IRON CARBON PHASE DIAGRAM

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Phase Diagrams Containing Three-Phase Reactions
 In the more complex binary phase diagrams, the type of melting
is sometimes used to describe the type of intermediate that
occurs along with a particular type of solid state reaction
 Congruently melting compounds are those that maintain their
specific composition right up to the melting point. This appears
as a localized “dome” in the liquidus region of the phase diagram
 Incongruent melting compounds do not occur directly from the
liquidus, but are formed by some form of solid-state reaction
 The five most common three-phase reactions that occur in phase
diagrams are:
 Eutectic – a liquid transforming into two new solids on cooling
 Peritectic – a liquid plus a solid transforms into a new solid
 Monotectic – a liquid transforms into a new liquid and a solid
 Eutectoid – a solid transforms into two new solids
 Peritectoid – two solids transforms into a new solid 89
Phase Diagrams Containing Three-Phase Reactions
Three phase reaction type, reaction equation and
appearance on a phase diagram
90
Phase Diagrams Containing Three-Phase Reactions
Three phase reaction types and reaction equations
Above Below Type
Homotectic
Monotectic
Eutectic
Catatectic
L
L
L
S1
L' + L“
L' + S
S1 + S2
S2 + L
Eutectic
Monotectoid
Eutectoid
S1
S1
S'1 + S2
S2 + S3
Eutectoid
Syntectic
Peritectic
L + L‘
L + S1
S
S2
Peritectic
Peritectoid S1 + S2 S3 Peritectoid
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 Locate a horizontal line (isotherm)
on the phase diagram. The
horizontal line, which indicates the
presence of a three-phase reaction,
represents the temperature at which
the reaction occurs under
equilibrium conditions
 Locate three distinct points on the
horizontal line: the two end points
plus a third point. The center point
represents the composition at which
the three-phase reaction occurs
 Write in reaction form the phase(s)
above the center point transforming
to the phase(s) below the point. In
most cases the reaction will be a
eutectic, eutectoid, peritectic, etc.
RULES OF THREE PHASE REACTIONS

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Development of Microstructure in
Eutectic Alloys
• Cooling of
liquid lead/tin
system at
different
compositions.
• Several types of
microstructures
forms during
slow cooling at
different
compositions.
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Development of Microstructure in
Eutectic Alloys
 Co less than 2 wt.% Sn
 In this case of lead – rich
alloy (0 – 2 wt.% of tin)
solidification proceeds in the
same manner as for
isomorphous alloys (e.g. Cu
– Ni) that was discussed
earlier.
 Result
o at extreme ends
o polycrystals of α grains
i.e. only one solid phase
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Alloys that exceed the
solubility limit
• Pb – Sn alloys between
2 – 19 wt.% Sn also
solidify to produce a
single solid solution,
however, as the solid-
state reaction
continues, a second
solid phase, β,
precipitates from the α
phase.
Development of Microstructure in
Eutectic Alloys
• The solubility of Sn in solid Pb at any temperature is given by the
solvus curve.
• Any alloy containing between 2% – 19 % Sn that cools past the solvus
exceeds the solubility resulting in the precipitation of the β phase. 95
Development of Microstructure in
Eutectic Alloys
• 2 wt.% Sn < Co <
19 wt.% Sn
• Result
o initially liquid + α
o then α alone
o finally two phases
 α polycrystals
 fine β phase
inclusions

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Development of Microstructure in
Eutectic Alloys
Alloys that exceed the
solubility limit
• The Pb – 61.9
wt.% Sn alloy has
the eutectic
composition.
• The eutectic
composition has
the lowest melting
temperature.
• The eutectic composition has no freezing range as solidification occurs at
one temperature (183 C in the Pb - Sn alloy).
• The Pb - Sn eutectic reaction forms two solid solutions and is given by:
L61.9 % Sn → α19 % Sn + β 97.5% Sn
• The compositions are given by the ends of the eutectic line. 97
Development of Microstructure in
Eutectic Alloys
• The Pb - Sn eutectic reaction :
L61.9 % Sn → α19 % Sn + β 97.5% Sn
• Co = CE
• Result
o eutectic
microstructure
(lamellar structure)
i.e. alternating layers
(lamellar) of α and β
phases
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Development of Microstructure in
Eutectic Alloys
Cooling curve for a
eutectic alloy is a simple
thermal arrest, since
eutectics freeze or melt at
a single temperature.
a) Atom redistribution during
lamellar growth of a Pb-Sn
eutectic. Sn atoms from the
liquid preferentially diffuse to the
b plates, and Pb atoms diffuse to
the a plates.
b) Photograph of the Pb-Sn eutectic.
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Development of Microstructure in
Eutectic Alloys
Hypoeutectic Alloy
• This is an alloy whose composition will be between the left-
hand-side of the end of the tie line and the eutectic
composition.
• For the Pb-Sn alloy, it
is between 19 wt.%
and 61.9 wt.% Sn.
• In the hypoeutectic
alloy, the liquid
solidifies at the liquidus
temperature producing
solid, α and is
completed by going
through the eutectic
reaction.

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Development of Microstructure in
Eutectic Alloys
Hypoeutectic Alloy  19 wt.% Sn < Co < 61.9 wt.% Sn
 Result
o initially liquid + α
o then α and eutectic
microstructure
 Just above TE:
Cα = 19 wt.% Sn and Cβ = 61.9 wt.% Sn
𝑊 =
𝑹
𝑹 + 𝑺
=
40 − 19
61.9 − 19
= 49 𝑤𝑡. % 𝑆𝑛
𝑊 = 1 − 𝑊 = 51 𝑤𝑡. % 𝑆𝑛
 Just below TE:
Cα = 19 wt.% Sn and Cβ = 97.5 wt.% Sn
𝑊 =
𝑹
𝑹 + 𝑺
=
40 − 19
97.5 − 19
= 27 𝑤𝑡. % 𝑆𝑛
𝑊 = 1 − 𝑊 = 73 𝑤𝑡. % 𝑆𝑛
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Development of Microstructure in
Eutectic Alloys
Hypereutectic Alloy
• This is an alloy whose composition will be between the right-
hand-side of the end of the tie line and the eutectic
composition.
• For the Pb-Sn alloy, it is
between 61.9 % and
97.5 % Sn.
• The primary or
proeutectic solid that
forms before the
eutectic phase is the b
phase which is different
from the eutectic solid
and leads to a variation
in microstructure.
a) A hypereutectic alloy of Pb-Sn and b) a hypoeutectic
alloy of Pb-Sn where the dark constituent is the Pb-rich α
phase and the light constituent is the Sn-rich β phase and
the fine plate structure is the eutectic.
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2
Hypoeutectic and Hypereutectic
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3
Strength of Eutectic Alloys
• Some eutectics can be strengthened by cold working.
• Adding grain refiners, or inoculants, during solidification can
decrease grain size.
• The amount and microstructure of the eutectic can also be
controlled.
• Each eutectic colony can nucleate and grow independently
having the orientation of the lamellae being identical.
Colonies in the Pb-Sn
eutectic and the effect of
growth rate, R, on the
interlamellar spacing, l, in
the eutectic, which follows
the relationship:
2
/
1

 cR
l

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Strength of Eutectic Alloys
 The lamellae orientation changes on crossing from one colony
boundary to another.
 By refining the colony size by inoculation, the strength can be
improved.
 The eutectic is strengthened by decreasing the interlamellar
spacing.
The interlamellar
spacing in a eutectic
microstructure.
Interlamellar spacing
 This is the distance between the center
of one α lamella to the center of the
next α lamella.
 A small interlamellar spacing indicates
that the amount of a → β interface area
is large.
 A small interlamellar spacing therefore
increases the strength of the eutectic.