6 Phase Diagram

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Phase Diagram BKTK-2011

Phase Diagram


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An Introduction

To have better properties, materials can be fabricated and be

composed of different compounds / atoms alloys material.

The properties of the alloys is strongly dependent on the

characteristic of microstructure of the alloys.

The characteristic on the alloys can be described using Phase

Diagram of the alloys. The characteristic is determined by elements

present, composition, heat treatment.

Valuable informations of alloys are often explainable from their PhaseDiagram


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Some definitions

Component : pure/stable compounds (metals or oxides) of which an

alloy is composed

System : material under consideration or possible alloys consisting of

the same components.

Solid solution : a solution that consists of at least two different

components.

Solubility limit: maximum concentration of solute can be dissolved in the

solvent in a solid solution.

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

chemical charateristics


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Some definitions (2)

Equilibrium : a state or condition where free energy of the system is

minimum (time independent property) a function of temperature,

pressure and composition.

Phase equillibrium: equillibrium state when it applies to a systemconsists of more than one phase.

Metastable state: a non equilibrium condition of a system since the

equilibrium state is very slowly to reach.


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The Gibbs phase rule*

P + F = C + N

Number of phases

Number of

noncompositional variables(temperature, pressure)Number of

components

Number of degrees of freedom

external controlled variables

*J. William Gibbs (1928)


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Water - Sugar


7/22Phase Diagram BKTK-2011

Nickel - Copper

Melting point

of Cu

Melting point

of Ni



The phase composition

T

emperature,

C

Composition, wt%

L (liquid)

A

B

C

L +

TA

TB

TC

CoCL C

Tie line / Isotherm



The phase amount

T

emperature,

C

Composition, wt%

L

A

B

C

L +

TA

TB

TC

CoCL C

R S

Lever rule

L mass fraction :

WL= (C-Co)/(C-CL)

mass fraction :W

= (Co-CL)/(C-CL)

CL

WL+C

W

= Co



Development of microstructure

equilibrium process



Development of microstructure

nonequilibrium process



Eutectic system

L (liquid)

+ L + L

+

A

B

C

F

G

H

E

(X) (Y)

liquidus

solidus

solvus

Composition (wt%)

Temperature(C)

TE

CECE CE

( ) ( ) ( )cooling

E E Eheating

L C C C



Microstructure of the eutectic system

L (liquid)

+ L + L

+

(X) (Y)Composition (wt%)

Temperatu

re(C)

A B C



Microstructure of the eutectic system (2)

(A) (B) (C)

primary

eutecticprimary

eutectic

Eutectic structure



Eutectoid system

+ +

+

E

(X) (Y)

solvus

Composition (wt%)

Temperatu

re(C)

( ) ( ) ( )cooling

E E Eheating

C C C


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Peritectic system

L

+ L

+ L +

P

(X) (Y)Composition (wt%)

Temperatu

re(C)

cooling

heatingL


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Systems with intermediate phases


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Iron-Carbon System

FCC

BCC

BCC

Iron carbide


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Iron-Carbon System (2)

Eutectic reaction at 1147C:

3(4.3 % ) (2.14 % ) (6.7 % )cooling

heatingL wt C wt C Fe C wt C

Eutectoid reaction at 727C:

3(0.76 % ) (0.022 % ) (6.7 % )cooling

heatingwt C wt C Fe C wt C


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Iron-Carbon System (3)

The most commond Fe-C alloys:

Pure iron (< 0.008 wt%C)

Steel (0.008 wt%C 2.14 wt%C)

Cast iron (2.14 wt%C

6.70 wt%C)

The microstructure of the alloys is strongly dependent on

both the C content and temperature treatment during

fabrication.


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Phase Diagram BKTK 2011

Heating or cooling through

hypoeutectoid

6 Phase Diagram

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