A eutectic system is a system of a homogeneous mixture of substances that either melts or solidifies at a particular given temperature that is lower than the melting point of any of the mixture of any of the constituent elements. This particular temperature is known as the eutectic point. Thus, for a liquid mixture eutectic point or the eutectic temperature is the lowest temperature at which a liquid can exist before solidifying in a eutectic system. Thus, the eutectic point definition is that the eutectic temperature is the lowest point of the melting temperature over all of the mixing ratios for the involved component species.
Eutectic System and Eutectic Temperature
The Eutectic system is defined already in the introduction. Upon the heating of any of the other mixture ratio and on reaching the eutectic temperature, one of the component’s of the lattice will melt first in a eutectic system, while the temperature of the entire system of the mixture has to increase for all the other components of the component lattices for melting. On the other hand, a non-eutectic mixture cools down, each of the components of the mixture will solidify (during the formation of its lattice) at a unique and different temperature, until all of the material is solid. The phase diagram is shown below:
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The terms by which the eutectic point is characterized on a phase diagram are the eutectic percentage ratio (on the atomic or molecular ratio axis (X-axis) of the diagram) and the eutectic temperature (on the Y-axis of the diagram). Not all the binary alloys have a eutectic point and the reason for this is that the valence electrons of the component species are not always compatible, in any of the mixing ratios, to form a new type of joint crystal lattice of the mixture. An example of this is the silver-gold eutectic system: the melting point and the freezing point meet at the pure element endpoints of the atomic ratio axis while slightly separating in the mixture region of the axis.
Phase Transition of the Eutectic System
The solidification of a eutectic system is defined as follows:
Liquid ⟶ ⍺ solid solution + 𝛽 solid solution (at eutectic temperature cooling)
This particular type of reaction is invariant because it is usually in thermal equilibrium. Another way to define such a system is the change in the Gibbs free energy totalling zero. Practically, this means that the liquid and the two solid solutions all coexist at the same time maintaining chemical equilibrium. There also occurs a thermal arrest for the time period of the change of phase during which the system temperature does not change at all.
The resulting macrostructure which is solid in nature forms a eutectic reaction depending on the few factors out of which the most important factor is how the two solid solutions nucleate resulting in further growth. The most common macrostructures formed are lamellar structures. Few other possible structures include rodlike, globular and acicular structures.
Compositions of the Non-eutectic Mixtures
The compositions of systems that are not eutectic in nature are classified as hypo- or hyper-eutectic. The hypoeutectic compositions are those with smaller percentage composition of species beta and a greater composition of species alpha than the eutectic composition (E) while hypereutectic solutions are defined as those with the more and higher composition of beta species and a lower composition of alpha-species. The non-eutectic temperature of a non-eutectic composition decreases than the liquid mixture, it will precipitate as one component of the mixture before any other. On the other hand, in a hyper eutectic solution, there will be a phase - the proeutectoid phase of beta species whereas a hypoeutectic solution will have an alpha proeutectic phase.
Eutectic Alloys and their Utility
The alloys having a eutectic system and a eutectic temp have are made up of two or more materials and by definition have a eutectic composition. When the non-eutectic alloy is solidified, its components become solidified at different temperatures having plasticity in the range of melting points. As opposed to this a very well-mixed eutectic alloy melts at a single and sharp temperature which is the eutectic point temperature. The different phase transformation that occurs during the event of solidification of a particular alloy composition can be understood by the vertical line starting from the liquid phase continuing to the solid phase on the phase diagram of that alloy. Examples of such eutectic alloys are given below:
Casting alloys such as aluminium-silicon alloy and cast iron alloy.
Experimental glassy materials with highly extreme and corrosive resistance.
Eutectic alloys are made of sodium and potassium that are usually liquid at room temperature and are used as a coolant in the experimental fast neutron nuclear reactors.
At eutectic point the system has the lowest temperature before the melting of the mixture and this point is lower than the lowest of the melting temperatures of the different components available in the mixture. Such characteristics of the eutectic system have many advantages and are therefore used in many of the alloy making processes.