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[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt1
Bruce Mayer, PE Engineering-45: Materials of Engineering
Bruce Mayer, PELicensed Electrical & Mechanical Engineer
Engineering 45
Metal PhaseMetal PhaseTransforms Transforms
(1)(1)
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt2
Bruce Mayer, PE Engineering-45: Materials of Engineering
Learning Goals.1 – Phase XformsLearning Goals.1 – Phase Xforms
Transforming one phase into another is a Function of Time:
Fe
(Austenite)
Eutectoid transformation
C FCC
Fe3C (cementite)
(ferrite)
+(BCC)
Understand How time & TEMPERATURE (t & T) Affect the Transformation Rate
Learn how to Adjust the Transformation RATE to Engineer NONequilibrium Structures
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt3
Bruce Mayer, PE Engineering-45: Materials of Engineering
Learning Goals.1 – PhaseX2Learning Goals.1 – PhaseX2
Fe
(Austenite)
Eutectoid transformation
C FCC
Fe3C (cementite)
(ferrite)
+(BCC)
Understand the Desirable mechanical properties of NONequilibrium-phase structures
Transforming one phase into another is a Function of Time:
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt4
Bruce Mayer, PE Engineering-45: Materials of Engineering
Classes of Phase XFormsClasses of Phase XForms
1. Diffusion Dependent – Single Phase• No Change in Either The Number or
Composition of Phases
• e.g.: Allotropic Transforms, Grain-Growth
2. Diffusion Dependent – MultiPhase• Two-Phase Structure; e.g. α + Mg2Pb in
Mg-Pb alloy system
3. DiffusionLess – MetaStable Phase• NonEquil Structure “Frozen” in Place
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt5
Bruce Mayer, PE Engineering-45: Materials of Engineering
Phase Xform → NucleationPhase Xform → Nucleation
Nuclei (seeds) act as the template to grow crystals
For a nucleus to form the rate of addition of atoms to the nucleus must be greater than rate of loss
Once nucleated, the new “structure” grows until reaching equilibrium
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt6
Bruce Mayer, PE Engineering-45: Materials of Engineering
Nucleation Driving ForceNucleation Driving Force
Driving force to nucleate increases as we increase ΔT• SuperCooling → Temp Below the eutectic
or, eutectoid
• SuperHeating → Temp Above the peritectic
Small Super Cooling → Few & Large Nuclei
Large Super Cooling → Rapid nucleation - many nuclei, small crystals
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt7
Bruce Mayer, PE Engineering-45: Materials of Engineering
Solid-State Reaction KineticsSolid-State Reaction Kinetics
“Kinetic” → Time Dependent Phase Xforms Often Require Changes
in Atom Position to Affect• Crystal Structure
• Local Chemical Composition
Atom Movement Requires DIFFUSION Diffusion is a TIME DEPENDENT
Physical Process
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt8
Bruce Mayer, PE Engineering-45: Materials of Engineering
Solidification by NucleationSolidification by Nucleation Homogeneous nucleation
• Nuclei form in the bulk of liquid metal• Requires supercooling (typically 80-300°C)
Heterogeneous nucleation• Much easier since stable “nucleus” is
already present at “defect” sites– Could be wall of a casting-mold or
impurities in the liquid phase
• Allows solidification with only 0.1-10ºC supercooling
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt9
Bruce Mayer, PE Engineering-45: Materials of Engineering
r* = critical nucleus: nuclei < r* shrink; nuclei>r* grow (to reduce energy)
Adapted from Fig.10.2(b), Callister 7e.
Homogeneous Nucleation & Energy EffectsHomogeneous Nucleation & Energy 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
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt10
Bruce Mayer, PE Engineering-45: Materials of Engineering
Solidification QuantifiedSolidification Quantified
TH
Tr
S
m
2*
Note: HS = strong function of T = weak function of T
r* decreases as T increases
For typical T r* ca. 100Å
HS = latent heat of solidification
Tm = melting temperature
= surface free energy
T = Tm - T = supercooling
r* = critical radius
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt11
Bruce Mayer, PE Engineering-45: Materials of Engineering
Phase Xform ProcessesPhase Xform Processes
Phase Transforms Typically Entail Two significant Time-Regions
1. Nucleation Formation of Very Small New-Phase “Starting” Particles
• Distribution is Usually Random, but can be assisted by “defects” in the Solid State
• Also Called the “Incubation” phase
T = const
Incubation
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt12
Bruce Mayer, PE Engineering-45: Materials of Engineering
Phase Xform Processes cont.Phase Xform Processes cont.
2. Growth New-Phase expands from the Nucleation “Starting” Particles to eventually Consume the Old-Phase
• If “Allowed” toProceed TheEquilibrium Phase-Fractions WillEventually Emerge
• This Stage of theXform ischaracterized by theTransformation Fraction, y
T = const
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt13
Bruce Mayer, PE Engineering-45: Materials of Engineering
Avrami Phase Xform KineticsAvrami Phase Xform Kinetics The Avrami Eqn
Describes the Kinetics of Phase Transformation
y
log (t)
Fixed T
0
0.5
1
t0.5
nktey 1• Where
– y New-Phase Fraction (0-1, 0-100%)
– t Time (s)
– k, n Time-Independent Constants (S-n, unitless)
%501 tr • Where
– t0.5 Time Needed for 50% New-PhaseFormation
RATE of Xform r
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt14
Bruce Mayer, PE Engineering-45: Materials of Engineering
Rcn Rate, r, as Fcn of TRcn Rate, r, as Fcn of T Temperature is a
Controlling Variable in the Heat Treating Process thru an Arrhenius Rln:
RTQAetr 5.01
• Where– R Gas Constant (8.31 J/mol-K)
– T Absolute Temperature (K)
– Q Activation Energy for the Reaction (J/mol)
– A Temperature-Independent Scalar (1/S)
– e.g. Cu Recrystallization
– In general, rate increases as T↑
135C 119C 113C 102C 88C 43C
1 10 102 104
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt15
Bruce Mayer, PE Engineering-45: Materials of Engineering
MetaStabilityMetaStability The Previous Eqn. Indicates that Rcn Rates
are Thermally Activated Typical Equilibrium Rcn Rates are Quite
Sluggish; Too slow to Be Maintained in a Practical Metal-Production Process• Most Metals are cooled More Rapidly Than
Equilibrium Conditions
Most Practical Metals are Thus SuperCooled and do NOT Exist in Equilibrium• They are thus MetaStable
– Quite Time-Stable; but Not Strictly in Equilibrium
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt16
Bruce Mayer, PE Engineering-45: Materials of Engineering
Recall Fe-C Eutectoid XformRecall Fe-C Eutectoid Xform
The Austenite to Ferrite+Cemtite Eutectoid Rcn Requires Large Redistribution of Carbon
ferrite
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6
.7
L
austenite
+L
+Fe 3C
Fe3C cementite+Fe3C
+
L+Fe3C
(Fe) Co, wt% C
Eutectoid:
0.7
7
727°C
T(°C)
T0
.02
2 Undercooling by T: Ttransf. < 727ºC
Equil. cooling: Ttransf. = 727ºC
Fe3C0.77wt%C
0.022wt%C6.7wt%C
Forms Pearlite• Can Equilibrium Cool:
727.5C → 726.5C; and SLOWLY
• Or Can UNDERCool by Amount T; say 727C → 600C; and QUICKLY
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt17
Bruce Mayer, PE Engineering-45: Materials of Engineering
Eutectoid Xform Rate ~ Eutectoid Xform Rate ~ TT Recall the Growth of Pearlite from Cooled
Austenite
pearlite growth direction
Austenite ()grain boundary
cementite (Fe3C)
ferrite ()
Diffusive flow of C needed
The →Pearlite Rcn Rate Increases with the Degree of UnderCooling (larger T)
675°C (T smaller)
1 10 102 103time (s)
0
50
100
y (
% p
earl
ite) 0
50
100
600°C (T larger)
650°C
% a
ust
enit
e
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt18
Bruce Mayer, PE Engineering-45: Materials of Engineering
Eutectoid Xform Rate ~ Eutectoid Xform Rate ~ T cont.1T cont.1 UnderCooling
Analogy• Liquid Water Can be
cooled below 32 °F (SuperCooled or UnderCooled)
• If any Ice Nucleates the Entire Liq body RAPIDLY Freezes
The Greater the SuperCooling, The More Rapid the Phase Transform
675°C (T smaller)
1 10 102 103time (s)
0
50
100
y (
% p
earl
ite) 0
50
100
600°C (T larger)
650°C
% a
ust
enit
e
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt19
Bruce Mayer, PE Engineering-45: Materials of Engineering
Eutectoid Xform Rate ~ Eutectoid Xform Rate ~ T cont.2T cont.2 More RAPID Xform
at LOWER Temps Seems to Contradict Arrhenius
675°C (T smaller)
1 10 102 103time (s)
0
50
100
y (
% p
earl
ite) 0
50
100
600°C (T larger)
650°C
% a
ust
enit
e
RTQAetr 5.01 Lower Rcn Rate is
Countered by Higher NUCLEATION rates for SuperCooled Conditions
Competing Process
max
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt20
Bruce Mayer, PE Engineering-45: Materials of Engineering
Nucleation and GrowthNucleation and Growth Transformation Rate Results from the
Combination of Nucleation AND Growth%
Pearl
ite
0
50
100
Nucleation regime
Growth regime
log (time)t50
• Nucleation Rate INcreases With SuperCooling (T↑)
• Grown Rate DEcreases with Super Cooling (T↑)
Examples
T just below TE Nucleation rate low
Growth rate high
pearlite colony
T moderately below TE
Nucleation rate med Growth rate med
Nucleation rate high
T way below TE
Growth rate low
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt21
Bruce Mayer, PE Engineering-45: Materials of Engineering
IsoThermal Xform DiagramsIsoThermal Xform Diagrams a.k.a. TIME-TEMP-
TRANSFORM (T-T-T) diagram• Example = Fe-C at
Eutectiod; C0 = 0.77 Wt%-Carbon At 675C– Moving Lt→Rt at 675C
notice intersection with 0% line → Incubation
Time 50% line →
Transformation Rate 100% line →
Completion
400
500
600
700
1 10 102 103 104 105
0%pearlite
100%
50%
Austenite (stable) TE (727°C)Austenite (unstable)
Pearlite
T(°C)
100
50
01 102 104
T=675°C
y,
% t
ran
sform
ed
time (s)
time (s)
isothermal Xform at 675°C
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt22
Bruce Mayer, PE Engineering-45: Materials of Engineering
IsoThermal Xform Dia. contIsoThermal Xform Dia. cont Notice
• Xform Lines make Asymptotic approach to TE
– LONG Xform Times for Equil Cooling
• Knee at Left on 0% line– Suggests Nucleation
Rate reaches a MAXIMUM (i.e.; it saturates at some large T; perhaps 727−550 C
400
500
600
700
1 10 102 103 104 105
0%pearlite
100%
50%
Austenite (stable) TE (727°C)Austenite (unstable)
Pearlite
T(°C)
100
50
01 102 104
T=675°C
y,
% t
ran
sform
ed
time (s)
time (s)
isothermal Xform at 675°C
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt23
Bruce Mayer, PE Engineering-45: Materials of Engineering
Rapid Cooling of Fe-C from Rapid Cooling of Fe-C from Eutectoid Composition; C0 = 0.77 wt%
Cool Rapidly: ~740C → 625C
1 10 102 103 104 105
time (s)
500
600
700
T(°C)
Austenite (stable)
Pearlite
0%pearlite
100%
50%
TE (727°C) • Persists for about 3S Prior to Pearlite Nucleation
• To 50% Pearlite at about 6S– r = 1/6S
• Transformation Complete at about 15S
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt24
Bruce Mayer, PE Engineering-45: Materials of Engineering
Pearlite vs Pearlite vs T - MorphologyT - Morphology
- Smaller T: colonies are larger
10 µ
m- Larger T: colonies are smaller
10 µ
m
TXform Just Below TE
• Higher T → C-Diffusion is Faster (can go Further)
• Pearlite is Coarser
TXform WELL Below TE
• Lower T → C-Diffusion is Slower (Shorter Diff-Dist)
• Pearlite is Finer
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt25
Bruce Mayer, PE Engineering-45: Materials of Engineering
Fe-C NonEquil Xform ProductsFe-C NonEquil Xform Products Bainite
• Ferrite, , lathes (strips) with long rods of Fe3C
Fe3C (cementite)
5 m
(ferrite)
Diffusion Controlled Formation• Bainite & Pearlite
Compete– Bainite Forms Below
The Boundary at About 540 °C
10 103 105
time (s)10-1
400
600
800
T(°C)Austenite (stable)
200
P
B
TE
0%
100%
50%
100% bainite
pearlite/bainite boundary100% pearlite
A
A
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt26
Bruce Mayer, PE Engineering-45: Materials of Engineering
Fe-C NonEquil XformFe-C NonEquil Xform Spherodite
• Ferrite, , Xtal-Matrix with spherical Fe3C “Globules”
• diffusion dependent
• heat bainite or pearlite for LONG times– T-T-T Diagram → ~104
seconds
• reduces -Fe3C Phase Boundary (driving force)
60 m
(ferrite)
Fe3C (cementite)
10 103 105time (s)10-1
400
600
800
T(°C)Austenite (stable)
200
P
B
TE
0%
100%
50%
A
A
Spheroidite100% spheroidite
100% spheroidite
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt27
Bruce Mayer, PE Engineering-45: Materials of Engineering
Fe-C NonEquil Xform ProductsFe-C NonEquil Xform Products Martensite
• A Diffusionless, and Hence Speed-of-Sound Rapid, Xform from FCC
• Poorly Understood Single Carbon-Atom Jumps Convert FCC Austenite to a Body Centered Tetragonal (BCT) Form x
x xx
x
xpotential C atom sites
Fe atom sites
Martentite needlesAustenite
60
m
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt28
Bruce Mayer, PE Engineering-45: Materials of Engineering
Martensite T-T-T DiagramMartensite T-T-T Diagram Martensite, M, is NOT
an Equil. Phase• Does NOT Appear on
the PHASE Diagram
• But it DOES Form– So Seen on Isothermal
Phase Xform Diagram
xForm →M is Rapid• %-Xformed to M
depends ONLY on Temperature
– A = Austenite
– P = Pearlite
– B = Bainite
– S = Spherodite
– M = Martensite
time (s)10 103 10510-1
400
600
800
T(°C)Austenite (stable)
200
P
B
TE
0%
100%50%A
A
S
M + AM + A
M + A
0%50%90%
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt29
Bruce Mayer, PE Engineering-45: Materials of Engineering
Martensite FormationMartensite Formation
slow cooling
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
(BCC) + Fe3C (FCC)
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt30
Bruce Mayer, PE Engineering-45: Materials of Engineering
WhiteBoard WorkWhiteBoard Work
None Today
Some Cool Pearlite• So Named
Because it Looks Like Mother-of-Pearl Oyster Shell– Under MicroScope
with Proper Mag & Lighting
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt31
Bruce Mayer, PE Engineering-45: Materials of Engineering
Appendix – 1-Xtal Turbine bldsAppendix – 1-Xtal Turbine blds
The blades are made out of a nickel-base superalloy with a microstructure containing about 65% of gamma-prime precipitates in a polycrystalline gamma matrix. The creep life of the blades is limited by the grain boundaries which are easy diffusion paths.
The blade is made out of a nickel-base superalloy with a microstructure containing about 65% of gamma-prime precipitates in a polycrystalline gamma matrix. It has been directionally-solidified, resulting in a columnar grain structure which mitigates grain-boundary induced creep.
The blade is made out of a nickel-base superalloy with a microstructure containing about 65% of gamma-prime precipitates in a single-crystal gamma matrix. The blade is directionally-solidified via a spiral selector, which permits only one crystal to grow into the blade.
The blade is made out of a nickel-base superalloy with a microstructure containing about 65% of gamma-prime precipitates in a polycrystalline gamma matrix. It has been Spiral-solidified, resulting in a single grain structure which eliminates grain-boundary induced creep.
http://www.msm.cam.ac.uk/phase-trans/2001/slides.IB/photo.html
[email protected] • ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt32
Bruce Mayer, PE Engineering-45: Materials of Engineering
Fe-C Phase TransformsFe-C Phase Transforms
Eutectoid Xform• Pearlite only
Hypo Eutectoid • Includes
ProEeutectiod α
ProEα