit-ik /
The martensitic transformation is the archetype of a displacive change, and the most spectacular. It is diffusionless — not a single atom hops or swaps places — yet an entire region of crystal snaps from one structure to another by a coordinated SHEAR, the way a deck of cards skews when you push its top sideways. It is named after martensite, the hard structure that forms when steel is quenched, but the same mechanism runs in many alloys, including the shape-memory ones.
In steel, face-centred-cubic austenite (gamma) is cooled so fast that carbon atoms have no time to diffuse out; instead the whole lattice shears, within a fraction of a microsecond, into a body-centred-tetragonal martensite (alpha-prime), with the trapped carbon straining it. The transformation is athermal (it advances as you keep cooling, not by holding time), often starts only below a martensite-start temperature Ms, and the atoms move less than one interatomic distance. Because the change is a shear on a specific plane, each martensite plate produces a visible tilt of the surface (a shape change), forms on a particular parent plane (its habit plane), and holds a fixed orientation relationship to the parent.
A pure shear would leave the plate badly misfitting its surroundings, so the lattice adds a lattice-invariant shear — fine internal twinning or slip — that leaves the overall shape change an invariant-plane strain (the habit plane is left undistorted). This is why martensite plates are internally twinned. The martensitic transformation is the backbone of steel hardening, the mechanism behind shape-memory and superelastic alloys (NiTi), and, honestly stated, a purely structural and mechanical transition — driven by cooperative displacement, not by atoms sorting themselves by diffusion.
Quench steel from the austenite field (FCC gamma) fast enough and carbon has no time to diffuse out; within under a microsecond the whole lattice shears into body-centred-tetragonal martensite (alpha-prime), the trapped carbon straining and hardening it. No atom hops to a new site — it is a diffusionless, cooperative shear, and it is why a blade can be hardened by quenching.
The martensitic transformation: a diffusionless cooperative shear turning FCC austenite instantly into BCT martensite (the heart of steel hardening).
The martensitic transformation is diffusionless: atoms move less than one interatomic spacing and never swap sites; it advances not by holding time but as you keep cooling (it is athermal), so it cannot be described as a diffusion-controlled process.