Question

Allotropic forms of iron in sequence is

• A. BCC alpha, HCP gamma and FCC gamma
• B. BCC alpha, FCC gamma and BCC delta
• C. FCC alpha, BCC delta and BCC gamma
• D. FCC gamma, BCC alpha and HCP delta

Hint:

Remember the iron phases with the acronym "A G D" for Alpha, Gamma, Delta. The crystal structures go from BCC -> FCC -> BCC. The first and last solid phases (alpha and delta) are both BCC. The intermediate high-temperature phase (gamma) is FCC.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Allotropy (or polymorphism) is the ability of a solid element to exist in more than one crystal structure. Pure iron exhibits allotropy, changing its crystal structure at specific temperatures upon heating from room temperature to its melting point. The question asks for the sequence of these structures.

Step 2: Detailed Explanation:
Let's trace the phases of pure iron as it is heated:

• From room temperature up to 912°C (1674°F): Iron exists as

alpha-ferrite (\(\alpha\)-Fe). It has a

Body-Centered Cubic (BCC) crystal structure. It is ferromagnetic up to the Curie temperature of 770°C.

• From 912°C to 1394°C (2541°F): The crystal structure transforms to

gamma-austenite (\(\gamma\)-Fe). Austenite has a

Face-Centered Cubic (FCC) crystal structure. This phase is non-magnetic.

• From 1394°C to its melting point at 1538°C (2800°F): The structure transforms back to a

Body-Centered Cubic (BCC) form, which is called

delta-ferrite (\(\delta\)-Fe).
So, the sequence of crystal structures with increasing temperature is: \[ \text{BCC (\(\alpha\)-ferrite)} \xrightarrow{912^\circ C} \text{FCC (\(\gamma\)-austenite)} \xrightarrow{1394^\circ C} \text{BCC (\(\delta\)-ferrite)} \xrightarrow{1538^\circ C} \text{Liquid} \] Now let's check the options:
(A) BCC alpha, HCP gamma and FCC gamma - Incorrect, gamma iron is FCC, not HCP.
(B) BCC alpha, FCC gamma and BCC delta - This matches the correct sequence perfectly.
(C) FCC alpha, BCC delta and BCC gamma - Incorrect, alpha iron is BCC, and the order is wrong.
(D) FCC gamma, BCC alpha and HCP delta - Incorrect order and delta iron is BCC, not HCP.

Step 3: Final Answer:
The correct sequence of allotropic forms of iron with increasing temperature is BCC alpha-ferrite, followed by FCC gamma-austenite, and finally BCC delta-ferrite.