martensite

Martensite is what you get when you cool austenite so fast the carbon atoms have no time to move: the crystal is forced to change shape without any diffusion. Instead of atoms strolling to new positions, the whole lattice shears cooperatively in an instant, trapping the carbon where it sits. This is the hard, brittle phase behind a quenched steel's cutting edge.

Rapid quenching skips past all the C-curves of the TTT and CCT diagrams and reaches the martensite-start temperature (Ms) with the carbon still dissolved. The FCC austenite then snaps into a body-centered-tetragonal (BCT) structure, like BCC iron but stretched along one axis by the trapped carbon squeezed into it. That stretched, carbon-jammed lattice makes dislocation motion extremely hard, so martensite is very hard and strong but brittle. It is diffusionless (athermal): the amount formed depends on how far below Ms you cool, not on how long you hold.

Martensite is the reason steel can be hardened at all, but as-quenched it is often too brittle to use and the quench builds dangerous internal stresses (a cracking risk). So it is almost always tempered afterward to trade a little hardness for much-needed toughness. Note that hardness rises with carbon content, and getting fully martensite through a thick part depends on hardenability, not just quench speed.