W A T K I N S - J O H N S O N C O M P A N Y Semiconductor Equipment Group

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Engineering 45
Metal PhaseTransforms (1)
Bruce Mayer, PE Licensed Electrical Mechanical
EngineerBMayer_at_ChabotCollege.edu
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Learning Goals.1 Phase Xforms

• Transforming one phase into another is a Function
  of Time

• Understand How time TEMPERATURE (t T) Affect
  the Transformation Rate
• Learn how to Adjust the Transformation RATE to
  Engineer NONequilibrium Structures

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Learning Goals.1 PhaseX2

• Transforming one phase into another is a Function
  of Time

• Understand the Desirable mechanical properties of
  NONequilibrium-phase structures

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Classes of Phase XForms

• Diffusion Dependent Single Phase
• No Change in Either The Number or Composition of
  Phases
• e.g. Allotropic Transforms, Grain-Growth
• Diffusion Dependent MultiPhase
• Two-Phase Structure e.g. a Mg2Pb in Mg-Pb
  alloy system
• DiffusionLess MetaStable Phase
• NonEquil Structure Frozen in Place

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Phase 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

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Nucleation Driving Force

• Driving force to nucleate increases as we
  increase ?T
• 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

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Solid-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

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Solidification by Nucleation

• Homogeneous nucleation
• Nuclei form in the bulk of liquid metal
• Requires supercooling (typically 80-300C)
• 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

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Homogeneous Nucleation Energy Effects
r critical nucleus nuclei lt r shrink
nucleigtr grow (to reduce energy)
Adapted from Fig.10.2(b), Callister 7e.
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Solidification Quantified
r critical radius
g surface free energy
Tm melting temperature
?HS latent heat of solidification
DT Tm - T supercooling
Note ?HS strong function of ?T
? weak function of ?T
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Phase Xform Processes

• Phase Transforms Typically Entail Two significant
  Time-Regions
• 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
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Phase Xform Processes cont.

• 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
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Avrami Phase Xform Kinetics

• The Avrami Eqn Describes the Kinetics of Phase
  Transformation

• Where
• y ? New-Phase Fraction (0-1, 0-100)
• t ? Time (s)
• k, n ? Time-Independent Constants (S-n, unitless)

• RATE of Xform r

• Where
• t0.5 ? Time Needed for 50 New-PhaseFormation

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Rcn Rate, r, as Fcn of T

• 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)

• Temperature is a Controlling Variable in the Heat
  Treating Process thru an Arrhenius Rln

• e.g. Cu Recrystallization
• In general, rate increases as T?

135?C
119?C
113?C
102?C
88?C
43?C
1
10
102
104
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MetaStability

• 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

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Recall Fe-C Eutectoid Xform

• The Austenite to FerriteCemtite Eutectoid Rcn
  Requires Large Redistribution of Carbon

• Forms Pearlite
• Can Equilibrium Cool 727.5C ? 726.5C and SLOWLY

• Or Can UNDERCool by Amount DT say 727C ? 600C
  and QUICKLY

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Eutectoid Xform Rate DT

• Recall the Growth of Pearlite from Cooled
  Austenite

• The g?Pearlite Rcn Rate Increases with the Degree
  of UnderCooling (larger DT)

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Eutectoid Xform Rate DT 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

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Eutectoid Xform Rate DT cont.2

• More RAPID Xform at LOWER Temps Seems to
  Contradict Arrhenius

Competing Process

• Lower Rcn Rate is Countered by Higher NUCLEATION
  rates for SuperCooled Conditions

max
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Nucleation and Growth

• Transformation Rate Results from the Combination
  of Nucleation AND Growth

• Nucleation Rate INcreases With SuperCooling (DT?)
  
• Grown Rate DEcreases with Super Cooling (DT?)

• Examples

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IsoThermal 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

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IsoThermal 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 DT perhaps ?
  727-550 C

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Rapid Cooling of Fe-C from g

• Eutectoid Composition C0 0.77 wt
• Cool Rapidly 740C ? 625C

• g Persists for about 3S Prior to Pearlite
  Nucleation
• To 50 Pearlite at about 6S
• r 1/6S
• Transformation Complete at about 15S

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Pearlite vs DT - Morphology

• 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

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Fe-C NonEquil Xform Products

• Bainite
• Ferrite, a, lathes (strips) with long rods of Fe3C

• Diffusion Controlled Formation
• Bainite Pearlite Compete
• Bainite Forms Below The Boundary at About 540 C

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Fe-C NonEquil Xform

• Spherodite
• Ferrite, a, Xtal-Matrix with spherical Fe3C
  Globules
• diffusion dependent
• heat bainite or pearlite for LONG times
• T-T-T Diagram ? 104 seconds
• reduces a-Fe3C Phase Boundary (driving force)

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Fe-C NonEquil Xform Products

• Martensite
• A Diffusionless, and Hence Speed-of-Sound Rapid,
  Xform from FCC g
• Poorly Understood Single Carbon-Atom Jumps
  Convert FCC Austenite to a Body Centered
  Tetragonal (BCT) Form

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Martensite 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 g?M is Rapid
• -Xformed to M depends ONLY on Temperature

• A Austenite
• P Pearlite
• B Bainite
• S Spherodite
• M Martensite

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Martensite Formation
? (FCC)

• ? (BCC) Fe3C

M martensite is body centered tetragonal (BCT)
Diffusionless transformation BCT if C gt
0.15 wt BCT ? few slip planes ? hard,
brittle
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WhiteBoard Work

• None Today

• Some Cool Pearlite

• So Named Because it Looks Like Mother-of-Pearl
  Oyster Shell
• Under MicroScope with Proper Mag Lighting

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Appendix 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-solidifie
d 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.I
B/photo.html
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Fe-C Phase Transforms

• Eutectoid Xform
• Pearlite only

• Hypo Eutectoid
• Includes ProEeutectiod a

ProEa