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Chapter 9: Problem 33

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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Question: Explain the formation of alternating layers in a eutectic alloy during solidification. Answer: The formation of alternating layers in a eutectic alloy during solidification occurs due to simultaneous nucleation, competitive growth, and minimization of interfacial energy between the two solid phases. As the eutectic alloy solidifies, the 伪 and 尾 phase particles nucleate and grow simultaneously while competing for space and atoms from the liquid phase. This results in alternating thin layers of 伪 and 尾 phases, as each restricts the growth of the other, forming a stable microstructure due to the minimization of interfacial energy between the phases.

Step by step solution

01

Introduction to eutectic alloy solidification

In a eutectic alloy, the solidification process consists in the formation of two distinct solid phases from a liquid phase during cooling. The eutectic composition refers to the specific proportion of the two components, which leads to the lowest possible melting point. It is represented as the "E" point in the phase diagram of the alloy.
02

Eutectic reaction and the phase diagram

The phase diagram of the alloy plots the equilibrium phases as a function of temperature and composition. In the eutectic reaction, a liquid phase of a specific composition cools and solidifies into two distinct solid phases with different compositions. The eutectic reaction can be represented as: Liquid (eutectic composition) -> 伪-phase + 尾-phase, where 伪 and 尾 are the two solid phases.
03

Formation of alternating layers

As the eutectic alloy starts solidifying at the eutectic temperature, the two solid phases nucleate simultaneously. The 伪 and 尾 phase particles grow and compete for the space and atoms from the liquid phase. The 伪-phase particles restrict the growth of the 尾-phase particles in some directions and vice versa. This results in alternating thin layers of 伪 and 尾 phases, as the layers grow perpendicular to the interface between the liquid and solid phases.
04

Microstructure stability

The alternating layers form a stable microstructure configuration due to the minimization of interfacial energy. The interfacial energy between the two solid phases is minimized, which lowers the free energy of the system and results in a stable microstructure. In summary, upon solidification of an alloy with eutectic composition, a microstructure consisting of alternating layers of the two solid phases forms due to the simultaneous nucleation, competitive growth, and minimization of the interfacial energy between the phases, resulting in a stable configuration.

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Most popular questions from this chapter

Cite three variables that determine the microstructure of an alloy.

An intermetallic compound is found in the aluminum-zirconium system that has a composition of \(22.8 \mathrm{wt} \%\) Al-77.2 wt \(\% \mathrm{Zr}\). Specify the formula for this compound.

A hypothetical A-B alloy of composition 40 \(\mathrm{wt} \% \mathrm{~B}-60 \mathrm{wt} \% \mathrm{~A}\) at some temperature is found to consist of mass fractions of \(0.66\) and \(0.34\) for the \(\alpha\) and \(\beta\) phases, respectively. If the composition of the \(\alpha\) phase is \(13 \mathrm{wt} \%\) B-87 wt \(\% \mathrm{~A}\), what is the composition of the \(\beta\) phase?

For alloys of two hypothetical metals \(\mathrm{A}\) and \(\mathrm{B}\), there exist an \(\alpha\), A-rich phase and a \(\beta\), B-rich phase. From the mass fractions of both phases for two different alloys provided in the following table (which are at the same temperature), determine the composition of the phase boundary (or solubility limit) for both \(\alpha\) and \(\beta\) phases at this temperature. $$ \begin{array}{lcc} \hline \begin{array}{c} \text { Alloy } \\ \text { Composition } \end{array} & \begin{array}{c} \text { Fraction } \\ \boldsymbol{\alpha} \text { Phase } \end{array} & \text { Fraction } \\ \hline 70 \mathrm{wt} \% \mathrm{~A}-30 \mathrm{wt} \% \mathrm{~B} & 0.78 & 0.22 \\ \hline 35 \mathrm{wt} \% \mathrm{~A}-65 \mathrm{wt} \% \mathrm{~B} & 0.36 & 0.64 \\ \hline \end{array} $$

What is the principal difference between congruent and incongruent phase transformations?
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