02a Phase Transform

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Materials Engineering

Phase transformation

Phase diagrams


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Phase Transformation

o Temperature, chemical composition andpressure can change the properties of materials

o Understanding what happens during heat

treating processes

o Understanding the development of the

microstructure

Why is it important for us?


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o Terminology of phase diagrams and phasetransformations

o Thermodynamics of phase transformations

o Phase diagrams

Todays topics



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Terminology

Component:

Pure metals and/or compounds of which an alloy is composed.e.g. in a copperzinc brass, the components are Cu and Zn

System:Series of possible alloys consisting of the same components,

but without regard to alloy composition

Solid solutionconsists at least two different types of atoms

the solute atoms occupy eithersubstitutional orinterstitial positions

in the solvent lattice



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Solubility limit

A maximum concentration of solute atoms that may dissolve in the solvent to

form a solid solution

The solubility of sugar in a

sugarwater syrup


Figures from Callister, Mat. Sci. andEng. An Introduction, 8th edition


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Phase

A homogeneous portion of a system that has uniform physical

and chemical characteristics.

1 phase

2 phases

Own distinct properties

Chemical composition

State of matter(e.g. water+ice)

Crystal structure

Homogeneous system:single-phase system

Heterogeneous system:two or more phases



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ThermodynamicsInternal energy U: the energy needed to create the system

Enthalpy (H=U+pV): U+ energy required to make room for it

by displacing its environmentEnthropy (S): expression of disorder or randomness, the energy

not available for work

Helmholtz free energy (F=U-TS)Gibbs free energy (free enthalpy)(G=H-TS)

A system is at equilibrium if its free energy is

at a minimum under some specifiedcombination of temperature, pressure, andcomposition.

The characteristics of the system do not

change with time.

Equilibrium

Gliquid

Gsolid

T

G



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Phase EquilibriumIn an equilibrium system the ratio of phases are constant.

1

2

3

Phase Equilibrium


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EquilibriumTwo component system (A-B)

(ideal solution: exchanging any two atoms does not change the enthalpy)

A B

S

Phase Equilibrium

Entropy, S

disorder or randomness

Processes reduce the state of order of theinitial systems.


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EquilibriumTwo component system (A-B), ideal solution

Phase Equilibrium


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LL+

CL C

CL C

Phase Equilibrium


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Metastable state

Often the state of equilibrium is never completely achieved because the

rate of approach to equilibrium is extremely slow.

nonequilibrium ormetastable state.

Metastable state or microstructure may persist indefinitely(changes only extremely slight)

Often, metastable structures are of more practical significance thanequilibrium ones.

Phase Equilibrium


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Phase or equilibrium diagram

Information about the phase structure of a particular system.

Parameters

Temperature

Pressure

Composition

Informations

State of the matter, crystal structure

Phase composition

Chemical composition of the phases

several different varieties

(e.g. composition-Temperature, pressure-Temperature)

Phase Diagrams


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One-component (or unary) phase diagram

Simplest, one-component system p-T phase diagram

Pressuretemperature phase diagram for H2O.

Note!

phases at

equilibrium

Phase Diagrams


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Binary phase diagrams

composition-Temperature

transition from one phase to

another, or the appearance or

disappearance of a phase.

Coppernickel phase diagram

Phase Diagrams


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Alloy (2)

(1) (2)

Point A:T=1100C

1 phase: , solid sol. of Cu and Ni

CNi=60%, CCu=40%C

Alloy (1)

Point C:T=1400C

1 phase: L, liquid

CNi=35%, CCu=65%

Phase Diagrams


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(1) (2)

C

Alloy (1)

Point B:T=1250C

2 phases: + L,

solid + liquid phase

Composition:CNi=35%, CCu=65%

Mass fraction of phasesW=? %, WL= ? %

Composition of phases:: CNi=? %, CCu=? %

L: CNi=? %, CCu=? %

Phase Diagrams

h i


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L

L

L

L

Phase Diagrams

Ph Di


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Lever rule

Mass fraction of phases

~ ~ L

CL

C0 C

Phase Diagrams

Ph Di


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Composition of phases

The chemical composition of the

phases is changing with thetemperature change.

~ ~ L

CL

C0 C

wt% of Ni

in liquid phase

wt% of Ni

in phase


Phase Diagrams

Ph Di


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4625

Liquid phase: 34% Ni phase: 46% Ni


Liquid phase: 35% Ni phase: -


Liquid phase: - phase: 35% Ni


Phase Diagrams

Ph Di


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Inhomogeneous and equilibrium phase

High cooling rateInhomogeneous

structure

Low cooling rateEquilibrium structure

Equilibrium solidification: only for extremely slow cooling rates.

diffusional processes (diffusion rates are lower for lower temperatures and forsolid phases)

Phase Diagrams

Phase Diagrams


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Inhomogeneous andequilibrium phase

Figure from Callister, Mat. Sci. and Eng.

An Introduction, 8th edition

Phase Diagrams

Phase Diagrams


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Eutectic reactionLiquid phase +(solid phases)

Lead-Tin system

Phase Diagrams

Phase Diagrams


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Lead-Tin system

Phase Diagrams

Phase Diagrams


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Lead-Tin system

Phase Diagrams

Phase Diagrams


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Eutectoid reaction( solid phase) +(solid phases)

Peritectic reaction( solid phase) + liquid phase

(solid phases)

Copperzinc

phase diagram

Phase Diagrams

Phase Diagrams


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Phase Diagrams

Ceramic Phase Diagrams


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The two components are compounds that share a common elementSimilar to metalmetal systems

Al2O2-Cr2O3phase diagram

Figure from Callister, Mat. Sci. and

Eng. An Introduction, 8th edition




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ZrO2-CaZrO3phase diagram




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BaTiO3 p-T phase diagram

S A Hayward and E K H Salje ,J. Phys.: Condens. Matter 14 No 36 2002 p 599-604


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02a Phase Transform