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: Understanding the Concept:
Phase transformations in steels can be categorized based on the mechanism of atomic movement.

• Diffusional Transformations: These require time for atoms (especially carbon) to move over relatively long distances to form new phases. The formation of pearlite and bainite are examples.

• Diffusionless (or Martensitic) Transformations: These occur extremely rapidly, without the long-range diffusion of atoms. Instead, there is a cooperative, shear-like movement of atoms over small distances, leading to a change in crystal structure.


Step 2: Detailed Explanation:

• Martensite: Martensite is formed when austenite is cooled so rapidly (quenched) that the carbon atoms do not have time to diffuse out of the crystal lattice to form cementite. The FCC structure of austenite transforms into a highly strained Body-Centered Tetragonal (BCT) structure. This transformation is diffusionless, athermal (dependent on temperature, not time), and involves a shear mechanism.

• Pearlite and Bainite: The formation of both pearlite and bainite involves the diffusion of carbon atoms to form ferrite and cementite. Thus, they are products of diffusional transformations.

• Austenite: Austenite is the high-temperature parent phase from which other microstructures are formed upon cooling. It is not a product of a diffusionless transformation in this context.


Step 3: Final Answer:
The only product listed that is formed via a diffusionless transformation mechanism is Martensite.