‘’PHASE DIAGRAMS’’

IE-114 Materials Science and General Chemistry

Lecture-10

There is a strong correlation between microstructure and

mechanical properties.

Phase diagrams provides information about:

- The development of microstructure during heating or cooling

can be understood from the phase diagrams.

- Melting, casting, crystallization, etc.

Importance of Phase Diagrams

Solubility Limit

At some specific temperatures , there

is a maximum concentration of solute

atoms that may dissolve in the solvent

to form a solid solution; this is called

solubility limit.

Phase-Diagram of Water-Sugar System

Question: What is the solubility limit at 20oC?

Solubility limit increases with T:

T = 20oC, solubility limit = 65 wt% sugar.

T = 100oC, solubility limit = 80 wt% sugar.

Answer: 65wt% sugar.

If Co < 65wt% syrup

If Co > 65wt% syrup + sugar.

Phase: Homogenous portion of a system that has uniform physical and

chemical characteristics

Aluminum-Copper Alloy

Component: Components are pure metals/or compounds of which alloy

is composed. e.g. Brass (Cu-Zn) ; components are Cu and Zn, Fe and C in

carbon steel, H2O and NaCl in salted water

Solid Solution: A solid solution consists of solute atoms, which

occupy either substitutional or interstitial positions in the solvent lattice

( , , etc,..)

Every pure material is considered to be a phase;

so also is every solid, liquid and gaseous solution

For example if a substance can exist in two or

more polymorphic forms (BBC and FCC) each of

these structures is a separate phase because

their physical properties are different.

Homogeneous system:

A single phase system

Mixture or heterogeneous system:

System of two or more phases.

Most of the metallic alloys,ceramics, polymeric and composite systems are

heterogeneous.

Phase Equilibria:

A system is said to be at equilibrium when the free energy, which is the

internal energy and randomness of the atoms, is at minimum under some

specified combination of temperature, pressure and composition.

Equilibrium Phase Diagrams

Phase diagram is also called equilibrium or constitutional diagram.

These diagrams defines the relationship between the temperature and

compositions or quantities of phases at equilibrium. External pressure

could also be another parameter affecting the phase distribution but it

remains constant at 1 atm in most of the applications.

- Isomorphous Binary Phase Diagrams

- Eutectic Binary Phase Diagrams

1) Isomorphous Binary Systems:

Binary systems are composed of two components and they are isomorphous since there is a complete solubility of liquids and solids.

Example: Cu-Ni

There are 2 phases :

- L (liquid)

- α-solid solution

There are 3 phase fields :

liquid, L+α, α

Liquidus line:The line separating L and α+L phases.

Solidus line: The line separating

α+L and α phases is called.

Cu Ni wt.% Ni

If we know T and Co, then we know the number and types of phases

present.

Examples:

1) Number and types of phases

1) Cu-35Ni Alloy at 1250oC

(Co = 35 wt.%Ni)

2 phases: α+L

2)Cu-60Ni Alloy at 1100oC

(Co = 60 wt.%Ni)

1 phase: α

From the phase diagrams we can learn the followings:

-Number and types of phases that are present at different temperatures for a fixed composition

-Composition of the phases

-Fractions of the phases

If we know T and Co, then we know the composition of each phase.

2) Composition of phases:

For Cu-35Ni Alloy (Co=35wt%Ni)

at T

A:

-Phases: 1(only Liquid)

-Composition of the alloy:

CL = Co (=35 wt%Ni, 65 wt%Cu)

at T

B:

-Phases: 2 (Liquid +α)

-Composition of the alloy:

CL = Cliquidus (32wt%Ni, 68 wt% Cu)

Cα = Csolidus (43wt%Ni, 57wt%Cu)

at T

D:

-Phases: 1 (only α)

-Composition of the alloy:

Cα=Co (35 wt%Ni, 65 wt%Cu)

3) Weight fractions (or percentage) of phases:

If we know T and Co, then we know the amount of each phase

(given in wt%).

WLS

R S

WR

R S

%731003243

3543wtx

= 27wt%

x100

x100

• Binary System (2 components)

•Isomorphous i.e., complete solubility of one

component in another;

phase field extends from

0 to 100wt% Ni.

Microstructural development during

cooling a Cu-Ni alloy

Binary Eutectic Systems

There are three phases : Liquid, and

Cu Ag

max. solubility of Cu

in Ag (8.8 wt%)

Eutectic line. This line shows the minimum

temperature for the liquid

phase existence.

Max.

Solubility

of Ag in Cu

8 wt% Ag at

7790C

SOLVUS

SOLIDUS LIQUIDUS

Melting point of

pure Cu Melting point of

pure Ag

Eutectic composition: 71.9wt%Ag, 28,1wt%Cu

Eutectic temperature: 779oC

Eutectic reaction: Liquid (71.9%) (8%Ag) + (91.2% Ag) Cooling

heating

For a 40wt%Sn-60wt%Pb alloy at 150oC, find:

1) the phases present

2) the compositions of the phases

For a 40wt%Sn-60wt%Pb alloy at 150oC, find:

--the phases present: +

--the compositions of the phases:

C = 11wt%Sn, 89wt%Cu

C = 99wt%Sn, 1wt%Cu

--the relative amounts of each phase:

x100

x100

Microstructural Development During

Cooling of Pb-Sn Alloys

• 18.3wt%Sn < Co < 61.9wt%Sn

• Room temperature microstructure: crystals and a eutectic microstructure

Cooling of an Alloy Having Eutectic Composition

alternating layers of and crystals.

The alloy having eutectic composition is called eutectic alloy

Hypoeutectic and Hypereutectic Alloy

For some alloy systems, discrete intermediate compounds rather than solid

solutions may be observed in phase diagrams. For example; Mg-Pb system.

These are called intermetallic compounds. The compound Mg2Pb is shown

as a vertical line on the diagram rather than a phase region since it exists

precisely at the composition defined.

Intermetallic Compounds

Eutectoid and Peritectic Reactions

Eutectoid reaction: 5600C and 74 wt% Zn-26 wt% Cu

Peritectic reaction: 5980C and 78.6 wt% Zn-21.4 wt% Cu

γ + cooling

heating

+ L cooling

heating

Consider Cu-Zn system.

Phase transformations can be classified according to whether or not there

is any change in composition.

Congruent and Incongruent Phase

Transformation

Phase transformations in which there is no

changes in composition are called as congruent transformations. The

opposite is incongruent

transformation. Allotropic transformations

are congruent as well as melting pure metals.

Eutectic, eutectoid or melting alloy systems

are incongruent transformations.

This is the most important system in manufacturing since primary

structural materials are essentially Fe-C alloys, such as, steel and

cast iron.

Iron Carbon System

STEEL IS an ALLOY OF;

Iron-Iron Carbide Equilibrium Phase Diagram

Phases and phase mixtures present in iron alloys;

• Ferrite (α)

• Cementite (Fe3C)

• Pearlite (ferrite + cementite)

• Austenite (γ)

• -ferrite

• Ledeburite (austenite + cementite)

Definition and Properties of Phases 1) Ferrite : -iron, Solid Solution, max. Carbon solubility 0.022%wt. at 727oC

BCC structure, SOFT

2) Cementite : Iron carbide(Fe3C), contains 6.67% wt. C

Orthorhombic structure, HARD and BRITTLE

3) Pearlite : Phase mixture (ferrite+cementite), Lamellar structure, contains ~0.8% wt. C

Produced from austenite decomposition

4) Austenite : -iron, Solid solution, stable at higher temperatures (>727oC)

Max. Carbon solubility is 2.14%wt. at 1147oC, FCC structure

HIGH TOUGHNESS

5) Ledeburite: Eutectic phase mixture(austenite+Fe3C), seen in cast irons

Contains 4.3 %wt. Carbon, forms at 1147oC

6) -ferrite : Solid solution, max. carbon solubility is 0.1%wt. At 1493oC

Invariant reactions in Fe-Fe3C Phase diagram

1) At 1493oC, 0.18 %wt C (PERITECTIC REACTION)

Liquid(l, 0.5%C)+ -ferrite( ,0.1%C) Austenite( , 0.18%C) cooling

heating

2) At 1147oC, 4.30 %wt C (EUTECTIC REACTION)

Liquid(l, 4.30 %C) Austenite( , 2.14 %C) + Cementite(Fe3C,6.67%C) cooling

heating

3) At 727oC, 0.77 %wt C (EUTECTOID REACTION)

Austenite( , 0.77 %C) Ferrite( , 0.022 %C) + Cementite(Fe3C,6.67%C) cooling

heating

PEARLITE

LEDEBURITE

CAST IRONS STEELS

Hypoeutectoid

Steel

Eutectoid

Steel

Hypereutectoid

Steel

Eutectoid Steel

Hypoeutectoid Steel

Hypereutectoid Steel

Effect of Carbon on Mechanical Properties

EXAMPLE1: For an annealed (cooled in equilibrium conditions after

austenitization) hypoeutectoid steel;

1) Determine the composition of the steel (C wt.%) if quantitative

metallographic analyses revealed 22% secondary ferrite (assume

solubility of carbon in ferrite is nil)

2) Calculate the weight fraction ratio of secondary ferrite and cementite

present in pearlite.

3) Determine the total amount of carbon dissolved in secondary ferrite at

room temperature for the corresponding steel if the total weight of the

steel part in kgs is equal to 80.(Room temperature solubility of carbon

in ferrite is 0.008%)

EXAMPLE2: The mass fraction of eutectoid cementite in iron carbon alloy is

0.109. On the basis of this information, is it possible to determine the

composition of the alloy? If so, what is its composition?