Eutectic, eutectoid, peritectoid, peritectic

Differences between Eutectic, Peritectic,
Eutectoid and Peritectoid
transformationstransformations
8/5/2018Prem Kumar Soni1

1. Eutectic Transformation
In an eutectic reaction, when a liquid solution of fixed
composition, solidifies at a constant temperature, forms a
mixture of two or more solid phases without an intermediate
pasty (लेई क तरह क लेई क तरह क )stage. This processलेई क तरह क लेई क तरह क
reverses on heating.
8/5/20182 Prem Kumar Soni

A eutectic reaction is a three-phase reaction, by
which, on cooling, a liquid transforms into two
solid phases at the same time. It is a
phase reaction, but a special one. For example:
liquid alloy becomes a solid mixture of alpha and
liquid alloy becomes a solid mixture of alpha and
beta at a specific temperature (rather than over a
temperature range).

In eutectic system, there is always a specific alloy, known as
eutectic composition, that freezes at a lower temp. than all
other compositions.
At the eutectic temp. two solids form simultaneously form a
single liquid phase.
The eutectic temp. & composition determine a point on theThe eutectic temp. & composition determine a point on the
phase diagram, called the eutectic point.

Eutectic or invariant point. Liquid and two solid phases exist in
equilibrium at the eutectic composition and the eutectic
temperature.
Note:
the melting point of the eutectic alloy is lower than that of thethe melting point of the eutectic alloy is lower than that of the
components (eutectic = easy to melt in Greek).
At most two phases can be in equilibrium within a phase field.
Single-phase regions are separated by 2-phase regions.

Binary alloy eutectic system can be classed as:
1. One in which, two metals are completely soluble in the
liquid state but are insoluble in each other in the solid
state.
2. two metals are completely soluble in the liquid state2. two metals are completely soluble in the liquid state
but are partly soluble in each other in the solid state.

1. Two metals completely soluble in the liquid state but 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 purposes they may be considered
insoluble.

 Alloy-1: 20% Cd and 80% Bi
 Contrary to alloy 3, in this case crystal of pure Bi form first, enriching the melt
with Cd.
 The composition of the melt (or liquid) moves to right until Ultimately the point E
is reached and the remaining liquid solidi-fies as eutectic (40% Cd and 60% Bi).
 Alloy-2: 40% Cd and 60% Bi (eutectic alloy)
 No solidification occurs until the melt reaches the eutectic temperature (140°)
 At the eutectic temperature, the two pure metals crystallize together to give a
characteristically line aggregate known as eutectic.
 Eutectic consists of alternate layers of Cd and Bi which form at the eutectic
temperature (140°C in this case).

Alloy-3: 80% Cd and 20% Bismuth.
 As the temperature falls to T1, crystal nuclei of pure Cd begin to form. Since
pure Cd is deposited, it follows that the liquid becomes richer in Bi; the
composition of liquid move s to left 3’ and as indicated by the diagram, no
further Cd deposits until temperature falls to T2.
 At T2 more Cd is deposited and dendrites begin to develop from the already formed At T2 more Cd is deposited and dendrites begin to develop from the already formed
nuclei.
 The growth of the Cd dendrites, on the one hand, and the consequent enrichment of
the remaining liquid in Bi, on the other, continues until the temperature has fallen to
140°C, the eutectic temperature in this case.
 The remaining liquid then contains 40% Cd and 60% Bi, the eutectic composition.

2. Two metals completely soluble in the liquid state, but only partly soluble in
the solid state

 Since most metals show some solubility for each other in the solid state, this type is
the most common and, therefore, the most common alloy system.
 Metals such as Pb-Sn and Pb-Sb are partly soluble in each other in the solid state.
 Fig. shows the Tin-Lead equilibrium diagram with micro-structures (of course)
obtained under non-equilibrium condition of solidification.
I. Tin will dissolve up to maximum of 2.6% Pb at the temperature, forming the solid
solution α.solution α.
II. Lead will dissolve up to a maximum of (100-80.5) i.e. 19 .5% tin at the eutectic
temperature, giving the solid solution β.
III. Slope of BA and CD indicate that the solubility of Pb in Sn (α) and that of Sn in Pb (β)
decrease as temperature falls
 Consider an alloy of composition Z (70% Pb-30% Sn). As the melt temperature
falls to T1, dendrites of composition Y will deposit.

 The alloy solidifies as a solid solution until at 183°C, the last layer of solid to form is
of composition C (80.5% Pb-19.5% Sn).
 The remaining liquid which has the eutectic composition (38% Pb-62% Sn) then
solidifies by depositing, in the form of a eutectic, i.e., alternate layers of α and β, of
compositions B and C respectively.
 If cooled slowly to room temperature the compositions of the solid solutions α and β
will follow the line BA and CD, i.e., α will become progressively poorer in lead and
β in tin.
 Take another alloy of composition Z' (95% Pb-5% Sn). When cooled slowly,
solidification starts at R and is complete at P, the resultant solid being a homogeneous
single phase, the β solid solution.
 As the alloy cools, the solvus line is reached at point Q. The β solution is now
saturated in tin. Below this temperature, under conditions of slow cooling, the excess
tin must come out of solution. Since tin is soluble in lead, the precipitate does not
come out as the pure metal tin, but rather the α solid solution.

2. Eutectoid Transformation
 Eutectoid reaction is an isothermal reversible reaction in which a solid phase (usually
solid solution) is converted into two or more intimately mixed solids on cooling, the
number of solids formed being the same as the number of component in the system.

Definition: A eutectoid reaction is a three-phase reaction by which,
on cooling, a solid transforms into two other solid phases at the
same time. If the bottom of a single-phase solid field closes (and
provided the adjacent two-phase fields are solid also), it does so
with a eutectoid point.
The eutectoid (eutectic-like) reaction is similar to the eutectic
reaction but occurs from one solid phase to two new solid phases. It
reaction but occurs from one solid phase to two new solid phases. It
also shows as V on top of a horizontal line in the phase diagram.
There are associated eutectoid temperature (or temperature),
eutectoid phase, eutectoid and proeutectoid microstructures.
Solid Phase 1 à Solid Phase 2 + Solid Phase 3

The eutectoid structure in iron has a special name: it is
called pearlite (because it has a pearly look). The schematic
and micrograph below show pearlite. It is important to note that
pearlite is not a phase, but a mixture of two phases: ferrite and
cementite.
During slow cooling of an iron-carbon alloy, pearlite forms byDuring slow cooling of an iron-carbon alloy, pearlite forms by
a eutectoid reaction as austenite cools below 727 °C (1,341 °F)
(the eutectoid temperature). Pearlite is a microstructure
occurring in many common grades of steels.

Eutectoid decomposition of iron as an example, austenite containing
0.8% C changes into ferrite (iron containing almost no carbon) and
cementite (Fe3C, containing 25 at% carbon). Hence carbon atoms
must diffuse together to form Fe3C, leaving ferrite.
Nuclei of small plates of ferrite and cementite form at the grainNuclei of small plates of ferrite and cementite form at the grain
boundaries of the austenite, and carbon diffusion takes place on a
very local scale just ahead of the interface (schematic below). Thus
the plates grow, consuming the austenite as they go, to form pearlite.

The difference is of % of Carbon present in the steel.
Eutectoid steel has 0.8% C.
Hypereutectoid steel has greater than 0.8% C
whereas hypoeutectoid steel has less than 0.8% C. ...
Thus, if the steel is hypoeutectoid it will produce
Thus, if the steel is hypoeutectoid it will produce
proeutectoid ferrite and if it is hypereutectoid it will
produce proeutectoid cementite.

3. Peritectoid Transformation
The peritectoid reaction is the transformation of two solid into a third
solid.

It is important to note that:
Ferrite is soft and ductile
Cementite is hard and brittle
Thus, combining these two phases in solution an alloy can
be obtained with intermediate properties. (Mechanicalbe obtained with intermediate properties. (Mechanical
properties also depend on the microstructure, that is, how
ferrite and cementite are mixed.)

4. Peritectic reaction
The peritectic reaction also involves three solid in
equilibrium, the transition is from a solid + liquid phase to
a different solid phase when cooling. The inverse reaction
occurs when heating.
occurs when heating.
Solid Phase 1 + liquid à Solid Phase 2

 It is the reaction that occurs during the solidification of some alloys where the liquid
phase reacts with a solid phase to give a solid phase of different structure.
 Assuming very slow rates of cooling, the peritectic reaction will occur only in those Pt-
Ag alloys that Contain between 12 and 69% silver (Ag).
 Consider a liquid (melt) of composition Z, i.e., containing 25% Ag. Solidification
commences at T1 and dendrites of α, initially of composition W, begin forming.
 Selective crystallization of α continues down to Tp, the peritectic temperature; when the
alloy reaches. this temperature, it is composed of solid α-dendrites of composition B andalloy reaches. this temperature, it is composed of solid α-dendrites of composition B and
liquid of composition D in the proportion α : liquid = RD : RB.

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Eutectic, eutectoid, peritectoid, peritectic

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