Eutectic Point

A eutectic system is a homogenous, solid mixture of two or more substances that form a super-lattice that melts or solidifies at a temperature lower than any of the individual ingredients' melting point. The term is most usually used to describe a mix of metals. Only when there is a precise ratio between the components does a eutectic system form.

The "Eutectic Point" is the lowest freezing point that can be reached for a eutectic combination (which means lowest melting point).

• This is the point in the solid-liquid system where all three phases, namely the liquid melt of the two metals and the solid phases of each of the components, are in equilibrium.

• The melting point of the mixture corresponding to the eutectic point is the lowest.

• At this stage, the system is invariant (i.e., the degrees of freedom or variance are zero) and has a fixed temperature and composition.

A eutectic system is a homogeneous mixture of substances that melts or solidifies at a single temperature lower than any of the constituents' melting points. This temperature is known as the eutectic temperature, and it is the lowest feasible melting temperature for the included component species for all mixing ratios. The eutectic temperature is shown as the eutectic point on a phase diagram.

Because one component's lattice melts at a lower temperature than the other's, non-eutectic mixture ratios will have distinct melting temperatures for their constituents. Each ingredient of a non-eutectic mixture solidifies (forms a lattice) at a different temperature as it cools until the entire mass solidifies.

Because the valence electrons of the component species are not always compatible in any mixing ratio to produce a new type of joint crystal lattice, not all binary alloys have eutectic points. The melt temperature (liquidus) and freeze temperature (solidus) of the silver-gold system, for example, meet at the pure element endpoints of the atomic ratio axis while slightly separating in the mixing area.

A eutectic system is a system of a homogeneous mixture of substances that either melts or solidifies at a particularly 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 ratios 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 pro eutectoid 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. 

Uses of Eutectic Alloys

Some uses are-

• For electrical protection of 3-phase motors for pumps, fans, conveyors, and other factory process equipment, NEMA Eutectic Alloy Overload Relays are used.  

• Eutectic soldering alloys, including traditional lead (Pb) and tin (Sn) alloys, sometimes with additional silver (Ag) or gold (Au) — especially Sn₆3Pb₃₇ and Sn₆2Pb₃6Ag₂ alloy formula for electronics — and newer lead-free soldering alloys, such as Sn₉₆.5Ag_3.5, composed of tin (Sn), silver (Ag), and copper (Cu).

• Aluminium-silicon and cast iron alloys are examples of casting alloys (at the composition of 4.3 per cent carbon in iron-producing and austenite-cementite eutectic)

• The application of ultrasonic energy to silicon chips bonds them to gold-plated substrates through a silicon-gold eutectic. See eutectic bonding for further information.

• Due to the removal of alloying components from the joint via diffusion during brazing, eutectic melting is only possible early in the process.

• Wood's metal and Field's metal for fire sprinklers are examples of temperature responsiveness.

• Mercury substitutes that aren't poisonous, such as galinstan

• Glassy metals with extraordinary strength and corrosion resistance are being tested.

• At room temperature, eutectic sodium and potassium alloys (NaK) are liquid and are employed as coolants in experimental fast neutron nuclear reactors.

Hence article discusses eutectic point and eutectic alloys and their utility etc. It covers all the important information that is important for a student.