Question

Why are eutectic reactions considered incongruent?

Answer 1

A eutectic reaction is one in which, upon cooling, a liquid transforms to two solid phases: L --> A + B

My textbook defines a congruent phase transformation as: "A transformation for which there is no change in composition for the phases involved".

The book gives eutectic reactions as an example of an incongruous phase transformation.

However, I do not understand why. If there is no solid phase present initially, how can the solid phase be considered to have changed composition?

Could anyone perhaps give me a clearer explanation of what a congruent reaction is? I have searched around but all references I can find give the same short sentence about composition.

Thank you

Answer 2

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A eutectic reaction is a three-phase reaction, by which, on cooling, a liquid transforms into two solid phases at the same time. It is a phase reaction, but a special one. For example: liquid alloy becomes a solid mixture of alpha and beta at a specific temperature (rather than over a temperature range).