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Chapter 9: Problem 28

Briefly explain why, upon solidification, an alloy of eutectic composition forms a microstructure consisting of alternating layers of the two solid phases.

Short Answer

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Answer: An alloy of eutectic composition forms a microstructure consisting of alternating layers of the two solid phases upon solidification due to the eutectic reaction. As the alloy cools down to the eutectic temperature, the liquid simultaneously transforms into both solid phases, with the distinct solid phases growing in alternate layers of uniform thicknesses. This minimizes interfacial energy between the two solid phases and results in a lamellar or rod-like microstructure, depending on the specific alloy and growth conditions.

Step by step solution

01

Understand the eutectic reaction

A eutectic reaction is a three-phase equilibrium reaction in which a liquid phase forms two distinct solid phases upon cooling. In an alloy, this occurs at a specific eutectic composition and temperature. The eutectic composition represents a unique proportion of two elements, at which the lowest melting point is achieved.
02

Formation of an alloy's microstructure during solidification

When an alloy solidifies, the microstructure is composed of two solid phases corresponding to the elements present in the alloy. During solidification, the two solid phases grow simultaneously. As the temperature drops, the solid-liquid equilibrium is maintained, and the levels of each element in the solid and liquid phases are altered.
03

Eutectic solidification

As the eutectic alloy cools down to the eutectic temperature, the liquid begins to transform simultaneously into the two solid phases. Due to the difference in diffusion rates, the solid phases grow into alternate layers with relatively uniform thicknesses. This happens because both solid phases want to minimize the interfacial energy between them by forming thin layers.
04

Eutectic microstructure

Upon reaching the eutectic temperature, the remaining liquid undergoes eutectic solidification, and the alternating layers of the two solid phases continue to form until the entire liquid phase solidifies. This results in a microstructure consisting of alternating layers of the two solid phases as a direct outcome of the eutectic reaction. The layers tend to have a lamellar or rod-like appearance depending on the specific alloy and growth conditions.

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Most popular questions from this chapter

For an alloy of composition \(74 \mathrm{wt} \% \mathrm{Zn}-26\) \(\mathrm{wt} \% \mathrm{Cu}\), cite the phases present and their compositions at the following temperatures: \(850^{\circ} \mathrm{C}, 750^{\circ} \mathrm{C}, 680^{\circ} \mathrm{C}, 600^{\circ} \mathrm{C}\), and \(500^{\circ} \mathrm{C}\).

A steel alloy is known to contain \(93.8\) wt \(\%\) \(\mathrm{Fe}, 6.0 \mathrm{wt} \% \mathrm{Ni}\), and \(0.2 \mathrm{wt} \% \mathrm{C}\). (a) What is the approximate eutectoid temperature of this alloy? (b) What is the proeutectoid phase when this alloy is cooled to a temperature just below the eutectoid? (c) Compute the relative amounts of the proeutectoid phase and pearlite. Assume that there are no alterations in the positions of other phase boundaries with the addition of Ni.

(a) Briefly describe the phenomenon of coring and why it occurs. (b) Cite one undesirable consequence of coring.

Is it possible to have an iron-carbon alloy for which the mass fractions of total ferrite and proeutectoid cementite are \(0.846\) and \(0.049\), respectively? Why or why not?

The mass fraction of \(e\) utectoid cementite in an iron-carbon alloy is \(0.104\). On the basis of this information, is it possible to determine the composition of the alloy? If so, what is its composition? If this is not possible, explain why.
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