Allotropy of iron: Delta-iron (the high-temperature phase of iron just below the melting point) possesses which crystal structure?

Introduction / Context:Pure iron exhibits allotropy—different crystal structures at different temperatures. Understanding these phases is important for interpreting phase diagrams, heat treatment responses, and high-temperature properties relevant to steelmaking and casting operations.

Given Data / Assumptions:

• Alpha-iron (ferrite) is stable at room temperature up to about 912°C with a BCC structure.
• Gamma-iron (austenite) is stable roughly from 912°C to 1394°C with an FCC structure.
• Delta-iron is stable from approximately 1394°C to 1539°C (melting point) with a BCC structure.

Concept / Approach:The question states that delta-iron possesses a body-centred cubic space lattice. This matches the accepted allotropic sequence: BCC (alpha) → FCC (gamma) → BCC (delta) as temperature increases to the melting point. The electronic and packing differences between BCC and FCC influence diffusion rates, solubility of carbon, and transformation kinetics in steels.

Step-by-Step Solution:Recall allotropy sequence with increasing temperature: alpha (BCC) → gamma (FCC) → delta (BCC).Identify delta-iron’s crystal structure: BCC.Compare with the statement: it claims BCC for delta-iron.Therefore, the statement is correct.

Verification / Alternative check:Iron–carbon phase diagram labels the delta-ferrite region near the liquidus as BCC, consistent with the statement.

Why Other Options Are Wrong:“Incorrect” would imply delta-iron is not BCC, which contradicts established metallurgical data.

Other placeholders do not apply; the core truth is the BCC nature of delta-iron.

Common Pitfalls:Confusing gamma (austenite, FCC) with delta; assuming the high-temperature phase must be FCC because gamma is.

Final Answer:Correct