Question

Based on Iron-Carbon diagram, when austenite decomposes during cooling process, an eutectoid mixture is formed. The nature of the element is

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

Hint:

Remember the definition of the eutectoid point on the Iron-Carbon diagram: - Temperature = 727 $^\circ$C, Carbon = 0.76%. - $\text{Austenite } (\gamma) \rightarrow \text{Ferrite } (\alpha) + \text{Cementite } (\text{Fe}_3\text{C}) = \textbf{Pearlite}$. Pearlite is a two-phase microstructural mixture, not a single individual phase.

Answer 1

The Correct Option is D

Concept: The Iron-Carbon ($\text{Fe}$-$\text{Fe}_3\text{C}$) equilibrium phase diagram contains three distinct isothermal invariant reactions: peritectic, eutectic, and eutectoid. The eutectoid reaction occurs during slow cooling when a single solid phase transforms directly into two distinct solid phases at a specific temperature and composition. For steel, this reaction occurs at a carbon concentration of 0.76% to 0.80% C and a temperature of 727 $^\circ$C. The high-temperature solid phase, Austenite ($\gamma$), decomposes into two new solid phases: \[ \gamma \text{ (Austenite)} \xrightarrow[\text{Cooling}]{\text{Eutectoid at } 727^\circ\text{C}} \alpha \text{ (Ferrite)} + \text{Fe}_3\text{C (Cementite)} \] This simultaneous cooperative growth forms alternating lamellar (layer-like) bands of soft, ductile ferrite and hard, brittle cementite. This unique lamellar microstructural mixture is named Pearlite. Under an optical microscope, its layered structure reflects light like mother-of-pearl.

Step 1: Evaluate the constituent phases and structures.
Let's analyze the microstructural terms listed in the options to clarify their definitions:
• Ferrite ($\alpha$): A solid solution of carbon in iron with a Body-Centered Cubic (BCC) crystal structure. It is soft and ductile, representing one of the individual component phases inside the eutectoid mixture.
• Cementite ($\text{Fe}_3\text{C}$): An iron carbide intermetallic compound containing 6.67% C by weight. It is hard and brittle, forming the second individual component phase inside the eutectoid mixture.
• Martensite: A metastable phase formed when austenite is quenched rapidly to room temperature, trapping carbon atoms in a diffusionless transformation that distorts the lattice into a Body-Centered Tetragonal (BCT) structure. It is a non-equilibrium phase and does not form via slow eutectoid cooling.
• Pearlite: The name given to the lamellar eutectoid mixture consisting of alternating layers of ferrite and cementite.

Step 2: Match the question description to the correct mixture.
The question asks for the name of the eutectoid mixture formed when austenite decomposes during cooling. Based on the phase diagram reaction, this mixture is Pearlite, matching Option (D).