1 Chapter 10 Phase Transformations in Metals (1) Development of Microstructure and Alteration of...
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1 Chapter 10 Chapter 10 Phase Phase Transformations Transformations in Metals (1) in Metals (1) Development of Development of Microstructure and Microstructure and Alteration of Mechanical Alteration of Mechanical Properties Properties
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Transcript of 1 Chapter 10 Phase Transformations in Metals (1) Development of Microstructure and Alteration of...
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Chapter 10Chapter 10Phase Phase
TransformationsTransformationsin Metals (1)in Metals (1)Development of Development of
Microstructure and Microstructure and Alteration of Mechanical Alteration of Mechanical
PropertiesProperties
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10.1 Introduction10.1 Introduction
The development of The development of microstructure in alloys involves microstructure in alloys involves phase transformation:phase transformation: Phase transformation need time, Phase transformation need time,
and cooling rate greatly modify and cooling rate greatly modify the phase transformation.the phase transformation.
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Phase TransformationPhase Transformation10.2 Basic Concepts10.2 Basic Concepts
Phase transformationPhase transformation as well as as well as the the transformation ratetransformation rate are are important to develop a specific important to develop a specific microstructure which in turn affect microstructure which in turn affect the mechanical properties of the mechanical properties of materials.materials.
For example, the tensile strength of For example, the tensile strength of Fe-C eutectoid alloy can be 700 MPa Fe-C eutectoid alloy can be 700 MPa to 2000 MPa depending on heat to 2000 MPa depending on heat treatment condition. treatment condition.
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10.2 Basic Concepts10.2 Basic Concepts
Three classifications of phase Three classifications of phase transformations:transformations: No change in composition, simple No change in composition, simple
diffusion-dependent transformationdiffusion-dependent transformation Composition change, diffusion-Composition change, diffusion-
dependent transformationdependent transformation Diffusionless transformationDiffusionless transformation
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The 1st kind of phase The 1st kind of phase transformation transformation
No change in composition, No change in composition, simple diffusion-dependent simple diffusion-dependent transformationtransformation SolidificationSolidification Allotropic transformation (αAllotropic transformation (αγ)γ) Recrystallization and grain growthRecrystallization and grain growth
No change in composition of phases. No change in composition of phases.
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Composition change, diffusion-Composition change, diffusion-dependent transformationdependent transformation Eutectoid reaction (Eutectoid reaction (γγ pearlite) pearlite)
The 2nd kind of phase The 2nd kind of phase transformation transformation
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The 3rd kind of phase The 3rd kind of phase transformationtransformation
Diffusionless transformationDiffusionless transformation Metastable phaseMetastable phase Martensitic transformation Martensitic transformation
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10.3 The Kinetics of Phase 10.3 The Kinetics of Phase TransformationTransformation
The progress of a phase The progress of a phase transformation can be broken down transformation can be broken down into two distinct stages:into two distinct stages: Nucleation Nucleation GrowthGrowth
There are two types of nucleation :There are two types of nucleation : Homogeneous Homogeneous HetergeneousHetergeneous
The growth begins once an embryo The growth begins once an embryo has exceeded the critical size, r*has exceeded the critical size, r*
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Solidification: Nucleation Solidification: Nucleation ProcessesProcesses
Homogeneous nucleationHomogeneous nucleation nuclei form in the bulk of liquid metalnuclei form in the bulk of liquid metal requires supercooling (typically 80-300°C max)requires supercooling (typically 80-300°C max)
Heterogeneous nucleationHeterogeneous nucleation much easier since stable much easier since stable ““nucleusnucleus”” is is
already presentalready present Could be wall of mold or impurities Could be wall of mold or impurities
in the liquid phasein the liquid phase allows solidification with only 0.1-10allows solidification with only 0.1-10ººC C
supercoolingsupercooling
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10.3 The Kinetics of Phase 10.3 The Kinetics of Phase TransformationTransformation
Homogeneous nucleationHomogeneous nucleation Nuclei of the new phase form uniformly Nuclei of the new phase form uniformly
throughout the parent phase. throughout the parent phase. Will occur spontaneously only when Will occur spontaneously only when
free energy change free energy change G is negative. G is negative. Fig. 10.1 solidification of a pure Fig. 10.1 solidification of a pure
material from liquid phase. material from liquid phase.
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10.3 The Kinetics of Phase 10.3 The Kinetics of Phase TransformationTransformation
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10.3 The Kinetics of Phase 10.3 The Kinetics of Phase TransformationTransformation
The are two contributions to the The are two contributions to the total free energy change that total free energy change that accompany a solidification accompany a solidification transformation:transformation: Free energy difference between the Free energy difference between the
solid and liquid phase:solid and liquid phase: Volume free Volume free energyenergy : : Gv < 0Gv < 0
Formation of the solid-liquid phase Formation of the solid-liquid phase boundary: boundary: surface free energysurface free energy : : >0 >0 23 4
3
4rGrG v
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10.3 The Kinetics of Phase 10.3 The Kinetics of Phase TransformationTransformation
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r* = critical nucleus: nuclei < r* shrink; nuclei>r* grow (to reduce energy)
Homogeneous Nucleation & Homogeneous Nucleation & Energy EffectsEnergy Effects
GT = Total Free Energy = GS + GV
Surface Free Energy- destabilizes the nuclei (it takes energy to make an interface)
24 rGS
= surface tension
Volume (Bulk) Free Energy – stabilizes the nuclei (releases energy)
GrGV3
3
4
volume unit
energy free volume G
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10.3 The Kinetics of Phase 10.3 The Kinetics of Phase TransformationTransformation
The subcritical particle is an embryo, The subcritical particle is an embryo, whereas the particle of radius whereas the particle of radius greater than r* is termed a nucleus. greater than r* is termed a nucleus.
Since r* and Since r* and G* appear at the G* appear at the maximum on the free energy-versus-maximum on the free energy-versus-radius curve of Fig. 10.2b, derivation radius curve of Fig. 10.2b, derivation of expressions for these two of expressions for these two parameters is a simple matter. parameters is a simple matter. equation 10.3 and 10.4equation 10.3 and 10.4
10.3 The Kinetics of 10.3 The Kinetics of Phase TransformationPhase Transformation
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vGr
2*
2
3
)(3
16*
vGG
m
mfv T
TTHG
)(
)1
)(2
(*TTH
Tr
mf
m
22
23
)(
1)
3
16(*
TTH
TG
mf
m
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10.3 The Kinetics of Phase 10.3 The Kinetics of Phase TransformationTransformation
T1>T2
10.3 The Kinetics of 10.3 The Kinetics of Phase TransformationPhase Transformation
The number of stable nuclei n* (radii The number of stable nuclei n* (radii greater than r*) is a function of greater than r*) is a function of temperature:temperature:
Another temperature-dependent Another temperature-dependent parameter is also important for parameter is also important for nucleation: rate of diffusion, the nucleation: rate of diffusion, the frequency atoms from the liquid attach to frequency atoms from the liquid attach to the solid nucleus.the solid nucleus.
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)*
exp(* 1 kT
GKn
)exp(2 kT
QKv d
d
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10.3 The Kinetics of Phase 10.3 The Kinetics of Phase TransformationTransformation
)exp()
*exp(* 3213 kT
Q
kT
GKKKvnKN d
d
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10.3 The Kinetics of Phase 10.3 The Kinetics of Phase TransformationTransformation
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10.3 The Kinetics of Phase 10.3 The Kinetics of Phase TransformationTransformation
Example Problem 10.1Example Problem 10.1 Computation of critical nucleus Computation of critical nucleus
radius and activation free energyradius and activation free energy Computing the number of atoms in a Computing the number of atoms in a
critical nucleus.critical nucleus.
10.3 The Kinetics of 10.3 The Kinetics of Phase TransformationPhase Transformation
Heterogeneous Nucleation Heterogeneous Nucleation The activation energy (energy barrier) The activation energy (energy barrier)
for nucleation (△G* ) is lowered when for nucleation (△G* ) is lowered when nuclei form on preexisting surfaces or nuclei form on preexisting surfaces or interfaces. interfaces.
The surface free energy The surface free energy is reduced. is reduced.
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10.3 The Kinetics of Phase 10.3 The Kinetics of Phase TransformationTransformation
Hetergeneous Nucleation
cosSLSIIL
10.3 The Kinetics of 10.3 The Kinetics of Phase TransformationPhase Transformation
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v
SL
Gr
2*
)()()(3
16hom*
2
3*
SGSG
Gv
SLhet
=30-90o, S()=0.01~0.5
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10.3 The Kinetics of Phase 10.3 The Kinetics of Phase TransformationTransformation
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10.3 The Kinetics of Phase 10.3 The Kinetics of Phase TransformationTransformation
The nucleation
rate is shifted to higher
temperature for
heterogeneous.
10.3 The Kinetics of 10.3 The Kinetics of Phase TransformationPhase Transformation
The growth rate is determined by the The growth rate is determined by the rate of diffusion and its temperature rate of diffusion and its temperature dependence is the same.dependence is the same.
The overall phase transformation rate The overall phase transformation rate is equal to some product of N and G is equal to some product of N and G
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)exp(kT
QCG
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10.3 The Kinetics of Phase 10.3 The Kinetics of Phase TransformationTransformation
10.3 The Kinetics of 10.3 The Kinetics of Phase TransformationPhase Transformation
The rate of transformation and the The rate of transformation and the time required for the transformation time required for the transformation (say 50%) is inversely proportional (say 50%) is inversely proportional to one another. to one another.
Rate of transformation : r = 1/ tRate of transformation : r = 1/ t0.50.5
t t 0.5 0.5 = time required for halfway to = time required for halfway to completion.completion.
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10.3 The Kinetics of Phase 10.3 The Kinetics of Phase TransformationTransformation
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Rate of transformationRate of transformation
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10.3 The Kinetics of Phase 10.3 The Kinetics of Phase Transformation-Transformation-
Kinetic considerations of solid-state Kinetic considerations of solid-state transformationtransformation
Rate of transformation : r = 1/ tRate of transformation : r = 1/ t0.50.5
t t 0.5 0.5 = time required for halfway to = time required for halfway to completion.completion.
Temperature will affect the rate of Temperature will affect the rate of transformation:transformation:
Q = activation energyQ = activation energy Phase transformation is a thermal activated Phase transformation is a thermal activated
process.process.
)(RT
Q
eAr
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10.3 The Kinetics of Phase 10.3 The Kinetics of Phase Transformation-Transformation-
Kinetic considerations of solid-Kinetic considerations of solid-state transformationstate transformation
Solid-state transformation did not occur Solid-state transformation did not occur instantly.instantly. Need composition redistribution or atomic Need composition redistribution or atomic
rearrangementrearrangement Energy increase for nucleation or new phase Energy increase for nucleation or new phase
boundaries.boundaries. Time-dependent process (kinetics): Time-dependent process (kinetics): Avrami Avrami
equationequation. .
k, n = time-independent constant ; k relates to r. k, n = time-independent constant ; k relates to r.
)exp(1 nkty
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Rate of transformationRate of transformation
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10.4 Metastable versus 10.4 Metastable versus Equilibrium statesEquilibrium states
Phase transformation is a time-Phase transformation is a time-dependent process, long time is dependent process, long time is required to reach equilibrium stage. required to reach equilibrium stage. Speed or rate of cooling is designed to Speed or rate of cooling is designed to
produce specific microstructure which produce specific microstructure which the phase may not the equilibrium one. the phase may not the equilibrium one. metastable phase. metastable phase.
Supercooling or superheating happens Supercooling or superheating happens very often in practical purpose. very often in practical purpose.
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Microstructure and Property Microstructure and Property Changes in Fe-C AlloysChanges in Fe-C Alloys
10.5 Isothermal Transformation 10.5 Isothermal Transformation DiagramsDiagrams
Eutectoid reaction, at 727Eutectoid reaction, at 727ooCC γγ pearlite (α+ Fe pearlite (α+ Fe33C)C)
See Fig. 10.12See Fig. 10.12 The lower the temperature, the fast The lower the temperature, the fast
the phase transforms. the phase transforms.
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10.5 Isothermal Transformation 10.5 Isothermal Transformation DiagramsDiagrams
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10.5 Isothermal Transformation 10.5 Isothermal Transformation DiagramsDiagrams
T~< 727T~< 727ooC (slightly lower):C (slightly lower): Phase transformation needs longer timePhase transformation needs longer time Microstructure becomes coarse pearlite.Microstructure becomes coarse pearlite.
T << 727T << 727ooC (much lower):C (much lower): Phase transformation is quick.Phase transformation is quick. Microstructure is fine pearlite.Microstructure is fine pearlite.
See Fig. 10.13, which is called See Fig. 10.13, which is called isothermal transformation diagram. isothermal transformation diagram. (Time-temperature-transformation, T-T-(Time-temperature-transformation, T-T-T plot)T plot)
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T-T-T plotT-T-T plot
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Cooling curve ABCD in T-T-T plot Cooling curve ABCD in T-T-T plot for eutectoid reactionfor eutectoid reaction
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(a) Coarse pearlite and (b) fine (a) Coarse pearlite and (b) fine pearlitepearlite
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End of section (1)End of section (1)
You have learned:You have learned: What is eutectoid phase transformation What is eutectoid phase transformation Phase transformation needs timePhase transformation needs time Cooling rate affect phase Cooling rate affect phase
transformation, and eventually transformation, and eventually microstructure. microstructure.
T-T-T plot to show how can the coarse T-T-T plot to show how can the coarse pearlite and fine pearlite be formed. pearlite and fine pearlite be formed.
