Question

Cast iron is

• A. Eutectic alloy
• B. Isomorphous alloy
• C. Peritectic alloy
• D. Peritectoid alloy

Hint:

Remember: The eutectic composition of iron–carbon system occurs at 4.3% carbon.

Answer 1

The Correct Option is A

Step 1: Understanding cast iron composition.
Cast iron contains more than 2% carbon, typically around 4.3% carbon, which corresponds to the eutectic composition in the iron–carbon phase diagram.
Step 2: Eutectic reaction in cast iron.
At 4.3% carbon, liquid iron transforms directly into a mixture of austenite and cementite at a constant temperature, which is known as a eutectic reaction.
Step 3: Analysis of other options.
(B) Isomorphous alloy: These alloys show complete solubility, which is not the case for iron–carbon systems.
(C) Peritectic alloy: Involves a different type of phase transformation.
(D) Peritectoid alloy: Occurs in solid-state reactions only.
Step 4: Conclusion.
Cast iron is correctly classified as a eutectic alloy.

Answer 2

The type of phase reaction that occurs at a given composition on the iron-carbon diagram is what distinguishes eutectic, peritectic, and peritectoid alloys, so locating cast iron's composition on that diagram settles the classification directly.

1. Eutectic alloy: Cast iron typically contains carbon in the range of about 2 to 4 percent, close to the eutectic composition of roughly 4.3 percent carbon on the iron-carbon diagram, where liquid iron transforms directly, at a single constant temperature, into a solid mixture of austenite and cementite; this is the defining eutectic reaction, and cast iron's composition and solidification behaviour place it in this category.
2. Isomorphous alloy: This term describes systems where the two components are completely soluble in each other in both liquid and solid states across the full composition range, which does not describe the iron-carbon system, since iron and carbon are only partially soluble and form distinct phases.
3. Peritectic alloy: The peritectic reaction in the iron-carbon system occurs at a very different, much lower carbon content and higher temperature region of the diagram, describing the formation of austenite from liquid and delta-ferrite, not the composition range relevant to cast iron.
4. Peritectoid alloy: A peritectoid reaction is a solid-state transformation between two solid phases forming a third, which is a different category of reaction entirely from the liquid-to-solid transformation that governs cast iron's solidification.

Since cast iron solidifies through a liquid-to-two-solid-phases transformation at a fixed temperature near the 4.3 percent carbon point, its behaviour matches the eutectic reaction rather than any of the other phase reaction types.

Therefore, the correct answer is Eutectic alloy.