4. 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.
5. 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.
6. 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.
7. 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)
9. Phase Equilibrium
• A system is at equilibrium if at constant
temperature, pressure and composition the
system is stable, not changing with time.
10. 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.
11. 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.
13. 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.
15. 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
16. 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)
17. 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
18. 3) The relative fractions of the phases
• 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.
19. 3) The relative fractions of the phases
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
20. 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
21. 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)
23. 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
25. 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.
28. 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.
30. 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
36. Calculation of relative amounts of
micro-constituents (Eutectic & α)
Amount of
Eutectic mixture:
We = P / (P+Q)
Amount of α:
Wα = Q / (P+Q)
37. Calculation of relative amounts of
micro-constituents (α & β)
Amount of α:
Wα =
(Q+R)/(P+Q+R)
Amount of β :
Wβ = P/(P+Q+R)
38. 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.
41. 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
42. 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
43. 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.
44. 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 %)
46. 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.
47. Microstructure of Eutectoid Steel
In the micrograph, the dark
areas are Fe3C layers, the
light phase is α-ferrite
48. 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)
50. 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)