Question

The product of diffusionless transformation is

• A. Martensite
• B. Pearlite
• C. Bainite
• D. Austenite

Hint:

Associate keywords:

• Diffusionless, Quenching, Athermal, Shear \(\rightarrow\) Martensite.

• Diffusion, Slow Cooling, Nucleation and Growth \(\rightarrow\) Pearlite or Bainite.

Answer 1

The Correct Option is A

Step 1: Separate the two transformation families.
Phase changes in steel fall into two very different camps: diffusional transformations, where carbon atoms have time to migrate and rearrange, and diffusionless transformations, where cooling is so fast that no atomic migration can occur at all.
Step 2: Examine what happens when there is no time to diffuse.
If austenite is quenched fast enough, carbon atoms get trapped exactly where they were sitting in the FCC lattice. The lattice cannot become BCC ferrite plus separate cementite because that would require carbon to move; instead, the whole lattice shears cooperatively into a highly strained body centered tetragonal structure, carrying all its original carbon along with it.
Step 3: Naming this structure.
This supersaturated, shear formed, carbon trapped phase is what metallurgists call martensite. Its formation is athermal, meaning it depends only on how far below the $M_s$ temperature you cool, not on how long you wait, which is the signature of a diffusionless change.
Step 4: Eliminating the other options.
Pearlite and bainite both require carbon to diffuse to build separate ferrite and cementite regions, so they are diffusional products, and austenite is simply the untransformed starting phase, not a product at all.
\[ \boxed{\text{Martensite}} \]