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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 University Iowa 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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Let’s do an experiment.Let’s heat a pure material so that it is a liquid at a uniform temperature, let it cool uniformly, and measure the temperature vs time.

t (sec)

T (°

C)

Tm

Freezing begins

Freezing ends

If we cool very slowly so that the system is always at equilibrium, then freezing will occur isothermally at Tm.

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Let’s do an experiment.

T (°

C)

Tm

∆T

Realistically, we do not observe an 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 liberated efficiently.

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

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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

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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

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Perspective

What is so special about the solid-liquid interface in metals?

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Early observation of dendrites

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Early observation of dendrites

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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 interfacesQuantification of driving forcesThermal and chemical partitioning

Kinetics

The diffusion of heat and soluteThe kinetics of atomistic processesNucleation kineticsInterface kinetics

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Critical Issues

The objective for today is to look at the evolution of cast microstructures from what may be a new viewpoint.

Competition

Selection

Instability (dynamic)

Local equilibrium

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Natural selection

If you want to study genetic – would you use antelope?

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Natural selection

Fruit flies

Atoms vibrate at ~10000 GHz,

- quite a prolific fruit fly!

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Competition and “natural selection”

In nature, everything is a competition, with many phenomena occurring simultaneously.

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Dynamic Instability

BUT – This is only a side view.

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Dynamic Instability

Front section viewSide view

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Dynamic Instability

Front section viewSide view

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Dynamic Instability

Front section viewSide view

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Dynamic Instability

Front section viewSide view

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Dynamic Instability

Front section viewSide view

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Dynamic Instability

Front section viewSide view

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Dynamic Instability

Front section viewSide view

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Dynamic Instability

Front section viewSide view

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Dynamic Instability

Top view

Frontsection view

Side view

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Dynamic Instability

Small fluctuationsor “perturbations” are NOT reinforced. Instead, they are counteracted, and the ball is returned to the original path.

Top view

A stable process

Frontsection view

Side view

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Dynamic InstabilityWhat happensin this case?

Top viewThe path might be straight.

Frontsection view

Side view

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Dynamic Instability

Any small “perturbations” would be reinforced, and the path would diverge.

Top view

An unstable process

Frontsection view

Side view

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Lesson learned

During phase transformations (actually always)

- The system relentlessly seeks the “best” path.

- Perturbations are ubiquitous.

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A simple (but useful) example

The evolution of a grain structure illustrates instability, competition, and selection.

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A simple example

The evolution of a grain structure:

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A simple example

The evolution of a grain structure:

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A simple example

The evolution of a grain structure:

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A simple example

The evolution of a grain structure:

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A simple example

The evolution of a grain structure:

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A simple example

The evolution of a grain structure:

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

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Competition within a single grain

During the growth of any given grain, every location is competing with every other location.

Which ones “win” and which ones “lose” depends on interfacial properties and how the crystal interacts with its surroundings.

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A simple (but useful) example

The evolution of a grain structure illustrates instability, competition, and selection.

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A simple example

The evolution of a grain structure:

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A simple example

The evolution of a grain structure:

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A simple example

The evolution of a grain structure:

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A simple example

The evolution of a grain structure:

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

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Solidification morphologiesIt is this competition within a growing grain that ultimately gives rise to most common solidification morphologies and casting microstructures.

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Dendritic grainsFor dendritic solidification, the final branch spacing sets the scale of microsegregation and porosity.

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A closer look

Let’s look at such a location in more detail.

S L

Let’s assume (momenarily) that the two phases are in equilibrium, so that the interface is not moving.

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At equilibrium

Typically, L-S interfaces in metals are atomisticallyrough.

S L

In addition, the interface continuously fluctuates with time.

EAM for pure Al (J.R. Morris)

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Interface motion

qLS

qq

q

This heat must be conducted away from the interface.

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Equiaxed vs directional growth

q

q

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Equiaxed vs directional growth

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Partitioning of soluteIn 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

S

L

Let’s now examine a full cooling path.

C

T

C

distance

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Partioning of solute

z

C

Distance (z)

S

LT

Region of constitutional supercooling.

z

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Instability criterion

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 the tips. The interface is morphologically unstable.

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Common growth modesConstrained (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 segregation length scale in dendritic growth.

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Morphological instability

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Morphological instability

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Dendritic structure

What is the length of the dendritic 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

When distance becomes on the 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

LS

S

L

This nonequilibriumsolute distribution results in a higher amount of eutectic constituent than predicted by the phase diagram.

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Examples of microstructure

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Eutectic solidification

αβ

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Eutectic solidification

Arrows to illustrate solute diffusion…

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Eutectic solidification

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Morphological selection

Ω=Iv(Pe)

∆Td α RV

λ

Observed behavior

* Interfacial properties, γ and µ, play a criticalrole in this selection.

∆T = aλV + b/λ

∆Tdα λV

V

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Summary of selection

Liquid

Solid

Local interfacial Conditions

IntrinsicBehavior

ExtrinsicContributorsPartitioning of heat

Partitioning of soluteDiffusion of heatDiffusion of soluteFluid convectionNucleation of new phases

(in Solid or Liquid) (G,Gc,V,K)(T,C,r,n)

Interface Stiffness&

Interface Mobility

Interface response

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Examples of simulations

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Dendritic grains

3-D alloy dendrite

J. A. Warren and W. L. George

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Prediction of grain structures

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Casting simulations

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Break time?

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Cast microstructures

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Dendrites in bronze

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Dendrites in brass

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Iron–carbon phase diagram

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Gray cast iron

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White cast iron

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?

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 morphologiesAll from the same composition of Al-Si.