PY3090 Preparation of Materials Lecture 1 Lecture 7.pdf · 4 Phase Transformations Nucleation...
Transcript of PY3090 Preparation of Materials Lecture 1 Lecture 7.pdf · 4 Phase Transformations Nucleation...
PY3090 PY3090 –– 77
PY3090PY3090Preparation of MaterialsPreparation of MaterialsLecture 7Lecture 7
Colm StephensColm StephensSchool of PhysicsSchool of Physics
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Phase transformationsPhase transformations
Fe3C
(cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
γ (austenite)
γ +L
γ + Fe3C
α + Fe3C
L+Fe3C
δ
Co, wt% C
1148°C
T(°C)
727°C
3
Eutectic, Eutectoid and Eutectic, Eutectoid and PeritecticPeritecticReactionsReactionsEutecticEutectic ReactionReaction
Eutectoid reactionEutectoid reaction
)2()1( phaseSolidphaseSolidLiquid +⇔
)3()2()1( phaseSolidphaseSolidphaseSolid +⇔
PeritecticPeritectic ReactionReaction
)3()2()1( phaseSolidphaseSolidphaseLiquid ⇔+
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Phase TransformationsPhase Transformations
NucleationNucleationnuclei act as seed points to grow crystalsnuclei act as seed points to grow crystalsfor nucleus to form for nucleus to form rate of addition of atoms > rate of lossrate of addition of atoms > rate of lossonce nucleated, growth once nucleated, growth equilibriumequilibrium
Driving force to nucleate increases as we increase Driving force to nucleate increases as we increase ΔΔTTsupercoolingsupercooling (eutectic, eutectoid)(eutectic, eutectoid)superheatingsuperheating ((peritecticperitectic))
Small Small supercoolingsupercooling few nuclei few nuclei -- large crystalslarge crystalsLarge Large supercoolingsupercooling rapid nucleation rapid nucleation -- many nuclei, many nuclei,
small crystalssmall crystals
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Solidification: Nucleation Solidification: Nucleation ProcessesProcesses
Homogeneous nucleation Homogeneous nucleation nuclei form in the bulk of liquid metalnuclei form in the bulk of liquid metalrequires requires supercoolingsupercooling (typically 80(typically 80--300300°°C max)C max)
Heterogeneous nucleationHeterogeneous nucleationmuch easier since stable much easier since stable ““nucleusnucleus”” is already presentis already present
Could be wall of mold or impurities in the liquid Could be wall of mold or impurities in the liquid phasephase
allows solidification with only 0.1allows solidification with only 0.1--1010ººC C supercoolingsupercooling
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Rate is r = 1 / t0.5
Avrami rate equation y = 1- exp (-k t n)k & n are constants for specific system
Rate of Phase TransformationRate of Phase TransformationAll out of material –transformation complete
log tFrac
tion
tran
sfor
med
, y Fixed T
maximum rate reached – now amount unconverted decreases so rate slows
0.5
1.0
t0.5
rate increases as surface area increases & nuclei grow
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In general, rate increases as T ↑
r = 1/t0.5 = A e -Q/RT
R = gas constantT = temperature (K)A = preexponential factorQ = activation energy
Rate of Phase TransformationsRate of Phase Transformations
• r often small: equilibrium not possible!
135°C 119°C 113°C 102°C 88°C 43°C
1 10 102 104
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Eutectoid Transformation RateEutectoid Transformation Rate
Course pearlite formed at higher T - softerFine pearlite formed at low T - harder
Diffusive flow of C needed
α
αγ γ
α
• Growth of pearlite from austenite:
γαααα
α
α
pearlite growth direction
Austenite (γ)grain boundary
cementite (Fe3C)Ferrite (α)
γ
• Eutectoid transformationrate increases with ΔT.
675°C (ΔT smaller)
0
50
y(%
pea
rlite
)600°C
(ΔT larger)650°C
100
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• Reaction rate is a result of nucleation and growth of crystals.
• Examples:
Nucleation and GrowthNucleation and Growth
% Pearlite
0
50
100
Nucleation regime
Growth regime
log(time)t0.5
Nucleation rate increases with ΔT
Growth rate increases with T
T just below TENucleation rate lowGrowth rate high
γpearlite colony
T moderately below TE
γ
Nucleation rate med .Growth rate med.
Nucleation rate highT way below TE
γ
Growth rate low
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Transformations & Transformations & SupercoolingSupercooling
• Can make it occur at: ...727ºC (cool it slowly)...below 727ºC (“supercool” it!)
• Eutectoid transf. (Fe-C System): γ ⇒ α + Fe3C0.76 wt% C
0.022 wt% C6.7 wt% C
Fe3C
(cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
γ (austenite)
γ+L
γ +Fe3C
α +Fe3C
L+Fe3C
δ
(Fe) Co, wt%C
1148°C
T(°C)
αferrite
727°C
Eutectoid:Equil. Cooling: Ttransf. = 727ºC
ΔTSupercooling by ΔTtransf. < 727°C
0.76
0.02
2
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Isothermal Transformation DiagramsIsothermal Transformation Diagrams• Fe-C system, Co = 0.76 wt% C• Transformation at T = 675°C.
100
50
01 102 104
T = 675°C
y,
% tr
ansf
orm
ed
time (s)
400
500
600
700
1 10 102 103 104 1050%
pearlite100%
50%
Austenite (stable) TE (727°C)Austenite (unstable)
Pearlite
T(°C)
time (s)
isothermal transformation at 675°C
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• Eutectoid composition, Co = 0.76 wt% C• Begin at T > 727°C• Rapidly cool to 625°C and hold isothermally.
Effect of Cooling History in FeEffect of Cooling History in Fe--C SystemC System
400
500
600
700
0%pearlite
100%
50%
Austenite (stable) TE (727°C)Austenite (unstable)
Pearlite
T(°C)
1 10 102 103 104 105
time (s)
γ γ
γγ γ
γ
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Transformations with Transformations with Proeutectoid MaterialsProeutectoid Materials
Hypereutectoid composition – proeutectoid cementite
α
CO = 1.13 wt% C
TE (727°C)
T(°C)
time (s)
A
A
A+C
P
1 10 102 103 104
500
700
900
600
800
A+
P
Fe3C
(cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
γ (austenite)
γ+L
γ +Fe3C
α+Fe3C
L+Fe3C
δ
(Fe) Co, wt%C
T(°C)
727°CΔT
0.76
0.02
2
1.13
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NonNon--Equilibrium Transformation Equilibrium Transformation Products: FeProducts: Fe--CC• Bainite:
--α strips with longneedles of Fe3C
--diffusion controlled.• Isothermal Transf. Diagram
FeFe33CC(cementite)(cementite)
5 μm
α (ferrite)
10 103 105
time (s)10-1
400
600
800
T(°C)Austenite (stable)
200
P
B
TE
0% 100%
50%
pearlite/bainite boundary
A
A
100% bainite
100% pearlite
120 μm
cf: Pearlite
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-- α grains with sphericalFe3C
-- diffusion dependent.
-- heat bainite or pearlite for long times (e.g. 18 h at 700°C)
-- reduces interfacial area (driving force)
SpheroiditeSpheroidite: Fe: Fe--C SystemC System
(ferrite)
60 μm
α
(cementite)
Fe3C
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• Martensite:--γ(FCC) to Martensite (BCT)
• Isothermal Transf. Diagram
• γ to M transformation..-- is rapid!-- % transf. depends on
T only.
Martensite: FeMartensite: Fe--C SystemC System
Martensite needlesAustenite
60 μ
m
10 103 105 t/s10-1
400
600
800
T(°C)Austenite (stable)
200
P
B
TE
0%
100%50%
A
A
M + AM + A
M + A
0%50%90%
xx x
xx
x potential C atom sites
Fe atom sites
(involves single atom jumps)
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γ (FCC) αα (BCC) + Fe(BCC) + Fe33CCslow cooling
Martensite FormationMartensite Formation
tempering
quench
M (BCT)
M = martensite is body centered tetragonal (BCT)
Diffusionless transformation BCT if C > 0.15 wt%BCT few slip planes hard, brittle
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Phase Transformations of AlloysPhase Transformations of Alloys
Effect of adding Effect of adding other elementsother elementsChange transition Change transition
temp.temp.
Cr, Ni, Mo, Si, Cr, Ni, Mo, Si, MnMnretard retard γγ αα + + FeFe33C C transformationtransformation
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Dynamic Phase TransformationsDynamic Phase Transformations
On the isothermal transformation diagram On the isothermal transformation diagram for 0.45 wt% C Fefor 0.45 wt% C Fe--C alloy, sketch and label C alloy, sketch and label the timethe time--temperature paths to produce the temperature paths to produce the following microstructures:following microstructures:a)a) 42% proeutectoid ferrite and 58% coarse 42% proeutectoid ferrite and 58% coarse
pearlitepearliteb)b) 50% fine pearlite and 50% bainite50% fine pearlite and 50% bainitec)c) 100% martensite100% martensited)d) 50% martensite and 50% austenite50% martensite and 50% austenite
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A + B
A + P
A + αA
BP
A50%
0
200
400
600
800
0.1 10 103 105time (s)
M (start)M (50%)M (90%)
Example Problem for Example Problem for CCoo = 0.45 wt%= 0.45 wt%a)a) 42% 42%
proeutectoid proeutectoid ferrite and 58% ferrite and 58% coarse coarse pearlitepearlite(amounts (amounts determined by determined by phase diagram)phase diagram)
first make first make ferriteferritethen pearlitethen pearlite
course pearlitecourse pearlite∴∴ higher higher TT
T (°C)
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a. the amount of pearlite and proeutectoid ferrite (α) note: amount of pearlite = amount of γ just above TE
Co = 0.45wt% CCα = 0.022 wt% CCpearlite = Cγ = 0.76 wt% C
%9.57100x =−−
=+ αγ
α
αγγ
CCCCo
pearlite = 58%proeutectoid α = 42%
Fe3C
(cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
γ (austenite)
γ+L
γ + Fe3C
α + Fe3C
L+Fe3C
δ
Co, wt% C
1148°C
T(°C)
727°C
CO
R S
CγCα
Example Problem for Example Problem for CoCo = 0.45 wt%= 0.45 wt%
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b)b) 50% fine 50% fine pearlite and pearlite and 50% bainite50% bainite
first make pearlitefirst make pearlitethen bainitethen bainite
fine pearlitefine pearlite∴∴ lower lower TT
T (°C)
A + B
A + P
A + αA
BP
A50%
0
200
400
600
800
0.1 10 103 105time (s)
M (start)M (50%)M (90%)
Example Problem for Example Problem for CCoo = 0.45 wt%= 0.45 wt%
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A + B
A + P
A + αA
BP
A50%
0
200
400
600
800
0.1 10 103 105time (s)
M (start)M (50%)M (90%)
Example Problem for Example Problem for CCoo = 0.45 wt%= 0.45 wt%
c)c) 100 % 100 % martensite martensite –– quench = quench = rapid coolrapid cool
d) 50 % martensite and 50 % austenite d)
c)
T (°C)
24
Mechanical Prop: Mechanical Prop: FeFe--C System (1)C System (1)
• More wt% C: TS and YS increase, %EL decreases.
• Effect of wt% C
Co < 0.76 wt% CHypoeutectoid
Pearlite (med)ferrite (soft)
Co > 0.76 wt% CHypereutectoid
Pearlite (med)Cementite
(hard)
300
500
700
900
1100
YS
TS(MPa)
wt% C0 0.5 1
hardness
0.76
Hypo Hyper
wt% C0 0.5 10
50
100%EL
Impa
ct e
nerg
y (Iz
od, f
t-lb)
0
40
80
0.76
Hypo Hyper
TS
EL
Izod
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Mechanical Prop: Mechanical Prop: FeFe--C System (2)C System (2)• Fine vs coarse pearlite vs spheroidite
• Hardness:• %RA:
fine > coarse > spheroiditefine < coarse < spheroidite
80
160
240
320
wt%C0 0.5 1
Brin
ellh
ardn
ess
fine pearlite
coarsepearlitespheroidite
Hypo Hyper
0
30
60
90
wt%CD
uctil
ity (%
AR
)
fine pearlite
coarsepearlite
spheroidite
Hypo Hyper
0 0.5 1
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Mechanical Prop: Mechanical Prop: FeFe--C System (3)C System (3)
• Fine Pearlite vs Martensite:
• Hardness: fine pearlite << martensite.
0
200
wt% C0 0.5 1
400
600
Brin
ellh
ardn
ess martensite
fine pearlite
Hypo Hyper
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Tempering MartensiteTempering Martensite• reduces brittleness of martensite,• reduces internal stress caused by quenching.
• decreases TS, YS but increases %RA• produces extremely small Fe3C particles surrounded by α.
9 μm
MPa
800
1000
1200
1400
1600
1800
30405060
200 400 600Tempering T (°C)
%RA
TS
YS
%RA
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Summary: Processing OptionsSummary: Processing OptionsAustenite (γ)
Bainite(α + Fe3C plates/needles)
Pearlite(α + Fe3C layers + a proeutectoid phase)
Martensite(BCT phase diffusionless
transformation)
Tempered Martensite (α + very fine
Fe3C particles)
slow cool
moderatecool
rapid quench
reheat
Stre
ngth
Duc
tility
Martensite T Martensite
bainite fine pearlite
coarse pearlite spheroidite
General Trends