Two Metals Completely Soluble in the Liquid State and Completely Insoluble in the solid
state
Technically, no two metals are completely insoluble in each other. However, in some cases
the solubility is so restricted that for practical purpose they may be considered insoluble.
The series of cooling curves for the pure metals and various alloys are shown in the figure
given below .
the cooling curves for the pure metals A and B show a single horizontal line at their freezing
points. As B is added to A, the temperature for the beginning of solidification is 60B. It can
be seen that over a wide range of compositions, a portion of the cooling curve that shows
the end of solidification occurs at a fixed temperature. The lower horizontal line at TE,
shown dotted in the above figure, is known as eutectic temperature. In one alloy, the
eutectic composition 40A-60B, complete solidification occurs at a single temperature, the
eutectic temperature.
The melting points of the two pure metals, points M and N, are plotted on the vertical lines
that represent the pure metals A and B. For an alloy having composition 80A-20B, the
points showing the beginning of solidification T1 and end of solidification TE are plotted as
shown. The same procedure is followed for all other alloys. The upper lines on the phase
diagram connecting the points M, E and N is the liquidus line and shows the beginning of
solidification. The point, at which the liquidus lines intersect, the minimum point E, is
known as the eutectic point. TE is called the eutectic temperature and 40A-60B the eutectic
composition. As solidus line is a continuous line connecting the melting points of pure
metals, the complete solidus line is MFGN.
It is common practice to consider alloys to the left of the eutectic composition
ashypoeutectic alloys and those to the right as hypereutectic .
The way solidification takes place and resulting microstructures at various stages can be
studied by following the slow cooling of different alloys. The process is explained by study
of slow cooling of Alloy 1 (eutectic composition), Alloy 2 (hypoeutectic alloys) and Alloy 3
(hypereutectic alloys) in the figure given below
Eutictic composition
Alloy 1 is the eutectic composition 40A-60B. As it is cooled from temperature T0, it remains
a uniform liquid solution until it reaches point E, on the horizontal eutectic temperature
line. the liquid must now start to solidify and the temperature cannot drop until the alloy is
completely solid. The liquid will solidify into a mixture of two phases as alternate of pure
metals (A&B).The reaction below show this transformation :
To understand how the eutectic solidifications occur let us assume solidify asmall amount
of pure metal A remaining liquid is enriched with B this lead to tends the composition has
shifted to the right . To restore the liquid composition to equilibrium layer of B will solidify.
If slightly too much B is solidified, the liquid composition will have shifted to the left,
requiringlayer of A to solidify to restore equilibrium. Therefore, at constant temperature,
the liquid solidifies alternately layers from pure A and pure B, resulting in extremely fine
mixture usually visible only under the microscope. This is known as eutectic mixture shown
at (4) in the below figure
In the above figure at (1), the alloy is in liquid state. At (2), as the alloy starts to solidify, it
forms alternate layers of pure A and pure B. This layered microstructure is known as
lamellar microstructure . The reason that a eutectic alloy forms in this way has to do with
the diffusion times required to form the solid
The grains grow by adding A to A and B to B until they encounter another grain as shown at
(3). Further nucleation sites will also continue to form within the liquid parts of the mixture
and solidification completes as shown at (4).
The change of this liquid of composition E into two solids at constant
temperature is known as the eutectic reaction and may be written as
Hypoeutectic alloys
Let consider the solidification of an Alloy 2 of composition of(80%A,20%B) at point 1 liquid
solid solution is exist .when temperature falls to the T1 crystal nuclei of A begin to form
(The temperature horizontal or tie-line, T1, cuts the liquidus at the chosen composition,
L1,and the other phase boundary is the 100% A ordinate) since pure A is deposited it
follows.
that the liquid which remains becomes correspondingly richer in B .Therefore the
composition of the liquid moves to the right no further deposition of A takesplace until the
temperature has fallen toT2 at point (3 ). When this happens more
A is deposited, and dendrites begin to develop from the nucleiwhich have already
formedThe amount of A and L2 can be calculated using the Lever Rule as under.
A (percent) = (x2L2 / T2L2) x 100 = (10 / 30) x 100 = 33 percent
L2 (percent) = (T2x2 / T2L2) x 100 = (20 / 30) x 100 = 63 percent
The growth of the A dendrites, on the one hand, and the consequent enrichment of the
remaining liquid in B ,on the other, continues until the temperature has fallen to TE.The
remaining liquid then contains 40% A and 60% , ie the eutectic point E has been reached at
point (4) Then a structure composed entirely of eutectic would be formed as outlined
above
.
Hypereutectic Alloys
Alloy 3, a hypereutectic alloy composed of 10A-90B, undergoes the same cooling process as alloy 2 except
that when the liquidus line is reached at temperature T3 the liquid deposits crystals of pure B instead of A
as shown at (2). As the temperature is decreased, more and more of B will solidify, leaving the liquid richer
in A. The amount of liquid gradually decreases, and its composition gradually moves down and to the left
along the liquidus line until the point E is reached at the eutectic temperature. The remaining liquid now
solidifies into the eutectic (A + B) mixture as shown at (5). After solidification the alloy will consist of 75
percent grains of primary B and 25 percent eutectic (A + B) mixture.The area below the solidus line and to
the left of the eutectic composition is labeled Solid A + Eutectic mixture and that to the right, Solid B +
Eutectic mixture.figure below show the microstructure of hypereutectic alloy .