In this article we will discuss about the phase diagram showing monotectic and syntectic reaction.

Phase Diagram Showing Monotectic Reaction:

In some alloy systems, two liquid solutions are seen to be immiscible (insoluble) only over a certain composition range, and in most cases (there are some exceptions), the mutual miscibility is found to increase with the rise of temperature.

For example, in Cu-Pb system, between the composition range of 36% Pb to 87% Pb, and just above 954°C, two liquid solutions, Liquid L1 (36 % Pb) and L2 (87% Pb) are present in equilibrium as these are immiscible in each other (the dome-shaped curve is miscibility gap having both L1 and L2).

As the temperature rises, their miscibility increases such that above temperature 990°C, both liquid solutions are miscible completely in each other, i.e., form a homogeneous liquid solution. L1 is a liquid solution of lead in copper, and L2 is a liquid solution of copper in lead, ABCDEF is a liquidus and AHEGF is the solidus.


Point B is a monotectic point, and 954°C is a monotectic-reaction tempera­ture. Monotectic-reactions are always asso­ciated with miscibility gaps in the liquid state. Fig. 3.34 illustrates that liquid miscibility gap lies just to the right of the monotectic point.

Monotectic reaction, in general, is:

In Cu-Pb system, the monotectic reaction is that, on cooling, a liquid (L1 = 36% Pb) at a constant temperature of monotectic reaction, transforms to another liquid (L2 = 87% Pb) and a solid (copper here):

i.e., liquid L1 should have composition of 36% Pb, and at 954°C (while cooling), the monotectic invariant reaction occurs to produce another liquid of 87% Pb and pure solid copper. In this diagram, the terminal solids are shown as pure copper and pure lead, because the solid solubility of Pb in Cu, and of Cu in Pb are almost negligibly small (at room temperature, solubility of Cu in Pb is less than 0.007%, and of Pb in Cu is 0.002 to 0.005%).

The cooling of Cu-15% Pb alloy has been shown in Fig. 3.35.

The alloy remains a liquid solution, on cooling, till it reaches its liquidus temperature T1, where freezing begins of pure copper. The composition of liquid starts changing along liquidus from T1 to B.


As the alloy cools, more dendrites of copper solidify till the monotectic temperature (954°C) is reached. Lever rule can be used to calculate the amount of liquid, L1 and copper, at a fraction of degree above monotectic temperature.

The chemical composition of this liquid, L1 is given by monotectic point, B (36% Pb)- (Use ‘tie-line’ JB):

Thus, monotectic reaction produces 24.43% of copper (solid) (82.76—58.33) and 17.24% of L2. As the alloy cools further, more solid copper solidifies from the liquid L2 till the alloy reaches eutectic temperature (326°C), where the liquid L2 has the eutectic composition (99.94 % Pb).


The amount of liquid transforming to eutectic mixture of Cu and Pb, can be calculated by using ‘tie-line HE’:

Thus, after the eutectic reaction, microstructure consists of 85% of grains of copper and 15% of eutectic mixture of Cu and Pb. This microstructure remains unchanged when the alloy is cooled to room temperature.

Zn-Bi, Cu-Te, Cu-Cr, Pb-Zn, Pb-Cr, Pb-Al etc. show monotectic reactions. Unfortunately, segregation invariably occurs in alloys, otherwise Cu-Pb alloys would have been very inexpensive bearing alloys.


The cooling of an alloy (Cu-60% Pb) is also illustrated in Fig. 3.35 (b) with its cooling curve in Fig. 3.35 (c). The homogeneous liquid solution of the alloy on reaching the liquidus at temperature, T2, gets saturated with Pb, and with further cooling, Pb is rejected from it.

As it separates, this Pb is much above its melting point, and appears as a second liquid. Since it is in contact with Cu-rich liquid, gets saturated with Cu. As the temperature falls, copper-rich liquid continuously rejects Pb, but remains saturated with it.

Thus, the amount of copper-rich liquid diminishes, while of lead- rich liquid increases, till the alloy reaches the monotectic temperature (954°C). Just slightly above this temperature, Lever rule could be used to calculate the amounts of these two liquids (L1 is copper-rich and L2 is lead-rich) using ‘BPD’ as ‘tie-line’.

The separation of Cu-Pb liquid into two immiscible liquids evolves little heat, and so it is difficult to detect this change easily by thermal analysis, although, it does delay cooling slightly (the slope of curve ‘WO’ in cooling curve in Fig. 3.35 (c) is slightly less than earlier).

At the monotectic temperature, monotectic reaction occurs. There is thermal arrest as this reaction occurs at a constant temperature, till all the copper-rich liquid (L1) is exhausted. Thus, by this reaction,

Lever rule can be used to calculate the amount of total L2 and solid copper at slightly below the monotectic temperature by using ‘JD’ as ‘tie-line’.

As the alloy cools further, more solid Cu separates from L2 till the alloy reaches the eutectic temperature, the amount of this liquid which transforms to eutectic mixture is-

At the eutectic temperature (326°C), this liquid weighing 60% of the alloy, undergoes eutectic reaction to result in 60% by weight of the eutectic mixture of solid Cu and Pb, the remaining 40% consists of the grains of copper. This microstructure remains unchanged as the alloy cools further to room temperature.

Phase Diagram Showing Syntectic Reaction:

In some rare systems, the following reaction occurs at a constant temperature:

in which a liquid of fixed composition reacts with another liquid of fixed composition to result in a solid.

To obtain 100% solid by the reaction, the amounts of both the liquids is fixed. If one of the liquids is in greater proportion, then extra part of this liquid remains unreacted alongwith the product solid after the syntectic reaction. All those alloys which fall in the composition range of the syn­tectic horizontal undergo the syntectic reaction.

Any such molten alloy, falling in the syntectic range of compositions, must be present as two different layers of liquids above the syntectic temperature. Freezing begins at syntectic isotherm, where L1 reacts with L2 to form ϒ. Normally, ϒ forms at the interface between liquid layers.

Once a thin ϒ layer is formed, diffusion becomes difficult, and further reaction becomes difficult, except with very long time at the reaction temperatures. Commonly, it is expected that the alloy freezes essentially in two independent parts with a minor reaction layer in between them.

In the best known relatively rare syntectic system of Na-Zn, no useful alloy exists.