Which statement about martensite is correct?

• A. It is a stable iron phase at high temperature.
• B. It is a metastable iron phase supersaturated in carbon produced by a diffusionless transformation from austenite.
• C. It is a ferrite-cementite lamellae in pearlite.
• D. It is produced by slow cooling in air.

Answer: (B)

Martensite forms when austenite is cooled very rapidly, so the transformation is diffusionless and occurs through a coordinated, shear-like rearrangement rather than atom-by-atom diffusion. This athermal, displacive change traps a high amount of carbon in solid solution, creating a metastable iron phase that is supersaturated with carbon. The result is a very hard, brittle microstructure because the carbon-induced lattice distortion (often modeled as a body-centered tetragonal arrangement) strengthens the iron matrix but also makes it susceptible to cracking. This is not a stable phase at high temperature—the stable high-temperature phase is austenite. It is also not the ferrite–cementite lamellae seen in pearlite, which forms by diffusion during slower cooling, nor is it produced by slow cooling in air.