martensite

A metastable, body-centered tetragonal (BCT) phase formed via a diffusionless, displacive shear transformation of austenite during rapid cooling; while most commonly associated with steels, martensitic transformations also occur in various other alloy systems.

Quenching a high-carbon steel blade in water triggers a diffusionless transformation from austenite to martensite, creating the high hardness required for an edge despite the resulting increase in brittleness.