Eutectic system:

Many binary systems have components which have limited solid solubility, e.g.: Cu-Ag, Pb-Sn. The regions of limited solid solubility at each end of a phase diagram are called terminal solid solutions as they appear at ends of the diagram.

Many of the binary systems with limited solubility are of eutectic type, which consists of specific alloy composition known as eutectic composition that solidifies at a lower temperature than all other compositions. This low temperature which corresponds to the lowest temperature at which the liquid can exist when cooled under equilibrium conditions is known as eutectic temperature. The corresponding point on the phase diagram is called eutectic point. When the liquid of eutectic composition is cooled, at or below eutectic temperature this liquid transforms simultaneously into two solid phases (two terminal solid solutions, represented by α and β). This transformation is known as eutectic reaction and is written symbolically as:

Liquid (L) ↔ solid solution-1 (α) + solid solution-2 (β)

This eutectic reaction is called invariant reaction as it occurs under equilibrium conditions at a specific temperature and specific composition which can not be varied. Thus, this reaction is represented by a thermal horizontal arrest in the cooling curve of an alloy of eutectic composition. A typical eutectic type phase diagram is shown in figure-4 along with a cooling curve.

As shown in figure-4, there exist three single phase regions, namely liquid (L), α and β phases. There also exist three two phase regions: L+α, L+β and α+β. These three two phase regions are separated by horizontal line corresponding to the eutectic temperature. Below the eutectic temperature, the material is fully solid for all compositions. Compositions and relative amount of the phases can be determined using tie-lines and lever rule. Compositions that are on left-hand-side of the eutectic composition are known as hypo-eutectic compositions while compositions on right-hand-side of the eutectic composition are called hyper-eutectic compositions. Development of micro-structure and respective cooling curves for eutectic alloys are shown in figure-5, 6, 7 and 8 for different compositions. The phase that forms during cooling but before reaching eutectic temperature is called pro-eutectic phase.

Figure-4: Typical phase diagram for a binary eutectic system.

In many systems, solidification in the solid + liquid region may lead to formation of layered (cored) grains, even at very slow cooling rates. This is as a result of very slow or no-diffusion in solid state compared with very high diffusion rates in liquids. The composition of the liquid phase evolves by diffusion, following the equilibrium values that can be derived from the tie-line method. However, new layers that solidify on top of the grains have the equilibrium composition at that temperature but once they are solid their composition does not change.

Figure-6: Cooling curve and micro-structure development for eutectic alloy that passes through terminal solid solution without formation of eutectic solid.

Invariant reactions: The eutectic reaction, in which a liquid transforms into two solid phases, is just one of the possible three-phase invariant reactions that can occur in binary systems those are not isomorphous. Schematically it can be shown as in figure-9. It represents that a liquid phase, L, transforms into two different solids phases (α and β) upon cooling during the eutectic reaction.

In the solid state analog of a eutectic reaction, called a eutectoid reaction, one solid phase having eutectoid composition transforms into two different solid phases. Another set of invariant reactions that occur often in binary systems are - peritectic reaction where a solid phase reacts with a liquid phase to produce a new solid phase, and in peritectoid reaction, two solid phases react to form a new solid phase. Peritectic reaction is commonly present as part of more-complicated binary diagrams, particularly if the melting points of the two components are quite different. Peritectic and peritectoid reactions do not give rise to micro-constituents as the eutectic and eutectoid reactions do. Another invariant reaction that involves liquid phase is monotectic reaction in which a liquid phase transforms into a solid phase and a liquid phase of different composition. Over a certain range of compositions the two liquids are immiscible like oil and water and so constitute individual phases, thus monotectic reaction can said to be associated with miscibility gaps in the liquid state. Example system for monotectic reaction: Cu-Pb at 954 ْ C and 36%Pb. Analog to monotectic reaction in solid state is monotectoid reaction in which a solid phase transforms to produce two solid phases of different compositions. Another notable invariant reaction that is associated with liquid immiscibility is syntectic reaction in which two liquid phases react to form a solid phase. All the invariant reactions are summarized in the table-1 showing both symbolic reaction and schematic part of phase diagram.