Allotropic Forms of Iron Which allotropic form of iron is stable at about 910° C in plain (unalloyed) iron under equilibrium conditions?

Introduction / Context:
Iron exhibits several allotropic forms (α, γ, δ) depending on temperature, each with distinct crystal structures and magnetic behavior. Knowing the stability ranges is essential for interpreting the iron–carbon diagram and heat treatment responses of steels.

Given Data / Assumptions:

• Purez (plain) iron under near-equilibrium heating conditions.
• Transformation temperatures are the conventional textbook values.
• Magnetic transition (Curie point) is distinguished from structural transitions.

Concept / Approach:
α-iron (ferrite, BCC) is stable up to ~910° C. At ~910° C, the structure transforms to γ-iron (austenite, FCC), which remains stable up to ~1394° C. The term β-iron is sometimes used to denote paramagnetic α-iron between ~768° C (Curie point) and ~910° C, but structurally it is still BCC. δ-iron (also BCC) is stable only at very high temperatures near melting (~1394–1538° C). Therefore, at about 910° C (just into the transformed region), the stable phase is γ-iron (austenite).

Step-by-Step Solution:
Below 910° C: α-iron (BCC) is stable.At ~910° C and above: transformation to γ-iron (FCC) occurs.Thus, around 910° C in the austenitic field, the correct allotrope is γ-iron.

Verification / Alternative check:
Refer to the standard Fe–C diagram: A3 for pure iron is ~910° C, above which γ (FCC) exists.

Why Other Options Are Wrong:
α-iron: stable below ~910° C, not above.β-iron: not a distinct crystal structure; it is paramagnetic α-iron (still BCC).δ-iron: appears at much higher temperature near melting.

Common Pitfalls:
Confusing the Curie point (~768° C) with structural transformation; magnetism changes at 768° C, crystal structure changes at ~910° C.

Final Answer:
γ-iron