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MetalsConservation
SummerInstitute
The structure of cast metalsThe structure of cast metals
Ralph E. NapolitanoRalph E. Napolitano
Department of Materials Science & EngineeringDepartment of Materials Science & EngineeringIowa State UniversityIowa State University
Ames, IowaAmes, Iowa
Metals Conservation Summer InstituteMetals Conservation Summer InstituteJune 1, 2005June 1, 2005
IOWA STATE UNIVERSITYMaterials Science & Engineering
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MetalsConservation
SummerInstitute
Lets do an experiment.
Lets heat a pure materialso that it is a liquid
at a uniform temperature, let it cool uniformly,
and measure the temperature vs time.
t (sec)
T(C
)
Tm
Freezingbegins
Freezingends
If we cool veryslowly so that
the system is always at
equilibrium, then freezing will
occur isothermally at Tm.
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MetalsConservation
SummerInstitute
Lets do an experiment.
T(C
)
Tm
T
Realistically, we do not observean isothermal arrest.
t (sec)
Even at the same temperature, the
liquid phase contains more heat than
the solid.
This heat Is liberated upon freezing.
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MetalsConservation
SummerInstitute
Driving force and the importance of rate
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MetalsConservation
SummerInstitute
Driving force and the importance of rate
In our freezing
example, the heat
may be liberated
too quickly to be
liberatedefficiently.
Mother Nature tries
to optimize this
efficiency using any
and all means
available.
Still
You are all very
familiar with one
consequence of
such optimization
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MetalsConservation
SummerInstitute
How do metals freeze?
Metals freeze in much the same way that water freezes into the familiar snowflakes.
The Rasmussen & Libbrecht Collection
M t l
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MetalsConservation
SummerInstitute
Goals for this lecture
I. Fundamentals of solidification
II. The structure of cast metals
III. A brief history of casting technology
IV. Modern casting techniques
M t l
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MetalsConservation
SummerInstitute
How do metals freeze?
Here we compare the snowflake structures to a transparent organic metal-analog.
M.E. Glicksman, NASA-IDGE, 1997.
The Rasmussen & Libbrecht Collection
Metals
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MetalsConservation
SummerInstitute
Perspective
What is so special about
the solid-liquid interfacein metals?
Metals
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MetalsConservation
SummerInstitute
Early observation of dendrites
Metals
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MetalsConservation
SummerInstitute
Early observation of dendrites
Metals
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MetalsConservation
SummerInstitute
Critical Issues
The critical issues are essentially the same for all (most)phase transformations
Thermodynamics
Phase stability (phase diagrams)The energy of interfaces
Quantification of driving forces
Thermal and chemical partitioning
Kinetics
The diffusion of heat and soluteThe kinetics of atomistic processes
Nucleation kineticsInterface kinetics
Metals
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MetalsConservation
SummerInstitute
Critical Issues
The objective for today is to look at the evolution of castmicrostructures from what may be a new viewpoint.
Competition
Selection
Instability (dynamic)
Local equilibrium
Metals
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ConservationSummerInstitute
Natural selection
If you want to study genetic would you use antelope?
Metals
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ConservationSummerInstitute
Natural selection
Fruit flies
Atoms vibrate at ~10000 GHz,
- quite a prolific fruit fly!
Metals
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ConservationSummerInstitute
Competition and natural selection
In nature, everything is a competition, with manyphenomena occurring simultaneously.
Metalsi
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ConservationSummerInstitute
Dynamic Instability
BUT This is only a side view.
MetalsC ti
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ConservationSummerInstitute
Dynamic Instability
Front section viewSide view
MetalsConservation
D i I t bilit
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ConservationSummerInstitute
Dynamic Instability
Front section viewSide view
MetalsConservation
D i I t bilit
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ConservationSummerInstitute
Dynamic Instability
Front section viewSide view
MetalsConservation
D i I t bilit
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ConservationSummerInstitute
Dynamic Instability
Front section viewSide view
MetalsConservation
D i I t bilit
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ConservationSummerInstitute
Dynamic Instability
Front section viewSide view
MetalsConservation
D i I t bilit
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ConservationSummerInstitute
Dynamic Instability
Front section viewSide view
MetalsConservation
Dynamic Instability
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SummerInstitute
Dynamic Instability
Front section viewSide view
MetalsConservation
Dynamic Instability
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SummerInstitute
Dynamic Instability
Front section viewSide view
MetalsConservation
Dynamic Instability
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SummerInstitute
Dynamic Instability
Top
view
Front
section
view
Side
view
MetalsConservation
Dynamic Instability
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SummerInstitute
Dynamic Instability
Small fluctuationsor perturbations are
NOT reinforced. Instead,they are counteracted,
and the ball is returned tothe original path.
Top
view
A stable process
Front
section
view
Side
view
MetalsConservation
S Dynamic Instability
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SummerInstitute
Dynamic Instability
What happensin this case?
Top
viewThe path might be straight.
Front
section
view
Side
view
MetalsConservation
S Dynamic Instability
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SummerInstitute
Dynamic Instability
Any small perturbationswould be reinforced, andthe path would diverge.
Top
view
An unstable process
Front
section
view
Side
view
MetalsConservation
Summer Lesson learned
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SummerInstitute
Lesson learned
During phase transformations (actually always)
- The system relentlessly seeks the best path.
- Perturbations are ubiquitous.
MetalsConservation
Summer A simple (but useful) example
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SummerInstitute
A simple (but useful) example
The evolution of a grain structure illustrates instability,
competition, and selection.
MetalsConservation
Summer A simple example
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SummerInstitute
A simple example
The evolution of a grain structure:
MetalsConservation
Summer A simple example
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SummerInstitute
A simple example
The evolution of a grain structure:
MetalsConservation
Summer A simple example
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SummerInstitute
p p
The evolution of a grain structure:
MetalsConservation
Summer A simple example
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Institute
p p
The evolution of a grain structure:
MetalsConservation
Summer A simple example
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Institute
p p
The evolution of a grain structure:
MetalsConservation
Summer A simple example
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Institute
p p
The evolution of a grain structure:
The size distribution is governed by the competition between nucleationand growth. Both depend on T and alloy variables in different ways.
MetalsConservation
Summer Competition within a single grain
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Institute
During the growth of any
given grain, everylocation is competingwith every otherlocation.
Which ones winand which oneslose depends on
interfacial propertiesand how the crystal
interacts with itssurroundings.
MetalsConservation
SummerI tit t
A simple (but useful) example
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Institute
The evolution of a grain structure illustrates instability,
competition, and selection.
MetalsConservation
SummerInstit te
A simple example
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Institute
The evolution of a grain structure:
MetalsConservation
SummerInstitute
A simple example
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Institute
The evolution of a grain structure:
MetalsConservation
SummerInstitute
A simple example
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Institute
The evolution of a grain structure:
MetalsConservation
SummerInstitute
A simple example
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Institute
The evolution of a grain structure:
The size distribution is governed by the competition between nucleationand growth. Both depend on T and alloy variables in different ways.
MetalsConservation
SummerInstitute
Solidification morphologies
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Institute
It is this competition within a growing grain that ultimately gives rise to
most common solidification morphologies and casting microstructures.
MetalsConservation
SummerInstitute
Dendritic grains
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For dendritic solidification, the final
branch spacing sets the scale ofmicrosegregation and porosity.
MetalsConservation
SummerInstitute
A closer look
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Lets look atsuch a location
in more detail.
S L
Lets assume (momenarily) that the two phases arein equilibrium, so that the interface is not moving.
MetalsConservation
SummerInstitute
At equilibrium
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Typically, L-S
interfaces in metalsare atomistically
rough.
S L
In addition, theinterfacecontinuouslyfluctuates with time.
EAM for pure Al (J.R. Morris)
MetalsConservation
SummerInstitute
Interface motion
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q
LS
q
This heat must be conducted away from the interface.
MetalsConservation
SummerInstitute
Equiaxed vs directional growth
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q
q
MetalsConservation
SummerInstitute
Equiaxed vs directional growth
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MetalsConservation
SummerInstitute
Partitioning of solute
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In an alloy, suppose we extract some heat, reducing
the temperature and moving the interface.
L
S
The excess solute is rejected into the liquid. Like the heat,this solute must be conducted away from the interface.
S
L
CS CL
C0L
CL
C0
CS
MetalsConservation
SummerInstitute
Partioning of solute
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S
L
Lets now examine a
full cooling path.
C
T
C
distance
MetalsConservation
SummerInstitute
Partioning of solute
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z
C
Distance (z)
S
LT
Region of constitutionalsupercooling.
z
MetalsConservation
SummerInstitute
Instability criterion
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z
T
Region of constitutional
supercooling.
CmG G>
What is really happening here?
The driving force at the
tips of the perturbations is
greater than behind thetips. The interface is
morphologicallyunstable.
MetalsConservation
SummerInstitute
Common growth modes
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Constrained (directional) growth gives rise to certain typical solidification
morphologies.
liquidus
solidus
G
Planar Cellular Dendritic
Cooling rate is given by GV, and the local solidification time is T/GV. This is the
time available for dendrite arm coarsening and therefore controls the final segregationlength scale in dendritic growth.
MetalsConservation
SummerInstitute
Morphological instability
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MetalsConservation
SummerInstitute
Morphological instability
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MetalsConservation
SummerInstitute
Dendritic structure
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What is the length of thedendritic region (Mushy Zone)?
How is this related to shrinkage
porosity and hot tearing?
When does branching stop?
What is the final spacing?
What solute distribution is
observed in the casting?
MetalsConservation
SummerInstitute
Columnar to equiaxed transition
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MetalsConservation
SummerInstitute
Branching limit
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When distance becomes onthe order of D/V, there is no
longer enough distance for
the solute gradient to cause
instability.
We model such a small
system by assuming perfect
mixing in the liquid and no
mixing in the solid phase.
MetalsConservation
SummerInstitute
The Gulliver-Scheil model
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LS
S
L
This nonequilibrium
solute distribution
results in a higher
amount of eutectic
constituent than
predicted by the phasediagram.
MetalsConservation
SummerInstitute
Examples of microstructure
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MetalsConservation
SummerInstitute
Eutectic solidification
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MetalsConservationSummerInstitute
Eutectic solidification
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Arrows to illustrate solute diffusion
MetalsConservationSummerInstitute
Eutectic solidification
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MetalsConservationSummerInstitute
Morphological selection
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=Iv(Pe)
Td RV
Observed behavior
* Interfacial properties,and , play a critical
role in this selection.
T = aV + b/
Td V
V
MetalsConservationSummerInstitute
Summary of selection
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Liquid
Solid
Local interfacial
Conditions
IntrinsicBehavior
Extrinsic
ContributorsPartitioning of heat
Partitioning of solute
Diffusion of heatDiffusion of solute
Fluid convection
Nucleation of new phases
(in Solid or Liquid) (G,Gc,V,K)
(T,C,r,n)
Interface Stiffness
&
Interface Mobility
Interface response
MetalsConservationSummerInstitute
Examples of simulations
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MetalsConservationSummerInstitute
Dendritic grains
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3-D alloy dendrite
J. A. Warren and W. L. George
MetalsConservationSummerInstitute
Prediction of grain structures
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MetalsConservationSummerInstitute
Casting simulations
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MetalsConservationSummerInstitute
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Break time?
MetalsConservationSummerInstitute
Cast microstructures
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MetalsConservationSummerInstitute
Dendrites in bronze
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MetalsConservationSummerInstitute
Dendrites in brass
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MetalsConservation
SummerInstitute
Ironcarbon phase diagram
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MetalsConservation
SummerInstitute
Gray cast iron
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MetalsConservation
SummerInstitute
White cast iron
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This can be heat treated to yield malleable cast iron.
MetalsConservation
SummerInstitute
Nodular (ductile) cast iron
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MetalsConservation
SummerInstitute
What can we measure in a cast microstructure?How can we measure it?
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Visual / Optical microscopy / SEM
Optical microscopy / SEM
EPMA / SEM-EDS-WDS
Visual / Optical microscopy
Optical microscopy
Optical microscopy
SEM / TEM / EDS / WDS / EPMA
Visual / Optical microscopy
Primary dendrite spacing
Secondary dendrite spacing
Dendritic chemical segregation profile
Grain size
Shrinkage porosity
Percent of secondary phases
Composition of secondary phases
Dendritic/Equiaxed transition
What can it tell us about the casting conditions?What can it tell us about the casting conditions?
Chemical composition, Growth velocity, thermal gradient, Pouring temperature,
mold materials, impurities, etc.
MetalsConservation
SummerInstitute
Diverse solidification morphologies
All from the same composition of Al-Si.
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