Which statement correctly describes martensite?

• A. A stable iron phase rich in carbon formed by diffusion.
• B. An alloy oxide formed during quenching.
• C. A metastable iron phase supersaturated in carbon that is the product of a diffusionless transformation from austenite.
• D. A mixture of ferrite and cementite in lamellar form.

Answer: (C)

Martensite forms in steel when austenite is cooled rapidly, causing a diffusionless transformation. The rearrangement of the iron lattice happens by a coordinated, shear-like shift to a body-centered tetragonal structure, and carbon atoms are trapped in solid solution because there isn’t time for diffusion during the fast quench. This carbon supersaturation and the displacive nature of the transform give martensite its characteristic very hard, brittle nature. It isn’t an oxide, it isn’t formed by diffusion-controlled reactions, and it isn’t a ferrite–cementite lamellar mixture like pearlite or bainite. Those descriptions describe other phases or products, not martensite.