Phase diagram and TTT diagram

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Phase diagram and TTT diagram
Which informations are obtained from phase
diagram or TTT diagram?

• Phase diagram
• Describes equilibrium microstructural development
  that is obtained at extremely sow cooling or
  heating conditions.
• Provides no information on time to take to form
  phase and on shapes, size and distribution of
  phase ? importance of kinetics
• TTT diagram
• For a given alloy composition, the percentage
  completion of a given phase transformation on
  temperature-time axes is described.

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10.1 Nucleation And Growth

• For a eutectic reaction
• L (XE) ? ? ? at TE

• (experiment)
• (1) Quench the liquid from Tm to some lower
  temperature
• (2) Measure the time for solidification, to go to
  completion at that temperature
• TTT diagram
• The time required for the liquid to transform to
  the eutectic microstructure is function of time

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Description of new phase from melt

• Homogeneous nucleation occurs within a
  homogeneous medium.
• Heterogeneous nucleation nucleation occurs at
  some structural imperfection such as foreign
  surface, and hence with reduced surface energy

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The change in free energy for homogeneous
nucleation

• For the transformation of liquid to solid
• L ? S
• and for forming a spherical nucleus
• ?GT total free energy change
• r radius of embryo or nucleus
• ? specific surface free energy
• ?GV volume free energy change

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at r r

• n number of spherical nucleus of radius r

Nuclei larger than critical size (r) are stable
and can continue to grow.
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Nucleation rate

• Nucleation rate, number of nuclei / unit
  volume / unit time

,where ?G energy barrier to form a nucleus
stable to grow. ?ED activation energy for
diffusion

• At T just below Tm,
• Diffusion rate is rapid but very few nuclei are
  formed.
• ? ?G ?
• At very low T (?T ?)
• Diffusion rate is extremely low but many nuclei
  are formed
• ? ?G ?
• At intermediate T
• Max.

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Growth of nuclei

• Growth of Nuclei
• Growth of nuclei is a diffusional process
• , where QD
  activation energy for self diffusion
• Transformation rate of a phase

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10.2 The TTT Diagram

• Temperature-time-transformation curve
• TTT diagrams represent specific thermal histories
  for the given microstructure.

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Isothermal transformation of eutectoid steel
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TTT diagram for eutectoid steel
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Transformations of austenite ? ? ? Fe3C

• A. Diffusional transformations
• 1) At slightly lower T below 727 ? ?T ltlt
• Coarse pearlite nucleation rate is very low.
  diffusion rate is very high.
• 2) As the Tt (trans. temp.) decreases to 500 ?
• Fine pearlite nucleation rate increases.
  diffusion rate decreases.
• Strength ? (MPa) 139 46.4 S-1 S
  intermetallic spacing

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A. Diffusional transformations

• 3) 250 ? lt Tt lt 500 ?, below the nose in TTT
  diagram.
• Driving force for the transformation (? ? ?
  Fe3C) is very high.
• Diffusion rate is very low.
• Nucleation rate is very high.
• ? ? ? Fe3C Bainite cementite in the
  form of needle type.

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B. Diffusionless Transformations - Martensitic
trans.

• When the austenite is quenched to temp. below
  Ms ? ? ? (martensite)
• Driving force for trans. of austenite ?
  extremely high.
• Diffusoin rate is extremely slow.
• instead of the diffusional migration of carbon
  atoms to produce separate ? and Fe3C phases, the
  matensite transformation involves the sudden
  reorientation of C and Fe atoms from the
  austenite (FCC) to a body centered tetragonal
  (bct) solid solution.

? ? ? (martensite), a solid solution super
saturated carbon atoms in ? shearlike
transformation ? very hard and brittle phase
martensite
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• 1) Diffusionless transformation ? no
  compositional change during transformation.
• 2) The trans. of ? ? ? starts at Ms temp. and
  finishes at Mf temp.
• 3) ? ? ? (BCT) c/a increases as the carbon
  content increases.

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• A more complete TTT diagram for eutectoid steel.

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• TTT diagram for a hypereutectoid composition
  (1.13 wt C)

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Heat treatment of steel

• A. Continuous cooling trans. diagram for
  eutectoid steels

• Normalizing heat the steel into ? region ? cool
  it in air ? fine pearlite
• Annealing heat the steel into ? region ? cool
  it in furnace (power off) ? coarse pearlite

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Tempering

• Tempering a process of heating a martensitic
  steel at a temp. below the eutectoid temp. to
  make it softer and more ductile.
• Fe3C particles precipitates from the ? phase
  ? tempered martensite ? spheroidite

spheroidite
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Martempering

• Martempering a modified quenching procedure
  used for steels to minimize distortion and
  cracking upon quenching.
• Austenitizing ? quenching in hot oil or molten
  salt at a temp. just above the Ms ? moderate
  cooling to Ms

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Austempering

• Austempering an isothermal treatment which
  produce a bainite structure in some carbon steels.

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10.3 Hardenability

• Hardenability
• Relative ability of a steel to be hardened in
  depth by quenching.
• Depends on
• 1. Alloy composition Cr, Ni, V, Mo ? increase
  hardenability
• 2. Austenite grain size

• Hardenability of a steel increases with an
  addition of alloying elements such as Cr, Mo, Ni,
  W, ? C curve move to the right direction in the
  TTT diagram.

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Jominy test

• Jominy Test
• Measures the hardenability of a steel
• Specimen 25 dia. X 100 mm long steel bar

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• Hardenability curves for various steels

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10.4 Precipitation Hardening

• A strengthening of alloy by generating a fine
  dispersion of second phase precipitates
  functioning as dislocation barriers.

Coarse precipitates form at grain boundaries in
an Al-Cu alloy
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The change in free energy from precipitation

• The change in free energy from precipitation of a
  second phase
• A decrease in ?GV of the precipitate.
• An increase in free energy due to the surface
  energy for the formation of interface.
• An increase in free energy due to local
  distortion in the vicinity of the precipitates.

Coherent precipitates
Incoherent precipitates
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Effects of aging time on strength and hardness
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Precipitation or age hardening

• ex) for Al rich Al-Cu alloy system
• Case 1 slow cooling from ? phase region
  develops relatively coarse precipitates isolated
  at grain boundaries, which produce little
  hardening.

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• Case 2
• Solution treatment heating into the single
  phase (?) region and then quenched to room temp.,
  which produce a supersaturated solid solution or
  a metastable phase.
• Aging reheating to some intermediate temp. so
  that the solid state diffusion of copper atoms in
  Al is sufficiently rapid to produce a fine
  dispersion of precipitates in matrix.

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Hardness vs. aging time for age hardenable alloy
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10.5 Annealing

• Cold work mechanical deformation of a metal at
  relatively low temperatures. Thus, cold work of a
  metal increases significantly dislocation density
  from 108 (annealed state) to 1012 cm/cm3, which
  causes the metal to be hardened. ex) rolling,
  forging, and drawing etc.

• cold work (A0 - Af)/A0 x 100, where A0 is
  the original cross-sectional area and Af is the
  final cross-sectional area after cold working.
• With increasing cold work, the hardness and
  strength of alloys are increased whereas the
  ductility of the alloys are decreased.

Cold-rolling
Cold-drawing
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• Annealed crystal (grain)
  deformed or strained crystal
• When a metal is cold worked, most of energy goes
  into plastic deformation to change the shaped and
  heat generation. However, a small portion of the
  energy, up to 5 , remains stored in the
  material. The stored energy is mainly in the form
  of elastic energy in the strain fields
  surrounding dislocations and point defects
  generated during the cold work.
• Annealing a cold worked grains are quite
  unstable due to the strain energy. By heating the
  cold worked material to high temperatures where
  sufficient atomic mobility is available, the
  material can be softened and a new microstructure
  can emerge. This heat treatment is called
  annealing where recovery and recrystallization
  take place.

Cold work
(high energy state)
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Recovery and recrystallization

• Recovery
• A low temperature annealing.
• The concentration of point defects is decreased
  and dislocation is allowed to move to lower
  energy positions without gross microstructural
  change.
• Modest effects on mechanical behavior while
  electrical conductivity increases significantly.
• Recrystallization occurs at 1/3 to 1/2 Tm.
• during recrystallization process, new equiaxed,
  strain-free grains nucleate at high-stress
  regions in the cold-worked microstructure, and
  hence hardness and strength decrease whereas
  ductility increases. Recrystallization temp. is
  that at which recrystallization just reaches
  completion in 1 hour.

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• The processed of recovery and recrystallization
  of a cold worked represent a structural
  transformation, not true phase transformations.
  The driving force for recovery and
  recrystallization is associated with the strain
  energy stored in the crystal as a result of cold
  work.
• ? the amount of cold work
• ? grain size before cold work ? number
  of strain-free nuclei
• ? annealing temp.

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Variation of recrystallization temperature with
percent cold work for iron
Influence of annealing temperature on the tensile
strength and ductility of a brass alloy
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Grain growth

• Grain growth A large concentration of grain
  boundaries (fine grain structure) is reduced by
  grain growth that occurs by high temp. annealing.
  The driving force for the grain growth is the
  reduction in the grain boundary surface energy.

Stages of the recrystallization and grain growth
of brass