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

Classifying an alloy as eutectic, peritectic, peritectoid, or isomorphous depends on what kind of reaction governs its solidification. Isomorphous systems involve complete solid solubility across all compositions, which iron and carbon do not exhibit. Peritectic and peritectoid reactions occur at different, specific regions of the iron-carbon diagram involving delta-ferrite or solid-state phase transformations, neither of which describes the way ordinary cast iron solidifies. Cast iron's composition, around 4.3 percent carbon, sits right at the point where liquid iron transforms directly into a solid mixture of austenite and cementite at one constant temperature, which is precisely the definition of a eutectic reaction, making cast iron a eutectic alloy.

Therefore, the correct answer is Eutectic alloy.

Answer 2

Defining each reaction type first and then matching cast iron's known solidification behaviour against those definitions is another reliable route to the same answer.

1. Eutectic alloy: A eutectic reaction is defined as liquid transforming into two solid phases simultaneously at one fixed temperature, and this is exactly what happens when cast iron solidifies near its 4.3 percent carbon composition, producing austenite and cementite together.
2. Isomorphous alloy: This describes a system with a single continuous solid solution across all compositions and temperatures, which does not apply to iron and carbon, since they form multiple distinct phases rather than one continuous solution.
3. Peritectic alloy: This describes liquid reacting with one existing solid phase to form a different solid phase, a reaction that occurs in the iron-carbon system only at very low carbon content, far from where cast iron's composition lies.
4. Peritectoid alloy: This describes a reaction between two solid phases forming a third solid phase without any liquid involved at all, which does not match cast iron's actual solidification from the liquid state.

Matching the definition of each reaction type to cast iron's real solidification path confirms that it undergoes a eutectic transformation.

Therefore, the correct answer is Eutectic alloy.