iron-iron carbide Phase diagrams

Contents
• Definitions and basic concepts:
 Component
 System
 Phases
 Solubility limit
 Microstructure
 Phase Equilibrium
• Phase Diagram
• Interpretation of phase diagram

Contents
• Lever’s Rule
• Eutectic Reactions
• Eutectoid Reactions
• Peritectic Reactions
• Cu-Ni Phase Diagram
• Pb-Sn Phase Diagram
• Al-Si Phase Diagram
• Iron-Iron Carbide Diagram

Component and System
• A component is defined as pure metals of
which an alloy is composed.
• A component is chemically recognizable, e.g.
Fe and C are the components in carbon steel.
• A binary alloy contains two components, and
a ternary alloys contain three.
• A system may relate to the series of possible
alloys consisting of same components, but
without regard to alloy composition.

Phase
• A phase is defined as a homogenous portion
of the system having uniform physical and
chemical characteristics.
• Every pure material is considered to be a
single phase.
• Each phase is separated by phase boundaries.
• A phase may contain one or two component.
• A single phase system is called as homogenous
and systems with two or more phases are
heterogeneous systems.

Solubility Limit
• A maximum amount of solute that can be
dissolved in the solvent to form a solid
solution is termed as solubility limit.
• For example, alcohol has unlimited solubility
in water, sugar has limited solubility, and oil is
insoluble in water.
• Cu and Ni are mutually soluble in any amount,
while C has limited solubility in Fe.
• The addition of solute in excess of this limit
results in the formation of two phase solution.

Microstructure
• Material physical properties and mechanical
behavior depend on microstructure.
• The microstructure is specified by the number
of phases, their proportions, and the manner
in which they are distributed.
• The microstructure of an alloy depends on
a. Alloying elements
b. Their concentrations and
c. The grain size (controlled by heat-treatment
process)

Phase Equilibrium
• A system is at equilibrium if at constant
temperature, pressure and composition the
system is stable, not changing with time.

Phase Diagram
• A phase diagram is a graphical representation
of the combinations of temperature, pressure,
composition, or other variables for which
specific phases exist at equilibrium.
• We will discuss phase diagrams for binary
alloys only and will assume pressure to be
constant at one atmosphere.
• The mechanical properties of engineering
materials depend strongly upon
microstructure.

Phase Diagram
• The purpose of phase diagram is to develop an
understanding of the phase transformations,
which occur under conditions of slow cooling
• Using phase diagrams, we can easily predict
the effects of compositional changes.
• Consider two components A and B, showing
complete solid solubility both in liquid as well
as solid state.

Phase Diagram
• Three phase region can be identified on the
phase diagram;
1. Liquid (L)
2. solid + liquid (α +L)
3. solid (α )
• Liquidus line separates liquid region from
(liquid + solid) region, above this line there lies
only liquid solution.
• Solidus line separates solid region from (liquid
+ solid) region, below this line only solid
solution is present.

Interpretation of Phase Diagram
For a given temperature and composition we
can use phase diagrams to determine:
1) The phases that are present
2) Composition of the each phase
3) The relative fractions of the phases

1) The phases that are present
Point A:
At 1100°C, the alloy
composition is
60% Ni & 40% Cu
(only α-phase)
Point B:
At 1250°C, 35% Ni &
65% Cu, system
contains two
phases (α +L)

2) Composition of each Phase
Point B:
At 1250°C, two phases
(α +L) are present.
Composition of each
phase can be found
by drawing a Tie-Line.
CL  31.5% Ni &
68.5 % Cu
Co  35% Ni
Cα  42.5% Ni &
57.5% Cu

• Lever rule is employed to find the relative
mass fractions of the phases present in the
alloy system.
• The lever rule is a mechanical
analogy to the mass balance
calculation.
• The tie line in the two-phase region is
analogous to a lever
balanced on a fulcrum.

Mass fractions:
Co = 35 wt. %,
CL = 31.5 wt. %,
Cα = 42.5 wt. %
WL = S / (R+S)
= (Cα - Co) / (Cα- CL)
Wα = R / (R+S)
= (Co - CL) / (Cα- CL)
WL = 0.68
Wα = 0.32

Eutectic Reactions
• Eutectic reaction is transition between liquid
and mixture of two solid phases, α + β at
eutectic concentration CE.
• Eutectic is a Greek word meaning easy to melt
Eutectic Reaction

Eutectoid Reactions
• The eutectoid (eutectic-like in Greek) reaction
is similar to the eutectic reaction but occurs
from one solid phase to two new solid phases.
• Upon cooling, a solid phase transforms into
two other solid phases (γ ↔ α + β)

Eutectic and Eutectoid Reactions

Peritectic Reactions
• A Peritectic reaction occurs when a solid and
liquid phase will together form a second solid
phase at a particular temperature and
composition upon cooling as,
L + α ↔ β
• Peritectic reactions are not as common as
eutectics and eutectoids, but they do occur in
some alloy systems.
• There is one in the Fe-C system

Cu-Ni Alloy Phase Diagram
• Cu-Ni alloy system presents one of the
simplest cases in which both components are
completely soluble in each other in solid as
well as in liquid state.
• The reasons of complete solubility are:
1. Both have same crystal structure (FCC)
2. Similar radii
3. Electro negativity
4. Valency
• Cu-Ni alloy is an example of Substitutional
Solid Solution.

Pb-Sn Alloy Phase Diagram
• Pb-Sn alloy system represents a phase
diagram that shows partial solid solubility.
• The α-phase is a solid solution of tin in lead at
the left side of the diagram.
• The β-phase is a solid solution of lead in tin at
the right side of the diagram.
• At eutectic temperature (183 °C), lead can
hold up to 18.3% tin in a single-phase solution
and tin can hold up to 2.2% lead within its
structure and still be single phase.

Pb-Sn Alloy Phase Diagram
• There are three single phase regions; α-phase
β-phase and the liquid phase.
• Two phase regions are also three; α + L, β +L,
α +β.
• Solvus line separates one solid solution from a
mixture of solid solutions. The Solvus line
shows limit of solubility

Calculation of relative amounts of
micro-constituents

micro-constituents (Eutectic & α)
Amount of
Eutectic mixture:
We = P / (P+Q)
Amount of α:
Wα = Q / (P+Q)

micro-constituents (α & β)
Amount of α:
Wα =
(Q+R)/(P+Q+R)
Amount of β :
Wβ = P/(P+Q+R)

Al-Si Alloy Phase Diagram
• Al-Si alloys differ from our "standard" phase
diagram in that aluminum has zero solid solubility
in silicon at any temperature.
• This means that there is no beta phase and so
this phase is "replaced" by pure silicon.
• The eutectic on this phase diagram contains
much more alpha than Si and so we expect the
eutectic mixture (alpha+Si) to be mainly alpha.
• For hypereutectic, primary Si forms first,
depleting the liquid of Si until it reaches the
eutectic composition where the remaining
solidification follows the eutectic reaction.

Single Phase Regions in Fe-Fe3C Phase
Diagram
1. Fe-C liquid solution
2. α-ferrite - solid solution of C in BCC Fe
o Stable form of iron at room temperature.
o The maximum solubility of C is 0.022 wt%
o Transforms to FCC γ-austenite at 912 °C
3. γ-austenite - solid solution of C in FCC Fe
o The maximum solubility of C is 2.14 wt %.
o Transforms to BCC δ-ferrite at 1395 °C
o Is not stable below the eutectoid temperature
(727 ° C) unless cooled rapidly

Single Phase Regions in Fe-Fe3C Phase
Diagram
3. δ-ferrite - solid solution of C in BCC Fe
o The same structure as α-ferrite
o Stable only at high T, above 1394 °C
o Melts at 1538 °C
4. Fe3C (iron carbide or cementite)
This intermetallic compound is metastable,
it remains as a compound indefinitely at room
T, but decomposes (very slowly, within several
years) into α-Fe and C (graphite) at 650 - 700
°C

Important things to remember
• C is an interstitial impurity in Fe. It forms a solid
solution with α, γ, δ phases of iron.
• Maximum solubility in BCC α-ferrite is limited
(max 0.022 wt% at 727 °C) - BCC has relatively
small interstitial positions.
• Maximum solubility in FCC austenite is 2.14 wt%
at 1147°C - FCC has larger interstitial positions.
• Cementite is very hard and brittle – can
strengthen steels. Mechanical properties also
depend on the microstructure, that is, how ferrite
and cementite are mixed.

Important things to remember
Three types of ferrous alloys:
1. Iron: less than 0.008 wt % C in α−ferrite at
room temperature.
2. Steels: 0.008 - 2.14 wt % C (usually < 1 wt % )
α-ferrite + Fe3C at room temperature.
3. Cast iron: 2.14 - 6.7 wt % (usually < 4.5 wt %)

Microstructure of Eutectoid Steel
• Microstructure depends on composition
(carbon content) and heat treatment.
• In the discussion, we consider slow cooling in
which equilibrium is maintained.
• When alloy of eutectoid composition (0.76 wt
% C) is cooled down slowly it forms a lamellar
or layered structure of two phases: α-ferrite
and cementite (Fe3C). This two phase
structure is called as Pearlite.

Microstructure of Eutectoid Steel
In the micrograph, the dark
areas are Fe3C layers, the
light phase is α-ferrite

Microstructure of Hypo-eutectoid
Steel
Compositions to the left of eutectoid point,
(0.022 - 0.76 wt % C) are termed as hypo-eutectoid
(less than eutectoid) Steels.
γ → Proeutectoid α + γ → Proeutectoid α + Pearlite
(Eutectoid α + Fe3C)

Microstructure of Hypo-eutectoid
Steel

Microstructure of Hyper-eutectoid
Steel
Compositions to the right of eutectoid point,
(0.76 – 2.14 wt % C) are termed as hyper-eutectoid
(greater than eutectoid) Steels.
γ → Proeutectoid Fe3C + γ→ Proeutectoid Fe3C + Pearlite
(Eutectoid Fe3C + α)

Microstructure of Hyper-eutectoid
Steel

iron-iron carbide Phase diagrams

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