Crystal structures of iron — phase identification: Which type of space lattice is exhibited by alpha-iron (ferrite) at room temperature?

Introduction / Context:Iron exhibits allotropic transformations, changing its crystal structure with temperature. Recognizing the structure of alpha-iron at ambient conditions is foundational for understanding steel heat treatment, diffusion, and mechanical behavior.

Given Data / Assumptions:

• Alpha-iron (ferrite) is stable below approximately 912°C.
• Question targets the room-temperature lattice type.
• Common structures: BCC, FCC, HCP.

Concept / Approach:At room temperature, pure iron exists as ferrite with a BCC lattice. Upon heating above about 912°C up to 1394°C, iron becomes austenite with FCC structure, and above that transforms again before melting. Martensite is a supersaturated, distorted tetragonal phase formed by rapid quenching in steels rather than a stable form of pure iron.

Step-by-Step Solution:Identify the temperature range for ferrite: below ~912°C.Recall that alpha-iron (ferrite) → BCC lattice.Eliminate FCC (austenite) and HCP (not a phase of pure iron at ambient).Select BCC as the correct lattice for alpha-iron.

Verification / Alternative check:Phase diagrams and metallurgy texts corroborate BCC ferrite at room temperature with low carbon solubility.

Why Other Options Are Wrong:FCC corresponds to gamma-iron (austenite); HCP is not a stable crystal structure for pure iron under normal conditions; “none of these” and “tetragonal martensitic” do not describe alpha-iron.

Common Pitfalls:Confusing ferrite and austenite stability ranges or equating martensite (a non-equilibrium phase) with base iron structures.

Final Answer:body-centred cubic (BCC) space lattice