What is Martensite, and how is it formed?

• A. A diffusionless, hard phase formed by rapid quenching from a high-temperature phase (supersaturated solid solution).
• B. A soft phase formed by slow cooling allowing diffusion.
• C. A brittle phase formed by annealing at elevated temperature.
• D. A ductile phase formed by precipitation of carbides during aging.

Answer: (A)

Martensite is a diffusionless, hard phase that forms when austenite is quenched rapidly. With the cooling being so fast that carbon cannot diffuse out, the steel undergoes a coordinated lattice distortion that changes the structure from austenite (typically face-centered cubic) to martensite (body-centered tetragonal). This rapid, shear-like transformation traps carbon in solution, producing a supersaturated solid solution and a highly distorted lattice, which makes martensite very hard and brittle.

If cooling is slow, carbon has time to diffuse and rearrange into more stable phases like pearlite or bainite, which are softer. Annealing or aging involves diffusion and precipitation processes, leading away from martensite. The other options describe diffusion-enabled pathways or different types of phases, not the diffusionless transformation that creates martensite.