Phase Transformations

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Phase Transformations

• Chapter 11
• Callister, 2001
• 5th Edition

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Why do we study phase transformations?

• The tensile strength of an Fe-C alloy of
  eutectoid composition can be varied between
  700-2000 MPa depending on the heat treatment
  process adopted. This shows that the desirable
  mechanical properties of a material can be
  obtained as a result of phase transformations
  using the right heat treatment process. In order
  to design a heat treatment for some alloy with
  desired RT properties, time and temperature
  dependencies of some phase transformations can be
  represented on modified phase diagrams.
• Based on this, we will learn
• Phase transformations in metals
• Microstructure and property dependence in Fe-C
  alloy system
• Precipitation HardeningCrystallization, Melting,
  and Glass Transition

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Phase Transformation in Metals

• Development of microstructure in both single- and
  two-phase alloys involves phase
  transformations-which involves the alteration in
  the number and character of the phases. Phase
  transformations take time and this allows the
  definition of transformation rate or kinetics.
• Phase transformations alter the microstructure
  and there can be three different classes of phase
  transformations
• Diffusion dependent transformations with no
  change in number of phases and composition (
  solidification of a pure metal, allotropic
  transformations, etc.)
• Diffusion dependent transformations with change
  in number of phases and composition (eutectoid
  reaction)
• Diffusionless transformations (martensitic
  transformation in steel alloys)

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• Kinetics of Solid State Reactions
  Transformations (formation of a new phase with a
  different composition and structure) involving
  diffusion depend on time (Chapter 6). Time is
  also necessary for the energy increase associated
  with the phase boundaries between parent and
  product phases.Moreover, nucleation, growth of
  the nuclei, formation of grains and grain
  boundaries and establishment of equilibrium take
  time. As a result we can say the transformation
  rate (progress of the transformation) is a
  function of time.
• In kinetic investigations, the fraction of
  reaction completed is measured as a function of
  time at constant T. Tranformation progress can be
  measured by microscopic examination or measuring
  a physical property (e.g., conductivity). The
  obtained data is plotted as fraction of the
  transformation versus logarithm of time.

k and n are time independent constants.
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• Temperature controls the kinetics of the
  transformations. For the recrystallization of Cu

For a specific temperature range, rate increases
according to
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• Multiphase transformations Phase transformations
  may be accomplished by varying temperature,
  composition and external pressure. Most of the
  phase transformations require some finite time to
  go to completion and the rate of transformation
  is important in the relationship between heat
  treatment and microstructure development.
• The rate of transformation to achieve the
  equilibrium state is very slow and equilibrium
  conditions are maintained if the heating/cooling
  is really slow. This is usually unpractical. In
  general, transformations are shifted to lower or
  higher temperatures for cooling and heating
  respectively. These phenomena are termed
  supercooling and superheating respectively. The
  more rapid the cooling or heating, the greater
  the supercooling or superheating. For example,
  Fe-C eutectoid reaction is typically displaced
  10-20C lower than the equilibrium transformation
  temperature.
• For many alloys, the preferred state is
  metastable state (intermediate between initial
  and eqm. states)

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• Microstructural and Property Changes in Fe-C
  Alloys
• Isothermal Transformation Diagrams
• Pearlite is a microstructural product of the
  transformation of

Temperature is important in this transformation
Each curve is obtained by rapidly cooling the
austenite to the temperature indicated.
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• The dependance of transformation to temperature
  and time can be analyzed best using the diagram
  below

Data for the construction of isothermal transforma
tion diagram is obtained from a series of plots
of the percentage transformation versus
logarithm of time investigated over a range of
temperatures.
727C
At T just below 727C, very long times (on the
order of 105 s) are required for
50 transformation and therefore transformation
rate is slow. The transformation rate
increases as T decreases, for example, at 540C
3 s is required for 50 completion.
isothermal transformation diagram for Fe-C alloy
of eutectoid composition
This type of diagram is valid for constant T.
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• This observation is in clear contradiction with
  the equation of

This is because in T range of 540C-727C, the
transformation rate is mainly controlled by the
rate of pearlite nucleation and nucleation rate
decreases with T increase. Q in this equation is
the activation energy for nucleation and it
increases with T increase. It has been found that
at lower T, the austenite decomposition is
diffusion controlled and the rate behavior can be
calculated using Q for diffusion which is
independent of T.
Isothermal phase diagrams are also called
time-temperature-transformation (T-T-T) plots.
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• An actual isothermal heat treatment curve on the
  isothermal transformation diagram

the thickness of the ferrite to cementite
layers is about 8 to 1.
rapid cooling
isothermal treatment
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• The layer thickness depends on temperature at
  which the isothermal transformation occurs. For
  example at T just below the eutectoid, relatively
  thick layers of both ferrite and cementite phases
  are produced. This structure is called coarse
  pearlite. At lower T, diffusion rates are slower,
  which causes formation of thinner layers at the
  vicinity of 5400C. This structure is called fine
  pearlite.

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• For Fe-C alloys of other compositions, a
  proeutectoid phase of ferrite or cementite will
  coexist with pearlite and therefore the
  isothermal transformation diagram has additional
  curves

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• Bainite is another microstructure formed as a
  result of transformation of austenite. Bainite
  consists of ferrite and cementite and diffusion
  processes take place as a result. This structure
  looks like needles or plates. There is no
  proeutectoid phase in bainite.

nose 5400C
2150C
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• Pearlitic and bainitic transformations are
  competitive and transformation from to another
  requires reheating. The kinetics of bainite
  formation follows the Equation relating the rate
  to temperature.
• If steel alloy with pearlitic or bainitic
  structure is heated to and left at a temperature
  below the eutectoid temperature (such as 7000C)
  for 18 to 24 hours, another microstructure,
  called spheroidite, forms.

ferrite
cementite
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• Another microstructure is formed when austenite
  is rapidly cooled or quenched to a relatively low
  temperature (ambient T) called martensite.
  Martensite is a single phase nonequilibrium
  structure and produced as a result of
  diffusionless transformation of austenite. The
  quenching rate should be very high to prevent
  carbon diffusion.
• FCC austenite undergoes a polymorphic
  transformation to a body centered tetragonal
  (BCT) martensite.

Fe
carbon
Steels can maintain their martensitic structure
indefinetely at RT. Since martensitic
transformation does not involve diffusion, it is
almost instantaneous. Therefore its
transformation is independent of time.
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Since martensite is in a nonequilibrium phase, it
does not appear on the phase diagram of Fe-Fe3C.
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• The austenite to martensite transformation is
  shown in the isothermal transformation diagram

Temperatures of these transformations change with
the composition of alloy and transformation to
martensite only depends on T not time. This type
of transformation is called athermal
transformation.
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• Steels in which C is the prime alloying element
  are called plain carbon steels, whereas alloy
  steels containg other elements, such as, Cr, Ni,
  Mo, W, etc.
• In the presence of other elements the isothermal
  transformaion diagrams may be different

Remarks about the diagram

• shifting to longer times the nose of austenite to
  pearlite transformation
• (a proeutectoid phase nose if it exists)
• 2. formation of separate bainite
• nose

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• Mechanical Behavior of Fe-C Alloys
• Pearlite Cementite is much harder but more
  brittle than ferrite. Therefore increasing the
  fraction of Fe3C will make the resulting material
  harder and stronger. Since Fe3C is brittle,
  increasing its content decreases ductility of the
  material.

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• The layer thickness is also important for the
  mechanical behavior of the material. Fine
  pearlite is harder and stronger than coarse
  pearlite. Coarse pearlite is more ductile than
  fine pearlite because of greater restriction to
  plastic deformation of the fine pearlite.

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• There is a large adherence between the two phases
  across a boundary of a and Fe3C. The strong and
  rigid cementite layers restricts the deformation
  of soft ferrite layers and as the phase boundary
  area increases per unit volume of the material,
  the degree of reinforcement is higher. In
  addition phase boundaries act like barriers for
  dislocation motions. This is why fine pearlite
  has a greater strength and hardness.
• Spheroidite has lower strength and hardness than
  pearlitic microstructures. This is becuase of the
  smaller phase boundary area in spheroiditic
  microstructures. Of all the steel alloys, those
  that are softest and weakest have a spheroidite
  microstructure. The spheroidized steels have
  higher ductility than coarse pearlite.

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• Bainite Bainitic steels have a finer structure
  and therefore they are stronger and harder than
  pearlitic ones. They have a good combination of
  strength and ductility

MartensiteThe hardest and strongest and the most
brittle form of the steel alloy. It has a
negligibly small ductility. Its hardness is
controlled by C content up to 0.6 wt rather than
its microstructure. These properties are the
results of effectiveness of the interstitial C
atoms in hindering dislocation motion and
existence of few slip systems for BCT structure.
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• Tempered Martensite Martensite is hard but also
  very brittle so that it can not be used in most
  of the applications. Any internal stress that has
  been introduced during quenching has a weakining
  effect. The ductility and toughness of the
  material can be enhanced by heat treatment called
  as tempering. This also helps to release any
  internal stress.

Tempering is performed by heating martensite to a
T below eutectoid temperature (2500C-6500C) and
keeping at that T for specified period of time.
The formation of tempered martensite is by
diffusional processes.
very small cementite particles dispersed homogeneo
usly.
ferrite matrix
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• Tempered martensite may be nearly as hard and
  strong as martensite, but with substantially
  enhanced ductility and toughness. The hardness
  and strength may be due to large area of phase
  boundary per unit volume of the material. The
  phase boundary acts like a barrier for
  dislocaitons. The continuous ferrite phase in
  tempered martensite adds ductility and toughness
  to the material.
• The size of the cementite particles is important
  factor determining the mechanical behavior.
• As the cementite particle size increases,
  material becomes softer and weaker. The
  temperature of tempering determines the cementite
  particle size. Since martensite-tempered
  martensite transformation involves diffusion, T
  increase will accelerate the diffusion and rate
  of cementite particle growth and rate of
  softening as a result.
• Heat treatment of martensite has two variables
• Temperature and time.

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This data is obtained for water
quenched eutectoid composition. As tempering
time increases the hardness decreases.
Overtempered martensite is relatively soft and
ductile.
This type of data is usually provided by the
steel manufacturer. For this data, martensite is
quenched in oil and tempering time is 1 hour.
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• Tempering of some steels may result in decrease
  in toughness and this is called temper
  embrittlement. These are the alloys with high
  concentrations of alloying elements, P, As, Ni,
  Cr, Sb and Sn. The presence of alloying elements
  increases the T at which the ductile-to-brittle
  transition occurs.
• This has been observed when steel is tempered at
  a temperature above about 5750C followed by slow
  cooling to RT or when tempering is carried out
  b/w 375-5750C.
• Crack propagation in this type of materials is
  intergranular, that is the fracture path follows
  the grain boundaries of the precursor austenite
  phase.
• We can avoid the temper embrittlement by
• compositional control
• tempering above 5750C or below 3750C followed by
  quenching to RT.

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• Summary of the Phase Transformations for Fe-C
  Alloy System

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• Precipitation Hardening is the enhancement of
  the strength and hardness by forming extremely
  small uniformly dispersed particles of a second
  phase within the original phase matrix. This can
  be done by phase tranformations at appropriate
  temperatures.
• Small particles in the new phase are called
  precipitates.
• For example Al-Cu, Cu-Be, Cu-Sn, Mg-Al and some
  ferrous alloys are precitation hardenable.
• Heat Treatments-Precipitation hardenable alloys
  usually contain two or more alloying elements.
  The simplest system is binary system A-B system.

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• There are conditions for the precipitation
  hardening to be applied
• considerably high solubility of the elements in
  each other
• solubility limit should rapidly decrease in
  concentration of the major element as T
  decreases.
• The composition of a preipitation hardenable
  alloy must be less than the maximum solubility.
• There are two different types of heat treatment
• 1) Solution heat treating all solute atoms are
  dissolved to form a single phase solid solution.
• Consider the alloy with C0 composition, which is
  heated to T0 -and waiting all ß phase to dissolve
  completely. At this point there is only a with C0
  composition. Then it is quenched to T1, which is
  usually RT so that any diffusion and formation of
  ß phase is prevented. The resulting material is a
  phase solid solution supersaturated with B atoms
  at T1.

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• 2) Precipitation heat treating Supersaturated a
  solid solution is heated to T2 within the aß two
  phase region, at which the diffusion rate is
  high.The ß precipitate phase forms as fine
  particles with a composition of Cß, which is
  called sometimes as aging. After aging time, the
  alloy is cooled to RT.

The behavior of a typical precipitation hardenable
alloy. The reduction in strength and
hardness after long time periods is called
overaging.
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• Mechanism of Hardening Precipitation hardening
  is commonly used in Al alloys with high strength.
  For example Al-Cu alloys.

a phase is a substitutional solid solution of Cu
in Al
? phase is intermetallic compound, CuAl2
During the initial hardening stage Cu atoms
cluster together in small discs (few atoms thick
and about 25 atoms in diameter) at countless
positions within a phase. The discs are very
small that they are not considered as particles.
With time and diffusion of Cu atoms, the discs
become particles and increase in size. The
particles undergoes two transition phases (?
and ?) before the formation of equilibrium ?
phase.
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• The strengthening process is improved by T
  increase.

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• Crystallization, Melting, and Glass Transition in
  Polymers
• Crystallization is the production of an ordered
  solid phase upon cooling from a liquid melt
  having no crystalline structure. The degree of
  crystallization controls the mechanical
  properties of the polymers. The crystallization
  of polymers follows the same steps as nucleation
  and growth processes.
• Time dependence of crystallization can be
  described by

(Avrami equation)
Since 100 crystallinity of the polymer is not
possible, the vertical axis is scaled
as normalized fraction crystallized. Fraction
1.0 is the highest level of crystallization.
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• Melting is the opposite of crystallization and
  occurs at melting temperature. Melting of a
  polymer takes place over a range of temperature
  and behavior of the polymer during melting
  depends on the history of the material, such as
  the temperature of crystallization, rate of
  heating.
• Glass transition This occurs in amorphous and
  semicrystalline polymers. As a result of glass
  transition, motions of molecular chains are
  restricted at lower temperatures. As T is
  decreased, a gradual transformation is observed
  from a liquid to a rubbery material and finally a
  rigid solid. The temperature of transition from
  rubbery to rigid solid phase is called glass
  transition temperature (Tg). Stiffness, heat
  capacity and thermal expansion coefficient are
  the major properties changed during this
  transition.
• Melting and glass transition temperatures are
  important since they define the upper and lower
  temperature limits of applications.

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• The temperature of melting and glass transition
  can be determined from a plot of specific volume
  (reciprocal of the density) versus temperature.

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• Factors influencing melting temperature
• Molecular chemistry and structure, chain
  stiffness (double bonds and aromatic groups
  lowers the flexibility), size and type of the
  side groups, molecular weight, degree of
  branching.
• Factors affecting glass transition temperature
• Molecular characteristics controlling the chain
  flexibility or stiffness control Tg to some
  degree. As chain flexibility is diminished glass
  transition temperature increases.
• Molecular weight also affects Tg