Question

Which of the following has bcc crystal structure?

• A. Ferrite
• B. Austenite
• C. Martensite
• D. Cementite

Hint:

Remember the crystal structures of steel phases:
- Ferrite ($\alpha$): BCC
- Austenite ($\gamma$): FCC
- Martensite: BCT (Body-Centered Tetragonal)
- Cementite ($Fe_3C$): Orthorhombic

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks to identify which of the given microstructural phases of steel (iron-carbon system) possesses a Body-Centered Cubic (BCC) crystal lattice structure at room temperature.

Step 2: Detailed Explanation:

• Ferrite ($\alpha$-iron):
Ferrite is a solid solution of interstitial carbon in alpha-iron ($\alpha$-Fe).
It is stable at room temperature and possesses a Body-Centered Cubic (BCC) crystal structure.
Due to the tight interstitial spaces in the BCC structure, ferrite can dissolve only a very small amount of carbon (maximum of $0.022 \text{ wt}\%$ at $727 \ ^\circ\text{C}$).

• Austenite ($\gamma$-iron):
Austenite is a solid solution of carbon in gamma-iron ($\gamma$-Fe).
It is stable at high temperatures (above $727 \ ^\circ\text{C}$ depending on carbon content) and has a Face-Centered Cubic (FCC) crystal structure.

• Martensite:
Martensite is formed by diffusionless, rapid quenching of austenite.
It has a highly distorted Body-Centered Tetragonal (BCT) crystal structure, which is a supersaturated solid solution of carbon in iron.

• Cementite ($Fe_3C$):
Cementite is an intermetallic compound of iron and carbon.
It has an Orthorhombic crystal structure containing 12 iron atoms and 4 carbon atoms per unit cell.

Step 3: Final Answer:
Ferrite possesses the Body-Centered Cubic (BCC) crystal structure.