Bg 5445-2005-Napolitano-structure of Cast Metals

7/25/2019 Bg 5445-2005-Napolitano-structure of Cast Metals

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MetalsConservation

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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

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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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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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Driving force and the importance of rate


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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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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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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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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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Perspective

What is so special about

the solid-liquid interfacein metals?

Metals


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Early observation of dendrites

Metals


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Early observation of dendrites

Metals


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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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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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Natural selection

Fruit flies

Atoms vibrate at ~10000 GHz,

- quite a prolific fruit fly!

Metals


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Competition and natural selection

In nature, everything is a competition, with manyphenomena occurring simultaneously.

Metalsi


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Dynamic Instability

BUT This is only a side view.

MetalsC ti


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Dynamic Instability

Front section viewSide view

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D i I t bilit


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Dynamic Instability


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D i I t bilit


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Dynamic Instability


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D i I t bilit


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Dynamic Instability


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D i I t bilit


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Dynamic Instability


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D i I t bilit


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Dynamic Instability


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Dynamic Instability


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Dynamic Instability


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Dynamic Instability


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Dynamic Instability


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Dynamic Instability


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Dynamic Instability

Top

view

Front

section

view

Side

view

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Dynamic Instability


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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

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S Dynamic Instability


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Dynamic Instability

What happensin this case?

Top

viewThe path might be straight.

Front

section

view

Side

view

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S Dynamic Instability


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Dynamic Instability

Any small perturbationswould be reinforced, andthe path would diverge.

Top

view

An unstable process

Front

section

view

Side

view

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Summer Lesson learned


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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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A simple (but useful) example

The evolution of a grain structure illustrates instability,

competition, and selection.

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Summer A simple example


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A simple example

The evolution of a grain structure:

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A simple example


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p p


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p p


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p p


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p p


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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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.

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SummerI tit t

A simple (but useful) example


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The evolution of a grain structure illustrates instability,

competition, and selection.

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A simple example


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A simple example


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A simple example


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A simple example


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The size distribution is governed by the competition between nucleationand growth. Both depend on T and alloy variables in different ways.

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Solidification morphologies


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It is this competition within a growing grain that ultimately gives rise to

most common solidification morphologies and casting microstructures.

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Dendritic grains


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For dendritic solidification, the final

branch spacing sets the scale ofmicrosegregation and porosity.

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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.

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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)

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Interface motion


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q

LS

qq

q

This heat must be conducted away from the interface.

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Equiaxed vs directional growth


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q

q

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Equiaxed vs directional growth


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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

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Partioning of solute


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S

L

Lets now examine a

full cooling path.

C

T

C

distance

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Partioning of solute


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z

C

Distance (z)

S

LT

Region of constitutionalsupercooling.

z

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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.

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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.

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Morphological instability


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Morphological instability


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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?

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Columnar to equiaxed transition


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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.

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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.

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Examples of microstructure


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Eutectic solidification


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Arrows to illustrate solute diffusion




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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


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


Examples of simulations


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Dendritic grains


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3-D alloy dendrite

J. A. Warren and W. L. George


Prediction of grain structures


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Casting simulations


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Break time?


Cast microstructures


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Dendrites in bronze


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Dendrites in brass


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Ironcarbon phase diagram


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Gray cast iron


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White cast iron


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This can be heat treated to yield malleable cast iron.

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Nodular (ductile) cast iron


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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.

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Diverse solidification morphologies

All from the same composition of Al-Si.


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Bg 5445-2005-Napolitano-structure of Cast Metals

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