Cooling an exactly eutectic solution: As a binary solution of eutectic composition is cooled, what happens before the eutectic temperature is reached?

Introduction / Context:
Eutectic systems are crucial in alloy design, soldering, and crystallisation. A eutectic composition freezes at a single, sharp temperature where two solid phases form simultaneously from the liquid.

Given Data / Assumptions:

• Binary system at exactly eutectic composition.
• Equilibrium cooling under negligible undercooling.
• No impurities shifting the phase diagram.

Concept / Approach:
At eutectic composition, the liquid remains entirely liquid until the eutectic temperature Te is reached. Then the eutectic reaction L → α + β occurs isothermally at Te, producing a characteristic fine microstructure.

Step-by-Step Solution:

Verification / Alternative check:
Compare to hypoeutectic or hypereutectic compositions, where primary crystals appear before Te; that contrast confirms the unique “no change until Te” behavior at the eutectic composition.

Why Other Options Are Wrong:

• Solvent freezing before Te: Occurs only off-eutectic (primary solid forms prior to Te).
• No solidification at or past Te: Contradicts the definition of the eutectic reaction.
• None: Not applicable since (b) is correct.

Common Pitfalls:
Confusing eutectic with azeotropic behavior or misreading phase diagrams; pay attention to composition versus temperature paths.

Final Answer:
It undergoes no phase change until the eutectic temperature is reached