Metallurgy — Definition Check on Austenite Is the following statement correct for materials science terminology: “Austenite is a solid solution of carbon or iron carbide (cementite) in gamma-iron (γ-Fe)”?

Introduction / Context:
Austenite is one of the fundamental phases in iron–carbon metallurgy. It forms the basis for many steel heat treatments such as hardening, normalising, and austempering. This question tests whether you know the precise definition of austenite and can distinguish it from other phases like cementite (iron carbide) and ferrite.

Given Data / Assumptions:

• The statement claims austenite is a solid solution of “carbon or iron carbide” in gamma-iron.
• Gamma-iron (γ-Fe) is the face-centred cubic form of iron stable at elevated temperatures in steels.
• Carbon solubility in γ-Fe is finite but significant compared with α-Fe.

Concept / Approach:
The precise definition is: austenite is a solid solution of carbon in gamma-iron (γ-Fe). Cementite (Fe3C), commonly called iron carbide, is a separate intermetallic compound phase, not something that dissolves into γ-Fe as “iron carbide.” Thus, including “iron carbide” in the definition is incorrect. When Fe3C coexists with austenite, it appears as a separate phase in the microstructure (e.g., ledeburite or pearlite constituents), not as solute in γ-Fe.

Step-by-Step Solution:
Identify the matrix: γ-Fe (austenitic iron) has an FCC lattice.Recognise the solute: interstitial carbon atoms occupy octahedral sites → austenite is C in γ-Fe.Contrast with cementite: Fe3C is a distinct compound; it does not define austenite by dissolution.Conclude the statement is wrong because it adds “iron carbide” to the definition.

Verification / Alternative check:
Iron–carbon phase diagram shows austenite as a single phase field (γ) with carbon in solution; Fe3C appears as a separate phase line/field.

Why Other Options Are Wrong:
Correct/true variants: they contradict the standard definition.“True for hypo-/hyper-eutectoid steels only”: steel composition does not change the phase definition.“Only true at very high carbon”: still incorrect; Fe3C remains a separate phase.

Common Pitfalls:
Confusing “phase present with” versus “dissolved in.” Cementite may coexist with austenite, but it is not dissolved as “iron carbide.”

Final Answer:
Incorrect