Allotropic Forms of Iron — Temperature Ranges Which allotropic form of iron is stable between 910°C and 1403°C under equilibrium conditions?

Introduction / Context:Pure iron exhibits several allotropic forms that appear over different temperature ranges. These phases—designated α, γ, and δ under modern nomenclature—have distinct crystal structures and magnetic properties, dictating transformation behavior in steels.

Given Data / Assumptions:

• Equilibrium temperature ranges for pure iron (no alloying additions).
• Interest in the phase stable from 910°C up to 1403°C.

Concept / Approach:Below ~910°C, α-iron (ferrite, BCC) is stable; between ~910°C and ~1403°C, γ-iron (austenite, FCC) is stable; from ~1403°C up to the melting point (~1538°C), δ-iron (BCC) reappears. The obsolete term β-iron referred to nonmagnetic α-iron just above the Curie point (~770°C) and is not a distinct crystal structure in modern usage.

Step-by-Step Solution:Identify the temperature interval: 910–1403°C.Match to phase: γ-iron (austenite, FCC) is stable in this range.Therefore, the correct selection is γ-iron.

Verification / Alternative check:Fe–C phase diagrams and metallurgy texts universally show the austenitic γ field occupying this interval in pure iron.

Why Other Options Are Wrong:

• α-iron: stable below 910°C (magnetic below ~770°C).
• β-iron: outdated term (nonmagnetic α), not a separate phase.
• δ-iron: stable only above ~1403°C up to melting.

Common Pitfalls:Confusing magnetic transition (Curie point) with structural transitions; the latter define α/γ/δ fields.

Final Answer:γ-iron