[email protected] ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 1 Bruce Mayer, PE Engineering-45:...

[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

[email protected]

Engineering 45

Metal PhaseMetal PhaseTransforms Transforms

(1)(1)



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



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:



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



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



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



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



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



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



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



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



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



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



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



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



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



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



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



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



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



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



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



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



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



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



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


200

P

B

TE

0%

100%

50%

A

A

Spheroidite100% spheroidite

100% spheroidite



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



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


200

P

B

TE

0%

100%50%A

A

S

M + AM + A

M + A

0%50%90%



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)



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



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



Fe-C Phase TransformsFe-C Phase Transforms

Eutectoid Xform• Pearlite only

Hypo Eutectoid • Includes

ProEeutectiod α

ProEα

Date post:	02-Jan-2016
Category:	Documents
Upload:	chloe-smith
View:	214 times
Download:	1 times

[email protected] ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 1 Bruce Mayer, PE Engineering-45:...

Documents