Chapter 11: Phase Transformations

1
Ferrite - BCC
Martensite - BCT
Austenite - FCC
Chapter 11 Phase Transformations
Fe3C (cementite)- orthorhombic
2
Phase Transformations

• Transformation rate
• Kinetics of Phase Transformation
• Nucleation homogeneous, heterogeneous
• Free Energy, Growth
• Isothermal Transformations (TTT diagrams)
• Pearlite, Martensite, Spheroidite, Bainite
• Continuous Cooling
• Mechanical Behavior
• Precipitation Hardening

3
Phase Transformations

• Phase transformations change in the number or
  character of phases.
• Simple diffusion-dependent
• No change in of phases
• No change in composition
• Example solidification of a pure metal,
  allotropic transformation, recrystallization,
  grain growth
• More complicated diffusion-dependent
• Change in of phases
• Change in composition
• Example eutectoid reaction
• Diffusionless
• Example metastable phase - martensite

4
Phase Transformations

• Most phase transformations begin with the
  formation of numerous small particles of the new
  phase that increase in size until the
  transformation is complete.
• Nucleation is the process whereby nuclei (seeds)
  act as templates for crystal growth.
• Homogeneous nucleation - nuclei form uniformly
  throughout the parent phase requires
  considerable supercooling (typically 80-300C).
• Heterogeneous nucleation - form at structural
  inhomogeneities (container surfaces, impurities,
  grain boundaries, dislocations) in liquid phase
  much easier since stable nucleating surface is
  already present requires slight supercooling
  (0.1-10ºC).

5
Supercooling

• During the cooling of a liquid, solidification
  (nucleation) will begin only after the
  temperature has been lowered below the
  equilibrium solidification (or melting)
  temperature Tm. This phenomenon is termed
  supercooling (or undercooling.
• The driving force to nucleate increases as ?T
  increases
• Small supercooling ? slow nucleation rate - few
  nuclei - large crystals
• Large supercooling ? rapid nucleation rate - many
  nuclei - small crystals

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Nucleation of a spherical solid particle in a
liquid

• The change in free energy DG (a function of the
  internal energy and enthalpy of the system) must
  be negative for a transformation to occur.
• Assume that nuclei of the solid phase form in the
  interior of the liquid as atoms cluster
  together-similar to the packing in the solid
  phase.
• Also, each nucleus is spherical and has a radius
  r.
• Free energy changes as a result of a
  transformation 1) the difference between the
  solid and liquid phases (volume free energy,
  DGV) and 2) the solid-liquid phase boundary
  (surface free energy, DGS).
• Transforming one phase into another takes time.

Liquid
DG DGS DGV
7
Homogeneous Nucleation Energy Effects
r critical nucleus for r lt r nuclei shrink
for r gtr nuclei grow (to reduce energy)
8
Solidification
Note ?Hf and ? are weakly dependent on ?T
9
Transformations Undercooling
g
Þ
a

Fe3C

Eutectoid transformation (Fe-Fe3C system)

For transformation to occur, must cool to below
727C
0.76 wt C
6.7 wt C
0.022 wt C

10
FRACTION OF TRANSFORMATION
Fraction transformed depends on time.
Transformation rate depends on T.
r often small equil not possible
2
11
Generation of Isothermal Transformation Diagrams
Consider
The Fe-Fe3C system, for Co 0.76 wt C A
transformation temperature of 675C.
100
T 675C
transformed
50
0
2
4
time (s)
1
10
10
12
Eutectoid Transformation Rate DT
Coarse pearlite ? formed at higher temperatures
relatively soft Fine pearlite ? formed at
lower temperatures relatively hard
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Nucleation and Growth
Reaction rate is a result of nucleation and
growth of crystals.
Examples
5
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Isothermal Transformation Diagrams

• 2 solid curves are plotted
• one represents the time required at each
  temperature for the start of the transformation
• the other is for transformation completion.
• The dashed curve corresponds to 50 completion.
• The austenite to pearlite transformation will
  occur only if the alloy is supercooled to below
  the eutectoid temperature (727C).
• Time for process to complete depends on the
  temperature.

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Isothermal Transformation Diagram
Eutectoid iron-carbon alloy composition, Co
0.76 wt C Begin at T gt 727C Rapidly cool
to 625C and hold isothermally.
Austenite-to-Pearlite
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Transformations Involving Noneutectoid
Compositions
Consider C0 1.13 wt C
Hypereutectoid composition proeutectoid
cementite
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Possible Transformations
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Coarse pearlite (high diffusion rate) and (b)
fine pearlite
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Bainite Non-Equil Transformation Products

• elongated Fe3C particles in a-ferrite matrix
• diffusion controlled
• a lathes (strips) with long rods of Fe3C

Martensite

Cementite
Ferrite
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Bainite Microstructure

• Bainite consists of acicular (needle-like)
  ferrite with very small cementite particles
  dispersed throughout.
• The carbon content is typically greater than
  0.1.
• Bainite transforms to iron and cementite with
  sufficient time and temperature (considered
  semi-stable below 150C).

21
Spheroidite Nonequilibrium Transformation

• Fe3C particles within an a-ferrite matrix
• diffusion dependent
• heat bainite or pearlite at temperature just
  below eutectoid for long times
• driving force reduction of a-ferrite/Fe3C
  interfacial area

10
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Pearlitic Steel partially transformed to
Spheroidite
23
Martensite Formation
Martensite needles
Austenite

• single phase
• body centered tetragonal (BCT) crystal structure
• BCT if C0 gt 0.15 wt C
• Diffusionless transformation
• BCT ? few slip planes ? hard, brittle
• transformation depends only on T of rapid
  cooling

24
An micrograph of austenite that was polished flat
and then allowed to transform into martensite.
The different colors indicate the displacements
caused when martensite forms.
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Isothermal Transformation Diagram

• Iron-carbon alloy with eutectoid composition.
• A Austenite
• P Pearlite
• B Bainite
• M Martensite

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Effect of Adding Other Elements
4340 Steel

• Other elements (Cr, Ni, Mo, Si and W) may cause
  significant changes in the positions and shapes
  of the TTT curves
• Change transition temperature
• Shift the nose of the austenite-to-pearlite
  transformation to longer times
• Shift the pearlite and bainite noses to longer
  times (decrease critical cooling rate)
• Form a separate bainite nose

nose
plain carbon steel

• Plain carbon steel primary alloying element is
  carbon.

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• Example 11.2
• Iron-carbon alloy with eutectoid composition.
• Specify the nature of the final microstructure (
  bainite, martensite, pearlite etc) for the alloy
  that is subjected to the following
  timetemperature treatments
• Alloy begins at 760C and has been held long
  enough to achieve a complete and homogeneous
  austenitic structure.
• Treatment (a)
• Rapidly cool to 350 C
• Hold for 104 seconds
• Quench to room temperature

Bainite, 100
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• Example 11.2
• Iron-carbon alloy with eutectoid composition.
• Specify the nature of the final microstructure (
  bainite, martensite, pearlite etc) for the alloy
  that is subjected to the following
  timetemperature treatments
• Alloy begins at 760C and has been held long
  enough to achieve a complete and homogeneous
  austenitic structure.
• Treatment (b)
• Rapidly cool to 250 C
• Hold for 100 seconds
• Quench to room temperature

Austenite, 100
Martensite, 100
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• Example 11.2
• Iron-carbon alloy with eutectoid composition.
• Specify the nature of the final microstructure (
  bainite, martensite, pearlite etc) for the alloy
  that is subjected to the following
  timetemperature treatments
• Alloy begins at 760C and has been held long
  enough to achieve a complete and homogeneous
  austenitic structure.
• Treatment (c)
• Rapidly cool to 650C
• Hold for 20 seconds
• Rapidly cool to 400C
• Hold for 103 seconds
• Quench to room temperature

Austenite, 100
Almost 50 Pearlite, 50 Austenite
Bainite, 50
Final 50 Bainite, 50 Pearlite
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Continuous Cooling Transformation Diagrams

• Isothermal heat treatments are not the most
  practical due to rapidly cooling and constant
  maintenance at an elevated temperature.
• Most heat treatments for steels involve the
  continuous cooling of a specimen to room
  temperature.
• TTT diagram (dotted curve) is modified for a CCT
  diagram (solid curve).
• For continuous cooling, the time required for a
  reaction to begin and end is delayed.
• The isothermal curves are shifted to longer times
  and lower temperatures.

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• Moderately rapid and slow cooling curves are
  superimposed on a continuous cooling
  transformation diagram of a eutectoid iron-carbon
  alloy.
• The transformation starts after a time period
  corresponding to the intersection of the cooling
  curve with the beginning reaction curve and ends
  upon crossing the completion transformation
  curve.
• Normally bainite does not form when an alloy is
  continuously cooled to room temperature
  austenite transforms to pearlite before bainite
  has become possible.
• The austenite-pearlite region (A---B) terminates
  just below the nose. Continued cooling (below
  Mstart) of austenite will form martensite.

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• For continuous cooling of a steel alloy there
  exists a critical quenching rate that represents
  the minimum rate of quenching that will produce a
  totally martensitic structure.
• This curve will just miss the nose where pearlite
  transformation begins

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• Continuous cooling diagram for a 4340 steel alloy
  and several cooling curves superimposed.
• This demonstrates the dependence of the final
  microstructure on the transformations that occur
  during cooling.
• Alloying elements used to modify the critical
  cooling rate for martensite are chromium, nickel,
  molybdenum, manganese, silicon and tungsten.

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Mechanical Properties

• Hardness
• Brinell, Rockwell
• Yield Strength
• Tensile Strength
• Ductility
• Elongation
• Effect of Carbon Content

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Mechanical Properties Influence of Carbon
Content
Pearlite (med)
ferrite (soft)
C0 lt 0.76 wt C
Hypoeutectoid
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Mechanical Properties Fe-C System
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Tempered Martensite

• Tempered martensite is less brittle than
  martensite tempered at 594 C.
• Tempering reduces internal stresses caused by
  quenching.
• The small particles are cementite the matrix is
  a-ferrite. US Steel Corp.

4340 steel
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Hardness as a function of carbon concentration
for steels
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Rockwell C and Brinell Hardness
Hardness versus tempering time for a
water-quenched eutectoid plain carbon steel
(1080) room temperature.
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Precipitation Hardening

• The strength and hardness of some metal alloys
  may be improved by the formation of extremely
  small, uniformly dispersed particles
  (precipitates) of a second phase within the
  original phase matrix.
• Other alloys that can be precipitation hardened
  or age hardened
• Copper-beryllium (Cu-Be)
• Copper-tin (Cu-Sn)
• Magnesium-aluminum (Mg-Al)
• Aluminum-copper (Al-Cu)
• High-strength aluminum alloys

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Phase Diagram for Precipitation Hardened Alloy

• Criteria
• Maximum solubility of 1 component in the other
  (M)
• Solubility limit that rapidly decreases with
  decrease in temperature (M?N).
• Process
• Solution Heat Treatment first heat treatment
  where all solute atoms are dissolved to form a
  single-phase solid solution.
• Heat to T0 and dissolve B phase.
• Rapidly quench to T1
• Nonequilibrium state (a phase solid solution
  supersaturated with B atoms alloy is soft,
  weak-no ppts).

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Precipitation Heat Treatment the 2nd stage

• The supersaturated a solid solution is usually
  heated to an intermediate temperature T2 within
  the ab region (diffusion rates increase).
• The b precipitates (PPT) begin to form as finely
  dispersed particles. This process is referred to
  as aging.
• After aging at T2, the alloy is cooled to room
  temperature.
• Strength and hardness of the alloy depend on the
  ppt temperature (T2) and the aging time at this
  temperature.

44
Precipitation Hardening
Particles impede dislocation motion. Ex
Al-Cu system Procedure
-- Pt A solution heat treat (get a solid
solution)
-- Pt B quench to room temp. (retain a solid
solution)
-- Pt C reheat to nucleate small q particles
within a phase.
At room temperature the stable state of an
aluminum-copper alloy is an aluminum-rich solid
solution (a) and an intermetallic phase with a
tetragonal crystal structure having nominal
composition CuAl2 (?).
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Precipitation Heat Treatment the 2nd stage

• PPT behavior is represented in the diagram
• With increasing time, the hardness increases,
  reaching a maximum (peak), then decreasing in
  strength.
• The reduction in strength and hardness after long
  periods is overaging (continued particle growth).

Small solute-enriched regions in a solid solution
where the lattice is identical or somewhat
perturbed from that of the solid solution are
called Guinier-Preston zones.
46
PRECIPITATION STRENGTHENING
Hard precipitates are difficult to shear.
Ex Ceramics in metals (SiC in Iron or Aluminum).
Result
24
47
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• Several stages in the formation of the
  equilibrium PPT (q) phase.
• supersaturated a solid solution
• transition (q) PPT phase
• equilibrium q phase within the a matrix phase.

48
Influence of Precipitation Heat Treatment on
Tensile Strength (TS), EL
2014 Al Alloy
TS peak with precipitation time. Increasing
T accelerates process.
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Effects of Temperature

• Characteristics of a 2014 aluminum alloy (0.9 wt
  Si, 4.4 wt Cu, 0.8 wt Mn, 0.5 wt Mg) at 4
  different aging temperatures.

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Aluminum rivets

• Alloys that experience significant precipitation
  hardening at room temp and after short periods
  must be quenched to and stored under refrigerated
  conditions.
• Several aluminum alloys that are used for rivets
  exhibit this behavior. They are driven while
  still soft, then allowed to age harden at the
  normal room temperature.