Martensite

Martensite is a metastable phase of iron-carbon alloys that forms in steel when austenite is rapidly cooled (quenched) from high temperature. The transformation from gamma-Fe (FCC) to martensite is diffusionless and is driven by a coordinated shear that distorts the lattice into body-centered tetragonal (BCT) martensite. Carbon atoms become trapped in supersaturated interstitial positions, producing a solid solution with high internal stresses.

The microstructure typically appears as needle-like or plate-like features; the exact morphology depends on carbon content and cooling rate. Martensite is extremely hard and strong, but brittle; its mechanical properties rise with carbon content but toughness usually falls as hardness increases.

The formation occurs below a temperature called Ms (martensite start) and finishes at Mf (martensite finish). Higher carbon contents lower the Ms temperature, and alloying elements can shift transformation temperatures. The transformation is rapid and diffusionless, occurring essentially as the temperature crosses Ms during quenching.

Tempering martensite reduces brittleness by permitting carbide precipitation and relaxation of internal stresses, producing tempered martensite with improved toughness while retaining much of the strength. In practice, martensite is central to the heat-treatment of steel, providing hard, wear-resistant microstructures for tools, dies, gears, and high-strength components. Martensite can also occur in other iron-containing alloys where rapid quenching of austenite is used to achieve a supersaturated, high-strength phase.