Eutectic system

Four eutectic structures: A) lamellar B) rod-like C) globular D) acicular.

The eutectic solidification is defined as follows:[6]

${\displaystyle {\text{Liquid}}{\xrightarrow[{\text{cooling}}]{\text{eutectic temperature}}}\alpha \,\,{\text{solid solution}}+\beta \,\,{\text{solid solution}}}$

This type of reaction is an invariant reaction, because it is in thermal equilibrium; another way to define this is the change in Gibbs free energy equals zero. Tangibly, this means the liquid and two solid solutions all coexist at the same time and are in chemical equilibrium. There is also a thermal arrest for the duration of the change of phase during which the temperature of the system does not change.[6]

The resulting solid macrostructure from a eutectic reaction depends on a few factors, with the most important factor being how the two solid solutions nucleate and grow. The most common structure is a lamellar structure, but other possible structures include rodlike, globular, and acicular.[7]

Compositions of eutectic systems that are not at the eutectic composition can be classified as hypoeutectic or hypereutectic. Hypoeutectic compositions are those with a smaller percent composition of species β and a greater composition of species α than the eutectic composition (E) while hypereutectic solutions are characterized as those with a higher composition of species β and a lower composition of species α than the eutectic composition. As the temperature of a non-eutectic composition is lowered the liquid mixture will precipitate one component of the mixture before the other. In a hypereutectic solution, there will be a proeutectoid phase of species β whereas a hypoeutectic solution will have a proeutectic α phase.[6]

The composition and temperature of a eutectic can be calculated from enthalpy and entropy of fusion of each components.[18]

The Gibbs free energy G depends on its own differential:

${\displaystyle G=H-TS\Rightarrow {\begin{cases}H=G+TS\\\left({\frac {\partial G}{\partial T}}\right)_{P}=-S\end{cases}}\Rightarrow H=G-T\left({\frac {\partial G}{\partial T}}\right)_{P}.}$

Thus, the G/T derivative at constant pressure is calculated by the following equation:

${\displaystyle \left({\frac {\partial G/T}{\partial T}}\right)_{P}={\frac {1}{T}}\left({\frac {\partial G}{\partial T}}\right)_{P}-{\frac {1}{T^{2}}}G=-{\frac {1}{T^{2}}}\left(G-T\left({\frac {\partial G}{\partial T}}\right)_{P}\right)=-{\frac {H}{T^{2}}}.}$

The chemical potential ${\displaystyle \mu _{i}}$ is calculated if we assume that the activity is equal to the concentration:

${\displaystyle \mu _{i}=\mu _{i}^{\circ }+RT\ln {\frac {a_{i}}{a}}\approx \mu _{i}^{\circ }+RT\ln x_{i}.}$

At the equilibrium, ${\displaystyle \mu _{i}=0}$, thus ${\displaystyle \mu _{i}^{\circ }}$ is obtained as

${\displaystyle \mu _{i}=\mu _{i}^{\circ }+RT\ln x_{i}=0\Rightarrow \mu _{i}^{\circ }=-RT\ln x_{i}.}$

Using and integrating gives

${\displaystyle \left({\frac {\partial \mu _{i}/T}{\partial T}}\right)_{P}={\frac {\partial }{\partial T}}\left(R\ln x_{i}\right)\Rightarrow R\ln x_{i}=-{\frac {H_{i}^{\circ }}{T}}+K.}$

The integration constant K may be determined for a pure component with a melting temperature ${\displaystyle T^{\circ }}$ and an enthalpy of fusion ${\displaystyle H^{\circ }}$:

${\displaystyle x_{i}=1\Rightarrow T=T_{i}^{\circ }\Rightarrow K={\frac {H_{i}^{\circ }}{T_{i}^{\circ }}}.}$

We obtain a relation that determines the molar fraction as a function of the temperature for each component:

${\displaystyle R\ln x_{i}=-{\frac {H_{i}^{\circ }}{T}}+{\frac {H_{i}^{\circ }}{T_{i}^{\circ }}}.}$

The mixture of n components is described by the system

${\displaystyle {\begin{cases}\ln x_{i}+{\frac {H_{i}^{\circ }}{RT}}-{\frac {H_{i}^{\circ }}{RT_{i}^{\circ }}}=0,\\\sum \limits _{i=1}^{n}x_{i}=1.\end{cases}}}$

${\displaystyle {\begin{cases}\forall i<n\Rightarrow \ln x_{i}+{\frac {H_{i}^{\circ }}{RT}}-{\frac {H_{i}^{\circ }}{RT_{i}^{\circ }}}=0,\\\ln \left(1-\sum \limits _{i=1}^{n-1}x_{i}\right)+{\frac {H_{n}^{\circ }}{RT}}-{\frac {H_{n}^{\circ }}{RT_{n}^{\circ }}}=0,\end{cases}}}$

which can be solved by

${\displaystyle {\begin{array}{c}\left[{\begin{array}{*{20}c}{\Delta x_{1}}\\{\Delta x_{2}}\\{\Delta x_{3}}\\\vdots \\{\Delta x_{n-1}}\\{\Delta T}\\\end{array}}\right]=\left[{\begin{array}{*{20}c}{1/x_{1}}&0&0&0&0&{-{\frac {H_{1}^{\circ }}{RT^{2}}}}\\0&{1/x_{2}}&0&0&0&{-{\frac {H_{2}^{\circ }}{RT^{2}}}}\\0&0&{1/x_{3}}&0&0&{-{\frac {H_{3}^{\circ }}{RT^{2}}}}\\\vdots &\ddots &\ddots &\ddots &\ddots &{\vdots }\\0&0&0&0&{1/x_{n-1}}&{-{\frac {H_{n-1}^{\circ }}{RT^{2}}}}\\{\frac {-1}{1-\sum \limits _{i=1}^{n-1}{x_{i}}}}&{\frac {-1}{1-\sum \limits _{i=1}^{n-1}{x_{i}}}}&{\frac {-1}{1-\sum \limits _{i=1}^{n-1}{x_{i}}}}&{\frac {-1}{1-\sum \limits _{i=1}^{n-1}{x_{i}}}}&{\frac {-1}{1-\sum \limits _{i=1}^{n-1}{x_{i}}}}&{-{\frac {H_{n}^{\circ }}{RT^{2}}}}\\\end{array}}\right]^{-1}.\left[{\begin{array}{*{20}c}{\ln x_{1}+{\frac {H_{1}^{\circ }}{RT}}-{\frac {H_{1}^{\circ }}{RT_{1}^{\circ }}}}\\{\ln x_{2}+{\frac {H_{2}^{\circ }}{RT}}-{\frac {H_{2}^{\circ }}{RT_{2}^{\circ }}}}\\{\ln x_{3}+{\frac {H_{3}^{\circ }}{RT}}-{\frac {H_{3}^{\circ }}{RT_{3}^{\circ }}}}\\\vdots \\{\ln x_{n-1}+{\frac {H_{n-1}^{\circ }}{RT}}-{\frac {H_{n-1}^{\circ }}{RT_{n-1}^{\circ }}}}\\{\ln \left({1-\sum \limits _{i=1}^{n-1}{x_{i}}}\right)+{\frac {H_{n}^{\circ }}{RT}}-{\frac {H_{n}^{\circ }}{RT_{n}^{\circ }}}}\\\end{array}}\right]\end{array}}}$

• Azeotrope, or constant boiling mixture
• Freezing-point depression

Look up eutectic in Wiktionary, the free dictionary.

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• Easterling, Edward (1992). Phase Transformations in Metals and Alloys. CRC. ISBN 978-0-7487-5741-1.
• Mortimer, Robert G. (2000). Physical Chemistry. Academic Press. ISBN 978-0-12-508345-4.
• Reed-Hill, R. E.; Reza Abbaschian (1992). Physical Metallurgy Principles. Thomson-Engineering. ISBN 978-0-534-92173-6.
• Sadoway, Donald (2004). "Phase Equilibria and Phase Diagrams" (PDF). 3.091 Introduction to Solid State Chemistry, Fall 2004. MIT Open Courseware. Archived from the original (PDF) on 2005-10-20. Retrieved 2006-04-12.